Spectral interferences, atomic spectroscopy

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). 

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... 

Why are spectral interferences less important in atomic absorption spectroscopy and atomic fluorescence spectroscopy than atomic emission spectroscopy ... 

The presence and concentration of various metallic elements in petroleum coke are major factors in the suitability of the coke for various uses. In the test method (ASTM D5056), a sample of petroleum coke is ashed (thermally decomposed to leave only the ash of the inorganic constituents) at 525°C (977°F). The ash is fused with lithium tetraborate or lithium metaborate. The melt is then dissolved in dilute hydrochloric acid and the resulting solution is analyzed by atomic absorption spectroscopy to determine the metals in the sample. However, spectral interferences may occur when using wavelengths other than those recommended for analysis or when using multielement hollow cathode lamps. 

Thus, spectral interferences in atomic spectroscopy are less likely than in molecular spectroscopy analysis. In any case, even the atomic lines are not completely monochromatic i.e. only one wavelength per transition). In fact, there are several phenomena which also bring about a certain broadening . Therefore, any atomic line shows a profile (distribution of intensities) as a function of wavelength (or frequency). The analytical selectivity is conditioned by the overall broadening of the lines (particularly the form of the wings of such atomic lines). 

In atomic spectroscopy, absorption, emission, or fluorescence from gaseous atoms is measured. Liquids may be atomized by a plasma, a furnace, or a flame. Flame temperatures are usually in the range 2 300-3 400 K. The choice of fuel and oxidant determines the temperature of the flame and affects the extent of spectral, chemical, or ionization interference that will be encountered. Temperature instability affects atomization in atomic absorption and has an even larger effect on atomic emission, because the excited-state popula-... 

The determination of organic selenium compounds is done preferably by GC coupled to element-or molecule-specific detectors, such as GC-AED or molecular mass spectrometric detection (GC-MS).240 In this case, ICP-MS detection does not yield the improvement in sensitivity otherwise seen, which is due to spectral interferences. Dietz et al.241 have compared the analytical figures of merit of three detector systems for GC (AED, atomic fluorescence spectroscopy (AFS), and ICP-MS), arriving at the conclusion that GC-AED is the most sensitive and most practical... 

Marshall, J., and Franks, J. (1990) Multielement analysis and reduction of spectral interferences using electrothermal vaporization inductively coupled plasma-mass spectrometry. Atomic Spectroscopy 11, 177-186. 

The determination of sodium by atomic absorption spectroscopy has been applied successfully by several workers using a variety of equipment. The first element to be determined by Alkemade and Milatz (A2), in fact, was sodium. While sensitivity in emission is slightly higher for this metal than sensitivity in absorption, sodium still counts as one of the most sensitive elements in atomic absorption spectroscopy. The absence of any spectral interference (P3) and the relative freedom from other interferences appear to offer promising advantages of absorption over emission also for this element. 

The development of fast and accurate procedures for the determination of calcium in biological materials represents one of the important early achievements of atomic absorption spectroscopy. The diflBculties encountered with calcium in emission flame photometry are well known (Dll, L6, S6, SIO), but spectral interferences and extreme dependency on flame temperature, serious obstacles in emission, are either nonexistent or of lower importance in absorption. Chemical interferences, however. 

Fernandez FJ, Giddings R. 1982. Elimination of spectral interference using Zeeman effect background correction. Atomic Spectroscopy 3 61-65. 

Because the atomic fluorescence is measured at a right angle to the source, spectral interferences are minimal and a simple cutoff filter may often be used to isolate the emission line. The intensity of the fluorescence is directly proportional to the analyte concentration. As the analyte concentration within the flame becomes large, self-absorption of resonance fluorescence becomes significant, as it does in flame emission spectroscopy. Under these conditions, the linearity of the instrumental response breaks down and a calibration curve must be used or the analyte solutions diluted accordingly. 

Two types of interference are encountered in atomic absorption spectroscopy Spectral interference is a result of the absorption of an interfering species that either completely overlaps with the signal of interest or lies so close to this signal that it cannot be resolved by the monochromator. Chemical interference may be a consequence of the various chemical processes that occur during atomisation and alter the absorption characteristics of the analyte. 

Spectral interference has been well studied and are probably best understood in atomic emission spectroscopy. The usual remedy to alleviate a spectral interference is to either increase the spectral resolution of the spectrometer (which often is not possible with a given type of instrument) or to select an alternative emission line. Three types of spectral interference can be discriminated 1. Direct wavelength coincidence with another emission line, 2. partial overlap of the hne under study with an interfering line in close proximity, 3. a linear or non-linear increase or decrease in background continuum (see Fig. 12.33). 

Because many elements have several strong emission Hnes, AES can be regarded as a multivariate technique per se. Traditionally, for quantitative analysis in atomic emission spectroscopy, a single strong spectral line is chosen, based upon the criteria of Hne sensitivity and freedom of spectral interferences. Many univariate attempts have been made to compensate spectral interferences by standard addition, matrix matching, or interelement correction factors. However, all univariate methods suffer from serious limitations in a complex and Hne-rich matrix. 

Chemical and ionization interferences frequently found in atomic absorption spectroscopy are suppressed in ICP analysis. Since all samples are converted to simple aqueous or organic matrices prior to analysis, the need for standards matched to the matrix of the original sample is eliminated. The requirement that the sample presented to the instrument must be a solution necessitates extensive sample preparation facilities and methods. More than one sample preparation method may be necessary per sample depending on the range of elements requested. Spectral interferences can complicate the determination of trace... 

ETV may also serve as sample introduction for inductively coupled plasma (ICP)-atomic emission spectroscopy (AES)/MS providing the possibility of in situ sample preparation by selective vaporization of different sample components, using appropriate heating programs. By the reduction/elimination of matrix components, spectral interferences can be minimized and matrix effects in the plasma decreased. 

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