Background spectral interference

In AAS, the excitation source inert gas emission offers a potential background spectral interference. The most common inert gases used in hollow cathode lamps are Ne and Ar. The data taken for this table and the other tables in this book on lamp spectra are from HCLs however, electrodeless discharge lamps emit very similar spectra. The emission spectra for Ne and Ar HCLs and close lines that must be resolved for accurate analytical results are provided in the following four tables. This information was obtained for HCLs and flame atom cells and should not be considered with respect to plasma sources. In the Type column, I indicates that the transition originates from an atomic species and II indicates a singly ionized species. 

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... 

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. 

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)... 

Spectral interferences may arise from the close proximity of other emission lines or bands to the analyte line or by overlap with it. They can often be eliminated or minimized by increasing the resolution of the instrumentation, e.g. changing from a filter photometer to a grating spectrophotometer. Alternatively, another analyte line can be selected for measurements. Correction for background emission is also important and is made by monitoring the emission from a blank solution at the wavelength of the analyte line or by averaging measurements made close to the line and on either side of it. 

Interferences in atomic absorption measurements can arise from spectral, chemical and physical sources. Spectral interference resulting from the overlap of absorption lines is rare because of the simplicity of the absorption spectrum and the sharpness of the lines. However, broad band absorption by molecular species can lead to significant background interference. Correction for this may be made by matrix matching of samples and standards, or by use of a standard addition method (p. 30 et seq.). 

Instrumental correction for background absorption using a double beam instrument or a continuum source has already been discussed (p. 325). An alternative is to assess the background absorption on a non-resonance line two or three band-passes away from the analytical line and to correct the sample absorption accordingly. This method assumes the molecular absorption to be constant over several band passes. The elimination of spectral interference from the emission of radiation by the heated sample and matrix has been discussed on page 324 et seq. 

The internal rotational relaxation times of 1-pyrene carboxaldehyde in sulfonate systems may offer some indication of the extent of probe binding to the inverted micelle. In the absence of any background fluorescence interference to the time-dependent anisotropy decay profile, the internal rotational relaxation time should correlate with the strength of binding with the polar material in the polar core. However, spectral interference from the aromatic moieties of sulfonates is substantial, so that the values of internal rotational relaxation time can only be used for qualitative comparison. 

The method of standard additions is a useful procedure for checking the accuracy of a determination and overcoming interferences when the composition of the sample is unknown. It should be noted that the method cannot be used to correct for spectral interferences and background changes. At least three aliquots of the sample are taken. One is left untreated to the others known additions of the analyte are made. The additions should preferably be about 0.5x, x and 2x, where x is the concentration of the unknown. It should also be noted that the volume of the addition should be negligible in comparison with the sample solution. This is to prevent dilution effects... 

Spectral interferences from the overlap of molecular bands and lines (e g. the calcium hydroxide absorption band on barium at 553.55 nm) cannot be so easily dismissed. Lead seems to be particularly prone to such non-specific absorption problems at the 217.0 nm line (e g. sodium chloride appears to give strong molecular absorption at this wavelength). This type of problem is encountered in practical situations, but can sometimes be removed by the technique of background correction (see Section 2.2.5.2). 

An interference check standard is a standard solution used to verify an accurate analyte response in the presence of possible interferences from other analytes present in the samples. For methods that have known interference problems arising from the matrix or that are inherent in the method, such as ICP-AES (spectral interference lines) and ICP-MS (isotope combinations with similar masses to analyte), these solutions are used in the batch. The interference check standard must be matrix matched to acid content of the samples. Acceptance criteria are set—for example, the magnitude of uncorrected background and spectral interference must not be greater than a stated value. 

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. 

With this technique, problems may arise with interference, such as background absorption—the nonspecific attenuation of radiation at the analyte wavelength caused by matrix components. To compensate for background absorption, correction techniques such as a continuous light source (D2-lamp) or the Zeeman or Smith-Hieftje method should be used. Enhanced matrix removal due to matrix modification may reduce background absorption. Nonspectral interference occurs when components of the sample matrix alter the vaporization behavior of the particles that contain the analyte. To compensate for this kind of interference, the method of standard addition can be used. Enhanced matrix removal by matrix modification or the use of a L vov platform can also reduce nonspectral interferences. Hollow cathode lamps are used for As, Cu, Cr, Ni, Pb, and Zn single-element lamps are preferred, but multielement lamps may be used if no spectral interference occurs. 

Physical Interferences Chemical Interferences The Literature Interpolation Problem Ionization Interferences Spectral Interferences Spectral Interferences in Flame AAS Background Correction in AAS Spectral Interferences in Flame AFS Spectral Interferences in Flame AES Conclusions about Interferences... 

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