D. Bax

The determination of selenium with hydride generation AAS-IV: Application to coal analysis

Abstract The destruction of coal with a H2SO4-HNO3 mixture was first studied. Quantitative recovery of the selenium requires the application of a cooling system on the destruction apparatus. Interference of the Se determination by NOx formed during the destruction is eliminated by sulphamic acid. The efficiency of the Se(IV) reduction with NaBH4 depends on the composition of the solution. A study of this effect led to the conclusion that the efficiency is increased if a halide anion is present during the reduction step. For practical reasons iodide was used. Results obtained with NBS standards were within the (broad) region certified. Several samples obtained from the Dutch Centre for Coal Specimens with unknown Se content were also analyzed. Se contents between 0.4 and 2.6 mg kg−1 were found, and RSD values varied between 3 and 7 %. The greater part of the RSD was attributed to the sampling and decomposition step. From the RSD values it was concluded that probably the greater part of the selenium is evenly distributed and not concentrated in some particles.

The determination of arsenic with hydride generation AAS. A study of the factors influencing the reactions in the absorption cuvette

Abstract The coexistence of AsH 3 , As and polymers of the type As, was studied by thermodynamic data and experimentally. The most obvious formula for the decomposition reaction is 2AsH 3 → 2As + 3H 2 . However, on thermodynamic grounds as well as on the basis of experiments there are strong objections against such a formula. Description of the decomposition reaction with H radicals as reactants does not agree with the amount of radicals present in, or near, the cuvette. Participation of H or OH radicals as catalysts is likely, however. No arguments have been found against a decomposition reaction formulated as 4AsH 3 +3O 2 → 4As + 6H 2 O. At the relevant temperatures As; molecules are thermodynamically more stable than As atoms. Experimentally it was found that dilution of AsH 3 with a carrier gas is imperative in order to prevent dimerization. When more carrier gas is used, the sample is further diluted and the residence time of the AsH 3 in the cuvette is further decreased. Therefore the sensitivity is decreased. The analytical implications of these investigations and the instrumental and chemical parameters influencing the determination of arsenic by hydride generation AAS are discussed.

Arsenic speciation in aqueous samples using a selective As(III)/As(V) preconcentration in combination with an automatable cryotrapping hydride generation procedure for monomethylarsonic acid and dimethylarsinic acid

Differentiation between As(III) and As(V) is accomplished using earlier developed selective preconcentration methods (carbamate and molybdate mediated (co)precipitation of As(III) and As(V) respectively) follewed by AAS detection of the (co)precipitates. Apart from this, separation of methylated arsenic species is performed by an automatable system comprising a continuous flow hydride generation unit in which monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) are converted into their corresponding volatile methylarsines, monomethylarsine (MMA) and dimethylarsine (DMA) respectively. These species are cryogenically trapped in a Teflon-line stainless stell U-tube packed with a gas chromatographic solid-phase and subsequently separated by selective volatilization. A novel gas drying technique by means of a “Perma Pure” dryer was applied successfully prior to trapping. Detection is by atomic absorption spectrometry (AAS). MMAA and DMAA are determined with absolute limits of detection of 0.2 and 0.5 ng, respectively. Investigation of the behaviour of the methylarsines in the system was conducted with synthesized73As labeled methylated arsenic species. It was found that MMA is taken through the system quantitatively whereas DMA is recovered for about 85%. The opumized system combined with selective As(III)/As(V) preconcentration has been tested out for arsenic speciation of sediment interstitial water from the “Chemiehaven” at Rotterdam. The obtained concentrations are 28.5, 26.8 and 0.60 ng·ml−1 for As(III), As(V) and MMAA, respectively, whereas the DMAA concentration was below 0.16 ng·ml−1.

Radiotracer examination of gas-liquid separators used in arsenic speciation by hydride generation — AAS

Two types of gas-liquid separators for use in on-line hydride generation AAS have been examined: the “classical” one, in which gas and liquid are separated by gravity and that based on diffusion through a pemeable tube. Results are presented for the determination of arsenic species (arsenite, As(III); arsenate, As(V); monomethylarsonic acid, MMAA; dimethylarsinic acid, DMAA). Yield (of hydride generation and gas-liquid separation) and response time are investigated as functions of the experimental variables. It is concluded that in conjunction with a cold trap, only the “classical” gas-liquid separator is satisfactory.