Gas chromatography

1. GAS CHROMATOGRAPHY
6th semester
Amar singh college

3. 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.

4.  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.
PRINCIPLES

5. 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.

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

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

8. INSTRUMENTATION

9. 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.
INSTRUMENTATION

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

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

12. 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.

13. 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.

14. 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.

15. Mode of Injections

16.  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.
D. Sample Injection System

17.  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.
D. Sample Injection System

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

19. D. Sample Injection System

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

21. 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.
D. Sample Injection System

22.  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.

23. 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.

24.  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.
E. Oven

25.  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.
Temperature Programming

26.  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.
Temperature Programming

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

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

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

30.  Less commonly used
 Made of glass or steel
 Length: 1 to 5 m
 Internal diameter: 2 to 4 mm
 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.
Cross-sectional view
of packed column
1. Packed Column

31.  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
1. Packed Column

32.  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
2. Capillary Column

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

34. Properties and characteristics of
GC Column

35. 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.

36.  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
G. Stationary Phase

37.  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.
G. Stationary Phase

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

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

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

41. G. Stationary Phase applications

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

44. 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.

45.  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.
H. Detectors

46.  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)
H. Detectors

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

48. 1. Flame Ionization Detector
(FID)

49.  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.
How does FID works?

50.  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.
1. FID

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

52. 2. Thermal Conductivity
Detector (TCD)

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

54.  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.
How does TCD works?

55.  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.
How does TCD works?

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

57. 3. Electron Capture
Detector (ECD)

58.  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 decreases
markedly.
How does ECD works?

59.  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.
3. ECD

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

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