Sample Collection/Prep:
 soil samples were collected, dried with sodium sulfate, to remove water, spiked with a deuterated
internal standard then extracted with hexane via a Soxhlet extractor
 the hexane sol'n
was concentrated
from 200 mL to
10 mL
 1ul was injected
into the GC/MS
through a
splitless
injection
 a temperature
program was
programmed into
the oven to
speed elution of
the PAHs
p. 683 Harris (5th)
Temperature Programming
For compounds with similar polarity, the elution order is predictable:
retention time  boiling point  molecular weight
For every and all compounds,
retention time  1/ vapour pressure  1/ column temperature
log (tr -tm) = a / T + b where a and b are constants and T is in kelvins
Lowering T → longer retention time + better resolution
Raising T → shorter retention time + poorer resolution
A compound is eluted from a gas chromatography column at an adjusted retention time t'r = 15.0 min
when the column temperature is 373 K. At 363 K, t'r = 20.0 min. Predict t'r at 353 K.
log (15.0) = (a / 373) + b
log (20.0) = (a / 363) + b
where a = 1.69 x 103 K and b = -3.36
At 353 K, log t'r = (1.69 X 103 1 353) -3.36
t'r = 27.1 min
1
2
3
4
5
p. 679 Harris (5th)
Stationary Phase
Several hundred liquid stationary phases are available for gas-liquid chromatography.
e.g.
Squalene (C30H62, a high M.W. saturated hydrocarbon, low vapor pressure, thermally stable,
non-polar) may be chosen for separation of members of a nonpolar homologous series such as
hydrocarbons (i.e., heptane, octane, nonane, etco)
Choice of liquid stationary phase for GC is based on the rule
"like dissolves like"
The stationary phase must be compatible with analytes
 similar polarity
 good solubilities for the analytes
6
7
8
9
10
11
http://www.sisweb.com/gc/agilent/db5.htm
Agilent/J&W DB-5 GC Columns
Features:
 (5%-Phenyl)methylpolysiloxane

Non-polar

Excellent general purpose
Applications:
column
 Amines, hydrocarbons, pesticides, PCBs, phenols,
Wide range of applications
sulfur compounds, flavors and fragrances
Low bleed
Similar Phases:
High temperature limit
 HP-5, Ultra-2, SPB-5, CP-Sil 8CB, Rtx-5, BP-5, OV-5,
007-2(MPS-5), SE-52, SE-54, XTI-5, PTE-5, HP-5MS,
Bonded and cross-linked
ZB-5, AT-5, MDN-5
Solvent rinsable






Wide range of column
dimensions available

Equivalent to USP Phase
G27
12
DB-5, the world's most popular capillary GC stationary phase, is most often a
chromatographer's first choice as a "scouting column" for method development.
It is similar to DB-1 except that 5% of the methyl groups bonded to the siloxane backbone are
substituted with phenyl groups. Though the phenyl contribution makes it slightly more polar than DB1, DB-5 is still considered to be a relatively non-polar stationary phase. Since the Durabond® process
crosslinks and bonds the stationary phase to the capillary wall, DB-5 columns are not harmed by
large solvent injections or solvent rinsing. And, as with DB-1, DB-5 displays excellent thermal
stability and low bleed levels.
13
14
When separating a mixture of compounds with a wide range of boiling points
e.g. , linear alkanes in a homologous series
Isothermal chromatography at a constant temperature ( e.g., 150oC)
 the more volatile compounds emerge very close together
+ the less volatile compounds may not even be eluted from the column within reasonable time.
15
Programmed temperature chromatography
 the temperature is increased from 50oC to 250oC at a rate of 8°C/min.
 separation between peaks is fairly "U';fonn + all of the compounds are eluted within reasonable time.
16
(a) What is the advantage of temperature programming in gas chromatography?
Low boiling solutes are separated well at low temperature, and the retention of high boiling solutes is reduced to a reasonable time at
high temperature.
17
Flame ionization detector (FID)
 responds to most organic hydrocarbons
 only 1 in 105 carbon atoms produces a CHO+ ion
 N2 carrier gas gives the best detection limit
 107 linear response range
18

not sensitive to H2, He, N2, O2, C02, CO, H2O, NH3, NO and H2S
19
Flame Ionization Detector
p. 691 Harris (5th)
p. 591 Harris (6th)

Eluate is burned in a H2/air flame.

Carbon atoms (except carbonyl and carboxyl carbons) produce CH radicals, which go on to produce CHO+ ions in the flame:
CH + O  CHO+ + eThe CHO+ ions carry electric current from the anode (or flame) to the cathode collector. This current is the detector signal.

20
Electron Capture Detector
p. 691 Harris (5th)
p. 592 Harris 6th
 particularly sensitive to compounds containing atoms with high .
e.g. halogens, carbonyls, nitrides, nitro-groups, condensed-ring aromatics, organometallic compounds

relatively insensitive to hydrocarbons, alcohols, and ketones

can detect trace levels of e--capturing compounds in the presence of non-capturing substances. e.g., halogenated compounds (CFCs) that
catalytically destroy ozone molecules in the stratosphere.

Gas entering the
=ncrgy electrons
containing
detector is ionized by high-e
("beta rays”) emitted from a foil
radioactive 63Ni.

Electrons thus
producing a small,
formed arc attracted to an anode,
steady current.

When analyte
affinity enter the
the electrons.
molecules with a high electron
detector, they capture some of
Reduction of the
peak) indicates the
electron affinity.
background current (i.e., a negative
detection of a compound with high
However, the detector
record a positive
 Carrier gas (or
N2 or 5%
response is usually reversed to
peak.
makeup gas) must be either
methane in Ar.
21

Moisture decreases the sensitivity.
New technology
The electron capture detector responds by varying the frequency of
voltage pulses between the anode and cathode to maintain a
constant current.
The ECD is extremely sensitive, with a detection limit comparable to
that of mass spectrometric selected ion monitoring.
22
Detector
Type
Flame ionization
Mass flow
(FID)
Support gases
Hydrogen and
air
Selectivity
Detectability
Dynamic
range
Most organic cpds.
100 pg
107
Universal
1 ng
107
Electron capture
Concentration Make-up
(ECD)
Halides, nitrates, nitriles, peroxides,
anhydrides, organometallics
50 fg
104-105
Nitrogenphosphorus
Mass flow
Hydrogen and
air
Nitrogen, phosphorus
10 pg
106
Flame
photometric
(FPD)
Mass flow
Hydrogen and
air possibly
oxygen
Sulphur, phosphorus, tin, boron, arsenic,
germanium, selenium, chromium
100 pg
103
Photo-ionization
Concentration Make-up
(PID)
Aliphatics, aromatics, ketones, esters,
aldehydes, amines, heterocyclics,
organosulphurs, some organometallics
2 pg
107
Hall electrolytic
conductivity
Halide, nitrogen, nitrosamine, sulphur
Thermal
conductivity
(TCD)
Concentration Reference
Mass flow
Hydrogen,
oxygen
23
24
25
10.C
2
4
22
19
3
In GC analysis, the column temperature
(a) must be above the boiling points of all organic analytes.
(b) must be lower than the boiling point of water.
(c) must be hot enough for each organic analyte to have sufficient vapour to be eluted.
(d) must be higher than the detector temperature.
(e) must not be higher than the injector temperature.
24.B/D
20
5
0
22
3
Bonus question What compounds cannot be determined b~
(a) Chlorinated pesticides.
(b) Water and aqueous solutions.
(c) Volatile organic compounds.
(d) Silanes and siloxanes.
(e) Diazepam and desmethyldiazepam.
25.E
The electron capture detector (ECD) is NOT particularly sensitive to
(a) halogen-containing molecules.
(b) conjugated carbonyls.
(c) nitriles.
(d) nitro compounds.
(e) alcohols and ketones.
8
10
5
7
21
26