Instrumental direct methods, trace analysis

Instrumental Direct Methods. In instrumental direct methods, after sampling and possibly sample preparation, the sample is analyzed directly. Instrumental direct determination methods are usually matrix-dependent relative methods. A mathematical correction of the matrix effect is possible only in particular cases (e.g.. X-ray fluorescence analysis, XRF). To compensate for systematic errors, therefore, standard reference materials are required, which must be very similar in composition to the sample to be analyzed (see Section 7.4.5). However, an optimum power of detection at high accuracy (freedom from systematic error) can be achieved only when the trace to be determined is present in isolated form in the highest possible mass concentration. In many cases of trace analysis, therefore, more complex multistage (compound) methods are required. 

The first (direct reading) method is fairly simple and results are available immediately. However, the instruments have limited sensitivity and must be recalibrated periodically. The second (absorption in a liquid or adsorption on a medium) and third (gas container) methods are generally considered more sensitive and more accurate method for trace analysis by gas chromatographs, infrared... 

However, it must be kept clear in mind that direct instrumental detection methods for trace substances are physically relative methods which require calibration, during which systematic errors, caused for instance by spectral and nonspectral interferences, may occur. Relative methods are in fact matrix-dependent and would require the analysis of Certified Reference Materials (CRMs) in order to guarantee the good quality of the analytical data. Unfortunately, CRMs are not available for polar snow and ice and hence the only way to assure the quality of the data is, whenever possible, to make careful intercomparisons of the techniques able to measure the same analytes with different approaches. 

Particularly in trace analysis, and in the absence of standard samples for calibration purposes, there still is no satisfactory alternative to relying at lea.st initially on wet-chemical multistep procedures. This entails a detour consisting of sample decomposition with subsequent separation and enrichment of the analyte(s) of interest relative to interfering matrix constituents. A suitable form of the analyte(s) is then subjected to the actual determination step, which may ultimately involve one or more of the direct instrumental methods of analysis. 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

Most of our understanding of the marine chemistry of trace metals rests on research done since 1970. Prior to this, the accuracy of concentration measurements was limited by lack of instrumental sensitivity and contamination problems. The latter is a consequence of the ubiquitous presence of metal in the hulls of research vessels, paint, hydrowires, sampling bottles, and laboratories. To surmount these problems, ultra-clean sampling and analysis techniques have been developed. New methods such as anodic stripping voltammetry are providing a means by which concentration measurements can be made directly in seawater and pore waters. Most other methods require the laborious isolation of the trace metals from the sample prior to analysis to eliminate interferences caused by the highly concentrated major ions. 

Analyte is measured at parts per million ( xg/g) to parts per trillion (pg/g) levels. To analyze major constituents, the sample must be diluted to reduce concentrations to the parts per million level. As we saw in the analysis of teeth, trace constituents can be measured directly without preconcentration. The precision of atomic spectroscopy, typically 1-2%, is not as good as that of some wet chemical methods. The equipment is expensive, but widely available. Unknowns, standards, and blanks can be loaded into an autosampler, which is a turntable that automatically rotates each sample into position for analysis. The instrument runs for many hours without human intervention. 

The application of the ASV technique to the analysis of seawater has been reviewed (6-9). Its advantages over other trace-metal methods are several (1) sea salts do not interfere (2) a high degree of sensitivity and selectivity can be achieved through electrolytic preconcentration, thus the analysis can be carried out directly in seawater, and few or no reagents need to be added (3) the measurement is easily automated (4) relatively inexpensive, rugged, and portable instrumentation can be built for field use (5) more than one metal may be measured at one time with a sensitivity approaching parts per trillion. 

A number of very useful and practical element selective detectors are covered, as these have already been interfaced with both HPLC and/or FIA for trace metal analysis and spe-ciation. Some approaches to metal speciation discussed here include HPLC-inductively coupled plasma emission, HPLC-direct current plasma emission, and HPLC-microwave induced plasma emission spectroscopy. Most of the remaining detection devices and approaches covered utilize light as part of the overall detection process. Usually, a distinct derivative of the starting analyte is generated, and that new derivative is then detected in a variety of ways. These include HPLC-photoionization detection, HPLC-photoelectro-chemical detection, HPLC-photoconductivity detection, and HPLC-photolysis-electrochemical detection. Mechanisms, instrumentation, details of interfacing with HPLC, detector operations, as well as specific applications for each HPLC-detector case are presented and discussed. Finally, some suggestions are provided for possible future developments and advances in detection methods and instrumentation for both HPLC and FIA. 

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