Simultaneous multielement determinations

Most commercial AAS systems have the monochromator, optics, and detector designed for the measurement of one wavelength at a time they are single-element instruments. There are a few systems available that do perform multielement determinations simultaneously, using an Echelle spectrometer (discussed in Chapter 2) and a bank of HCLs all focused on the atomizer. The limitation to this approach is not the sources or the spectrometer or the detector, but the atomizer. The atomizer can only be at one set of conditions, and those conditions will not necessarily be optimum for all of the elements being measured. There will be a tradeoff in detection limits for some of the elements. 

This analytical method, based on TXRF, enables a large number of trace elements to be determined simultaneously. The range is suitable for different areas of the sea. The motivation to use TXRF resulted mainly from the characteristic features of the method its high detection power, its universal calibration curve, which eliminates the need for matrix-dependent standard samples or standard-addition procedures, the simple preparation of the sample films, and of course the possibility of multielement determination. 

Depth profile of elements in seawater near hydrothermal vents. [From T. Akagi and H. Haraguchi, Simultaneous Multielement Determination of Trace Metals Using W mL of Seawater by Inductively Coupled Plasma Atomic Emission Spectrometry with Gallium Coprecipitation and Microsampling Technique Anal. Chem. 1990, 62.81.]... 

Metals can be conveniently determined by emission spectroscopy using inductively coupled plasma (ICP). A great advantage of ICP emission spectroscopy as applied to environmental analysis is that several metals can be determined simultaneously by this method. Thus, multielement analysis of unknown samples can be performed rapidly by this technique. Another advantage is that, unlike atomic absorption spectroscopy, the chemical interference in this method is very low. Chemical interferences are generally attributed to the formation of molecular compounds (from the atoms) as well as to ionization and thermochemical effects. The principle of the ICP method is described below. 

Multielement determination (sequential or simultaneous) faster analysis time minimal chemical interaction detection limits and sensitivity fall in between that of flame and graphite furnace measurements. 

Because different elements have different spectrochemical properties, optimum analytical conditions may vary from element to element (69,70). Since all elements are determined simultaneously with an image detector spectrometer, compromise analytical conditions must be employed. Brost, et al. (71 ) have described a response parameter which can be used to determine the optimum compromise analytical conditions. Because the optimum compromise analytical conditions for a given determination depend on the expected analytical concentrations of the elements present in the sample, meaningful multielement detection limits cannot be reported without reference to a particular sample type. The many reported multielement detection limits which appear in the literature simply indicate the detection limits obtained under a particular arbitrary set of conditions, and do not necessarily represent the detection limits obtainable under optimum analytical conditions for a particular sample type. Thus the detection limits achieved in an actual multielement determination are more often likely to be compromise-limited rather than instrument-limited. 

Figure 7 illustrates the usefulness of this optical arrangement. Radiation from a multielement hollow cathode lamp containing Mn and Cr was allowed to fall on one fiber-optic strand. Radiation from a second hollow cathode lamp containing Li was incident on a second fiber optic strand. Individual and composite spectra are shown in the figure. With this optical system, lithium can be determined simultaneously with Cr and Mn by atomic emission, or lithium could be used as an internal standard for the analysis. To do this with a conventional one-dimensional dispersive system would require a wavelength window from 403 nm to 671 nm, resulting in poor resolution. 

Simultaneous Multielement Determinations by Atomic Absorption and Atomic Emission with a Computerized Echelle Spectrometer/Imaging Detector System... 

EUTRON ACTIVATION ANALYSIS IS A VERY SENSITIVE TECHNIQUE for trace element determinations in various samples. If there are no elements that mutually interfere, the purely instrumental version of this method is often chosen for its established advantages such as accuracy, speed, sensitivity, simultaneous multielement determination, and sample preservation (1). For these reasons, instrumental neutron activation analysis (INAA) was applied to samples taken from a series of metal-working residues excavated at Tel Dan, Israel, from 1985 to 1986. 

Inductively coupled plasma-atomic emission spectrometry was investigated for simultaneous multielement determinations in human urine. Emission intensities of constant, added amounts of internal reference elements were used to compensate for variations in nebulization efficiency. Spectral background and stray-light contributions were measured, and their effects were eliminated with a minicomputer-con-trolled background correction scheme. Analyte concentrations were determined by the method of additions and by reference to analytical calibration curves. Internal reference and background correction techniques provided significant improvements in accuracy. However, with the simple sample preparation procedure that was used, lack of sufficient detecting power prevented quantitative determination of normal levels of many trace elements in urine. 

Simultaneous multielement determinations using plasma sources have gained in popularity. Such determinations make it possible to form correlations and to reach conclusions that were impossible with single-element determinations. For example, trace metal determinations can aid in determining the origins of petroleum products found in oil spills or in identifying sources of pollution. 

In a system for coherent forward scattering, the radiation of a primary source is led through the atom reservoir (a flame or a furnace), across which a magnetic field is applied. When the atom reservoir is placed between crossed polarizers scattered signals for the atomic species occur on a zero-background. When a line source such as a hollow cathode lamp or a laser is used, determinations of the respective elements can be performed. In the case of a continuous source, such as a xenon lamp, and a multichannel spectrometer simultaneous multielement determinations can also be performed. The method is known as coherent forward scattering atomic spectrometry [309, 310]. This approach has become particularly interesting since flexible multichannel diode array spectrometers have became available. 

Broekaert J. A. C. (1976) Application of hollow cathode excitation coupled to vidicon detection to the simultaneous multielement determination of toxic elements in airborne dust -... 

We have used ICP-AES with a SIT detector for the simultaneous multielement determination of trace elements in 100 uL volumes of dilute whole blood, blood serum and urine samples. The analyte solutions were ultrasonically nebulized at a rate of 1.2 ml/min. to achieve high nebulization efficiency without... 

Plasma emission spectrometry, especially ICP- and DCP-sources, has a fixed place in modern trace element analysis. In spite of the relatively small number of relevant elements detectable by these techniques for the biomedical and environmental fields of application, plasma emission spectrometry can deliver a lot of possibly important information. The main advantages are the multielement character of the technique (sequentially or simultaneously), nearly chemical interference free measurements, control of physical interferences, a relatively high level of accuracy and precision, high specificity, fast multielement determinations (especially in case of a simultaneous device), low sample consumption and in general a wide range of detectable elements (Table 13). 

In many cases, ACSV has been utilized as a singleelement method. However, a number of ligands allow the simultaneous determination of several elements. For example, catechol allows the determination of Cu, Fe, V, and U in a single measurement, with well-separated reduction peaks. Furthermore, multielement ACSV methods for HMDE have been developed recently, whereby with the use of a mixture of ACSV ligands up to six trace metals (Cu, Pb, Cd, Ni, Co, and Zn) can be determined simultaneously in coastal and estuarine waters. 

Although various attempts have been made over the past three decades to simultaneously determine a number of elements using AAS, the strong point of this technique is the selective and specific determination of one element at a time. This obviously does not exclude a rapid change from one element to the next, i.e., a sequential multielement determination. 

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