Direct line overlap

The presence of a concomitant element can lead to additional absorption lines within the recorded spectral interval. Again, they are only of interest when they directly overlap with the analyte line and cannot be separated in time in the case of GF measurements. 

An example of a direct line overlap and its elimination is given in Section 8.1.4, where the zinc concentration is determined on its resonance line at 213.856 nm in the presence of high amounts of iron, having a weak close-by absorption line at 213.859 nm. 

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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. 

Electronic excitation temperatures in a helium MIP are on the order of4000 K, permitting the excitation of the halogens, C, N, H, and other elements that cannot be excited in a flame atomizer. The lower temperature results in less spectral interference from direct line overlap than in ICP or high-energy sources, but also causes more chemical interference. 

The CCD detector permits not only simultaneous background correction but complete correction of spectral interferences such as structured background from molecular species such as OH and NO. The resolution permits collection of the spectra from these species and removal of them from the absorbance of the sample. Direct-line overlap can also be corrected, if the matrix has an additional absorption line within the registered spectral range of the detector. Direct-line overlap is rare and usually is due to line-rich matrices such as Fe, so the system can reliably correct for the direct overlap of Fe on Zn, for example. 

The MIP operates at lower power than the ICP and at microwave frequencies instead of the RFs used for ICP. Because of the low power, an MIP cannot desolvate and atomize liquid samples. Therefore, MIPs have been limited to the analysis of gaseous samples or very fine (1-20 pm diameter) particles. Helium is the usual plasma gas for an MIP source. Electronic excitation tanperatures in a helium MIP are on the order of 4000 K, permitting the excitation of the halogens, C, N, H, O, and other elements that cannot be excited in a flame atomizer. The lower temperature results in less spectral interference from direct-line overlap than in ICP or high-energy sources, but also causes more chemical interference. 

For the case where analyte and background absorption coincide both spectrally and temporally, as shown in the series of model spectra in Figure 5.6, it is possible to correct for fine-stmctured background absorption using reference spectra of the interfering molecule(s). Spectral interferences caused by other atoms because of direct line overlap will be treated in the next section. 

There is a direct line overlap between an iron line at 213.859 nm with the zinc line, which has to be taken into consideration whenever zinc has to be determined in the presence of high iron concentrations. However, the presence of a second iron line within the spectral window of HR-CS AAS at 213.970 nm makes a correction of this interference easy using least-squares BC (refer to Section 8.1.4). There is also a direct line overlap of the zinc resonance line with a weak copper line at 213.853 nm, which requires special attention in the presence of high copper containing matrices (refer to Section 8.1.5). 

High-purity copper is one of the important materials used in electrical devices, and the content of several trace elements has to be controlled carefully, as they have a significant influence on the quality and the properties of the product. Huang et al. [67] investigated the use of HR-CS F AAS for this purpose because of the superior detection power of this technique and the improved background measurement and correction capabilities. The authors found the well-known direct line overlap between the weak copper line at 213.853 nm... 

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. 

Figure 7.41 Direct spectral overlap of Pt and Cr emission lines. No background correction technique can solve this problem. A line with no interference must be found, an interelement correction factor must be applied or the elements must be separated chemically. [From Boss and Fredeen, courtesy of PerkinElmer Inc. (www.perkinelmer.com).]...

In some cases the interference caused by a background spectrum line which directly coincides with the desired analyte can be avoided by background correction. For example, the OH molecular line overlaps the A11 line at 308.2 nm. Because the OH signal is relatively constant, it can be subtracted out with the blank signal. However, this will add background noise that can interfere with the determination at low analyte concentrations. This type of problem is much greater when the spectral line originates from a concomitant line which directly overlaps the analyte line, like the coincidence of the Cu I line at 213.859 nm with the Zn I line at 213.856 nm, or that of the... 

Direct spectral line interference occurs when the spectral line energy of two or more elements reaches the detector circuit. One type of spectral line interference involves spectral line overlap. This occurs because spectral lines have a finite linewidth. If the spectral energies of two lines overlap, the result is spectral interference regardless of the resolving power of the spectral isolation system of the spectrometer. At high flame temperatures, when... 

Figure 1.48a explains schematically the appearance of resonances between the fixed frequency

Figure 7.41 shows a case of direct spectral overlap between two emission lines, one from Pt and one from Cr. A high-resolution spectrometer will limit the number of direct spectral overlaps... 

When the interference is from the plasma emission background, there are background correction options available with most commercial instrumentation. The region adjacent to the line of interest can be monitored and subtracted from the overall intensity of the line. If direct spectral overlap is present, and there are no alternative suitable lines, the interelement equivalent concentration (lEC) correction technique can be employed. This is the intensity observed at an analyte wavelength in the presence of 1000 mg of an interfering species. It is expressed mathematically as ... 

When a C2 symmetry axis of the chromophore is in direct line with the bond which links the chromophore to the chain, the photophysical behaviour is simplified. When this situation is not met, different chromophore orientations lead to more than one type of excimer overlap. Bulky chromophores (carbazolyl, anthracyl) may interact in the excited state in the conformation which is very close to the conformation present in the ground state, producing also partial overlap excimers. 

Positive changes in Be emission intensities are due to direct line or wing overlaps. Negative deviations are due to interference at a background correction point. 

Figure 5.15 Transient far-infrared conductivity spectra in bare and dye-sensitized ZnO and Ti02 nanoparticles. Symbols (left axis) measured data lines (right axis) calculated mobility of directly photogenerated electrons (solid) and injected electrons (dashed). These lines overlap in the left graph.

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