Atomic dynamic range

The MaxEnt valence density for L-alanine has been calculated targeting the model structure factor phases as well as the amplitudes (the space group of the structure is acentric, Phlih). The core density has been kept fixed to a superposition of atomic core densities for those runs which used a NUP distribution m(x), the latter was computed from a superposition of atomic valence-shell monopoles. Both core and valence monopole functions are those of Clementi [47], localised by Stewart [48] a discussion of the core/valence partitioning of the density, and details about this kind of calculation, may be found elsewhere [49], The dynamic range of the L-alanine model... 

Brenner et al. [ 169] applied inductively coupled plasma atomic emission spectrometry to the determination of calcium (and sulfate) in brines. The principal advantage of the technique was that it avoided tedious matrix matching of calibration standards when sulfate was determined indirectly by flame techniques. It also avoided time-consuming sample handling when the samples were processed by the gravimetric method. The detection limit was 70 ig/l and a linear dynamic range of 1 g/1 was obtained for sulfate. 

The potential for the employment of plasma emission spectrometry is enormous and it is finding use in almost every field where trace element analysis is carried out. Some seventy elements, including most metals and some non-metals, such as phosphorus and carbon, may be determined individually or in parallel. As many as thirty or more elements may be determined on the same sample. Table 8.4 is illustrative of elements which may be analysed and compares detection limits for plasma emission with those for ICP-MS and atomic absorption. Rocks, soils, waters and biological tissue are typical of samples to which the method may be applied. In geochemistry, and in quality control of potable waters and pollution studies in general, the multi-element capability and wide (105) dynamic range of the method are of great value. Plasma emission spectrometry is well established as a routine method of analysis in these areas. 

Dodson has used molecular dynamics to study atom-surface dynamics for silicon atom energies ranging from 10 to lOOeV incident on a silicon (111) surface . In this study a modified form of the Tersoff potential was... 

Non-linear concentration/response relationships are as common in pesticide residue analysis as in analytical chemistry in general. Although linear approximations have traditionally been helpful the complexity of physical phenomena is a prime reason that the limits of usefulness of such an approximation are frequently exceeded. In fact, it should be regarded the rule rather than the exception that calibration problems cannot be handled satisfactorily by linear relationships particularly as the dynamic range of analytical methods is fully exploited. This is true of principles as diverse as atomic absorption spectrometry (U. X-ray fluorescence spectrometry ( ), radio-immunoassays (3), electron capture detection (4) and many more. 

In a flame, as the concentration of atoms increases, the possibility increases that photons emitted by excited atoms in the hot region in the centre will collide with atoms in the cooler outer region of the flame, and thus be absorbed. This self-absorption effect contributes to the characteristic curvature of atomic emission calibration curves towards the concentration axis, as illustrated in Fig. 4.4. The inductively coupled plasma (ICP) tends to be hotter in the outer regions compared with the centre—a property known as optical thinness—so very little self-absorption occurs, even at high atom concentrations. For this reason, curvature of the calibration curve does not occur until very high atom concentrations are reached, which results in a much greater linear dynamic range. 

In the past, much atomic emission work has been performed on atomic absorption instruments which use a flame as the excitation source. However, these have been surpassed by instruments which utilise a high-temperature plasma as the excitation source, owing to their high sensitivity and increased linear dynamic range. 

Although AFS combines, for some elements, the advantages of the large linear dynamic range typical of atomic emission techniques and the high selectivity of atomic absorption, the method has so far not found widespread use. 

Figure 14.7—Characteristics of a hollow cathode lamp (element As). Because the dynamic range in atomic absorption is narrow (two decades), the choice of a second wavelength of weaker absorption avoids the need to dilute a solution when an element is present in high concentration.

The ionization cross section is a constitutive property and is the sum of the atomic cross sections. Knowing the elemental composition of a gas enables prediction of a response. Hydrogen is the carrier gas used. The detector is universal and capable of an MDL of 3x10 lg/see. It has the advantage of a wide linear dynamic range of 10 1. Some exception to the linear dynamic range and predictability of response has been made and certainly rests in the diversity of experimental conditions. 

Most important features of atomic spectrometry are the element specific detection and the superior sensitivity. Features such as the large dynamic range, the relative freedom from matrix effects even when atomic spectrometry is coupled to chromatography can be used more extensively to save time and to earn more accurate data using coupling techniques. 

The calibration of atomic spectrometers can be handled much easier than that of conventional IC detectors using the large dynamic range of ICP techniques. Those simple off-line calibrations had been used for ICP-AES and ICP-MS in on-line preconcentration applications. With its ability to decide between isotopes the ICP-MS is well suited for isotope dilution analysis (IDMS), a calibration tool which increases the accuracy, the results and saves time due to reduced calibration work. The use of IDMS in combination with on-line coupling methods allows a significant speedup of the usually to IDMS applied time consuming separation processes. 

XRF offers a unique approach for rapid, non-destructive elemental analysis of liquids, powders, and solids. Although the first row transition elements are the most sensitive, elements from atomic number 12 (magnesium) and greater can be measured over a dynamic range from trace (ppm) to major (percent) element concentrations. EDXRF is well suited for qualitative elemental identification of unique samples, while WDXRF excels at high precision quantitative analysis. 

In another review, Magee and Honig [24] discuss three important aspects of depth profiling by SIMS depth resolution, dynamic range and sensitivity. First, the depth resolution is a measure of the profile quality. They point out that the depth resolution is limited by atomic mixing effects and the flatness of the sputtered crater within the analyzed area. Second, the dynamic range of depth profiles is limited by crater edge... 

The performance of analytical methods based on atomic spectrometry is characterized by Pgures of merit, such as LoDs and limits of quantiPcation (LoQs), linear dynamic range, and precision and accuracy of measurements. 

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