Page 166 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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7.5.2 Cold Vapour Generation.

This applies solely to mercury as it is the only analyte that has an appreciable atomic vapour pressure
at room temperature. The 253.7 nm line is usually used for mercury atomic absorption, but the
transition is spin forbidden, and relatively insensitive. The 184.9 nm line is potentially 20-40 times
more sensitive, but at this wavelength most flame gases and the atmosphere absorb strongly. Thus,
flame methods for mercury are not noted for their sensitivity (typical flame detection limits are in the
range 1-0.1 µg ml ). If chemical reduction is employed, mercury can be brought into the vapour phase
-1
without the need to use a flame, and detection limits are dramatically improved.

Four main methods have been used to bring mercury into the vapour phase:

(i) Reduction-aeration: this is by far the most common of the methods used to bring mercury into the
vapour phase. Mercury in aqueous solution is treated with a reducing agent and then swept out of
solution by bubbling a gas through it. The most typical reducing agent used is tin(II) chloride, although
tin(II) sulphate has also been used. A few workers have used sodium borohydride, but this can pose
safety problems, and the evolution of large amounts of hydrogen increases the quenching if AFS
detection is employed.

(ii) Heating: the sample is pyrolysed and combusted.

(iii) Electrolytic amalgamation: mercury is plated on to a copper cathode during electrolysis. The
cathode is then heated as in (ii), to release the mercury.

(iv) Direct amalgamation: mercury is collected on a silver or gold wire, from which it is released by
heating. This method may be employed with (i) or (ii) as a concentration method. Specialist commercial
instruments have been developed that pyrolyse samples and then trap the mercury evolved on gold.

A system for cold vapour AAS is shown in Fig. 7.3. The evolved mercury vapour is passed to a long
path-length absorption cell, usually constructed of Pyrex glass tubing with silica end windows. A
transient absorption peak is observed. In some systems, a recirculating pump is used to cycle the
mercury vapour around the system and achieve a steady reading.
Such arrangements as in Fig. 7.3 are exceedingly simple and can be built 'in-house'. A hollow-cathode
lamp is used as a source and the cell can be taped to the end of the burner. Air from a small compressor
can be used for aeration. As an enclosed cell is used, problems can arise from water condensation and
spray. The cell can be heated to minimize such