Selection of Analytical Lines

Moreover, as the entire radiation passes through the atomizer, any temporal change in the transmittance, for example of the flame gases, is corrected in the same way, a feature that is obviously not available in an optical double-beam system. This function is of particular importance for the measurement and correction of background absorption, which will be treated in detail in Section 5.2. 

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]. 

All these limitations do not exist in HR-CS AAS, firstly because the radiation intensity of the source is, even down to 200 nm, high enough to provide a significantly better SNR than the line sources in conventional LS AAS. In a first approximation the radiation intensity, and hence the SNR for all lines is of the same magnitude, although it degrades somewhat in the far-UV. Secondly, the resolution of the monochromator is such that only about two times the FWHM of the line, or less, is detected by the analytical pixel. As the source emits a continuum, and the analytical pixel is adjusted in such a way that it is always in the line center, all the phenomena that are associated with a line source, sueh as line shift, self-absorption or the presence of other lines emitted by the lamp, are nonexistent. 

Although AAS is predominantly used as a technique for trace analysis, it has also been applied for the determination of main components, such as calcium in cement [21], lead and tin in soft solders [149], or chromium, iron and nickel in stainless steel [16] using 

In HR-CS AAS again these problems are essentially non-existent for the same reasons as given above. Firstly, because of the relatively constant, very intense emission of the primary radiation source, there are no weak lines, i.e. the same high SNR ratio will be obtained on all analytical lines, independent of their spectral origin. The only criteria that will have an influence will be the absorption coefficient and the population of the low excitation level when non-resonance lines are used. Secondly, because of the high resolution of the monochromator, and as the entire spectral environment of the analytical line becomes visible in HR-CS AAS, potential spectral interferences can easily be detected, and in addition cannot influence the actual measurement, except in the rare case of direct line overlap. But even in this case HR-CS AAS provides an appropriate solution, as will be discussed in Section 5.2.3. 

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Although the relative intensities of spectral lines in the ICP differ from those observed in the DC arc and AC spark, the published tables are invaluable for the selection of analyte lines in ICP sources, and the identification of spectral interferences in the spectrometer bandwidths. However, spectral lines are emitted by ICP sources that are not emitted by DC arcs and sparks. In order to facilitate spectral line selection in ICP-AES, numerous spectral line atlases are now available which list the best analytical lines and the potential interferences due to coincidences from major and minor constituents. Simulated... 

The choice of analytical lines and calibrations was dictated primarily by the requirements for quality control of purified rare earth elements. The line pairs selected covered the determination of the two rare earths of lesser and of greater atomic number than the matrix element. Because yttrium normally falls between different pairs of rare earths depending upon specific separation procedures used, calibrations were also established for the determination of yttrium in each of the lanthanide elements. In addition, line pairs were also selected for the determination of all other naturally occurring rare earths in yttrium because this element is often used as a carrier for other reu e earths. 

GC chromatographic retention times were recorded to facilitate identification by retention index data. Chromatographic methods were used to indicate the presence of any impurities in the commercial chemicals. Samples of the reference substances are available on request and the collection of spectra and other information will be made available in printed format and on-line through the Internet. Van Lierop and co-workers [5] give an overview of the work done to establish the reference collection and the spectral atlas, which together will assist enforcement laboratories in the characterisation of plastics and the selection of analytical methods for additives that may migrate. 

When a first column of a very short length (and therefore a low selectivity) is used (this is especially suitable for multiresidue methods), we talk about an on-line precolumn (PC) switching technique coupled to LC (PC-LC or solid-phase extraction (SPE)-LC). This is particulary useful for the enrichment of analytes, and enables a higher sample volume to be injected into the analytical column and a higher sensitivity to be reached. The sample is passed through the precolumn and analytes are retained, while water is eliminated then, by switching the valve, the analytes retained in the precolumn are transferred to the analytical column by the mobile phase, and with not just a fraction, as in the previous cases. 

Depending on the kind of sorbent in the precolumn connected on-line to the analytical column, the retention of analytes in the precolumn may be more or less selective. 

LC-GC is a very powerful analytical technique because of its selectivity and sensitivity in analysing complex mixtures and therefore it has been used extensively to determine trace components in environmental samples (2, 5,77). LC allows preseparation and concentration of the components into compound types, with GC being used to analyse the fractions. The advantages of on-line LC-GC over the off-line System are, first, the less sample which is required and, secondly, that there is less need for laborious sample pretreatment because the method is automated (78). 

Because the two analytical lines differ in wavelength, an internal standard can never compensate absorption and enhancement effects completely. If Cases II and IV %re avoided in selecting ah internal standard, the use of such a standard will usually prove satisfactory. Special cases may require special calibration curves run with the disturbing elements present. 

It is sometimes convenient to use a properly chosen scattered line in the background for comparison to avoid having to add an internal standard to the sample.37 The experience of the Applied Research Laboratories shows that the effects of variations or fluctuations in the equipment and of particle size in powdered samples can be eliminated satisfactorily in this way. In some cases, absorption and enhancement effects are also adequately compensated. We have already mentioned (7.8) that scattered reference line and the analytical line will be subject to considerably different absorption effects if the two lines differ appreciably in wavelength. Everything depends upon the selection of a satisfactory scattered reference line, and this is done empirically. 

Determination of appropriate measuring and analysis methods. Decisions must be made on the selection of appropriate and available measuring and or analytical equipment and tools. The characteristics of the methods must be discussed in terms of specificity, accuracy, precision, sensitivity of the methods, and locations of measuring and/or sampling (off-line, at-line, on-line, in-line, non-invasive). 

In on-line extraction the process is coupled directly ( hyphenated ) to the analytical technique used for further analysis of the extract (either spectroscopy or, more frequently, chromatography, because of the limited selectivity of extraction). Common examples include SFE-GC, SFE-SFC, SFE-HPLC, SFE-FTIR,... 

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