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The mechanisms controlling the generation of PbH 4 by reaction of inorganic Pb(II) with aqueous NaBH 4 were investigated both in the presence and in the absence of the additive K 3 Fe(CN) 6 . For the first time PbH 4 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 NaBH 4 . In the presence of K 3 Fe(CN) 6 , deuterium labelled experiments clearly indicated that the generated PbH 4 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 K 3 Fe(CN) 6 by NaBH 4 , the rate of hydrolysis of NaBH 4 both in the presence and in the absence of K 3 Fe(CN) 6 , and the rate of dihydrogen evolution following NaBH 4 hydrolysis. The effect of delayed addition of Pb(II) to NaBH 4 -HCl- K 3 Fe(CN) 6 , and K 3 Fe(CN) 6 to NaBH 4 -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 K 3 Fe(CN) 6 additive., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

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 [ABSTRACT FROM AUTHOR]

The efficiency of chemical generation of arsanes from inorganic arsenic, monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA), to arsane, AsH3, monomethylarsane, CH3AsH2 (MMA), and dimethylarsane, (CH3)2AsH (DMA), has been investigated in different reaction media with the aim to better elucidate the mechanisms controlling their generation process and to find the experimental conditions to implement a nonchromatographic arsenic speciation analytical method, which is based on the selective determination of some arsenic species. Studies were performed by continuous flow hydride generation coupled with atomic spectrometry (CF-HG-AS), using different reductants such as borane–ammonia (AB), borane-tert-butylamine (TBAB), and sodium tetrahydridoborate (THB) in HCl and HClO4 media, in the presence or absence of l-cysteine (Cys). The efficiency of HG processes for MMA and DMA is mainly controlled by the reactivity of the substrates with the borane, which could be strongly influenced by the formation of ion couples. The protonation of arsane did not play a significant role in the employed reaction system. By taking advantage of the different reactivity pattern of As species in selected generation conditions, DMAA and MMAA could be selectively determined in 0.5 and 10 M HClO4 solutions, respectively, in the presence of Cys, with AB as the reducing agent. The presence of Cys as a masking agent and the peculiar reducing properties of AB ensured a good control of interferences, as far as it has been observed for Co(II), Ni(II), Cu(II), Fe(II), Fe(III). The overall time needed to complete the prereduction step has been verified for MMAA and DMAA at different acidities in order to achieve the best selectivity. The selective determination of DMAA with AB/Cys in HClO4 has been optimized and applied to certified reference materials (CRMs) of natural waters CASS-4, SLRS-4, and NASS-4 (NRCC). The estimation of DMAA concentration allows us to correct the concentration of As(III) for the interference of DMAA in the selective determination of As(III) according to a selective HG method recently reported. [ABSTRACT FROM AUTHOR]

Unknown and controversial aspects related to the mechanisms of hydrolysis of borane complexes and to the mechanisms of chemical vapor generation for trace element determination in strongly acidic media (0.01-10 M HCl) have been investigated and clarified. The overall hydrolysis rates of borane complexes (BH4-, H3N-BH3) in the acidity range of 0.2-10 M HCl were several orders of magnitude lower than those predicted by kinetics laws and obtained in the pH range of 3.8-14. The decomposition of the borane complexes takes place stepwise and proceeds through the formation of hydroboron intermediates, LxBH4-xn (x = 1, 2, 3), where L could be one or more species among the donor groups H2O, NH3, OH-, and Cl- and n is the charge of the hydroboron species (n = 0, +1, -1, depending on L). Some intermediates present surprisingly long lifetimes at elevated acidities and play a key role in determining both the overall hydrolysis rates of borane complexes and the reactivity of Hg(II), As(III), Sb(III), Bi(III), Se(IV), Te(IV), and Sn(IV) in chemical vapor generation for trace element determination. Atomic absorption experiments demonstrated that almost all trihydroboron species (LBH3n, dihydroboron species (L2BH2n, and monohydroboron species (L3BHn) play an active role in the generation of elemental mercury and stibine. Some of these intennediates are inactive or play a marginal role in the generation of arsine, bismuth-me, and hydrogen selenide. Hydrogen telluride is preferentially formed by those hydroboron species, which are stable in strongly acidic conditions, while the same species are unreactive in the generation of stannane. The collected experimental evidence is in agreement with the general reactivity of the elements in chemical vapor generation techniques and, together with other literature data, definitely rule out the hypothesis of "nascent hydrogen" as a possible mechanism of chemical vapor generation by borane complex derivatization. [ABSTRACT FROM AUTHOR]

The behavior of NaBH 4 (THB) and the amine boranes, NH 3 BH 3 (AB), tert butylNH 2 BH 3 (TBAB), Me 2 NHBH 3 (DMAB) was investigated in continuous flow chemical vapor generation of H 2 Se from aqueous Se IV coupled with atomic absorption spectrometry. Unexpected higher efficiency of H 2 Se 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, HClO 4 , H 2 SO 4 , HNO 3 , 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 H 2 SO 4 . 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 HClO 4 reaction media and in 0.1 mol L −1 cysteine +0.1 mol L −1 HCl or HClO 4 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 HClO 4 ), the hydrolysis of amine boranes takes place in two steps, generating a first amount of H 2 (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 H 2 Se 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. [ABSTRACT FROM AUTHOR]

The complex reactivity of the system As-AH-RSH-THB (As = As(III), As(V); AH = HC1, HClO4, CH3COOH; RSH = L-cysteine (Cys); THB = NaBH4) was investigated using continuous flow (CF) hydride generation (HG) coupled either with atomic absorption (AAS) or atomic fluorescence specfrometry (AFS). AsH3 generation was examined in the presence of Cys by varying acidity and the type of acid, the mixing sequence, and the reaction time of reagents. The strong depression of arsane generation, which is typically observed in the range of acidity of 0.2–2 M HCl, can be addressed to the low reaction rate of thiolborane, hydroboron complexes, or both toward those As-(III) substrates that are formed in the same reaction environment. The simultaneous presence of Cys-borane and As(III)-Cys species is at the origin of the gap of the arsane generation efficiency in the 0.2–2 M HCl acidity range. The selective formation of Cys-borane complexes, which are formed faster than As(III)-thiol complexes, can be achieved by a careful choice of the mixing sequence of the reagents. The simultaneous mixing of sample, Cys, and THB is able to reduce substantially the gap of the arsane generation efficiency in the 0.2–2 M HCl acidity range. These properties were employed to implement a simple method for selective determination of As(III) in samples containing inorganic arsenic: (i) Total inorganic arsenic is determined by sample treatment with 0.2 M Cys for 30 min, acidity 0.1 M HCl, followed by CF-HG- AFS; (ii) As(III) is selectively determined in 0.005 M CH3-COOH in the presence of Cys using a chemifold setup allowing the simultaneous mixing of sample, 0.2 M Cys and 0.1 M THB. The selectivity, measured from the ratio between the slopes of calibration graphs As(III)/As(V), is 220. The interference effects of Cu(II), Fe(III), Ni(II), Co(II), Ag(I), Pd(II), and Pt(IV) can be kept under control using the simultaneous mixing of all the reagents. The tolerance toward the interferences was almost the same as that obtained by allowing the formation of As(III)-Cys complexes (offline sample pretreatment with Cys for 30 min). The method was tested with the application to the natural waters and mineral well waters analysis employing CF-HG-AFS. [ABSTRACT FROM AUTHOR]

A systematic study has been developed in order to clarify the mechanism of hydride generation using different borane complexes [sodium tetrahydroborate(III), NaBH4 (THB); borane–ammonia complex, H3B–NH3 (AB); borane–tert-Butylamine complex, H3B–NH2C(CH3)3 (TBAB)], as derivatizing reagents. Stannane, stibine and bismuthine were generated in a continuous flow reaction system at different acidities in the pH range of 1.38–12.7. The pH of sample solution was pre-equilibrated on-line in a mixing loop by the addition of appropriate solution before the reaction with the derivatizing reagent in a reaction loop. The generated hydrides were delivered to a miniature argon hydrogen flame atomizer and free atoms detected by atomic absorption spectrometry (AAS). The effect of pH on the relative sensitivity has been investigated by varying both the mixing loop volume (4, 15 and 50 μl) and reaction loop volume (100 and 500 μl). The mixing rates of the solutions have been also tested to avoid any undesired effect arising from the incomplete mixing of the solution in the flow reaction system. The generation of hydrides using on-line pre-equilibration of pH can be observed also in alkaline or neutral conditions, while the generation of the same hydrides is observed only in acidic solution if the equilibration of pH was performed off-line. Stannane generation using amineboranes has never been reported before. Kinetic calculations were performed in order to estimate the concentration of nascent hydrogen arising from the decomposition of the derivatizing agents in the flow reaction system. It has been found that in many cases, the mechanism of nascent hydrogen failed to explain the generation of the hydrides. The direct action of BH4- and H3B-X species (X=ammonia or amino group) on the analyte element, present in solution in a suitable chemical form, is the only possible mechanism of hydride formation in a wide range of solution acidities, from pH 4.5 up to pH 12.7. The possible role that pH and different acid species, HA, could play in the modification of derivatization reaction is discussed. [Copyright &y& Elsevier]