Measurement of the Spectrum

The coupling of gas chromatographs and FT-IR spectrometers (GC-IR) has been accomplished by three approaches. In the first, and by far the simplest, the

Eicman GA (1990) In: Clement RE (ed.) Gas Chromatography, Biochemical, Biomedical and Clinical Applications, Ch. 14. New York: Wiley Interscience. (An account of the GC of organometallic compounds.)

Hill HH and McMinn DG (eds) (1992) Detectors for Capillary Chromatography. New York: Wiley Interscience. (An excellent book with chapters on the ELCD (by Hall), the NPD (by Patterson) and GC-FTIR (by Gurka) in particular.)

GC column is connected directly to a heated flow-through cell. For capillary GC, this cell is usually fabricated from a 10-cm length of heated glass tubing with an internal diameter of & 1 mm. The inside bore of this tube is coated with a thick enough film of gold to be highly reflective to infrared (IR) radiation. IR-transparent windows (for example made of potassium bromide) are attached to both ends of the tube. IR radiation entering one window is multiply reflected down the gold-coated interior bore before emerging from the other window, giving rise to the name light-pipe for this device. The effluent from the GC column is passed into one end of the tube and out of the other via heated fused-silica transfer lines. The entire unit is held at a temperature between 250 and 300°C to preclude the condensation of semi-volatile materials.

Infrared radiation from an incandescent source, such as an SiC Globar, is collimated and passed through a rapid-scanning interferometer so that each wavelength in the spectrum is modulated at a different frequency. The beam of radiation is then focused onto the first window of the light-pipe and the infrared beam emerging from the second window is refocused onto a sensitive detector (typically a liquid-nitrogen-cooled mercury cadmium telluride (MCT) photoconductive detector). A typical system is illustrated schematically in Figure 1. The signal measured in this way is known as an interferogram and the Fourier transform of the interferogram yields a single-beam spectrum. By calculating the ratio of a single-beam spectrum measured when a component is present in the light-pipe to one measured when only the helium carrier gas is present, the transmittance spectrum, T (v), of the component is obtained. The trans-mittance spectrum is usually immediately converted to an absorbance spectrum, A(v), by the standard Beer's law operation, A(v) = — log10 T(v), as the relative intensities of bands in absorbance spectra are independent of the concentration of the analyte, thereby allowing spectral library searching to be

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Solar Panel Basics

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