Atomic spectrometry physical interferences

Different analytical procedures have been developed for direct atomic spectrometry of solids applicable to inorganic and organic materials in the form of powders, granulate, fibres, foils or sheets. For sample introduction without prior dissolution, a sample can also be suspended in a suitable solvent. Slurry techniques have not been used in relation to polymer/additive analysis. The required amount of sample taken for analysis typically ranges from 0.1 to 10 mg for analyte concentrations in the ppm and ppb range. In direct solid sampling method development, the mass of sample to be used is determined by the sensitivity of the available analytical lines. Physical methods are direct and relative instrumental methods, subjected to matrix-dependent physical and nonspectral interferences. Standard reference samples may be used to compensate for systematic errors. The minimum difficulties cause INAA, SNMS, XRF (for thin samples), TXRF and PIXE. 

Regarding historical insight and descriptions of principles and fundamentals of flame atomic emission spectrometry, a chapter on flame photometry appeared in the first edition of Treatise on Analytical Chemistry (Vallee and Thiers 1965) covering the flame and burner, photometer/spec-trometer, fundamental discussion of excitation and processes within the flame, cation and anion interferences and handling of analytical samples. In an analogous, expanded, detailed and excellent treatment of EAES in the second edition of the Treatise on Analytical Chemistry, Syty (1981) discusses types of flames used for excitation, processes within flames, spectral, chemical and physical interferences and remedies. 

For all the techniques of optical atomic spectrometry, the samples (solutions and/or solid samples) must be converted into an atomic vapour. The sensitivity is strongly dependent on the yield of this process, as are the chemical and physical interferences, i.e. the specificity of the method in general. For the first approach, the atomization of the sample is proportional and the occurrence of chemical and/or physical interferences is inversely proportional to the excitation temperature. Therefore the temperature available in the atomization stage should be as high as possible. The classical excitation sources used in atomic spectrometry like flame, graphite furnace, arc and spark are well known. The temperature available, especially in a flame or in the graphite furnace, is around 3000°C. Due to the Boltzmann-distribution... 

In atomic spectrometry, the sample is introduced, by means of a sampling device, into a high-temperature source or atom cell (plasma, flame, etc.). Here, the sample is vaporized, e.g., by thermal evaporation or sputtering. It is important to supply as much energy as possible, so that the volatilization processes, which involve a physical or chemical equilibrium, result in complete atomization, irrespective of the state of aggregation, solid state structure, or chemical composition of the sample. This is very important, both to ensure maximum sensitivity and to minimize matrix interference in the analysis. The effectiveness of the volatilization processes involved, the plasma temperature, and the number densities of the various plasma components will all influence sample atomization. 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

The procedure is strictly analogous to that used for absorbance measurements in UV and visible molecular spectrometry (p. 355). To avoid interference from emission by excited atoms in the flame and from random background emission by the flame, the output of the lamp is modulated, usually at 50 Hz, and the detection system tuned to the same frequency. Alternatively, a mechanical chopper which physically interrupts the radiation beam, can be used to simulate modulation of the lamp output. 

The disadvantages of electrothermal atomisation (ETA) — atomic absorption spectrometry (AAS) are the physical, chemical and spectral interferences, these being more severe than with flame atomic absorption spectrometry (FAAS), and which depend critically upon the experimental and operational conditions within the atomiser and the nature of the chemical pretreatment used. It is not intended to discuss here the theoretical aspects of these interferences which have been reviewed excellently elsewhere [2], but it is pertinent to consider briefly how these interferences affect the various stages of the analysis and how they may be minimised. 

Interferences are physical or chemical processes that cause the signal from the analyte in the sample to be higher or lower than the signal from an equivalent standard. Interferences can therefore cause positive or negative errors in quantitative analysis. There are two major classes of interferences in AAS, spectral interferences and nonspectral interferences. Nonspectral interferences are those that affect the formation of analyte free atoms. Nonspectral interferences include chemical interference, ionization interference, and solvent effects (or matrix interference). Spectral interferences cause the amount of light absorbed to be erroneously high due to absorption by a species other than the analyte atom. While all techniques suffer from interferences to some extent, AAS is much less prone to spectral interferences and nonspectral interferences than atomic anission spectrometry and X-ray fluorescence (XRF), the other major optical atomic spectroscopic techniques. 

---