Fourier transform near-infrared analysers

For a wavelength A and an optical retardation 6, one expects that the amplitude of the resultant beam after recombination (the interference signal) will be 

Therefore for fixed v and a linearly changing 6, the output of the ideal two-beam interferometer is a cosine function. In other words, it is a cosine modulator of the original d.c. light source. The modulation frequency observed in the a.c. output of the FTIR spectrometer detector is dependent on the rate at which 8 is increased or decreased, and is given by 

It is commonplace that FTIR-based analysers are the predominant technology for mid-IR applications. This arises from a unique tie-in between the inherent advantages of the FTIR method and serious limitations in the mid-IR range. The most serious problem for mid-IR spectroscopy is the very low emissivity of mid-IR sources combined with the low detectivity of mid-IR thermal detectors. This causes a direct and severe conflict between the desire for 

The argument for the applicability (or lack of it) of the above factors in FTIR-based NIR applications centres on the issues of optical throughput and detector responsivity. NIR sources are comparatively very bright, and detector responsivities may be many orders of magnitude better than in the mid-IR. Hence in order to avoid detector 

Finally, however, it is the advantages (3) and (4) which really have a marked effect on NIR applications. The wavelength precision and reproducibility achievable with a good FTIR analyser design translates into long-term spectroscopic stability both within one analyser and across multiple analysers, so that measurement errors between analysers are in the region of a single milliabsorbance unit. For the untroubled development of NIR calibration models, and their maintainability over time, this of itself is critically important. 

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Infrared analyses are conducted on dispersive (scanning) and Fourier transform spectrometers. Non-dispersive industrial infrared analysers are also available. These are used to conduct specialised analyses on predetermined compounds (e.g. gases) and also for process control allowing continuous analysis on production lines. The use of Fourier transform has significantly enhanced the possibilities of conventional infrared by allowing spectral treatment and analysis of microsamples (infrared microanalysis). Although the near infrared does not contain any specific absorption that yields structural information on the compound studied, it is an important method for quantitative applications. One of the key factors in its present use is the sensitivity of the detectors. Use of the far infrared is still confined to the research laboratory. 

In order to demonstrate the usefulness of the method, SERS spectra of indolino-spironaphthopyran (SPP, 4) and indolino-spironaphthoxazine (SPOX, 3) (see Scheme 7) were recorded at low concentration (10 7-10-8 A/) and their spectral characteristics analysed.48 Figure 8 shows the SERS spectrum of 4 in Ag colloids (spectrum a) along with the corresponding near-infrared Fourier transform (NIR-FT) Raman spectrum of the neat compound (spectrum b). Strong differences between the SERS and spontaneous Raman spectra are apparent. As stated above, these differences arise mainly from the particular orientation of the adsorbed spiropyran with to the metal surface in SERS experiments. 

The most attractive sensors now being developed are the Fourier transform infrared spectrometer (FTIR) and the near-infrared (NIR) spectrometer for the on-line measurement of composition changes in complex media during cultivation. The FTIR measurements are based on the type and quantities of infrared radiation that a molecule absorbs. The NIR measurements are based on the absorption spectra following the multi-regression analyses. These sensors are not yet available for fermentation processes. 

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