Transcript Gas Chromatography

Gas Chromatography (GC; GLC; GSC) 1
“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
Schematic GC apparatus

Liquid sample of
ca. 0.1ml 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.
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INJECTOR PORT
D
SIGNAL
COLUMN
OVEN
CARRIER
GAS
Impurities in styrene
60m Innowax, 2ml/min He, 1ml split 80:1, 80C (9min), 5C/min to 150C
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Schematic gas chromatograph
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Carrier gas (high purity, unreactive, cheap): N2, He, H2
Flow control
» constant, reproducible flow rate
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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
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Powerful analytical tool — why?
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Very large separating power
– para (138.4C), ortho (144.4C) & meta (139.1C) xylene
High speed of analysis
– no sample pre-treatment usually
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Quantitative analysis
– excellent
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High sensitivity
– 58 ppm of phenylacetylene (#11) in styrene sample
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Qualitative analysis the Achilles heel!
– Just because peak at 18 min is labelled a-methylstyrene
Simple to use and operate
– unskilled, automatic, low cost
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Qualitative analysis
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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
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Mixture of unknown alcohols
n-amyl
knowns
Quantitative analysis
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5.00 ml of a soln containing an internal standard, S,
of concentration 100 mg/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)
0.80
– area ratio, A = (AX / AS)
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0.20 0.40
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 mg
\ WX = 356 mg
Detectors — 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
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Electron capture
– ECD — #5 halothane in blood analysis
– very high sensitivity, very selective
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Thermal conductivity
– TCD or HWD or katharometer
– robust, universal, low sensitivity
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Mass spectrometer MSD — expensive but worth it
– excellent for identification
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Flame ionisation detector(s)
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FID (basic design)
– mix H2 and carrier, burn in
clean dust-free air
– collect ions formed
– current  eluting cpds
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COLLECTOR
ELECTRODE
AFID (N/P sensitive)
– surround jet by alkali salt
– surface catalysed reactions
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FPD (collect photons emitted)
– Sulphur mode
394 nm
– Phosphorus mode 526 nm
AIR
SIGNAL
FLAME
HYDROGEN
CARRIER
Flame ionization detector
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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
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Flame photometric detector
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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
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Flame photometric detector
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Sulphur mode; 394 nm
– large solvent peak
– small hydrocarbon peak
(pentadecane) for 4,000 ng
– dodecanethiol (IS) 20 ng
– methyl parathion
20 ng
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Phosphorus mode; 526 nm
– tiny solvent peak
– tributyl phosphate (IS) 20 ng
– methyl parathion
20 ng
Same sample in both cases
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Hot Wire Detector (TCD)
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Tungsten-rhenium
filaments
COLUMN
GAS FLOW
CURRENT
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CARRIER
GAS FLOW
– Current of 0.3 A at 16 V
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Temperature of filament?
 350 C but depends on
thermal conductivity of gas
flowing over hot wire
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Resistance of wire changes
as T changes
– Pre & post column detection
SIGNAL
Thermal conductivity detector
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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
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Electron capture detector
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Radioactive source emits
b-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
15
+
amplifier
63
Ni or
carrier gas
signal
3
H
ECD: organohalogen pesticides
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Column DB-210+ 15 m x 0.53 mm id; film 1.0 mm
He carrier; 100-220C at 3C/min. 600pg each
– 2-lindane; 4-aldrin; 9-dieldrin; 13-DDT
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Electron capture detector
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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
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ECD; biphenyls at 30 ppb each
MDQ: 10 fg lindane in 2ml injection
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The column
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Two kinds
– capillary (WCOT: 0.2 to 5 mm 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
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Liquid phase
– low vapour pressure over operating range & thermally stable
– chemically inert to solutes
– good solvent for solutes used and low viscosity
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Temperature
– isothermal
– programmed (linear, reproducible)
Packed columns (SS, glass)
¼ or ½“od; coiled, U-shaped
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Solid support
–
–
–
–
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uniform pore diameter (10mm 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
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Effect of column temperature
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Increasing the column
temperature reduces
retention times
– biggest effect on longest
times
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Conflict: analysis time
versus resolution
Temperature programming
sidesteps problem
– initial, final, rate of climb
and timings
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Solute classes
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Based on H-bonding capability
– Weak bond
I
Polyalcohols, amino alcohols, etc
II
Alcohols
III
Ethers, ketones
IV
Aromatics, olefins, halocarbons
V
Saturated hydrocarbons
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“Liquid” phase — the heart of the GC
‘Polarity’
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Squalane — the standard phase with zero polarity
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Silicone gum SE30/OV-1
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Dexsil 300
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Di-nonylphthalate
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OV-210 silicone
100-300C
220
50-400C
470
0-150C
790
20-275C
1500
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Polyethylene glycol (CarboWax) 60-225C
2300
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OV-275 silicone
4200
100-275C
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RTX-200 (trifluoropropylmethyl polysiloxane)
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Stabilwax (Carbowax PEG 20M)
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Specialised applications
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Pyrolysis
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Headspace analysis
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Multicolumn techniques
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Hyphenated
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Preparative GC
– brake lining dust
– black peppercorns or cola can
– dual
– back-flushing
– heart-cutting
– GC + MS
– GC/FTIR
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Pyrogram
Headspace analysis of 0.1% cpd in water
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Multi-column techniques
A B C D
SV
B
D C
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A
D
B A*
D
Vent D+C
A B C D
SV
D
C
B
DC
Vent most of A
A B C D
SV
D+C
D
B A
D
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C
Backflushing to vent
» speeding up analysis of A, B by not bothering with C, D
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Heart-cutting
» analyse for B in the presence of large amounts of interfering A
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Dual column for difficult separations
» 1st column can separate A & B but not C & D; 2nd col vice-versa