Lead analytical line

The situation is clearly different in the case of Pb determination, in particular at the most sensitive analytical line at 217.001 nm, as shown in Figure 4.19a and b. Lead requires an atomization temperature of at least 1700°C, and the phosphates are already significantly volatilized under these conditions, resulting in a pronounced molecular absorption. Fortunately, in the case of lead, the atomic absorption pulse is well resolved from the molecular structures, both in wavelength and in time, as shown in Figure 4.19c. There are no molecular structures... 

With quartz fibers it is easier to lead the optical emission into the spectral apparatus, however, it should be mentioned that the transmittance decreases seriously below 220 nm. This may give rise to detector noise limitations for analytical lines at lower wavelengths. 

Furthermore, the radiant density of the D2 lamp in a large part of the spectrum is fairly low. Hence, the procedure limits the number of analytical lines which can be used and the number of elements that can be determined. As the spectral radiance of the D2 lamp is generally low as compared with that of a hollow cathode lamp, the latter must be operated at a low radiant output (low current), which means that detector noise limitations and poor detection limits are soon encoun-terd. Finally, as work is carried out with two primary radiation sources, which are difficult to align as they have to pass through the same zone of the atom reservoir, this may lead to further systematic errors. 

Fig. 12 Trend lines for composting. The composting lead analytes were water, propanol,...

It is important in AA measurements that the emission line width coming from the radiation source is narrower than the absorption line width of the atoms studied. In principle, a high resolution monochromator is not needed to separate the analyte line from the other lines of the spectrum, but in practice, the spectral bandpass of the source should be equal or less than the absorption line width. Otherwise, artificially low absorbance values are obtained leading to reductions in sensitivity. In the AA technique the use of continuum sources (quartz-halogen filament lamps and deuterium and xenon arc lamps) with reasonably priced monochromators is not satisfactory. This is demonstrated in Figure 17. In the case of (A) the emission of radiation is continuous for the whole spectral bandwidth. The energy absorbed by the atoms of the analyte is small in comparison to the whole... 

Usually, in LS AAS, the most sensitive analytical line is used for the determination of an element, because AAS is mostly applied for trace and ultra-trace analysis, which obviously requires the highest sensitivity. Another reason for using the most sensitive line is that it makes it possible to apply higher dilution in the case of complex sample matrices, and hence to avoid potential interferences. On occasions, however, the most sensitive line is not recommended in LS AAS, as it does not provide the best SNR, as in the case of the 217.001 nm lead line. Another reason not to recommend use of the most sensitive line might be a strongly non-linear working curve due to the presence of other lines in the lamp spectmm that cannot be excluded even with a 0.2 nm bandwidth, as in the case of the 340.725 nm cobalt, the 232.003 nm nickel, or the 244.791 nm palladium line [150]. 

The presence of a concomitant element can lead to additional absorption lines within the recorded spectral interval. Again, they are only of interest when they directly overlap with the analyte line and cannot be separated in time in the case of GF measurements. 

Silva et al. [131] investigated the determination of lead in soil and sediment slurries with ruthenium permanent modifier using LS AAS and HR-CS AAS, and comparing the analytical lines at 217.001 nm and 283.306 nm. It is well known that the former line provides about two-times higher sensitivity, but it is rarely used in conventional LS AAS because of its inferior SNR, its shorter linear working range, and its higher susceptibility... 

The emission spectrum from a hollow cathode lamp includes, besides emission lines for the analyte, additional emission lines for impurities present in the metallic cathode and the filler gas. These additional lines serve as a potential source of stray radiation that may lead to an instrumental deviation from Beer s law. Normally the monochromator s slit width is set as wide as possible, improving the throughput of radiation, while being narrow enough to eliminate this source of stray radiation. 

Most aroma chemicals are relatively high boiling (80—160°C at 0.4 kPa = 3 mm Hg) Hquids and therefore are subject to purification by vacuum distillation. Because small amounts of decomposition may lead to unacceptable odor contamination, thermal stabiUty of products and by-products is an issue. Important advances have been made in distillation techniques and equipment to allow routine production of 5000 kg or larger batches of various products. In order to make optimal use of equipment and to standardize conditions for distillations and reactions, computer control has been instituted. This is particulady well suited to the multipurpose batch operations encountered in most aroma chemical plants. In some instances, on-line analytical capabihty is being developed to work in conjunction with computer controls. 

For the purposes of analytical chemistry, four kinds of monochromatic beams need to be considered. (The quotation marks are to remind the reader that the beams under discussion are not always truly monochromatic.) Three kinds of beams—those produced by Bragg reflection (4.9), filtered beams (4.6), beams in which characteristic lines predominate over a background that can be neglected— will be discussed later ( 6.2). The fourth kind of beam contains monochromatic x-rays that are a by-product of our atomic age and that promise to grow in importance they are given off by radioactive isotopes. These x-rays must not be confused with 7-rays (11.1), which also originate from radioactive atoms but which differ from x-rays because the transformation that leads to radiation involves the nucleus. 

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