Hydride generation technique methods

Hydride generation techniques for the volatilization and separation of alkyl leads have been infrequently used as the organolead hydrides are not very stable. The commonest derivatization reagents are NaBtC IRR or Grignard reagents. The normal separation systems and methods of detection are used. 

The hydride generation technique is a technique in which volatile metal hydrides are formed by chemical reaction of the analyte solutions with sodium borohydride. The hydrides are guided to the path of the light, heated to relatively low temperatures, and atomized. It is useful because it provides an improved method for arsenic, bismuth, germanium, lead, antimony, selenium, tin, and tellurium. 

The recommended procedure for the determination of arsenic and antimony involves the addition of 1 g of potassium iodide and 1 g of ascorbic acid to a sample of 20 ml of concentrated hydrochloric acid. This solution should be kept at room temperature for at least five hours before initiation of the programmed MH 5-1 hydride generation system, i.e., before addition of ice-cold 10% sodium borohydride and 5% sodium hydroxide. In the hydride generation technique the evolved metal hydrides are decomposed in a heated quartz cell prior to determination by atomic absorption spectrometry. The hydride method offers improved sensitivity and lower detection limits compared to graphite furnace atomic absorption spectrometry. However, the most important advantage of hydride-generating techniques is the prevention of matrix interference, which is usually very important in the 200 nm area. 

Azad et al. [ 186] used a similar technique for the determination of selenium in soil extracts using a nondispersive spectrometer, with which it was possible to observe fluorescence from the 196.1, 214.3 and 204.0 lines simultaneously, thus enabling a detection limit of 10 ng/ml to be observed using discrete sample introduction via the hydride generation technique. In this method, soil... 

The EPA has approved special methods for arsenic (EPA Method 7061) and selenium (EPA Method 7741) analysis by gaseous hydride generation technique, which offer a unique combination of selectivity and sensitivity. This technique has an advantage of being able to isolate these elements from complex matrices that may cause interferences in analysis with other techniques. 

The measurement of Bi in body fluids and tissues may be achieved using either ETA—AAS or hydride generation techniques. Rooney [101] compared these procedures and reported that the latter was the method of choice. The severe molecular absorption interferences at 213.2 nm necessitate some form of chemical pretreatment for ETA—AAS. Thomas et al. 

The ability to monitor trace levels of a number of heavy metals in a variety of samples is an important feature of modern environmental chemistry. Hence, sensitive analytical methods are required. When faced with the task of analyzing very low concentrations of antimony, bismuth and tin the hydride generation method is the first choice because of the improved sensitivity and lower detection limits as compared to many other techniques. The hydride generation technique includes the use of a reductant, such as a NaBH4 solution, to separate the volatile metal hydrides from the sample solution and the subsequent determination with atomic absorption after decomposition of the hydrides in a heated quartz cell. 

Hydride Generation [9]. The hydride generation technique is probably the most sensitive for direct ICP-AES measurement/detection (Figure 2.17). The sensitivity of this procedure is 50 to 300 times greater than that by direct nebulisation. The method is relatively free from interferences, as it involves separation of the metals as hydride gases from the sample solution after reaction with sodium borohydride in the presence of acid. The technique is limited to the elements As, Bi, Ge, Pb, Se, Sb, Sn and Te, which are known to form readily volatile covalent hydrides. The hydrides are purged directly into the plasma where they are atomised, excited and measured by ICP-AES in the normal way. 

The validation has the objective to identify, during the method development process, all sources of error and eliminate them or to quantify their contribution to the total uncertainty of the determination. For hydride generation techniques particular attention must be given to the quantitative transformation of all As species into hydrides (arsenobetaine, arsenosugars etc). Several types of adapted materials must be prepared to test all steps of the process (from simple calibrant solutions or mixtures to spiked fish tissue samples). If they exist CRMs should be used for validating trueness. Laboratory RMs must be prepared for the establishment of control charts when the method is under statistical control [27, 28]. 

Most selenium measurements are complicated by the presence of other elements in the sample. In some cases selenium can be separated from interferences by distillation as the bromide, by high pressure liquid chromatography, or by solvent extraction (1, 12, 13). The hydride generation technique provides good separation of selenium from interferences in the atomic absorption technique that has been developed. However, separation from arsenic and mercury is not accomplished, and these elements and the nitrate ion have been found to interfere (14,15). However, the effect of these species on the measurement is not significant for the method developed by the Project. 

Hydride generation techniques provide a method for introducing samples containing arsenic, antimony, tin. selenium, bismuth, and lead into an atomizer as a gas. Such a procedure enhances the detection limits for these elements bv a factor of lOio l(K). Because several of those species are highl> toxic, delerrnining them at... 

Trivalent monomethylated and dimethylated arsenic species have also been reported in lake water (58,68,69). These arsenicals are probably methylarsonous acid and dimethylarsinous acid, although their precise chemical structures in natural waters have not been demonstrated. Most analytical methods for determining arsenic species in water samples convert the original arsenic species into volatile hydrides, which then serve as the analytes. Since the trivalent methylated arsenicals generate the same analyte as their respective pentavalent analogues, they must be separated before the hydride generation step so that they can be determined independently. Solvent extraction has been used to effect this separation (58). Possibly, the presence of these trivalent methylated arsenicals has been underestimated because few studies include a solvent separation step. However, in one smdy at least, dimethylarsenic in estuarine and coastal waters, as determined by hydride generation techniques, was shown to be present solely as the pentavalent dimethylarsinate species in three out of the four samples tested (50). 

Two modes of operation can be applied for the hydride generation technique (i) In the normal batch system, the whole sample is reduced in a hydride generator and the hydride formed transported in a carrier gas stream to an absorption tube (ii) In the flow injection (FIA) technique all stages of the hydride generation method take place in a fully automated closed system. The FIA system is discussed in section 6.3. 

Initially hydride generation and cold vapour techniques were developed for the quantitative determination of the hydride-forming elements and mercury by atomic absorption spectrometry (Chapters, Sections 6.2 and 6.3), but nowadays these methods are also widely used in plasma atomic emission spectrometry. In the hydride generation technique, hydride-forming elements are more efficiently transported to the plasma than by conventional solution nebulization, and the production and excitation of free atoms and ions in the hot plasma is therefore more efficient. Spectral interferences are also reduced when the analyte is separated from the elements in the sample matrix. Both continuous (FIA) and batch approaches have been used for hydride generation. The continuous method is more frequently used in plasma AES than in AAS. Commercial hydride generation systems are available for various plasma spectrometers. 

The higher reactivity of this reagent allows its use for the formation of volatile hydrides of antimony, arsenic, bismuth, germanium, lead, selenium, tellurium, and tin. This method is not only superior to the Zn/HCl method due to the wider range of elements that are accessible, but also with respect to speed, efficiency of the reaction, and reduced contamination. The reaction is essentially completed within 10—30 s, and the reagent is typically added into the acidified samples as 0.1—10% (w/v) solution. These factors contribute to the ease of automation which has been a key factor in the success of the hydride generation technique. 

Organic tin may be isolated from the matrix after extraction by various solvents or by hydride generation techniques. Measurement of tin is generally done by chromatographic methods sometimes coupled with sophisticated detection procedures. 

In addition to the above widely used atomization techniques, mention must also be made of the hydride generation technique . The method is based on conversion of a hydride forming element (As, Bi, Ge, Sb, Se, Sn, Te) in an acidified sample to volatile hydride and transport of the released hydride to an atomizer (hydrogen diffuse flame, graphite furnace, heated quartz tube) where they are atomized to give free analyte atoms. Sodium borohydride is almost exclusively used as an agent for conversion of analyte to hydride ... 

Arsenic is both toxic and cai cinogenic element. It is necessary to have a fast, reliable and accurate method for determination of ai senic in water. The hydride-generation atomic fluorescence spectrometry (HG AFS) is one of the simple and sensitive techniques for the determination of this element in various types of waters. 

In summary, the hydride generation method cannot adequately differentiate between aquated SnIV and Sn11, which may coexist in certain, especially anaerobic, environments found in marine waters. Inorganic tin, speciated as tin (IV) , should probably be regarded as total reducible inorganic tin until more discriminatory techniques become available [578,580]. 

It has been reported that the differential determination of arsenic [36-41] and also antimony [42,43] is possible by hydride generation-atomic absorption spectrophotometry. The HGA-AS is a simple and sensitive method for the determination of elements which form gaseous hydrides [35,44-47] and mg/1 levels of these elements can be determined with high precision by this method. This technique has also been applied to analyses of various samples, utilising automated methods [48-50] and combining various kinds of detection methods, such as gas chromatography [51], atomic fluorescence spectrometry [52,53], and inductively coupled plasma emission spectrometry [47]. 

These methods were used to determine arsenic in certified sediments (Table 12.15). Conventional inductively coupled plasma atomic emission spectrometry is satisfactory for all types of samples, but its usefulness was limited to concentrations of arsenic greater than 5pg g-1 dry weight. Better detection limits were achieved using the flow-injection-hydride generation inductively coupled plasma technique in which a coefficient of variation of about 2% for concentrations of lOpg g 1 were achieved. 

For the determination of organotin compounds (tributyltin, triphenyltin, triethyltin, and tetra-ethyltin) a MAE is proposed before the normal phase (NP) HPLC/UV analysis [35], In organotin and arsenic speciation studies, hydride generation is the most popular derivatization method, combined with atomic absorption and fluorescence spectroscopy or ICP techniques [25,36], Both atmospheric pressure chemical ionization (APCI)-MS and electrospray ionization ESI-MS are employed in the determination of butyltin, phenyltin, triphenyltin, and tributyltin in waters and sediments [37], A micro LC/ESI-ion trap MS method has been recently chosen as the official EPA (Environmental Protection Agency) method (8323) [38] it permits the determination of mono-, di-, and tri- butyltin, and mono-, di-, and tri-phenyltin at concentration levels of a subnanogram per liter and has been successfully applied in the analysis of freshwaters and fish [39], Tributyltin in waters has been also quantified through an automated sensitive SPME LC/ESI-MS method [40],... 

The most useful chemical species in the analysis of arsenic is the volatile hydride, namely arsine (AsH3, bp -55°C). Analytical methods based on the formation of volatile arsines are generally referred to as hydride, or arsine, generation techniques. Arsenite is readily reduced to arsine, which is easily separated from complex sample matrices before its detection, usually by atomic absorption spectrometry (33). A solution of sodium borohydride is the most commonly used reductant. Because arsenate does not form a hydride directly, arsenite can be analyzed selectively in its presence (34). Specific analysis of As(III) in the presence of As(V) can also be effected by selective extraction methods (35). 

To implement an easy and automated means for chemical vapour generation procedures (hydride generation for arsenic, selenium, etc., and cold vapour mercury), which allows for a reduction on the interferences caused by first-row transition metals (such as copper and nickel). FI methods may be readily coupled with almost all the atomic-based spectroscopic techniques (including graphite furnace atomisers). 

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