This document provides an overview of gas chromatography (GC). It discusses the basic principles and components of GC, including how samples are vaporized and separated based on interactions with a stationary phase in the column. The main components described are the carrier gas, injector, column, oven and detectors. Different types of chromatography are classified based on the stationary and mobile phases. Key terms like retention time, capacity factor, and selectivity are also defined.

1. Contents
▶ Introduction
▶ Chromatographic Classifications
▶ Definition
▶ Working Principle
▶ Basic Terms
▶ GC Instrumentation
▶ Main Components
▶ GC Modes of Separation
▶ Detectors
▶ QualitativeAnalysis
▶ QuantitativeAnalysis
2

2. Chromatographic
Classifications:
▶ Gas Chromatography
▶ Gas / Liquid (partition)
▶ Gas / Solid (adsorbent)
▶ Liquid Chromatography
▶ Paper
▶ Column
▶ Liquid / Liquid (partition)
▶ Liquid / Solid (adsorption)
▶ Gel permeation
▶ Ion exchange
▶ Thin layer
4

3. Definition:
▶ Gas Chromatography (GC) is an analytical procedure employed for
separating compounds based primarily on their volatilities. GC offers
both qualitative and quantitative information for individual compounds
present in a sample. The differential partitioning into the stationary
phase allows the compounds to be separated both in time and space. Gas
chromatography (GC) is a common type of chromatography used
in analytical methods for separating and analyzing compounds that can
be vaporized without decomposition.
▶ Basic Principle of GC – Sample vaporized by injection into a heated
system, eluted through a column by inert gaseous mobile phase and
detected.
7

4. Working Principle of GC:
▶ A gas chromatograph uses a flow-through narrow tube known as
the column, through which different chemical constituents of a sample
pass in a gas stream (carrier gas, mobile phase) at different rates
depending on their various chemical and physical properties and their
interaction with a specific column filling, called the stationary phase.
▶ As the chemicals exit the end of the column, they are detected and
identified electronically.
▶ The function of the stationary phase in the column is to separate
different components, causing each one to exit the column at a different
time (retention time).
8

5. Working Principle of GC:
▶ Other parameters that can be used to alter the order or time of retention
are the carrier gas flow rate, column length and the temperature.
▶ In a GC analysis, a known volume of gaseous or liquid analyte is
injected into the "entrance" (head) of the column, usually using a micro
syringe (or, solid phase micro extraction fibers, or a gas source
switching system).
▶ As the carrier gas sweeps the analyte molecules through the column, this
motion is inhibited by the adsorption of the analyte molecules either
onto the column walls or onto packing materials in the column.
▶ The rate at which the molecules progress along the column depends on
the strength of adsorption, which in turn depends on the type of
molecule and on the stationary phase materials.
9

6. Working Principle of GC:
▶ Since each type of molecule has a different rate of progression, the
various components of the analyte mixture are separated as they
progress along the column and reach the end of the column at different
times (retention time).
▶ A detector is used to monitor the outlet stream from the column; thus,
the time at which each component reaches the outlet and the amount of
that component can be determined.
▶ Generally, substances are identified (qualitatively) by the order in which
they emerge (elute) from the column and by the retention time of the
analyte in the column.
10

7. Working Principle of GC: 11

8. Basic
Terms:
▶ Retention Time (tR): The total time that a compound spends in both the
mobile phase and the stationary phase i.e. the time between sample
injection and an analytical peak reaching a detector at the end of the
column. The time taken for the mobile phase to pass through the column
is called tM.
▶ Dead Time (tm): The time that a non-retained compound spends in the
mobile phase, which also is the amount of time the non-retained
compound spends in the column.
▶ Adjusted Retention Time (T ’): The time that a compound spends in the
R
stationary phase. It is the difference between the dead time and the
retention time for a compound
Tr
’= tr - tm
12

9. Basic Terms:
▶ Capacity Factor (or Partition Ratio) (k’): The ratio of the mass of the
compound in the stationary phase relative to the mass of the compound
in the mobile phase.
▶ Phase Ratio (b): The phase ratio relates the column diameter and the
film thickness of the stationary phase. The phase ratio is unitless and
constant for a particular column and represents the volume ration β.
▶ Distribution Constant (KD): The distribution constant is a ratio of the
concentration of a compound in the stationary phase relative to the
concentration of the compound in the mobile phase.
13

10. Basic Terms:
▶ Selectivity (or Separation Factor) (α): It is a ratio of the capacity factors
of two peaks. It is always greater than or equal to one. The higher the
selectivity, the more will be the separation between two compounds or
peaks.
▶ Linear Velocity (u): It is the speed at which the carrier gas or mobile
phase travels through the column.
▶ Efficiency: It is related to the number of compounds that can be
separated by the column.
▶ Retention Volume: VR = tR*F (retained) VM = tM*F (non- retained)
▶ F = average volumetric flow rate (mL/min)
▶ VR and VM both depend on pressure inside the column and temperature
of the column.
14

11. Basic Terms:
▶ Pressure drop factor (j): Is used to calculate average pressure from inlet
pressure Pinlet and outlet pressure Poutlet .
j = 3[(Pinlet / Poutlet )2 -1]/ 2 [(Pinlet / Poutlet )3 -1]
▶ Corrected Retention Volume:
VR
0 = j*tR*F (reatined) VM = j* tM*F (non- retained)
0
▶ Specific Retention Volume:
Vg = [(VR
0- VM )/ MS]*[273/Tcolumn];
0
MS = mass of stationary phase.
Vg = [K/ ρstationary]*[273/T column];
K = partition ratio; ρ stationary = density of stationary phase.
15

12. GC Instrumentation: Block Diagram 16

13. Main
Components:
▶ Carrier Gas:
▶ Injector:
▶ Column:
▶ Oven:
▶ Detectors:
17

14. Three major
types:
▶ Gas - Solid chromatography (stationary phase: solid)
▶ Gas - Liquid chromatography (stationary phase: immobilized
liquid)
▶ Gas - Bonded phase (relatively new)
19

15. Carrier Gas:
▶ He (common),
▶ Others: N2, H2,Ar andAir.
▶ Safety, availability, non-flammability, cost and efficiency are factors for
gas selection.
▶ Purity of 99.995 % and higher is considered for selection as well.
▶ Pinlet = 10-50 psig
▶ F = 25-150 mL/min for packed column
▶ F = 1-25 mL/min for open tubular column
20

16. Injector:
▶ Transfers the analyte into the column.
▶ It provides the means to introduce a sample into a continuous flow of
carrier gas.
▶ Injectors are usually heated to ensure analyte’s transfer to a gas phase.
▶ Volatile liquid or gaseous sample is injected through a septum.
▶ Vapor is swept through column.
▶ Types:
1. Split/ Splitless
2. On – Column
3. PTV Injector
4. P/T (Purge and Trap) System
21

17. Split/ Splitless Injector
▶ Usually consists of heated liner (a glass sleeve, prior to the column
(200–300 °C).
▶ A sample is introduced into a heated small chamber via a syringe
through a septum – the heat facilitates volatilization of the sample and
sample matrix.
▶ The carrier gas then either sweeps the entirety (splitless mode) or a
portion (split mode) of the sample into the column.
▶ In split mode, a part of the sample/carrier gas mixture in the injection
chamber is exhausted through the split vent.
▶ Split injection is preferred when working with samples with high
analyte concentrations (>0.1%) whereas splitless injection is best suited
for trace analysis with low amount of analytes (<0.01%).
22

18. Split/ Splitless Injector
▶ In splitless mode the split valve opens after a pre-set amount of time to
purge heavier elements that would otherwise contaminate the system.
▶ This pre-set (splitless) time should be optimized, the shorter time (e.g.,
0.2 min) ensures less tailing but loss in response, the longer time (2 min)
increases tailing against signal.
▶ – Split (dilution) only part of sample is introduced to the column 1:25 -
1:600
▶ – Splitless – all the sample is introduced (but only for limited time
period)
23

19. Split/ Splitless Injector 24

20. Split Injector 25

21. On-Column Injector
▶ The sample is here introduced directly into the column in its entirety
without heating or at a temperature below the boiling point of the
solvent.
▶ The low temperature condenses the sample into a narrow zone.
▶ The column and inlet can then be heated, releasing the sample into the
gas phase.
▶ This ensures the lowest possible temperature for chromatography and
keeps samples from decomposing above their boiling point.
▶ Analytes are injected directly on the column.
▶ This technique is suitable for thermally unstable compounds.
27

22. On-Column Injector 28

23. PTV Injector
▶ Temperature-programmed sample introduction was first described by Vogt in
1979.
▶ Originally Vogt developed the technique as a method for the introduction of
large sample volumes (up to 250 µL) in capillary GC.
▶ Vogt introduced the sample into the liner at a controlled injection rate.
▶ The temperature of the liner was chosen slightly below the boiling point of the
solvent.
▶ The low-boiling solvent was continuously evaporated and vented through the
split line.
▶ Based on this technique, Poy developed the Programmed Temperature
Vaporizing injector; PTV.
▶ By introducing the sample at a low initial liner temperature many of the
disadvantages of the classic hot injection techniques could be circumvented.
29

24. PTV Injector 30

25. P/T (Purge-and-Trap)
System
▶ An inert gas is bubbled through an aqueous sample causing insoluble
volatile chemicals to be purged from the matrix.
▶ The volatiles are 'trapped' on an absorbent column (known as a trap or
concentrator) at ambient temperature.
▶ The trap is then heated and the volatiles are directed into the carrier gas
stream.
▶ Samples requiring pre concentration or purification can be introduced
via such a system, usually hooked up to the S/SL port.
31

26. Sample
Injection:
▶ The real chromatographic analysis starts with the introduction of the sample
onto the column.
▶ The technique of on-column injection, often used with packed columns, is
usually not possible with capillary columns.
▶ The injection system in the capillary gas chromatograph should fulfill the
following two requirements:
▶ The amount injected should not overload the column.
▶ The width of the injected plug should be small compared to the spreading due
to the chromatographic process.
▶ Failure to comply with this requirement will reduce the separation capability of
the column.
▶ As a general rule, the volume injected, Vinj and the volume of the detector cell,
Vdet should be about 1/10 of the volume occupied by the portion of sample
containing the molecules of interest (analytes) when they exit the column.
32

27. General criteria for injection system selection 33

28. Injectors 34
True Cold
On Column
Split/ Splitless PTV Packed and
Wide born Column

29. Sample
Injection:
▶ Some general requirements which a good injection technique should
fulfil are:
1) It should be possible to obtain the column’s optimum separation
efficiency.
2) It should allow accurate and reproducible injections of small amounts
of representative samples.
3) It should induce no change in sample composition.
4) It should not exhibit discrimination based on differences in boiling
point, polarity, concentration or thermal/catalytic stability.
5) It should be applicable for trace analysis as well as for undiluted
samples.
35

30. Sample
Injection:
36

31. Sample Injection: Features
▶ Volume Injected is typically 0.1-3μL (liquid)
▶ The injected volume is limited by the volume of solvent as a vapour phase.
▶ At 200°C and pressure on column 100 kPa
▶ 1 μL of hexane (l) forms 222 μL (g)
▶ 1 μL of methylene chloride (l) forms 310 μL (g)
▶ 1 μL of water (l) form 1111 μL (g)
▶ Volume of vapour > then volume of injector = backflash (system
contamination)
▶ Concentration
▶ Is defined by column retaining capacity
▶ Columns with a thicker film thickness (a stationary phase) retain more of the
analyte.
37

32. On Column Injection:
▶ On column injection for samples which would decompose at higher
temperatures
▶ Injects the sample directly on the column or the guard column.
▶ All the sample is introduced on the column.
▶ Also all interfering components are injected.
▶ In past, the column has to be ca. 0.53 mm I.D. so the syringe needle can
fit in.
38

33. Features
▶ Split injection - A fraction of a solute (solvent) is injected, therefore
peaks are sharp.
▶ Splitless injection – for trace analysis. The split valve is closed and
most of the sample is introduced on the column. The volume of the gas
going through the injector is only ca. 1 ml/min. Thus, sample
components are transferred to the column for long time. Thus peak is
tailing.
▶ Splitless time - If the split valve is opened after certain time 20 - 120 s,
the transfer of sample is stopped. Still the transfer can be prolonged,
causing an increased peak width.
▶ Solvent trapping - Injecting the sample to the column at temperature
bellow boiling point of a solvent <20°C, after 30s (splitless time) a fast
increase in the temperature to 20°C above solvent’s boiling point. Fast
transfer from gas to liquid and again to the gas phase sharpens the
elution band.
39

34. Injectors: 40

35. Columns
:
▶ Gas chromatography columns are of two designs: packed and capillary.
▶ Packed columns are typically a glass or stainless steel coil (typically 1-5
m total length and 5 mm inner diameter) that is filled with the stationary
phase, or a packing coated with the stationary phase.
▶ Capillary columns are a thin fused-silica (purified silicate glass)
capillary (typically 10-100 m in length and 250 mm inner diameter) that
has the stationary phase coated on the inner surface.
▶ Capillary columns provide much higher separation efficiency than
packed columns but are more easily overloaded by too much sample.
41

36. Columns
:
▶ Columns: Separate the analytes. 2-50 m coiled stainless
steel/glass/Teflon.
▶ The main chemical attribute regarded when choosing a column is
the polarity of the mixture, but functional groups can play a large part in
column selection.
▶ The polarity of the sample must closely match the polarity of the column
stationary phase to increase resolution and separation while reducing run
time.
▶ The separation and run time also depends on the film thickness (of the
stationary phase), the column diameter and the column length.
42

37. Columns:
▶ Packed
1. Solid particles either porous or non-porous coated with thin (1 μm) film of
liquid
2. 3 - 6 mm ID; 1 - 5 m length
▶ Capillary (open tubular) silica columns
1. 0.1 - 0.5 mm I.D. (internal diameter); 15 - 100 m length
2. Inner wall modified with thin (0.1-5 μm) film of liquid (stationary phase)
3. Easy to install
4. Well defined stationary phase
5. Optimal flow rate depends on carrier gas, I.D., film thickness
6. As the linear velocity, I.D. and film thickness increases so is the van Deemter
curve steeper.
43

38. Columns: 44
Capillary Column
Packed Column

39. Columns: Stationary
Phases
▶ The most common stationary phases in gas-chromatography columns are
polysiloxanes, which contain various substituent groups to change the polarity of the
phase.
▶ The nonpolar end of the spectrum is polydimethyl siloxane, which can be made more
polar by increasing the percentage of phenyl groups on the polymer.
▶ For every polar analytes, polyethylene glycol (a.k.a. carbowax) is commonly used as
the stationary phase.
▶ After the polymer coats the column wall or packing material, it is often cross-linked to
increase the thermal stability of the stationary phase and prevent it from gradually
bleeding out of the column.
▶ Small gaseous species can be separated by gas-solid chromatography. Gas-solid
chromatography uses packed columns containing high-surface-area inorganic or
polymer packing.
▶ The gaseous species are separated by their size and retention due to adsorption on the
packing material.
45

40. Columns: Stationary
Phases
▶ Stationary Phases: Must have:
1. Low volatility
2. Thermal stability
3. Chemical inertness
4. Solvation properties giving suitable values for k’, α.
▶ Stationary phases are usually bonded and/or cross-linked
• bonding - covalent linking of stationary phase to support.
• cross-linking - polymerization reactions after bonding to join individual
stationary phase molecules.
46

41. Column Stationary
Phases:
▶ Packed
• liquid coated silica particles (<100-300 mm diameter) in glass tube
• best for large scale but slow and inefficient.
▶ Capillary/Open Tubular
• wall-coated (WCOT) <1 mm thick liquid coating on inside of silica tube
•support-coated (SCOT) 30 mm thick coating of liquid, coated support on
inside of silica tube
• best for speed and efficiency but only small samples.
47

42. Columns
:
48

43. Some Common Stationary Phases –
GLC
50

44. Stationary Phase Compound
Selection:
▶ The polarity of the solute is crucial for the choice of stationary phase
compound, which in an optimal case would have a similar polarity as
the solute.
▶ Common stationary phase compounds in open tubular columns are
cyanopropylphenyl dimethyl polysiloxane, carbowax polyethylene
glycol, biscyanopropyl cyanopropylphenyl polysiloxane and diphenyl
dimethyl polysiloxane.
▶ For packed columns more options are available. Solid stationary phase
adsorbents are SiO2 (silica gel), Al2O2 (alumina), charcoal and Na/ Ca
Al Silicates.
51

45. GC - Modes of
Separation:
1. Isothermal GC
2. Programmed temperature GC
3. Programmed pressure GC
▶ Temperature Effect
 Increase in temperature
 Decreases retention time
 Sharpens peak
52

46. Oven
:
▶ 0-400 °C ~ average boiling point of sample.
▶ Accurate to <1 °C.
▶ The column(s) in a GC is/are contained in an oven, the temperature of
which is precisely controlled electronically.
▶ The rate at which a sample passes through the column is directly
proportional to the temperature of the column.
▶ The higher the column temperature, the faster the sample moves through
the column.
▶ However, the faster a sample moves through the column, the less it
interacts with the stationary phase and the less the analytes are
separated.
53

47. Oven
:
▶ A method which holds the column at the same temperature for the entire
analysis is called "isothermal".
▶ Most methods, however, increase the column temperature during the
analysis, the initial temperature, rate of temperature increase (the
temperature "ramp"), and final temperature manipulations are called the
temperature program.
▶ A trade-off is maintained between the length of analysis and level of
separation.
54

48. Selecting Temperature
Conditions:
▶ Temperature of injector: ensures evaporation of sample, but do not
decompose it (200 – 300 °C).
▶ Temperature of the column (GC oven).
▶ Effect of injection.
▶ For the split injection– no specific requirements.
▶ For the splitless and on column injection – solvent trapping technique
▶ Oven temperature - optimized to improve the separation.
▶ Temperature of the detector: has to be high enough to prevent
condensation of analytes on the detector.
55

49. Detectors
:
▶ The detector is placed at the exit of the column.
▶ It is employed to detect and provide a quantitative measurement of the
various constituents of the sample as they emerge from the column in
combination with the carrier gas.
▶ The choice of a particular type of detector is governed by the following
factors:
 High sensitivity, sufficient enough to provide adequate signal for even small sample
 Response should be linear
 Non distorted shape of peak
 Detector temperature must not condense the eluted vapours in it.
 Insensitive to changes in flow rate
 Good reproducibility
56

50. Name detectors
1. Kathorometer or Thermal Conductivity Detector (TCD)
2. Flame Ionisation Detector (FID)
3. Electron Capture Detector (ECD)
57

51. Thermal Conductivity Detector TCD
▶ This common detector relies on the thermal conductivity of matter
passing around a tungsten -rhenium filament with a current traveling
through it.
▶ In this set up, helium or nitrogen serves as the carrier gas because of
their relatively high thermal conductivity which keeps the filament cool
and maintains uniform resistivity and electrical efficiency of the
filament.
▶ However, when analyte molecules elute from the column, mixed with
carrier gas, the thermal conductivity decreases and this causes a detector
to loose response.
▶ The response is due to the decreased thermal conductivity causing an
increase in filament temperature and resistivity resulting in fluctuations
in voltage.
60

52. Thermal Conductivity Detector TCD
▶ Detector sensitivity is proportional to filament current while it's
inversely proportional to the immediate environmental temperature of
that detector as well as flow rate of the carrier gas.
▶It is rugged and has wide range and also it is non – destructive. However
sensitivity is non – uniform.
61
Selectivity: All compounds except for carrier gases.
Sensitivity: 5 – 20 ng
Temperature: 150 – 250 °

53. Flame Ionization Detector
FID
62
▶ In this common detector, electrodes are placed adjacent to a flame fueled
by hydrogen / air near the exit of the column, and when carbon containing
compounds exit the column they are pyrolyzed by the flame.
▶ This detector works only for organic / hydrocarbon containing compounds
due to the ability of the carbons to form cations and electrons upon
pyrolysis which generates a current between the electrodes.
▶ The increase in current is translated and appears as a peak in a
chromatogram. FIDs have low detection limits (a few picograms per
second, but they are unable to generate ions from carbonyl containing
carbons.
Selectivity: Compounds with C – H bonds.
Sensitivity: 0.1 – 10 ng
Temperature: 250 – 400 °

54. Flame Ionization Detector FID
▶ FID compatible carrier gasses include nitrogen, helium, and argon.
▶ These are rugged, sensitive and have wide dynamic range, however they
are destructive and are not sensitive to non - combustibles.
63

55. Electron Capture Detector
ECD
▶ It uses a radioactive beta particle (electron) source to measure the degree
of electron capture. ECD is used for the detection of molecules
containing electronegative / withdrawing elements and functional
groups like halogens, carbonyl, nitriles, nitro groups, and organo -
metalics.
▶ In this type of detector, either nitrogen or 5% methane in Ar is used as
the mobile phase carrier gas.
▶ The carrier gas passes between two electrodes placed at the end of the
column and adjacent to the anode (negative electrode) that resides in a
radioactive foil such as 63Ni.
64

56. Electron Capture Detector ECD
▶ The radioactive foil emits a beta particle (electron) which collides with
and ionizes the carrier gas to generate more ions resulting in a current.
▶ When analyte molecules with electronegative / withdrawing elements or
functional group electrons are captured, it results in a decrease in current
generating a detector response.
65

57. Electron Capture Detector ECD 66
Selectivity: Halogens, Nitrates and conjugated carbonyls
Sensitivity: 0.1 – 10 pg
Temperature: 300 – 400 °