Introduction

Chemical vapor generation (CVG) of gaseous hydrides and volatile metal species by aqueous boranes, MBH4 (M = Na, K), and amine-boranes is one of the most powerful sample introduction techniques for ultra-trace determination of some elements (Ge, Sn, Pb, As, Sb, Bi, Se, Te, Hg, Zn, Hg, Cu, etc.) by atomic and mass spectrometry [1,2,3]. In recent years, several investigations have been devoted to CVG systems using both NaBH4 (THB) and amine-boranes, NH3·BH3 (AB), t-BuNH2·BH3 (TBAB), Me2NH·BH3 (DMAB), and Me3N·BH3 (TMAB), in order to clarify unknown and controversial aspects related to their reactivity, in particular the mechanism of hydrolysis and hydrogen transfer [4,5,6,7,8,9].

In a recent study on CVG of H2Se using different aqueous boranes, we found some anomalous results compared with that foreseen by the literature [10]. It was observed that the efficiency of generation of H2Se increases in the order TBAB > AB > THB; this is an inverse correlation in comparison with CVG of SbH3 and Hg0 where the generation efficiency increases in the order THB > AB > TBAB [3]. Moreover, in CVG of SbH3 and Hg0, there is a good linear correlation between log ki and log CR, where ki is the second-order rate constants of the kinetic equation d [R3N·BH3]/dt = − ki [R3N·BH3] [H3O+] of the hydrolysis of amine-boranes:

$$ {R}_3\mathrm{N}\cdotp {\mathrm{BH}}_3+{\mathrm{H}}_3{\mathrm{O}}^{+}\to {R}_3{\mathrm{NH}}^{+}+\mathrm{B}{\left(\mathrm{OH}\right)}_3+{3\mathrm{H}}_2\ \left(R=\mathrm{alkyl},\mathrm{H}\right) $$
(1)

and CR is the concentration of borane giving 50% generation efficiency [3].

Also, it was found [10] that the rate of hydrogen generation by the acid hydrolysis of amine-boranes (AB, TBAB, and DMAB) at acidities in the range from 0.01 to 5 mol L−1 H+ was much slower than predicted using the rate constants of hydrolysis of amine-borane reported in the literature [11,12,13]. Furthermore, at elevated acidities (> 3 mol L−1 H+), hydrogen was evolved in two distinct steps, the first being the faster one and leading to the release of about one of the three moles of hydrogen generated after complete hydrolysis [10], confirming a previous observation on AB [14].

The anomalous results obtained for CVG of H2Se ([H+] > 0.5 mol L−1) and for the acid hydrolysis of amine-boranes ([H+] > 0.1 mol L−1) were ascribed to the occurrence of a reaction pathway for reaction 1 that starts with the loss of molecular hydrogen [10]:

$$ {R}_3\mathrm{N}\cdotp {\mathrm{BH}}_3+{\mathrm{H}}_3{\mathrm{O}}^{+}\to {\left[\left({\mathrm{H}}_2\mathrm{O}\right){\mathrm{R}}_3{\mathrm{NBH}}_2\right]}^{+}+{\mathrm{H}}_2\ \left(R=\mathrm{alkyl},\mathrm{H}\right) $$
(2)

The ionization mechanism starting with reaction 2 differs from that involved in the currently accepted mechanism of hydrolysis of amine-boranes [11,12,13], which starts with the displacement of protonated amine:

$$ {R}_3\mathrm{N}\cdotp {\mathrm{BH}}_3+{\mathrm{H}}_3{\mathrm{O}}^{+}\to \left({\mathrm{H}}_2\mathrm{O}\right){\mathrm{BH}}_3+{\mathrm{R}}_3{\mathrm{NH}}^{+}\ \left(R=\mathrm{alkyl},\mathrm{H}\right) $$
(3)

The difference between reactions 2 and 3, which are only the first step of reaction 1, has important implications for the understanding of the mechanisms that operate in CVG. In most of the cases, the hydrolysis products of the borane reagent are the effective derivatization species generating the volatile molecule employed in the analytical determination [14].

However, the identification of borane intermediates generated during experiments lasting a few minutes, in aqueous solutions and in the presence of gas bubbles due to hydrogen evolution, is not a trivial task using spectrometric techniques such as MS, NMR, or FTIR. In this study, we applied, for the first time, the direct analysis in real time coupled with high-resolution mass spectrometry (DART-Orbitrap) to the identification of boranes species formed during the hydrolysis of amine-boranes at different acidities. Despite some limitations emerged during these studies, DART-Orbitrap experiments allow the collection of evidences that, together with those previously obtained on the rate of hydrogen evolution [10], represent a useful support to the understanding of the reactivity of amine-boranes in CVG system. Furthermore, these results reveal new aspects of the reactivity of aqueous amine-boranes not covered by the current literature.

Experimental

Borane tert-butylamine (TBAB) and borane dimethylamine (DMAB) complexes (assay 97%) were purchased from Sigma-Aldrich. Solutions of 0.2 mol L−1 TBAB and 1.0 mol L−1 DMAB were prepared in deionized water and were stable for several days at room temperature. Solutions of 0.1, 0.5, and 5.0 mol L−1 HCl were prepared from analytical grade 37% HCl.

Mass spectrometric identification of reaction intermediates was performed by using a direct analysis in real time (DART) ion source (IonSense, Saugus, MA, USA), which was positioned 17 mm away from the orifice of a LTQ linear ion trap-Orbitrap mass spectrometer working in positive ion mode (Thermo Fisher Scientific Inc., Bremen, Germany). The mass spectrometer was tuned and calibrated according to the manufacturer’s operating instructions and a resolution of 30,000 was employed for all experiments. The DART was operated with helium as carrier gas at 3.5 L min−1 with a temperature of 80 °C and 200 °C for TBAB and DMAB, respectively. All other DART operating conditions were standard settings: needle current = 9990 mA; needle voltage = 2500 V; discharge electrode current = 7 mA; discharge electrode voltage = 149 V; grid electrode current = 10 mA; grid electrode voltage = 249 V. The lower m/z that the Orbitrap could acquire is 50.

Hydrolysis experiments were performed by mixing a volume of aqueous amine-borane with HCl solutions (see Table 1) in a 10-mL vial, under stirring.

Table 1 Reaction conditions for hydrolysis of amine-boranes
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A DIP-it™ glass stick (IonSense Inc., USA) was employed to sample the solution of amine-borane before the DART-Orbitrap analysis. The glass stick was dipped into the solution at regular intervals over the range of reaction times from 10 to 600 s and exposed to the DART ion source in order to collect a scan of the investigated amine-borane solution. Signal acquisition was performed with Xcalibur Thermo Tune Plus software (Thermo Fisher Scientific). For the collection of the mass spectra of the solid, the glass stick was dipped several times into the solid reagents until a few crystals were collected on its surface. At this point, such crystals were exposed to the DART. For both the solutions and the solid reagents, the signal acquisition was started about 30 s before sample exposure and terminated about 30 s after removal of the sample from the ion source.

Results

DART mass analysis of amine-boranes in solid form and in aqueous solution

Positive ion, DART-Orbitrap mass spectra were obtained for both solid and aqueous solution (pH ~ 8) of amine-boranes. The results are reported in Table 2 for TBAB and in Table 3 for DMAB.

Table 2 DART-Orbitrap full scan mass spectra of TBAB, in solid and in aqueous solution (pH ~ 8)
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Table 3 DART-Orbitrap full scan mass spectra of DMAB, in solid and in aqueous solution (pH ~ 8)
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The spectra do not exhibit intense signals for the expected [M + H]+ ion or the molecular ion M+•. As already reported for alkanes [15] and methylarsanes [16], the most characteristic signals associated with the amine-borane molecules, R3N·BH3 (R = alkyl, H), are those arising from the hydride abstraction. Both TBAB and DMAB ionized positively giving the characteristics signals at m/z [M − H−]+, C4H13NB+ (3a, Table 2) and C2H9NB+ (2b, Table 3), respectively. The amine-borane cations can evolve by further hydride loss and the replacement of the hydride by hydroxyl group (4a and 5a for TBAB, Table 2; 3b and 4b for DMAB, Table 3). The protonated amine is well detectable for TBAB (2a, Table 2) but not for DMAB because the molecular weight of Me2NH2+ is < 50 Da. This evidence indicates that in DART the most likely ionization pathways for amine-boranes can be schematized as follows (Scheme 1):

Scheme 1
scheme 1

DART ionization pathways for DMAB and TBAB

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Scheme 2
scheme 2

Proposed structure of selected species detected by DART-Orbitrap (see ESM for mass spectra, Figs. S1–S3)

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Other pathways seem to be related to the identity of amine. The borane cation of DMAB evolves by dihydrogen loss giving 1b (Table 3), a pattern that was not observed for TBAB. For TBAB, the formation of dimeric species (7a, 8a, 9a, Table 2) that are not observed for DMAB was instead evident.

Time evolution of mass spectra during hydrolysis of amine-boranes

The similarities between the ionization pathways taking place in DART (Scheme 1) and those taking place during hydrolysis (reactions 2 and 3) indicate that amine-boranes in DART undergo a process that could be defined as a hydrolysis in gaseous phase. For this reason, a particular attention was dedicated to the evaluation of mass spectra under hydrolysis conditions in order to identify possible artifacts or interferences.

The DART-Orbitrap mass spectra obtained during hydrolysis are reported in Table 4 for TBAB and in Table 5 for DMAB.

Table 4 DART-Orbitrap full scan mass spectra of TBAB in 0.45 M HCl (reaction time 30 s. Species 3a, 4a, 5a, 6a, 7a, 8a, and 9a reported in Table 2 were not detected. No ions other than those reported in this table were observed during hydrolysis using other acidity conditions (1.67 × 10−2 and 8.3 × 10−2 mol L−1 HCl))
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Table 5 DART-Orbitrap full scan mass spectra of DMAB under acidic conditions
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Depending on the acidity, the mass spectra of amine-borane during hydrolysis present some striking differences compared with those obtained in aqueous solution and, as an important feature, they present a temporal evolution.

Figure 1 compares the mass spectrum of TBAB in aqueous solution (Fig. 1a) with that obtained after 30 s hydrolysis in 0.45 mol L−1 HCl (Fig. 1b). The boron-containing ions 10a, 11a, and 12a (see ESM, Fig. S3, for mass spectra of 12a) and the ion 10x, which does not contain boron, are the most abundant species; these species were not detected in aqueous solution nor in the solid reagent. At the same time, the ions from 3a to 9a detected from aqueous solution and arising from ionization of TBAB (Fig. 1a and Table 2) were not present under these hydrolysis conditions. Molecular formulae could be assigned (Table 4) to the ions at m/z 184.2142 (10a), 201.1840 (10x), 240.2768 (11a), and 254.3056 (12a).

Fig. 1
figure 1

Comparison of mass spectra of 1.67 × 10–1 mol L−1 TBAB in aqueous solution (a) and during hydrolysis in 0.45 mol L−1 HCl at reaction time of 30 s (b). See Tables 2 and 4 for molecular formula and Scheme 2 for structures. See ESM, Fig. S3, for mass spectra of 12a. Instrumental conditions for DART-MS are reported in the “Experimental” section

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Table 4 summarizes ions detected during hydrolysis of TBAB in 0.45 mol L−1 HCl. No ions other than those reported in Table 4 were observed during hydrolysis using other acidity conditions (1.67 × 10−2 and 8.3 × 10−2 mol L−1 HCl). Ion 12a is the product of hydride abstraction from the N-tert-butyl, cyclotriborazane, 12. Ions 11a and 10a are most likely formed in the DART source by sequential loss of borane and tert-butyl fragments from 12, respectively (Table 4).

While these species are not detectable at low acidities (Fig. 2a), their appearance and disappearance in the mass spectra are well correlated with the reaction time at higher acidities (Fig. 2b). This evidence strongly supports the hypothesis that 12 is a species that exists in solution and is formed during hydrolysis of TBAB, whereas 10a and 11a are species that are formed from 12 in the DART source.

Fig. 2
figure 2

DART-MS measurements. Variation of the relative abundance of selected ions during the hydrolysis of TBAB in 1.67 × 10−2 (a) and 8.3 × 10−2 mol L−1 HCl (b). 12 is detected as three different ions, 12a, 10a, and 9a, arising from the indicated fragment losses (see ESM). Instrumental conditions for DART-MS are reported in the “Experimental” section

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Figure 3 compares mass spectra of DMAB in aqueous solution and during hydrolysis in 0.4 mol L−1 HCl at 10- and 60-s reaction times. At 60 s, the mass spectra are dominated by ion 6b (Fig. 3c) that is also detectable at 10 s (relative abundance about 0.8% (Fig. 3b), but not in aqueous solution (Fig. 3a and Table 2) or in the solid reagent (Table 2). Ion 5b is also present in the mass spectrum of an aqueous solution, even if at lower relative abundance, but not in that of the solid (Table 2). By increasing the acidity to 4.0 mol L−1 HCl, new ions appear at m/z 74.0589 Da (7x), 91.1221 Da (8b), 128.1023 Da (9b), and 163.0750 Da (10b), but none of these contains boron (Table 5). The relative abundance of the ions observed in the mass spectrum of DMAB changes with the reaction time and hydrolysis conditions, as shown in Fig. 4.

Fig. 3
figure 3

Mass spectra of 0.2 mol L−1 DMAB in aqueous solution (a) and during hydrolysis in 0.4 mol L−1 HCl at reaction time of 10 s (b) and 60 s (c). See Tables 3 and 5 for molecular formulae and structure. See ESM, Fig. S2, for mass spectra of 6b. Instrumental conditions for DART-MS are reported in the “Experimental” section

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Fig. 4
figure 4

Variation of the relative abundance of selected ions (see Tables 3 and 5) during the hydrolysis of 0.2 mol L−1 DMAB in 0.08 mol L−1 HCl (a), 0.4 mol L−1 HCl (b), and 4.0 mol L−1 HCl (c). Instrumental conditions for DART-MS are reported in the “Experimental” section

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The most interesting boron-containing species is ion 6b. It is detectable only after 6-min hydrolysis in 0.08 mol L−1 HCl (relative abundance about 3%, Fig. 4a), while it becomes the most abundant ion from 60- to 300-s reaction time in 0.4 mol L−1 HCl (Fig. 4b). No ions other than those reported in Table 5 were observed during hydrolysis using lower acidity conditions (8.0 × 10−2 mol L−1 HCl).

Under strong acidic conditions (4.0 mol L−1 HCl, Fig. 4c), 6b is detected after 30-s hydrolysis as the major species that disappears rather quickly from the mass spectrum. Ion 6b is therefore related to a species formed during hydrolysis of DMAB, that is, bis-dimethylamine boronium itself, (Me2NH)2BH2+.

Ion 5b can be detected in aqueous solution (about 0.6%, Table 2), but its relative abundance remained low even during hydrolysis in both 0.08 and 0.4 mol L−1 HCl (< 6%; Fig. 4a, b). However, species 5b is undetectable during hydrolysis in 4.0 mol L−1 HCl. Possibly, 5b can be generated in the DART source by hydride abstraction from both species 6c and 6d (Table 3), which, in turn, could be formed during hydrolysis of DMAB.

In conclusion, the time evolution of mass spectra during hydrolysis, using various acidic conditions, permits the selection of at least two species formed during hydrolysis, 12 from TBAB and 6b from DMAB, and not from artifacts in the DART source. The appearance of species 12 and 6b during hydrolysis is almost simultaneous with the disappearance of the amine-borane, confirming that 12 and 6b are species arising from reactions involving the original substrate.

Limitations of DART-Orbitrap experiments

Not all the species that are formed during hydrolysis experiments could be detected by DART-Orbitrap due to ionization and/or volatilization limitations or being outside of the mass range of the Orbitrap (< 50 Da). In the hydrolysis at low acidities, for TBAB, the displacement of amine forms 2a (tBuNH3+) whereas in the hydrolysis of DMAB, the protonated amine Me2NH2+ could not be detected (< 50 Da). For both TBAB and DMAB, the species BH3(H2O) and its hydrolysis products (H2O)BH2OH and (H2O) BH (OH)2 were outside the Orbitrap mass range. In this case, the hydrolysis pathway starting with reaction 3 could not be fully supported by direct evidence. Considering the data on the rate of hydrogen evolution [10], it can be concluded that, at low acidities, there is no evidence against the proposed hydrolysis pathway starting with reaction 3.

A further evidence indicating that not all the species present in solution are detected is the hydrolysis of DMAB in 4.0 mol L−1 HCl. In this case, no boron-containing ions were detectable after 90-s hydrolysis. According to the experimental data, under these conditions, the hydrolysis of DMAB is far from completion, as about 60–70% of the total available hydrogen is evolved at 90–120-s reaction time [10]. This means that a significant fraction of hydrolysable borane species are still present in solution, but they were not detected by DART-Orbitrap.

Discussion

The hydrolysis behavior of amine-boranes

Details of the displacement mechanism starting with reaction 3 are reported in Scheme 3.

Scheme 3
scheme 3

Hydrolysis mechanism proposed by Kelly et al. [11, 13] and Ryschkewitsch [12]

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The displacement of protonated ammine is the rate determining step of hydrolysis. The above mechanism of amine-boranes was proposed and supported by evidence reported by Ryschkewitsch [12] and by Kelly et al. [11, 13].

The ionization mechanism of hydrolysis of amine-boranes starting with reaction 2 is similar to the one described by Kreevoy and Hutchins for the hydrolysis of NaBH4 and NaBH3CN (Scheme 4) and proposed by the authors also for the hydrolysis of amine-boranes [8, 17, 18]:

Scheme 4
scheme 4

Hydrolysis mechanism proposed by Kreevoy and Hucthins [17, 18]

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Even if the displacement mechanism for the hydrolysis of amine-boranes according to Scheme 3 appears to be accepted in the literature [8, 19, 20], there are several reasons indicating that the mechanism of hydrolysis of amine-boranes presented some controversial aspects. Kelly and Marriot, who proposed the displacement mechanism, in their study on the reexamination of the mechanism of hydrolysis of AB [13], did not exclude the occurrence of a hydrolysis pathway according to Scheme 4, but they conclude that the achieved experimental evidences were difficult to reconcile with such a reaction scheme. The kinetics experiments conducted by Kelly and Marriot were performed in the range from pH/pD from 4.3 to 6.1 [13], very far from the CVG conditions that gave the anomalous results described above [10]. Furthermore, on the contrary of ionization mechanism of Scheme 4, the displacement mechanism of Scheme 3 is not able to explain the H-D exchange which prevails on hydrolysis at elevated acidities in more hydrolytic stable amine-boranes [21].

The evidence obtained in this work, integrated with those on the rate of hydrogen evolution reported recently [10], indicates that the displacement mechanism (Scheme 3) is not able to explain the behavior of amine-boranes at increasing acidities. In particular, it appears evident that, depending on acidity, the hydrolysis of amine-boranes can take place following two different reaction pathways. At low acidities, the displacement of protonated amine takes place according to Scheme 3, whereas by increasing the acidity the hydrolysis starts with the ionization of amine-borane, similarly according to reaction 4. However, even the mechanisms of hydrolysis reported in Scheme 4 cannot explain the formation of 12 and 6b species.

The only reasonable hypothesis is that, at high acidity, the hydrolytic stable boronium species [22] reacts preferentially with the amine-borane forming dimeric species which can evolve both by rearrangement of further polymerization reactions. This hypothesis fits with the evidence that amine-borane was disappearing from solution during the formation of either 12 or 6b (Figs. 1, 2, 3, and 4). This type of reactivity is closer to that of amine-boranes undergoing dehydrogenation using various types of catalysts in organic media. The formation of 12 and similar cyclotriborazanes is reported in dehydrocoupling reactions of TBAB and other primary amine-boranes with various types of catalysts [19, 20, 23, 24] whereas the formation of boronium cation 6b is reported in the dehydrocoupling of DMAB using PtII complexes [25]. Stephens et al. [26] investigated dehydrogenation of ammonia borane initiated by either Brønsted or Lewis acids in organic solvents and described details of the mechanism of the reaction:

$$ {3\mathrm{H}}_3\mathrm{N}\cdotp {\mathrm{B}\mathrm{H}}_3+\mathrm{acid}\to \left[\mathrm{intermediates}\right]\to {\mathrm{H}}_2+{\left[{\mathrm{B}\mathrm{H}}_2{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}\kern0.5em +{\mathrm{B}}_2{\mathrm{H}}_5\left(\upmu -{\mathrm{NH}}_2\right) $$
(4)

where the boronium species, [BH2(NH3)2]+, is similar to 6b.

One important conclusion of Stephens et al. [26] on the mechanism of acid-initiated dehydrogenation of ammonia borane was that the reaction starts most likely with the formation of a boronium species, according to a reaction similar to 2. We believe that the occurrence of similar mechanisms, originating with the formation of boroniun species at elevated acidities, causes the deviations from the kinetics and mechanisms of hydrolysis of aqueous amine-boranes which are proposed in the literature (Schemes 3 and 4). The acid hydrolysis behavior of ammine-boranes, which is consistent with the available experimental evidences, can be summarized in the following reaction scheme (Scheme 5):

Scheme 5
scheme 5

Acid hydrolysis reaction pathways of amine-boranes

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Mechanism of CVG using amine-boranes

The interpretation of the reactivity of the analytical substrates of HgII [3], SbIII [3, 6, 9], BiIII [6], SnIV [9], AsIII, AsV, methylarsenicV acids [6, 7], and SeIV [10, 27] undergoing CVG to hydrides by amine-boranes must be revised in light of the herein reported evidence. In particular, it is of interest for the elucidation of CVG mechanisms to understand which borane species can play a role in the derivatization reaction.

Depending on acidity, different borane intermediates are formed during CVG by aqueous boranes.

At low acidity, i.e., pH > 2, the amine-boranes, R3N·BH3 (R = alkyl, H), hydrolyzes slowly according to reaction pathway I (Scheme 5), which is the displacement mechanism of Scheme 3. In this case, the derivatizing species can be R3N·BH3, the amine-borane itself, BH3(H2O), and the hydridoboron species arising from its hydrolysis, (H2O)BH2OH and (H2O)BH(OH)2 [8, 28]. The hydrolysis products of amine-boranes at low acidities are therefore the same that are formed from hydrolysis of BH4−. However, in this case, their rate of formation is controlled by the displacement of protonated amine, which in this case coincides with ki, the second-order rate constant of the kinetic equation [8, 28] (see the “Introduction” section).

By increasing the acidity, the amine-borane starts to be ionized according to reaction pathway II (Scheme 5) forming borane cations, [R3NBH2]+ (R = alkyl, H), and other B-H containing species that are formed by a series of polymerization and/or rearrangements starting with the reaction of [R3NBH2]+ with R2NH·BH3. In strong acidic conditions ([H+] > 1 mol L−1), ionization of amine-boranes like TBAB and AB is as fast as the mixing step or reagent [10] and, most likely, the slowly hydrolyzing [R3NBH2]+ and its derivatives, such as 12 and 6b, are the most abundant species available for CVG derivatization.

Conclusions

The use of DART-Orbitrap allowed the detection of amine-boranes both in solid and in near aqueous solutions, highlighting the unique capability of this analytical arrangement to interrogate multiphase, non-dilute systems often in harsh pH conditions. Amine-boranes, in solid or in aqueous solution (pH ~ 8), exhibited an ionization pathway that starts with both the displacement of protonate amine and with the hydride abstraction, the latter behavior being similar to alkanes [15] and arsanes [16]. Striking differences are observed for mass spectra of amine-boranes during hydrolysis under various acidic conditions. We identified N-tert-butyl, cyclotriborazane, and bis(dimethylamino)boronium cation during the hydrolysis of TBAB and DMAB, respectively. The formation of these intermediates could not be justified by the currently proposed mechanism of hydrolysis reported in the literature [11,12,13, 17]. The evidences obtained in the present work along with those reported recently [10] highlight the current limits on the knowledge of the mechanisms that govern the acid hydrolysis of the amine-boranes. In particular, a single mechanism could not explain the hydrolysis of amine-boranes in a wide range of acidity, 0 < pH < 6. The mechanisms starting with the displacement of the amine (R3N·BH3 + H3O+ → R3NH+ + BH3(H2O)) [11,12,13] and the ionization reaction (R3N·BH3 + H3O+ → [(H2O)R3NBH2] + + H2) [17] can coexist and they can explain the apparent anomalous results reported recently both in CVG of H2Se and in kinetics of amine-boranes hydrolysis [10]. At low acidity, the displacement mechanism I (Scheme 5) prevails, whereas by increasing the acidity, the hydrolysis of amine-borane is governed by the ionization mechanism II (Scheme 5). The intermediate boranes/hydridoboron species formed in the different hydrolysis pathways play a decisive role in determining the reactivity of a CVG system.

Our results represent a further confirmation that the borane reagents (NaBH4, R3N·BH3) employed in CVG of volatile species are not the only reactive molecules in the hydride generation, but another plethora of boranes intermediates may enter the CVG reactivity and be better suited for the formation of volatile species of certain elements [8].