Non-element-specific absorption

Instrumentation for diode laser based AAS is now commercially available and the method certainly will expand as diode lasers penetrating further into the UV range become available, especially because of their analytical figures of merit that have been discussed and also because of their price. In diode laser AAS the use of monochromators for spectral isolation of the analyte lines becomes completely superfluous and correction for non-element specific absorption no longer requires techniques such as Zeeman-effect background correction atomic absorption or the use of broad band sources such as deuterium lamps. 

For the case of both electrically conducting and electrically non-conducting samples, laser ablation combined with AAS may be useful. In this case AAS measurements can be performed directly at the laser plume. Measurement of the non-element specific absorption will be very important, because of the presence of particles, molecules and radicals as well as due to the emission of continuum radiation. In addition, the absorption measurements should be made in the apprppriate zones. When applying laser ablation for direct solids sampling, the atomic vapor produced can also be led into a flame for AAS work, as has previously been described by Kantor et al. [299] in their early work. 

In AAS, systematic errors are often due to non-element specific absorption, which necessitates the use of background correction procedures. The absence of nonelement specific absorption can only be expected in the analysis of sample solutions with low matrix concentrations by flame AAS, but in furnace AAS work background correction is required particularly for matrix loaded solutions. The determination of the non-element specific absorption can be performed in several ways. 

In addition, undissodated molecules, which may be oxides arising from the dissociation of ternary salts (MgO, ZnO, etc.), but also radicals and molecular species arising from solvent residues, such as OH, PO, SO2, etc., may cause non-element-specific absorption. In addition, Rayleigh scattering of primary radiation by non-evaporated solid particles may occur, which also leads to radiation losses. Both phenomena necessitate the application of suitable techniques for background correction in most analyses of real samples by furnace AAS (see Section 4.6). Furthermore, emission by the furnace itself may give rise to continuum radiation, which can lead to systematic errors. 

Fig. 86. Principle of background correction with the D2-lamp technique (A), Al spectral bandwidth of monochromator, BG non-element-specific background absorption, As element-specific absorption signal. (B), Optical...

We will concentrate here on correction using a continuous emission lamp. The method consists of measuring, alternatively, the atomic absorption from the line of the element and the non specific absorption from a continuous spectrum lamp, over an range centred on the line and defined by the monochromator bandwidth. As this is much greater than the width of the line being analysed, we can consider that the second measurement corresponds solely to continuous (non specific) absorption. Continuous spectrum lamps used to correct the background arc ... 

The measure of absorption specific to an element may be subject to interference from non specific absorptions and various interactions that may be corrected or compensated for by a variety of methods. 

Molecular compounds from the matrix which are not dissociated may, however, lead to broadband absorption spectra whereas small solid particles in the flame may diffuse the light over an extensive range of wavelengths. When these non specific absorptions are superimposed on the atomic absorption at the wavelength of the element to be determined, it is necessary to measure the non specific absorptions and correct the total absorption. 

The instrument is calibrated for a given element for each series of samples. Direct calibration requires detailed knowledge of the milieu to be analysed. Precise results depend on the composition of the calibration solutions being as close as possible to that of the solutions to be analysed. The standard addition method can be u.sed in cases where it is not possible to produce external reference solutions similar to the solutions to be analysed. This method should be used with considerable caution because it assumes that the absorption is due solely to the element under analysis and, in particular, that the non specific absorption is fully corrected for. If this is not the case, any interfering absorption leads to an overestimate of the values observed. On the other hand, this method does have the advantage of eliminating the matrix effect. 

Spectral interferences of analyte lines with other atomic spectral lines are of minor importance as compared with atomic emission work. Indeed, it is unlikely that resonance lines emitted by the hollow cathode lamp coincide with an absorption line of another element present in the atom reservoir. However, it may be that several emission lines of the hollow cathode are within the spectral bandwidth or that flame emission of bands or a continuum occur. Both contribute to the non-absorbed radiation, by which the linear dynamic range decreases. Also, the nonelement specific absorption (see Section 4.6) is a spectral interference. 

A non exhaustive description of the history of X-ray Absorption Spectroscopy (XAS) can be found in Ref. 1. The modem EXAFS (Extended X-ray Absorption Fine Structure) technique began in the early seventies of the last century. It corresponds to the concomitance of both theoretical and experimental developments. Between 1969 and 1975, Stem, Sayers and Lytle succeeded in interpreting theoretically the X-ray Absorption Structures observed above an absorption edge [2], while during the same period, the advent of synchrotron radiation (SR) sources reduced drastically the acquisition time of a spectrum if compared to data obtained with conventional X-ray tubes. XAS provides essential information about the local atomic geometry and the electronic and chemical state of a specific atom, for almost any element of the periodic table (Z>5). This prime tool for... 

---