A miniaturized flow batch chemical vapor generation system for Hg determination in fish by ICP-MS

A new flow batch (FB) system for chemical vapor generation (CVG) is proposed for mercury (Hg) determination in fish. An inductively coupled plasma mass spectrometer was used as a detector. Low-cost peristaltic mini pumps were used to propel the solutions and different configurations of FB systems (reactor/gas/liquid separator) were studied. The proposed configuration of the FB-CVG system allows good sensitivity, low limit of detection (LOD) and low consumption of reagents and sample solutions. In summary, only 1 mL of reductant, 1 mL of acid and 0.16 mL of sample are needed. The proposed method has good linearity, precision (better than 5 %), LOD of 0.008 μg g-1 and LOQ of 0.012 μg g-1, and high sample throughput, allowing 90 measurements/h. The accuracy of the method was evaluated through the analysis of a certified reference material (DOLT-4 Dogfish Liver), whose result is in good agreement with certified value (t-test with 95 % confidence level) and the quantification limit meets current legislations, of 1.0 μg g-1 (Brazil) and 0.3 μg g-1 (EU). In addition, analyte recovery test was done, where Hg recovery was better than 95 %, demonstrating the good analytical performance of the method. To demonstrate the applicability of the method, five samples of fish tissue (muscle) were analyzed. The proposed FB-CVG system, in addition to being low cost, is robust and requires only the volume of reagents necessary for Hg vapor generation, producing a very low amount of waste. It can be concluded that the proposed system can be used for routine analysis for Hg determination in fish tissue. It is worth noting that with the appropriate adjustments, the system can be coupled to different Hg detectors.

Sensitive and isotopic interference-free analysis of Sb using hydride generation-microwave plasma torch-mass spectrometry under ambient condition

A sensitive and isotopic interference-free analysis method for Sb was developed based on hydride generation-microwave plasma torch-mass spectrometry (HG-MPT-MS). Compared to the conventional ICP-MS, MPT coupled to an ion trap mass spectrometer enabled much "softer" ionization of Sb under ambient condition, which provided multi-detection modes and various ion forms, such as Sb+, SbO+, SbO2-, SbO++H2O and so on. These ion formations can be easily regulated by tuning capillary voltage and tube lens voltage, which facilitated elimination of isotopic interference during analysis, for instance the interference of 123Te on 123Sb could be effectively excluded by optimizing parameters of capillary voltage and tube lens voltage. The potential application of HG-MPT-MS for Sb isotope ratio analysis was also demonstrated, which could be determined in different forms, e.g., 123Sb/121Sb or 123Sb16O/121Sb16O. The value of 123Sb/121Sb was determined to be 0.75110 ± 0.00038 (2σ, n > 50). In addition, the detection limit, linearity and spike recovery were also studied. Overall, HG-MPT-MS performed equally well on detection limit (0.05 μg/L) with ICP-MS or HG-AFS. The linearity (R2 = 0.998) was checked in the concentration range of 10-500 μg/L. Spike recovery were evaluated with two soil samples, and the obtained spike recovery ranged 90-100 %. In general, HG-MPT-MS was expected to be a versatile tool for study the biochemical or geochemical behaviors of Sb and other hydride forming elements under ambient condition in a much simpler and more efficient way.

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.

Effect of additives on cadmium chemical vapor generation and reliable quantification of generation efficiency

Chemical vapor generation (CVG) of cadmium was optimized based on response from atomic absorption spectrometry (AAS) with a heated quartz tube atomizer (QTA). Effect of several modifiers on analytical performance was studied. These additives were: inorganic salts of Cr³⁺, Ti⁴⁺ and Co²⁺ and their on-line synthesized complexes with KCN and thiourea, respectively. The use of these additives resulted in sensitivity enhancement, better repeatability and correspondingly in improvement of overall CVG efficiency. The latter was quantified by two independent approaches: a) by means of ^115mCd radioactive indicator, b) from comparison of sensitivities obtained with conventional solution nebulization and with CVG, both coupled simultaneously to inductively coupled plasma mass spectrometry. Both approaches provided comparable results. The highest efficiency, between 60 and 70%, was reached in the presence of Cr³⁺/KCN and Ti⁴⁺/KCN while 19% was achieved in Co²⁺/ascorbic acid/thiourea environment. Highly irreproducible results with low CVG efficiency ranging from 2.5 to 15% were reached in the absence of any additives. The generated cadmium species were identified to be mostly free atoms regardless of the additives presence or their absence. Cr³⁺/KCN environment was selected as the most robust for CVG of Cd reaching sensitivity of 6.6 s ng^-1 Cd and limit of detection of 60 pg mL^-1 Cd (9 pg Cd absolute) with detection by QTA-AAS.

Comparison of the performance of atmospheric pressure glow discharges operated between a flowing liquid cathode and either a pin-type anode or a helium jet anode for the Ga and In determination by the optical emission spectrometry

Novel atmospheric pressure glow discharge (APGD) microplasma systems, sustained between a miniaturized flowing liquid cathode (FLC) and either a pin-type anode or a He nozzle jet were investigated for the determination of Ga and In by the optical emission spectrometry (OES). The most influential working parameters, i.e., solution flow rate, acid concentration, discharge current, and He flow rate, were optimized for both studied systems. Furthermore, the effect of the addition of low molecular weight organic compounds (LMWOCs) into the FLC solution on the signals intensity of Ga and In was investigated. Subsequently, the impact of concomitant ions on the signals intensity of Ga and In was thoroughly studied and it was established that both studied methods are relatively resistant to matrix effects. Under the optimized conditions, the detection limits (DLs, assessed on the basis of the 3σ criterion) of the studied elements were similar for both discharges and ranged between 1.8 and 2.3 μg L^-1 for Ga and 0.37-0.40 μg L^-1 for In. The received DLs were therefore better than those obtained for other spectrometric methods being premised upon microplasma systems and comparable with those obtained by currently employed large-scale instrumentation. The system with the pin-type anode was successfully applied for the Ga and In determination in four leachates of solders and electronic scrap as well as river water, using external calibration with simple standard solutions. The received results were compared to those obtained from ICP-OES or ICP-MS measurements and their recoveries were fallen within the range of 98-114%, confirming the excellent accuracy and reliability of the developed FLC-APGD-OES method.

Speciation of germanium in environmental water reference materials by hydride generation and cryotrapping in combination with ICP-MS/MS

A method for the speciation analysis of the three main species of germanium in environmental waters, namely inorganic germanium (iGe), monomethyl germanium (MMGe) and dimethyl germanium (DMGe), has been developed. Germanium species were volatilized by hydride generation (HG) prior to their preconcentration/separation in a semi-automated cryogenic trap (cryotrapping, CT) and detection by ICP-MS/MS. A procedure to minimize the iGe blanks from the chemicals and water is reported. One mL of water can be analyzed without any pretreatment. After application of this procedure, and the careful optimization of all experimental variables, limits of detection (LOD) of 0.015, 0.005 and 0.003 ng L^-1 have been obtained for iGe, MMGe and DMGe, respectively. Standard addition experiments did not show any significant matrix effect, and, therefore, external calibration was used for sample analysis. In the Tris-HCl + L-Cysteine reaction media, additional experiments did not reveal any significant demethylation of MMGe to iGe in the process of HG-CT, which could affect the accuracy of the analysis in seawater. The method has been applied to the analysis of iGe, MMGe and DMGe in certified reference materials of unspiked natural waters: CASS-4, CASS-5 and CASS-6 (nearshore seawater); NASS-5 and NASS-7 (seawater); SLRS-4, SLRS-5 and SLRS-6 (river water).

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.

AuNCs-Catalyzed Hydrogen Selenide Oxidation: Mechanism and Application for Headspace Fluorescent Detection of Se(IV)

The excellent fluorescence property of Au nanoclusters (AuNCs) has received great attention for various chemosensing and biorelated applications, but the sample matrix is still an important problem that causes undesirable fluorescence variation. On the one hand, hydride generation (HG) is an effective strategy to separate the target analyte from the complex sample matrices, but the implementation of HG with AuNC-based fluorescent assays was not realized. On the other hand, due to the ultrasmall size of AuNCs and good catalytic performance of Au, AuNCs are also featuring intriguing catalytic applications. Herein, we proposed a new type of AuNC-based fluorescence assay for Se(IV) detection, in which hydride generation of Se(IV) was coupled with the fluorescence/catalytic dual functions of AuNCs. In a batch hydride generation mode, Se(IV) was first converted to volatile H₂Se. When it spread in the headspace to contact with AuNCs supported paper, AuNC-catalyzed oxidation of H₂Se by O₂ to yield elemental selenium occurred, which further deposited on the surface of AuNCs to induce fluorescence quenching. The catalytic effect of AuNCs was studied in depth via both experimental and theoretical (density functional theory) investigations. Three main steps for H₂Se oxidation were identified, with energy barriers in the presence of AuNCs significantly lower than those without. Benefiting from the reduced matrix interference by hydride generation and the unique catalysis/fluorescence of AuNCs, the proposed assay featured high selectivity, good sensitivity, and simplicity, with successful applications for selenium detection in real samples.

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.

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.

Step-by-Step Design and Synthesis of Au@SiO2@Phenyl-azathiacrown for SERS-Based Specific Quantification of Inorganic Mercury

Direct SERS‐based quantification of inorganic metal species has been a problem, because they have a small Raman cross‐section or even no vibrational mode. Here, we report a new strategy for SERS‐based quantification of such metal species, as exemplified by inorganic mercury (Hg^II) in waters. Step‐by‐step design and synthesis from azathioethers [3, 9‐dithia‐6‐monoazaundecane (DMA) and 3,6,12,15‐tetrathia‐9‐monoazaheptadecane (TTM)] to an azathiacrown [7‐aza‐1,4,10,13‐tetrathiacyclohexadecane (NS4)] demonstrate an improved S‐pulling effect and size‐fit specificity towards Hg^II to form Hg−S bonds. Modification of NS4 on the surface of Au@SiO₂ by using a 4‐(bromomethyl)benzoic linker enabled direct SERS‐based specific quantification of Hg^II for the first time, in which the ultrathin layer (ca. 2 nm) that covered the Au core (55 nm) could be a barrier preventing the Au core from having direct interaction with the Hg^II, and with phenyl serving as an internal standard (IS). The ratio of the Hg−S SERS band intensity at 270 cm⁻¹ to that of IS [(γCC+γCCC) at 1046 cm⁻¹] was practically proportional to the concentration of Hg^II, eliminating the inevitable uncertainties encountered in SERS‐based measurements. Such a methodology is expected to pave a new way for SERS‐based quantification of inorganic metal species when specific complexing substrates and suitable ISs are designed.

A comparative evaluation of different ionic liquids for arsenic species separation and determination in wine varietals by liquid chromatography - hydride generation atomic fluorescence spectrometry

The application of different ionic liquids (ILs) as modifiers for chromatographic separation and determination of arsenite [As(III)], arsenate [As(V)], dimethylarsonic acid (DMA) and monomethylarsonic acid (MMA) species in wine samples, by reversed-phase high performance liquid chromatography coupled to hydride generation atomic fluorescence spectrometry detection (RP-HPLC-HG-AFS) was studied in this work. Several factors influencing the chromatographic separation of the As species, such as pH of the mobile phase, buffer solution concentration, buffer type, IL concentration and length of alkyl groups in ILs were evaluated. The complete separation of As species was achieved using a C18 column in isocratic mode with a mobile phase composed of 0.5% (v/v) 1-octyl-3-methylimidazolium chloride ([C8mim]Cl) and 5% (v/v) methanol at pH 8.5. A multivariate methodology was used to optimize the variables involved in AFS detection of As species after they were separated by HPLC. The ILs showed remarkable performance for the separation of As species, which was obtained within 18min with a resolution higher than 0.83. The limits of detection for As(III), As(V), MMA and DMA were 0.81, 0.89, 0.62 and 1.00μg As L(-1). The proposed method was applied for As speciation analysis in white and red wine samples originated from different grape varieties.

Glossary of terms used in extraction (IUPAC Recommendations 2016)

Approaches for analytical-scale extraction are developing rapidly as new strategies are implemented to improve sample throughput, to minimize material use in laboratory methods, and to develop on-site capabilities. In this contribution, definitions and recommendations for symbols for the terms used in analytical extraction are presented. Exhaustive, microextraction, elevated temperature, microwave- and ultrasound-assisted, parallel batch, flow through systems, and membrane extraction approaches are discussed. An associated tutorial titled “Extraction” provides a detailed introduction to the topic.

Hydride generation coupled to microfunnel-assisted headspace liquid-phase microextraction for the determination of arsenic with UV-Vis spectrophotometry

In this research, a microfunnel-assisted headspace liquid-phase microextraction technique has been used in combination with hydride generation to determine arsenic (As) by UV-Vis spectrophotometry. The method is based on the reduction of As to arsine (AsH3) in acidic media by sodium tetrahydroborate (NaBH4) followed by its subsequent reaction with silver diethyldithiocarbamate (AgDDC) to give an absorbing complex at 510 nm. The complexing reagent (AgDDC) has been dissolved in a 1:1 (by the volume ratio) mixture of chloroform/chlorobenzene microdroplet and exposed to the generated gaseous arsine via a reversed microfunnel in the headspace of the sample solution. Several operating parameters affecting the performance of the method have been examined and optimized. Acetonitrile solvent has been added to the working samples as a sensitivity enhancement agent. Under the optimized operating conditions, the detection limit has been measured to be 0.2 ng mL(-1) (based on 3sb/m criterion, n b = 8), and the calibration curve was linear in the range of 0.5-12 ng mL(-1). The relative standard deviation for eight replicate measurements was 1.9 %. Also, the effects of several potential interferences have been studied. The accuracy of the method was validated through the analysis of JR-1 geological standard reference material. The method has been successfully applied for the determination of arsenic in raw and spiked soft drink and water samples with the recoveries that ranged from 91 to 106 %.

Selective hydride generation- cryotrapping- ICP-MS for arsenic speciation analysis at picogram levels: analysis of river and sea water reference materials and human bladder epithelial cells

An ultra sensitive method for arsenic (As) speciation analysis based on selective hydride generation (HG) with preconcentration by cryotrapping (CT) and inductively coupled plasma- mass spectrometry (ICP-MS) detection is presented. Determination of valence of the As species is performed by selective HG without prereduction (trivalent species only) or with L-cysteine prereduction (sum of tri- and pentavalent species). Methylated species are resolved on the basis of thermal desorption of formed methyl substituted arsines after collection at -196°C. Limits of detection of 3.4, 0.04, 0.14 and 0.10 pg mL-1 (ppt) were achieved for inorganic As, mono-, di- and trimethylated species, respectively, from a 500 μL sample. Speciation analysis of river water (NRC SLRS-4 and SLRS-5) and sea water (NRC CASS-4, CASS-5 and NASS-5) reference materials certified to contain 0.4 to 1.3 ng mL-1 total As was performed. The concentrations of methylated As species in tens of pg mL-1 range obtained by HG-CT-ICP-MS systems in three laboratories were in excellent agreement and compared well with results of HG-CT-atomic absorption spectrometry and anion exchange liquid chromatography- ICP-MS; sums of detected species agreed well with the certified total As content. HG-CT-ICP-MS method was successfully used for analysis of microsamples of exfoliated bladder epithelial cells isolated from human urine. Here, samples of lysates of 25 to 550 thousand cells contained typically tens pg up to ng of iAs species and from single to hundreds pg of methylated species, well within detection power of the presented method. A significant portion of As in the cells was found in the form of the highly toxic trivalent species.

The Determination of Arsenic Compounds: A Critical Review

A large number of publications describe the determination of arsenic in “environmental” samples in the broadest sense, a substantial subset of which focus on plant-based foodstuffs. There is a considerable interest in the inorganic arsenic content of food, especially rice, as there is recent evidence that concentrations may be high enough to exceed acceptable risk thresholds. The methodology for the determination of arsenic in rice is critically evaluated and results (a) for a rice flour reference material (National Institute of Standards SRM 1568a, certified only for total arsenic) and (b) a recent proficiency test (run by the European Commission's Joint Research Centre Institute for Reference Materials and Measurement) are examined. Difficulties with this particular analysis may lie in the sample preparation stages, over which there is still disagreement with regard to species stability, though a simple, hot-water extraction may be sufficient. High performance liquid chromatography separations with plasma-source mass spectrometry detection are popular; however, chromatographic separations are often not adequately described, the enhancement effect of carbon-containing species is often overlooked, and the fate of chlorine-containing species, responsible for an isobaric overlap interference, often obscure. Compound-dependent responses, for which there is a plenty of evidence, are almost never acknowledged or discussed.

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.

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

The formation of volatile products during the reaction of As(III: ), As(V: ), MeAsO(OH)(2), and Me(2)AsO(OH) with aqueous NaBH(4) has been investigated, and the formation of arsanes, diarsanes, and triarsanes has been detected. The presence of triarsanes is reported here for the first time. Diarsanes and triarsanes are likely formed in condensation cascade reactions, whereas trimethylarsane arises via the transfer of a methyl group. The formation of volatile by-products is considerably reduced by increasing the acidity of the medium and the concentration of NaBH(4) or by the addition of thiols, such as cysteine. A reaction scheme is proposed which reconciles the evidence reported herein and elsewhere in the literature that is valid for both analytical (trace analysis) and non-analytical reaction conditions.