conditions the simple proportionality expressed in equation (8.2) applies. Straightforward calibration
with standards is thus possible. Over seventy elements may be detected and measured by ICP-AES and
in principle could be determined simultaneously by using an instrument fitted with a polychromator
having the appropriate number of channels. Two related approaches to the dispersion and measurement
of emitted radiation are in use.
In the first of these the radiation is dispersed by conventional optics and its spectrum presented in a
conventional format. Figure 8.13 illustrates this. In order to carry out simultaneous measurements on
this type of instrument, each spectroscopic line will require a separate channel. Apart from the high cost
of providing a large number of channels, physical limitations in fitting additional ones become severe
after 35 or 40, and most simultaneous instruments are limited to this number. There is also a potential
technical limitation on the quality of the data produced in these circumstances. If a single sample
injection is used, the instrumental conditions must necessarily represent a compromise for all elements
to be determined, and the best output for individual analytes will not always be obtained. Furthermore,
reprogramming in order to change the portfolio of elements can be slow and expensive.
Many laboratories will require the flexibility to deal with smaller and varying groups of elements. An
alternative approach in widespread use, is to make measurements on a sequential basis, with the
instrumental parameters being optimized in turn by a preprogrammed computer. Using this system, a
group of 10–12 elements can be determined in 2–3 minutes. Notwithstanding this flexibility, in
laboratories where large numbers of samples are routinely analysed for large numbers of elements,
simultaneous instruments capable of analysing for a portfolio of 30–35 elements may well be
employed. This would be typified by analyses in the quality control of potable water.
The second approach is to use Echelle Optics in conjunction with a two-dimensional array of radiation
detectors. Echelle optical systems employ a combination of prism and diffraction grating light
dispersion at right angles in order to improve the resolution of the spectrum. A two-dimensional array
of light-sensitive microchips enables a very wide range of wavelengths, and hence elements, to be
detected and measured simultaneously. The spectrum is displayed in the form of an echellogram and is
illustrated by Figure 8.14. Up to 73 elements may be measured at the same time. The application of
echelle optics in plasma spectrometry is by no means new, but it is their combination with the new
microchip technology which has led to these developments by which the range of simultaneous
operation can be greatly extended. Similarities in the overall pattern measurement techniques can be
seen with regard to the use of diode array detectors in column chromatography (Chapter 7).
Plasma sources are characterized by the great stability of the discharge