A Beginner’s Guide to ICP-MS - University of São Paulo

38 SPECTROSCOPY 16(4) APRIL 2001 www.spec t roscopyo n l i n e . com

Amazingly, 18 years after the com-mercialization of inductively cou-pled plasma mass spectrometry(ICP-MS), less than 4000 systemshave been installed worldwide. If

you compare this number with anotherrapid multielement technique, inductivelycoupled plasma optical emission spec-trometry (ICP-OES), first commercial-ized in 1974, the difference is quite signif-icant. In 1992, 18 years after ICP-OES wasintroduced, more than 9000 units hadbeen sold, and if you compare it with thesame time period that ICP-MS has beenavailable, the difference is even more dra-matic. From 1983 to the present day,more than 17,000 ICP-OES systems havebeen installed — more than four timesthe number of ICP-MS systems. If thecomparison is made with all atomic spec-troscopy instrumentation (ICP-MS, ICP-OES, graphite furnace atomic absorption[GFAA] and flame atomic absorption[FAA]), the annual turnover for ICP-MSis less than 7% of the total atomic spec-troscopy market — 400 units comparedto approximately 6000 atomic spec-troscopy systems. It’s even more surpris-ing when you consider that ICP-MS of-fers so much more than the othertechniques, including two of its most at-tractive features — the rapid multiele-ment capabilities of ICP-OES, combinedwith the superb detection limits of GFAA.

ICP-MS — ROUTINE OR RESEARCH?Clearly, one of the reasons is price — anICP-MS system typically costs twice asmuch as an ICP-OES system and threetimes more than a GFAA system. But in acompetitive world, the “street price” of anICP-MS system is much closer to a top-of-the-line ICP-OES system fitted with sam-pling accessories or a GFAA system thathas all the bells and whistles on it. So ifICP-MS is not significantly more expen-

sive than ICP-OES and GFAA, why hasn’tit been more widely accepted by the ana-lytical community? I firmly believe thatthe major reason why ICP-MS has notgained the popularity of the other traceelement techniques is that it is still con-sidered a complicated research tech-nique, requiring a very skilled person tooperate it. Manufacturers of ICP-MSequipment are constantly striving tomake the systems easier to operate, thesoftware easier to use, and the hardwareeasier to maintain, but even after 18 yearsit is still not perceived as a mature, rou-tine tool like flame AA or ICP-OES. Thismight be partially true because of the rel-ative complexity of the instrumentation;however, in my opinion, the dominantreason for this misconception is thatthere has not been good literature avail-able explaining the basic principles andbenefits of ICP-MS in a way that is com-pelling and easy to understand for some-one with very little knowledge of thetechnique. Some excellent textbooks (1,2) and numerous journal papers (3–5)are available that describe the fundamen-tals, but they tend to be far too heavy for

a novice reader. There is no question inmy mind that the technique needs to bepresented in a more user-friendly way tomake routine analytical laboratories morecomfortable with it. Unfortunately, thepublishers of the “for Dummies” series ofbooks have not yet found a mass (excusethe pun) market for writing one on ICP-MS. So until that time, we will be present-ing a number of short tutorials on thetechnique, as a follow-up to the posterthat was included in the February 2001issue of Spectroscopy.

During the next few months, we will bediscussing the following topics in greaterdepth:• principles of ion formation • sample introduction • plasma torch/radio frequency genera-

tor • interface region• ion focusing • mass separation • ion detection • sampling accessories• applications.

We hope that by the end of this series,we will have demystified ICP-MS, made it

TUTORIALTUTORIALS P E C T R O S C O P Y

A Beginner’s Guide to ICP-MSPart I

R O B E R T T H O M A S

Figure 1. Generation of positively charged ions in the plasma.

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GENERATION OF IONS IN THE PLASMAWe’ll start this series off with a brief de-scription of the fundamental principleused in ICP-MS — the use of a high-temperature plasma discharge to gener-ate positively charged ions. The sample,

typically in liquid form, is pumped intothe sample introduction system, which ismade up of a spray chamber and nebu-lizer. It emerges as an aerosol and eventu-ally finds its way — by way of a sample in-jector — into the base of the plasma. As ittravels through the different heatingzones of the plasma torch it is dried, va-porized, atomized, and ionized. Duringthis time, the sample is transformed froma liquid aerosol to solid particles, theninto a gas. When it finally arrives at theanalytical zone of the plasma, at approxi-mately 6000–7000 K, it exists as excitedatoms and ions, representing the elemen-tal composition of the sample.The excitation of the outer electron ofa ground-state atom, to producewavelength-specific photons of light, isthe fundamental basis of atomic emission.However, there is also enough energy inthe plasma to remove an electron from itsorbital to generate an ion. It is the genera-tion, transportation, and detection of sig-nificant numbers of these positivelycharged ions that give ICP-MS its charac-teristic ultratrace detection capabilities.It is also important to mention that, although ICP-MS is predominantly usedfor the detection of positive ions, negativeions (such as halogens) are also pro-duced in the plasma. However, becausethe extraction and transportation of nega-tive ions is different from that of positiveions, most commercial instruments arenot designed to measure them. Theprocess of the generation of positivelycharged ions in the plasma is shown con-ceptually in greater detail in Figure 1.


a little more compelling to purchase, andultimately opened up its potential as aroutine tool to the vast majority of thetrace element community that has not yetrealized the full benefits of its capabilities.

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Figure 3. Conversion of a chromium ground-state atom (Cr0) to anion (Cr1).

Figure 2. Simplified schematic of a chromium ground-state atom(Cr0).

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Figure 5. Relative abundance of the naturally occurring isotopes of all the elements (6). Reproduced with the permission of PerkinElmerInstruments (Norwalk, CT).


Table I. Breakdown of the atomic structure of copper isotopes.

63Cu 65Cu

Number of protons (p1) 29 29Number of electrons (e2) 29 29Number of neutrons (n) 34 36Atomic mass (p1 1 n) 63 65Atomic number (p1) 29 29Natural abundance 69.17% 30.83%Nominal atomic weight 63.55*

* Calculated using the formulae 0.6917n 1 0.3083n 1 p1 (referenced to theatomic weight of carbon)

Figure 4. Mass spectra of the two copper isotopes — 63Cu1 and65Cu1.

APRIL 2001 16(4) SPECTROSCOPY 41

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ION FORMATIONFigures 2 and 3 show the actual processof conversion of a neutral ground-stateatom to a positively charged ion. Figure 2shows a very simplistic view of thechromium atom Cr0, consisting of a nu-cleus with 24 protons (p1) and 28 neu-trons (n), surrounded by 24 orbiting elec-trons (e2) (It must be emphasized thatthis is not meant to be an accurate repre-sentation of the electrons’ shells and sub-shells, but simply a conceptual explana-tion for the purpose of clarity). From thiswe can say that the atomic number ofchromium is 24 (number of protons), andits atomic mass is 52 (number of protons1 neutrons).

If energy is then applied to thechromium ground-state atom in the formof heat from a plasma discharge, one ofthe orbiting electrons will be stripped offthe outer shell. This will result in only 23electrons left orbiting the nucleus. Be-cause the atom has lost a negative charge(e2) but still has 24 protons (p1) in thenucleus, it is converted into an ion with anet positive charge. It still has an atomicmass of 52 and an atomic number of 24,but is now a positively charged ion andnot a neutral ground-state atom. Thisprocess is shown in Figure 3.

NATURAL ISOTOPESThis is a very basic look at the process,because most elements occur in morethan one form (isotope). In fact,chromium has four naturally occurringisotopes, which means that thechromium atom exists in four differentforms, all with the same atomic numberof 24 (number of protons), but with differ-ent atomic masses (numbers of neu-trons).

To make this a little easier to under-stand, let’s take a closer look at an ele-ment like copper, which has only two dif-ferent isotopes — one with an atomicmass of 63 (63Cu) and the other with anatomic mass of 65 (65Cu). They both havethe same number of protons and elec-trons, but differ in the number of neu-trons in the nucleus. The natural abun-dances of 63Cu and 65Cu are 69.1% and30.9%, respectively, which gives copper anominal atomic mass of 63.55 — thevalue you see for copper in atomic weightreference tables. Details of the atomicstructure of the two copper isotopes areshown in Table I.

When a sample containing naturally oc-curring copper is introduced into the

plasma, two different ions of copper,63Cu1 and 65Cu1, are produced, whichgenerate different mass spectra — one atmass 63 and another at mass 65. This canbe seen in Figure 4, which is an actualICP-MS spectral scan of a sample contain-ing copper. It shows a peak for the 63Cu1

ion on the left, which is 69.17% abundant,and a peak for 65Cu1 at 30.83% abun-dance, on the right. You can also seesmall peaks for two Zn isotopes at mass64 (64Zn) and mass 66 (66Zn) (Zn has a to-tal of five isotopes at masses 64, 66, 67,68, and 70). In fact, most elements haveat least two or three isotopes and manyelements, including zinc and lead, havefour or more isotopes. Figure 5 is a chartthat shows the relative abundance of thenaturally occurring isotopes of all theelements.

During the next few months, we willsystematically take you on a journeythrough the hardware of an ICP massspectrometer, explaining how each majorcomponent works, and finishing the se-ries with an overview of how the tech-nique is being used to solve real-world ap-plication problems. Our goal is to presentboth the basic principles and benefits ofthe technique in a way that is clear, con-cise, and very easy to understand. Wehope that by the end of the series, youand your managers will be in a better po-sition to realize the enormous benefitsthat ICP-MS can bring to your laboratory.

REFERENCES(1) A. Montasser, Inductively Coupled Plasma

Mass Spectrometry (Wiley-VCH, Berlin,1998).

(2) F. Adams, R. Gijbels, and R. Van Grieken,Inorganic Mass Spectrometry (John Wileyand Sons, New York, 1988.).

(3) R.S. Houk, V. A. Fassel, and H.J. Svec, Dy-namic Mass Spectrom. 6, 234 (1981).

(4) A.R. Date and A.L. Gray, Analyst 106,1255 (1981).

(5) D.J. Douglas and J.B. French, Anal.Chem. 53, 37 (1982).

(6) Isotopic Composition of the Elements: PureApplied Chemistry 63(7), 991–1002(1991).

Robert Thomas is the principal of his ownfreelance writing and scientific consultingcompany, Scientific Solutions, based inGaithersburg, MD. He can be contacted byemail at [email protected] or viahis web site at www.scientificsolutions1.com.◆

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56 SPECTROSCOPY 16(5) MAY 2001 www.spec t roscopyo n l i n e . com

Part II of Robert Thomas’ series on induc-tively coupled plasma mass spectrometrylooks at one of the most critical areas of theinstrument — the sample introduction sys-tem. He discusses the fundamental princi-ples of converting a liquid into a fine-droplet aerosol suitable for ionization inthe plasma, and provides an overview ofthe different types of commercially avail-able nebulizers and spray chambers.

The majority of inductively coupledplasma mass spectrometry (ICP-MS) applications involve the analysisof liquid samples. Even though spec-troscopists adapted the technique

over the years to handle solids, it was de-veloped in the early 1980s primarily to an-alyze solutions. There are many ways ofintroducing a liquid into an ICP massspectrometer, but they all basicallyachieve the same result — they generatea fine aerosol of the sample so it can beefficiently ionized in the plasma dis-charge. The sample-introduction area hasbeen called the Achilles heel of ICP-MSbecause it is considered the weakestcomponent of the instrument, with only1–2% of the sample finding its way intothe plasma (1). Although there has re-cently been much improvement in thisarea, the fundamental design of an ICP-MS sample introduction system has notdramatically changed since the techniquewas first introduced in 1983.

Before discussing the mechanics ofaerosol generation in greater detail, let uslook at the basic components of a sampleintroduction system. Figure 1 shows theproximity of the sample introduction arearelative to the rest of the ICP mass spec-trometer, while Figure 2 represents theindividual components.

The mechanism of introducing a liquidsample into analytical plasma can be con-sidered as two separate events — aerosol

generation using a nebulizer and dropletselection by way of a spray chamber.Sharp carried out a thorough investiga-tion of both processes (2).

AEROSOL GENERATIONAs mentioned previously, the main func-tion of the sample introduction system isto generate a fine aerosol of the sample. Itachieves this purpose with a nebulizerand a spray chamber. The sample is nor-mally pumped at ~1 mL/min via a peri-staltic pump into the nebulizer. A peri-staltic pump is a small pump with lots ofminirollers that rotate at the same speed.The constant motion and pressure of therollers on the pump tubing feed the sam-ple to the nebulizer. The benefit of a peri-staltic pump is that it ensures a constantflow of liquid, irrespective of differencesin viscosity between samples, standards,and blanks. After the sample enters thenebulizer, the liquid is broken up into afine aerosol by the pneumatic action of

gas flow (~1 L/min) smashing the liquidinto tiny droplets, which is very similar tothe spray mechanism of a can of deodor-ant. Although pumping the sample is themost common approach to introducing it,some pneumatic nebulizers, such as theconcentric design, don’t need a pump be-cause they rely on the natural venturi ef-fect of the positive pressure of the nebu-lizer gas to suck the sample through thetubing. Solution nebulization is conceptu-ally represented in Figure 3, which showsaerosol generation using a nebulizer witha crossflow design.

DROPLET SELECTIONBecause the plasma discharge is ineffi-cient at dissociating large droplets, thespray chamber’s function is primarily toallow only the small droplets to enter theplasma. Its secondary purpose is tosmooth out pulses that occur during thenebulization process, due mainly to theperistaltic pump. Several ways exist to en-


A Beginner’s Guide to ICP-MSPart II: The Sample-Introduction System


Figure 1. ICP-MS system diagram showing the location of the sample introduction area.

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MAY 2001 16(5) SPECTROSCOPY 57

(4). Therefore, general-purpose ICP-OESnebulizers that are designed to aspirate1–2% dissolved solids, or high-solids neb-ulizers such as the Babbington, V-groove,or cone-spray nebulizers, which are de-signed to handle as much as 20% dis-solved solids, are not ideal for use withICP-MS. The most common of the pneu-matic nebulizers used in commercial ICPmass spectrometers are the concentricand crossflow designs. The concentric de-sign is more suitable for clean samples,while the crossflow is generally more tol-erant to samples containing higher levelsof solids or particulate matter.

Concentric design. In the concentric neb-ulizer, the solution is introduced througha capillary tube to a low-pressure regioncreated by a gas flowing rapidly past theend of the capillary. The low pressure andhigh-speed gas combine to break up thesolution into an aerosol, which forms atthe open end of the nebulizer tip. Figure5 illustrates the concentric design.

Concentric pneumatic nebulizers canprovide excellent sensitivity and stability,particularly with clean solutions. How-ever, the small orifices can be plagued byblockage problems, especially if largenumbers of heavy matrix samples areaspirated.

Crossflow design. For samples that con-tain a heavier matrix or small amounts ofundissolved matter, the crossflow designis probably the best option. With this de-sign the argon gas is directed at right an-gles to the tip of a capillary tube, in con-trast to the concentric design, where thegas flow is parallel to the capillary. Thesolution is either drawn up through thecapillary tube via the pressure created bythe high-speed gas flow or, as is most

common with crossflow nebulizers,forced through the tube with a peristalticpump. In either case, contact between thehigh-speed gas and the liquid streamcauses the liquid to break up into anaerosol. Crossflow nebulizers are gener-ally not as efficient as concentric nebuliz-ers at creating the very small dropletsneeded for ICP-MS analyses. However,the larger diameter liquid capillary andlonger distance between liquid and gasinjectors reduce clogging problems.Many analysts feel that the small penaltypaid in analytical sensitivity and precisionwhen compared with concentric nebuliz-ers is compensated by the fact that thecrossflow design is far more rugged forroutine use. Figure 6 shows a cross sec-tion of a crossflow nebulizer.

Microflow design. A new breed of nebu-lizers is being developed for ICP-MScalled microflow nebulizers, which aredesigned to operate at much lower sam-ple flows. While conventional nebulizershave a sample uptake rate of about1 mL/min, microflow nebulizers typicallyrun at less than 0.1 mL/min. They arebased on the concentric principle, but

sure only the small droplets get through,but the most common way is to use adouble-pass spray chamber where theaerosol emerges from the nebulizer andis directed into a central tube running thewhole length of the chamber. Thedroplets travel the length of this tube,where the large droplets (greater than~10 µm in diameter) fall out by gravityand exit through the drain tube at the endof the spray chamber. The fine droplets(~5–10 µm in diameter) then pass be-tween the outer wall and the central tube,where they eventually emerge from thespray chamber and are transported intothe sample injector of the plasma torch(3). Although many different designs areavailable, the spray chamber’s main func-tion is to allow only the smallest dropletsinto the plasma for dissociation, atomiza-tion, and finally ionization of the sample’selemental components. Figure 4 presentsa simplified schematic of this process.

Let us now look at the different nebu-lizer and spray chamber designs that aremost commonly used in ICP-MS. This ar-ticle cannot cover every type available be-cause a huge market has developed overthe past few years for application-specificcustomized sample introduction compo-nents. This market created an industry ofsmall OEM (original equipment manufac-turers) companies that manufacture partsfor instrument companies as well as sell-ing directly to ICP-MS users.

NEBULIZERSBy far the most common design used forICP-MS is the pneumatic nebulizer,which uses mechanical forces of a gasflow (normally argon at a pressure of20–30 psi) to generate the sampleaerosol. The most popular designs ofpneumatic nebulizers include concentric,microconcentric, microflow, and cross-flow. They are usually made from glass,but other nebulizer materials, such asvarious kinds of polymers, are becomingmore popular, particularly for highly cor-rosive samples and specialized applica-tions. I want to emphasize at this pointthat nebulizers designed for use with ICP-optical emission spectroscopy (OES) arenot recommended for ICP-MS. This factresults from a limitation in total dissolvedsolids (TDS) that can be put into the ICP-MS interface area. Because the orificesizes of the sampler and skimmer conesused in ICP-MS are so small (~0.6–1.2mm), the concentration of matrix compo-nents must generally be kept below 0.2%


Figure 2. Diagram of the ICP-MS sampleintroduction area.

Figure 3. Conceptual representation ofaerosol generation with an ICP-MSnebulizer.

Figure 4. Simplified representation of theseparation of large and fine droplets in thespray chamber.

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they usually operate at higher gas pres-sure to accommodate the lower sampleflow rates. The extremely low uptake ratemakes them ideal for applications withlimited sample volume or where the sam-ple or analyte is prone to sample intro-duction memory effects. These nebuliz-ers and their components are typically

constructed from polymer materials suchas polytetrafluoroethylene (PTFE), per-fluoroalkoxy (PFA), or polyvinylidene flu-oride (PVDF). In fact, their excellent cor-rosion resistance means that they havenaturally low blank levels. This character-istic, together with their ability to handlesmall sample volumes such as vapor-phase decomposition (VPD) applications,makes them an ideal choice for semicon-ductor labs that are carrying out ultra-trace element analysis (5). A typical mi-croflow nebulizer made from PFA isshown in Figure 7.

SPRAY CHAMBERSLet us now turn our attention to spraychambers. Basically two designs are usedin commercial ICP-MS instrumentation— double pass and cyclonic spray cham-bers. The double pass is by far the most

common, with the cyclonic type gainingin popularity. Another type of spray cham-ber based on the impact bead design(first developed for flame AA and thenadapted for ICP-OES) was tried on theearly ICP-MS systems with limited suc-cess, but is not generally used today. Asmentioned earlier, the function of thespray chamber is to reject the largeraerosol droplets and also to smooth outpulses produced by the peristaltic pump.In addition, some ICP-MS spray cham-bers are externally cooled (typically to2–5 °C) for thermal stability of the sam-ple and to minimize the amount of solventgoing into the plasma. This can have anumber of beneficial effects, dependingon the application, but the main benefitsare reduction of oxide species and theability to aspirate volatile organicsolvents.


Figure 5. Diagram of a typical concentric nebulizer. Figure 6. Schematic of a crossflow nebulizer.

Figure 7. A typical concentric microflownebulizer. Printed with permission from Ele-mental Scientific (Omaha, NE).

Figure 8. Schematic of a Scott double-pass spray chamber (shownwith crossflow nebulizer). Printed with permission of PerkinElmerInstruments (Norwalk, CT).

Figure 9. Schematic of a cyclonic spray chamber (shown withconcentric nebulizer).

58 SPECTROSCOPY 16(5) MAY 2001

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60 SPECTROSCOPY 16(5) MAY 2001 www.spec t roscopyo n l i n e . com

Double pass. By far the most commondesign of double-pass spray chamber isthe Scott design, which selects the smalldroplets by directing the aerosol into acentral tube. The larger droplets emergefrom the tube and, by gravity, exit thespray chamber via a drain tube. The liq-uid in the drain tube is kept at positivepressure (usually by way of a loop),which forces the small droplets back be-tween the outer wall and the central tube,where they emerge from the spray cham-ber into the sample injector of the plasmatorch. Scott double-pass spray chamberscome in a variety of shapes, sizes, andmaterials, but are generally consideredthe most rugged design for routine use.Figure 8 shows a Scott spray chambermade of a polysulfide-type material, cou-pled to a crossflow nebulizer.

Cyclonic spray chamber. The cyclonicspray chamber operates by centrifugalforce. Droplets are discriminated accord-ing to their size by means of a vortex pro-duced by the tangential flow of the sam-ple aerosol and argon gas inside thechamber. Smaller droplets are carried

with the gas stream into the ICP-MS,while the larger droplets impinge on thewalls and fall out through the drain. It isgenerally accepted that a cyclonic spraychamber has a higher sampling effi-ciency, which, for clean samples, trans-lates into higher sensitivity and lower de-tection limits. However, the droplet sizedistribution appears to be different from adouble-pass design, and for certain typesof samples, can give slightly inferior pre-cision. An excellent evaluation of the ca-pabilities of a cyclonic spray chamber wasmade by Beres and co-workers (6). Fig-ure 9 shows a cyclonic spray chamberconnected to a concentric nebulizer.

Many other nonstandard sample intro-duction devices are available that are notdescribed in this particular tutorial, suchas ultrasonic nebulization, membrane de-solvation, flow injection, direct injection,electrothermal vaporization, and laser ab-lation. However, they are becoming moreand more important, particularly as ICP-MS users are demanding higher perfor-mance and more flexibility. For that rea-son, they will be addressed in a separate

Circle 51

tutorial at the end of this series.

REFERENCES(1) R. A. Browner and A.W. Boorn, Anal.

Chem. 56, 786–798A (1984).(2) B.L. Sharp, Analytical Atomic Spectrome-

try 3, 613 (1980).(3) L.C. Bates and J.W. Olesik, Journal of An-

alytical Atomic Spectrometry 5(3), 239(1990).

(4) R.S. Houk, Anal. Chem. 56, 97A (1986).(5) E. Debrah, S. A. Beres, T.J. Gluodennis,

R.J. Thomas, and E.R. Denoyer, AtomicSpectroscopy, 197–202 (September 1995).

(6) S. A. Beres, P. H. Bruckner, and E.R. De-noyer, Atomic Spectroscopy, 96–99(March/April 1994).

Robert Thomas is the principal of his ownfreelance writing and scientific marketingconsulting company, Scientific Solutions,based in Gaithersburg, MD. He specializesin trace element analysis and can be con-tacted by e-mail at [email protected] or via his web site at www.scientificsolutions1.com. ◆

TRAINING COURSESThermo Nicolet (Madison, WI) is offering afree Spring 2001 Spectroscopic SolutionsSeminar Series. The seminars will coverbasic FT-IR spectroscopy, microspec-troscopy, dispersive Raman microscopy,Raman spectroscopy, and specializedsampling techniques. This year’s sched-ule includes the following seminars:

May 22 at the Syracuse Sheraton,Syracuse, NY; May 24 at the Wynd-ham Westborough Hotel, Westbor-ough, MA; May 24 at the MarriottOak Brook, Oak Brook, IL; June 5 atthe East Lansing Marriott, East Lans-ing, MI; June 5 at the Embassy Suites,Overland Park, KS; June 7 at the Sher-aton Indianapolis, Indianapolis, IN;June 12 at the Delta Meadowvale Conference Center, Mississauga, Ontario, Canada; June 12 at the Em-bassy Suites, Brookfield, WI; July 10at the Coast Terrace Inn, Edmonton,Alberta, Canada; and July 26 at the AlaMoana Hotel, Honolulu, HI.For more information, contact Thermo

Nicolet, (800) 201-8132 fax: (608) 273-5046, e-mail: [email protected], web site: www.thermonicolet.com. ◆

“News Spectrum” continued from page 13

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26 SPECTROSCOPY 16(6) JUNE 2001 www.spec t roscopyo n l i n e . com

Part III of Robert Thomas’ series on induc-tively coupled plasma–mass spectroscopy(ICP-MS) looks at the area where the ionsare generated — the plasma discharge. Hegives a brief historical perspective of someof the common analytical plasmas usedover the years and discusses the compo-nents that are used to create the ICP. Hefinishes by explaining the fundamentalprinciples of formation of a plasma dis-charge and how it is used to convert thesample aerosol into a stream of positivelycharged ions.

Inductively coupled plasmas are by farthe most common type of plasmasources used in today’s commercialICP–optical emission spectrometry(OES) and ICP-MS instrumentation.

However, it wasn’t always that way. In theearly days, when researchers were at-tempting to find the ideal plasma sourceto use for spectrometric studies, it wasunclear which approach would prove tobe the most successful. In addition toICPs, some of the other novel plasmasources developed were direct currentplasmas (DCP) and microwave-inducedplasmas (MIP). A DCP is formed when agas (usually argon) is introduced into ahigh current flowing between two orthree electrodes. Ionization of the gasproduces an inverted Y-shaped plasma.Unfortunately, early DCP instrumenta-tion was prone to interference effects andalso had some usability and reliabilityproblems. For these reasons, the tech-nique never became widely accepted bythe analytical community (1). However,its one major benefit was that it could as-pirate high levels of dissolved or sus-pended solids, because there was no re-strictive sample injector for the solidmaterial to block. This feature alonemade it attractive for some laboratories,and once the initial limitations of DCPs

were better understood, the techniquebecame more accepted. In fact, for thosewho want a DCP excitation source cou-pled with an optical emission instrumenttoday, an Echelle-based grating using asolid-state detector is commerciallyavailable (2).

Limitations in the DCP approach led tothe development of electrodeless plasma,of which the MIP was the simplest form.In this system, microwave energy (typi-cally 100–200 W) is supplied to the plasmagas from an excitation cavity around aglass or quartz tube. The plasma dis-charge in the form of a ring is generatedinside the tube. Unfortunately, eventhough the discharge achieves a veryhigh power density, the high excitationtemperatures exist only along a central fil-ament. The bulk of the MIP never getshotter than 2000–3000 K, which means itis prone to very severe matrix effects. Inaddition, they are easily extinguished dur-

ing aspiration of liquid samples. For thesereasons, they have had limited success asan emission source, because they are notconsidered robust enough for the analysisof real-world, solution-based samples.However, they have gained acceptance asan ion source for mass spectrometry (3)and also as emission-based detectors forgas chromatography.

Because of the limitations of the DCPand MIP approaches, ICPs became thedominant focus of research for both opti-cal emission and mass spectrometricstudies. As early as 1964, Greenfield andco-workers reported that an atmospheric-pressure ICP coupled with OES could beused for elemental analysis (4). Althoughcrude by today’s standards, the systemshowed the enormous possibilities of theICP as an excitation source and most defi-nitely opened the door in the early 1980sto the even more exciting potential of us-ing the ICP to generate ions (5).


A Beginner’s Guide to ICP-MSPart III: The Plasma Source


Iondetector

MSinterface

ICP torchIon optics

Spraychamber

Nebulizer

Mass separationdevice

Turbomolecularpump

Turbomolecularpump

Mechanicalpump

Radio frequencypower supply

Figure 1. Schematic of an ICP-MS system showing the location of the plasma torch and radiofrequency (RF) power supply.

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nents that are used to generate thesource: a plasma torch, a radio frequency(RF) coil, and RF power supply. Figure 1shows their proximity to the rest of theinstrument; Figure 2 is a more detailedview of the plasma torch and RF coil rela-tive to the MS interface.

The plasma torch consists of three con-centric tubes, which are usually madefrom quartz. In Figure 2, these are shownas the outer tube, middle tube, and sam-ple injector. The torch can either be one-piece with all three tubes connected, or itcan be a demountable design in whichthe tubes and the sample injector are sep-arate. The gas (usually argon) used toform the plasma (plasma gas) is passedbetween the outer and middle tubes at aflow rate of ;12–17 L/min. A second gasflow, the auxiliary gas, passes betweenthe middle tube and the sample injectorat ;1 L/min and is used to change theposition of the base of the plasma relativeto the tube and the injector. A third gasflow, the nebulizer gas, also flowing at;1 L/min carries the sample, in the formof a fine-droplet aerosol, from the sampleintroduction system (for details, see PartII of this series: Spectroscopy 16[5],56–60 [2001]) and physically punches achannel through the center of the plasma.The sample injector is often made frommaterials other than quartz, such as alu-mina, platinum, and sapphire, if highlycorrosive materials need to be analyzed.It is worth mentioning that although ar-gon is the most suitable gas to use for allthree flows, there are analytical benefitsin using other gas mixtures, especially inthe nebulizer flow (6). The plasma torch

THE PLASMA TORCHBefore we take a look at the fundamentalprinciples behind the creation of an in-ductively coupled plasma used in ICP-MS, let us take a look at the basic compo-


Plasma

Nebulizer gas

Interface Outertube

RFcoil

Sample injector

RF power

Middletube

Plasmagas

Auxiliarygas

Figure 2. Detailed view of a plasma torchand RF coil relative to the ICP-MS interface.

Quartztorch

Collision-inducedionization of argon

Formation of inductivelycoupled plasma

Sample introducedthrough sample injector

Loadcoil

Tangential flowof argon gas

Electromagneticfield High voltage

spark

(a) (b) (c)

(d) (e)

Figure 3. (right) Schematic of an ICP torchand load coil showing how the inductivelycoupled plasma is formed. (a) A tangentialflow of argon gas is passed between theouter and middle tube of the quartz torch.(b) RF power is applied to the load coil,producing an intense electromagnetic field.(c) A high-voltage spark produces free elec-trons. (d) Free electrons are accelerated bythe RF field, causing collisions and ioniza-tion of the argon gas. (e) The ICP is formedat the open end of the quartz torch. Thesample is introduced into the plasma viathe sample injector.

Circle 21

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JUNE 2001 16(6) SPECTROSCOPY 29

Initialradiationzone

Normalanalyticalzone

10,000 K

Preheatingzone

6500 K

6000 K

7500 K 8000 K

Figure 4. Different temperature zones in the plasma.

Droplet (Desolvation) Solid (Vaporization) Gas (Atomization) Atom (Ionization) Ion

From sample injector To mass spectrometer

M(H2O)1 X2 (MX)n MX M M1

Figure 5. Mechanism of conversion of a droplet to a positive ion in the ICP.

is mounted horizontally and positionedcentrally in the RF coil, approximately10–20 mm from the interface. It must beemphasized that the coil used in an ICP-MS plasma is slightly different from theone used in ICP-OES. In all plasmas,there is a potential difference of a fewhundred volts produced by capacitivecoupling between the RF coil and theplasma. In an ICP mass spectrometer,this would result in a secondary dis-charge between the plasma and the inter-face cone, which could negatively affectthe performance of the instrument. Tocompensate for this, the coil must begrounded to keep the interface region asclose to zero potential as possible. I willdiscuss the full implications of this ingreater detail in Part IV of this series.

FORMATION OF AN ICP DISCHARGELet us now discuss the mechanism of for-mation of the plasma discharge. First, atangential (spiral) flow of argon gas is di-rected between the outer and middle tubeof a quartz torch. A load coil, usually cop-per, surrounds the top end of the torchand is connected to a radio frequencygenerator. When RF power (typically750–1500 W, depending on the sample) isapplied to the load coil, an alternatingcurrent oscillates within the coil at a ratecorresponding to the frequency of thegenerator. In most ICP generators thisfrequency is either 27 or 40 MHz. ThisRF oscillation of the current in the coilcauses an intense electromagnetic field tobe created in the area at the top of thetorch. With argon gas flowing throughthe torch, a high-voltage spark is appliedto the gas, which causes some electronsto be stripped from their argon atoms.These electrons, which are caught up andaccelerated in the magnetic field, thencollide with other argon atoms, strippingoff still more electrons. This collision-

induced ionization of the argon continuesin a chain reaction, breaking down thegas into argon atoms, argon ions, andelectrons, forming what is known as aninductively coupled plasma discharge.The ICP discharge is then sustainedwithin the torch and load coil as RF en-ergy is continually transferred to itthrough the inductive coupling process.The sample aerosol is then introducedinto the plasma through a third tubecalled the sample injector. This wholeprocess is conceptionally shown inFigure 3.

THE FUNCTION OF THE RF GENERATORAlthough the principles of an RF powersupply have not changed since the workof Greenfield (4), the components havebecome significantly smaller. Some of theearly generators that used nitrogen or airrequired 5–10 kW of power to sustain theplasma discharge — and literally took uphalf the room. Most of today’s generatorsuse solid-state electronic components,which means that vacuum power ampli-fier tubes are no longer required. Thismakes modern instruments significantlysmaller and, because vacuum tubes werenotoriously unreliable and unstable, farmore suitable for routine operation.

As mentioned previously, two frequen-cies have typically been used for ICP RFgenerators: 27 and 40 MHz. These fre-quencies have been set aside specificallyfor RF applications of this kind, so theywill not interfere with other communica-tion-based frequencies. The early RF gen-erators used 27 MHz, while the more re-cent designs favor 40 MHz. Thereappears to be no significant analytical ad-vantage of one type over the other. How-ever, it is worth mentioning that the 40-MHz design typically runs at lower powerlevels, which produces lower signal inten-sity and reduced background levels. Be-

cause it uses slightly lower power, thismight be considered advantageous when it comes to long-term use of thegenerator.

The more important consideration isthe coupling efficiency of the RF genera-tor to the coil. The majority of modernsolid-state RF generators are on the orderof 70–75% efficient, meaning that 70–75%of the delivered power actually makes itinto the plasma. This wasn’t always thecase, and some of the older vacuumtube–designed generators were notori-ously inefficient; some of them experi-enced more than a 50% power loss. An-other important criterion to consider isthe way the matching network compen-sates for changes in impedance (a mater-ial’s resistance to the flow of an electriccurrent) produced by the sample’s matrixcomponents or differences in solventvolatility. In older crystal-controlled gen-erators, this was usually done with servo-driven capacitors. They worked very wellwith most sample types, but because theywere mechanical devices, they struggledto compensate for very rapid impedancechanges produced by some samples. As aresult, the plasma was easily extin-guished, particularly during aspiration ofvolatile organic solvents.

These problems were partially over-come by the use of free-running RF gen-erators, in which the matching networkwas based on electronic tuning of smallchanges in frequency brought about bythe sample solvent or matrix components.The major benefit of this approach wasthat compensation for impedancechanges was virtually instantaneous be-cause there were no moving parts. Thisallowed for the successful analysis ofmany sample types that would probablyhave extinguished the plasma of acrystal-controlled generator.

S P E C T R O S C O P Y T U T O R I A L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Clic

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IONIZATION OF THE SAMPLETo better understand what happens to thesample on its journey through the plasmasource, it is important to understand thedifferent heating zones within the dis-charge. Figure 4 shows a cross-sectionalrepresentation of the discharge alongwith the approximate temperatures fordifferent regions of the plasma.

zone before it eventually becomes a posi-tively charged ion in the analytical zone.To explain this in a very simplistic way,let’s assume that the element exists as atrace metal salt in solution. The first stepthat takes place is desolvation of thedroplet. With the water moleculesstripped away, it then becomes a verysmall solid particle. As the sample movesfurther into the plasma, the solid particlechanges first into a gaseous form andthen into a ground-state atom. The finalprocess of conversion of an atom to anion is achieved mainly by collisions of en-ergetic argon electrons (and to a lesserextent by argon ions) with the ground-state atom (7). The ion then emergesfrom the plasma and is directed into theinterface of the mass spectrometer (fordetails on the mechanisms of ion genera-tion, please refer to Part I of this series:Spectroscopy 16[4], 38–42 [2001]). Thisprocess of conversion of droplets intoions is represented in Figure 5.

The next installment of this series willfocus on probably the most crucial areaof an ICP mass spectrometer — the inter-face region — where the ions generatedin the atmospheric plasma have to besampled with consistency and electricalintegrity by the mass spectrometer,which is under extremely high vacuum.

REFERENCES(1) A.L. Gray, Analyst 100, 289–299 (1975).(2) G.N. Coleman, D.E. Miller, and R.W.

Stark, Am. Lab. 30(4), 33R (1998).(3) D.J. Douglas and J.B. French, Anal.

Chem. 53, 37-41 (1981).(4) S. Greenfield, I.L. Jones, and C.T. Berry,

Analyst 89, 713–720 (1964).(5) R.S. Houk, V. A. Fassel, and H.J. Svec,

Dyn. Mass Spectrom. 6, 234 (1981).(6) J.W. Lam and J.W. McLaren, J. Anal.

Atom. Spectom. 5, 419–424 (1990).(7) T. Hasegawa and H. Haraguchi, ICPs in

Analytical Atomic Spectrometry, A.Montaser and D.W. Golightly, Eds., 2d ed.(VCH, New York, 1992).

Robert Thomas is the principal of his ownfreelance writing and scientific marketingconsulting company, Scientific Solutions,based in Gaithersburg, MD. He specializesin trace-element analysis and can be con-tacted by e-mail at [email protected] or via his web site at www.scientificsolutions1.com. ◆


As mentioned previously, the sampleaerosol enters the injector via the spraychamber. When it exits the sample injec-tor, it is moving at such a velocity that itphysically punches a hole through thecenter of the plasma discharge. It thengoes through a number of physicalchanges, starting at the preheating zoneand continuing through the radiation

Circle 22

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www.spec t roscopyo n l i n e . com

The interface region is probably themost critical area of the whole induc-tively coupled plasma mass spec-trometry (ICP-MS) system. It cer-tainly gave the early pioneers of the

technique the most problems to over-come. Although we take all the benefitsof ICP-MS for granted, the process of tak-ing a liquid sample, generating an aerosolthat is suitable for ionization in theplasma, and then sampling a representa-tive number of analyte ions, transportingthem through the interface, focusingthem via the ion optics into the massspectrometer, finally ending up with de-tection and conversion to an electronicsignal, are not trivial tasks. Each part ofthe journey has its own unique problemsto overcome but probably the most chal-lenging is the movement of the ions fromthe plasma to the mass spectrometer.Let’s begin by explaining how the ion-

sampling process works, which will givereaders an insight into the many prob-lems faced by the early researchers.

SAMPLING THE IONSFigure 1 shows the proximity of the inter-face region to the rest of the instrument.The role of the interface is to transportthe ions efficiently, consistently, and withelectrical integrity from the plasma,which is at atmospheric pressure (760Torr), to the mass spectrometer analyzerregion, which is at approximately 1026

Torr. One first achieves this by directingthe ions into the interface region. The in-terface consists of two metallic coneswith very small orifices, which are main-tained at a vacuum of ;2 Torr with a me-chanical roughing pump. After the ionsare generated in the plasma, they passthrough the first cone, known as the sam-pler cone, which has an orifice diameter

of 0.8–1.2 mm. From there they travel ashort distance to the skimmer cone,which is generally sharper than the sam-pler cone and has a much smaller orifice(0.4–0.8 mm i.d.). Both cones are usuallymade of nickel, but they can be made ofmaterials such as platinum that are farmore tolerant to corrosive liquids. To re-duce the effects of the high-temperatureplasma on the cones, the interface hous-ing is water-cooled and made from a ma-terial that dissipates heat easily, such ascopper or aluminum. The ions thenemerge from the skimmer cone, wherethey are directed through the ion optics,and finally are guided into the mass sepa-ration device. Figure 2 shows the inter-face region in greater detail; Figure 3shows a close-up of the sampler andskimmer cones.

CAPACITIVE COUPLING This process sounds fairly straight-forward but proved very problematic dur-ing the early development of ICP-MS be-cause of an undesired electrostatic(capacitive) coupling between the loadcoil and the plasma discharge, producinga potential difference of 100–200 V. Al-though this potential is a physical charac-teristic of all inductively coupled plasmadischarges, it is particularly serious in anICP mass spectrometer because the ca-pacitive coupling creates an electrical dis-charge between the plasma and the sam-pler cone. This discharge, commonlycalled the pinch effect or secondary dis-charge, shows itself as arcing in the re-gion where the plasma is in contact withthe sampler cone (1). This process isshown very simplistically in Figure 4.

If not taken care of, this arcing cancause all kinds of problems, including anincrease in doubly charged interferingspecies, a wide kinetic energy spread ofsampled ions, formation of ions gener-


A Beginner’s Guide to ICP-MSPart IV: The Interface Region


Iondetector

MSinterface

ICP torchIon optics

Spraychamber

Nebulizer


Turbomolecularpump

Turbomolecularpump

Mechanicalpump

Radio power supply

Figure 1. Schematic of an inductively coupled plasma mass spectrometry (ICP-MS) system,showing the proximity of the interface region.

26 SPECTROSCOPY 16(7) JULY 2001

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ated from the sampler cone, and a de-creased orifice lifetime. These problemswere reported by many of the early re-searchers of the technique (2, 3). In fact,because the arcing increased with sam-pler cone orifice size, the source of thesecondary discharge was originallythought to be the result of an electro-gas-dynamic effect, which produced an in-crease in electron density at the orifice(4). After many experiments it was even-tually realized that the secondary dis-charge was a result of electrostatic cou-pling of the load coil to the plasma. Theproblem was first eliminated by ground-ing the induction coil at the center, whichhad the effect of reducing the radio fre-quency (RF) potential to a few volts. Thiseffect can be seen in Figure 5, taken fromone of the early papers, which shows thereduction in plasma potential as the coil isgrounded at different positions (turns)along its length.

Originally, the grounding was imple-mented by attaching a physical ground-ing strap from the center turn of the coilto the interface housing. In today’s instru-mentation the grounding is achieved in anumber of different ways, depending onthe design of the interface. Some of themost popular designs include balancingthe oscillator inside the circuitry of theRF generator (5); positioning a groundedshield or plate between the coil and theplasma torch (6); or using two interlacedcoils where the RF fields go in opposingdirections (7). They all work differentlybut achieve a similar result of reducing oreliminating the secondary discharge.

ION KINETIC ENERGYThe impact of a secondary discharge can-

not be overestimated with respect to its ef-fect on the kinetic energy of the ions beingsampled. It is well documented that the en-ergy spread of the ions entering the massspectrometer must be as low as possible toensure that they can all be focused effi-ciently and with full electrical integrity bythe ion optics and the mass separation de-vice. When the ions emerge from the ar-gon plasma, they will all have different ki-netic energies based on their mass-to-change ratio. Their velocities should all besimilar because they are controlled byrapid expansion of the bulk plasma, whichwill be neutral as long as it is maintained atzero potential. As the ion beam passesthrough the sampler cone into the skim-mer cone, expansion will take place, but itscomposition and integrity will be main-tained, assuming the plasma is neutral.This can be seen in Figure 6.

Electrodynamic forces do not play arole as the ions enter the sampler or theskimmer because the distance overwhich the ions exert an influence on eachother (known as the Debye length) issmall (typically 1023–1024 mm) com-pared with the diameter of the orifice(0.5–1.0 mm) (8), as Figure 7 shows.

It is therefore clear that maintaining aneutral plasma is of paramount impor-tance to guarantee electrical integrity ofthe ion beam as it passes through the in-terface region. If a secondary dischargeis present, it changes the electrical char-acteristics of the plasma, which will affectthe kinetic energy of the ions differently,depending on their mass. If the plasma isat zero potential, the ion energy spread isin the order of 5–10 eV. However, if a sec-ondary discharge is present, it results ina much wider spread of ion energies en-

tering the mass spectrometer (typically20–40 eV), which makes ion focusing farmore complicated (8).

BENEFITS OF A WELL-DESIGNEDINTERFACEThe benefits of a well-designed interfaceare not readily obvious if simple aqueoussamples are analyzed using only one setof operating conditions. However, it be-comes more apparent when many differ-ent sample types are being analyzed, re-quiring different operating parameters. Atrue test of the design of the interface oc-curs when plasma conditions need to bechanged, when the sample matrixchanges, or when a dry sample aerosol isbeing introduced into the ICP-MS. Ana-lytical scenarios like these have the po-tential to induce a secondary discharge,change the kinetic energy of the ions en-tering the mass spectrometer, and affectthe tuning of the ion optics. It is therefore


Plasmatorch

Ionoptics

~2 Torrvacuum

SampleconeSkimmer

cone

RF coil

Figure 2. Detailed view of the interfaceregion.

Interfacehousing

Samplercone

Skimmercone

Figure 3. Close-up view of the sampler andskimmer cones. (Courtesy PerkinElmer Instruments, Norwalk, CT.)

Secondarydischarge

Figure 4. Interface area affected by sec-ondary discharge.

100

80

60

40

20

00.0 0.5 1.0 1.5

Load coil turns

Plas

ma

pote

ntia

l

2.0 2.5 3.0

n

n

n

n

n

n

n

Figure 5. Reduction in plasma potential asthe load coil is grounded at different posi-tions (turns) along its length.

JULY 2001 16(7) SPECTROSCOPY 27

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28 SPECTROSCOPY 16(7) JULY 2001 www.spec t roscopyo n l i n e . com

critical that the interface groundingmechanism can handle these types ofreal-world applications, of which typicalexamples include• The use of cool-plasma conditions. It isstandard practice today to use cool-plasma conditions (500–700 W power and1.0–1.3 L/min nebulizer gas flow) tolower the plasma temperature and reduceargon-based polyatomic interferences

such as 40Ar16O, 40Ar, and 38ArH, in thedetermination of difficult elements like56Fe, 40Ca, and 39K. Such dramaticchanges from normal operating condi-tions (1000 W, 0.8 L/min) will affect theelectrical characteristics of the plasma. • Running volatile organic solvents. Ana-lyzing oil or organic-based samples re-quires a chilled spray chamber (typically220 °C) or a membrane desolvation sys-

tem to reduce the solvent loading on theplasma. In addition, higher RF power(1300–1500 W) and lower nebulizer gasflow (0.4–0.8 L/min) are required to dis-sociate the organic components in thesample. A reduction in the amount of sol-vent entering the plasma combined withhigher power and lower nebulizer gasflow translate into a hotter plasma and achange in its ionization mechanism. • Reducing oxides. The formation of ox-ide species can be problematic in somesample types. For example, in geochemi-cal applications it is quite common to sac-rifice sensitivity by lowering the nebulizergas flow and increasing the RF power toreduce the formation of rare earth ox-ides, which can interfere spectrally withthe determination of other analytes. Un-fortunately these conditions have the po-tential to induce a secondary discharge.• Running a “dry” plasma. Samplingaccessories such as membrane desolva-tors, laser ablation systems, and elec-trothermal vaporization devices are beingused more routinely to enhance the flexi-bility of ICP-MS. The major difference be-tween these sampling devices and a con-ventional liquid sample introductionsystem, is that they generate a “dry” sam-ple aerosol, which requires totally differ-ent operating conditions compared with aconventional “wet” plasma. An aerosolcontaining no solvent can have a dramaticeffect on the ionization conditions in theplasma.

Even though most modern ICP-MS in-terfaces have been designed to minimizethe effects of the secondary discharge, it


Ion optics

Skimmer Sampler

760 Torr3–4 TorrExpansion ofion beam

Ion beamSampleaerosol

Figure 6. The composition of the ion beam is maintained, assuming a neutral plasma.

Interfacecone

Debyelength

Orificediameter

Figure 7. Electrodynamic forces do not af-fect the composition of the ion beam enter-ing the sampler or the skimmer cone.

Circle 17

“Tutorial” continued on page 34

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34 SPECTROSCOPY 16(7) JULY 2001 www.spec t roscopyo n l i n e . com

of the vendor) have been checked to en-sure compliance with the regulations.The audit should be planned and shouldcover items such as the design and pro-gramming phases, product testing and re-lease, documentation, and support; a re-port of the audit should be produced afterthe visit. Two published articles have cov-ered vendor audits in more detail (3, 4).

The minimum audit is a remote vendoraudit using a checklist that the vendorcompletes and returns to you. This isusually easy to complete, but the writer ofthe checklist must ensure that the ques-tions are written in a way that can be un-derstood by the recipient, because lan-guage and cultural issues could affect aremote checklist. Moreover, there is littleway of checking the answers you receive.However, for smaller software systems —and some spectrometers fall into this cat-egory — a remote audit is a cost-effectiveway of getting information on how a ven-dor carries out its development process,so long as you know and understand itslimitations.

SYSTEM SELECTION: PART TWOIf the vendor audit, price quote, instru-ment, and software are all acceptable,you’ll be raising a capital expenditurerequest (or whatever it is called in yourorganization) and then generating a pur-chase order. The quote and the purchaseorder are a link in the validation chain;they provide a link into the next phase ofthe validation life cycle: qualification. Thepurchase order is the first stage in defin-ing the initial configuration of the system,as we’ll discover in the next article in thisseries.

REFERENCES(1) R.D. McDowall, Spectroscopy 16(2),

32–43 (2001).(2) IEEE Standard 1012-1986, “Software Vali-

dation and Verification Plans,” Institute ofElectronic and Electrical Engineers, Pis-cataway, NJ, USA.

(3) R.D. McDowall, Sci. Data Mgmt. 2(2), 8(1998).

(4) R.D. McDowall, Sci. Data Mgmt. 2(3), 8(1998). ◆

F O C U S O N Q U A L I T Y . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .

shouldn’t be taken for granted that theycan all handle changes in operating con-ditions and matrix components with thesame amount of ease. The most notice-able problems that have been reportedinclude spectral peaks of the cone mater-ial appearing in the blank (9); erosion ordiscoloration of the sampling cones;widely different optimum plasma condi-tions for different masses (10); and in-creased frequency of tuning the ion op-tics (8). Of all these, probably the mostinconvenient problem is regular opti-mization of the lens voltages, becauseslight changes in plasma conditions canproduce significant changes in ion ener-gies, which require regular retuning ofthe ion optics. Even though most instru-ments have computer-controlled ion op-tics, it becomes another variable thatmust be optimized. This isn’t a majorproblem but might be considered an in-convenience for a high–sample through-put lab. There is no question that theplasma discharge, interface region, andion optics all have to be designed in con-cert to ensure that the instrument canhandle a wide range of operating condi-tions and sample types. The role of theion optics will be discussed in greater de-tail in the next installment of this series.

“Tutorial” continued from page 34

Circle 22, 23

REFERENCES(1) A.L. Gray and A.R. Date, Analyst 108,

1033 (1983).(2) R.S. Houk, V. A. Fassel, and H.J. Svec, Dy-

namic Mass Spectrosc. 6, 234 (1981).(3) A.R. Date and A.L. Gray, Analyst 106,

1255 (1981).(4) A.L. Gray and A.R. Date, Dynamic Mass

Spectrosc. 6, 252 (1981).(5) S.D. Tanner, J. Anal. At. Spectrom. 10,

905 (1995).(6) K. Sakata and K. Kawabata, Spectrochim.

Acta 49B, 1027 (1994).(7) S. Georgitus, M. Plantz, poster paper pre-

sented at the Winter Conference onPlasma Spectrochemistry, FP4, FortLauderdale, FL (1996).

(8) D.J. Douglas and J.B. French, Spec-trochim. Acta 41B, 3, 197 (1986).

(9) D.J. Douglas, Can. J. Spectrosc. 34, 2(1089).

(10) J.E. Fulford and D.J. Douglas, Appl. Spec-trosc. 40, 7 (1986).

Robert Thomas has more than 30 years ex-perience in trace element analysis. He is theprincipal of his own freelance writing andscientific consulting company, Scientific So-lutions, based in Gaithersburg, MD. He canbe contacted by e-mail at [email protected] or via his web site atwww.scientificsolutions1.com.◆

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bodo

of the vendor) have been checked to ensure compliance with the regulations. The audit should be planned and should cover items such as the design and programming phases, product testing and release, documentation, and support; a report of the audit should be produced after the visit. Two published articles have covered vendor audits in more detail (3, 4). The minimum audit is a remote vendor audit using a checklist that the vendor completes and returns to you. This is usually easy to complete, but the writer of the checklist must ensure that the questions are written in a way that can be understood by the recipient, because language and cultural issues could affect a remote checklist. Moreover, there is little way of checking the answers you receive. However, for smaller software systems — and some spectrometers fall into this category — a remote audit is a cost-effective way of getting information on how a vendor carries out its development process, so long as you know and understand its limitations. SYSTEM SELECTION: PART TWO If the vendor audit, price quote, instrument, and software are all acceptable, you’ll be raising a capital expenditure request (or whatever it is called in your organization) and then generating a purchase order. The quote and the purchase order are a link in the validation chain; they provide a link into the next phase of the validation life cycle: qualification. The purchase order is the first stage in defining the initial configuration of the system, as we’ll discover in the next article in this series. REFERENCES (1) R.D. McDowall, Spectroscopy 16(2), 32–43 (2001). (2) IEEE Standard 1012-1986, “Software Validation and Verification Plans,” Institute of Electronic and Electrical Engineers, Piscataway, NJ, USA. (3) R.D. McDowall, Sci. Data Mgmt. 2(2), 8 (1998). (4) R.D. McDowall, Sci. Data Mgmt. 2(3), 8 (1998). u



38 SPECTROSCOPY 16(9) SEPTEMBER 2001 www.spec t roscopyo n l i n e . com

Part V of the series on inductively coupledplasma–mass spectrometry (ICP-MS) takesa detailed look at the ion focusing system— a crucial area of the ICP mass spec-trometer where the ion beam is focused be-fore it enters the mass analyzer. Sometimesknown as the ion optics, it comprises one ormore ion lens components, which electro-statically steer the analyte ions from the in-terface region into the mass separation de-vice. The strength of a well-designed ionfocusing system is its ability to produce lowbackground levels, good detection limits,and stable signals in real-world samplematrices.

Although the detection capability ofICP-MS is generally recognized asbeing superior to any of the otheratomic spectroscopic techniques, itis probably most susceptible to the

sample’s matrix components. The inher-ent problem lies in the fact that ICP-MS isrelatively inefficient; out of every millionions generated in the plasma, only one ac-tually reaches the detector. One of themain contributing factors to the low effi-ciency is the higher concentration of ma-trix elements compared with the analyte,which has the effect of defocusing theions and altering the transmission charac-teristics of the ion beam. This is some-times referred to as a space charge ef-fect, and it can be particularly severewhen the matrix ions have a heavier massthan the analyte ions (1). The role of theion focusing system is therefore to trans-port the maximum number of analyteions from the interface region to themass separation device, while rejecting asmany of the matrix components and non-analyte-based species as possible. Let usnow discuss this process in greater detail.

ROLE OF THE ION OPTICSFigure 1 shows the position of the ion op-tics relative to the plasma torch and inter-face region; Figure 2 represents a moredetailed look at a typical ion focusingsystem.

The ion optics are positioned betweenthe skimmer cone and the mass separa-tion device, and consist of one or moreelectrostatically controlled lens compo-nents. They are not traditional optics thatwe associate with ICP emission or atomicabsorption but are made up of a series ofmetallic plates, barrels, or cylinders thathave a voltage placed on them. The func-tion of the ion optic system is to take ionsfrom the hostile environment of theplasma at atmospheric pressure via theinterface cones and steer them into themass analyzer, which is under high vac-uum. As mentioned in Part IV of the se-ries, the plasma discharge, interface re-gion, and ion optics have to be designed

in concert with each other. It is absolutelycritical that the composition and electricalintegrity of the ion beam is maintained asit enters the ion optics. For this reason itis essential that the plasma is at zero po-tential to ensure that the magnitude andspread of ion energies are as low aspossible (2).

A secondary but also very importantrole of the ion optic system is to stop par-ticulates, neutral species, and photonsfrom getting through to the mass ana-lyzer and the detector. These speciescause signal instability and contribute tobackground levels, which ultimately af-fect the performance of the system. Forexample, if photons or neutral speciesreach the detector, they will elevate thebackground noise and therefore degradedetection capability. In addition, if particu-lates from the matrix penetrate fartherinto the mass spectrometer region, theyhave the potential to deposit on lens com-

A Beginner’s Guide to ICP-MSPart V: The Ion Focusing System


Figure 1. Position of ion optics relative to the plasma torch and interface region.

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making it into the mass spectrometer.The first method is to place a groundedmetal stop (disk) behind the skimmercone. This stop allows the ion beam tomove around it but physically blocks theparticulates, photons, and neutral speciesfrom traveling downstream (3). The other

approach is to set the ion lens or mass an-alyzer slightly off axis. The positivelycharged ions are then steered by the lenssystem into the mass analyzer, while thephotons and neutral and nonionic speciesare ejected out of the ion beam (4).

It is also worth mentioning that somelens systems incorporate an extractionlens after the skimmer cone to electro-statically pull the ions from the interfaceregion. This has the benefit of improvingthe transmission and detection limits ofthe low-mass elements (which tend to bepushed out of the ion beam by the heav-ier elements), resulting in a more uni-form response across the full mass rangeof 0–300 amu. In an attempt to reducethese space charge effects, some olderdesigns have used lens components to ac-celerate the ions downstream. Unfortu-nately this can have the effect of degrad-ing the resolving power and abundancesensitivity (ability to differentiate an ana-lyte peak from the wing of an interfer-ence) of the instrument because of themuch higher kinetic energy of the accel-erated ions as they enter the massanalyzer (5).

DYNAMICS OF ION FLOWTo fully understand the role of the ion op-tics in ICP-MS, it is important to have anappreciation of the dynamics of ion flowfrom the plasma through the interface re-gion into the mass spectrometer. Whenthe ions generated in the plasma emergefrom the skimmer cone, there is a rapidexpansion of the ion beam as the pres-sure is reduced from 760 Torr (atmos-pheric pressure) to approximately 1023 to1024 Torr in the lens chamber with aturbomolecular pump. The compositionof the ion beam immediately behind thecone is the same as the composition infront of the cone because the expansionat this stage is controlled by normal gasdynamics and not by electrodynamics.One of the main reasons for this is that,in the ion sampling process, the Debyelength (the distance over which ions ex-ert influence on each other) is small com-pared with the orifice diameter of thesampler or skimmer cone. Consequentlythere is little electrical interaction be-tween the ion beam and the cone and rel-atively little interaction between the indi-vidual ions in the beam. In this way,compositional integrity of the ion beam ismaintained throughout the interface re-gion (6). With this rapid drop in pressurein the lens chamber, electrons diffuse out

ponents and, in extreme cases, get intothe mass analyzer. In the short term thiswill cause signal instability and, in thelong term, increase the frequency ofcleaning and routine maintenance.

Basically two approaches will reducethe chances of these undesirable species

Figure 2. A generic ion focusing system, showing position of ion optics relative to the inter-face cones and mass analyzer.

Figure 3. Extreme pressure drop in the ion optic chamber produces diffusion of electrons, re-sulting in a positively charged ion beam.

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of the ion beam. Because of the small sizeof the electrons relative to the positivelycharged ions, the electrons diffuse far-ther from the beam than the ions, result-ing in an ion beam with a net positivecharge. This is represented schematicallyin Figure 3.

The generation of a positively chargedion beam is the first stage in the chargeseparation process. Unfortunately, the net

positive charge of the ion beam meansthat there is now a natural tendency forthe ions to repel each other. If nothing isdone to compensate for this, ions with ahigher mass-to-charge ratio will dominatethe center of the ion beam and force thelighter ions to the outside. The degree ofloss will depend on the kinetic energy ofthe ions: those with high kinetic energy(high mass elements) will be transmitted

in preference to ions with medium (mid-mass elements) or low kinetic energy(low-mass elements). This is shown inFigure 4. The second stage of charge sep-aration is therefore to electrostaticallysteer the ions of interest back into thecenter of the ion beam by placing volt-ages on one or more ion lens compo-nents. Remember, however, that this ispossible only if the interface is kept atzero potential, which ensures a neutralgas-dynamic flow through the interfaceand maintains the compositional integrityof the ion beam. It also guarantees thatthe average ion energy and energyspread of each ion entering the lens sys-tems are at levels optimum for mass sepa-ration. If the interface region is notgrounded correctly, stray capacitance willgenerate a discharge between the plasmaand sampler cone and increase the ki-netic energy of the ion beam, making itvery difficult to optimize the ion lenssystem (7).

COMMERCIAL ION OPTIC DESIGNSOver the years, there have been many dif-ferent ion optic designs. Although they allhave their own characteristics, they per-form the same basic function: to discrimi-nate undesirable matrix- or solvent-basedions so that only the analyte ions aretransmitted to the mass analyzer. Themost common ion optics design used to-day consists of several lens components,which all have a specific role to play inthe transmission of the analyte ions (8).With these multicomponent lens sys-tems, the voltage can be optimized onevery lens of the ion optics to achieve thedesired ion specificity. Over the years thistype of lens configuration has proven tobe very durable and has been shown toproduce very low background levels, par-ticularly when combined with an off-axismass analyzer. However, because of theinteractive nature of parameters that af-fect the signal response, the more com-plex the lens system the more variableshave to be optimized, particularly if manydifferent sample types are being ana-lyzed. This isn’t such a major problem be-cause the lens voltages are all computer-controlled, and methods can be stored forevery new sample scenario. Figure 5 is acommercially available multicomponentlens system, with an extraction lens andoff-axis quadrupole mass analyzer, show-ing how the ion beam is deflected into themass analyzer, while the photons and

Figure 4. The degree of ion repulsion will depend on kinetic energy of the ions: those withhigh kinetic energy (green with red +) will be transmitted in preference to ions with medium(yellow with red +) or low kinetic energy (blue with red +).

Figure 5. Schematic of a multicomponent lens system with extraction lens and off-axisquadrupole mass analyzer (courtesy of Agilent Technologies [Wilmington, DE]).

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neutral species travel in a straight lineand strike a metal plate.

Another, more novel approach is to usejust one cylindrical ion lens, combinedwith a grounded stop positioned just in-side the skimmer cone as shown inFigure 6 (9).

With this design, the voltage is dynam-ically ramped on-the-fly, in concert withthe mass scan of the analyzer (typically aquadrupole). The benefit of this approachis that the optimum lens voltage is placedon every mass in a multielement run toallow the maximum number of analyteions through, while keeping the matrixions to an absolute minimum. This is rep-resented in Figure 7, which shows a lensvoltage scan of six elements: lithium,cobalt, yttrium, indium, lead, and ura-nium, at 7, 59, 89, 115, 208, and 238 amu,respectively. We can see that each ele-ment has its own optimum value, which isthen used to calibrate the system, so thelens can be ramp-scanned across the fullmass range. This type of approach is typi-cally used in conjunction with a groundedstop to act as a physical barrier to reduceparticulates, neutral species, and photonsfrom reaching the mass analyzer and de-tector. Although this design producesslightly higher background levels, it of-fers excellent long-term stability withreal-world samples. It works well formany sample types but is most effectivewhen low mass elements are being deter-mined in the presence of high-mass–matrix elements.

It is also worth emphasizing that anumber of ICP-MS systems offer what isknown as a high-sensitivity interface.

These all work slightly differently butshare similar components. By using acombination of slightly different conegeometry, higher vacuum at the inter-face, one or more extraction lenses, andslightly modified ion optic design, they of-fer as much as 10 times the sensitivity ofa traditional interface (10). However, thisincreased sensitivity is usually combinedwith inferior stability and an increase inbackground levels, particularly for sam-ples with a heavy matrix. To get aroundthis degradation in performance onemust usually dilute the samples beforeanalysis, which limits the systems’ applic-ability for real-world samples (11). How-ever, they have found a use in non-liquid-based applications in which highsensitivity is crucial, for example in theanalysis of small spots on the surface of ageological specimen using laser ablationICP-MS. For this application, the instru-ment must offer high sensitivity becausea single laser pulse is used to ablate thesample and sweep a tiny amount of thedry sample aerosol into the ICP-MS (12).

The role of the ion focusing systemcannot be overestimated. It affects thebackground noise level of the instrument.It has a huge impact on both long- andshort-term signal stability, especially inreal-world samples, and it also dictatesthe number of ions that find their way tothe mass analyzer. However, it must beemphasized that the ion optics are only asgood as the ions that feed it, and for thisreason it must be designed in concertwith both the plasma source and the in-terface region. There is no question thatthis area is crucial to the design of thewhole ICP mass spectrometer. In the

next part of the series we will discuss theheart of the ICP mass spectrometer: themass analyzer.

REFERENCES(1) J. A. Olivares and R.S. Houk, Anal. Chem.

58, 20 (1986).(2) D.J. Douglas and J.B. French, Spec-

trochim. Acta 41B(3), 197 (1986).(3) E.R. Denoyer, D. Jacques, E. Debrah, and

S.D. Tanner, At. Spectrosc. 16(1), 1(1995).

(4) D. Potter, American Lab (July 1994).(5) P. Turner, paper presented at Second In-

ternational Conference on Plasma SourceMass Spec, Durham, UK, 1990.

(6) S.D. Tanner, D.J. Douglas, and J.B.French, Appl. Spectrosc. 48, 1373 (1994).

(7) R. Thomas, Spectroscopy 16(7), 26–34(2001).

(8) Y. Kishi, Agilent Technologies ApplicationJournal, August (1997).

(9) S.D. Tanner, L.M. Cousins, and D.J. Dou-glas, Appl. Spectrosc. 48, 1367 (1994).

(10) I.B. Brenner, M. Liezers, J. Godfrey, S.Nelms, and J. Cantle, Spectrochim. ActaPart 53B(6–8), 1087 (1998).

(11) B.C. Gibson, presented at Surrey Interna-tional Conference on ICP-MS, London,UK, 1994.

(12) T. Howe, J. Shkolnik, and R.J. Thomas,Spectroscopy 16(2), 54 (2001).


Figure 6. Schematic of a single ion lens andgrounded stop system (not to scale[courtesy of PerkinElmer Instruments{Norwalk, CT}]).

Figure 7. A calibration of optimum lens volt-ages is used to ramp-scan the ion lens inconcert with the mass scan of the analyzer.The signals have been normalized for com-parison purposes.

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44 SPECTROSCOPY 16(10) OCTOBER 2001 www.spec t roscopyo n l i n e . com

Part VI of the series on inductively coupledplasma–mass spectroscopy (ICP-MS) fun-damentals deals with the heart of the sys-tem — the mass separation device. Some-times called the mass analyzer, it is theregion of the ICP mass spectrometer thatseparates the ions according to their mass-to-charge ratio (m/z). This selectionprocess is achieved in a number of differentways, depending on the mass separationdevice, but they all have one common goal:to separate the ions of interest from all theother nonanalyte, matrix, solvent, and argon-based ions.

Although inductively coupledplasma–mass spectroscopy (ICP-MS) was commercialized in 1983,the first 10 years of its developmentprimarily used traditional quadru-

pole mass filter technology to separatethe ions of interest. These worked excep-tionally well for most applications butproved to have limitations in determiningdifficult elements or dealing with more-complex sample matrices. This led to the

development of alternative mass separa-tion devices that pushed the capabilitiesof ICP-MS so it could be used for morechallenging applications. Before we dis-cuss these different mass spectrometersin greater detail, let’s take a look at the lo-cation of the mass analyzer in relation tothe ion optics and the detector. Figure 1shows this in greater detail.

As we can see, the mass analyzer is po-sitioned between the ion optics and thedetector and is maintained at a vacuum ofapproximately 1026 Torr with a secondturbomolecular pump. Assuming the ionsare emerging from the ion optics at theoptimum kinetic energy (1), they areready to be separated according to theirmass-to-charge ratio by the mass ana-lyzer. There are basically four kinds ofcommercially available mass analyzers:quadrupole mass filters, double focusingmagnetic sector, time-of-flight, andcollision–reaction cell technology. Theyall have their own strengths and weak-nesses, which we will discuss in greaterdetail in the next few installments of this

column. Let’s first begin with the mostcommon of the mass separation devicesused in ICP-MS, the quadrupole mass filter.

QUADRUPOLE MASS FILTERTECHNOLOGYDeveloped in the early 1980s,quadrupole-based systems represent ap-proximately 90% of all ICP mass spec-trometers used today. This design wasthe first to be commercialized; as a result,today’s quadrupole ICP-MS technology isconsidered a very mature, routine, high-throughput, trace-element technique. Aquadrupole consists of four cylindrical orhyperbolic metallic rods of the samelength and diameter. They are typicallymade of stainless steel or molybdenum,and sometimes have a ceramic coatingfor corrosion resistance. Quadrupolesused in ICP-MS are typically 15–20 cm inlength and about 1 cm in diameter andoperate at a frequency of 2–3 MHz.

BASIC PRINCIPLES OF OPERATION By placing a direct current (dc) field onone pair of rods and a radio frequency(rf) field on the opposite pair, ions of a se-lected mass are allowed to pass throughthe rods to the detector, while the othersare ejected from the quadrupole. Figure 2shows this in greater detail.

In this simplified example, the analyteion (black) and four other ions (colored)have arrived at the entrance to the fourrods of the quadrupole. When a particu-lar rf-dc voltage is applied to the rods, thepositive or negative bias on the rods willelectrostatically steer the analyte ion ofinterest down the middle of the four rodsto the end, where it will emerge and beconverted to an electrical pulse by the de-tector. The other ions of different mass-to-charge ratios will pass through thespaces between the rods and be ejected


Beginner’s Guide to ICP-MSPart VI — The Mass Analyzer


Iondetector

MSinterface

ICP torchIon optics

Spraychamber

Nebulizer


Turbomolecularpump

Turbomolecularpump

Mechanicalpump

RFpowersupply

Figure 1. Schematic of an ICP-MS system showing the location of the mass separation device.

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OCTOBER 2001 16(10) SPECTROSCOPY 45

from the quadrupole. This scanningprocess is then repeated for another ana-lyte at a completely different mass-to-charge ratio until all the analytes in a mul-tielement analysis have been measured.The process for the detection of one par-ticular mass in a multielement run is rep-resented in Figure 3. It shows a 63Cu ionemerging from the quadrupole and beingconverted to an electrical pulse by the de-tector. As the rf-dc voltage of the quadru-pole — corresponding to 63Cu — is re-peatedly scanned, the ions as electricalpulses are stored and counted by a multi-channel analyzer. This multichannel data-acquisition system typically has 20 chan-nels per mass, and as the electrical pulsesare counted in each channel, a profile ofthe mass is built up over the 20 channels,

corresponding to the spectral peak of63Cu. In a multielement run, repeatedscans are made over the entire suite ofanalyte masses, as opposed to just onemass represented in this example.

Quadrupole scan rates are typically onthe order of 2500 atomic mass units(amu) per second and can cover the en-tire mass range of 0–300 amu in about0.1 s. However, real-world analysis speedsare much slower than this, and in practice25 elements can be determined in dupli-cate with good precision in 1–2 min.

QUADRUPOLE PERFORMANCE CRITERIATwo very important performance specifi-cations of a mass analyzer govern its abil-ity to separate an analyte peak from aspectral interference. The first is resolv-

S P E C T R O S C O P Y T U T O R I A L . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .

+

+

+

+

++

+Quadrupole rods

-

+

Ionflow

Figure 2. Schematic showing principles of a quadrupole mass filter.

Quadrupole

Quadrupolemass scancontrollerMultichannel data

acquisition system

Ch l

Ions (electricalpulses)

Mass scan (amu)

63

1n

Detector

63Cu

63Cu scan

Copper ion — m/z 63

1 2 3 4 5 6 7 ............ 20

Figure 3. Profiles of different masses are built up using a multichannel data acquisition system.Circle 33

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ing power (R), which in traditional massspectrometry is represented by the fol-lowing equation: R 5 m/Dm, where m isthe nominal mass at which the peak oc-curs and Dm is the mass difference be-tween two resolved peaks (2). However,for quadrupole technology, the term reso-lution is more commonly used, and isnormally defined as the width of a peak at10% of its height. The second specifica-tion is abundance sensitivity, which is the

signal contribution of the tail of an adja-cent peak at one mass lower and onemass higher than the analyte peak (3).Even though they are somewhat relatedand both define the quality of a quadru-pole, the abundance sensitivity is proba-bly the most critical. If a quadrupole hasgood resolution but poor abundance sen-sitivity, it will often prohibit the measure-ment of an ultratrace analyte peak next toa major interfering mass.

RESOLUTIONLet us now discuss this area in greater de-tail. The ability to separate differentmasses with a quadrupole is determinedby a combination of factors includingshape, diameter, and length of the rods, fre-quency of quadrupole power supply, oper-ating vacuum, applied rf-dc voltages, andthe motion and kinetic energy of the ionsentering and exiting the quadrupole. Allthese factors will have a direct impact onthe stability of the ions as they travel downthe middle of the rods and thus thequadrupole’s ability to separate ions of dif-fering mass-to-charge ratios. This is repre-sented in Figure 4, which shows a simpli-fied version of the Mathieu mass stabilityplot of two separate masses (A and B) en-tering the quadrupole at the same time (4).

Any of the rf-dc conditions shown un-der the light blue plot will allow onlymass A to pass through the quadrupole,while any combination of rf-dc voltagesunder the yellow plot will allow only massB to pass through the quadrupole. If theslope of the rf-dc scan rate is steep, repre-sented by the light blue line (high resolu-tion), the spectral peaks will be narrow,and masses A and B will be well sepa-rated (equivalent to the distance betweenthe two blue arrows). However, if theslope of the scan is shallow, representedby the red line ( low resolution), the spec-tral peaks will be wide, and masses A andB will not be so well separated (equiva-lent to the distance between the two redarrows). On the other hand, if the slopeof the scan is too shallow, represented bythe gray line (inadequate resolution), thepeaks will overlap each other (shown bythe green area of the plot) and themasses will pass through the quadrupolewithout being separated. In theory, theresolution of a quadrupole mass filter canbe varied between 0.3 and 3.0 amu. How-

S P E C T R O S C O P Y T U T O R I A L . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F(rf)

F(dc

)

High resolutionA

BLow

resolution

Inadequateresolution

Overlap

Figure 4. Simplified Mathieu stability dia-gram of a quadrupole mass filter, showingseparation of two different masses, A (lightblue plot) and B (yellow plot).

Resolution

Inte

nsity

High resolution(0.3 amu)

Normal resolution (1.0 amu)

Low resolution (3.0 amu)

Figure 5. Sensitivity comparison of aquadrupole operated at 3.0, 1.0, and 0.3amu resolution (measured at 10% of itspeak height).

Resolution 0.70 amuResolution 0.50 amu

30,000

20,000

10,000

062 63 64 65 66

Mass

Puls

e in

tens

ity

Figure 6. Sensitivity comparison of two cop-per isotopes, 63Cu and 65Cu, at resolutionsettings of 0.70 and 0.50 amu.

Circle 34

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OCTOBER 2001 16(10) SPECTROSCOPY 47

ever, improved resolution is always ac-companied by a sacrifice in sensitivity, asseen in Figure 5, which shows a compari-son of the same mass at a resolution of3.0, 1.0, and 0.3 amu.

We can see that the peak height at 3.0amu is much larger than the peak heightat 0.3 amu but, as expected, it is alsomuch wider. This would prohibit using aresolution of 3.0 amu with spectrally com-plex samples. Conversely, the peak widthat 0.3 amu is very narrow, but the sensi-tivity is low. For this reason, a compro-mise between peak width and sensitivityusually has to be reached, depending onthe application. This can clearly be seenin Figure 6, which shows a spectral over-lay of two copper isotopes — 63Cu and65Cu — at resolution settings of 0.70 and0.50 amu. In practice, the quadrupole isnormally operated at a resolution of0.7–1.0 amu for most applications.

It is worth mentioning that mostquadrupoles are operated in the first sta-bility region, where resolving power istypically ;400. If the quadrupole is oper-ated in the second or third stability re-gions, resolving powers of 4000 (5) and9000 (6), respectively, have beenachieved. However, improving resolutionusing this approach has resulted in a sig-nificant loss of signal. Although there areways of improving sensitivity, other prob-lems have been encountered, and as a re-sult, to date there are no commercial in-struments available based on this design.

Some instruments can vary the peakwidth on-the-fly, which means that the res-olution can be changed between 3.0 and0.3 amu for every analyte in a multiele-ment run. For some challenging applica-tions this can be beneficial, but in realitythey are rare. So, even thoughquadrupoles can be operated at higherresolution (in the first stability region),

until now the slight improvement has notbecome a practical benefit for most rou-tine applications.

ABUNDANCE SENSITIVITYWe can see in Figure 6 that the tails of

the spectral peaks drop off more rapidlyat the high mass end of the peak com-pared with the low mass end. The overallpeak shape, particularly its low mass andhigh mass tail, is determined by the abun-dance sensitivity of the quadrupole,


Mass (amu)

Inte

nsity

Quadrupole

Mass (M)

M 1 M 1

Ion exitingIon entering

Figure 7. Ions entering the quadrupole areslowed down by the filtering process andproduce peaks with a pronounced tail orshoulder at the low-mass end.

1 ppm of monoisotopic arsenic (75As)

75 75

Doubly charged 151Eu (75.5 amu)

(a) (b)

Figure 8. A low abundance sensitivity specification is critical to minimize spectral interfer-ences, as shown by (a) a spectral scan of 50 ppm of 151Eu++ at 75.5 amu and (b) an ex-panded view, which shows how the tail of the 151Eu++ elevates the spectral background of1 ppb of As at mass 75.

Circle 35

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which is affected by a combination of fac-tors including design of the rods, fre-quency of the power supply, and operat-ing vacuum (7). Even though they are allimportant, probably the biggest impactson abundance sensitivity are the motionand kinetic energy of the ions as they en-ter and exit the quadrupole. If one looksat the Mathieu stability plot in Figure 3, itcan be seen that the stability boundariesof each mass are less defined (not sosharp) on the low mass side than they areon the high mass side (4). As a result, thecharacteristics of ion motion at the lowmass boundary is different from the highmass boundary and is therefore reflectedin poorer abundance sensitivity at the lowmass side compared with the high massside. In addition, the velocity (and there-fore the kinetic energy) of the ions enter-ing the quadrupole will affect the ion mo-tion and, as a result, will have a directimpact on the abundance sensitivity. Forthat reason, factors that affect the kineticenergy of the ions, like high plasma po-tential and the use of lens components toaccelerate the ion beam, will degrade theinstrument’s abundance sensitivity (8).

These are the fundamental reasonswhy the peak shape is not symmetricalwith a quadrupole and explains why thereis always a pronounced shoulder at thelow mass side of the peak compared tothe high mass side — as represented inFigure 7, which shows the theoreticalpeak shape of a nominal mass M. We cansee that the shape of the peak at onemass lower (M 2 1) is slightly differentfrom the other side of the peak at onemass higher (M 1 1) than the mass M.For this reason, the abundance sensitivityspecification for all quadrupoles is alwaysworse on the low mass side than on thehigh mass side and is typically 1 3 1026

at M 2 1 and 1 3 1027 at M 1 1. In otherwords, an interfering peak of 1 millioncounts per second (cps) at M 2 1 wouldproduce a background of 1 cps at M,while it would take an interference of 107

cps at M 1 1 to produce a background of1 cps at M.

BENEFITS OF GOOD ABUNDANCESENSITIVITYFigure 8 shows an example of the impor-tance of abundance sensitivity. Figure 8ais a spectral scan of 50 ppm of the doublycharged europium ion — 151Eu11 at 75.5amu (a doubly charged ion is one withtwo positive charges, as opposed to a nor-mal singly charged positive ion, and ex-hibits a m/z peak at half its mass). We cansee that the intensity of the peak is sogreat that its tail overlaps the adjacentmass at 75 amu, which is the only avail-able mass for the determination of ar-senic. This is highlighted in Figure 8b,which shows an expanded view of the tailof the 151Eu11, together with a scan of 1ppb of As at mass 75. We can see veryclearly that the 75As signal lies on thesloping tail of the 151Eu11 peak. Mea-surement on a sloping background likethis would result in a significant degrada-tion in the arsenic detection limit, particu-larly as the element is monoisotopic andno alternative mass is available. This ex-ample shows the importance of a lowabundance sensitivity specification inICP-MS.

DIFFERENT QUARDUPOLE DESIGNSMany different designs of quadrupole areused in ICP-MS, all made from differentmaterials with various dimensions,shapes, and physical characteristics. Inaddition, they are all maintained atslightly different vacuum chamber pres-sures and operate at different frequen-

cies. Theory tells us that hyperbolic rodsshould generate a better hyperbolic (el-liptical) field than cylindrical rods, result-ing in higher transmission of ions athigher resolution. It also tells us that ahigher operating frequency means ahigher rate of oscillation — and thereforeseparation — of the ions as they traveldown the quadrupole. Finally, it is verywell accepted that a higher vacuum pro-duces fewer collisions between gas mole-cules and ions, resulting in a narrowerspread in kinetic energy of the ions andtherefore less of a tail at the low massside of a peak. However, given all thesespecification differences, in practice theperformance of most modern quadrupoleICP-MS instrumentation is very similar.

So even though these differences willmainly be transparent to users, there aresome subtle variations in each instru-ment’s measurement protocol and thesoftware’s approach to peak quantitation.This is a very important area that we willdiscuss it in greater detail in a future col-umn. The next part of the series will con-tinue with describing the fundamentalprinciples of other types of mass analyz-ers used in ICP-MS.

REFERENCES(1) R. Thomas, Spectroscopy 16(9), 38–44

(2001).(2) F. Adams, R. Gijbels, and R. Van Grieken,

Inorganic Mass Spectrometry (John Wileyand Sons, New York, 1988).

(3) A. Montasser, Ed. Inductively CoupledPlasma Mass Spectrometry (Wiley-VCH,Berlin, 1998).

(4) P.H. Dawson, Ed., Quadrupole Mass Spec-trometry and its Applications (Elsevier,Amsterdam, 1976; reissued by AIP Press,Woodbury, NY, 1995).

(5) Z. Du, T.N. Olney, and D.J. Douglas, J.Am. Soc. Mass Spectrom. 8, 1230–1236(1997).

(6) P.H. Dawson and Y. Binqi, Int. J. MassSpectrom., Ion Proc. 56, 25 (1984).

(7) D. Potter, Agilent Technologies ApplicationNote, 228–349 (January, 1996).

(8) E.R. Denoyer, D. Jacques, E. Debrah, andS.D. Tanner, At. Spectrosc. 16(1), 1(1995).


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A Beginner’s Guide to ICP-MS - University of São Paulo

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