H.A. Das

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.

The separation of 99mTc(Sn) EHDP complexes by HPLC and GPC

For the characterization of the multi-component bone-scan agent 99mTc(Sn)EHDP we have analysed the complex mixture with reversed phase ion pair chromatography (IPC) and soft gel permeation chromatography (GPC). With IPC five major complexes were found within a separation-time of 40 min. To avoid decomposition of the complexes during separation, the concentrations of EHDP and the reductant Sn(II) in the eluent had to be identical to the EHDP and Sn(II) concentrations used for the preparation of the complexes. To investigate the stability of the complexes we applied separation by IPC, followed by re-analysis of the fractions within 1 h and after 21-25 h. It appears that there is a state of equilibrium between the five complexes. Within 1 h after isolation the separated complexes were still more than 90% in their original form while after 20 h considerable amounts of the other complexes are found. The two complexes with the largest retention time with IPC are the most stable ones. When the total mixture was re-analysed after 26 h all five components appeared to be still present, but the relative amount of the most stable component had increased. Using GPC for the separation of the complex mixture, we found four major peaks within a separation time of 14 h. The elution orders of the complexes with the two separation methods are opposite.

A system for high performance liquid chromatography of 99mTc-compounds with on-line radiometric detection and data processing

Abstract A system for HPLC with on-line γ activity (two channels) and u.v.-detection is described, including software for the on-line data processing with a microcomputer. It was developed for the analysis of 99 m Tc-diphosphonates. Example of some of its features are given using the separation of 99 m Tc and 113 Sn labeled Tc(Sn)EHDP complexes with ion-pair-chromatography.

Solid-Phase Extraction of As(III) from Aqueous Samples Using On-Column Formation of As(III)-Trispyrrolidinedithiocarbamate

Abstract Two methods are presented for the selective preconcentration of As(III) by using complexation-type stationary phases. These phases are prepared: (1) by loading a C18-bonded silica with the cetyltrimethylammonium-pyrrolidinedithiocarbamate (CTA+-PDC−) ion-pair, or (2) by converting a strong anion-exchange (SAX) resin from the quaternary ammonium form into the pyrrolidinedithiocarbamate (PDC−) form. When As(III)-containing solutions are passed through such stationary phases, the immobilized PDC− yields the non-polar As(III)-trispyrrolidinedithiocarbamate (As(PDC)3) which is retained on the non-loaded second part of the C18 cartridge (method 1), or on an unloaded C18 cartridge coupled in series with the SAX cartridge (method 2). For both methods parameters influencing the preconcentration have been investigated by using the 74As(III) radiotracer; 74As(V) was applied to study the selectivity of both methods. A selective preconcentration of As(III) is possible. The retention mechanisms are discussed. ...

Determination of the apparent diffusion constants in water containing granular solids

Abstract Diffusion of inert radiotracers in a miniaturized system can be used to measure the apparent diffusion constants of trace constituents through water containing granular solids. The diffusion behaviour of reactive microconstituents can be interpreted in terms of the free volume and an extra retardation due to interaction.

The effect of the reduction method on the composition of 99mTc-EHDP complexes

The composition of 99mTc-ethane-1-hydroxy-1, 1-diphosphonate complexes, prepared with the reducing agent NaBH4 and by electrolytic reduction, was analysed by reversed-phase ion-pair chromatography and soft-gel permeation chromatography. The results were compared with corresponding data for complexes prepared with the traditional reducing agent tin(II). Significant differences between the chromatograms of the three preparations were found. They were interpreted in terms of the occurrence of different complexes in the three preparations.

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.

Improvement of the reproducibility of ion-pair HPLC of 99mTc(Sn)EHDP complexes and the influence of the Sn(II) concentration on the composition of the reaction mixture.

The use of a non-volatile modifier and the simultaneous exclusion of oxygen by flushing with helium in ion-pair HPLC of 99mTc(Sn)EHDP-complexes prevents spurious oxidation during the separation process. It thus enables the omission of the reducing agent from the eluent solution and the determination of the labelling percentage from the TcO4(1)-peak. In addition, retention times become more reproducible.