Hydride generation analyte oxidation state

Analytical. Arsenic oxidation state determinations were per-formed by hydride generation-flame atomic absorption spectroscopy (AAS) at the University of Arizona Analytical Center. The analytical procedures are discussed in Brown, et al. (12). 

Fiydride generation (and cold-vapor) techniques significantly improve atomic absorption spectrometry (AAS) concentration detection limits while offering several advantages (1) separation of the analyte from the matrix is achieved which invariably leads to improved accuracy of determination (2) preconcentration is easily implemented (3) simple chemical speciation may be discerned in many cases and (4) the procedures are amenable to automation. Disadvantages with the approach that are frequently cited include interferences from concomitant elements (notably transition metals), pH effects, oxidation state influences (which may be advantageously used for speciation) and gas-phase atomization interferences (mutual effect from other hydrides). 

The theory behind hydride generation is beyond the scope of this text, but in brief, if a sample is mixed with a reducing agent, such as sodium tetrahy-droborate, then some elements, e.g. As, Bi, Ge, Pb, Se, Sb, Sn and Te, will form hydrides. It must be noted that the efficiency of hydride formation depends on the oxidation state of the analyte. Arsenic(III)... 

Multiple hydride-forming elements can be determined simultaneously by this process. However, as previously mentioned, the hydride generation reaction is critically dependent on the oxidation state or form of the analyte element. For example, a prereductant, such as KI, must be added to samples containing As, prior to the hydride reduction step, to convert it to As " ", which is the active species for the hydride formation reaction. Without this prereducing step,As will be missed in a total As determination. 

Chemical modifications can be achieved by the addition of chemical reagents to the solvent in the transport line before the sample has been injected. The analyte elements undergo reactions with the reagents during the time that they are transported to the nebulizer, the transport tube acting as the chemical reaction vessel. The length of the transport line and the flow rate of the carrier liquid control the time of the reaction. After the reaction is completed and before the sample is introduced into the plasma, separations can be carried out on the analyte species. Two examples of this process include the separation of volatile species formed by a hydride generation reaction by a gas-liquid separator and the separation of complexed analyte species by a continuous liquid—liquid solvent extraction.This procedure has been effectively used for speciation measurements to isolate a specific form or oxidation state of the analyte prior to elemental analysis. 

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