Trace metal techniques characteristics

Table 12.3 Characteristics of some trace metal techniques ...

Table 12.3 Characteristic of some trace, metal technique ...

ICP-MS not only offers extremely low detection limits in the sub parts per trillion (ppt) range, but also enables quantitation at the high parts per million (ppm) level. This unique capability makes the technique very attractive compared to other trace metal techniques such as ETA, which is limited to determinations at the trace level, or FAA and ICP-OES, which are traditionally used for the detection of higher concentrations. In Chapter 1 we will present an overview of ICP-MS and explain how its characteristic low detection capability is achieved. 

Because of the complex nature of most biological samples, a single fractionation technique may not be adequate for the separation of the wide range of molecules present. Better resolution of some molecules is obtainal when properties other than differences in size are exploited. These include differences in ionic characteristics, affinity for other molecules and hydrophobicity. In separations that involve any one or more of these properties, the sample constituents interact with the column material and are then eluted with a suitable eluant. As a consequence of this interaction, and the use of eluants, whose properties may not closely resemble those of the medium found in vivo, the metal may dissociate from the ligand. In addition, as the complexity of the sample increases it is difficult to predict the behaviour of the various constituents. Undesirable effects leading to irreversible interaction between some molecules in the sample and the column packing material, degradation and decomposition of some constituents may result. Furthermore, it may be difficult to rid the column of certain trace metal contamination. 

Inductively coupled plasma emission (ICP) spectroscopy is used to determine the amount of trace metals and silica on a membrane. This technique uses inductively coupled plasma to excite atoms and ions that emit electromagnetic radiation and wavelengths that are characteristic of a particular element. The intensity of the emission is indicative of the concentration of that element in the sample. 

It is well known that the trace metal content of a sediment is often to a large extent a function of its chemical and mineralogical characteristics. It is, therefore, very important to use a reliable normalising technique for reporting trace metal concentrations. The key sediment characteristic is surface area or particle size since many of the trace metal binding components (e.g. organic matter, Fe and Mn oxides and hydroxides) are very well correlated with both characteristics. 

Heated vaporization atomic absorption (HVAA) has been described extensively. HVAA differs from conventional atomic absorption in that an electrically heated device replaces the flame. The characteristics of HVAA are microliter sample consumption, sensitivity down to picogram quantities, and applicability to a wide variety of solutions. These characteristics have led to its widespread use. In petroleum analyses, this technique has been used to determine relatively high levels (ppm) of lead in gasoline, metals in used oils, and nickel and vanadium in crude oils (18). The Trace Metals Project has extended application of this technique to the determination of Be, Cd, Co, Cr, Mn, Mo, Sb, and V at the 10-ng/g level. 

The analysis for operation of the QCM in a liquid is somewhat more complex because of the viscous coupling. The shear mode vibration of an AT crystal is parallel to the surface and induces the adjacent liquid to move because of the no-slip boundary condition expected at the interface. Applications of the QCM to electrochemical systems have advanced rapidly from the first studies [72-74], which used the technique to investigate the electrodeposition of Au, and of trace metals from solutions. Indeed, the sensitivity of the QCM for liquid investigations has been calibrated by electrochemical methods [75-78] and found to be identical to that for gas phase studies. The general characteristics of the QCM in electrochemical... 

H. Zang and W. Davidson. Performance characteristics of the technique of diffusion gradients in thin films (DGT) for the measurement of trace metals in aqueous solution. Anal. Chem., 67 3391-3400,1995. 

The aim of this chapter is to give the reader a flavor of the application potential of ICP-MS, and a better understanding of why it is the fastest-growing trace element technique available today. If there is one conunon theme that runs through many of these applications, it is the unparalleled detection limits the technique has to offer, for both the total and speciated form of the element. When this is combined with its rapid multielement characteristics, isotopic measurement capability, freedom from interferences, and ease of use, it is clear that it is only a matter of time before ICP-MS becomes the dominant technique for trace metal analysis. 

The source unit must vaporize and excite a portion of the sample, which is generally used as one of the electrodes between which the electric discharge takes place. No single excitation source is ideally suited for all applications of emission spectrochemistry. Trace impurities in metals, alloying constituents in high concentrations, biological substances, ceramics, slags, oils, nonconductors, refractories—all may require different excitation techniques and sample preparation procedures. Table 1 summarizes the important characteristics of the commonly used spectrochemical source units. 

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