Spectral Line Interferences

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

Other sources of background include spectral line (nonanalyte atomic fluorescence) and spectral band (molecular fluorescence) interferences. Spectral line interferences are caused by the presence of another element that can absorb source radiation and emit fluorescence sufficiently close to the analyte wavelength to be collected by the detection system. Spectral band interferences involve the absorption of source light by a molecule whose fluorescence is collected by the detection system. Nonanalyte atomic fluorescence and molecular fluorescence are minimized by the use of a narrow line source and a nonresonance transition. This is in contrast to AES, where spectral interferences are sufficiently severe that a high-resolution monochromator is required. 

The quantum theory of spectral collapse presented in Chapter 4 aims at even lower gas densities where the Stark or Zeeman multiplets of atomic spectra as well as the rotational structure of all the branches of absorption or Raman spectra are well resolved. The evolution of basic ideas of line broadening and interference (spectral exchange) is reviewed. Adiabatic and non-adiabatic spectral broadening are described in the frame of binary non-Markovian theory and compared with the impact approximation. The conditions for spectral collapse and subsequent narrowing of the spectra are analysed for the simplest examples, which model typical situations in atomic and molecular spectroscopy. Special attention is paid to collapse of the isotropic Raman spectrum. Quantum theory, based on first principles, attempts to predict the. /-dependence of the widths of the rotational component as well as the envelope of the unresolved and then collapsed spectrum (Fig. 0.4). 

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. 

Spark sources are especially important for metal analysis. To date, medium-voltage sparks (0.5-1 kV) often at high frequencies (1 kHz and more), are used under an argon atmosphere. Spark analyses can be performed in less than 30 s. For accurate analyses, extensive sets of calibration samples must be used, and mathematical procedures may be helpful so as to perform corrections for matrix interferences. In arc and spark emission spectrometry, the spectral lines used are situated in the UV (180-380nm), VIS (380-550nm) and VUV (<180 nm) regions. Atomic emission spectrometry with spark excitation is a standard method for production and product control in the metal industry. 

Table 8.7). Thus, intensity and concentration are directly proportional. However, the intensity of a spectral line is very sensitive to changes in flame temperature because such changes can have a pronounced effect on the small proportion of atoms occupying excited levels compared to those in the ground state (p. 274). Quantitative measurements are made by reference to a previously prepared calibration curve or by the method of standard addition. In either case, the conditions for measurement must be carefully optimized with reference to the choice of emission line, flame temperature, concentration range of samples and linearity of response. Relative precision is of the order of 1-4%. Flame emission measurements are susceptible to interferences from numerous sources which may enhance or depress line intensities. 

Spectral interferences are due to substances in the flame that absorb the same wavelength as the analyte, causing the absorbance measurement to be high. The interfering substance is rarely an element, however, because it is rare for another element to have a spectral line at exactly the same wavelength, or near the same wavelength, as the primary line of the analyte. However, if such an interference is suspected, the analyst can tune the monochromator to a secondary line of the analyte to solve the problem. 

A chemical interference is one in which the sample matrix affects the chemical behavior of the analyte. A spectral interference is one that interferes with accurate measurement of the desired spectral line. 

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. 

The atomic absorption characteristics of technetium have been investigated with a technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 /ig/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. Only calcium, strontium, and barium cause severe technetium absorption suppression. Cationic interferences are eliminated by adding aluminum to the test solutions. The atomic absorption spectroscopy can be applied to the determination of technetium in uranium and its alloys and also successfully to the analysis of multicomponent samples. 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

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. 

R. M. Herman. Scalar and vector collisional interference in the vibration-rotation absorption spectra of H2 and HD. In R. J. Exton, ed., Spectral Line Shapes 4, p. 351, Deepak, Hampton, VA, 1987. 

Calculations indicate that about 1% of the transitions between complex configurations has values of F < 10-4. Computed line strengths may have large percentage errors for F < 0.1. Because of such interference of separate terms (so-called angular effects) some spectral lines disappear for certain ions. Figure 31.2 illustrates this statement for the example of the transition 2p53d — 2p6 for the neon isoelectronic sequence. Indeed,... 

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. 

Several types of interference effects may contribute to inaccuracies in the determination of major and minor elements. The interferences can be classified as spectral, physical, and chemical. Spectral interferences involve an overlap of a spectral line from another element, unresolved overlap of molecular band spectra, background contribution from continuous or recombination phenomena, and background contribution from stray light from the line emission of high-concentration elements. The second effect may require selection of an alternative wavelength. The third and fourth effects can usually be compensated by a background correction adjacent to the analyte line. 

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. 

To reduce the detrimental effects of spectral interferences on element quantitation, laboratories select the spectral lines that are least affected by the background, and use the background compensation and interelement correction routines as part of the analytical procedure. The instrument software uses equations to compensate for overlapping spectral lines the effectiveness of these equations in eliminating spectral interferences must be confirmed at the time of sample analysis. That is why laboratories analyze a daily interelement correction standard (a mixture of all elements at a concentration of 100mg/l) to verify that the overlapping lines do not cause the detection of elements at concentrations above the MDLs. 

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. 

At high concentrations especially, a number of elements produce significant concentrations of polyatomic species in flames. Such species absorb, and may therefore cause spectral interference. However the molecular absorption spectra are very wide compared with the atomic spectral lines. Figure 6, for example, shows how the presence of CaOH species in flames may interfere in the determination of barium by AAS. The formation of any solid particles in the flame causes scatter, which also causes an apparent broad band absorption, especially at lower wavelengths. 

The situation is more complicated in the applications of the statistical method to radiation phenomena. Some of the older attempts have not as yet yielded clear results. Among these are the connection of the interference limit for large path differences with the average time between collisions undergone by the center of emission,2,6 or the remark that, on account of the corresponding Doppler effect, the thermal motion of the sources of emission creates a lower limit for the width of fine spectral lines.218... 

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