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Various chemicals whether in the gas or liquid phase absorb energy from specific regions of the electromagnetic radiation spectrum. Thus infrared radiation passed through two parallel chambers to a detection device may be used to record the difference in infrared absorption. The difference is sometimes detected in a Golay cell which consists of two infrared absorption chambers separated by a diaphragm whose movement is detected by a capacitance change with respect to an adjacent electrode. The infrared radiation is passed through a chopping disc so that each cell in turn receives the radiation causing alternating displacement of the diaphragm in the Golay cell. The diaphragm will be displaced to one side or other depending on the relative quantities of infrared light reaching the absorption chambers. An electronic circuit to amplify, demodulate and linearize the output is connected to the capacitance transducer.
One of the cells is filled with a reference gas while the other is filled with a background gas plus a small flow-through of the gas being sampled. The modification in absorption is used to detect the molar fraction of the sample gases. Problems arise where the absorption spectra of the gases being tested overlap (e.g. nitrous oxide and halothane, carbon dioxide and carbon monoxide). The device requires a long stabilization period and the results will be affected by changes in atmospheric pressure.
Such instruments can be used for the analysis of gases in the lung function laboratory. They may also be called gas chromatographs.
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