Analytical Working Range

FIGURE 21.3 Analytical working ranges for major AS techniqnes. 

Sample throughput is typically the number of samples that can be analyzed in a given amount of time. For most techniques, analyses performed at the limit of detection or analyses for which the best precision is required will be more time consuming than the less demanding ones. In cases where this is not the limiting factor, the number of elements to be determined per sample and the analytical technique will determine the sample throughput. Let us take a brief look at the sample throughput capability of each technique. 

As with flame AA, ETA is basically a single-element technique, although multielement instrumentation is available from some vendors. Because of the need to thermally and sometimes chemically pretreat the sample to remove solvent and matrix components prior to atomization, ETA has a relatively low sample throughput. A typical graphite furnace determination normally requires 2-3 min per element per replicate, although multielement systems are capable of achieving up to six elements in the same amount of time. 

ICP-OES is commercially available as either a scanning instrument (elements determined sequentially) or a fixed-channel instrument (elanents determined simultaneously). The simultaneous design is usually faster, but both systems offer exceptional sample throughput capability and can determine up to 20-30 elements in a few minutes. However, when determination of only a few elanents is required, ICP-OES probably is not the best techniqne because of the relatively long read delay times of 60-90 s required to wash-out/wash-in a sample and wait for the signal to reach equilibrium. 

ICP-MS is also a rapid multielement technique. The sample throughput of a quad-rupole-based ICP mass spectrometer, which represents the majority of instruments being used for routine applications, is similar to that of a simultaneous ICP-OES system, and is typically 20-30 elemental determinations in a few minutes, depending on such factors as the concentration levels and the precision required. 

A Comparison of Detection Limits between ICP-MS and the Other AS Instrumentation in pg/L ppb 

Element Flame AA Hg/Hydride CFAA ICP-OES ICP-MS Element Flame AA Hg/Hydride CFAA ICP-OES ICP-MS 3 CfQ 

If no DL data is shown, it means the AS technique is not ideally suited to determine that element. Source Copyright 2003-2007, all rights reserved, PerkinElmer Inc. 

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In non-competitive or reagent excess immunoassays [26], an excess of immu-noreagent (antibody or antigen) is used so that all the analyte forms an immunocom-plex that is further quantified and related to the analyte concentration in the sample. These assays are also known as immunometric assays and their advantages over competitive assays include higher sensitivity, precision, and analyte working range. 

When compared with optical spectrometric techniques of elemental analysis, the techniques based on mass spectrometry provide an increase in sensitivity and in analytical working range of some orders of magnitude. For instance, the detection limits with ICP-MS are three orders of magnitude better than ICP-optical emission spectrometry (ICP-OES). Figure 1.45 shows the maximum sensitivity obtained for the different elements, using an ICP-MS coupling with a quadrupole. 

In this chapter, two aspects of analysis are considered. Firstly, the digestion step of contaminated soil samples is discussed. Secondly, the various techniques of determination of all relevant elements are considered. Important criteria for selecting an analytical technique include detection limits, analytical working range, sample throughput, cost, interferences, ease of use and the availability of proven methodology. These criteria are discussed below for the major atomic spectroscopic techniques. 

Figure 4.4 Analytical working ranges for the major atomic spectroscopic techniques.

Analytical working range 2-3 orders of magnitude (nonlinear)... 

Spectrophotometry is an excellent alternative for the determination of inorganic compounds. It is characterized by a wide analytical working range,... 

The atomic spectroscopic techniques discussed in Chapters 6 and 7 and ICP-MS discussed in Chapters 9 and 10 each have advantages and disadvantages in the determination of elements in real samples. A summary of the capabilities of each technique and a comparison of the techniques is given in Appendix 7.B. Most major instrument companies have selection guides on their websites that compare their atomic spectroscopy instruments by DL, analytical working range, sample throughput, interferences, cost, and so on. 

Graphite furnace applications are well-documented, though not as complete as flame AA. It has exceptional detection limit capabilities but with a limited analytical working range. Sample throughput is less than that of other atomic spectroscopy techniques. Operator skill requirements are much more extensive than for flame AA. 

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