Theoretical study of gas chromatography and its application in day to day life

1. M.Pharm 1st Sem, Seminar
Presentation
Sub :- THEORY OF GAS CHROMATOGRAPHY
Presented by :- Satyaki Aparajit Mishra (Pharmaceutics)
School of Pharmaceutical Sciences, S‘O’A University, Bhubaneswar

2. Gas chromatography[1]
Gas chromatography is common type of
chromatography for separating and analyzing
compounds that can be vaporized without
decomposition. Typical uses of GC include testing
the purity of a particular substance, or separating
the different components of a mixture.

3.  In gas chromatography, the mobile phase is a
carrier gas, usually an inert gas such as helium
or an unreactive gas such as nitrogen.
 The stationary phase is a microscopic layer of
liquid or polymer on an inert solid support,
inside a piece of glass or metal tube called a
column .
 The instrument used to perform gas
chromatography is called a gas chromatograph.

5. Retention time

6. Theoretical Plate
 An imaginary unit of the column
where equilibrium has been
established between S.P & M.P
 It can also be called as a
functional unit of the column
HETP – Height Equivalent to a
Theoretical Plate
 Efficiency of a column is
expressed by the number of
theoretical plates in the column
or HETP
 If HETP is less, the column is ↑
efficient.
 If HETP is more, the column is ↓
efficient

7. HETP=>
(length of the column)
(no of theoretical plates)
HETP is given by Van Deemter equation
HETP= A + B + C.u
u
A = Eddy diffusion term or multiple path
diffusion which arises due to packing of the
column
B = Molecular diffusion, depends on flow rate
C = Effect of mass transfer, depends on flow rate
u = Flow rate

8. Van Deemter Equation
 The minimum value of HETP at a particular flow
velocity using Van Deemter equation achieves a
minimum value at a particular flow.
 At this flow rate, the resolving power of column is
maximum.
 From the previous equation, A is the EDDY
diffusion term. The mobile phase moves through
the column which is packed with the stationary
phase.
 B is the longitudinal diffusion constant. The
concentration of analyte is less at the edges of
band at the centre.
 C is the resistance to mass transfer. The analyte
takes a certain time to equilibrate b/w the
stationary and the mobile phase.
 The higher velocity of mobile phase results in
stronger affinity of analyte for the stationary
phase .

10. Separation factor (S)[1]
Ratio of partition co-efficient of the two components to
be separated.
If more difference in partition co-efficient b/w two
compounds, the peaks are far apart & S is more. If
partition co-efficient of two compounds are similar,
then peaks are closer
Resolution (R)[1]
The true separation of 2 consecutive peaks on a
chromatogram is measured by resolution
It is the measure of both column & solvent efficiencies
R = 2d
W1+W2

11. Separation factor
Resolution

12. Carrier gas
 The carrier gas must be chemically inert.
Commonly used gases include nitrogen,
helium, argon, and carbon dioxide.
 The choice of carrier gas is often
dependant upon the type of detector which
is used.
 The carrier gas system also contains a
molecular sieve to remove water and other
impurities.

13. Sample injection port[3]
 For optimum column efficiency, the sample should not
be too large, and should be introduced onto the column
as a “slug" of vapour - slow injection of large samples
causes band broadening and loss of resolution
 The most common injection method is where a
microsyringe is used to inject sample through a rubber
septum into a flash vapouriser port at the head of the
column.
 The temperature of the sample port is usually about
50°C higher than the boiling point of the least volatile
component of the sample.
 For packed columns, sample size ranges from tenths of
a µl up to 20 µl

14. Sample injection port

15. Detectors[2]
 There are many detectors which can be used in gas
chromatography. Different detectors will give different types
of selectivity.
 A non-selective detector responds to all compounds except
the carrier gas.
 a selective detector responds to a range of compounds with a
common physical or chemical property .
 a specific detector responds to a single chemical compound.
EXAMPLES :-
 Thermal conductivity detector
 Flame ionization detector
 Electron capture detector

16. Ideal properties of Detectors[2]
The requirements of an ideal
detector are-
 Applicability to wide range of samples
 Rapidity
 High sensitivity
 Linearity
 Response should be unaffected by
temperature, flow rate…
 Non destructive
 Simple & inexpensive

18. Application of Gas
chromatography[1]
 In general, substances that vaporize below
300°C (and are stable up to that
temperature) can be measured
quantitatively.
 In assuring the quality of products in the
chemical industry; or measuring toxic
substances in soil, air or water. GC is very
accurate if used properly and can measure
picomoles of a substance in a 1 µl liquid
sample, or parts-per-billion concentrations
in gaseous sample
 The hydrocarbons are separated using a
capillary column and detected with an FIDs.

19.  Helps in detecting the steroid drugs used
by the athletes in international sports
competition and the steroids
administered to the animals in traces are
carried out by GLC
 In food analysis, it helps in the separation
of lipids, proteins, carbohydrates
preservatives, colours, as well as
vitamins, steroids, drug and pesticide
residues involved.
 GLC, monitors the hazardous pollutants
like formaldehyde, carbon monoxide,
trichloroethylene, benzene, acrylonitrile.

20. Advantages of Gas
Chromatography[1]
 Strong separation power
 Sensitivity is high for which a small sample is
sufficient for analysis
 Time saving (analysis is short)
 Gives good precision and accuracy
 Low cost effect
 Technique doesn’t require highly skilled persons

21. References
1. [1]Chatwal G.R., Anand S.K.; Instrumental
Methods of Chemical Analysis 5th Revised and
enlarged edition 2002. Reprint 2010 ; [Page
2.673-2.700]
2. [3]Prathap, et al. International Journal of
Pharmacy 2013; 3(1): 160-165 (A typical review
on Pharmaceutical Analysis of Gas
Chromatography-Mass Spectrophotometry)
3. [2]Skoog D.A., Holler F.J., Crouch S.R.; Principles
of Instrumental Analysis. Sixth Edition,
Thomson Brooks/Cole, USA, 2007.

22. THANK YOU