Page 73 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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the furnace, or deposited a carbide lining on the inner wall (e.g. using lanthanum salts).

The furnace is heated by low voltage (usually 10 V) and high current (up to 500 A) from a well
stabilized step-down transformer. For optimum precision, the voltage should be well stabilized, often by
a feedback loop which may be temperature feed-back based (see Section 3.6.1). A rapid rise-time of the
temperature is also preferable, because of theoretical considerations of peak shapes. This has
implications for power supply design and furnace design, as will be shown below. Currently, furnaces
are available that reach temperatures of up to 3000°C, and temperatures of 2500°C should be reached in
less than 2 s in a well designed furnace.

The furnace is purged with an inert gas, usually nitrogen or argon. Argon, with a small addition of
methane, is also used to provide continuous pyrolytic coating. There are some chemical effects
between nitrogen and certain elements, e.g. titanium, vanadium and barium, (extremely refractory
nitrides are formed) and the rate of diffusion of argon is less. This latter effect means that slightly
larger signals are usually observed in argon. A gas flow that sweeps into the tube and out of the centre
hole has been shown to reduce problems of background scatter. The flow may often be stopped during
atomization to prevent dilution.

The whole atomizer may be water cooled to improve precision and increase the speed of analysis. The
tube is positioned in place of the burner in an atomic absorption spectrometer, so that the light passes
through it. Liquid samples (5-100 mm ) are placed in the furnace, via the injection hole in the centre,
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often using an autosampler but occasionally using a micro-pipette with a disposable, 'dart-like' tip.
Solid samples may also be introduced; in some designs, this may be achieved using special graphite
boats. The sample introduction step is usually the main source of imprecision and may also be a source
of contamination. The precision is improved if an autosampler is used. These samplers have been of
two types: automatic injectors and a type in which the sample was nebulized into the furnace prior to
atomization. This latter type was far less common.

The power supply controlling the furnace can be programmed so as to dry the sample after injection,
ash it at an intermediate temperature (say 500°C) and atomize it (Fig. 3.5). The temperature and
duration of each of these steps can usually be controlled over a wide range. Optimizing the operating
conditions of the furnace (or 'programming the furnace') is a vital step in the development of
analytical methods. In the drying phase, the solvent must be driven off without problems of 'spitting'.
Drying of organic solvents tends to give particular problems, and the ashing conditions are most
critical. It is essential to remove organic matter by pyrolysis and as many volatile components of the
matrix as possible, but to avoid any loss of the analyte, either as the element or as a volatile salt, such as
a halide.