Spectra Evaluation

International or in-house standards in combination with fundamental parameters software, lead to the same accuracy as conventional analysis using regression analysis of standards. Provided that accurate standards are available, the main factors that determine the accuracy of XRF are the matrix absorption correction and (in the case of EDXRF) the spectrum evaluation programme, i.e. correction for spectral overlap and background. 

Spectral databases of all known carbohydrate structures are undoubtedly useful for the identification of the carbohydrates at hand. Most of the oligosaccharide structures contained in the SweetDB are appended with H- and/or C-NMR spectra. Computer Aided Spectrum Evaluation of Regular Polysaccharides (CASPER) at htq) //www.casper.organ.su.se/casper/ is a tool for the analysis of the primary structures of oligosaccharides and for polysaccharides with repeating units based on NMR spectroscopy... 

CASPER Computer Aided Spectrum Evaluation of Regular Polysaccharides... 

Spectrum evaluation is a crucial step in X-ray analysis, as important as sample preparation and quantification. As with any analytical procedure, the final performance of X-ray analysis is determined by the weakest step in the process. Spectrum evaluation in EDXRF analysis is more critical than in WDXRF spectrometry because of the relatively low resolution of the solid-state detectors employed. 

Starting with the rather unreliable comparison by human eye and then by photography, emission spectrum evaluation made spectacular improvements as a result of electronics and instrumentation, via photoelectric line densitometers introduced by Lundegardh in 1929, to the commercial manufacture of today s direct reading spectrometers. 

The significance of convolution/deconvolution exceeds the boundaries of probability theory. Some nuclear spectra can also be described by a convolution-type integral. For instance, a transmission Mossbauer spectrum (see O Chap. 25 in Vol. 3) is the convolution of two functions one of which is characteristic of the source of radiation while the other of the absorber (sample). The latter function contains all the parameters that the spectroscopist can be interested in, which explains why some methods of spectrum evaluation include the calculation of deconvolution as well. 

In case 3, the nuclear character is only expressed approximately, but the approximates ( can be considered as constants from the viewpoint of differentiation and therefore the following minimum condition/merit function is accepted in the practice of spectrum evaluation... 

The a priori knowledge of the matrix Tand the vector C is normally an essential condition of spectrum evaluation. From the point of view of analytical applications, the a priori determination of the Tmatrix is of fundamental importance. Namely, this transformation enables the experimenter to specify a set of peaks as a pattern representing one particular Mossbauer species in the sample. 

Another, so-called model-independent way of spectrum evaluation is done by obtaining and analyzing transformed patterns. For a thin absorber, the spectrum shape S( ) can be described as the convolution of a density function /( ) with a Lorentzian function L( ) ... 

Usually, the positron source NaCl is sealed between very thin metal (Ni, Ti) or polymer (kapton, Mylar) foils. The thickness (i.e., surface density) of these foils is around 1-2 mg/cm, but several percents of positrons annihilate in this wrapping even in the case of the thinnest foil. During spectrum evaluation, positrons annihilated in the source foil should be taken into account. 

The tendency toward fully automated gamma-ray spectrum evaluation can introduce errors. The degree of the potential for error varies among the software packages, nevertheless, there are some common risk factors that must be checked and evaluated by the user. These factors lie in... 

Systematic comparison of the theoretical and experimental calibration methods proves that the determination of elemental concentrations can be performed with an overall accuracy of 3-5% this number includes the errors appearing in spectrum evaluation as well. 

Analytical features. As a typical example of the form in which experimental data appear during PIXE analysis, Fig. 33.4 presents an X-ray spectrum obtained on a sample of coarse fraction atmospheric aerosol deposited on a polycarbonate membrane filter of 7 pm thickness. The spectrum was taken with protons of 2 MeV bombarding energy in a run of 1,200 s and beam intensity of 30 nA. The physical conditions in this case correspond to the approximation of the thin uniform homogeneous sample. Full and dashed lines show curves fitted by the spectrum evaluation code to the total spectrum and to its continuous background, respectively. The general features of the spectrum can be well explained on the basis of the simplified diagrams in Fig. 33.1. 

TABLE 19.4 Spectrum Evaluation Results of Sro.95Ca0.0sFe0.5Co0 sOs-i m TMS and EMS Modes... 

Widmalm et al7 studied and C NMR chemical shifts assignment of mono-, di-, tri- and tetrasaccharides (totally 43 compounds) and applied them for NMR chemical shift predictions of oligosaccharides depicted in Fig. 1 using CASPER program. The CASPER (computer assisted spectrum evaluation of regular polysaccharides) software contains chemical shift database and empirical spectra simulation routine optimized for carbohydrates. The calculated chemical shifts are in good to excellent agreement with those from and C NMR experiments. 

Informs on broad spectrum evaluation of economic feasibility of the product, including its corrosion control. 

---