brief description of GC principle and instrumentation

1. Gas chromatography
Team(4240-4255)

2. content
 introduction
 Principle
 Applications
 Advantages disadvantages
 Practical method
 Instrumentation

3. What is gas chromatography?
 Gas chromatography (GC) is a common type of
chromatography used in analytical chemistry for
separating and analyzing compounds that can be
vaporized without decomposition.

4. Principle
 gas chromatography(gas liquid chromatography) runs
on principle of partitioning of volatile samples with
gaseous mobile phase and liquid stationary phase .
 Velocity of a compound through the column depends
upon affinity for the stationary phase ,The components
partitioned into gas come out first while other come
later.

5. GC is in principle similar to HPLC, TLC, but
has several notable differences.
 the process of separating the compounds in a mixture carried
out between a liquid stationary phase and a gas mobile
phase(partitioning), whereas in column chromatography the
stationary phase is a solid and the mobile phase is a
liquid(adsorption).
 the column through which the gas phase passes is located in
an oven where the temperature of the gas can be controlled,
whereas column chromatography has no such temperature
control.

6. Applications
 criteria of compounds to be analysed by GC
1-volatility 2-thermostability
Ex 1- Aromatics (benzene, toluene,)
2- Hydrocarbons
3-Flavors and Fragrances
4- Permanent gases (H2, N2, O2, Ar, CO2,, CH4)
5- Pesticides, Herbicides

7. Advantages of gas chromatography.
 High resolution power compared to other methods.
 High sensitivity.
 High accuracy and precision.
 analysis of sample very quickly(minutes even seconds)
 Small sample needed (µl-μg)

8. Disadvantages of gas chromatography
 Limited to volatile sample.
 Not suitable for thermally labile samples.
 Samples be soluble and don’t react with the column.
 During injection of the gaseous sample proper
attention is required.

9. How dose GC works.

10. Instrumentation
Carrier gas
system
Gas clean
filter
Sample Introduction
System
column
Detection System
Computer
ChemStation
Integrator

11. Carrier gas system
Types of carrier gas
-H2(with safety precautions)
-N2
-He
-Argon

12. Criteria of carrier gas
 Should meet purity of 99.9995%
 free from oxygen so we use oxygen trap why???
as significant damages can be done to the column if it is
heated above 70℃with even trace amounts of O2 in the
column

13. sample introduction system
The sample must be of a suitable size
and introduced instantaneously as a
PLUG OF VAPOR why??
As Slow injection/oversize causes peak
broadening and poor resolution.

14. sample introduction system
the most common injection method is
where a micro syringe is used to inject
sample through a rubber septum into a
flash vaporizer 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.

15. Column
It is the heart of GC where separation
of sample take place.

16. 1. Types of GC columns

17. A)Packed column
Liquid stationary
phase coat

18. • Packed columns contain a finely divided, inert,
solid support material (commonly based on
diatomaceous earth) coated with liquid stationary
phase.
• Most packed columns are 1.5 - 10m in length and
have an internal diameter of 2 - 4mm
• The efficiency of a gas chromatographic column
increases rapidly with decreasing particle diameter
of the packing

19. Disadvantages
 Less application: fixed gas analysis
 Lower column efficiency than that of capillary
columns (smaller in length)
Advantages
 Larger sample capacity

20. B) capillary column
Polyamide
Provides great mechanical
strength and flexibility
• Most widely used.
• Capillary columns is 30 up to 100 m in length (increased efficacy)
why??

21. Types of capillary column
1- wall-coated open tubular (WCOT)
 consist of a capillary tube whose walls are coated with
liquid stationary phase.
2- support-coated open tubular (SCOT)
 the inner wall of the capillary is lined with a thin layer of
support material such as diatomaceous earth, onto which
the stationary phase has been adsorbed
SCOT columns are generally less efficient than WCOT
columns.
Both types of capillary column are more efficient than packed columns

22. Advantages
High column efficiency (large number of theoretical
plates due to long column length, up to 100 m)
Disadvantages
Small sample capacity.

23. Effect of stationary film thickness on
capillary column efficacy.

24. Effect of stationary film thickness
on capillary column efficacy.

25. Detection System

26. Characteristics of the Ideal Detector
 Adequate sensitivity (high signal to noise ratio ) S/N >3
 Good stability and reproducibility
 A wide temperature range
 A short response time independent of flow rate
 High reliability and ease of use
 The detector should be nondestructive

27. Typical GC Detectors

28. A)flame ionization detector(FID)
• Most common detector for GC
• In an FID, effluent from the column is
directed into a small air-hydrogen
flame causing ionization of most
carbon atoms (except C=O) produce
radicals (CHO+) in the flame.
• atoms and the ions are collected at an
electrode to create a current to be
measured. This current is proportional
to the number of molecules present
CH + O→ CHO+ + e-

29. Advantages
 universal detector for organics.
 does not respond to common inorganic compounds.
 mobile phase impurities not detected.
 carrier gases not detected.
 limit of detection of FID is 1000x better than TCD.
Disadvantage
 destructive detector.

30. b)Thermal conductivity detector
• When an analyte elutes and the thermal
conductivity of the column effluent is
reduced, the filament heats up and changes
resistance.
• This resistance change is often sensed by a
Wheatstone bridge circuit which produces
a measurable voltage change

31. b)Thermal conductivity detector
The thermal conductivities of helium and
hydrogen (commonly used carrier gases
for TCD) are roughly 6~10 times greater
than those of most organic compounds.
so
even small amounts of organic species
cause relatively large decreases in the
thermal conductivity of the column
effluent, which results in a marked rise in
the temperature of the detector .

32. Advantages
 Simplicity
 nondestructive
Disadvantages
 Low sensitivity

33. c) Mass Spectrometry Detector (MS)
One of the most powerful detectors for gas chromatography