定量分析31气相色谱分析 Gas Chromatography

Thermal Conductivity Detectors
The thermal conductivity detector, which was one of the earliest detectors for gas chromatography, still finds wide application. This device consists of an electrically heated source whose temperature at constant electric power depends on the thermal conductivity of the surrounding gas. The heated element may be a fine platinum, gold, or tungsten wire or, a small thermistor. The advantages of the thermal conductivity detector are its simplicity, its large linear dynamic range, its general response to both organic and inorganic species, and its nondestructive character, which permits collection of solutes after detection. The chief limitation of the thermal conductivity detector is its relatively low
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 50oC above the boiling point of the least volatile component of the sample.
Column Thermostating
Reproducible retention time require control of the column temperature to within a few tenths of a degree. For this reason, the coiled column is ordinarily housed in a thermostated oven. The optimum temperature depends on the boiling point of the sample components. A temperature that is roughly equal to or slightly above the average boiling point of a sample results in a reasonable elution period. For samples with a broad boiling range, it may be necessary to employ temperature programming.
Flame-Ionization Detectors
The flame-ionization detector (FID) is the most widely used and generally applicable of all detectors for gas chromatography. Most organic compounds, when pyrolyzed in a hot flame, produce ionic intermediates that conduct electricity through the flame. Detection involves monitoring the current produced by collecting electrons and ions produced by the combustion process at biased electrodes. The FID exhibits a high sensitivity, a large linear response, and low noise. A limitation of the FID is that it destroys the sample during the combustion step.
Two types of columns are encountered in gas-liquid chromatography, capillary, and packed. The latter can accommodate larger samples and are generally more convenient to use than the former. Capillary columns have become of considerable importance because of their unparalleled resolution. In old days the vast majority of gas chromatography has been carried out on packed columns but capillary column is becoming popular day by day.
Electron-capture detectors are highly sensitive and have the advantage of not altering the sample significantly.
Gas Chromatographic Columns and Stationary Phases
Electron-Capture Detectors
The electron-capture detector (ECD) has become one of the most widely use detectors for environmental samples because this detector selectively responds to halogen-containing organic compounds, such as pesticides and polychlorinated biphenyls. In this detector, the sample eluate from a column is passed over a radioactive emitter, usually nickel-63. An electron from the emitter causes ionization of the carrier gas (often nitrogen) and the production of a burst of electrons.
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. range, high reliability, and uniform response for a wide variety of chemical species or, a predictable and selective response toward one or more classes of solutes and nondestructive of sample.
Carrห้องสมุดไป่ตู้er 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/in2) and provide flow rates of 25 to 50 mL/min.
Quantitative Analysis 定量分析
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.