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