Comprehensive Guide to Gas Chromatography: Techniques and Instruments

1. Chapter 26 & 27 Introduction to Chromatographic Separations and Gas Chromatography

2. Introduction... • In gas chromatography (GC), the sample is vaporized and injected onto the head of a chromatographic column. Elution is brought about by the flow of an inert gaseous mobile phase.

3. Gas liquid chromatography is based upon the partition of analyte between a gaseous mobile phase and a liquid phase immobilized on the surface of an inert solid.

4. Retention Volume... The specific retention volume, Vg is defined as follows: Vg = [JF(tr-tm)]/W(273/Tc) where, J = the pressure drop factor, F = average flow rate, tr and tm = retention times W = mass of stationary phase and Tc = column temperature in Kelvin.

5. Instruments for GC... The schematic of a gas chromatograph is shown below:

6. Carrier Gas Supply... Carrier gases, which must be chemically inert, include helium, argon, nitrogen, carbon dioxide, and hydrogen. The choice of gases is often dictated by the detector used. Associated with the gas supply are pressure regulators, gauges, and flow meters.

7. In addition, the carrier gas system often contains a molecular sieve to remove water or other impurities.

8. Sample Injector System... The most common method of injection involves the use of a micro-syringe to inject a liquid or gaseous sample through a silicone rubber diaphragm or septum into a flash vaporizer port located at the head of the column.

10. Column Configurations and Column Ovens... Two general types of columns are encountered in gas chromatography: (1) packed (2) open tubular or capillary

11. Chromatographic columns vary in length from less than 2m to 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into and oven for thermostating, they are usually formed as coils having diameters of 10 to 30cm.

13. Characteristics Of The Ideal Detector... 1. Adequate sensitivity 2. Good stability and reproducibility. 3. A linear response to analyses that extends over several orders of magnitude. 4. A temperature range from room temperature to at least 400 deg C. 5. A short response time that is independent of flow rate. 6. High reliability and ease of use. 7. Similarity in response toward all analyses. 8. Nondestructive of sample.

14. Flame Ionization Detector... • The flame ionization detector (FID) is one of the most widely used and generally applicable detectors for gas chromatography. The effluent from the column is mixed with hydrogen and air and then ignited electrically.

15. A diagram of a typical FID is shown in below.

17. Thermal Conductivity Detector... • A very early detector for gas chromatography is based upon changes in the thermal conductivity of the gas stream brought about by the presence of analyte molecules. This device is sometimes called a katharometer.

19. Thermoionic Detector... • The thermoionic detector (TID) is selective toward organic compounds containing phosphorous and nitrogen. It is similar in structure to the flame detector.

20. Electron Capture Detector (ECD)... Electron-capture detector (ECD) operates in much the same way as a proportional counter for measurement of X-radiation. Here the effluent from the column passes over a beta-emitter, such as nickel-63 or tritium (adsorbed on platinum or titanium foil). An electron from the emitter causes ionization of the carrier gas (often nitrogen) and the production of a burst of electrons.

22. Atomic Emission Detector (AED)... • The newest commercially available gas-chromatographic detector is based upon atomic emission. In this device, the eluent is introduced into a microwave-energized helium plasma that is coupled to a diode-array optical emission spectrometer.

24. Packed Columns... • Present day packed columns are fabricated from glass, metal (stainless steel, copper, aluminum), or Teflon tubes that typically have lengths of 2 to 3 m and inside diameters of 2 to 4mm. These tubes are densely packed with a uniform, finely divided packing material, or solid support, that is coated with a thin layer of stationary liquid phase. The tubes are formed as coils having diameters of roughly 15cm.

25. Solid Support Materials... • The solid support in a packed column serves to hold the stationary phase in place so that as large a surface area as possible is exposed to the mobile phase. • The ideal support consists of small, uniform spherical particles with good mechanical strength and a specific surface area of at least 1m/g.

26. Particle Size Supports... • The efficiency of a gas-chromatographic column increases rapidly with decreasing particle diameter of the packing. The pressure difference required to maintain a given flow rate of carrier gas, however, varies inversely as the square of the particle diameter

27. Open Tubular Columns... • Open tubular, or capillary, columns are of two basic types, namely, wall-coated open tubular (WCOT) and support-coated open tubular(SCOT). WCOT columns are simply capillary tubes coated with a thin layer of the stationary phase. In SCOT columns the inner surface of the capillary is lined with a thin film of a support material, such as diatomaceous earth.

28. Desirable properties for the immobilized liquid phase in a gas liquid chromatographic column include... • low volatility • thermal stability • chemical inertness • solvent characteristics such that k’ and alpha values for the solutes to be resolved fall within a suitable range.

29. Bonded and Cross-linked Stationary Phases... • The purpose of bonding and cross-linking is to provide a longer-lasting stationary phase that can be rinsed with a solvent when the film becomes contaminated.

30. Chiral Stationary Phases... • There has been recent developments of stationary phases which can separate chiral compounds. One method is to form a derivative of the analyte with an optically active reagent that forms a pair of diasteromers that can be separated on an achiral column. The alternative method is to use a chiral liquid as the stationary phase.

32. Film Thickness... • Film thickness primarily affect the retentive character and the capacity of a column. Thick films are used with highly volatile analytes, because such films retain solutes for a longer time and thus provide a greater time for separation to take place. Thin films are useful for separating species of low volatility in a reasonable time.

33. The Retention Index... • The retention index, I, was first proposed by Kovats in 1958 as a parameter for identifying solutes from chromatograms. The retention index for any given solute can be derived from a chromatogram of a mixture of that solute with at least two normal alkanes having retention times that bracket that of the solute

34. Below is a diagram of GC/MS a instrument

35. References... • http://www.acs.org • http://www.cas.org • http://www.chemcenter/org • http://www.sciencemag.org • http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html