AA Mercury cold vapor and Hydride generation
Atomic absorbtion: Mecury and Asenic
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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 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.
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
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.
Signature: Dhanlal De Lloyd, Chem. Dept, The University of The West Indies, St. Augustine campus
The Republic of Trinidad and Tobago.
Copyright: delloyd2000© All rights reserved.