Hydride generation samples

Several elements (including As, Bi, Ge, Pb, Sb, Se, Sn, and Te) form volatile hydrides when reacted with sodium borohydride at room temperature. By introducing the analyte as a volatile hydride, high-transport efficiencies, and therefore improved detection limits, can be achieved. Often as importantly, much of the sample matrix is not introduced into the ICP because those species do not form volatile compounds. Commercial hydride generation sample introduction systems are available. 

It should be pointed out that few elements are present in most natural waters at concentrations where flame spectroscopic techniques are directly applicable. Those that are include calcium, magnesium, sodium, potassium, and, in some samples and if conditions are very carefully optimized, manganese, iron, and aluminium. Zinc, and sometimes cadmium, may be determined directly by AFS. Mercury and hydride-forming elements may be determined if cold vapour and hydride generation sample introduction techniques are employed, as discussed in... 

M. Segura, Y. Madrid, C. Camara, Evaluation of atomic fluorescence and atomic absorption spectrometric techniques for the determination of arsenic in wine and beer by direct hydride generation sample introduction, J. Anal. Atom. Spectrom., 14 (1999), 131-135. 

H. Matusiewicz, M. Mikolajczank, Determination of As, Sb, Sn, and Hg in beer and wort by direct hydride generation sample introduction - electrothermal AAS, J. Anal. Atom Spectrom., 482 (2001), 652-657. 

Buckley WT, Budac JJ, Godfrey DV, et al. 1992. Determination of selenium by inductively coupled plasma mass spectrometry utilizing a new hydride generation sample introduction system. Anal Chem 64(7) 724-729. 

FIGURE 5.2 Plumbing arrangement for hydride generation sample introduction system. 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

Air (particulate lead) Collection of sample onto cellulose acetate filter dissolution in HN03 with heat addition of HCI / H202 and reaction in hydride generator with sodium borohydride to generate lead hydride AAS 8 ng/L 100-101 Nerin et al. 1989... 

Aroza I, Bonilla M, Madrid Y, et al. 1989. Combination of hydride generation and graphite furnace atomic absorption spectrometry for the determination of lead in biological samples. J Anal Atmos Spectra 4 163-166. 

Samanta G, Chakraborti D. 1996. Flow injection hydride generation atomic absorption spectrometry (FI-HG-AAS) and spectrophotometric methods for determination of lead in environmental samples. Environmental Technology 17(12) 1327-1337. 

Sturgeon et al. [59] have described a hydride generation atomic absorption spectrometry method for the determination of antimony in seawater. The method uses formation of stibene using sodium borohydride. Stibine gas was trapped on the surface of a pyrolytic graphite coated tube at 250 °C and antimony determined by atomic absorption spectrometry. An absolute detection limit of 0.2 ng was obtained and a concentration detection limit of 0.04 pg/1 obtained for 5 ml sample volumes. 

Bertine and Lee [60] have described hydride generation techniques for determining total antimony, Sb (V), Sb (III), Sb-S species and organo-antimony species in frozen seawater samples. 

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]. 

Valkirs et al. [105] have conducted an interlaboratory comparison or determinations of di- and tributyltin species in marine and estuarine waters using two methods, namely hydride generation with atomic absorption detection and gas chromatography with flame photometric detection. Good agreement was obtained between the results of the two methods. Studies on the effect of storing frozen samples prior to analysis showed that samples could be stored in polycarbonate containers at - 20 °C for 2 - 3 months without significant loss of tributyltin. 

An ultasensitive simultaneous multi-element method of determination for As, Se, Sb and Sn in aqueous solution, consists of hydride generation, collection in a cryogenic trap and end analysis by GC-PID (photoionization detector) LOD ca 1 ng Sn/L for a 28 mL sample. No drying or CO2 scrubbing is necessary before the cold trap35. 

Instead of AES the molecular emission of SnO can be stimulated in an oxycavity placed in a H2/N2 flame, and measured at 408 nm. The recommended sample preparation consists of hydride generation and concentration by cold-trap collection LOD 80 pg Sn(II)/L in a 1 mL sample36. 

The analytical response of inorganic and organic tin compounds of formula R SnX 4 was studied for direct hydride generation and measurement with a non-dispersive AFD (atomic fluorescense detector). Tributyltin and phenyltin compounds gave unsatisfactory results. This was corrected by warming the sample with a dilute Br2-HN03 solution37. 

A method for tributyltin in sediments consists of extraction with anhydrous acetic acid, hydride generation, cold trapping and end analysis by GC-AAS using a quartz furnace75. Reduction with NaBFLi followed by solvent extraction, concentration and GC-FPD was proposed for simulaneous determination of di- and tributyltin residues in sea water LOD 10 ng/L for 1 L sample, with 87.1-98.4% of Sn recovery76. 

There are several sample introduction methods that are used in conjunction with ICP, including nebulization, electrothermal evaporation, gas chromatography, hydride generation, and laser ablation [30]. Laser ablation combined with ICP (LA-ICP) is useful for analysis of solids. In such a source the sample is positioned in a chamber prior to the ICP source, the ablation cell. Argon gas at atmosperic pressure flows through the cell towards the ICP source. The sample is irradiated by a laser beam and... 

The manifold for hydride generation is shown in Fig. 12.7. The operating conditions are as follows forward power 1400W, reflected power less than 10W, cooling gas flow 12L nr1, plasma gas flow 0.12L nr1, injector flow, 0.34L m 1. The standard deviation of this procedure was 0.02pL 1 arsenic and the detection limit O.lpg L-1. Results obtained on a selection of standard reference sediment samples are quoted in Table 12.14. 

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. 

All four dissolution procedures studied were found to be suitable for arsenic determinations in biological marine samples, but only one (potassium hydroxide fusion) yielded accurate results for antimony in marine sediments and only two (sodium hydroxide fusion or a nitricperchloric-hydrofluoric acid digestion in sealed Teflon vessels) were appropriate for determination of selenium in marine sediments. Thus, the development of a single procedure for the simultaneous determination of arsenic, antimony and selenium (and perhaps other hydride-forming elements) in marine materials by hydride generation inductively coupled plasma atomic emission spectrometry requires careful consideration not only of the oxidation-reduction chemistry of these elements and its influence on the hydride generation process but also of the chemistry of dissolution of these elements. 

In this procedure methylstannanes were generated directly in buffered sample, purged and cryogenically trapped on the head of a chromatographic column. Gas flow was diverted from the hydride generator while the purge tube was filled with up to lOmL of fluid, usually consisting of 5mL of sample and 5mL of buffer (saturated... 

In this method approximately 19 samples of marine sediment were oven dried at 110°C then digested with nitric acid-perchloric acid and hydrofluoric acid-hydrochloric acid. The digested solution is made up to 50ml of an equal volume mixture of 6M hydrochloric acid and 2M nitric acid. 0.1ml or less of the digest was pipetted into the hydride generator, followed by 1ml 2M acetic acid, diluted to 100ml with double distilled water and reacted with sodium borohydride. 

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