Gas Chromatography by mirola

GAS CHROMATOGRAPHY

By Mirola Afroze, SO, DRiCM, BCSIR

Prepared by Mirola Afroze, SO, DRiCM, BCSIR

Gas Chromatography Page | 2

INTRODUCTION

Chromatography is probably the most powerfuland versatile technique available to the modernanalyst. In a single step process it can separate amixture into its individual components andsimultaneously provide an quantitative estimateof each constituent. Samples may be gaseous,liquid or solid in nature and can range incomplexity from a simple blend of twoentantiomers to a multi component mixturecontaining widely differing chemical species.

Furthermore, the analysis can be carried out, atone extreme, on a very costly and complexinstrument, and at the other, on a simple,inexpensive thin layer plate.



DEFINITION OF CHROMATOGRAPHY

The term chromatography is difficult to definerigorously because the name has been applied tosuch a variety of systems and teclmiques.

All of these methods, however, have in commonthe use of a stationary phase and a mobilephase.

Components of a mixture are carried throughthe stationary phase by the flow of a gaseous orliquid mobile phase, separations being based ondifferences in migration rates among the samplecomponents.



Chromatography may be defined as a separationprocess that is achieved by distributing thecomponents of a mixture between two phases, astationary phase and a mobile phase.

Those components held preferentially in thestationary phase are retained longer in thesystem than those that are distributed selectivelyin the mobile phase.

As a consequence, solutes are eluted from thesystem as local concentrations in the mobilephase in the order of their increasingdistribution coefficients with respect to thestationary phase; ipso facto a separation isachieved.



HISTORY OF CHROMATOGRAPHY

The first scientist to recognize chromatographyas an efficient method of separation was theRussian botanist Tswett (1), who used a simpleform of liquid-solid chromatography to separatea number of plant pigments.

The colored bands he produced on the adsorbentbed evoked the term chromatography for thistype of separation (color writing).

Although color has little to do with modernchromatography, the name has persisted and,despite its irrelevance, is still used for allseparation techniques that employs the essentialrequisites for a chromatographic separation, viz.a mobile phase and a stationary phase.



The technique, as described by Tswett waslargely ignored for a along time and it was notuntil the late 1930s and early 1940s that Martinand Synge(2) introduced liquid-liquidchromatography by supporting the stationaryphase, in this case water, on silica in a packedbed and used it to separate some acetyl aminoacids.

In their paper, they recommended replacing theliquid mobile phase by a suitable gas, as thetransfer of sample between the two phaseswould be faster, and thus provide more efficientseparations.In this manner, the concept of gaschromatography was created but again, littlenotice was taken of the suggestion and it wasleft to Martin himself and A. T. James to bringthe concept to practical reality nearly a decadelater

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BASIC CHROMATOGRAPHIC TERMINOLOGY

• Chromatograph: Instrument employed for achromatography.

• Stationary phase: Phase that stays in placeinside the column. Can be a particular solid orgel-based packing (LC) or a highly viscousliquid coated on the inside of the column(GC).

• Mobile phase: Solvent moving

through the column, either a liquid inLC or gas in GC.

• Eluent: Fluid entering a column.

• Eluate: Fluid exiting the column.

Gas Chromatography



• Elution: The process of passing the mobilephase through the column.

• Chromatogram: Graph showing detector

response as a function of a time.

• Flow rate: How much mobile phase passed /minute (ml/min).

• Linear velocity: Distance passed by mobilephase per 1min in the column (cm/min).

• tm: time at which an unretained analyte ormobile phase travels through the column.

• Adjusted retention time : tr = tr-tm



• Relative retention (separation factor) α=t’r2/t’ 1 a ratio of relative retention times α >1,indicates quality of the separation; ↑α =greater separation

• Capacity factor k = (tr-tm)/tm ↑k = greaterretention used to monitor performance of thecolumn α = k2/k1

• Retention time tr: Retention time is the timeelapsed between the point of injection of thesample and the point of appearance of thepeak of the chromatogram.

• Retention volume: Under a given set ofoperating conditions a constant volume of gasis required to elute a component from thecolumn. Thus, the volume measured from thepoint of injection to the projection of the peakof chromatogram is known as retentionvolume



GAS CHROMATOGRAPHY

Gas chromatography (GC) is a powerful andwidely used tool for the separation,identification and quantitation of components ina mixture.

In early 1900s, Gas chromatography wasdiscovered by Mikhail Semenovich Tsvett as aseparation technique to separate compounds.



In this technique, a sample is converted to thevapor state and a flowing stream of carrier gas(often helium or nitrogen) sweeps the sampleinto a thermally-controlled column.

This technique uses a gas as the mobile phase,and the stationary phase can either be a solid ora non-volatile liquid in which case small inertparticles such as diatomaceous earth are coatedwith the liquid so that a large surface area existsfor the solute to equilibrate with.

GC is divided into two major categories:• Gas Solid Chromatography (GSC)• Gas Liquid Chromatography (GLC)



Gas Solid Chromatography (GSC): GSCutilizes as the stationary phase a glass or metalcolumn filled with powder adsorbent where theadsorbent is a solid of large surface area. InGSC, retention of solutes is dependent largelyupon differences in adsorption properties of thesolutes for the powder adsorbent as they passthrough the stationary phase.Gas Liquid Chromatography (GLC): GLCwhere the adsorbent is a non-volatile liquidcoated on an inert solid support or capillarycolumn in which the inside wall will be coatedwith nonvolatile liquid. In GLC, the retention ofsolutes is dependent largely upon the partitionco-coefficient of the solutes for the nonvolatileliquid of the stationary phase.In both cases, the mobile phase or the carriergas is an inert gas which is made to flow at aconstant rate through a packed column that is asmall diameter tube containing the adsorbent.

Figure: Schematic of a Gas Chromatography

Gas Chromatography


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WORKING PRINCIPLES OF GASCHROMATOGRAPHY

For separation or identification the sample mustbe either a gas or have an appreciable vapourpressure at the temperature of the column – itdoes not have to be room temperature. Thesample is injected through a self sealing disc (arubber septum) into a small heated chamberwhere it is vaporised if necessary. Although thesample must all go into the column as a gas,once it is there the temperature can be below theboiling point of the fractions as long as theyhave appreciable vapour pressures inside thecolumn. This ensures that all the solutes passthrough the column over a reasonable time span.The injector oven is usually 50–100 °C hotterthan the start of the column.



The sample is then taken through the column byan inert gas (known as the carrier gas) such ashelium or nitrogen which must be dry to avoidinterference from water molecules. It can bedried by passing it through anhydrous copper(II)sulphate or self indicating silica (silicaimpregnated with cobalt(II) chloride).Unwanted organic solvent vapours can beremoved by passing the gas through activatedcharcoal. The column is coiled so that it will fitinto the thermostatically controlled oven.

The temperature of the oven is kept constant fora straightforward separation, but if there are alarge number of solutes, or they have similaraffinities for the stationary phase relative to themobile phase, then it is common for thetemperature of the column to be increasedgradually over a required range. This is done byusing computer control, and gives a better



separation if solute boiling points are close anda faster separation if some components arerelatively involatile.

The solutes progress to the end of the column, toa detector. The sample components can bedetected by a suitable detector at the exit. With asuitable differential detector at the exit of thecolumn, the signal obtained is proportional tothe instantaneous concentration of the dilutecomponent in the binary gas mixture

Once a mixture has been separated by GC itscomponents need to be identified. The materialto be identified by GC is run through thecolumn so that its retention time (the time forthe components to pass through the column) canbe determined.



INSTRUMENTATION OF GASCHROMATOGRAPHY

The instrumentation of gas chromatographyincludesi. Carrier gasii. Sample portiii. Columniv. Detectorv.Monitor & Recorder



CARRIER GAS

The carrier gas plays an important role, andvaries in the GC used. Carrier gas must be dry,free of oxygen and chemically inert mobile-phase employed in gas chromatography.

Typical carrier gases include helium, nitrogen,argon, hydrogen and air. Which gas to use isusually determined by the detector being used,for example, a DID requires helium as thecarrier gas.

When analyzing gas samples, however, thecarrier is sometimes selected based on thesample's matrix, for example, when analyzing amixture in argon, an argon carrier is preferred,because the argon in the sample does not showup on the chromatogram.



Safety and availability can also influence carrierselection. The purity of the carrier gas is alsofrequently determined by the detector, thoughthe level of sensitivity needed can also play asignificant role.

Typically, purities of 99.995% or higher areused.

Carrier gases that are used in gas

chromatography:

• Helium is generally used because of itsExcellent thermal conductivityInertnessLow densityGreater flow rate

LimitationsIt is highly expensive



• HydrogenBetter thermal conductivityLower density

LimitationsIt may react with unsaturated

compoundsIt creates fire and explosive hazards

• NitrogenIt is inexpensive

LimitationsIt gives reduced sensitivity

• AirAir is used as carrier gas only when the

oxygen in the air is useful for the detector orseparations



SAMPLE PORT /SAMPLE INJECTIONSYSTEM

The sample injection system is very importantbecause very small amount of sample is used.The system must introduce the sample in areproducible manner and must vaporizeinstantly so that the sample will enter thecolumn easily.

Liquid samples are generally introduced buthypodermic syringe and solid sample must bedissolved in volatile liquids (acetone) for directintroduction

Syringe containing sample is introduced througha septum, injection port oven temperatureheated to temperatures that ensures fastvolatilization of sample, i.e. above the b.p. of allsample components, usually 275º C.



Splitless Injection: (where the split vent isclosed) attempts to transfer all of the sample tothe column and is used for trace analysis.

Split Mode: only a small portion (maybe 1-10%of the sample moves into the column, and therest is sent to waste. This is used when theanalytes are in high concentration and wouldoverload the column



SEPARATION COLUMNThe columns can be constructed of metal orglass tube. It can be any length from a fewcentimers to over a hundred meters. It can becoiled, straight or bent.

The choice of column depends on the sampleand the active measured. The main chemicalattribute regarded when choosing a column isthe polarity of the mixture, but functionalgroups can play a large part in column selection.

The polarity of the sample must closely matchthe polarity of the column stationary phase toincrease resolution and separation whilereducing run time. The separation and run timealso depends on the film thickness (of thestationary phase), the column diameter and thecolumn length.



Two types of columns are used in GC:

Packed columns are usually made of stainlesssteel or glass and contain a packing of finelydivided, inert, solid support material that iscoated with a liquid or solid stationary phase. InGSC, columns are packed with size gradedadsorbent or porus polymers while in GLC, thepacking is prepared by coating the liquid phaseover a size graded inert solid support. Thenature of the coating material determines whattype of materials will be most stronglyadsorbed. Thus numerous columns are availablethat are designed to separate specific types ofcompounds.



Capillary columns have a very small internaldiameter, on the order of a few tenths ofmillimeters, and lengths between 25-60 metersare common. The inner column walls are coatedwith the active materials (WCOT columns),some columns are quasi solid filled with manyparallel micro-pores (PLOT columns) and solidsupported coated with stationary liquid phase(SCOT). Most capillary columns are made offused-silica with a polyimide outer coating.These columns are flexible, so a very longcolumn can be wound into a small coil.


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Figure: Capillary and Packed Columns

Figure: Capillary columns

Gas Chromatography



The Stationary Liquid Phase

The column is the heart of the gaschromatograph, and the success or failure of aparticular separation depends to a large extenton the choice of the stationary liquid phase. Anumber of points should be considered whenselection of a liquid phase is made:

i. The solute solvent interaction forces thatcontribute to the selectivity of the liquid phase.Solution properties such as the polarity,chemical interactions, and hydrogen bondingand other cohesive forces will affect theseparation.

ii. Prediction of retention behavior Types ofsolution, their concentration and use ofmultistage columns, mixed columns determinesthe prediction of retention times.



iii. The temperature limitations of the liquidphases The liquid phase should have lowvolatility and no tendency to decompose at theoperating temperature.

iv. The possibility of irreversible reactions onthe column Irreversible reactions between thesolute and the liquid phase or its impurities ordegradation products may limit the usefulness ofcertain solvents.

The solute solvent interactionA. Cohesion forces:Dipole-dipole interactions- results from theinteraction between two permanent dipoles.

Induction forces- results from an interactionbetween a permanent dipole in either solute orsolvent and induced dipole in the other.



Dispersion forces- arise from electric fieldproduced by the very rapidly varying dipolesformed between nuclei and electrons.Dispersion forces are always present in anysolute-solvent system.B. Hydrogen bonding:Hydrogen bonding is one type of orientationforce that is very important in GC. Hydrogenbonds are intermediate in strength between thestrong chemical bonds. It can be classified intofive groups-Class (I) consists of compoundcapable of forming networks of multiplehydrogen bonds, Class (II) composed ofcompounds containing both a donor atom(ONF) and an active hydrogen atom, Class (III)consists of compounds having donor atoms butno active hydrogen, Class (IV) made up ofmolecules containing active hydrogen atoms butno donor atoms and Class (V) compounds ofthis group show no hydrogen bonding at all.



C. PolarityIn general, polar solutes will be retained to agreater extent as the polarity of the solvent isincreased. Conversely, non-polar solutes areretained to a greater extent as the polarity of thesolvent is decreased.

Thumb rule: Polar solvents dissolve polarcompounds and non-polar solvents dissolvenon-polar compounds



DETECTORS

The detector is that device and associatedequipment used to sense and measure the smallamounts of the components present in thecarrier gas stream leaving the chromatographiccolumn. The impulse received from the elute ofthe column in the form of solute vapor is sensedby the detector. It in turn converts this impulseinto an electrical signal proportional to theconcentration of the solute in the carrier gas.This signal is then amplified and recorded as apeak in the chromatograph.In all types of detectors, when the carrier gaspassing they give a zero signal but when acomponent is eluted, it is detected and a signalproportional to the concentration of thatcomponent is produced.



Classification of detectors:

Depending upon selectivity detectors areclassified asNon-selective detectors: Responds to all

compounds except the carrier gas (e.g. TCD,FID, (PID)Selective detectors: Respond to a range ofcompounds with common physical andchemical properties (e.g. FID, ECD, PID, N andP detector, Sulfur chemiluminiscence)Specific detectors: Respond to a single chemicalcompound

Detectors are also classified asI. Integral and differentialII. Destructive (FID, NPD, FPD) andnondestructive (e.g. TCD, ECD, PID)



Characteristics of an ideal detectorFunctional

• Sensitivity• Stability• Versatility• Proportionality• Reactivity• Response Time• Signal Recording

Non Functional• Simplicity• Cost and availability• Robustness• Safety



Detectors that are used in GC• Thermal conductivity detector (TCD)• Flame ionization detector (FID)• Electron capture detector (ECD)• Discharge ionization detector (DID)• Flame photometric detector (FPD)• Hall electrolytic conductivity detector

(HECD)• Helium ionization detector (HID)• Nitrogen phosphorous detector (NPD)• Photoionization Detector (PID)• Mass selective detector (MSD)



Flame Ionization Detection (FID)The most commonly used detector is the flameionization detector (FID) it is a general carbondetector. It does not detect compounds that donot contain carbon such as nitrogen(N2),oxygen(O2), or water. The presence of N, O, orS in a carbon compound will tend to decreasethe response of the FID.

The Carbon atoms (C-C bonds) are burned in ahydrogen flame. The hydrogen can be suppliedether from a cylinder or from an electrolytichydrogen generator. The hydrogen must bepure to avoid background noise. A charcoalfilter is often placed in the hydrogen supply lineto remove any organic contaminants.



The response of the detector depends on theflow of the hydrogen, air and the makeup gas (ifit is used).

A certain amount of inert gas is needed foroptimum response of the detector. Generallythe flow from a capillary is too low so a makeupgas is used to provided the inert gas flow.

The makeup gas has other beneficial effectssuch as stabilizing the detector, prolonging thelifetime of the jet, and purging any unsweptareas of the detector.

It is also very important to adjust the air andhydrogen gas flows for optimum response.



Mechanism: Compounds are burned in ahydrogen-air flame. Carbon containingcompounds produce ions that are attracted to thecollector. The number of ions hitting thecollector is measured and a signal is generated.

Selectivity: Compounds with C-H bonds. Apoor response for some non-hydrogencontaining organics (e.g., hexachlorobenzene)

Sensitivity: 0.1-10 ng

Linear range: 105-107

Gases: Combustion - hydrogen and air; Makeup- helium or nitrogen

Temperature: 250-300°C,and 400-450°C forhigh temperature analyses



Figure Flame ionisation detector



The Nitrogen–Phosphorus Detector (NPD)

The NPD is fundamentally similar to the flameionisation detector. They both work by formingions and subsequently detecting them as aminute electrical current.

However, a major difference arises in the waythe ions are formed. The eluate (exit gases) ofthe GC is forced through a jet in the presence ofair and hydrogen gas.

The mixture passes over the surface of a heatedrubidium salt in the form of a bead. The excitedrubidium atoms (Rb*) selectively ionisenitrogen and phosphorus.



The ions formed allow a small electric current toflow between two charged surfaces which,under different operating conditions, gives aresponse to either nitrogen or phosphoruscontaining compounds, or both.

To differentiate between nitrogen andphosphorus containing compounds the retentiontime of each solute in the column is used.

Mechanism: Compounds are burned in aplasma surrounding a rubidium bead suppliedwith hydrogen and air. Nitrogen andphosphorous containing compounds produceions that are attracted to the collector.

The number of ions hitting the collector ismeasured and a signal is generated.



Selectivity: Nitrogen and phosphorous

containing compounds

Sensitivity: 1-10 pg

Linear range: 104-10-6

Gases: Combustion - hydrogen and air; Makeup- helium

Temperature: 250-300°C



Figure The nitrogen–phosphorus detector



Thermal Conductivity Detector (TCD)

Measures the changes of thermal conductivitydue to the sample. A detector cell contains aheated filament with an applied current. Ascarrier gas containing solutes passes through thecell, a change in the filament current occurs.The current change is compared against currentin a reference cell.

The difference is measured and a signal isrecorded. When a separated compound elutesfrom the column , the thermal conductivity ofthe mixture of carrier gas and compound gas islowered.The filament in the sample column becomeshotter than the control column. The imbalancebetween control and sample filamenttemperature is measured and a signal isrecorded.



Mechanism: A detector cell contains a heatedfilament with an applied current. As carrier gascontaining solutes passes through the cell, achange in the filament current occurs. Thecurrent change is compared against the currentin a reference cell. The difference is measuredand a signal is generated.

Selectivity: All compounds except for the carriergas

Sensitivity: 5-20 ng


Gases: Makeup: same as the carrier gas




Figure: Thermal conductivity detector (TCD)



Electron Capture Detector (ECD)Another useful detector is the electron capturedetector (ECD) It is an excellent detector formolecules containing an electronegative groupsuch as Cl or F etc. (or derivitized molecules) Itis probably the second most common detectorafter the FID. It is most often used for the tracemeasurement of halogen compounds inenvironmental applications for detectinginsecticide and herbicide residues.The ECD uses a radioactive source such as Ni63

which produces Beta particles which react withthe carrier gas producing free electrons. Theseelectrons flow to the anode producing anelectrical signal . When electrophillicmolecules are present, they capture the freeelectrons, lowering the signal. The amount oflowering is proportional to the amount ofanalyte present. It is sensitive down to 10-15butthe dynamic range is only about 104.



Mechanism: Electrons are supplied from a 63Nifoil lining the detector cell. A current isgenerated in the cell. Electronegativecompounds capture electrons resulting in areduction in the current. The amount of currentloss is indirectly measured and a signal isgenerated.

Selectivity: Halogens, nitrates and conjugatedcarbonyls

Sensitivity: 0.1-10 pg (halogenated

compounds); 1-100 pg (nitrates); 0.1-1 ng(carbonyls)


Gases: Nitrogen or argon/methane




Figure: Electron Capture Detector (ECD)



Photoionization DetectorThe photoionization detector (PID) uses a UVlamp (xenon, krypton or argon lamp, dependingon the ionization potential of the analytes) toionize compounds. The ionization produces acurrent between the two electrodes in thedetector. The detector is non-destructive andcan be more sensitive than an FID for certaincompounds (substituted aromatics and cycliccompounds for example).

Mechanism: Compounds eluting into a cell arebombarded with high energy photons emittedfrom a lamp. Compounds with ionizationpotentials below the photon energy are ionized.The resulting ions are attracted to an electrode,measured, and a signal is generated.



Selectivity: Depends on lamp energy. Usuallyused for aromatics and olefins (10 eV lamp).

Sensitivity: 25-50 pg (aromatics); 50-200 pg(olefins)Linear range: 105-106

Gases: Makeup - same as the carrier gasTemperature: 200°C

Figure: Photoionization Detector



Atomic Emission DetectorOne of the newest gas chromatography detectorsis the atomic emission detector (AED). TheAED is quite expensive compared to othercommercially available GC detectors, but can bea powerful alternative. The strength of the AEDlies in the detector's ability to simultaneouslydetermine many of the elements in analytes thatelute from the column.It uses microwave energy to excite heliummolecules (carrier gas) which emit radiationwhich breaks down molecules to atoms such asS, N, P, Hg, As, etc. These excited moleculesemit distinctive wavelengths which can beseparated by a grating and sent to the detector(typically a photodiode array) which producesthe electrical signal. The atomic emissiondetector is very sensitive (10-15) and has adynamic range of 104.



Figure: Atomic Emission Detector



Thermal Conductivity DetectorThe thermal conductivity detector (TCD)consists of an electrically-heated wire orthermistor. The temperature of the sensingelement depends on the thermal conductivity ofthe gas flowing around it. Changes in thermalconductivity, such as when organic moleculesdisplace some of the carrier gas, cause atemperature rise in the element which is sensedas a change in resistance. Low molecular weightgases have high conductivities so hydrogen andhelium are often used as carrier gases. Nitrogenand argon have similar conductivities to manyorganic volatiles and are often not used.However if nitrogen is used as a carrier gas, thedetector can be used to measure hydrogen orhelium. TCD’s are often used to measurelightweight gases or water (compounds forwhich the FID does not respond).



The TCD is not as sensitive as other detectorsbut it is a universal detector and is non-destructive. However, modern detectors calledmicro-TCD’s have very small cell volumes, andnew electronics that produce much highersensitivities and wider linear ranges. Due to itsincreased sensitivity, and the fact that it is auniversal non-destructive detector, it is againbecoming more popular for certain applications.

Figure: Thermal Conductivity Detector



MASS SPECTROMETER (MS)The Mass spectrometer is the only detectorwhich does not require higher temperature thanis the temperature of GC column. The outlet ofcapillary column of GC is placed directly toionization source. The MS employed consists ofEI or CI ionization and RF analyzer such asquadrupole or ion trap.

Mechanism: The detector is maintained undervacuum. Compounds are bombarded withelectrons (EI) or gas molecules (CI).Compounds fragment into characteristic chargedions or fragments. The resulting ions arefocused and accelerated into a mass filter. Themass filter selectively allows all ions of aspecific mass to pass through to the electronmultiplier. All of the ions of the specific massare detected. The mass filter then allows thenext mass to pass through while excluding all



others. The mass filter scans stepwise throughthe designated range of masses several times persecond. The total number of ions are counted foreach scan. The abundance or number of ions perscan is plotted versus time to obtain thechromatogram (called the TIC). A massspectrum is obtained for each scan which plotsthe various ion masses versus their abundanceor number.

Selectivity: Any compound that producesfragments within the selected mass range. Maybe an inclusive range of masses (full scan) oronly select ions (SIM).Sensitivity: 1-10 ng (full scan); 1-10 pg (SIM)Linear range: 105-106

Gases: NoneTemperature: 250-300°C (transfer line), 150-250°C (source)



Table: GC Detectors



APPLICATION of GAS CHROMATOGRAPHY

Qualitative analysisGenerally chromatographic data is presented asa graph of detector response (y-axis) againstretention time (x-axis).

This provides a spectrum of peaks for a samplerepresenting the analytes present in a sampleeluting from the column at different times.

Retention time can be used to identify analytesif the method conditions are constant. Also, thepattern of peaks will be constant for a sampleunder constant conditions and can identifycomplex mixtures of analytes.

Gas Chromatography


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Qualitative analysisThe area under a peak is proportional to theamount of analyte present. By calculating thearea of the peak using the mathematical functionof integration, the concentration of an analyte inthe original sample can be determined.

Concentration can be calculated using acalibration curve created by finding the responsefor a series of concentrations of analyte, or bydetermining the response factor of an analyte.

Gas Chromatography


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High Efficiency of separationThe technique has strong separation power andeven complex mixture can be resolved intoconstituents

Speed of separationThe analysis is completed in a very short time

The sensitivity in detection of compoundsThe sensitivity is quite high and it ismicromethod and only a few mg of sample issufficient for analysis.

AccuracyIt gives good precision and accuracy



ApplicationIt has wide application for most groups ofpharmaceutical agents

PersonnelThe technique is suitable for routine analysisbecause the operation of a gas chromatographand related calculations does not require highlyskilled persons.

Limitations of Gas ChromatographyThe primary limitation of gas chromatography isthat the sample must be capable ofvolatilized/vaporiaed without undergoingdecompositions. The samples must be thermallystable to prevent degradation when heated. Inaddition, sample cannot be used for furtheranalysis once separated.

Gas Chromatography by mirola

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