Gas Chromatography.pptx

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=Kc 1/n or Langmuir, i.e., X/m=K 1 C 1 +K 2 C 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 K 1 ,K 2 and K 3 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- A carrier gas which is maintained at a high pressure p. It is delivered to the instrument at a rapid and reproducible rate. A sample injection system The separation column One or more detectors Thermostated chambers for the temperature regulation of the column and detectors. 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 Packed column Open tubular column Wall coated open tubular column 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-CH 2 -CH 2 -(O-CH 2 -CH 2 ) 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) 65 Ni or 3 H 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 H 2 -air flame All organic compounds Little or no response to CO, CO 2 , CS 2 , O 2 , H 2 O, NH 3 , inert gases (use a Thermal Conductivity Detector for these gases) Linear from the minimum detectable limit through concentrations 10 7 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 10 4 -10 6 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: Good solvent property of the compound. Differential partitioning of sample components. Low vapour pressure at the column temperature . 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: Natural or Ballastic Linear (commonly used) 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: V R = t R F c t R is time in minutes on the time axis from the point of injection to the peak maximum. F c 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 500 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.

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