Spectral interference interferences

As the light collection capacity of a dispersive monochromator is frequently low, the use of filters can lead to more precise measurements of emission signals if the bandwidth is suflBciently narrow to avoid spectral interference. Interference filters with a bandwidth of 5 nm are available, and for maximum selectivity these should be used with near parallel light (L7). In atomic absorption the light collection capacity of the monochromator is frequently unimportant as the source intensity is high and the cross section of the optimum absorption zone of the flame is small. 

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. 

An important question to consider when using a flame as an atomization source, is how to correct for the absorption of radiation by the flame. The products of combustion consist of molecular species that may exhibit broad-band absorption, as well as particulate material that may scatter radiation from the source. If this spectral interference is not corrected, then the intensity of the transmitted radiation decreases. The result is an apparent increase in the sam-... 

Minimizing Spectral Interferences The most important spectral interference is a continuous source of background emission from the flame or plasma and emission bands from molecular species. This background emission is particularly severe for flames in which the temperature is insufficient to break down refractory compounds, such as oxides and hydroxides. Background corrections for flame emission are made by scanning over the emission line and drawing a baseline (Figure 10.51). Because the temperature of a plasma is... 

The variety of AES techniques requires careful evaluation for selecting the proper approach to an analytical problem. Table 4 only suggests the various characteristics. More detailed treatment of detection limits must include consideration of spectral interferences (191). AES is the primary technique for metals analysis in ferrous and other alloys geological, environmental, and biological samples water analysis and process streams (192). 

BeryUium aUoys ate usuaUy analyzed by optical emission or atomic absorption spectrophotometry. Low voltage spark emission spectrometry is used for the analysis of most copper-beryUium aUoys. Spectral interferences, other inter-element effects, metaUurgical effects, and sample inhomogeneity can degrade accuracy and precision and must be considered when constmcting a method (17). 

By for the most simple acid to work with in ICPMS is nitric acid. This has minimal spectral interferences and in concentradons under 5% does not cause excessive wear to the sample cones. Other acids cause some spectral interferences that often must be minimized by dilution or removal. When HF is used, a resistant sampling system must be installed that does not contain quartz. 

To produce an analytical method, the operator must select the power level of the plasma, the wavelength for each element (preferably free from spectral interferences), and the vewing height at which the plasma is to be seen for each element. Further, it may be necessary to apply background correction intervals are set using the graphics capability. 

These factors may be broadly classified as (a) spectral interferences and (b) chemical interferences. 

Spectral interferences in AAS arise mainly from overlap between the frequencies of a selected resonance line with lines emitted by some other element this arises because in practice a chosen line has in fact a finite bandwidth . Since in fact the line width of an absorption line is about 0.005 nm, only a few cases of spectral overlap between the emitted lines of a hollow cathode lamp and the absorption lines of metal atoms in flames have been reported. Table 21.3 includes some typical examples of spectral interferences which have been observed.47-50 However, most of these data relate to relatively minor resonance lines and the only interferences which occur with preferred resonance lines are with copper where europium at a concentration of about 150mgL 1 would interfere, and mercury where concentrations of cobalt higher than 200 mg L 1 would cause interference. 

With flame emission spectroscopy, there is greater likelihood of spectral interferences when the line emission of the element to be determined and those due to interfering substances are of similar wavelength, than with atomic absorption spectroscopy. Obviously some of such interferences may be eliminated by improved resolution of the instrument, e.g. by use of a prism rather than a filter, but in certain cases it may be necessary to select other, non-interfering, lines for the determination. In some cases it may even be necessary to separate the element to be determined from interfering elements by a separation process such as ion exchange or solvent extraction (see Chapters 6, 7). 

Selection of an alternative resonance line will overcome spectral interferences from other atoms or molecules and from molecular fragments. 

In order to make this exercise as useful and as interesting as possible, we will take steps to insure that our synthetic data are suitably realistic. We will include difficult spectral interferences, and we will add levels of noise and other artifacts that might be encountered in a typical, industrial application. 

Of the four strategies given above, the best condition for obtaining independent data for quality control (QC) are satisfied when INAA and RNAA results are compared, because the use of RNAA dramatically improves the selectivity of signal measurement and eliminates or greafiy reduces the measurement uncertainty sotuces, such as spectral interferences. A variety of radiochemical separations and... 

Nelissen [157] has adopted a standard approach for the determination of FRs in polyester compounds (Figure 3.26). The analysis is complicated by spectral interferences of PET/PBT and by the complexity of FR structures, notably DBDPO, Saytex BT 93 W (ethylene-bis-tetrabromophthalimide), PDBS 80, Pyrochek 68PB, Saytex HP7010, Saytex 8010, FR1808, FR 1025, F 2400, BC 52 and BC 58 (brominated polycarbonate oligomers). 

Table 7.33 reports the main characteristics of GC-ICP-MS. Since both GC and ICP-MS can operate independently and can be coupled within a few minutes by means of a transfer line, hyphenation of these instruments is even more attractive than GC-MIP-AES. GC-ICP-MS is gaining popularity, probably due to the fact that speciation information is now often required when analysing samples. Advantages of GC-ICP-MS over HPLC-ICP-MS are its superior resolution, resulting in sharper peak shapes and thus lower detection limits. GC-ICP-MS produces a dry plasma when the separated species reach the ICP they are not accompanied by solvent or liquid eluents. This reduces spectral interferences. Variations on the GC-ICP-MS... 

Little spectral interference from the mobile phase... 

Analyte dilution sacrifices sensitivity. Matrix matching can only be applied for simple matrices, but is clearly not applicable for complex matrices of varying composition. Accurate correction for matrix effect is possible only if the IS is chosen with a mass number as close as possible to that of the analyte elements). Standard addition of a known amount of the element(s) of interest is a safe method for samples of unknown composition and thus unknown matrix effect. Chemical separations avoid spectral interference and allow preconcentration of the analyte elements. Sampling and sample preparation have recently been reviewed [4]. 

Practically all classical methods of atomic spectroscopy are strongly influenced by interferences and matrix effects. Actually, very few analytical techniques are completely free of interferences. However, with atomic spectroscopy techniques, most of the common interferences have been studied and documented. Interferences are classified conveniently into four categories chemical, physical, background (scattering, absorption) and spectral. There are virtually no spectral interferences in FAAS some form of background correction is required. Matrix effects are more serious. Also GFAAS shows virtually no spectral interferences, but... 

Any difference in the behaviour of the analyte atoms in the sample and in the standard implies an interference. AAS using a line source for excitation suffers little spectral interference. Background interference in AAS is more important. This nonspecific absorption is caused by ... 

High matrix concentration (pronounced spectral interferences, high background absorbances)... 

ICP-AES and ICP-MS analyses are hampered in almost all cases by the occurrence of sample matrix effects. The origins of these effects are manifold, and have been traced partly to physical and chemical aerosol modifications inside sample introduction components (nebulisation effects). Matrix effects in ICP-AES may also be attributed to effects in the plasma, resulting from easily ionised elements and spectral background interferences (most important source of systematic errors). Atomic lines are usually more sensitive to matrix effects than are ionic lines. There exist several options to overcome matrix interferences in multi-element analysis by means of ICP-AES/MS, namely ... 

Today, ICP-AES is an indispensable inorganic analytical tool. However, because of the high plasma temperature, ICP-AES suffers from some severe spectral interferences caused by line-rich spectra of concomitant matrix elements such as Fe, Al, Ca, Ni, V, Mo and the rare-earth elements. This is at variance with AAS. The spectral interference can of course be minimised by using a (costly) high-resolution spectrometer. On the other hand, the high temperature of the ICP has the advantage of reducing chemical interferences, which can be a problem in AAS. 

AFS instruments are mainly used to detect the vapour-forming elements, such as those that form hydrides (As, Bi, Ge, Pb, Se, Sb, Sn and Te). AFS is less prone to spectral interferences than either AES or A AS. Detection limits in AFS are low, especially for elements with high excitation energies, such as Cd, Zn, As, Pb, Se and Tl. In recent years, the use of AFS has been boosted by the production of specialist equipment that is capable of determining individual analytes at very low concentrations (at the ng L-1 level). The analytes have tended to be introduced in a gaseous form. AFS methods and instrumentation have been reviewed [214-216], see also ref. [17]. 

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