Application of direct analysis in real time to study chemical vapor generation mechanisms: reduction of dimethylarsinic(
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RESEARCH PAPER
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 Enea Pagliano 1 & Massimo Onor 2 & Zoltan Mester 1 & Alessandro D’Ulivo 2 Received: 10 June 2020 / Revised: 1 August 2020 / Accepted: 18 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract 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 DARTHRMS, 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. Keywords Chemical vapor generation . Tetrahydridoborate(1-) . Arsenic . Mechanisms . Mass spectrometry
Introduction Chemical vapor generation (CVG) of volatile arsanes by reaction of inorganic As(III, V) and methylated As(V) acids with aqueous boranes (MBH4, M = Na, K; R3NBH3, R = H, alkyl), besides its analytical relevance for trace arsenic determination and speciation [1–12], represents an useful tool for the investigation of the mechanisms controlling aqueousphase CVG [13–16]. The good stability of arsane and methylated arsanes, combined with the use of mass spectrometry and deuterated reagents, allowed the identification of the mechanism of hydrogen transfer [13, 15, 17, 18] and reaction intermediates [13, 16, 17], taking place in CVG under various reaction conditions. The present state of knowledge can be outlined with the aids of Schemes 1 and 2. In general, an As(V) substrate, * Alessandro D’Ulivo [email protected] 1
National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada
2
C.N.R, Institute of Chemistry of Organometallic Compounds, S.S. of Pisa, Via. G Moruzzi, 1, 56124 Pisa, Italy
R2AsO(OH) (R = OH, Me), is converted to the corresponding arsanes, R2AsH (R = H, Me), by the stepwise, direct transfer of hydride from borane to arsenic substrate (Scheme 1). The hydride transfer takes place through the formation of analyteborane complexes, ABC, between the analytical substrate and the borane or a hydridoboron
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