Page 163 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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generating their hydrides and atomizing these, in either a flame, an electrically heated tube or a plasma.
Initially, the method was applied to arsenic and selenium, both of which pose difficulties in AAS
because of their low-wavelength, primary resonance lines. The hydrides were generated by zinc-
hydrochloric acid reduction and collected in a balloon, before expulsion into an argon-hydrogen
diffusion flame. A better reducing agent is sodium borohydride, because the hydrides are formed more
rapidly and a collection reservoir is not needed. A 1% w/v aqueous solution is usually sufficient,
provided that there is vigorous stirring of the acidified sample. This reagent can be used to generate the
hydrides of antimony, arsenic, bismuth, germanium, lead, selenium, tellurium and tin. The replacement
of inefficient nebulization by gaseous sample transport improves the detection limit of all the elements
except lead (the improvement for germanium is not dramatic), where the limiting factor is presumably
the difficulty in forming plumbane. If the lead is oxidized prior to reduction, improvements in
detectability are also observed with this element.

Arsenic and antimony also give responses which vary according to valence state, the +5 state giving
poorer responses than the +3 state. These elements are therefore reduced in iodide solution or by -
L
cysteine prior to hydride generation. Selenium and tellurium in the +6 state must be reduced to the +4
state prior to hydride generation because the +6 state does not form a hydride. This is normally
achieved by heating with hydrochloric acid. This process is often accelerated by the use of microwave
technology. It must also be noted that many of the arsenic and selenium compounds that occur naturally
in nature do not form hydrides. Examples include arsenobetaine (a very common arsenic-containing
compound in marine organisms) and selenomethionine. These compounds are very stable, even to
attack by concentrated nitric acid, and must be broken down to arsenic and selenium ions by much
harsher conditions, e.g. perchloric acid or by photolysis. Severe underestimates of arsenic and selenium
contents are often obtained because the analyst has failed to destroy completely non-hydride forming
compounds such as these.

Considerable inter-element interference effects have been reported on the actual hydride-forming step.
Elements easily reduced by sodium borohydride (e.g. silver, gold, copper, nickel) give rise to the
greatest suppressions. These interfering ions may be removed by the addition of masking agents that
complex with them.

7.5.1.1 Instrumentation

Hydride generation (HG) systems make use of a gas-liquid separator to separate the gaseous hydrides
from the liquid reagents prior to introduction