Hydride Generation

Hydride generation is a common method for the detection of metalloids such as As, Bi, Ge, Pb, Sb, Se, Sn and Te, although other vapours, for example Hg or alkylated Cd, may also be determined.

From: Encyclopedia of Spectroscopy and Spectrometry (Third Edition) , 2017

Arsenic Speciation in Algae

Bin Hu , ... Chi Xu , in Comprehensive Analytical Chemistry, 2019

5.1 Hydride generation (HG)

HG is based on the formation of gaseous hydride for some elements, with the presence of some reducing agents, e.g., NaBH4, KBH4. Arsenic can be transformed into volatile species by HG; actually, NaBH4 or KBH4 is also a derivatizing agent for arsenic species. The formed volatile species can be trapped by using low temperature, solvents, or adsorbents/SPME fibres. Till now there are also some strategies used for arsenic speciation in algae including HG-AAS [138], HG-AFS [139] and FI-HG-AAS [140,141]. Mania et al. [138] reported a HG-AAS method to analyse fish, seafood and algae. They found Hijiki algal contained the highest inorganic arsenic (iAs) content among various algae (the mean content of iAs: 102.7 vs 0.41   mg/kg). Diaz et al. [140] used FI-HG-AAS method to separate inorganic As from all As species in 14 algae samples. They found that inorganic arsenic concentrations ranged between 0.15 and 1.06   mg/kg. Similarly, Farias et al. [141] used similar method to calculate inorganic As ratio in algae of the Antarctic region.

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Flow Analysis with Atomic Spectrometric Detectors

Afredo Sanz-Medel J. Enrique Sánchez Uría , Alberto Menéndez García , in Analytical Spectroscopy Library, 1999

9.2.3 Continuous flow hydride generation/nebulization with ICP–OES detection

Hydride generation (HG) in a continuous flow ( Figure 9.3) is probably the more common method for coupling hydride generation to ICP–OES, since proposed in 1978 for the improved determination of As, Bi, Sb, Se, and Te by Thompson et al. 15

However, a combination of HG and conventional nebulization in a single manifold is possible. Huang et al. 16 proposed such a useful system by using a modified concentric nebulizer (Figure 9.5). The sample is introduced by a peristaltic pump via pneumatic nebulization; the same peristaltic pump drives by another channel a NaBH4 solution that flows over the hydride generation device (a sort of spoon made in glass) where the hydride formation takes place when the spray of acid sample and reductant mix in the glass device. The nebulization chamber acts as a gas–liquid separator. This Huang's design, shown in Figure 9.5, allows analytes of the sample to reach the plasma in two different ways: as a volatile hydride in the case of the elements forming hydrides and as a liquid aerosol for non-hydride-forming elements. Of course, very important improvements in the detection limits (LD) of elements forming hydrides are observed, without degradation of LDs attained for non-hydride forming elements. The introduction system was later modified 17 in order to attain a smoother reaction of hydride formation. This system was also used to generate continuously hydrides from organic solvents. 17

Figure 9.5. Schematic of the nebulizer-hydride generator system

 (reproduced from Reference 16 by permission of Elsevier Science).

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Flow Analysis with Atomic Spectrometric Detectors

A. de Diego , ... O.F.X. Donard , in Analytical Spectroscopy Library, 1999

12.2.1.2.1 Hydride generation (HG)

Hydride generation was first used by Holak for total determination of arsenic by atomic absorption spectroscopy. 23 It is now well established for elements such as arsenic, bismuth, antimony, selenium, tin, germanium and tellurium. 24 Some of the early applications for speciation analysis were conducted for the determination of methylated forms of selenium in water samples. 11 The technique also gives access to the determination of redox inorganic species such as arsenic (III and V), 25 antimony (III and V) 26 and selenium (IV and VI). 27 The determination of organometallic forms including monomethylarsonate, dimethylarsinate, dimethylselenide, dimethyldiselenide and diethylselenide is also possible. 28 Speciation analysis of organo-germanium species after reaction with NaBH4 was reported by Hambrick et al. 29 Methyl and butyl tins are also rapidly converted into their gaseous derivatives by hydride generation. 30 Despite the fact that hydride generation was initially considered not to be suitable for mercury speciation analysis due to possible reduction and conversion of the analytes to Hg0, this technique has recently been demonstrated to be fully applicable for mercury speciation purposes. Mercury hydrides can be readily formed before total reduction to Hg0 and their half-life prior to total reduction allows them to be used in an analytical procedure. 21, 31, 32 Immediate cryotrapping considerably facilitates this procedure. 33 This technique has recently been successfully applied to the simultaneous determination of inorganic mercury and methylmercury in alkaline extracts of bio-tissues using an HG–CT–GC–QFAAS system. 33

The general reaction of hydride generation can be written as follows for alkylated tin compounds:

(12.1) R n S n 4 − n liquid + NaB H 4 → H + R n Sn H 4 − n gas + H 2 gas R : methyl ethyl , or butyl group n : 1 , 2 or 3

Important production of hydrogen during the reaction improves the purging efficiency of the volatile hydrides from the vessel. It may also generate over-pressure problems in the system. Many elements may form hydrides which will also be stripped simultaneously. This fact can be an advantage for simultaneous detection, but may also generate later interferences. A careful selection of the experimental conditions (pH and volume of reagent) results in selective conversion to hydrides, minimizing the number of species trapped in the column, and in reduction of potential interferences in the later detection step. The optimal conditions for hydride generation with regards to a large array of species have been reported in the literature for several inorganic and organometallic compounds and are summarized in Table 12.1. In general, the hydritization yield of the reaction is optimal for a pH slightly below the pKa of the species to be converted. 43 It is worth noting that most of the compounds suitable to be analysed by atomic absorption after hydride generation are naturally present in the environment. 7

Table 12.1. Derivatizing conditions reported in the literature for several inorganic and organometallic compounds

Species Reagent Derivatization conditions Sample pretreatment Reference
Inorganic Sn MexSn(4   −   x)+ EtxSn(4   −   x)+ n-BuxSn(4   −   x)+ NaBH4 1   ml of 4% aqueous NaBH4 1   ml of HAc 2   mol   dm–3 [34]
Inorganic Sn MexSn(4   −   x)+ NaBH4 2   ×   1   ml of 4% aqueous NaBH4 4   ml of Tris-HCl 2   mol   dm–3; pH   ~   6.5 [25]
Inorganic Sn MexSn(4   −   x)+ NaBH4 1   ml of 4% aqueous NaBH4 in NaOH 0.02   mol   dm–3 0.2   ml of HNO3 5   mol   dm–3; pH   ~   2 [35]
Inorganic Sn MexSn(4   −   x)+ n-BuxSn(4  − x)+ NaBH4 2   ×   1.5   ml of 4% aqueous NaBH4 2   ml of HNO3 5   mol   dm–3; pH   ~   2 [30]
n-BuxSn(4   −   x)+ Et3Sn+ NaBH4 2   ×   2.5   ml of 6% aqueous NaBH4 2   ml of HNO3 5   mol   dm–3; pH   ~   1.6 [36]
BuxSn(4   −   x)+ NaBEt4 0.13   ml of 1% aqueous NaBEt4 pH   ~   4.1 [37, 38]
Inorganic Ge NaBH4 6   ml of 20% aqueous NaBH4 in NaOH 5   ml of Tris-HCl 1.9   mol   dm–3  +   10   ml [29]
MexGe(4   −   x)+ 0.06   mol   dm–3 per 100   ml of sample of 30% NaCl   +   1   ml of EDTA 0.2   mol   dm–3 per 100   ml of sample
AsIII NaBH4 2   ml of 2% aqueous NaBH4 1 –3  ml of 5% potassium biphtalate; pH   ~   1.6 [28]
Asv NaBH4 4   ×   2   ml of 2% aqueous NaBH4 5   ml of 10% oxalic acid; pH   ~   1.-1.5 [28]
Monomethylarsine Dimethylarsine
Trimethylarsine MexPb(4   −   x)+ NaBEt4 3   ml of 0.43% aqueous NaBEt4 pH   ~   4.1 [39]
MeHg+ NaBEt4 0.05   ml of 1% aqueous NaBEt4 Acetate buffer solution; pH   ~   4.9 [40]
Hg2   + NaBH4 I ml of 0.4% aqueous NaBH4 pH   ~   4 [21]
MeHg+ LiB(C2H5)3H 0.1% solution of LiB(C2H5)3H in THF pH   ~   4 [21]
Hg2   +, MeHg+ NaBH4 10   ml of 4% aqueous NaBH4 0.5   ml of acetate buffer 1   mol   dm–3; pH   ~   4.9 [41]
Hg2   +, MeHg+ NaBEt4 0.05   ml of 1% aqueous NaBEt4 2   ml of acetate buffer; pH   ~   4.5 [19]
Hg2   +, MeHg+ Me2Hg, Et2Hg NaBH4 0.8   ml of 6% aqueous NaBH4 HCl 0.01   mol   dm–3; pH   ~   2 [32]
MeHg+ NaBEt, 10   ml of 0.01   % aqueous NaBEt4 0.5   ml of acetate buffer 5   mol   dm–3; pH   ~   4.5 [13]
Hg2   +, MeHg+ NaBH4 5   ml of 4% aqueous NaBH4 0.15   ml of HCl 12   mol   dm–3; pH   ~   1.5 [33]
Me3Sn+, Me2Sn2+, MeSn3   + Hg2   +, MeHg+ Me3Pb+, Me2Pb2   + NaBEt4 0.1   ml of 0.3% aqueous NaBEt4 Acetate buffer; pH   ~   5 [42]

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ARSENIC

A. Raab , J. Feldmann , in Encyclopedia of Analytical Science (Second Edition), 2005

Hydride Generation Coupled to Gas Chromatography (HG–GC)

HG in its simplest form allows the speciation of inorganic AsIII and AsV. For the speciation of samples containing more than these two hydride-active species, a combination with GC is necessary. Every arsenic species, with the exception of those carrying four carbon bonds, can be transformed into arsines by borohydride. Most of the arsines are volatile and can be separated by GC. Depending on the pH used during hydride formation a distinction between tri- and pentavalent arsenic is in most cases possible. Trivalent arsenic species form hydrides already at pH 7, whereas pentavalent ones are only reactive at pH 1 (Table 2). Measuring the same sample with both pHs gives therefore the ratio of penta- to trivalent arsenic. It has to be kept in mind that the efficiency of the hydride formation is species dependent and there might be pentavalent arsenic species present in the sample that form hydrides already at pH 7, like DMASV. HG is routinely used for the determination of inorganic arsenic species and in combination with cryotrapping and GC for the separation of monomethylated and dimethylated arsenic, since these species are easy to volatilise. Dimethylated arsenosugars, which can as well form volatile arsenic species during HG, have not yet been successfully separated by this technique.

Table 2. Arsenic species and the arsines formed by treatment with borohydride

Species Solution pH 1 Solution pH 7 (buffered)
As(OH)3 AsH3 AsH3
H3AsO3 AsH3 –
VMAIII MeAsH2 MeAsH2
MAV MeAsH2 –
DMAIII Me2AsH Me2AsH
DMAV Me2AsH –
TMAO Me3As Me3As
DMASV Me2AsH Me2AsH

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Nebulization systems

John A. Burgener , Yoseif Makonnen , in Sample Introduction Systems in ICPMS and ICPOES, 2020

2.10.4 Hydride generation

Hydride generation will be discussed more in Chapter 8. However, spray chambers are an integral part of hydride generation, so they need to be mentioned here also.

Very briefly, sodium borohydride will react with many elements to produce volatile compounds. If the volatile compounds can be separated from the liquid sample and sent into the plasma, these compounds will be dramatically concentrated compared to their levels in the liquid, and provide much lower detection limits than possible otherwise. The hydride generation process allows analysis of many elements not otherwise sensitive on an ICPOES or AA instrument [278, 279]. The original hydride generation methods simply used a bucket with a plug on top to add the NaBH4, and a second hole on top to direct the volatile compounds to the flame. This was very slow and required washing out the system after each sample [280]. If samples with high organic solvent concentrations were analyzed, they bubbled and frothed and often blew out the AA flame with all the organic solvents traveling with the volatiles. Since then, continuous flow mixing and hydride generation systems were developed, allowing a small amount of sample to mix with a small amount of NaBH4, minimizing the frothing and bubbling, and improving the transfer of the volatile compounds [281].

Current spray chambers used for hydride generation include many variations on the concept, but all have a small area of interaction between the NaBH4 and the sample, and all have a simple path for the volatiles to be directed to the plasma or AA flame. Usually, the continuous flow systems clean themselves out effectively as they operate, so there is not a need to take the system apart after each sample [282, 283]. Jobin Yvon have a patented hydride generation chamber called the Concomitant Metals Analyzer (or CMA) that is essentially a cyclonic design with a small puddle forming at the bottom where the reaction takes place, and then the remaining reacted sample goes down the drain while the volatiles go to the torch (Fig. 2.53) [284].

Fig. 2.53

Fig. 2.53. CMA chamber details.

 Image by John Burgener.

A newer design is the Agilent Multi-mode Sample Introduction System (MSIS) (Fig. 2.54), which is also based on a cyclonic spray chamber design, with a standard nebulizer input and an additional reaction area in the center [285–287]. The reaction area is a conical post in the center with a capillary to deliver the sample, and a reagent delivery tube above it that delivers the NaBH4 to the tip of the conical post. Both sample and NaBH4 arrive at the space at the tip of the post, and flow down the post while reacting. The hydrides forming are immediately released into the chamber gas flow, and swept into the plasma [288]. With only a thin film forming on the post, the volatile compounds can easily escape from the liquid, so the MSIS is able to provide volatile compounds of elements that normally break down in the reaction liquid, and otherwise would not escape into the gas in the chamber. The MSIS is the only reaction chamber at present able to provide hydride separations of platinum group elements. The MSIS also allows for the simultaneous determination of hydride- and non-hydride-forming elements [281, 288–290].

Fig. 2.54

Fig. 2.54. MSIS chamber details and photo.

 Image by John Burgener.

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Flow Analysis with Atomic Spectrometric Detectors

Angel Morales-Rubio , Miguel de la Guardia , in Analytical Spectroscopy Library, 1999

Selenium

Se is commonly determined in waters by on-line hydride generation, 39,143–150 but in recent years new approaches have appeared in the literature 151,152

Hydride generation of Se through reaction with NaBH4 is most suitable for specific determination of Se(IV) 39,143 and also for speciation of Se(IV), Se(VI) and other selenium compounds based on the selective elution of previously preconcentrated species on a Dowex 1   ×   8 column, 144 the on-line pre-reduction of Se(VI) to Se(IV) carried out in convective heating systems 145 or with a microwave-assisted treatment. 146 Microwaves have also been employed for post-column derivatization in HPLC followed by HG–AFS 147 or HG–AAS. 148

On-line preconcentration procedures for FI–AAS determination of Se in waters have also been reported using ion exchangers 39,144,153 or coprecipitation with La(OH)3. 143,149

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Atomic Absorption Spectrometry | Vapor Generation☆

Alessandro D'Ulivo , in Encyclopedia of Analytical Science (Third Edition), 2019

Applications

Hydride generation techniques are superior to direct solution analysis in several ways. However, the attraction offered by enhanced detection limits is offset by the relatively few elements to which the technique can be applied, potential interferences, as well as limitations imposed on the sample preparation procedures in that strict adherence to valence states and chemical form must be maintained. Cold-vapor generation of mercury currently provides the most desirable means of quantitation of this element, although detection limits lower than AAS can be achieved when it is coupled to other means of detection (e.g., nondispersive atomic fluorescence or microwave induced plasma atomic emission spectrometry).

Applications of both vapor generation techniques have been widespread in that waters and effluents, metallurgical, clinical, biological, agricultural, geological, and environmental samples have all been analyzed at both the trace and ultratrace levels for these analytes. The reader is referred to the Further Reading section for an extensive compilation of specific applications.

Currently, detection power is primarily limited by reagent contamination. Progress in the widespread implementation of FI techniques, which feature online sample preparation and pretreatment capabilities as well as capabilities for rapid automation, should facilitate a further revolution in the use of vapor generation techniques in atomic spectroscopy as will, without doubt, the further development of photochemical vapor generation techniques. 19,40

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Vapor generation

Ian D. Brindle , in Sample Introduction Systems in ICPMS and ICPOES, 2020

8.1.2 Hydride generation

Hydride generation is the oldest of the vapor generation techniques that was used for the determination of arsenic a century before Alan Walsh introduced atomic absorption spectrometry to the analytical community. John Marsh developed a forensic method for the determination of arsenic that was based on dissolving zinc in hydrochloric acid to generate arsine (and stibine), that was stripped from solution by the hydrogen produced in the reaction [15, 16]. a Although Holak is usually credited with the development of vapor generation in his work on arsenic [17], Brandenberger and Bader developed the cold vapor method for mercury 2 years earlier [18, 19]. Holak's technique was a batch process, based on trapping the evolved arsine in a packed U-tube, cooled in liquid nitrogen, and then release of the arsine to an atomic absorption spectrometer for measurement of the transient signal.

The development of plasmas as excitation sources for atomic spectrometry, and the development of multi-element capabilities, arising from improvements in (echelle based) spectrometers and two-dimensional detector arrays, such as the charge-coupled device (CCD) gave rise to the potential of the application of interfacing vapor generation with these powerful spectrometers for multi-element determinations. Early work on the formation of atoms in heated quartz tubes, which were specifically designed for application to absorption spectrometric methods, has largely been supplanted by plasma-based instruments, although several single-element based methods, based on atomic fluorescence systems, still find valuable applications in a number of areas where a single element is the target of the measurement [20].

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Hazardous Metals in the Environment

Kurt J. Irgolic , in Techniques and Instrumentation in Analytical Chemistry, 1992

11.4.4.3.9 HG-DC plasma atomic emission spectrometry

Analytical systems combining hydride generation with DC-plasma atomic emission spectrometers have only been used infrequently for the determination of total arsenic. Seawater and wastewater samples have been treated with zinc and hydrochloric acid and the evolved arsine (and stibine) collected in a trap cooled by liquid nitrogen and thus separated from the gaseous hydrogen. The detection limit for arsenic was 8  ng [282]. Continuous hydride generation systems were coupled to DC-plasma emission spectrometers for the determination of total arsenic in water, digested canned tuna fish [285], soil extracts, and digested reference materials (NBS Orchard Leaves, NIES, Japan, No. 6 Mussel) [286] with detection limits in the 4 to 10   ppb range. These systems can be used for the simultaneous determination of all hydride-forming elements.

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Flow Analysis with Atomic Spectrometric Detectors

J. Dĕdina , in Analytical Spectroscopy Library, 1999

8.5.8 Selective hydride generation

In general, the selective HG (see Section 8.2.2) can be performed successfully in the batch arrangement as well as in a flow mode since the optimum conditions for HG of individual analyte forms are controlled mainly by the composition of the reaction mixture. The only advantage of flow methods in this respect is the influence of the length of reaction coil on the hydride release efficiency in the case of As(V) (see Section 8.5.2). This can be used to control the efficiency of arsine release from As(V) instrumentally. Yet, for reasons discussed in Section 8.4, the flow modes are much more popular for the selective HG also for other analyte forms.

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