Gas Chromatography
Injector
Port
Flow Control
Detector
Recorder
Recorder
Column
Column Oven
Carrier Gas
The GC system consists of gas supplies for the
mobile phase (and the detector, if needed),
flow controls for the gasses, an injector, an oven
for heating the column, a detector, and a data
recording device.
Sample Preparation
1.
2.
3.
4.
Direct Injection of Gasses
Dynamic Headspace
SPME (Solid Phase Microextraction)
As a Liquid Via a Syringe
Direct Injection of Gasses
Gasses can be sampled with a simple gas-tight syringe
and introduced directly into the GC injector.
Another method of direct injection of gasses, uses a
sample loop similar to the ones used in HPLC.
The sample loop is filled with the gas sample and the
valve is turned to divert the sample to the GC column.
Direct Injection of Gasses
Unlike HPLC, temperature and pressure must be carefully
controlled because the mass of the sample will depend on
these factors as gasses expand and contract depending
on the temperature.
Cryofocusing:
Larger volumes of gasses can be introduced into a specialized
injector where the analytes are focused on a cold surface
and then volatilized by rapid heating.
Dynamic Headspace
While direct injection of gasses can be used, many volatile
analytes are often in too low of a concentration to
be quantified using most common detectors. For this reason,
volatiles in the headspace are often concentrated using
dynamic headspace or purge-and-trap methods.
Dynamic Headspace
Typically the sample is placed in a closed container and a flow
of inert gas such as nitrogen is used to purge the headspace
onto an adsorbent or a cryogenic trap.
Cryogenic traps are more complex
than adsorbent traps but they are the
least selective and will contain
virtually any aroma compound in
the sample. However, cryogenic
traps will also concentrate water
(the most abundant volatile in food),
and the aqueous sample must
be further processed.
Dynamic Headspace
Common adsorbents, do not trap much water, and the traps
can be thermally desorbed directly into the injector of a gas
chromatograph. Tenax is one of
the most common trapping
materials because its high
thermal stability makes it
the best choice for thermal
desorption.
Dynamic Headspace
T able 1. P hysica l P ro perties o f S o m e A dso rba nts U sed fo r P reco nce ntratio n o f O rga nic
V o lat iles fro m the V apo r P hase.
o
C o m po und
S urfac e area
M ean po re D ia.(Å )
T e m peratureL im it( C )
2
(m /g)
T ena x G C
19-30
720
450
C hro m o so rb 102
300-400
90
250
C hro m o so rb 105
600-700
500
200
P orapak Q
630-840
75
250
X A D -4
750
50
200
X A D -7
458
80
150
S o urce. S ucan et al. (1).
Solid Phase Microextraction
Sampling devices (manual and autosampler)
consist of a coated silica fiber inside a
hollow needle. The coating on the fiber
adsorbs and concentrates volatiles from
the headspace inside the sample container.
The needle protects the fiber which can be
inserted through the septa of the GC injector
for direct analysis.
Some Common Fiber Coatings Used with SPME
C o ating M ateria l
U ses
T ype
PDM S
N o npo lar se m ivo lat iles (M W 60-275)
A bso rbent,
N o npo lar
P o lyacrylate
P o lar se m ivo lat ile s (M W 80-300)
A bso rbent,
P o lar
P D M S /C arbo xe n
T race-le ve l vo lat iles (M W 30-225)
A d so rbent,
B ipo lar
P D M S /D V B
V o lat iles, nitro gen co m po unds (M W 50-300) A d so rbent,
B ipo lar
C W /D V B
A lco ho ls and po lar co m po unds (M W 40-275) A d so rbent,
P o lar
D V B /C arb./P D M S V o lat iles a nd se m ivo lat iles (M W 40-275)
A d so rbent,
B ipo lar
P D M S = Po lyd im ethylsilo xa ne, D V B = D ivinylbe nze ne, C W = C arbo w a x
Quantification of Analytes with SPME
External calibration curves can only be used if the standards are
made up in the same matrix as the test sample. For complex
samples such as foods, internal calibrations such as isotopic
dilution or standard addition should be used. Care must be
taken to ensure that the response is linear in the concentration
range of the sample and the spiked sample, and multiple standard
addition is advised whenever practical.
Percent Change in Peak Area Due to the Addition of Salt
C o m po und
15% N aC l
E tha no l
38
Z -3-H exa no l
220
H exyl A lco ho l
200
E thyl B ut yrate
170
L ina lo o l
300
L im o ne ne
25
-37
 -P ine ne
S o urce. Steffe n and P aw lisz yn (6).
36% N aC l
94
1000
700
920
2170
32
-26
42% N aC l
150
1040
4200
880
2230
-25
-49
Injection of Liquid Samples in Gas Chromatography
Much more difficult than you would think!
The most common method of introducing a sample into the GC
is with a microliter syringe, hence this process has become
known as injection.
Syringe Loading Methods
Hot Needle Injection
Can help reduce bias in the injection of complex samples
Pull the sample up into the barrel of the syringe and
then pre-heat the needle for several seconds before injecting.
This reduces the condensation of high boiling point
compounds on the walls of the needle.
Split/Splitless Injectors
...are probably the most common types
Splitless Injection,
(where the split vent is closed)
attempts to transfer all of the
sample to the column and is
used for trace analysis.
Split Mode,
only a small portion
(maybe 1-10% of the sample
moves into the column, and
the rest is sent to waste. This
is used when the analytes are
in high concentration and would
overload the column.
How a Split/Splitless Injector Works
Backflash
When a liquid solvent is transferred into the hot injector, it vaporizes
and expands. If the injector volume is not large enough to contain the
expansion, a problem called backflash can occur. The sample expands
so much that it can reach to the septum purge area and part of the
sample can be lost through the septum purge. The sample may also
condense on the cooler area of the upper injector, and may
re-volatilize slowly. Backflash can result in poor peak area
reproducibility, solvent peak tailing, ghost peaks or non-linear
increases in peak area as injection volume increases.
Determining Liquid Volumes for Injection
Table 4. Liner Volumes
Theoretical
1.0 mm ID
59 mL
2.0 mm ID
236 mL
3.0 mm ID
530 mL
4.0 mm ID
942mL
Effective
30 mL
118 mL
265 mL
471 mL
Solvent Expansion Volumes
o
T able 5. So lve nt E xpansio n V o lu m es( injecto r 250 C and 10 psig)
0.5 m L
H 2O
710*
E xpa nsio n V o lu m e s m L (vapo rized)
CS2
C H 2 C l2
H exa ne
212
200
98
Isoo ctane
78
1.0 m L
1420*
423
401
195
155
2.0 m L
2840*
846*
802*
390
310
5.0 m L
7100*
2120*
2000*
975*
775*
In jection volum e
(Liquid)
Ways to Reduce Backflash
1. Optimize sleeve Pack with glass wool or use double gooseneck
2. Inject less
3. Decrease injector temperature
4. Increase head pressure (higher flow rate into column)
Cool On-Column Injection
This method places the liquid sample directly on a cooled column.
and then the column inlet is rapidly heated to vaporize the sample.
Advantages:
highly reproducible
minimal molecular weight discrimination
backflash is not a problem
minimal thermal degradation
Disadvantages:
difficult to handle the small needles required
solvent overload (peak splitting)
contamination with non-volatiles
Temperature Programmable Injector
Temperature programmable injectors are a combination
of split/splitless injectors and cool on-column inlets.
Advantages:
more flexibility than on-column injection
better than on-column injection for dirty samples
can concentrate sample using solvent purge mode
Disadvantages:
cost
complexity