Atomic optical spectrometry selectivity

Plasma sources were developed for emission spectrometric analysis in the late-1960s. Commercial inductively coupled and d.c. plasma spectrometers were introduced in the mid-1970s. By comparison with AAS, atomic plasma emission spectroscopy (APES) can achieve simultaneous multi-element measurement, while maintaining a wide dynamic measurement range and high sensitivities and selectivities over background elements. As a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multi-element trace determinations. With several techniques, absolute detection limits are below the ng level. 

Table 8.76 shows the main characteristics of voltammetry. Trace-element analysis by electrochemical methods is attractive due to the low limits of detection that can be achieved at relatively low cost. The advantage of using standard addition as a means of calibration and quantification is that matrix effects in the sample are taken into consideration. Analytical responses in voltammetry sometimes lack the predictability of techniques such as optical spectrometry, mostly because interactions at electrode/solution interfaces can be extremely complex. The role of the electrolyte and additional solutions in voltammetry are crucial. Many determinations are pH dependent, and the electrolyte can increase both the conductivity and selectivity of the solution. Voltammetry offers some advantages over atomic absorption. It allows the determination of an element under different oxidation states (e.g. Fe2+/Fe3+). 

Different analytical techniques such as ICP-OES (optical emission spectrometry with inductively coupled plasma source), XRF (X-ray fluorescence analysis), AAS (atomic absorption spectrometry) with graphite furnace and flame GF-AAS and FAAS, NAA (neutron activation analysis) and others, are employed for the trace analysis of environmental samples. The main features of selected atomic spectrometric techniques (ICP-MS, ICP-OES and AAS) are summarized in Table 9.20.1 The detection ranges and LODs of selected analytical techniques for trace analysis on environmental samples are summarized in Figure 9.15.1... 

This section starts with a discussion of selectivity for the most extended analytical atomic techniques based on optical spectrometry. Then, aspects such as detection limits (DLs), linear ranges, precision, versatility and sample throughput will be presented. The section ends with a brief comparison of the... 

Atomic Fluorescence Spectrometry. A spectroscopic technique related to some of the types mentioned above is atomic fluorescence spectrometry (AFS). Like atomic absorption spectrometry (AAS), AFS requires a light source separate from that of the heated flame cell. This can be provided, as in AAS, by individual (or multielement lamps), or by a continuum source such as xenon arc or by suitable lasers or combination of lasers and dyes. The laser is still pretty much in its infancy but it is likely that future development will cause the laser, and consequently the many spectroscopic instruments to which it can be adapted to, to become increasingly popular. Complete freedom of wavelength selection still remains a problem. Unlike AAS the light source in AFS is not in direct line with the optical path, and therefore, the radiation emitted is a result of excitation by the lamp or laser source. 

Atomic spectroscopic methods are used for the qualitative and quantitative determination of more than 70 elements. Typically, these methods can detect parts-per-million to parts-per-billion amounts, and, in some cases, even smaller concentrations. Atomic spectroscopic methods are, in addition, rapid, convenient, and usually of high selectivity. They can be divided into two groups optical atomic spectrometry and atomic mass spectrometry. ... 

Figure 3 Illustrates the problem faced by the IAEA in the broader context of their trace element laboratory intercomparison program. These data show the reported results of 16 laboratories for measurements of arsenic in the horse kidney intercomparison sample (H-8), based on various versions of atomic absorption spectrometry, optical emission spectrometry, neutron activation analysis, and Induced X-ray emission analysis. The objective of the horse kidney intercomparison was to assess (and refine) analytical methods for the determination of essential and toxic trace elements in this surrogate for human kidney (2). Kidney, as the main target organ which accumulates toxic elements, was of special Interest with respect to cadmium. Horse kidney, which contains similar levels of cadmium to the human kidney cortex, was selected for the development and maintenance of methods having a demonstrated level of quality to assure reliable biological monitoring of this element. Participants were Invited to analyze some 24 additional trace elements, however.

Although originally FIA was conceived as a special technique for delivery of a sample segment into the instrument, the combination of flow injection as a sample pretreatment tool with atomic spectrometry has been shown to be of great potential for enhancing the selectivity and sensitivity of the measurements. Moreover, contamination problems are reduced due to the closed system used, making this interface suitable for ultratrace determination of metal species. Hyphenated techniques such as FIA/ SIA with flame atomic absorption spectrometry, inductively coupled plasma (ICP)-optical emission spectrometry, and ICP-mass spectrometry (MS) have been exploited extensively in recent years. The major attraction of FIA-ICP-MS is its exceptional multi-elemental sensitivity combined with high speed of analysis. In addition, the possibility of... 

Inductively Coupled Plasma. Atomic Fluorescence Spectrometry. Atomic Mass Spectrometry Inductively Coupled Plasma. Chemiluminescence Liquid-Phase. Enzymes Enzyme-Based Electrodes. Fluorescence Instrumentation. Ion-Selective Electrodes Overview. Optical Spectroscopy Detection Devices. Sensors Overview. Voltammetry Overview. 

For atomic emission spectrometry, selectivity is achieved by isolation of the spectral line at the exit. slit of the spectrometer. This puts high demands on the optical quality of the spectral apparatus. In atomic absorption and fluorescence, se-... 

Atomic spectrometric methods of analysis essentially make use of equipment for spectral dispersion to achieve their selectivity. In optical atomic spectrometry, this involves the use of dispersive as well as nondispersive spectrometers, whereas in the case of atomic spectrometry with plasma ion sources mass spectrometric equipment is used. In both cases, suitable data acquisition and processing systems are built into the instruments. 

For determination of the elements, mainly spectrometric techniques are used here. Depending on the kind of element and the expected concentration level, the following methods are applied flame atomic emission spectrometry (flame AES), flame atomic absorption spectrometry (flame AAS), inductively coupled plasma optical emission spectrometry (ICP-OES), electrothermal atomisation (graphite furnace) atomic absorption spectrometry (ETA-AAS), inductively coupled plasma mass spectrometry (ICP-MS), spectrophotometry and segmented flow analysis (SFA). Besides, potentiometry (ion selective electrodes (ISE)) and coulometry will be encountered. In many cases, more than one method is described to determine a component. This provides a reference, as well as an alternative in case of instrumental or analytical problems. 

About 70 elements of the Periodic Table can be determined by optical techniques of atomic spectrometry. AAS techniques are basically considered as single element (particularly so for ETAAS, where the lamp and the atomisation conditions have, as a rule, to be selected individually for each element). This feature determines that the sample throughput in AAS (especially with ETAAS) is comparatively low. 

Since the mid-1960s, a variety of analytical chemistry techniques have been used to characterize obsidian sources and artifacts for provenance research (4, 32-36). The most common of these methods include optical emission spectroscopy (OES), atomic absorption spectroscopy (AAS), particle-induced X-ray emission spectroscopy (PIXE), inductively coupled plasma-mass spectrometry (ICP-MS), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray fluorescence spectroscopy (XRF), and neutron activation analysis (NAA). When selecting a method of analysis for obsidian, one must consider accuracy, precision, cost, promptness of results, existence of comparative data, and availability. Most of the above-mentioned techniques are capable of determining a number of elements, but some of the methods are more labor-intensive, more destructive, and less precise than others. The two methods with the longest and most successful histoty of success for obsidian provenance research are XRF and NAA. 

In both total and sequential dissolutions, the result is a solution containing the components of rocks and soils. This solution is then analyzed by different methods. Mostly, spectroscopic methods are used atomic absorption and emission spectroscopic methods, ultraviolet, atom fluorescence, and x-ray fluorescence spectrometry. Multielement methods (e.g., inductively coupled plasma optical emission spectroscopy) obviously have some advantages. Moreover, elec-troanalytical methods, ion-selective electrodes, and neutron activation analysis can also be applied. Spectroscopic methods can also be combined with mass spectrometry. 

The primary aim of this book is to provide readers interested in solid sample pretreatment with an overview of available techniques for development of this step of the analytical process. The title of the book is intended to reflect that it is mainly concerned with the dissolution or removal of target analytes from solid samples. Once they have selected the technique most closely fitting their intended purpose, readers can obtain a deeper knowledge about the technique of choice in the specialized literature — in fact, providing a thorough description of each of the wide variety of sample pretreatment techniques available at present was obviously outside the scope of a book like this. In fact, only those aspects that can be illustrated with reasonable concision are dealt with specifically in it. For identical reasons, the book does not touch on the subsequent steps of the analytical process. The authors therefore assume that the reader will be acquainted with the general principles of chromatography in its different variants, as well as with those of commonplace molecular optical and electroanalytical techniques, and atomic and mass spectrometries. 

Atomic spectrometric methods of analysis essentially make use of equipment for spectral dispersion so as to isolate the signals of the elements to be determined and to make the full selectivity of the methodology available. In optical atomic spectrometry, this involves the use of dispersive as well as of non-dispersive spectrometers. The radiation from the spectrochemical radiation sources or the radiation which has passed through the atom reservoir is then imaged into an optical spectrometer. In the case of atomic spectrometry, when using a plasma as an ion source, mass spectrometric equipment is required so as to separate the ions of the different analytes according to their mass to charge ratio. In both cases suitable data acquisition and data treatment systems need to be provided with the instruments as well. 

Fig. 2.3. Absorbance as a function of optical density for selected shock tube investigations employing OH electronic absorption spectrometry. The unmarked curve represents the semi-empirical relationship derived in Reference 37, evaluated at a pressure (5 1 atm) and temperature (1520 K) typical of recombination experiments in an argon diluent. Tlie curves labelled 6 1, 3 1 and 1 3 were empirically determined over a selected range of recombination pressures and temperatures for mixtures dilute in argon with those particular initial H2/O2 ratios (Reference 32). The curve identified by HJ (Reference 24) was empirically determined in a 1 % Hg-l % 02-98 % Ar mixture at 1300 K for a selected range of pressures. The cross-hatched area represents the approximate range of absorbances and optical densities observed with an atomic bismuth line source (Reference 41). Also shown are the line HH derived from photographic spectroscopy using instrumental definition of absorption line centres on a continuum (Reference 48), and a solid circle (beyond the range of the abscissa) denoting the photoelectric absorbance reported in Reference 47 for a continuum source at an optical density of 750 x 10" moles liter cm.

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