Gas Chromatography

Gas Chromatography

separation by partition between gaseous mobile phase and liquid stationary phase supported by inert packing

developed in 1941 by Martin and Synge applications did not come till 1950s

Specific Retention Volume

Vg - depends on nature of solute and stationary phase, independent of many of the column variables


Tc P - PH2OF = Fm * ---- * ------------T Pwhere F =>av. flow rate within column

Fm =>flow rate at bubble meterTc =>column temperature, KT =>gas temp. at bubble meter, Kp =>pressure at end of columnpH2O =>vapor pressure of water


VR = tR *F

where VR => retention volumetR =>retention time


VM = tM *F

where VM =>volume of component not retained

tM =>retention time of unretained component


VR & VM depend on av. p within column


VRo & VMo => corrected retention vol.

VRo = j*tR *F

VMo = j*tM *F

3((pi /p)2 - 1)where j = ---------------------2((pi /p)3 - 1)

where pi =>initial pressure on column

Specific Retention Volumetherefore,

VRo - Vmo 273Vg = -------------- * -------

W Tcor

j*F(tR - tM ) 273 Vg = ---------------- * -------

W Tcwhere T => 273 K

W => wt of stationary phase


Relationship between Vg and K, the partition coefficient

j*F*tM *K' 273Vg = ---------------- * -------

W Tc



VMo*K' 273Vg = ------------- * --------

W Tc



Vs *K 273Vg = ---------- * --------

W Tc

where Vs =>vol of stationary phase


Ws = -----Vs

where s =>density of stationary phase


thus

K 273V g = ------ * --------s Tc

thus Vg is a function of K and swhen Tc = 273 K

Schematic of a gas chromatograph

Apparatus

Carrier Gas Supplychemically inert gases:

He, Ar, N2 & H2high purity, 99.9995% pure or better

Apparatus

Carrier Gas Supplypressure regulators:

- reduce pressure of gas- control the flow rate

gaugesflow meters: rotameters

soap bubble

A soap-bubble flow meter

Apparatus

Sample Injection System introduces "plug" of sample onto front of

column microsyringe thru septum sampling valve

Injector

Rotary Sample Valve

Apparatus

Columncolumn preparationcolumn: glass, stainless steel, copper, or aluminum capillary packed

ApparatusColumn

packed solid support

small, uniform spherical particles inert at elevated temperatures examples:

firebrick alumina diatomaceous earth ChromosorbR A, G, P, W and T

ChromosorbR Packing Materials and Their Uses

A Used in preparative columns because of its highcapacity for liquid phase.

G Used for separation of polar compounds.P Used primarily for hydrocarbon work.W Used for separating highly polar or reactive

compounds such as water, hydrazine, sulfur dioxideand the halogens.

T Used for separtion of polar compounds.

Chemical Analysis of Some Selected Supports

FirebrickC22

Celite 545 ChromosorbP

ChromosorbW

SiO2 89.7 89.9 89.2 91.2Al2O3 5.1 3.6 5.1 4.1Fe2O3 1.55 1.65 1.50 1.15TiO2 0.30 0.30 0.30 0.25CaO 1.30 1.75 0.90 0.40MgO 0.90 0.70 1.00 0.65

Apparatus

Liquid Phase low volatility thermal stability chemical inertness solvent characteristics tR falls within proper limits

Apparatus

Liquid Phase classifications:

Non-polar polar intermediate hydrogen bonding specific

Apparatus

Column Thermostatingtemperature: slightly below or equal to av. b.p. of

components allows for tR between 2 and 30 min programmable

Effect of temperature on gas chromatograms:

(a.) isothermal at 45oC; (b.) isothermal at 145oC; (c.) programmed at 30oC to 180oC.

Apparatus

Detectors Thermal Conductivity Detector (TCD) Flame Ionization Detector (FID) Electron Capture Detectors (EC or ECD) Flame Photometric Detectors Fourier Transform Infrared Detector (FTIR) Mass Spectrometer (MS) Other: UV, FT-NMR

Apparatus

Detectors Thermal Conductivity Detector (TCD)

universal relies on change of cooling ability of carrier gas

over filaments as solutes elute few ppt sensitivity nondestructive

Thermal Conductivity Detector Cell

Apparatus

Detectors Flame Ionization Detector (FID)

sample burned in H2 /air flame sample must be combustible must use electrometer flame resistance 1012 ppm sensitivity destructive

Flame Ionization Detector

Apparatus

Detectors Electron Capture Detectors (EC or ECD)

must have

emitter source, H3 usually Radioactive

rays cause carrier gas to emit

electrons which are collected by solute particles particularly sensitive to halogens, few ppbs destructive

Electron-Capture Detector

Apparatus

Detectors Atomic Emission Detectors

similar to FID sample burns in plasma emitting light

characteristic to selected elements uses low band pass filter to be specific to

selected elemental emissions especially sensitive to P and S, several ppbs destructive

Atomic Emission Detector

Apparatus

Detectors Fourier Transform Infrared Detector

(FTIR) first technique which tried to really identify

solutes difficulty with sample cell transmission of IR

radiation to detector nondestructive

Light Pipe for GC/IR Instrument

Apparatus

Detectors Mass Spectrometer (MS)

another technique aimed at identifying solutes main difficulty, changing from highly

pressurized gas mixture to vacuum containing isolated components

destructiveOther: UV, FT-NMR

Capillary Gas Chromatography/Mass Spectrometer.

Jet Separator

Apparatus

Detectors Other:

UV FT-NMR

Applications

1.separation2.complete analysis: quantitative as well as

qualitative better if use more informative detector: MS,

FTIR, UV, NMR

Quantitative Analysis

integration of area under peak triangulation "weighing paper dolls" mechanical electronic

Qualitative Analysis

TR vs. VRVR better depends only on K, analyte, column T, Ps


Relative Retention Times(tR )Y - tm = ----------------(tR )X - tm


Retention Indexgood for homologous series, straight line

log(tR ')x - log(tR ')nIX = 100 * ----------------------------- + 100 * nlog(tR ')n+1 - log(tR)n

where n => # carbons in parafinsTR ' = (tR - tm )


use 2 paraffin's whose tR bracket the value for the unknown

Summary

single chromatograph inconclusive 3 chromatographs with columns of

varying polarity, comparing retention index - good way of identifying analyte

Slide Number 1Gas ChromatographySpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSpecific Retention VolumeSchematic of a gas chromatographApparatusApparatusA soap-bubble flow meterApparatusInjectorRotary Sample ValveApparatusApparatusChromosorbR Packing Materials and Their UsesChemical Analysis of Some Selected SupportsApparatusApparatusApparatusEffect of temperature on gas chromatograms:ApparatusApparatusThermal Conductivity Detector CellApparatusFlame Ionization DetectorApparatusElectron-Capture DetectorApparatusAtomic Emission DetectorApparatusLight Pipe for GC/IR InstrumentApparatusCapillary Gas Chromatography/Mass Spectrometer.Jet SeparatorApparatusApplicationsQuantitative AnalysisQualitative AnalysisQualitative AnalysisQualitative AnalysisQualitative AnalysisSummary

Gas Chromatography

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