Chapter 27 - Chapter 27 Gas Chromatography In gas...

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Chapter 27 Gas Chromatography 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. The mobile phase does not interact with molecule of the analyte; its only function is to transport the analyte through the column. Gas-liquid chromatography is based upon the partition of the analyte between a gaseous mobile phase and a liquid phase immobilized on the surface of an inert solid.
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INSTRUMENTS FOR GC Carrier Gas-Supply Carrier gases, which must be chemically inert, include helium, nitrogen, and hydrogen. Associated with the gas supply are pressure regulators, gauges, and flow meters. In addition, the carrier gas system often contains a molecular sieve to remove water or other impurities.
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Sample Injection System Column efficiency requires that the sample be of suitable size and be introduced as a “plug” of vapor; slow injection of oversized samples causes band spreading and poor resolution. The most common method of sample injection involves the use of microsyringe to inject a liquid or gaseous sample through a self-sealing, silicone-rubber diaphragm or septum into a flash vaporizer 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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Sample Injection System For quantitative work, more reproducible sample sizes for both liquids and gases are obtained by means of a rotary sample valve. Errors due to sample size can be reduced to 0.5% to 2% relative. The sampling loop is filled by injection of an excess of sample. Rotation of the valve by 45 deg then introduces the reproducible volume ACB into the mobile phase.
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Column Configurations Two general types of columns are encountered in gas chromatography, packed and open tubular , or capillary . Chromatographic columns vary in length from less than 2 m to 50 m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into an oven for thermostating, they are usually formed as coils having diameters of 10 to 30 cm.
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Column Ovens Column temperature is an important variable that must be controlled to a few tenths of a degree for precise work. Thus, the column is ordinarily housed in a thermostated oven. The optimum column temperature depends upon the boiling point of the sample and the degree of separation required. Roughly, a temperature equal to or slightly above the average boiling point of a sample results in a reasonable elution time (2 to 30 min). For samples with a broad boiling range, it is often desirable to employ temperature programming, whereby the column temperature is increased either continuously or in steps as the separation proceeds.
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Detection Systems Characteristics of the Ideal Detector: The ideal detector for gas chromatography has the following characteristics: 1. Adequate sensitivity
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Chapter 27 - Chapter 27 Gas Chromatography In gas...

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