GC Detectors

Gas chromatography detectors (GC detectors) identify solutes as they exit a chromatographic column. As solutes are eluted from the column they interact with the gas chromatography detector. Gas chromatography detectors convert this interaction into an electrical signal that is sent to the data system. The magnitude of the signal is plotted from the time of injection and a chromatogram is generated. Gas chromatography detectors use one of several technology types to identify solutes as they exit the column. Choices include flame ionization, atomic emission, electron capture, photoionization, flame photometric, chemiluminescence spectroscopy, and nitrogen phosphorous.

Thermal conductivity and flame ionization are two types of gas chromatography detectors. Thermal conductivity GC detectors have an electrically-heated wire or thermistor. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity cause a temperature rise in the element, which is sensed as a change in resistance. Flame ionization gas chromatography detectors consist of hydrogen or an air flame and a collector plate. The effluent from the GC column passes through the flame, which breaks down organic molecules and produces ions. The ions are collected on a biased electrode and produce an electric signal.

Gas chromatography detectors which use atomic emissions or electron capture gas are also available. Atomic emission GC detectors simultaneously determine the atomic emissions of many elements in analytes that elute from the GC capillary column. As the eluant exits the capillary column it is fed into a microwave-powered plasma (or discharge) cavity where the components are destroyed and their atoms are excited by the energy of the plasma. These excited particles emit light that is separated into individual lines via a photodiode array. Electron capture gas chromatography detectors use a radioactive Beta emitter (electrons) to ionize some of the carrier gas and produce a current between a biased pair of electrodes. When organic molecules contain electronegative functional groups such as halogens, phosphorous, and nitro, groups pass by the GC detector and capture some of the electrons while reducing the current measured between electrodes.

Photoionization gas detectors, chemiluminescence spectroscopy gas detectors, and nitrogen phosphorous gas detectors are all types of gas chromatography detectors. Photoionization GC detectors use ultraviolet light as a means of ionizing an analyte exiting from a GC column, allowing electrodes to collect the ions. The generated current is a measure of the analyte concentration. Chemiluminescence spectroscopy gas chromatography detectors use quantitative measurements of the optical emission from excited chemical species to determine analyte concentration. Typically, the emission is measured from energized molecules rather than excited atoms. The bands of light determined by this technique emerge from molecular emissions and are broader and more complex than bands which originate from atomic spectra. Nitrogen phosphorous gas chromatography detectors burn the compound in the plasma surrounding a rubidium bead is supplied with hydrogen and air. Nitrogen and phosphorous-containing compounds produce ions that are attracted to the collector. A signal is generated from the measurement of the number of ions hitting the collector.

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