Wavelength choice

Many manufacturers have a number of different models available, differing in wavelength choices and cell geometrii ( ) = Not known whether r.m.s., peak or peak-to-peak noise is specified, typ. = Typical. 

The water peaks with maxima near 10,300 cm- (970 nm) 8330 cm- (1200 nm), and 6900 cm" (1450 nm) at room temperature shift towards higher wavenumber (lower wavelength) with increasing temperature, and appear to consist of an unresolved pair of peaks with an isosbestic point between them. See Figure 6.1 for an illustration of the temperature effect at the first overtone. The isosbestic point for the 10,300-cm (970-nm) band is 10,100 cm (990 mn), for the 8330-cm- (1200-nm) band is about 8400 cm- (1190 nm), ° and for the 6900-cm (1450-nm) band is at about 6960 cm" (1440 nm)." The presence of an isosbestic point indicates that NIR calibrations for water independent of temperature may be possible by judicious wavelength choice. 

Fig. 10.51. Laser-induced fluorescence spectra for an experimental tumour (a human colon adenocarcinoma inoculated in rat muscle) and normal surrounding tissue. The rat had been injected with a dose of hematoporphyrin derivative about two days before the investigation. Two excitation wavelengths, 337 and 405 nm, are chosen to illustrate the influence of the wavelength choice [10.185]...

All of the previously mentioned statistics are useful in determining both the feasibility of an NIR method as well as calibration equation validity for well-known applications. The statistics allow us to determine the fitness of a particular calibration they allow us to select the proper numbers and locations of wavelength choices using purely statistical criteria and they give us some indication of the ability of a particular calibration to accurately predict analyte concentrations in samples never previously presented to the instrument. The use of bias and slope adjustments to improve calibration or prediction statistics requires some precautionary statements. As illustrated in Figure 7.2, if monitor... 

The multitude of laser wavelengths that are available today means that the choice of wavelength for a project is dictated by the sample properties rather than what instrumentation is available. Some of the considerations for wavelength choice have already been mentioned in the text, but it is probably useful to summarize what needs to be considered ... 

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