J. T. van Elteren

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.

Technetium speciation: non-size effects in size-exclusion chromatography

Abstract The predominant separation technique used in technetium speciation, e.g., in plant material, is size-exclusion chromatography. However, interactions of technetium compounds with the stationary phase, i.e., non-size effects, have been reported. To evaluate these effects, the interaction of three technetium compounds, the pertechnetate anion (TcO4−), the anionic Tc-diethylenetriaminepentaacetate complex (Tc-DTPA), and the cationic [Tc(V)O2(1,4,8,11-tetraazacyclotetradecane)]+ complex (Tc-cyclam) with three different types of size-exclusion packing material, Sephadex G-25, Zorbax GF-250 and HEMA-SEC BIO 1000 has been studied. The effects of the composition of the mobile phase and the column temperature on the distribution coefficients of the three technetium compounds was measured. It is shown that non-size effects play a dominant role in the separation of these technetium compounds.

A dual radiotracer speciation technique with emphasis on probing of artefacts: a case study for technetium and spinach (Spinacia oleracea L.)

Abstract Speciation is required for obtaining information on chemical reactivity, toxicity and bioavailability of an element. In this work, a novel analytical speciation methodology, using a dual radiotracer technique is described. The approach is illustrated with the speciation of 99 Tc , a pure β−-emitter, in spinach plants. The actual leaf concentration of two technetium species, i.e. 99 TcO − 4 and 99 TcX (plant-formed Tc species), is determined as a function of incubation time with 99 TcO − 4 in the substrate. Addition of 95 m TcO − 4 or 99 m TcO − 4 , both containing a γ-emitting radioisotope of technetium, to the substrate allows easy radiotracing, while addition of 99 m TcO − 4 or 95 m TcX yield tracers during the analytical homogenisation procedure makes it possible to correct for procedural losses and redox conversions. For TcO−4 significant procedural losses were observed, while for TcX procedural recovery was nearly quantitative. Significant procedural conversions of TcO−4 or TcX were not observed. Speciation of other elements might benefit from this novel methodology.

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.

Preservation of As(III) and As(V) in some water samples

This paper reports on the behavior of arsenite [As(III)] and arsenate [As(V)] in some water samples at storage under several conditions (pH=2/natural pH, 4°C/20°C). The investigation was carried out using73As as a radiotracer for both forms and with the aid of earlier developed simple speciation methods for differentiation between arsenite and arsenate. Although arsenate is the thermodynamically stable arsenic form, it was observed that arsenate in deionized water is completely converted to the trivalent state; this phenomenon took place in about one week. By monitoring the radioactive As(III) and As(V) over a period of one month in two natural water samples, a fresh water and a sea water sample, it could be concluded that no adsorption occurs on the surface of polyethylene containers, independent of storage conditions. During that period, storage at natural pH values results for both water samples in a gradual oxidation of As(III); the oxidation rate is higher for storage at 20°C. At pH=2 As(III) is fairly stable in fresh water at both storage temperatures. However, in sea water a fast oxidation of As(III) is observed (complete oxidation within 3 d at both temperatures). As(V) is stable at all storage conditions studied.

KINETICS IN A EU(III)-HUMIC ACID SYSTEM BY ISOTOPIC EXCHANGE WITH 152MEU3+AND SIZE EXCLUSION CHROMATOGRAPHY : A FEASIBILITY STUDY

The work presented illustrates the feasibility of using radiotracers with size exclusion chromatography (SEC) as a means of studying the kinetics of metal-ligand interactions in environmental systems. To simulate such systems a pre-equilibrated Eu(III)-humic acid model system was chosen in which 152mEu3+ as a radiotracer was used to study the isotopic exchange between Eu3+ and Eu(III)-humic acid complexation.

Search for plant-induced technetium species with size-exclusion and reversed-phase ion-pair chromatography

In this work an attempt is made to have a closer look at technetium species (TcX) induced by spinach plants, grown on TcO−4 containing nutrient solutions. To this end, size-exclusion chromatography (SEC) and reversed-phase ion-pair chromatography (RP-IPC) were used: The performance was established with model compounds, i.e., anionic Tc-DTPA and cationic Tc-cyclam, used as mimics for TcX. Under the conditions applied, the information retrieved from high performance RP-IPC was minimal due to strong interactions of the technetium species with the Nucleosil C18 stationary phase. However, in low-pressure SEC non-size effects allowed differentiation of TcX into two distinct classes of technetium compounds, i.e., TcX1 and TcX2. From retention behavior in SEC, it was possible to speculate on the chemical properties of these two technetium species.

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.