Matrix absorption

When not enough is known about the matrix, absorption effects (7.3) cannot be estimated, and comparison of standard and unknown (7.8) may be unreliable. Somewhat less likely consequences of such lack of knowledge are errors due to interfering lines and to shifts in x-rav spectra, which can affect both the analytical-line intensity and the background (8.8). Sometimes a quick examination of all the characteristic lines that can be excited in a sample is helpful in giving useful knowledge about the matrix. 

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. 

Matrix absorption, secondary fluorescence and scattering phenomena limit sensitivity and precision in many cases, especially with dense matrices. The sensitivity falls off with atomic number elements with Z < 15 are particularly difficult to analyse. Analysis is characteristic of surface layers (5-500 pm depth) only for a solid specimen. Instruments are often large, complicated and costly. 

The currently accepted spectroscopic assignments were obtained by a combination of multiple techniques. Leyva et applied matrix absorption and emission spectroscopy along with flash photolysis techniques. Chapman and LeRoux obtained the matrix IR spectrum of cyclic ketenimine K and Hayes and Sheridan obtained the matrix IR and UV-Vis spectrum of triplet phenylnitrene and cyclic ketenimine K. Schuster and co-workers applied time resolved IR and UV-Vis spectroscopy and demonstrated the formation of cyclic ketenimine K in solution, the species that absorbs strongly at 340 nm. 

A numerical matrix correction technique is used to linearise fluorescent X-ray intensities from plant material in order to permit quantitation of the measurable trace elements. Percentage accuracies achieved on a standard sample were 13% for sulfur and phosphorus and better than 10% for heavier elements. The calculation employs all of the elemental X-ray intensities from the sample, relative X-ray production probabilities of the elements determined from thin film standards, elemental X-ray attenuation coefficients, and the areal density of the sample cm2. The mathematical treatment accounts for the matrix absorption effects of pure cellulose and deviations in the matrix effect caused by the measured elements. Ten elements are typically calculated simultaneously phosphorus, sulfur, chlorine, potassium, calcium, manganese, iron, copper, zinc and bromine. Detection limits obtained using a rhodium X-ray tube and an energy-dispersive X-ray fluorescence spectrometer are in the low ppm range for the elements manganese to strontium. 

Phase 2 - data preprocessing. There are many ways to process spectral data prior to multivariate image reconstruction and there is no ideal method that can be generally applied to all types of tissue. It is usual practice to correct the baseline to account for nonspecific matrix absorptions and scattering induced by the physical or bulk properties of the dehydrated tissue. One possible procedure is to fit a polynomial function to a preselected set of minima points and zero the baseline to these minima points. However, this type of fit can introduce artifacts because baseline variation can be so extreme that one set of baseline points may not account for all types of baseline variation. A more acceptable way to correct spectral baselines is to use the derivatives of the spectra. This can only be achieved if the S/N of the individual spectra is high and if an appropriate smoothing factor is introduced to reduce noise in the derivatized spectra. Derivatives serve two purposes they minimize broad... 

Table I lists several XRD analytical methods recently developed in the NIOSH laboratories. For each analyte, the analytical range, detection limit and analytical precision are listed. The method numbers refer to the NIOSH Manual of Analytical Methods (2.). As indicated in the table, there are several NIOSH methods available for free silica analysis. Method No, P CAM 109 incorporates the internal standard approach as developed by Bumsted (3.), The other two methods S-315 and P CAM 259 are based on the substrate standard method. The major difference between the two is the direct sampling on silver membrane filters (S-315). This paper will address the various methods of quantitation, sample collection and procedures for matrix absorption corrections that have been used in this laboratory for the analysis of crystalline particulate contaminants in the workplace.

Proponents of the internal standard procedure have questioned the validity of the substrate standard method to adequately correct for matrix absorption, Leroux and coworkers JLA) have presented data which support the method in addition, several measurements were performed in this laboratory to verify the validity of the method ( ), Mixtures of chrysotile in talc (1-7%) were prepared and various quantities "spiked" on silver filters. Table III illustrates the results obtained after correcting for matrix absorption as compared with the uncorrected data. Overall, there is excellent agreement between the corrected weight and the "spiked" weight. 

The trend in industrial hygiene work is to identify the particular species responsible for an occupational health problem, although assessment of exposures to inorganic materials previously has most often been based on elemental analysis When a solid inorganic compound is to be identified and quantified, X-ray diffraction should be among the approaches considered This paper has outlined the use of X-ray powder diffraction as a tool for the identification and quantitation of crystalline particulates It has been shown that the substrate standard method is the preferred quantitative procedure for several reasons (1) easy adaptability to most analytes (2) fast analysis time (as compared to the internal standard procedure) and (3) accurate determination of matrix absorption effects While there are a number of reasons why a given compound may not be amenable to this technique, it is likely that the list of analytes will be added to in the future ... 

Common UV-MALDI matrices are listed in Table 1.2 with the class of compounds with which they are used. The matrices used with IR lasers, such as urea, caboxylic acids, alcohols and even water, are often closer to the natural solutions than the highly aromatic UV-MALDI matrices. In addition, there are many more potential matrices for IR-MALDI owing to the strong absorption of molecular compounds at IR wavelengths, even if the correlation between ion formation and matrix absorption in IR-MALDI is not clear [36],... 

No. Conf igurat ion Li Doublet-States State Energy (cm Li -Xe-Matrix Absorption ... 

For KBr powder (Uvasol, E. Merck) a reflectance of 0.873 at 1724 cm was reported (Reinecke et al., 1988). At such a level, the matrix absorption must be included in a quantitative evaluation. Similarly, the scattering properties of the analyte material must be taken into account at higher concentrations. Several approaches have been proposed (cf. review by Korte, 1990b)... 

Matrix absorption. As the composition of the sample changes, so does its absorption coefficient. As a result there are changes both in the absorption of the... 

When only a single element is to be determined and its concentration range is narrow, the matrix composition is essentially constant and so are the effects of matrix absorption and enhancement. The analytical problem is then reduced to the preparation of a single calibration curve, which is often linear. Figure 15-9 shows an example, for analysis in the parts-per-million (ppm) range. In this particular case, the background, represented by the intercept on the ordinate, is very high. 

Figure 8. Overlay of the matrix absorption spectrum of triplet phenyl nitrene with the transient absorption spectrum of dehydroazepine.

The nature of the dependences n (u>) and nn(w) are known to be dependent on the signs of the coefficients fi and n2, respectively (1). Therefore, the experimental results for the ratio (5.53) can, in principle, be used to identify the spatial dispersion effects of the crystal matrix. However, the decay of the excited states of both the impurity and the matrix makes this difficult. If the levels of these states are wide enough then it becomes practically impossible to sneak up on the frequency fli(O) and to distinguish the impurity absorption from the matrix absorption. 

A second source of uncertainty Is associated with the quantities comprising the overall callhratlon factor A, such as recovery. Instrumental detection efficiency, matrix absorption or scattering, etc. If A Is determined as a random variable each time X (concentration) is estimated, then there is no problem its random error is automatically taken into account through error... 

Unfortunately, most /r, values have not been measured and fi is unknown for any given mixture. Hence, W, cannot be calculated from I,. However, as first pointed out by Chung (1974a), the matrix absorption effect can be eliminated by calculating a ratio ... 

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