J. Agterdenbos

Determination of traces of inorganic anions by means of high-performance liquid chromatography on zipaxsax columns

Zipax-SAX pellicular beads are used as the anion-exchanger material ; a high-pressure packing technique is described. A Zipax-SAX column (200 × 4.5 mm) is used in a separation system with eluent suppression and conductivity detection as in ion-chromatography. Good separation of chloride, nitrite, bromide, nitrate and sulfate is obtained with 1.4 × 10-3 M succinate or adipate eluents at pH 7. A complete separation takes about 6 min at a flow rate of 3 ml min-1. Detection limits of 2 μg l-1 chloride, 4 μg l-1 nitrate and 10 μg l-1 sulfate can be reached if 2 ml of sample is preconcentrated.

A study of arsenic(III) and arsenic(V) reduction and of arsine decomposition in hydride-generation atomic absorption spectrometry

Abstract The difference in behaviour of arsenic(III) and arsenic(V) in their conversion to arsine by reduction with sodium tetrahydroborate, and the role of the addition of oxygen and hydrogen to the carrier gas in the production of arsenic atoms are described.

Determination of arsenic(III/V) in aqueous samples by neutron activation analysis after sequential coprecipitation with dibenzyldithiocarbamate

Abstract A simple and sensitive method for distinguishing between As (III) and As (V) is presented. It is based on sequential coprecipitation with dibenzyldithiocarbamate (DBDTC) of As (III) and with a mixture of potassium iodide and thiosulphate. The precipitates are collected successively on 0.45-μm membrane filters and arsenic is determined by neutron activation of the filters and subsequent γ-spectrometry. Optimum conditions for the two steps of the sequential coprecipitation technique were determined using 74 As (III) and 74 As (V) tracers. Under the optimized conditions some other heavy metal tracers were applied to study the selectivity of the method; of these, Cd and Fe were quantitatively and Ag and Zn partly recovered. The accuracy of the method was checked by determination of the total arsenic concentration in some water reference materials by applying the second step of the coprecipitation technique. The values found were well within the certified ranges. The applicability of the method to the speciation of arsenic in a fresh water sample is demonstrated. As (III) and As (V) could be determined with relative standard deviations of 3 and 5%, respectively, and a detection limit of 0.02 μg 1 −1 . Investigation of the behaviour of some other arsenic species, e.g., monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA), in the sequential coprecipitation technique showed that neither MMAA nor DMAA is coprecipitated in the first step; in the second step, MMAA is coprecipitated quantitatively whereas DMAA is not coprecipitated at all. This implies that MMAA interferes in the determination of As(V).

Determination of arsenate in aqueous samples by precipitation of the arsenic(V)—molybdate complex with tetraphenylphosphonium chloride and neutron activation analysis or hydride generation atomic absorption spectrometry

Abstract Precipitation of As(V) from aqueous samples is achieved by complexation of As(V) with molybdate followed by formation of an insoluble precipitate with tetraphenylphosphonium chloride (TPP + Cl − ). The selectivity of the method was studied by investigating the behaviour of other arsenic species [As(III), monomethylarsonic acid and dimethylarsinic acid] using 73 As-labelled species. It follows that differentiation between As(V) and the methylated arsenic acids is excellent, but that some As(III) may precipitate. Combination with selective coprecipitation using dibenzyldithio-carbamate for preliminary As(III) removal yields accurate results when used with neutron activation analysis or hydride generation atomic absorption spectrometry. The competition of phosphate with As(V) for complexation with molybdate limits the use to samples with phosphate concentrations −1 . Results for some real water samples are presented. The results of both detection methods are in good agreement.

Study of processes in the hydride generation atomic absorption spectrometry of antimony, arsenic and selenium

Abstract Studies of the decomposition rate of the reducing agent sodium tetrahydroborate in alkaline and acidic media and of the reaction rate of the formation of the hydrides under the usual analytical conditions are described. The stripping of the hydrides with different lengths of the stripping coil, with different amounts of hydrogen in the carrier gas and with sodium hydroxide added during and after the stripping process are discussed. Some evidence for the existence of an intermediate during the decomposition reaction of the sodium tetrahydroborate is given. The role of temperature, hydrogen and oxygen during the atomization of the hydrides in an electrically heated quartz cuvette is discussed. Under certain conditions, antimony atoms form dimers or elemental antimony precipitates in the heated cuvette.

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.

Linear calibration in ion chromatography by calculating total amounts of sample from measured conductivity data

Abstract Calibration graphs in ion chromatography are generally not linear, if a suppressor column and a conductivity detector are employed. The main reason for this is that the dissociation equilibrium of the eluent (which is a weak acid after eluent suppression) is shifted by H + ions from the sample, which are formed in the suppressor column. Therefore a formula is derived in which the suppression of the eluent dissociation by the sample is considered. Application of this formula for chloride, nitrate and sulphate samples in succinate and carbonate eluents results in linear calibration graphs in the range 0–40 mg l − when peak areas are used.

Study of interferences by transition metal ions in hydride generation atomic absorption spectrometry of antimony

Abstract The mechanism of the interference of metal ions in the hydride generation atomic absorption spectrometry (AAS) of antimony was studied. Experiments on the decomposition rate of sodium tetrahydroborate in acidic media were done in the presence of interfering metals, with and without the addition of masking agents. The results were compared with experiments on the sensitivity of AAS in the presence of the same interfering metals with and without the addition of the same masking agents. AAS experiments are described in which the product of the reaction between sodium tetrahydroborate and one of the interfering metals was present during hydride formation. This reaction product was certainly one of the causes of interference, but the contribution of the catalytic decomposition of sodium tetrahydroborate to interference is not clear.

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.