Page 122 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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(up to six orders of magnitude) because of its optical thinness. Because of the high temperature most
samples are completely atomized and the technique suffers from few chemical interferences compared
with the flame. However, the nature of the sample introduction system means that it does suffer from
sample transport effects. Such effects are caused by the sample and standard solutions having
different physical characteristics such as viscosity, surface tension and volatility. This can result in
different nebulization and sample transport efficiencies, and manifest itself as apparent suppression or
enhancement of the analytical signal for the sample compared with the standard. Another type of
interference is caused by a large excess of easily ionizable elements (EIEs), usually the alkali or
alkaline earth elements, in the sample matrix. Unless the plasma operating conditions are carefully
optimized a suppression in the analytical signal can result. It is thought that the EIEs cause a reduction
in the excitation temperature, or cause ambipolar diffusion of the analyte out of the central channel.
These types of interference can be overcome by matrix matching the samples and standards, i.e.
ensuring that the matrix in which the standard is prepared is the same as that of the sample. This is
possible for samples such as sea-water, blood and urine, or a well characterized sample which is
analysed on a routine basis, although for the majority of samples the matrix will be unknown, in which
case the method of standard additions can be used.

It is also possible to use an internal standard to correct for sample transport effects, instrumental drift
and short-term noise, if a simultaneous multi-element detector is used. Simultaneous detection is
necessary because the analyte and internal standard signals must be in-phase for effective correction. If
a sequential instrument is used there will be a time lag between acquisition of the analyte signal and the
internal standard signal, during which time short-term fluctuations in the signals will render the
correction inaccurate, and could even lead to a degradation in precision. The element used as the
internal standard should have similar chemical behaviour as the analyte of interest and the emission line
should have similar excitation energy and should be the same species, i.e. ion or atom line, as the
analyte emission line.

Spectroscopic interferences can also manifest themselves, either as an increase in the continuum
background emission or as line overlap, especially if samples with a complex matrix, or organic
solvents, are analysed. An increase in the continuum background emission can be easily compensated
for by subtraction of the background adjacent to the analytical line. For a sloping background then
measurements must be made on both sides of the line and usually the mean value is subtracted. These
options are summarized in Fig. 4.18. Line overlap is a particular problem when an element, present in
large excess in the matrix, has an emission line close to,