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Like all spectroscopic techniques, AES suffers from some interferences, e.g. spectral interferences. Such interferences for AES can be classified into two main categories, i.e. spectral overlap and matrix effects. Spectral interferences are probably the most well known and best understood. The usual remedy to alleviate a spectral interference is to either increase the resolution of the spectrometer or to select an alternative spectral emission line.
11.3.3 Inductively Coupled Plasma-Mass Spectrometry
Inductively coupled plasma-mass spectrometry combines the benefits of the ICP with mass spectrometry. The major instrumental development required in order to establish ICP-MS was the efficient coupling of an ICP, operating at atmospheric pressure, with a mass spectrometer, which operates under high vacuum (Figure 11.16). The development of a suitable interface held the key to the establishment of the technique. The only instrumental alteration is in how the analyte is observed. In AES, the ICP torch is positioned vertically, so that emission can (normally) be observed at right angles by the (atomic emission) spectrometer. In MS, the ICP torch is positioned horizontally, so that ions can be extracted from the ICP directly into the mass spectrometer (Figure 11.17). As a consequence of this horizontal positioning of the ICP torch in relation to the mass spectrometer, all species that enter the plasma are transferred into the MS unit.
How would you introduce a sample into an ICP-MS system?
The sample introduction devices used for ICP-AES can be similarly used
202 Methods for Environmental Trace Analysis
Figure 11.16 Layout of a commercially available inductively coupled plasma-mass spectrometry (ICP-MS) system. From applications literature published by VG Elemental. Reproduced by permission of Thermo Elemental, Winsford, Cheshire, UK.
Figure 11.17 Schematic diagram of the inductively coupled plasma/mass spectrometer interface. From Dean, J. R., Atomic Absorption and Plasma Spectroscopy, ACOL Series, 2nd Edn, Wiley, Chichester, UK, 1997. Reproduced with permission of the University of Greenwich.
The quadrupole mass spectrometer acts as a filter, transmitting ions with a preselected mass/charge ratio. The transmitted ions are then detected with a channel electron multiplier. ICP-MS can be operated in two distinctly different modes, i.e. with the mass filter transmitting only one mass/charge ratio, or where the DC and RF values are changed continuously. The former would allow single-ion
Instrumental Techniques for Trace Analysis
monitoring (SIM), while the latter allows multi-element analysis. This permits the analyst to carry out fast repetitive analyses of a pre-determined set of elements.
Quadrupole mass analysers are capable of only unit mass resolution, i.e. they can observe integral values of the mass/charge ratio only (204, 205, 206, etc.). This limited resolution leads to interferences. The type of interferences can be broadly classified, according to their origin, into isobaric, molecular and matrix-dependent. Just as in AES, some overlap of elements can occur (isobaric), and just as in the other case, such information is well documented. However, other types of interference can occur, with these being the result of the acid(s) used to prepare the sample and/or the argon plasma gas (polyatomics). In addition, the formation of oxide, hydroxide and doubly charged species is possible. Finally, the occurrence of matrix-interferences results in signal enhancement or depression with respect to atomic mass.
11.3.4 Other Techniques
Anodic stripping voltammetry (ASV) is an electroanalytical technique used for the analysis of trace metals in solution. The apparatus for this consists of three electrodes located in an electrolytic cell. These electrodes are a working electrode, e.g. a mercury-drop, a reference electrode and a counter electrode. Sample is placed in the cell, together with a supporting electrolyte, e.g. 0.1 moll-1 acetate buffer at pH 4.5. Dissolved oxygen is removed from the solution by bubbling nitrogen or argon through the cell. By holding the working electrode at a small negative potential (with respect to the reference electrode), the metal ions in solution are attracted to the electrode and deposited. This process is aided by stirring of the solution. By careful control of the deposition time and stirring rate, the amount of metal deposited is proportional to its original concentration. After a specified time, the working electrode potential is slowly changed to become less negative (in the positive direction). At specific potentials, the deposited metal on the surface of the working electrode is oxidized, and hence returned to the solution. This process is monitored by plotting the current change between the working electrode and counter electrode against the potential. The resultant voltammogram (Figure 11.18) can be used to determine the concentration of metals, e.g. lead, copper and zinc, in solution at trace levels.