Forensic Analytical Chemistry

1. AN INTRODUCTION
TO
GAS CHROMATOGRAPHY
PRESENTED BY:
TEJASVI

2. CONTENTS
 INTRODUCTION
 BASIC PRINCIPLE
 INSTRUMENTATION
 FORENSIC APPLICATIONS
 LIMITATIONS
 REFFRENCES

3. INTRODUCTION
 Like other forms of chromatography, Gas
chromatography is a method which separates
a mixture into its constituents.
 The theory of this technique was developed
by Martin and Synge ,but first experiment was
performed by Martin and James in 1951 on
lower fatty acids.
 First commercial Gas chromatogram was
available after 1955.

4. BASIC PRINCIPLE
 Different components of the volatile sample are
separated between stationary phase and the
mobile phase on the basis of their partition
coefficient.
 Components are eluted out from the column on
the basis of their affinity for the stationary
phase.
 Each substance passing through the column
will have characteristics retention time which is
used for qualitative analysis.
 Quantitative data is obtained from evaluation
of peak area.

6. INSTRUMENTATION
 MOBILE PHASECARRIER GAS
 SAMPLE INJECTION SYSTEM
 COLUMNS
 CHOICE OF STATIONARY PHASE
 DETECTORS
 RECORDER

8. MOBILE PHASE
 It should be inert ,easily available, cheap.
 Hydrogen ,Helium, Argon etc. are commonly
used carrier gases.
 Carrier gas system also contain a molecular
sieve.
 Choice of gas depends upon type of detector
used.

9. SAMPLE INTRODUCTION
SYSTEM
 Liquid samples are introduced with the help of
hypodermic syringe through a self sealing
rubber septum.
 The sample should be volatile in nature.
 Split and splitless injection systems are used.
 The system should maintain at specific
temperature.

10. SAMPLE INJECTION SYSTEM

11. COLUMNS
COLUMNS
CAPILLARY
WCOT
SCOT
PACKED

12. PACKED COLUMNS
These are made from diatomaceous
earth.
Length of column varies from 2 to 5
meters.
Internal diameter ranges from 2 to 4
mm.
Carrier gas flow rate is 20 to 50
ml/min.

13. PACKED COLUMN

14. CAPILLARY COLUMN
 These are made from fused silica.
 Length varies from 10 to 100 m.
 Diameter varies from 0.2 to 0.3 m.
 Carrier gas flow 2 to 5 ml/min.
 External coating of polyamide.

15. CONT………
 Capillary columns are of two types
 WCOT : WALL COATED OPEN TUBULAR
 Inner walls are coated with liquid stationary
phase.
 SCOT : SPOT COATED OPEN TUBULAR
 Inner walls are lined with diatomaceous earth
material.
 Stationary phase absorbed .

16. CHOICE OF STATIONARY
PHASE
S.No. Stationary Phase Compounds to be
separated
1. Apiezone L (hydrogen
hydrocarbon grease)
Barbiturates &
amphetamines
2. SE-30, OV-01, OV-101
(Dimethyl silicon
polymer)
Many drugs depending
upon their molecular
weight
3. SE-60 (cynoethyl silicon) Steroids
4. Polyster Fatty acids & Barbiturates

17. DETECTOR
Requirement for detectors are :
 High sensitivity.
 Physically suitable .
 Capable of operable up to a maximum
temperature range 350 degree centigrade .
 No response to undesired compounds .
 Concentrate level to be measured.
 It should respond at small volume .

18. THERMAL CONDUCTIVITY
DETECTOR
 This is oldest type of detector .
 Filament temperature increases which cause
increase in resistance .
 Least sensitive detector .
 Sensitivity depends upon carrier gas filament
current and flow rate of carrier gas .

19. THERMAL CONDUCTIVITY DETECTOR

20. ELECTRON CAPTURE
DETECTOR
 Electron affinity .
 Electro negative constituents capture low
energy electron .
 Decrease in current directly proportional to
concentration .
 Halogenated compounds such as pesticides
,insecticides ,DDT , aldrin etc and certain
noxious gases .

21. ELECTRON CAPTURE DETECTOR

22. FLAME IONIZATION
DETECTOR
 Based on electrical conductivity .
 Insulator become conductive if electrons are
present .
 Effluent is mixed with water and air and
ignited .
 Organic compounds burning in flame
produces ions and electrons which an conduct
electricity .
 High sensitivity, larger linear response range
low noise .

23. FLAME IONISATION DETECTOR

24. FORENSIC APPLICATION
 This technique is used in forensic analytical
work for qualitative and quantitative analysis
of wide range of evidential material such as
authentication of petroleum products .
 Analysis of illicit liquor and alcohol ,
beverages , paint chips , insecticides and
pesticides etc.

25. LIMITATIONS
 Sample should be volatile in nature and
stable in vapor state.
 Sample to be detected should be in
gaseous state.
 Tentative identification
 Temperature should be maintained.

26. REFERENCES
 Willard.H.H, Instrumental Method of Analysis
(1986), Wadsworth Publishing Company, USA;
pp 542-552
 Mendham.J, Denney.R.C, Textbook of
Quantitative Chemical Analysis (2000), Dorsling
Kindersley Pvt. Ltd.; pp 317-334
 Wilson.K, Walker.J, Principles & Techniques of
Biochemistry and molecular biology; pp 541-545
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