Chemical generation of arsane and methylarsanes with amine boranes. Potentialities for nonchromatographic speciation of arsenic

High sensitivity atomic fluorescence spectroscopy for the detection of AsIII by selective electrolysis of arsenic on nanoflowers-like Fe/NFE

Modified electrosynthetic sample introduction technique is a reliable means of solving the problem of high sensitivity analysis of trace arsenite. This article attempts to achieve selective electroreduction of AsIII through the construction of electrode surfaces with different structures and materials from the perspective of interface reactions. Among the four transition metal modifiers, the iron modified nickel foam electrode with nano-flower structure documented higher efficiency in inducing arsenic reduction and better species selectivity. Systematic electrochemical and spectroscopic tests suggest that strong adsorption effect between Fe and AsIII, appropriate hydrogen evolution potential, and catalytic activity jointly promote efficient electroreduction of AsIII. Optimization based on electrode materials and electrolysis conditions, with high sensitivity, wide linear range (0.1-50 μg L-1), and excellent species selectivity, this paper offers an efficient and economic sample introduction method for trace AsIII/V selective atomic spectroscopy direct determination.

Development of a Fluorophore with Enhanced Unorthodox Chalcogen Bonding for Highly Sensitive Detection of Trimethyl Arsine Vapor

In this work, we report the design of novel fluorophores that bear three benzothiadiazole and benzoselenadiazole groups, respectively, for sensitive detection of trimethyl arsine vapor. In particular, the fluorophore with the benzoselenadiazole groups can form stronger chalcogen bonding with trimethyl arsine than the fluorophore with the benzothiadiazole groups, which in turn triggers much faster and more sensitive fluorescence responses. On the basis of this novel mechanism, fluorescence detection of trimethyl arsine vapor with rapid response (∼3 s), high sensitivity (the theoretical LOD is 0.44 ppb), and high selectivity is achieved on bundled nanofibers from the fluorophore with the benzoselenadiazole groups. Here, the new fluorescence sensor may find wide applications in health and environmental monitoring, arsenic distribution recognition in soil, and arsenic mines exploration.

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.

Application of direct analysis in real time to the study of chemical vapor generation mechanisms: identification of intermediate hydrolysis products of amine-boranes

In order to elucidate controversial results emerging in chemical vapor generation (CVG) for trace element determination, we conducted a series of experiments devoted to the identification of intermediates formed by acid hydrolysis of amine-boranes. For the first time, direct analysis in real time coupled with high-resolution mass spectrometry (DART-Orbitrap) was applied for detection of this class of compounds. Mass spectra of both solid amine-boranes and their aqueous solutions (pH ~ 8, no hydrolysis) were acquired for understanding their ionization pathway. Mass spectra of aqueous solutions of t-BuNH₂·BH₃ and Me₂NH·BH₃ were acquired under conditions that are employed in CVG (0.017-4.0 mol L^-1 HCl, 0.167-0.2 mol L^-1 borane reagent). The results disclose a reactivity driven by pH of amine-boranes undergoing hydrolysis. At low acidity, the hydrolysis proceeds according to the currently accepted displacement mechanisms (i.e., R₃N·BH₃ + H₃O⁺ → R₃NH⁺ + H₂OBH₃). At higher acidity, N-tert-butyl, cyclotriborazane, and bis(dimethylamino)boronium were identified, for the first time, during the hydrolysis of t-BuNH₂·BH₃ and Me₂NH·BH₃, respectively. Formation of these intermediates was ascribed to a hydrolysis pathway starting with the ionization of the amine-borane, (i.e., R₃N·BH₃ + H₃O⁺ → [(H₂O)R₃NBH₂] ⁺ + H₂). The new evidence explains the anomalous behavior observed in CVG by amine-borane derivatization, and updates the currently accepted mechanisms for the acid hydrolysis of amine-boranes. Graphical Abstract.

Validation and inter-laboratory study of selective hydride generation for fast screening of inorganic arsenic in seafood

It is advisable to monitor and regulate inorganic arsenic (iAs) in food and feedstuff. This work describes an update and validation of a method of selective hydride generation (HG) with inductively coupled plasma mass spectrometry (ICP-MS) for high-throughput screening of iAs content in seafood samples after microwave-assisted extraction with diluted nitric acid and hydrogen peroxide. High concentration of HCl (8 M) for HG along with hydrogen peroxide in samples of a same concentration as used for extraction leads to a selective conversion of iAs to volatile arsine that is released and transported to the detector. A minor contribution from methylarsonate (≈20% to iAs) was found, while HG from dimethylarsinate, trimethylarsine oxide is substantially suppressed (less than 1% to iAs). Methodology was applied to Certified Reference Materials (CRMs) TORT-3, DORM-3, DORM-4, DOLT-4, DOLT-5, PRON-1, SQID-1 and ERM-CE278k, in some of them iAs has been determined for the first time, and to various seaweed samples from a local store. The results were always compared with a reference method and selectivity of iAs determination was evaluated. An inter-laboratory reproducibility was tested by comparative analyses of six fish and four seaweed samples in three European laboratories, with good agreement of the results. The method of HG-ICP-MS is sensitive (limit of detection 2 μg kg^-1 iAs), well suited for screening of large number of samples and selective at iAs concentration levels at which maximum limits are expected to be set into EU legislation for marine samples.

Behavior and kinetic of hydrolysis of amine boranes in acid media employed in chemical vapor generation

The behavior of NaBH₄ (THB) and the amine boranes, NH₃BH₃ (AB), tertbutylNH₂BH₃ (TBAB), Me₂NHBH₃ (DMAB) was investigated in continuous flow chemical vapor generation of H₂Se from aqueous Se^IV coupled with atomic absorption spectrometry. Unexpected higher efficiency of H₂Se generation was obtained with amine boranes compared to THB (TBAB > AB > THB) using millimolar concentration of reductant (0.001-0.1 mol L^-1) under strongly acidic conditions (HCl, HClO₄, H₂SO₄, HNO₃, 0.5-5 mol L^-1 H⁺). Analytical applicability of the CVG system was tested by the determination of Se^IV in natural water samples certified reference materials, using 0.01 mol L^-1 TBAB in 0.5 M H₂SO₄. In order to explain this unexpected higher efficiency of amine boranes with respect of THB, the kinetic of hydrolysis of AB, TBAB and DMAB was investigated in acid media typically employed in chemical vapor generation for trace element determination. The kinetic was investigated by monitoring the rate the hydrogen gas evolved during hydrolysis, using a laboratory made thermostated reaction cell. Kinetics were measured for AB, TBAB and DMAB in 0.1, 0.5, 5 mol L^-1 HCl or HClO₄ reaction media and in 0.1 mol L^-1 cysteine +0.1 mol L^-1 HCl or HClO₄ buffer, for reaction times from 0 to 30 min. Under strongly acidic conditions, the rates of hydrogen evolution produced by amine boranes hydrolysis appear to be much slower than those predicted by a pseudo-first order reaction and using the rate constants reported in the literature. This suggests that, at elevated acidities (5 mol L^-1 HCl or HClO₄), the hydrolysis of amine boranes takes place in two steps, generating a first amount of H₂ (0.67-1.15 mol) much faster than the remaining about 2 mol. This evidence indicates a different mechanism of hydrolysis to the one accepted in the literature for amine boranes. The relatively high efficiencies of H₂Se observed with amine borane reduction of inorganic Se^IV at elevated acidities can be addressed to the action of borane intermediates, most probably amine borane cations, formed during amine borane hydrolysis in the same reaction conditions.