Condensation cascades and methylgroup transfer reactions during the formation of arsane, methyl- and dimethylarsane by aqueous borohydride and (methyl) arsenates

Unveiling the mechanisms behind the chemical vapor generation of plumbane for trace analysis of lead

The mechanisms controlling the generation of PbH₄ by reaction of inorganic Pb(II) with aqueous NaBH₄ were investigated both in the presence and in the absence of the additive K₃Fe(CN)₆. For the first time PbH₄ has been identified in analytical chemical vapor generation (CVG) by using gas chromatographic mass spectrometry (GC-MS), which allows the use of deuterium labelled experiments. In the absence of the additive, under reaction conditions typically employed for trace lead determination by CVG, Pb(II) is converted to solid species and no volatile lead species can be detected by either atomic or mass spectrometry for Pb(II) concentration up to 100 mg L^-1. In alkaline conditions Pb(II) substrates are unreactive towards NaBH₄. In the presence of K₃Fe(CN)₆, deuterium labelled experiments clearly indicated that the generated PbH₄ is formed by the direct transfer of hydride from borane to lead atoms. Kinetic experiments were carried out to evaluate the rate of reduction of K₃Fe(CN)₆ by NaBH₄, the rate of hydrolysis of NaBH₄ both in the presence and in the absence of K₃Fe(CN)₆, and the rate of dihydrogen evolution following NaBH₄ hydrolysis. The effect of delayed addition of Pb(II) to NaBH₄-HCl- K₃Fe(CN)₆, and K₃Fe(CN)₆ to NaBH₄-HCl-Pb(II) reaction mixtures on the efficiency of plumbane generation was investigated by continuous flow CVG coupled with atomic fluorescence spectrometry. The collected evidences, complemented with thermodynamic considerations and literature data, have made it possible to clarify long-standing controversial aspects related to the mechanism of plumbane generation and the role of K₃Fe(CN)₆ additive.

A mass spectrometric study of hydride generated arsenic species identified by direct analysis in real time (DART) following cryotrapping

Hydride generation (HG) coupled to cryotrapping was employed to introduce, separately and with high selectivity, four gaseous arsanes into a direct analysis in real time source for high-resolution mass spectrometry (DART-HR-MS). The arsanes, i.e., arsane (AsH₃), methylarsane (CH₃AsH₂), dimethylarsane ((CH₃)₂AsH), and trimethylarsane ((CH₃)₃As), were formed under HG conditions that were close to those typically used for analytical purposes. Arsenic containing ion species formed during ambient ionization in the DART were examined both in the positive and negative ion modes. It was clearly demonstrated that numerous arsenic ion species originated in the DART source that did not accurately reflect their origin. Pronounced oxidation, hydride abstraction, methyl group(s) loss, and formation of oligomer ions complicate the identification of the original species in both modes of detection, leading to potential misinterpretation. Suitability of the use of the DART source for identification of arsenic species in multiphase reaction systems comprising HG is discussed.

Application of direct analysis in real time to study chemical vapor generation mechanisms: reduction of dimethylarsinic(V) acid with aqueous NaBH4 under non-analytical conditions

The aqueous-phase reaction of dimethylarsinc acid (DMAs(V)) with NaBH₄ (THB) was studied under non-analytical conditions (1000 μg/mL As, 0.1 M HCl, 1% NaBH₄) with the aim of identifying intermediates and reaction products. The use of direct analysis in real time (DART) with high-resolution mass spectrometry (HRMS), in combination with two different chemical vapor generation systems, allowed the identification of some species not detected by GC-MS such as Me₂As-AsMe-AsMe₂ and the arsonium species [Me₃As-AsMe₂]⁺ and [Me₂As-AsMe₂-AsMe₂]⁺. Many other methylated species of arsenic containing up to four arsenic atoms have been observed. Unfortunately, the oxidation mechanism that took place in the DART source interfered with the identification of some of those species formed in solution following THB reduction. The species identified by DART-HRMS, together with those previously identified by GC-MS (Me₂AsH, Me₂AsOH, Me₃As, Me₃AsO, Me₂AsAsMeH, Me₂AsAsMe₂, and Me₂As-O-AsMe₂)' enabled the formulation of hypotheses on the possible reaction pathways and revealed an aqueous-phase reactivity of DMAs(V) which could not be explained on the basis of current knowledge. Graphical Abstract.

Inorganic Arsenic Determination in Food: A Review of Analytical Proposals and Quality Assessment Over the Last Six Years

Here we review recent developments in analytical proposals for the assessment of inorganic arsenic (iAs) content in food products. Interest in the determination of iAs in products for human consumption such as food commodities, wine, and seaweed among others is fueled by the wide recognition of its toxic effects on humans, even at low concentrations. Currently, the need for robust and reliable analytical methods is recognized by various international safety and health agencies, and by organizations in charge of establishing acceptable tolerance levels of iAs in food. This review summarizes the state of the art of analytical methods while highlighting tools for the assessment of quality assessment of the results, such as the production and evaluation of certified reference materials (CRMs) and the availability of specific proficiency testing (PT) programmes. Because the number of studies dedicated to the subject of this review has increased considerably over recent years, the sources consulted and cited here are limited to those from 2010 to the end of 2015.

The binomial distribution of hydrogen and deuterium in arsanes, diarsanes, and triarsanes generated from As(III)/[BH(n)D(4-n)]- and the effect of trace amounts of Rh(III) ions

Recent studies of the formation of arsane in the borohydride/arsenate reaction demonstrate the occurrence of condensation cascades whereby small quantities of di- and triarsanes are formed. In this study, the isotopic composition of these di- and triarsanes was examined using deuterium labelled borohydrides. A statistical model was employed to construct the mass spectra of all diarsane and triarsane isotopologues (As(2)H(n)D(4-n) and As(3)H(n)D(5-n)) from the mass spectra of isotopically pure compounds (As(2)H(4), As(2)D(4), As(3)H(5), and As(3)D(5)). Subsequent deconvolution of the experimental mixed spectra shows that incorporation of hydrogen closely follows the binomial distribution, in accord with arsane formation. The H/D distribution in arsane, diarsane, and triarsane isotopologues is binomial in the absence of any interference. However, this is significantly altered by the presence of some transition metals; presented here, for the first time, are the effects of Rh(III). The presence of Rh(III) in the As(III)/[BD(4)](-) system entails the incorporation of hydrogen into the arsanes arising from the solvent, altering the expected binomial H/D distribution.