Continuum source method

Background absorption can be compensated for or minimized by using sample-like standards or matrix modification, or by moving to an interference-free line if possible. However, the actual background correction methods are (i) Two line method (ii) Continuum source method (iii) Smith-Hieftje method (iv) Methods using the Zeeman effect. 

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. 

Various approaches to measuring flame temperature are well described in Gaydon s book on flames (see Appendix C). The best methods are spectroscopic rather than those which use thermocouples. The sodium line reversal method is perhaps the easiest. Sodium is added to the flame and the sodium D lines viewed against a bright continuum source (e g. a hot carbon tube). When the flame is cooler than the source the lines appear dark because of absorption. When the flame is hotter than the tube, the bright lines stand out in emission. The current to the tube, which will have been precalibrated for temperature readings by viewing the tube with an optical pyrometer, is adjusted until the lines cannot be seen. At this reversal point, the flame and tube temperature should be equal. 

A. F. Silva, D. L. G. Borges, B. Welz, M. G. R. Vale, M. M. Silva, A. Klassen, U. Heitmann, Method development for the determination of thallium in coal using solid sampling graphite furnace atomic absorption spectrometry with continuum source, high-resolution monochromator and CCD array detector, Spectrochim. Acta, 59B (2004), 841. 

One of the main practical problems with the use of AAS is the occurrence of molecular species that coincide with the atomic signal. One approach to remove this molecular absorbance is by the use of background correction methods. Several approaches are possible, but the most common is based on the use of a continuum source, D2. In the atomization cell (e.g. flame) absorption is possible from both atomic species and from molecular species (unwanted interference). By measuring the absorption that occurs from the radiation source (HCL) and comparing it with the absorbance that occurs from the continuum source (D2) a corrected absorption signal can be obtained. This is because the atomic species of interest absorb the specific radiation associated with the HCL source, whereas the absorption of radiation by the continuum source for the same atomic species will be negligible. 

The occurrence of molecular absorbance and scatter in AAS can be overcome by the use of background correction methods. Various types of correction procedures are common, e.g. continuum source, Smith-Hieftje and the Zeeman effect. In addition, other problems can occur and include those based on chemical, ionization, physical and spectral interferences. 

Aside from the extensively studied volatility interferences, it has been demonstrated that the conventional method of background correction which is based on the use of a continuum source (D2), is subject to spectral interferences from iron and for phosphate decomposition products (presumably PO and P2) (Saeed and Thomassen, 1981). Even though these spectral interferences are highly reduced by matrix modification with either nickel, a nickel/platinum or a nickel/palladium matrix modifier, the use of a Zeeman based instrument is highly recommended (Bauslaugh et al., 1984 Radziuk and Thomassen, 1992 Hoenig, 1991). 

Atomic fluorescence flame spectrometry is receiving increased attention as a potential tool for the trace analysis of inorganic ions. Studies to date have indicated that limits of detection comparable or superior to those currently obtainable with atomic absorption or flame emission methods are frequently possible for elements whose emission lines are in the ultraviolet. The use of a continuum source, such as the high-pressure xenon arc, has been successful, although the limits of detection obtainable are not usually as low as those obtained with intense line sources. However, the xenon source can be used for the analysis of several elements either individually or by scanning a portion of the spectruin. Only chemical interferences are of concern they appear to be qualitatively similar for both atomic absorption and atomic fluorescence. With the current development of better sources and investigations into devices other than flames for sample introduction, further improvements in atomic fluorescence spectroscopy are to be expected. 

Although the presently realised continuum-source AA spectrometers are still operated sequentially for multi-element detection, it may be anticipated that, with use of suitable optics and multi-array detectors, this method wiU become a truly simultaneous multi-element technique [10]. 

Multielement atomic absorption spectroscopy is complicated by the need for a multielement emission source. Some multielement hollow cathode lamps are available and continuum sources are possibilities. Another method is to place several hollow cathode lamps along the focal plane of a spectrometer, at positions corresponding to the desired wavelengths, thus permitting all the radiation to emerge from a single slit. In this approach the... 

In spite of these limitations, continuum source background correction may be used with good accuracy for many analyses. It offers low cost, wide applicability, operation at high frequencies, and little degradation in detection limits or linear dynamic range. It is commonly found in commercial instrumentation alone or with other methods of correction. 

In the mid-1960s, the availability of tunable ultraviolet radiation laboratory sources led to the replacement of electron beams with photon sources. Rare gas resonance lamps producing continuum sources of radiation that could then be passed through monochromators allowed the field of photoionization mass spectrometry to develop. The determination of photoion yield as a function of ionization energy, the photoionization efficiency curve, led to determination of ionization potentials with accuracies exceeding those of electron impact methods. Modern photoionization experiments often utilize laser or synchrotron light sources with narrow bandwidths and may employ collimated molecular beam sources that reduce the effects of... 

---