Spectral line overlap

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. 

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. 

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. 

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

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... 

Several approaches are possible to minimize the effects of spectral band-spectral line overlap. One method is to use a high resolution spectrometer with narrow slit widths. This method frequently resolves the band into its separate components, thus permitting better separation of spectral line and spectral band components. Another method is to determine if another line is available for use in a different spectral region. For example, there is less OH band interference with the copper 3274.0 A line than with the copper line at 3247.0 A. Cobalt at 2873.1 A is in a region of strong CH band interference, while the cobalt line at 3453.5 A is not. 

Spectral line interference is less critical in atomic absorption than it is in flame emission. This is due to the fact that absorption is usually concerned with one spectral line only for each element and that, by proper modulation of the source signal, extraneous spectral lines that do not actually overlap the desired line are not detected. It is wise, however, to use as narrow a slit width as possible to keep the spectral band pass of the monochromator to a minimum. Actual spectral line overlap cannot be corrected by this means. If spectral line overlap occurs, as might happen, e.g., with palladium at 3404.6 A and cobalt at 3405.1 A, the only solutions are (1) to use another spectral line of the element or (2) remove the offending element from the analytical sample. 

NMR structure determination of large membrane proteins is hampered by broad spectral lines, overlap, and ambiguity of signal assignment. Chemical shift and NOE assignment can be facilitated by amino acid-selective isotope labeling in a cell-free protein synthesis system. Researchers report the cell-free synthesis of subunits a and c of the proton channel of Escherichia coli ATP synthase in a solution form in a mixture of phosphatidylcholine derivatives. In comparison, subunit a was purified from the cell-free system and from the bacterial cell membranes. NMR speetra of both preparations were similar, indicating that the procedure for cell-free synthesis produces protein structurally similar to that prepared from the cell membranes. 

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. 

R. I. Botro. In Developments in Atomic Plasma Spectrochemical Analysis. (R. M. Barnes, ed.) Heyden, Philadelphia, 1981. Describes merhod for correction of overlapping spectral lines when using a polychromaror for ICP-OES. 

Spectral overlap of emission and absorption wavelengths Is a potential cause of Interference In atomic absorption spectrometry (57) Thus, (a) the emission line of Fe at 352.424 nm Is close to the resonance line of N1 at 352.454, (b) the emission line of Sb at 217.023 nm Is close to the resonance line of Pb at 216.999 nm, and (c) the emission line of As at 228.812 nm Is close to the resonance line of Cd at 228.802 (57). To date, these practically coincident spectral lines have not been reported to be of practical Importance as sources of analytical Interference In atomic absorption analyses of biological materials. 

This type of interference normally takes place when the absorption of an interfering species either overlaps or lies veiy near to the analyte absorption, with the result that resolution by the monochromator almost becomes impossible, Hollow-cathode-source invariably give rise to extremely narrow emission-lines, hence interference caused due to overlap of atomic spectral lines is rather rare. 

Remedy The overlapping of this nature may be eliminated either by prior chemical separation or by selection other spectral lines. 

With a horizontally oriented sample (a = 0°), the spectrum of the labeled bR in Figure 48(b) should display three quadrupole splittings corresponding to the three labeled methyl groups on the retinal. It is apparent, however, that the expected three pairs of resonances are not resolved because of spectral overlap of the broadened lines. A computer simulation approach was used to analyze the spectral line shapes despite the overlap, but much qualitative information about the cyclohexene ring can be gained by simple inspection of the experimental data in Figure 48. 

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- ). 

Spectral interferences. These interferences result from the inability of an instrument to separate a spectral line emitted by a specific analyte from light emitted by other neutral atoms or ions. These interferences are particularly serious in ICP-OES where atomic spectra are complex because of the high temperatures of the ICP. Complex spectra are most troublesome when produced by the major constituents of a sample. This is because spectral lines from other analytes tend to be overlapped by lines from the major elements. Examples of elements that produce complex line spectra are Fe, Ti, Mn, U, the lanthanides and noble metals. To some extent, spectral complexity can be overcome by the use of high-resolution spectrometers. However, in some cases the only choice is to select alternative spectral lines from the analyte or use correction procedures. 

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