تسجيل الدخول

تسجيل

Instrument calibration , Time scale of measurement (time constant, averaged scans)

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  431

2026-11-14

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Instrument calibration

Time scale of measurement (time constant, averaged scans)

In analogue dispersive instruments, the scan speed refers to the rate of rotation of the monochromator diffraction grating, which in turn controls the rate-of-change of the wavelength (wavenumber) that comes from the monochromator exit slit. The instrument has an inherent controllable response time (time constant). Scanning too rapidly means that measured peaks are distorted and flattened (reduced intensity): the instrument is being run too fast for the electronics to cope with the changing signal from the detector. A scan should be fast enough to avoid time wastage, yet slow enough to leave the spectrum undistorted. The longer the response time (i.e. the larger the time constant), the lower the noise and the longer the time required to measure spectra. The choice of time response versus scan speed is sample dependent and needs to be selected according to experience. In a FTIR spectrometer, the scan speed is set by the oscillation rate of the moving mirror and the computer Fourier transformation speed. A typical spectrum can be accumulated in a second or so. The instrument operator has no control of this. The time to measure an FTIR spectrum is set by the number of scans accumulated and averaged, and the speed of the computer.