Introduction to Gas Chromatography

1. GAS CHROMATOGRAPHY
(GC)

2. INTRODUCTION
• GC is one of the most utilized of all the chromatographic
techniques.
• Major requirement is that the sample must be capable of
being volatilized without undergoing decomposition.
• The sample is vaporized and injected onto the head of the
chromatographic column. The elution is brought about by
flow of inert gaseous mobile phase.
• Mobile phase does not interact with the sample molecules
but just transport the sample through the column.
• Quite similar to column chromatography except that the gas
is used as the mobile phase instead of a liquid.

3. TYPES OF GC
Gas-liquid Chromatography-
• It is the widely used type of GC.
• The mobile phase is a gas and the stationary phase is
a thin layer of a non-volatile liquid bound to a solid
support.
• A “partition” process occurs.
Gas-solid Chromatography-
• It utilizes a solid adsorbent as the stationary phase.
• A selective adsorption process takes place.

4. PRINCIPLE OF GC SEPARATION
• When a gas or vapour comes in contact with an adsorbent,
certain amount of it gets adsorbed on the solid surface.
• The phenomenon takes place according to the Laws of
Fruendlich, i.e.,
X/m=Kc1/n
or Langmuir, i.e.,
X/m=K1C1+K2C
where, X is the mass of gas or vapour sorbed in mass m
of the sorbent.
C is the vapour concentration in the gas phase
K1,K2 and K3 are constants.

5. • Similarly, if the vapour or gas comes in contact with a
liquid, a fixed amount of it gets dissolved in the liquid.
• The phenomenon takes place according to Henry’s law of
partition, i.e.,
X/m = KC
• Both the phenomena are selective and there are different K
values for different vapour-sorbent pairs.

6. INSTRUMENTATION OF GC

7.  The basic components are-
1) A carrier gas which is maintained at a high pressure p. It
is delivered to the instrument at a rapid and reproducible
rate.
2) A sample injection system
3) The separation column
4) One or more detectors
5) Thermostated chambers for the temperature regulation of
the column and detectors.
6) An amplification and recording system.

8. CARRIER GAS
• Most widely used carrier gases are hydrogen, helium,
nitrogen and air.
• Must be at a constant flow rate so that retention times &
retention volumes may be equated.
• Moves continuously throughout the instrument.
• Carries the sample vapour through the column to detector.
IDEAL PROPERTIES OF CARRIER GAS:
• Should be chemically inert.
• Should be compatible with detector.
• Should be highly purified.

9. INJECTION PORT
• Should be versatile, rapid and
quantitative.
• Should introduce sample to
column as a sharp, symmetric
band.
• Must introduce sample in a
reproducible manner and must
vaporize it instantaneously so
that the sample will enter the
column in a single slug.

10. • Liquid samples are introduced by hypodermic syringe
through a self-sealing rubber septum.
• Solid samples must be dissolved in volatile liquids for
introduction or may be introduced directly if they can be
liquefied.
• Gas samples require special gas sampling valves for
introduction into the carrier gas stream.

11. GC COLUMNS
 The most common column
shapes are the coiled helix
and the U-tube.
 Types of columns
1. Packed column
2. Open tubular column
a) Wall coated open tubular
column
b) Support coated open
tubular column

12. THE STATIONARY LIQUID PHASE
PROPERTIES OF STATIONARY LIQUID PHASE
• Low volatility
• Thermally stable
• Chemically inert
The immobilized liquid phase must generate different
partition ratios for different solutes.
The polarity of the stationary phase should match the
sample components of that polarity.
Polar stationary phases contain –CN, -CO and –OH groups.

13. HO-CH2-CH2-(O-CH2-CH2)n-OH
polyethylene glycol

14. DETECTORS IN GC
The ideal detector should:
• be highly sensitive
• give rapid & linear response to changes in solute
vapour concentration
• be sensitive to wide range of solute vapours.
• Response should be unaffected by flow rate of
carrier gas & temperature
• be highly reliable& reproducible response

15. ELECTRON CAPTURE DETECTOR
(ECD)
• 65Ni or 3H ionizes carrier gas which
produces electrons.
• Commonly used carrier gases-
argon and nitrogen.
• Highly sensitive to certain
molecules.
• Specially recommended for alkyl
halides, conjugated carbonyls,
nitriles, nitrates and
organometallics.
• Useful for insecticide analysis.

16. FLAME IONIZATION DETECTOR (FID)
• Responds to compounds that
produce ions when burned in
an H2-air flame
• All organic compounds
• Little or no response to CO, CO2, CS2, O2,
H2O, NH3, inert gases (use a Thermal
Conductivity Detector for these gases)
• Linear from the minimum
detectable limit through
concentrations 107 times the
minimum detectable limit.

17. THERMAL CONDUCTIVITY (TCD)
• Also called as Katharometer.
• Sensitivity of detector depends upon
the difference between thermal
conductivity of carrier gas alone and
that of compound eluted from column.
• Mounted in a Wheatstone bridge
alignment.
• The heating element is made of either
platinum, gold or tungsten.
• Helium is the mobile phase of choice
when using a TCD.

18. THERMIONIC DETECTOR
• Selective towards organic compounds
containing phosphorus and nitrogen.
• Response to phosphorus is 10 times
more than nitrogen and 104-106 more
than carbon atom.
• Compared with FID, it is 500 times
sensitive for phosphorus and 50 times
sensitive for nitrogen.
• Structurally similar to FID.

19. SUBSTRATES
 The solid support is generally coated with a high boiling
liquid known as the substrate which acts as the immobile
phase in GLC.
 The general requirements for the liquid phase are:
1) Good solvent property of the compound.
2) Differential partitioning of sample components.
3) Low vapour pressure at the column temperature .
4) High thermal stability.

20. TEMPERATURE CONTROL AND
SIGNAL AMPLIFICATION
 Temperature programming facilitates controlled increase of
even temperature during an analysis.
 The latter peaks also become sharp and emerge quickly.
 Thus, the components of a wide boiling range mixture may be
resolved efficiently.
 The temperature programming may be carried out in three
different modes. These are:
1) Natural or Ballastic
2) Linear (commonly used)
3) Matrix or Multicellular

21. RECORDER
• The amplified signal from the electrometer recorded on
millivolt strip recorder to produce response against time.

22. EVALUATION
 The efficiency of a column is expressed by the number (N) of
theoretical plates in the column or by the height equivalent of a
theoretical plate (HETP).
 The larger the number of theoretical plates or the smaller the
HETP, the more efficient the column is for separations.
RETENTION VOLUME
The uncorrected or experimental retention volume for a
chromatogram is given by:
VR = tRFc
tR is time in minutes on the time axis from the point of injection to
the peak maximum.
Fc is the volumetric flow rate in millilitres per minute.

23. BRANCHES OF GC
• Packed Column GC
• Capillary Column GC
• Preparative Solid GC
• Programmed Temperature GC
• Gas Chromatography – Mass Spectroscopy (GC–MS)

24. ADVANTAGES OF GC
• Fast analysis (in minutes or even seconds).
• Requires only very small samples (in µl or µg) with little
preparation.
• High resolution.
• Reliable, relatively simple and cheap.
• Non-destructive.
• Highly accurate quantification
• Good at separating complex mixtures into components
• Only instrument with the sensitivity to detect volatile organic
mixtures of low concentrations
• Equipment is not very complex (sophisticated oven)

25. DISADVANTAGES
• Limited to volatile samples.
• Analytes should have boiling point below 5000 C.
• Not suitable for thermally liable samples.
• Some samples may require intensive preparation.
• Samples should be soluble and not react with the column.
• Requires spectroscopy (usually MS) to confirm the peak
identity.

26. APPLICATIONS OF GC
QUALITATIVE ANALYSIS
• By comparing the retention times or volumes of the
unknown to the retention times or volumes of a series of
standards.
• By collecting the individual components as they emerge
from the chromatograph and subsequently identifying these
compounds by other methods.

27. QUANTITATIVE ANALYSIS
• It depends on the fact that the area under a single
component elution peak is proportional to the
quantity of the detected component.

28. APPLICATIONS TO NATURAL PRODUCTS
• Separation, identification and determination of volatile
compounds of Ziziphora persica.
• Separation of Artemisia volatile oils.
• Analysis of peppermint oil for the components of α and β-
pinene, limonene, menthone, isomenthone, menthol,
isomenthol, pulegone and methyl acetate.
• The lavender or citrus oils containing linalyl oxides,
linalool, linalyl acetate, borneol, bornyl acetate, α-terpenoid
are resolved.

29. MISCELLANEOUS APPLICATIONS
• In analysis of foods, the separation and identification of
lipids, proteins, carbohydrates, preservatives etc. Pesticides
and trace elements are involved.
• GLC finds valid applications in drug analysis. Some
examples are analysis of commercial drug preparations,
analyze drug samples, blood, urine sample and stomach
contents.

30. THANKS