Multi-element-specific detector

Fig. 2.4. Chromatogram of a solution containing arsenite, arsenate, methylarsonic acid, dimethylarsinic acid, phenylarsonic acid, selenite, and phosphate recorded with an ARL 34000 simultaneous inductively coupled argon plasma emission spectrometer as the multi-element-specific detector [Hamilton PRP-1 resin-based reverse-phase column, Waters Associates Inc. Model 6000A high pressure liquid chromatograph, 0.1 ml injected flow rate 1.5 ml min", mobile phases 0.002 M aqueous HTAB at pH 9.6 to 250 sec, 99/1 (v/v) H2O/CH3COOH 250-1100 sec, 90/10 (v/v) H20/dimethylformamide 1100-1700 sec. ICP As 189.0 nm, P 241.9 nm, Se 203.9 nm, integration time 5 sec]. Redrawn from Spectrochimica Acta [11] by permission of Pergamon Press and the authors.

The second major environmental application of FFF has been the use of an element-specific detector, usually in series with a UV detector, to provide elemental composition data along with the PSD. Graphite-furnace atomic absorption spectrometry has been used off-line on fractions collected from the FFF run. However, the multi-element detection, low detection limits and capability to function as an online detector have made inductively coupled plasma mass spectrometry (ICP-MS) the ideal detector for FFE85-86 The sample introduction system of the ICP-MS is able to efficiently transport micron-sized particles into the high-temperature plasma,... 

The association of a spectrometer with a liquid chromatograph is usually to aid in structure elucidation or the confirmation of substance identity. The association of an atomic absorption spectrometer with the liquid chromatograph, however, is usually to detect specific metal and semi-metallic compounds at high sensitivity. The AAS is highly element-specific, more so than the electrochemical detector however, a flame atomic absorption spectrometer is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed, then multi-element detection is possible as already discussed. Such devices, used as a LC detector, are normally very expensive. It follows that most LC/AAS combinations involve the use of a flame atomic absorption spectrometer or an atomic spectrometer fitted with a graphite furnace. In addition in most applications, the spectrometer is set to monitor one element only, throughout the total chromatographic separation. 

The dispersed light is focused on an outlet slit. One or more outlet slits can be used to select the wavelengths to be transmitted to the detector while the others are suppressed. When several outlet slits and detectors are used in the same spectrometry, the system is termed polychromatic. In this case, each outlet slit is aligned to an emission line for a specific element and can be used for simultaneous multi-element analysis. 

The single comparator method, discussed by Simonits et al. (1975), is often a reasonable compromise between the absolute method and multi-comparator method. In this method, the specific activities of the elements of interest are measured relative to a comparator. In future irradiations, only the comparator is measured and is used to calculate the specific activities of the other elements. This technique can work to advantage when frequent irradiations are required and when the irradiation positions and detector geometries can be closely duplicated. Massart and Hoste (1968) found the single comparator method was advantageous for the determination of rare earths which were separated from each other and counted on Nal(Tl) detectors. This method has little advantage where one is measuring rare earths which are not separated from each other since only one mixed rare earth standard needs to be irradiated. If multi-element standards are irradiated each time, the experimenter has the confidence that differences in reactor operation conditions, which are often out of his control, have not yielded differences in relative specific activities that could lead to errors. 

Chromatography has as its basis the transformation of a complex multi-component sample into a time-resolved, separated analyte stream, usually observed in the analog differential signal mode. The chromatographic sample is thus distinctive in that analytes are changing in nature and with time. An essential feature of chromatographic instrumentation is a detector for qualitative and quantitative determination of the components resolved by the column this should respond immediately and predictably to the presence of solute in the mobile phase. An important class of solute properly detectors are those giving Selective , or Specific information on the eluates. Spectral property detectors such as the mass spectrometer, the infrared spectrophotometer and the atomic emission spectrometer fall into this class. Such detectors may be element selective , structure or functionality selective or property selective . 

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