Noise FTIR spectrometer

In an industrial-design FTIR spectrometer, a modified form of the G enzel interferometer is utilized.A geometric displacement of the moving mirrors by one unit produces four units of optical path difference (compared with two units of optical difference for a Michelson type interferometer). The modified Genzel design reduces the time required to scan a spectrum and further reduces the noise effects asstxiated with the longer mirror translation of most interferometers. 

The more sophisticated treatment of Ingle and Crouch [7] comes very close but also misses the mark for an unexplained reason they insert the condition ... it is assumed there is no uncertainty in measuring Ert and Eot... . Now in fact this could happen (or at least there could be no variation in AEr) for example, if one reference spectrum was used in conjunction with multiple sample spectra using an FTIR spectrometer. However, that would not be a true indication of the total error of the measurement, since the effect of the noise in the reference reading would have been removed from the calculated SD, whereas the true total error of the reading would in... 

For an FTIR spectrometer, all of the spectral elements are observed for time T. Thus, the time spent sampling the resolution Av is also equal to T. Since the signal-to-noise ratio improves as the square root of the observation time... 

Until the early 1980s, most IR spectrometer systems were double-beam dispersive grating spectrometers, similar in operation to the double-beam system for UV/VIS spectroscopy described in Chapter 2. These instruments have been replaced almost entirely by FTIR spectrometers because of the advantages in speed, signal-to-noise ratio, and precision in determining spectral frequency that can be obtained from a modern multiplex instrument. There are NIR instruments that are part of double-beam dispersive UV /VIS/NIR systems, but many NIR instruments are stand-alone grating instruments. 

The signal-to-noise ratio (SNR) of FTIR spectrometers is direcdy proportional to the optical throughput (ff). For measurements at high resolution (approximately <2 cm ), throughput is determined by the allowable divergence of the infrared... 

The application of FT IK in chemistry, its unique features, and the relevant instrumentation are well documented [34,35]. In brief, an FTIR spectrometer is based on a Michelson interferometer that provides a spectrum in the time domain which is Fourier-transformed by a computer to a spectrum in the frequency domain. The sample can be scanned repeatedly, and the accumulated spectra can be averaged, thus producing a representative IR spectrum of a very high signal to noise ratio. This enables the measurement of samples containing a very low concentra-... 

The basic transmission infrared spectroscopy technique has been a workhorse for chemical analysis for many years. As computers and computing techniques became more advanced (FTIR) spectrometers were developed. The introduction of FTIR revolutionised infrared (IR) spectroscopy because for the first time weak signals could be cleared of noise thanks to co-addition and Fourier analysis of many scans. 

Normal sample, modern FTIR spectrometer 1-4 minute scan time, with spectral resolution of 2-4 cm providing a signal-to-noise ratio (S/N) of between 1000 1 and 10,000 1. If a spectrum is to be used for qualitative purposes only, then a S/N of approximately 1000 1 to 2000 1 is normally adequate and is typically obtained in well under 1 min often within a few seconds. For quantitative apphcations, or applications involving spectral subtraction or resolution enhancement techniques, higher signal-to-noise performance is required. Note that scanning for more than 256 scans may approach a point of diminishing returns in terms of spectral quality and S/N improvement (limited by the law, and the performance of the... 

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