Investigation of the role of hollow cathode (vaporization)

temperature on the performance of particle beam-hollow cathode

atomic emission spectrometry (PB-HC-AES)

Melissa A. Dempster,a W. Clay Davis,a R. Kenneth Marcus*a and Paula R. Cable-Dunlapb

aDepartment of Chemistry, Howard L. Hunter Laboratory, Clemson University, Clemson,South Carolina, 29634-0973, USA

bWestinghouse Savannah River Company, Building 735-A, Aiken, South Carolina, 29808,USA

Received 19th July 2000, Accepted 31st October 2000First published as an Advance Article on the web 24th January 2001

An evaluation of the effect of cathode temperature on analyte emission responses for the particle beam-hollow

cathode glow discharge atomic emission spectroscopy (PB-HC-AES) system is described. A series of Cu salts

were introduced as both nebulized solutions and dry particulates to examine the effect of vaporization

temperature on the resulting Cu I emission intensity. For the analysis of aqueous samples, a high-ef®ciency

thermoconcentric nebulizer was used to generate a ®ne aerosol of the solution ¯owing at rates of 1.5 mL min21.

A momentum separator±particle beam interface was employed for desolvation and subsequent introduction of

dry analyte particles into a heated hollow cathode glow discharge source for vaporization, atomization and

excitation. In the case of dry particle analysis, the particle beam interface was modi®ed to allow introduction of

samples by vacuum action through a 1 mm id stainless-steel tube. The effects of both vaporization temperature

and analyte particle size were investigated by monitoring Cu emission intensity as well as by examining

collected particles by scanning electron microscopy. Results indicate an optimum vaporization temperature of

200±300 ³C for this group of Cu salts, with increased analyte emission intensities obtained through the

introduction of smaller-sized particles.

Introduction

Glow discharge (GD) techniques have become valuable toolsfor direct solids elemental analysis of both conductive and non-conductive samples by atomic absorption, atomic emission andmass spectrometries due to their ef®cient atomization, excita-tion and ionization processes.1±3 However, recent efforts in thislaboratory have focused on the development of a glowdischarge source coupled with an atomic emission spectrometeras an element speci®c detector for both liquid chromatographiceluents4±10 and airborne particulates.11 In order to effectivelyemploy this source for these types of real-time analyses, aninterface is needed to allow for continuous sample introductionto the plasma. This is accomplished through use of the particlebeam (PB) interface, originally developed by Willoughby andBrowner12 for organic LC-MS applications.

The use of glow discharge devices for indirect analysis ofsolution samples by the sputter atomization of solutionresidues has been investigated by several researchers.13±19 Inthese applications, dried residues from liquid samples arechanged into the gas phase using cathodic sputtering, with theatomic population sampled by atomic absorption,13,14 emis-sion,15,16 ¯uorescence,17 or mass spectrometries.18,19 Althoughthis fairly simple method is capable of giving impressive levelsof detection, the processes of sample deposition and solventevaporation can be labor intensive and time consuming.Implementation of the PB interface provides a means ofcontinuous sample introduction for GD devices, therebyimproving sample throughput and enabling the use of thesesources for detection in chromatographic applications. In thecase of nebulized solutions, previous studies indicate analyteparticles in the range of 1±10 mm diameter are effectively

transported through the interface, with detection limits of1±100 ppb obtained for a wide variety of analyte species.6±8

Owing to its simplicity of operation and compatibility with awide range of solvent polarities and ¯ow rates,20 the PB hasfound increasing use as a valuable sample introductionmechanism for liquid chromatography/mass spectrometry.The PB interface permits detection of LC eluents whilemaintaining natural chromatographic characteristics such asretention/elution quality and solvent gradient compatibility.Separation and enrichment of the analyte from the solventthrough the processes of nebulization (aerosol formation),desolvation and momentum separation is accomplished withthe PB interface, ultimately transporting dry analyte particlesinto the low pressure (1023 Torr) MS ionization source. In thistechnique, a nebulizer is employed to produce primary aerosolsfrom the LC ¯ow in order to aid analyte desolvation. Afterpassing through a heated desolvation chamber, the resultingmixture of solvent vapor and analyte particles enters a two-stage momentum separator. The lighter nebulizer gas andsolvent vapor are skimmed and pumped away as they passthrough the differential pumping stages while the heavieranalyte particles are transported through the interface to thedetection source for subsequent vaporization and excitation orionization. Differential pumping of the momentum separatorprovides enrichment of the analyte through removal of solvent,as well as reduction of pressure to match the vacuumrequirements of the detection source. Removal of solventbefore analyte introduction into the GD source also helps toavoid problems with band emission and signal perturbationscaused by the presence of water vapor in the glow dis-charge,21,22 because the low pressures and powers employed inthe GD are inherently inef®cient for the desolvation and

DOI: 10.1039/b005811o J. Anal. At. Spectrom., 2001, 16, 115±121 115

This journal is # The Royal Society of Chemistry 2001

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vaporization of liquid samples in comparison with atmosphericpressure ¯ames and plasmas.

In terms of real-time sampling of particulate matter, the PBinterface allows continuous introduction of dry particulates tothe glow discharge source while atmospheric gases aredifferentially pumped away. The United States EnvironmentalProtection Agency regulates particles having a diameter of10 mm or less, and has recently proposed a standard limiting thedensity of particles of less than 2.5 mm in diameter on an annualbasis.23 However, there currently are no criteria in place whichrefer to the chemical composition of these airborne particles.24

It is likely that more extensive regulations will emerge as thepathogenic effects of particle composition become moreevident. Most spectrochemical methods currently availablefor analysis of particulates involve collection of analyteparticles on quartz ®ber ®lters prior to techniques such asacid dissolution25,26 or laser ablation27,28 for introduction to aninductively coupled plasma atomic emission or mass spectro-metry system. Unfortunately, the collection process requiredfor these types of analyses precludes the ability to perform real-time monitoring. By utilizing the PB interface, particulates canbe continuously introduced to the glow discharge atomicemission spectrometry system for elemental analysis. Similar tothe aforementioned nebulized solutions, the mixture of analyteparticles and atmospheric gases passes through the differentialpumping stages of the momentum separator. Here, the lightervapors are pumped away, while the heavier particulates passthrough the interface into the plasma source for analysis. Theabove apparatus could feasibly be employed for applicationssuch as direct, remote exhaust stack monitoring.

Thorough assessments of nebulization/desolvation charac-teristics, discharge operating parameters, and hollow cathodedimensions have been studied previously for the particle beam-hollow cathode glow discharge atomic emission spectrometry(PB-HC-AES) system.5±8,10,11 Nebulization variables includingliquid ¯ow rate, capillary size and temperature have beenoptimized, and the effects of glow discharge He pressure,current, and hollow cathode size on emission intensity for bothmetals and nonmetals have been studied. Preliminary investi-gations of the role of hollow cathode temperature in theeffective vaporization, atomization, and excitation of analytespecies for the PB-HC-AES system indicated an overallincrease in emission intensity with temperature; however, themaximum operating temperature was limited (v300 ³C) by themelting point of the PTFE insulators used.11 Althoughpromising data have been obtained thus far using atemperature of 220 ³C, it can be envisioned that elevatedcathode temperatures would lead to more ef®cient vaporizationand, therefore, increased analyte emission intensity. Slevin andHarrison29 explained that the mode of entrance of material intothe hollow cathode discharge is considerably affected bycathode temperature. Atomization occurs as a result ofcathodic sputtering at cool temperatures, both sputtering andselective volatilization in uncooled cathodes, and primarilyvolatilization at high temperatures (#2000 K). It is importantto emphasize here that the gas-phase temperature (v500 K)2 ofthis plasma is not suf®cient to dissociate particles of any size, asmass spectrometry experiments clearly show that even smallmolecules can remain intact.11 As such, all vaporization mustbe the result of the collision of the particles with the heatedwalls of the HC. Therefore, higher cathode temperatures maynaturally be expected to improve volatilization, thoughpyrolysis would be a negative consequence of too high of asurface temperature. By the same token, the low density of theplasma volume (v10 Torr) would suggest that the gas-phasetemperature of the discharge would probably not be appreci-ably effected by the applied heating.

Further characterization of the PB-HC-AES system ispresented here in terms of the effect of hollow cathodetemperature and analyte particle size on the resulting emission

response for a series of copper salts. Optical emission intensitiesover a range of cathode temperatures were determined both fornebulized solutions introduced through ¯ow injection modeand for dry particulates sampled through the particle ``sniffer''interface. These responses were correlated to analyte particlesize by examining collected particulates using scanning electronmicroscopy. By optimizing these parameters, it is hoped thatthe utility of this system will be enhanced as an element-speci®cdetector for both liquid chromatographic eluents and dryparticulates.

Experimental

Glow discharge source

The hollow cathode glow discharge atomic emission source wassimilar to that employed in the previous studies.5 The hollowcathode (3 mm id, 25 mm length) was mounted at the center ofa stainless-steel ``thermoblock'' that ensures easy access to theplasma via optical monitoring. In order to allow the use ofhigher thermoblock temperatures, the 1 mm thick PTFEinsulator surrounding the hollow cathode was replaced withone machined from boron nitride. The source vacuum port,discharge gas inlet, pressure gauge, electrical feedthroughs andheating components were also placed on the thermoblock. Theentire glow discharge source was heated for more ef®cientsample vaporization and atomization by a pair of cartridgeheaters (Model SC 2515, Scienti®c Instrument Services,Ringoes, NJ, USA), with the block temperature monitoredby means of a W±Re thermocouple. The overall gas pressure ofthe glow discharge source (a combination of helium dischargegas and other gaseous sources from the particle beam interface)was monitored by a vacuum gauge (DV-24, Teledyne Hastings-Raydist, Hampton, VA, USA). The hollow cathode dischargewas powered by a Bertan (Hicksville, NY, USA) Model 915-1Nsupply, operating in a constant-current mode, over the range of30±100 mA.

Particle beam interface

A Thermabeam (Extrel Corp., Pittsburgh, PA, USA) particlebeam LC-MS interface was employed to couple both the liquiddelivery system and particle sampler with the hollow cathodeglow discharge source. In the case of liquid sampling, theinterface consisted of a thermoconcentric nebulizer, a heatedspray chamber, and a two-stage momentum separator, asillustrated in Fig. 1.4±6,9 The liquid sample was introduced viaan HPLC pump into a fused-silica capillary (110 mm id)mounted within a stainless-steel tube (1.6 mm od). A dcpotential difference applied across the stainless-steel tubingresulted in resistive heating along the tube, producing a thermalcomponent to the aerosol formation. Helium gas ¯owed(#360 mL min21) through the gap between the central fused-silica capillary and outer stainless-steel tubing as a sheath gas toimprove heat conduction and to promote the breakup of theliquid at the capillary tip (i.e., pneumatic nebulization). Theresulting ®ne aerosol was directed into a steel spray chamberwrapped in heating tape (150±200 ³C). A separate 6 mm id inletallowed addition of a supplemental ¯ow (250 mL min21) of Hegas.6 The separation of the aerosol mixture was achieved in thesubsequent two-stage momentum separator that skims out therelatively low mass nebulizer gas and solvent molecules,effectively enriching the analyte beam in the differentialpumping regions. The pressures in the two differential pumpingstages were #10 Torr and %0.1 Torr, respectively. Afterpassing through the momentum separator, the resultantbeam of dry particles entered the hollow cathode glowdischarge volume where they were vaporized, atomized andexcited. In the case of dry particle introduction, the nebulizerand desolvation chamber were replaced with a 1 mm id

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stainless-steel ``sniffer'' (Fig. 1), which enabled particle sam-pling via vacuum action.11 As the mixture of particulates andatmospheric vapors was pulled into the particle beam interface,the gases were skimmed away and the particles weretransported to the hollow cathode source for analysis byoptical emission spectroscopy.

Sample preparation and delivery

Aqueous stock solutions of copper nitrate (Cu(NO3)2, HighPurity Standards, Charleston, SC, USA), cupric chloride(CuCl2?2H2O, Matheson Coleman and Bell, Norwood, OH,USA), copper sulfate (CuSO4, Aldrich Chemical Co., Milwau-kee, WI, USA), cupric acetate (Cu(C2H3O2)2?H2O, AlliedChemical, Morristown, NJ, USA) and ethylenediamine±tetraacetic acid cupric disodium salt (C10H12N2O8Cu-Na2?2H2O, Sigma Chemical Co., St. Louis, MO, USA) wereprepared with HPLC-grade water from analytical-reagentgrade inorganic salts. The solvent and analyte solutions weredelivered in a continuous mode at a rate of 1.5 mL min21 froma solvent reservoir to the nebulizer by a Waters (Division ofMillipore, Milford, MA, USA) Model 510 high-performanceliquid chromatography pump. When operated in the ¯owinjection mode, the sample was introduced to the liquid(HPLC-grade H2O, Fisher Scienti®c, Pittsburgh, PA, USA)¯ow through a manual Rheodyne (Cotati, CA, USA) Model7520 sample injector with a 200 mL sample loop volume. Nochromatographic separations were performed in this study.

In the case of direct particle introduction, dry samples of thecopper salts listed above as well as standard reference materials(SRM) 1648, Urban Particulate Matter, and 1649a, UrbanDust [National Institute of Standards and Technology (NIST),Gaithersburg, MD, USA] were weighed (0.30 mg) into 1/2-dram vials. Each vial was then held up to the ``sniffer'' interfaceto allow the particles to enter the system through vacuumaction.

Optical spectrometer and data acquisition system

The hollow cathode optical emission was sampled by use of aplano-convex, fused-silica lens (45 mm diameter, 10 cm focallength) such that the image of the excitation region was focused#1 : 1 on the entrance slit of the monochromator. A 0.24 m

Czerny±Turner spectrometer equipped with a 2400 groove permm holographic grating (Digikrom 240, CVI Laser Corp.,Albuquerque, NM, USA) was employed for optical monitor-ing, with the scanning range, slit width, spectral calibration andwavelength selection adjusted via the monochromator controlinterface. Atomic emission signals detected by a photomulti-plier tube (Hamamatsu Model R955, Bridgewater, NJ, USA)were then converted into voltage signals with an analog currentmeter. A Macintosh IIsi computer was employed to record theoutput of the current meter via a National Instruments (Austin,TX, USA) NB-MIO-16X interface board. For this particularapplication, an X±Y recorder-type program within theNational Instruments LabView 2 software environment hasbeen developed to record the optical responses. Finally, theobtained digital data were processed and managed in the formof Microsoft (Seattle, WA, USA) Excel ®les.

Particle collection and scanning electron microscopy

A metallic platform was designed for the purpose of collectingresidues of analyte particles passing through the particle beaminterface into the hollow cathode for subsequent examinationwith a scanning electron microscope. A layer of aluminumfoil was fastened around a stainless-steel support(3462.660.2 mm) and inserted into the center of the hollowcathode, perpendicular to the incoming beam of particles, foroptimum analyte particle collection ef®ciency. Each solution-phase sample containing 20 ppm Cu was introduced in acontinuous-¯ow mode at 1.5 mL min21 for a period of 15 minto allow for a suf®cient amount of analyte particles toaccumulate on the aluminum collector. To reduce theabsorption of moisture from the atmosphere, each collectorwas stored in an oven at 100 ³C after sampling, then placed in adesiccator for transport to the scanning electron microscope. AJEOL (Peabody, MA, USA) JSM-5410 scanning electronmicroscope (SEM) was utilized to examine the images ofanalyte particles collected within the hollow cathode. Thealuminum collectors were placed on the sample stage andimaged using an accelerating voltage of 15 kV and magni®ca-tions of 35±1500. For comparison of the collected nebulizedanalyte particles with their corresponding dry particulates, asmall amount of each dry sample was attached to a sample stubwith double-sided tape and carbon-coated to enhance SEM

Fig. 1 Diagrammatic representation of the particle beam-hollow cathode glow discharge atomic emission spectroscopy (PB-HC-AES) apparatus forparticle introduction by solution nebulization and direct aspiration.

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imaging. These samples were examined under the sameconditions as described above. This method is different from`collecting' dry particulates within the hollow cathode, whereparticles of certain sizes may not pass through the particlebeam interface due to the location of the skimmers in themomentum separator.

Results and discussion

Effect of hollow cathode temperature on analyte response for Cusolutions

The quantity of heat available for vaporization of analyteparticles plays a role in the resulting atomization and excitationprocesses, as demonstrated previously for dry particulates.11

To gain a more thorough understanding of this relationship fornebulized solutions, optical emission responses for a group ofCu salts were examined over a range of vaporizationtemperatures. Fig. 2 shows the effect of hollow cathode

temperature on the emission response of Cu I at 324.7 nmfor 20 ppm Cu solutions of a series of Cu salts. The transientpeak areas are calculated within Microsoft Excel by ®rstsubtracting the background for a time scale analogous to thatof the peak, followed by integrating the total signal from thepeak starting point over a ®xed peak width. The triplicate200 mL injections at each temperature generally producevariations of less than 10% RSD, as indicated by the errorbars, with only a few replicates showing greater deviation.

The observed responses suggest that, at the lower tempera-tures (50±100 ³C), there is not enough heat available foref®cient vaporization. The analyte intensity for each Cusolution increases with temperature, reaching a maximumvalue in the 200±300 ³C temperature range, then decreases asthe temperature continues to rise toward 450 ³C. Becausetemperature controls vaporization and in part atomization ofanalyte species, a continuous increase of emission intensity withtemperature might be expected. However, this trend was notobserved at all here.

In order to further investigate these results, analyte particlesfrom nebulized solutions of CuSO4 were collected over therange of hollow cathode temperatures for examination byscanning electron microscopy. The resulting images aredisplayed in Fig. 3. Fig. 3a and b illustrates the analytescollected at 50 and 150 ³C, which seem to have dried as a ®lmresidue on the collector rather than existing in the hollowcathode region as de®ned particles. This supports the loweremission intensity for CuSO4 observed at these temperatures asthe analyte may be slightly wet as it enters the hollow cathode,resulting in reduced vaporization ef®ciency. It is likely that thethermal energy at these lower temperatures is just suf®cient todry the particles, which would in turn be sputtered from thesurface of the hollow cathode. In addition, a decreased atompopulation could result from the presence of water vapor in theplasma if sputtering were the mechanism for atomization.30

Very early work in the development of the PB-HC-AESindicated that very little water vapor was introduced in the HCas evidenced in the absence of OH band emission.4 It is possiblethat minute amounts of water carried into the system via theparticles could yield a suf®cient emission to be used as a

Fig. 2 Effect of hollow cathode temperature on the response of Cu I324.7 nm emission for 20 ppm Cu in copper nitrate, copper sulfate,cupric chloride, cupric acetate, and EDTA±cupric disodium salt. Errorbars represent the range of values for triplicate 200 mL injections ofeach solution. Discharge current 40 mA; source pressure 2 Torr He;nebulizer temperature 280 ³C.

Fig. 3 Scanning electron microscopy images of nebulized CuSO4 solutions (20 ppm Cu) collected at hollow cathode temperatures of (a) 50 ³C, (b)150 ³C, (c) 250 ³C, (d) 350 ³C, and (e) 450 ³C. Collection time 15 min at 1.5 mL min21; source pressure 2 Torr He; nebulizer temperature 280 ³C.

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diagnostic here. As the temperature is increased to 250 ³C(Fig. 3c), the analyte appears in the form of well-de®nedparticles of #2±5 mm diameter. These small, dry particles areeasily vaporized at this temperature, as evidenced in Fig. 2.However, at higher hollow cathode temperatures of 350 and450 ³C (Fig. 3d and e), the collected CuSO4 appeared as afused, charred mass. It is possible that the aluminum collectorsused in this study may have been at a slightly lower temperaturethan the cathode wall due to cooling by the He gas sweepingthrough the center of the hollow cathode; however, thetemperature difference should be minimal as the system isallowed to equilibrate before particle collection is initiated.Although Cu metal has a fairly high melting point of 1083 ³C,these salts typically possess much lower melting points in therange of #100±600 ³C.31 Therefore, it is feasible that, insteadof enhancing vaporization ef®ciency, the elevated temperaturesmay cause pyrolysis or decomposition of the analyte, resultingin the reduced Cu I emission intensities displayed in Fig. 2. Infact, the CuSO4 decomposes at temperatures above 300 ³C,with one of the likely products, CuS, possessing a melting pointof 1100 ³C.31 As such, the evidence presented in Figs. 2 and 3 isconsistent with the chemistry of this salt.

Ideally, the Cu salt solutions in Fig. 2 should display similarCu emission responses, as each contains 20 ppm Cu. However,the integrated intensities vary by almost an order of magnitudein the optimum vaporization temperature range of 250±300 ³C. These differences may be attributed to the correspond-ing particle sizes of the nebulized Cu salts. To determine thesizes of particulates reaching the hollow cathode for eachanalyte, particles from nebulized solutions of each Cu salt werecollected at a hollow cathode temperature of 250 ³C for SEMexamination. The resulting images are shown in Fig. 4. Anexamination of the collected cupric acetate (Fig. 4a) revealsparticles of the order of 1±10 mm, but it appears that only asmall quantity of analyte reached the hollow cathode source.Previous studies have indicated that the momentum separator,in its current con®guration, allows particulates of #1±10 mm totraverse the particle beam interface.7 It is possible that themajority of the nebulized cupric acetate particles had a

diameter of v1 mm and were, therefore, not effectivelytransported across the particle beam interface, resulting inthe low Cu I emission intensities in Fig. 2. These results couldbe improved by adjusting the distance between the skimmers inthe momentum separator to improve transport ef®ciency of thecupric acetate. Images of copper sulfate (Fig. 3c) and coppernitrate (Fig. 4b) show particles of #5 mm. These particles aretransported effectively and vaporized fairly easily, as evidencedby their corresponding emission intensities in Fig. 2. Collectedparticles of cupric chloride and EDTA±CuNa2, whichexhibited the highest intensities, appear to have very smalldiameters of the order of #1 mm, as shown in Fig. 4c and d.Based on these SEM images, it may be expected that the twoanalytes should produce relatively low emission intensities dueto the small number of particles present. However, most of thetiny particles are probably vaporized quickly and, therefore, donot stick to the aluminum collector. The SEM photographssupport the correlation between particle size and emissionintensity, as smaller particles generally produce higherintensities due to more ef®cient vaporization. A similar trendwas found by Brenner and co-workers32 for the analysis of avariety of elements in geological samples by spark ablationinductively coupled plasma atomic emission spectrometry. Theeffects of particle size in vaporization ef®ciency, and ultimatelythe analytical performance of this device, must be evaluated, asthe technique is employed in speci®c application areas.

Comparison of analyte responses for dry particles and nebulizedsolutions

Analyte particles reaching the hollow cathode source fromboth nebulized solutions (where particulates are formed as thesolvent evaporates) and direct dry particle introduction couldconceivably have different size ranges. Differences in particlediameters will de®nitely affect the rate and temperature atwhich optimum vaporization occurs. Fig. 5a and b comparesCu I emission responses over a range of vaporizationtemperatures (50±450 ³C) for CuSO4 and EDTA±CuNa2,respectively, introduced as both nebulized solutions and drypowders. The optimum vaporization temperatures for each

Fig. 4 Scanning electron microscopy images of nebulized Cu solutions (20 ppm Cu) collected at a hollow cathode temperature of 250 ³C: (a)Cu(C2H3O2)2, (b) Cu(NO3)2, (c) CuCl2, (d) EDTA±CuNa2. Collection time 15 min at 1.5 mL min21; source pressure 2 Torr He; nebulizertemperature 280 ³C.

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form of introduced analyte were slightly different, with the dryparticulates requiring more heat for maximum emissionintensity. The nebulized solutions also resulted in muchhigher overall emission intensities, even though the totalamount injected (4 mg Cu) was actually much lower than thatintroduced with the dry powders (120 mg Cu for CuSO4, 44 mgCu for EDTA±CuNa2).

It can be hypothesized from these results that the dryparticles were either inef®ciently transported across the particlebeam interface, or were too large to be vaporized effectivelywhen they reach the hollow cathode. In order to compare theparticle sizes of the dry powder to those collected previouslyfrom nebulized solutions, attempts were made to collect CuSO4

and EDTA±CuNa2 introduced through the particle sampler.Unfortunately, the particles were too dry to adhere to thealuminum collector, and were swept out of the hollow cathoderegion by the discharge gas. To enable examination of thesepowders by SEM, a small amount was af®xed to a sample stubwith double-sided tape. The resulting micrographs aredisplayed in Fig. 6a and b. It is obvious from these imagesthat the dry particles are much larger (200±500 mm for CuSO4,10±60 mm for EDTA±CuNa2) than those collected from thenebulized solutions as seen in Fig. 3c and 4d (5 mm for CuSO4,1 mm for EDTA±CuNa2). The large discrepancy betweenparticle sizes explains the higher vaporization temperaturerequired for the dry particulates as well as the much lower Cu Iemission intensities as compared to those obtained for thenebulized solutions. Since the majority of particles reaching thehollow cathode source through the particle beam interface areof the order of 1±10 mm in diameter as determined fornebulized solutions,7 it is very conceivable that the dryparticulates in this case are too large and heavy to effectivelypass through the momentum separator. Incorporation of agrinding step with a mortar and pestle before analysis ofcommercial analytes would be bene®cial in reducing theparticle sizes prior to analysis.

Effect of hollow cathode temperature on analyte response forcollected dry particles

Fig. 7 shows Cu I emission responses over a range ofvaporization temperatures (100±450 ³C) for two NIST stan-dard reference materials, Urban Particulate Matter (SRM1648) and Urban Dust (SRM 1649a), introduced as drypowders. The optimum vaporization temperature for eachSRM occurs at 200 ³C. This value is signi®cantly lower than thetemperature required for maximum emission intensity of bothcopper sulfate and EDTA±CuNa2 introduced as dry particles

Fig. 5 Effect of hollow cathode temperature on the response of Cu I324.7 nm emission for both dry particles and 20 ppm Cu solutions of(a) EDTA±CuNa2 and (b) CuSO4. Error bars represent the range ofvalues for triplicate 0.30 mg samples of dry particles and 200 mLinjections of solution. Discharge current 40 mA; source pressure 2 TorrHe; nebulizer temperature 280 ³C.

Fig. 6 Scanning electron microscopy images of dry (a) CuSO4 and (b)EDTA±CuNa2 particles.

Fig. 7 Effect of hollow cathode temperature on the response of Cu I324.7 nm emission for Urban Particulate Matter (NIST SRM 1648)and Urban Dust (NIST SRM 1649a). Error bars represent the range ofvalues for triplicate 0.30 mg samples of dry particles. Discharge current40 mA; source pressure 2 Torr He.

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(350 ³C). Based on particle size information provided by NIST,77.7% of the Urban Particulate Matter has a diameter of 45 mmor less, with most particles being #10 mm. As can be seen in thescanning electron micrographs of these SRMs, Fig. 8a and b,most of the particulates appear to be of the order of #10 mm.The lower optimum vaporization temperature for these dryparticles most likely stems from their smaller sizes, which are inline with those typically produced through nebulization ofliquid analyte solutions. It is evident from these results thatanalyte particle diameter signi®cantly in¯uences the vaporiza-tion process; therefore, particle sizing instrumentation shouldultimately be integrated with this system to correlation sizeswith analyte emission intensities.

Conclusions

The effect of hollow cathode (vaporization) temperature on theobserved emission intensities for the PB-HC-AES system hasbeen examined. Studies of Cu I emission in salt solutions for arange of cathode temperatures show optimum signal intensitiesat moderate temperatures of 200±300 ³C. Although an increasein intensity with higher temperatures was initially anticipated, alower optimum temperature range is conceivable based on thelow melting points of this group of Cu salts. Analyte particlesize also appears to play an important role in the vaporization,atomization, and excitation processes, as smaller particles tendto produce greater emission intensities. As a result, it would bebene®cial to incorporate in-line particle sizing capabilities with

this system to enable correlation of analyte emission intensitieswith corresponding particle sizes. Future work will includeimplementation of an array detector to enable simultaneous,multi-element analysis. It is believed that the continuedcharacterization and improvements in optical instrumentationwill enhance the PB-HC-AES system for elemental speciationanalysis of both liquid and dry particle samples.

Acknowledgements

Financial support from the South Carolina UniversitiesResearch and Education Foundation under Grant # SC0101is greatly appreciated.

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Fig. 8 Scanning electron microscopy images of dry (a) UrbanParticulate Matter (NIST SRM 1648) and (b) Urban Dust (NISTSRM 1649a) particles.

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