Gas chromatography is a technique used to separate components of a vaporized sample. It works by partitioning the components between a mobile gaseous phase and a stationary phase within a column. The sample is injected and vaporized, then transported through the column by the mobile phase gas. As the components pass through the column they are separated and detected. Common detectors include the flame ionization detector (FID), thermal conductivity detector (TCD), and electron capture detector (ECD). The FID responds to organic compounds, the TCD is universal but less sensitive, and the ECD selectively detects halogen-containing compounds.

1. GAS CHROMATOGRAPHY
LECTURE 8

2. PRINCIPLES
In Gas Chromatography, the components
of a vaporized sample are separated as a
result of being partitioned between a
mobile gaseous phase and a liquid or a
solid stationary phase held in the column.

3. PRINCIPLES
 A sample is being injected at the
inlet/injector and vaporized into the
chromatographic column.
 The sample is transported through the
column by the flow of inert gaseous
mobile phase.
 As the sample passes through the
column, they are separated and
detected electronically by detector.

4. Gas Chromatography
 Gas is called “carrier gas”.
 Typical carrier gas: helium or nitrogen.
 Pressure from a compressed gas
cylinder containing the carrier gas is
sufficient to create the flow through
the column.

5. Gas Chromatography
There are two types.
 Gas-liquid chromatography (GLC)
mobile phase – gas Shortened to Gas
Chromatography
stationary phase - liquid
 Gas-solid chromatography (GSC)
mobile phase – gas
stationary phase - solid

6. INSTRUMENTATION
A. Carrier gas
B. Flow regulator
C. Injector
Thermostated
D. Column oven
E. Detector
Integrator
F. Integrator
G. Display system
-printer/monitor

7. INSTRUMENTATION

8. INSTRUMENTATION
Injection port and detector must be kept
warmer than the column,
1. To promote rapid vaporization of the
injected sample.
2. To prevent sample condensation in
the detector.

9. Samples must be…..
 Volatile
 Thermally stable.
 When injected onto the head of a
chromatographic column and
vaporized.

10. Mobile phase
 Mobile phase transports the analytes
(sample) through column.
 Mobile phase can not interact with the
molecules of the analyte.
 Referred as carrier gas.

11. A. Carrier gas
 Must be chemically inert.
 Most common carrier gas is Helium(He)
 Some specific detectors are using
Nitrogen gas(N2), Hydrogen gas(H2),
Carbon dioxide gas(CO2) and Argon.
 The carrier gas should not contain
traces of water or oxygen. Both are
harmful to the stationary phase.

12. B. Flow regulator
 The function of flow regulator is to
control the flow rate of the carrier gas
using the pressure regulators, gauges and
flow meters.
 The pressure at the head of the column
is stabilized
 mechanically OR
 through the use of an electronic
device.

13. C. Injectors
 Functions
1. An inlet for the sample.
2. To vaporize and mix the sample with the
carrier gas before the sample enters the
head of the column.
 Temperature is set about 50°C higher than
boiling point of the least volatile component
of the sample.
 Modes of injection and characteristics of
injectors vary depending on type of column
used whether split/splitless.

14. Mode of Injections

15. D. Sample Injection System
 Column efficiency requires sample to
be……
1. Of a suitable size
2. Introduced as a “plug” of vapor
 Band broadening and poor resolution
are caused by…….
1. Slow injection.
2. Oversized sample.

16. D. Sample Injection System
 Sample introduction usually……
1. In the form of neat liquid or solution.
2. Introduced in a small volumes.
a. 1 μL - 20 μL for packed column.
b. 1 x 10-3 μL for capillary column.

17. D. Sample Injection System
Examples
 Direct injection using microsyringe
 Loop injectors
 Auto samplers
 Headspace

18. D. Sample Injection System

19. D. Sample Injection System
LOOP INJECTORS
DIRECT INJECTION
USING MICROSYRINGE

20. D. Sample Injection System
Headspace
A headspace sample is normally prepared in a
vial containing the sample, the dilution solvent, a
matrix modifier and the headspace.
Volatile components from complex sample
mixtures can be extracted from non-volatile
sample components and isolated in the
headspace or gas portion of a sample vial.
A sample of the gas in the headspace is
injected into a GC system for separation of all of
the volatile components.

21.  G = the gas phase (headspace)
The gas phase is commonly referred to
as the headspace and lies above the
condensed sample phase.
 S = the sample phase
The sample phase contains the
compound(s) of interest. It is usually in
the form of a liquid or solid in
combination with a dilution solvent or a
matrix modifier.
Once the sample phase is introduced into
the vial and the vial is sealed, volatile
components diffuse into the gas phase
until the headspace has reached a state
of equilibrium as depicted by the
arrows. The sample is then taken from
the headspace.

22. E. Oven
 Must have sufficient space to hold the
column.
 Can be heated to the desired
temperature for analysis.
 Atmosphere inside the oven is
constantly agitated by forced
ventilation which has small thermal
inertia.
 Reproducible of retention time,tR which
require control of the column
temperature within a few tenths of a
degree.

23. E. Oven
 Optimum temperature depends on
the boiling points of the sample
components.
 A temperature that is roughly ≥ the
average boiling point of the sample
results in a reasonable elution period.
 Samples with broad boiling range,
necessary to employ temperature
programming.

24. Temperature Programming
 Definition:
A technique in which the column
temperature is increased either
continuously or in steps as the
separation proceeds.
 In general, optimum resolution is
associated with minimal temperature.
 Low temperature, result in longer
elution times hence slower analysis.

25. Temperature Programming
 Using Temperature programming, low
boiling point constituents are separated
initially at temperatures that provide
resolution.
 As separation proceeds, column
temperature is increased so that the
higher boiling point constituents come
off the column with good resolution
and at reasonable lengths of time.

26. Isothermal Elution
A technique in which the column
temperature is constantly maintained
throughout the separation.

27. Isothermal at 1500C
Temperature
programmed:
500C to 2500C at
80C/min

28. F. Columns
Two types of columns
1.Packed column
2.Capillary column
Capillary Column:
Packed column:
10-100m in length, very small i.d
1-5m in length, 2-4mm i.d

29. 1. Packed Column
 Less commonly used
 Made of glass or steel
 Length: 1 to 5 m
Cross-sectional view
 Internal diameter: 2 to 4 mm of packed column
 These column is densely packed with uniform,
finely divided solid support, coated with thin
layer (0.05 to1μm) of stationary liquid phase.
 Accommodate larger samples.

30. 1. Packed Column
 Carrier gas flow between 10 – 40 mL/min.
 Not well adapted for trace analysis.
 Contain an inert & stable porous support on
which the stationary phase can be
impregnated(coated) or bound.
 Advantages:
1. Large sample size
2. Ease & convenience of use

31. 2. Capillary Column
 Widely used in GC analysis
 Also known as open tubular column
 Length: 10 – 100 m
 Coiled around a light weight of metallic
support.
 Types of capillary column
I. FSOT (Fused Silica Wall Coated) - i.d. 0.1 -
0.3 mm
II. WCOT (Wall Coated) - i.d. 0.25 – 0.75 mm
III. SCOT (Support Coated) - i.d. 0.5 mm

32. 2. Capillary Column
 Advantages:
1. High resolution
2. Short analysis time
3. High sensitivity

33. Properties and characteristics
of GC Column

34. G. Stationary Phase
Desirable properties for the immobilized liquid
stationary phase:
Low volatility (ideally the boiling point of the
liquid at least 1000C higher than the maximum
operating temperature for the column)
Thermal stability.
Chemical inertness.
Solvent characteristics such as k and α values
for the solutes to be resolved fall within a suitable
range.

35. G. Stationary Phase
 Separation principles
 Use the principle of “like dissolve like”
where like refers to the polarity of the
analyte and the immobilized liquid
stationary phase.
 Polarity of organic functional group in
increasing order
 Aliphatic hydrocarbons<olefins<aromatic
hydrocarbons<halides<ethers< esters/
aldehydes/ketones<alcohols/amines<
amides<carboxylic acids<water

36. G. Stationary Phase
 Polarity of the stationary phase should match
that of sample components.
 When the match is good, the order of elution
is determined by the boiling point of the
eluents.

37. The choice of stationary phase should match
that of sample components.
Non polar Stationary phase Polar Stationary phase

38. Polarity of Stationary Phase
Water
Polar Carboxylic acids
Amides
Alcohol/amines
Esters/aldehydes/ketones
Ethers
Halides
Aromatic hydrocarbons
Olefins
Non-polar
Aliphatic hydrocarbons

39. Non-polar Polar
Aliphatic hydrocarbons < esters/aldehydes/ketones < alcohols/amines < water
Pentane, Hexane Acetone, 3-pentanone Propanol, Butanol
Heptane, Octane Methyl ethyl ketone Pentanol

40. G. Stationary Phase
applications

42. G. Stationary Phase
 Many liquid statationary phase are based on
polysiloxanes or polyethylene glycol (PEG)
H H H H
HO C C O C C OH Polyethylene glycol (PEG)
Use for separating polar species
H H H H
n
R R R
R Si O Si O Si R
Polydimethyl siloxane, the R
groups are all CH3. (Non-polar)
R R R
n

43. H. Detectors
 Some detectors are universal.
 They are sensitive to almost every
compound that elutes from the column.
 Most detectors are selective.
 They are sensitive to a particular type of
compound. Give response that is
dependent on the concentration of
analyte in the carrier gas.
 Yield(produce) simple chromatogram.

44. H. Detectors
 Characteristics of ideal detector
1. High reliability & ease to use.
2. Similarity response toward all solutes
or alternatively a high predictable &
selective response toward one or
more classes of solute.
3. Detector should be nondestructive.

45. H. Detectors
 Characteristics of ideal detector
4. Adequate sensitivity.
5. Good stability and reproducibility.
6. Linear response to solutes that
extends over several orders of
magnitude.
7. Temperature range (from room
temperature to at least 400 0C)

46. H. Detectors
 Several types of detectors.
1. Flame Ionization Detector (FID)
2. Thermal Conductivity Detector (TCD)
3. Electron Captured Detector (ECD)

47. 1. Flame Ionization
Detector (FID)

48. How does FID works?
 Effluent from the column is passes
through a small burner fed H2 and air.
 Combustion of the organic
compounds flowing through the flame
creates charged particles (ionic
intermediates are responsible for
generating a small current between
the two electrodes).
 The burner, held at ground potential
acts as one of the electrodes.
 The second electrode called as a
collector, is kept at a positive voltage
& collects the current that is
generated.
 Signal amplified by electrometer that
generate measurable voltage.

49. 1. FID
 Advantages
 Rugged
 Sensitive (10-13 g/s)
 Wide dynamic range (107)
 Signal depends on number of C atoms
in organic analyte - mass sensitive not
concentration sensitive.

50. 1. FID
 Disadvantages
 Weakly sensitive to carbonyl, amine,
alcohol & amine groups.
 Not sensitive to non-combustibles
analyte such as H2O, CO2, SO2, NOx.
 Destructive method.

51. 2. Thermal Conductivity
Detector (TCD)

52. 2. TCD
 A universal detector.
 Has a moderate sensitivity.
 Less satisfactory with carrier gas whose
conductivities closely resemble those
of most sample components.

53. How does TCD works?
 Consists of an electrically heated
source whose temperature at
constant electric power depends
on the thermal conductivity of the
surrounding gas.
 The electrical resistance of this
element (fine platinum, gold or
tungsten wire or thermistor)
depends on the thermal
conductivity of the gas.
 Operating principles relies on the
thermal conductivity of the
gaseous mixture.
 The thermal conductivity affects
the resistance of the thermistor as
a function of temperature.

54. How does TCD works?
 Twin detectors are normally used
One located ahead of sample
injection chamber and the other
immediately beyond the column or
alternatively, the gas stream can be
split.
 When the solutes elutes from the
column there is a change in the
composition of the mobile phase
thus in the thermal conductivity.
 this results in a deviation from thermal
equilibrium, causing a variation in the
resistance of one the filament.
 this variation is proportional to the
concentration of the analyte,
provided its concentration in the
mobile phase is low.

55. 2. TCD
 Advantages
 Simple
 Large linear dynamic range
 Responds to both organic and
inorganic species
 Nondestructive; permits collection of
solutes after detection.
 Disadvantage
 Relatively low sensitivity.

56. 3. Electron Capture
Detector (ECD)

57. How does ECD works?
 Sample elute from a column is passed over a radioactive β
emitter, usually nickel-63.
 An electron from the emitter causes ionization of carrier gas
(often N2) and the production of a burst of electrons.
 In the absence of organic species, a constant standing of
current.
 In the presence of organic molecules containing electronegative
functional groups that tend to capture electrons, the current

58. 3. ECD
 Most widely used for environmental samples
 Advantages
 Selectively responds to halogen-containing
organic compounds such as pesticides and
polychlorinated biphenyls.
 Highly sensitive towards halogens, peroxides,
quinones and nitro groups.
 Disadvantages
 Insensitive to functional groups such as
amines, alcohols and hydrocarbons.

59. H. Detectors
 Other detectors
 Nitrogen-Phosphorous Detector
(NPD)
 Flame Photometry Detector (FPD)
 Mass spectrometer (GC-MS)

60. H. Detectors
Detector Principle of Principle class of
operation compound detected
FID Ionization of solute Organics
molecules in a
flame
TCD Thermal Any samples
conductivity
ECD Current Compounds containing
electronegative
elements