Chapter 31 - Chapter 31 Gas Chromatography GAS-LIQUID...

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Chapter 31 Gas Chromatography GAS-LIQUID CHROMATOGRAPHY In gas chromatography, the components of a vaporized sample are fractionated as a consequence of being partitioned between a mobile gaseous phase and a liquid stationary phase held in a column. In performing a gas chromatographic separation, 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. The mobile phase does not interact with molecules of the sample; its only function is to transport the sample species through the column.
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Carrier Gas System The gaseous mobile phase in gas chromatography must be chemically inert. Helium is the most common mobile phase, although argon, nitrogen, and hydrogen are also used. These gases are available in pressurized tanks. Pressure regulators, gauges, and flow meters are required to control the flow rate of the gas. Pressures at the column inlet usually range from 10 to 50 psi(lb/in 2 ) and provide flow rates of 25 to 50 mL/min.
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Sample Injection System Column efficiency requires that the sample be of a suitable size and be introduced as a plug of vapor; slow injection or oversized samples cause band spreading and poor resolution. Calibrated microsyringes are used to inject liquid samples through a rubber or silicone septum into a head sample port located at the head of the column. The sample port is ordinarily about 50 o C above the boiling point of the least volatile component of the sample.
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Detectors Detector devices for gas chromatography must respond rapidly to minute concentration. The solute concentration in the carrier gas at any instant is no more than a few parts per thousand. Moreover, the time during which a peak passes the detector is very short, which requires that the device be capable of exhibiting its full response during this brief period. Desirable properties for a detector include high sensitivity, linear response, stability, reproducibility, wide temp.
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