Line overlap

Minimizing Spectral Interference A spectral interference occurs when an analyte s absorption line overlaps with an interferant s absorption line or band. As noted previously, the overlap of two atomic absorption lines is seldom a problem. On the other hand, a molecule s broad absorption band or the scattering of source radiation is a potentially serious spectral interference. 

Over the last seventeen year s the Analytical center at our Institute amassed the actual material on the application of XRF method to the quantitative determination of some major (Mg, Al, P, S, Cl, K, Ti, Mn, Fe) and trace (V, Cr, Co, Ni, Zn, Rb, Sr, Y, Zr, Nb, Mo, Ba, La, Ce, Pb, Th, U) element contents [1, 2]. This paper presents the specific features of developed techniques for the determination of 25 element contents in different types of rocks using new Biaiker Pioneer automated spectrometer connected to Intel Pentium IV. The special features of X-ray fluorescence analysis application to the determination of analyzed elements in various types of rocks are presented. The softwai e of this new X-ray spectrometer allows to choose optimal calibration equations and the coefficients for accounting for line overlaps by Equant program and to make a mathematic processing of the calibration ai ray of CRMs measured by the Loader program. 

An excellent example is cited by Salmon and Blackledge51 to show that qualitative x-ray data can be given semiquantitative status even when the characteristic lines overlap. These investigators incorporated empirical corrections for the overlapping in constructing calibration... 

So basically there is no point in integrating a broad-band decoupled carbon spectrum. This is not so much of a drawback as it sounds, because the signals are distributed over a range of more than 200 ppm, so that line overlap is very unusual. 

FIGURE 10.2 The spectrum of the DMPO 011 adduct. The rapidly tumbling adduct affords an isotropic spectrum split by 14N (I = 1) in three lines, each of which is split by the P proton (I = 1/2) in two lines. Overlap of lines, due to AN AH, gives a 1 2 2 1 intensity pattern. 

Epoxies are good candidates for solid state C studies because of their relative chemical simplicity but even so some spectral lines overlap, as was shown in Fig. 2. We enquire into the limits of resolution to see what improvements can be expected. 

These are the only type of interference that do not require the presence of analyte. For AAS the problem of spectral interference is not very severe, and line overlap interferences are negligible. This is because the resolution is provided by the lock and key effect. To give spectral interference the lines must not merely be within the bandpass of the monochromator, but actually overlap each other s spectral profile (i.e. be within 0.01 nm). West [Analyst 99, 886, (1974)] has reviewed all the reported (and a number of other) spectral interferences in AAS. Most of them concern lines which would never be used for a real analysis, and his conclusion is that the only real problem is in the analysis of copper heavily contaminated with europium The most commonly used copper resonance line is 324.754 nm (characteristic concentration 0.1 pg cm- ) and this is overlapped by the europium 324.753 nm line (characteristic concentration 75 pg cm- ). 

Spectroscopic interferences can also manifest themselves, either as an increase in the continuum background emission or as line overlap, especially if samples with a complex matrix, or organic solvents, are analysed. An increase in the continuum background emission can be easily compensated for by subtraction of the background adjacent to the analytical line. For a sloping background then measurements must be made on both sides of the line and usually the mean value is subtracted. These options are summarized in Fig. 4.18. Line overlap is a particular problem when an element, present in large excess in the matrix, has an emission line close to. 

Two opposing forces are at the points marked X and Y in a map similar to the one [in Fig. 3]. The commander of each force wishes to occupy as much of the area as he can and knows the other does too. But each commander wishes to avoid an armed clash and knows the other does too. Each must send forth his troops with orders to take up a designated line and to fight if opposed. Once the troops are dispatched, the outcome depends only on the lines that the two commanders have ordered their troops to occupy. If the lines overlap, the troops will be assumed to meet and fight, to the disadvantage of both sides. If the troops take up positions that leave any appreciable space unoccupied between them, the situation will be assumed unstable and a clash inevitable. Only if the troops are ordered to occupy identical lines or lines that leave virtually no unoccupied space between them will a clash be avoided. In that case, each side obtains successfully the area it occupies, the advantage going to the side that has the most valuable area in terms of land and facilities. (1960, p. 62)... 

As discussed, XRD has for many years been the standard, everyday characterization method for solid catalysts, and in almost every laboratory in this field there is access to an X-ray diffractometer. This instrument allows a wide variety of different characterizations, but there are also limitations of such equipment. For example, the limited resolution of an in-house diffractometer may often be insufficient for a detailed analysis. This point is illustrated in Fig. 5a, which shows the diffractogram of an industrial type steam-reforming catalyst consisting of nickel crystallites on a spinel support (35). The Ni(lll) and the spinel(400) lines overlap so that a detailed analysis is impossible. This problem can be overcome if the XRD... 

The smaller bond angles prevent normal overlap along the intemuclear axis (dotted line). Overlap occurs outside the intemuclear axis and a bent bond is formed (also referred to as a r bond). The r bond is weaker than a a bond due to nonoptimal overlap and thus is of higher energy. This type of strain is termed angle strain (sometimes called Baeyer strain). It comes about because normal bond angles are not possible and thus nonoptimal overlap results. Angle strain... 

Spectral interferences, such as line overlaps, are prevalent and must be corrected for accurate quantitative analysis. With a scanning instrument it may be possible to move to an interference free line. With a direct reader, sophisticated computer programs apply mathematical corrections based on factors previously determined on multi-element standards. 

The most common interferences are absorption and/or enhancement of the element of interest by other elements in the matrix. Line overlaps may also occur. In the analysis of solids, particle size and geological effects can be important. Computer programs are available to correct for all of these interferences. 

This is a crude assumption. However, it appears that a quantum picture of discrete rotational lines, placed in the submillimeter wavelength range (ca. from few to 150 cm-1), is essentially determined by a form of a molecule only for a gas. In the case of a liquid, discrete spectrum is not revealed, since separate rotational lines overlap due to strong intermolecular interactions, which become of primary importance. So, due to these interactions and the effect of a tight local-order cavity, in which molecules reorient, the maximum of the absorption band, situated in the case of vapor at 100 cm-1, shifts in liquid water to... 

Spectral interferences from ion-atom recombination, spectral line overlaps, molecular band emission, or stray light can occur that may alter the net signal intensity. These can be avoided by selecting alternate analytical wavelengths and making background corrections. 

When absorption lines overlap, the absorption coefficient is a summation fry = g. ... 

According to the results presented in Fig. 1, as density decreases, the diffraction lines of diamond are broadened, the tails of diffraction lines overlap, and a halo corresponding to the formation of a fine-crystalline ( amorphous )... 

Some examples of spectral line overlap are known.15 For example, europium at 324.7530 nm interferes in the determination of copper at 324.7540, but europium does not interfere in copper determination at 327.3962 (see Figure 5). The fact that the interference occurs only at one analytical wavelength confirms that it is spectral in nature, since the extent of physical, chemical, or ionization interferences would be similar at all wavelengths. 

Again, in HR-CS AAS these problems are essentially nonexistent for the same reasons as given above. Firstly, because of the relatively constant, very intense emission of the primary radiation source, there are no more weak lines that is, the same high SNR will be obtained on all analytical lines, regardless of their spectral origin. The only factors that will have an influence will be the absorption coefficient and the population of the low excitation level in case nonresonance lines are used. Secondly, because of the high resolution of the monochromator, and the visibility of the entire spectral environment of the analytical line in HR-CS AAS, potential spectral interferences can easily be detected, and in addition cannot influence the actual measurement, except in the rare case of direct line overlap. However, even in this case, HR-CS AAS provides an appropriate solution, as discussed in the previous section. 

The shifts were applied to a measured S02 spectrum [21], with band positions shown in the last column of Table 5.1. The coefficients of the linear fit iov — v are also given in Table 5.1. The model S-MIF signatures computed during SO2 photolysis (see below) exhibit little sensitivity to whether the Ran et al. shifts (black squares in Figure 5.4) are used versus the Ran et al. shifts from 200 to 220 nm and the linear fit from 190 fo 200 nm. (All resulfs presented below use the latter scheme). Flowever, the S-MIF results were different (e.g., smaller A S/A S ratio) if only the linear fit was used from 190 fo 220 nm, possible indicating that fortuitous line overlap was more likely with the purely linear fit, an effect not expected with actual spectra. 

There are troublesome spectral interferences, spectral line overlapping in ICP-OES, and polyatomic interferences in ICP-MS. 

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