Mercury and Arsenic Atomic Absorption Methods

Mercury Cold Vapor

Free mercury atoms can exist at room temperature can be analysed using atomic absorption without employing flame and graphite furnace techniques at high temperatures. Mercury is reduced in solution using stannous chloride or sodium borohydride in a closed system. The reaction quantitatively releases mercury (from the sample solution) and is carried by a stream of air or argon through a quartz sample sell placed in the light path of an AA instrument for analysis. The detection limit for mercury by this cold vapor technique is approximately 0.02 mg/L.

Hydride Generation

Hydride generation sampling systems are similar to cold vapor mercury systems, except that the hydrides generated must be heated in air/acetylene flame or electrically to create atoms in the free state. Samples are reacted in an external system with a reducing agent,usually sodium borohydride. Gaseous reaction products are then carried to a sampling cell in the light path of an AA spectrometer.

The following metals can be analysed by HGAAS.

As

Bi

Ge

Pb

Sb

Se

Sn

Te

The nebulizer required in AAS is not used in HGAAS. Continuous flow system or batch flow systems can be used.
Acid contents of samples and standards of 10% to 50% are common; this is much much higher than in normal AAS.

The reaction of many metalloid oxyanions with sodium borohydride and HCl produces a volatile hydride: H₂Te, H₂Se, H₃As, H₃Sb, etc. The oxidation state of the metalloid is crucial and care must be taken to produce the specific metalloid oxidation state before the sample is introduced into the hydride generation system.
For example, HGAAS analysis of selenium requires the Se(IV) oxidation state (selenite). Se(VI), the more highly oxidized state of the element (selenate), responds erratically and non reproducibly in the system. All selenium in Se calibration standards and samples must be in the Se(IV) form for analysis. Oxidize all Se in the sample to selenate using conc.nitric acid or hydrogen peroxide (decomposing the excess oxidant) and then reducing the contained selenate to selenite with boiling HCl. After that reduction step, the final acid content is made up to the required content and is immediately introduced into the hydride generation module.
The sodium borohydride and hydrochloric acid reagents feed into the hydride generation reaction vessel must be optimized and may be different for different elements. Example concentrations are 0.35% NaBH₄ and 50% HCl.