30 AnalyticaZ A tomic Spectroscopy 1.6 VAPOUR GENERATION 1.6.1 Hydride Generation A novel method for generating ASH, has been dscribed by Rigin and Verkhoturov (1608). They used electrolytic reduction in KOH medium using platinum electrodes. Improvements in sensitivities for the determination of As and Sb havc been achicved by collecting the liberated hydrides in a liquid-N, cold trap before introduction to the atomizer (942, 1258, 1291).Vijan and Wood (295) have improved the detection limit for determination of Pb to 0.1 ng ml-1 by the addition of H,O, to the samples. They suggest that the improved sensi- tivity is brought about by the oxidation of Pb to a metastable tetravalent state. The current trend is to the use of ekctrically-heated silica tube furnaces, rather than flame and flame-heated silica tube atomizers, for the decomposition of volatile metalPart I : Fundamentals and Znsttumentation 31 hydrides.An electrically-heated furnace has been described where the silica tube is sup- ported by two ceramic holders, which contain the heating elements (307). This design permits rapid replacement of the silica tube alone. The growing popularity of electrically- heated furnaces is also reflected by the fact that one is now available from an instrument manufacturer (702, 1259, 1451).Other references of interest - Interferences: 215, 949, 961, 1175, 1635. Non-dispersive AFS detection: 1069. Se recovery: 927. 1.6.2 Mercury Determination Some minor modifications to the Hg gencration system have been described: (i) a sample decomposition train that allows samples containing up to 50% organic material to be analyzed (304); (ii) the use of a sintered-glass bubbler in an Hg generation vessel-the lig is collected by absorption on the sintered glass before removal - to double the sensi- tivity of the method (353); (iii) a dual-channel Hg generation system to incrcase the sample throughput rate (59).The addition of ammonia solution to samples containing chloride was claimed to give a significant improvement in the AAS signal (859). Thorburn Burns et al. have demon- strated that an alkaline Sn(II)/Cd reduction method improves recoveries and precision in the analysis of organo-Hg compounds (1030). Other references of interest - Sample preparation: 832, 1375.