1 gas chromatography (gc; glc; gsc) “basic gas chromatography” by mcnair, wiley, 1997 “modern...

1Gas Chromatography (GC; GLC; GSC)“Basic Gas Chromatography” by McNair, Wiley, 1997“Modern Practise in GC” by Grob, pp. 900“Gas Chromatography” by Willett, pp. 250 (Wiley) Martin and Synge

– 1941 idea; 1952 instrument– 1969 Nobel prize

Manufacturers – Perkin Elmer, Hewlett Packard, Shimadsu, Phillips, Carlo

Erba, Varian, etc– price? inexpensive; many per laboratory

Separation technique — pure n’ simple– partitioning between two phases

2Schematic GC apparatus Liquid sample of

ca. 0.1l volume injected via a syringe into heated injector port where it is rapidly volatilised and swept by a stream of flowing (carrier) gas thru a column & out via a detector.

D SI GNALI NJECT OR PORT

COLUMNOVEN

CARRI ERGAS

3Impurities in styrene 60m Innowax, 2ml/min He, 1l split 80:1, 80C (9min), 5C/min to 150C

4Schematic gas chromatograph Carrier gas (high purity, unreactive, cheap): N2, He, H2

Flow control» constant, reproducible flow rate

Injection port (sample inlet, microlitre syringe) Oven — thermostatted at constant T or linear rate Column Detector Data processing

» retention time (volume)» peak area» recorder, integrator, microprocessor, computer, etc

5Powerful analytical tool — why? Very large separating power

– para (138.4C), ortho (144.4C) & meta (139.1C) xylene High speed of analysis

– no sample pre-treatment usually Quantitative analysis

– excellent High sensitivity

– 58 ppm of phenylacetylene (#11) in styrene sample Qualitative analysis the Achilles heel!

– Just because peak at 18 min is labelled -methylstyrene Simple to use and operate

– unskilled, automatic, low cost

6Qualitative analysis Identification based on

retention times (volumes)

Not conclusive even by comparing two or more different columns

If analytes known then reasonable supposition

Unambiguous?– GC + MS or– GC + IR

Mixture of unknown alcohols

knowns

n-amyl

7Quantitative analysis 5.00 ml of a soln containing an internal standard, S, of

concentration 100 g/ml were added to a soln of unknown, X. Chromatography of the mixture gave an area ratio of (AX / AS) = 0.81 ± 0.01.

Calibration of known weight ratio mixes gave:– weight ratio, W = (WX / WS) 0.20 0.40 0.80– area ratio, A = (AX / AS) 0.23 0.46 0.91

Calculate weight of X in unknown.» By least-squares: A = 1.132 W + 0.005 so if A = 0.81 then

W = 0.711 but WS = 500 g WX = 356 g

8Detectors — key components Flame ionization family

– the parent FID — workhorse, quasi-universal, reliable– flame photometric FPD — #6 sulphur/phosphorus detector– alkali flame AFID or nitrogen/phosphorus NPD or TID

Electron capture – ECD — #5 halothane in blood analysis– very high sensitivity, very selective

Thermal conductivity– TCD or HWD or katharometer– robust, universal, low sensitivity

Mass spectrometer MSD — expensive but worth it– excellent for identification

9Flame ionisation detector(s) FID (basic design)

– mix H2 and carrier, burn in clean dust-free air

– collect ions formed– current eluting cpds

AFID (N/P sensitive)– surround jet by alkali salt– surface catalysed reactions

FPD (collect photons emitted)– Sulphur mode 394 nm– Phosphorus mode 526 nm

AI R

HYDROGEN

CARRI ER

SI GNAL

COLLECT ORELECT RODE

FLAME

10Flame ionization detector MDQ — 5 picograms / second Response — quasi-universal Linearity — excellent (over 106) Stability — flow and temperature insensitive Temperature limit — 400 C Carrier gas — Nitrogen, helium or hydrogen Summary

– Rugged– non-responsive to water and air (“inorganics”)– destructive and – very widely used

11Flame photometric detector MDQ — 1 nanogram S (394 nm); 0.1 ng P (526 nm) Response — effectively only S and P compounds Linearity — moderate (104) Stability — good Temperature limit — 400 C Carrier — nitrogen Summary

– very selective– flame needs clean hydrogen/air supply– expensive but invaluable for pesticide and air pollution work

12Flame photometric detector Sulphur mode; 394 nm

– large solvent peak– small hydrocarbon peak

(pentadecane) for 4,000 ng– dodecanethiol (IS) 20 ng– methyl parathion 20 ng

Phosphorus mode; 526 nm– tiny solvent peak– tributyl phosphate (IS) 20 ng– methyl parathion 20 ng

Same sample in both cases

13Hot Wire Detector (TCD)

Tungsten-rhenium filaments– Current of 0.3 A at 16 V

Temperature of filament?350 C but depends on

thermal conductivity of gas flowing over hot wire

Resistance of wire changes as T changes– Pre & post column detection

CURRENTCARRI ERGAS FLOW

COLUMNGAS FLOW

SI GNAL

14Thermal conductivity detector

MDQ — 10 nanograms (about 50 ppm) Response — universal (all except the carrier) Linearity — moderate (104) Stability — flow and temperature sensitive Carrier — hydrogen or helium Temperature limit — 400 C Summary

– non-destructive and simple to operate (portable)– moderate stability and sensitivy– used for fixed gas analysis, eg, H2, N2, O2, CO2, Ar, etc

15Electron capture detector Radioactive source emits

-particles (fast electrons) which are converted into slow electrons by collision with N2 carrier gas

These are captured by molecules to form a slower moving anions

Reduction in current as compound flows through detector

amplifi er signal

63Ni or 3H

+

carr ier gas

16ECD: organohalogen pesticides

Column DB-210+ 15 m x 0.53 mm id; film 1.0 m He carrier; 100-220C at 3C/min. 600pg each

– 2-lindane; 4-aldrin; 9-dieldrin; 13-DDT

17Electron capture detector MDQ — very high sensitivity (picogram range) Response — very selective (halogenated compounds only)

Linearity — Poor ( 500 to 104) Stability — fair Temperature limit — 220 C (3H) or 350 C (Ni) Carrier — nitrogen or argon + 10% methane Summary

– easily contaminated, carrier must be dry– non-destructive– requires license for radioactive source

18ECD; biphenyls at 30 ppb eachMDQ: 10 fg lindane in 2l injection

19The column Two kinds

– capillary (WCOT: 0.2 to 5 m film thickness, PLOT) » 0.3mm id 50m 300,000 plates 0.01ml 2 ml/min

– packed » 3mm id 2m 3,000 plates 10ml 40 ml/min

Liquid phase– low vapour pressure over operating range & thermally stable – chemically inert to solutes– good solvent for solutes used and low viscosity

Temperature– isothermal– programmed (linear, reproducible)

20Packed columns (SS, glass)

¼ or ½“od; coiled, U-shaped Solid support

– uniform pore diameter (10m or less)– large inert surface area (AW, treated with DMCS)– regularly shaped, uniformly sized (mesh nos.)– eg Chromosorb W/AW/DMCS 100-120 mesh

Preparation (5% X on Y):– slurry 5g liquid phase X with 100g solid support Y in

small quantity of suitable solvent– Rotovap off solvent, pack column– Leave overnight at highest safe temperature in oven

with flow of carrier

21Effect of column temperature Increasing the column temperature reduces retention times– biggest effect on longest

times Conflict: analysis time

versus resolution Temperature programming

sidesteps problem– initial, final, rate of climb

and timings

22Solute classes

Based on H-bonding capability

– Weak bond I Polyalcohols, amino alcohols, etc II AlcoholsIII Ethers, ketonesIV Aromatics, olefins, halocarbons V Saturated hydrocarbons

23“Liquid” phase — the heart of the GC

‘Polarity’

Squalane — the standard phase with zero polarity

Silicone gum SE30/OV-1 100-300C 220 Dexsil 300 50-400C 470 Di-nonylphthalate 0-150C 790 OV-210 silicone 20-275C 1500 Polyethylene glycol (CarboWax) 60-225C 2300 OV-275 silicone 100-275C 4200

24RTX-200 (trifluoropropylmethyl polysiloxane)

25

Stabilwax (Carbowax PEG 20M)

26Specialised applications Pyrolysis

– brake lining dust Headspace analysis

– black peppercorns or cola can Multicolumn techniques

– dual– back-flushing– heart-cutting

Hyphenated– GC + MS– GC/FTIR

Preparative GC

Pyrogram

27Headspace analysis of 0.1% cpd in water

28Multi-column techniques

Backflushing to vent» speeding up analysis of A, B by not bothering with C, D

Heart-cutting» analyse for B in the presence of large amounts of interfering A

Dual column for difficult separations» 1st column can separate A & B but not C & D; 2nd col vice-versa

A B C D SV D C B A * DD C B

A B C D SV B A DD C

A B C D SV

D C

DD + C B A

V e n t m o s t o f A

V e n t D + C