Chapter 26 & 27. Introduction to Chromatographic Separations and Gas Chromatography. 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.
Introduction to Chromatographic Separations and Gas Chromatography
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.
The schematic of a gas chromatograph is shown below:
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.
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.
Two general types of columns are encountered in gas chromatography:
(2) open tubular or capillary
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.
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.