Vapour Generation – an alternative technique in Atomic Absorption Spectroscopy

Typical-Hydride-forming-elements

Typical Hydride forming elements

Trace metals are routinely analysed in ppm and ppb or even sub- ppb levels using conventional flame and graphite furnace Atomic Absorption Spectroscopy techniques. The vapour generation techniques offer an alternative for low-level determinations.

Cold vapour technique for Mercury analysis

Mercury is an element which has assumed global concern due to its high toxicity. It is a unique metal which exists as liquid at room temperature and this feature helps determine it without the requirement of high temperatures of the flame or graphite furnace. Mercury present in the combined state in the sample is reduced to the metal state by stannous chloride or sodium borohydride in a closed system, stripped from solution and carried by a stream of argon gas through a quartz sample cell positioned in the light path of the atomic absorption spectrometer. For low-level determinations vapour can be first concentrated on a gold coated graphite disk in a graphite furnace before carriage to the sample tube. The technique is limited to mercury as no other element exists in free atomic state at room temperature.

Hydride Vapour generation technique

The technique is similar to the cold vapour technique except that the volatile metal hydride generated requires heating in a flame or furnace to generate atoms in the ground state. Detection limits of some of the elements are improved10 – 100 times in comparison to the flame or furnace analysis.

Comparative detection limits (ppb)

Element  Air- acetylene flame  Graphite furnace  Hydride technique
As 100 0.3 0.01
Bi 0.2 0.02
Sb 30  0.2 0.02
 Se 1.0 0.01
 Sn  0.2  0.04

Metals such asAs,Sb,Sn,Be and Pb form volatile hydrides which can be detected at low concentration levels using this technique. The required oxidation state is crucial and the element should be oxidised to this state prior to hydride formation

Table of oxidation states

Element  Usual state  Required oxidation state
Arsenic As^5^+ As^3^+
Antimony Sb^5^+ Sb^3^+
Selenium Se^5^+ Se^4^+
Mercury Hg^2^+ Hg^2^+
Bismuth Bi^3^+ Bi^3^+

The element is oxidized to the required oxidation state and subsequently reduced by sodium borohydride to form the volatile metal hydride and swept to the quartz sample tube mounted in the flame or furnace for conversion of the metal hydride to the ground state atoms. Ions such as mercury and bismuth which are already in the required oxidation state in the sample do not require the initial oxidation step.

The main advantage of the technique is enhanced sensitivity and freedom from matrix interferences as the element of interest is isolated from all other components of the sample matrix.

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