Power of detection

Table 8.31 Power of detection of optical atomic emission spectrometric methods...

The power of detection of any atomic spectrometric method of analysis is conveniently expressed as the lower limit of detection (l.o.d) of the element of interest. The l.o.d. is derived from the smallest measure x which can be accepted with confidence as genuine and is not suspected to be only an accidentally high value of the blank measure. The value ofx at the 99.7% confidence level (so called 3s level) is given by... 

Figure 9.2 Power of detecting and measuring concentration range of inorganic mass spectrometric methods, (j. S. Becker and H.J. Dietze, Int. J. Mass Spectrom., 228, /27 (2000). Reproduced by permission of Elsevier.)...

The reason for this reticent attitude stems from the specific culture in which analytical chemistry grew into a major scientific discipline and as a consequence of the development of another distinct activity, namely chemical analysis. In its diversity of approaches, analytical chemistry is complex, it contains a variety of different techniques, some of them more reliable and accurate than others. As a discipline, analytical chemistry cannot be treated as a collection of general, simple, absolute or dogmatic concepts. It is an immensely practical subject. Its driving force is power of detection, reliability (traceability could be a tool for achieving this) and efficiency and cost [5],... 

Hydride and cold-vapour techniques represent a special combination of chemical separation and pre-enrichment with AAS determination, resulting in higher powers of detection for elements with volatile hydrides, eg, As, Bi, Se, Sb, Hg. Recent literature on vapour generation has been reviewed by Hill et al. (1991). Some examples of the use of hydride generation for the analysis of plant material are given by Muse et al. (1989), Leuka et al. (1990) and Ainsworth and Cooke (1990). Hydride generation can also be used with ICP-EAS (see below) and applications have been reviewed (Nakahara, 1991). 

When designing instruments for atomic spectrometry the central aim is to realize fully the figures of merit of the methods. They include the power of detection and its relationship to the precision, the freedom from spectral interferences causing systematic errors and the price/performance ratio, these being the driving forces in the improvement of spectrochemical methods (Fig. 7). 

Here, up represents the noise of the photoelectrons. When the photon flux is n, Up x VW up, is the dark current noise of the photomultiplier and is proportional to the dark current itself, up is the flicker noise of the source and is proportional to the signal and uA is the amplifier noise resulting from electronic components. The last contribution can usually be neglected, whereas up is low for very stable sources (e.g., glow discharges) or can be compensated for by simultaneous line and background measurements. As up, x Ip, one should use detectors with low dark current, then the photon noise of the source limits the power of detection. 

In many cases it is not the background signal from the source or the measurement system but blank contributions that limit the power of detection, the limiting standard deviation is often the standard deviation of the blank measurements and this value must be included in Eq. (139) [44]. From the calibration function the detection limit then is obtained as ... 

Thirdly, it has to be considered that it is often possible, e.g. through selection of the appropriate gas flows, to realize a long residence time in the plasma. This favors volatilization and dissociation of the analyte. Both of these effects enable the high absolute power of detection that can usually be achieved with electrothermal vaporization to be reached, as compared with other sample introduction techniques. 

Electrothermal atomization, because of its high analyte vapor generation efficiency (in theory 100%), allows it to obtain extremely high absolute as well as concentration power of detection with any type of atomic spectrometry. In the case of two-... 

Transport phenomena occur particularly when transporting the vapors themselves. They disappear completely when the sample is inserted directly into the signal generation source, where it is evaporated thermally. This approach is known from work with graphite or metal probes in atomic absorption, where for example W wire cups and loops are used. The technique is also used in plasma spectrometry with the inductively coupled plasma (ICP), both in atomic emission [189-191] and in mass spectrometry [192]. Its absolute power of detection is extremely high and the technique can be used both for the analysis of dry solution residues as well as for the volatilization of microamounts of solids. 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

In order to obtain a maximum power of detection, the atomization efficiency should be as high as possible. Therefore, an optimization of the form of the spray chamber and also of the nebulizer gas flow is required. Furthermore, the primary radiation should be well selected by the monochromator and the amount of non-absorbed radiation reaching the detector should be minimized by selection of the appropriate observation zone with the aid of a suitable illumination system. 

The high absolute power of detection of electrothermal AAS is due to the fact that the sample is completely atomized and brought in the vapor phase as well as to the fact that the free atoms are kept in the atom reservoir for a long time. The signals obtained are transient, as discussed earlier. 

For the determination of traces and ultratraces of Hg, As, Se, Te, As and Bi the formation of the volatile mercury vapor or of the volatile hydrides of the appropriate elements is often used, respectively. This allows a high sampling efficiency to be achieved and accordingly a high power of detection. The absorption measure-... 

Coherent forward scattering (CFS) atomic spectrometry is a multielement method. The instrumentation required is simple and consists of the same components as a Zeeman AAS system. As the spectra contain only some resonance lines, a spectrometer with just a low spectral resolution is required. The detection limits depend considerably on the primary source and on the atom reservoir used. When using a xenon lamp as the primary source, multielement determinations can be performed but the power of detection will be low as the spectral radiances are low as compared with those of a hollow cathode lamp. By using high-intensity laser sources the intensities of the signals and accordingly the power of detection can be considerably improved. Indeed, both Ip(k) and Iy(k) are proportional to Io(k). When furnaces are used as the atomizers typical detection limits in the case of a xenon arc are Cd 4, Pb 0.9, T11.5, Fe 2.5 and Zn 50 ng [309]. They are considerably higher than in furnace AAS. 

Trace and ultratrace determinations are now very important for chemicals. For solid chemicals dissolution again may put limitations on the power of detection achievable due to contamination during the dissolution procedure. Graphite furnace atomic absorption together with plasma mass spectrometry are now of great importance for the analyzing acids used e.g. in the treatment of surfaces in microelectronic components. 

There is some difference in the standard deviations when line and background intensities are measured simultaneously, as in the case of photographic detection or with modern CCD spectrometers, or when they are measured sequentially as done in slew scanning systems. Indeed, in the first case the fluctuations of line intensities and background intensities for a considerable part are correlated, especially at low signal levels and thus partially cancel. This may lead to a considerable gain in power of detection. 

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