Selectivity of wavelength

P.C. Thijssen, L.J.P. Vogels, H.C. Smit and G. Kateman, Optimal selection of wavelengths in spectrophotometric multicomponent analysis using recursive least squares. Z. Anal. Chem., 320(1985) 531-540. 

In conclusion, the gas phase photofragmentation of Co (C0) CCH shows a wavelength dependence that, with the proper selection of wavelength, gives the experimentalist access to a range of interesting unsaturated metal and metal-carbon clusters. 

In contrast to previously described instruments, the dispersive system in these instruments is movable in order to focus each wavelength on the fixed exit slit. A step motor allows a selection of wavelengths to be pre-programmed to sequentially... 

The besl isolation of radiant energy can he achieved with flame spectrometers that incorporate either a prism sir grating monochromator, those with prisms having variable gauged entrance and exii slits. Both these spectrometers provide a continuous selection of wavelengths with resolving power sufficient lo separate completely most of the easily excited emission lines, and afford freedom from scattered radiation sufficient lo minimize interferences. Fused silica or quartz optical components are necessary to permit measurements in Ihe ultraviolet portion of the spectrum below 350 nanometers Sec also Analysis (Chemical) Atomic Spectroscopy Photometers and Spectra Instruments. 

In the case of new analytical problems, decisions must be taken on the operating conditions of the instrument, the selection of wavelengths and the calibration ranges. Once ail the preceding stages have been completed, the measurements of the blank, standards and subsequently samples can be conducted automatically the solutions are automatically introduced using a sample changer. The measurements are recorded and processed and the element concentration levels calculated by computer. 

When compared to using PRESS as the optimization criterion based on cross-validation, the SEP for an independent set improves the optimization speed several hundred percents. To avoid selection of wavelength combinations specific to the prediction set, it is necessary to validate the predictive ability of selected wavelengths by using additional prediction sets. In addition, the PRESS value for the calibration spectra should also be acceptable. 

Selection of wavelengths that give good detection limits. 

The initial selection of wavelength range for S should consider the shape of the spectrum (mentioned above). 

In the case of tryptophan synthase, qualitative examination of the RSSF spectra has resulted in the direct detection of most of the expected reaction intermediates, and in the elucidation of the sequence of catalytic events, information crucial to the determination of the reaction mechanism. The RSSF data also provide a rational approach both for the selection of wavelengths for the detailed analysis of the dependence of relaxation rates on substrate concentrations by SWSF and for the accurate determination of isoabsorptive points by singlewavelength methods (85, 86). The presence of apparent isoabsorptive points during one or more phases of a multistep reaction simplifies gready the interpretation of physical events observed during either RSSF or SWSF rapid-kinetic studies. 

A solution can only be revisited in the next iteration if it is a neighbor N x) of the current solution. Here we consider the selection of wavelength in a multivariate calibration problem (cf. Section 6.2) ... 

A spectrophotometer is used to determine the percent transmittance or the absorbance of a sample (usually a solution) as a function of wavelength. The Bausch and Lomb Spectronic 20 Spectrophotometer sacrifices some photometric accuracy in percent transmittance or absorbance in return for simplicity of operation and relatively low cost (about 950). It allows continuous selection of wavelength throughout the 340-600 nm range accurate to +2.5 nm, and reading of absorbance accurate to 1-3% over the range from 0 to 1.5. 

In fact, many PCR models include more factors than are actually necessary because some of the eigenvectors are not related to any of the constituents of interest. Ideally, a PCR model should be built by performing a selection on the scores (much like a selection of wavelengths for an ILS model) to determine which factors should be used to build a model for each constituent. In practice, this is a difficult process, in terms of both developing the selection rules and making it simple to perform. Most commercial chemometric software packages do not support this type of PCR model. 

Because the wavelengths are produced by a radio frequency-driven transducer, random selection of wavelengths is possible. That is, unlike the grating or FT instruments, an analyst does not have to scan an entire spectrum—merely the desired wavelengths or specific sections of the spectrum. This is the feature that makes the acoustooptic tunable filter (AOTF) faster than the FT instruments for selected applications using partial scanning measurements. 

A more useful method of standardization would not require transfer samples to be analyzed. A general method based loosely on MSC has also demonstrated success when there are relatively minor performance differences between the original and second instruments [11,12]. Here a local selection of wavelengths from each spectrum are regressed against the mean spectrum to build a transfer function. Consequently, the spectra from the second instrument are not transformed to look like the spectra from the first instrument. Instead, spectral from both instruments are transformed to lie in a common multidimensional space. 

Second, the presence of high concentrations of constituents, which absorb in the same area, can cause the selection of wavelengths in alternate areas for one or more constituents. For example, for highly fibrous oilseeds, wavelengths selected for the determination of oil may be in the 12- or 1700-nm areas rather than in the traditional 2300-nm area. 

When the DAD is used, a selection of wavelengths may be available for individual monitoring as well as the selection of the best wavelength for quantitation in consideration of background noise that may be largely chemical noise or interferences from the matrix. In the case of PNAs, the full absorption spectrum for benzo [a] pyrene is given in Figure 2. 

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