gas chromatography

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- μ g)

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