Hydride/vapour generation

Table 5.3 sets out the advantages and disadvantages of the batch and continuous flow techniques. The introduction of continuous-flow hydride/vapour-generation has substantially advanced the value and acceptance of the technique for trace elemental analysis. Appfied Research Laboratories (now part of Fisons Elemental), P.S. Analytical and Varian have all introduced continuous-flow hydride/vapour-generation systems, whilst Perkin Ehner has used the flow injection modification to automate the techniques with their instrumentation. 

Figure S.2 shows a schematic diagram of the automatic hydride/vapour-generator system designed by P.S. Analytical. This has been widely used to determine hydrideforming elements, notably arsenic, selenium, bismuth, tellurium and antimony, in a wide range of sample types. To provide a wide range of analyses on a number of matrices the chemistry must be very well defined and consistent. Goulden and Brooksbank s automated continuous-flow system for the determination of selenium in waste water was improved by Dennis and Porter to lower the detection levels and increase relative precision [10, 11]. The system described by Stockwell [9] has been specifically developed in a commercial environment using the experience outlined by Dennis and Porter.

Figure S.4 shows a calibration graph of arsenic concentrations obtained by using a Perkin Elmer 2100 atomic-absorption system bnked to a P.S. Analytical hydride/vapour generator (PSA 10.003). An electrically heated tube has been used in this work and the spectral source was an electrodeless discharge lamp. Alternatively, a flame-heated tube can be used.

Hydride/vapour generation techniques provide extremely good sensitivity. When coupled to continuous flow methodologies for use in routine analysis, simple and reliable analytical techniques are provided. TTie extension of chemistries and sample transfer systems to provide analytical protocols to cope with a wider range of elemental analyses should be pursued in the search for lower detection levels. While multi-element techniques offer very low levels of detection, the use of specific single element analytical instruments with detection capabihties similar to those described above may be the best route for routine laboratories with high sample throughput. 

SEC. S.2] HYDRIDE/VAPOUR-GENERATION TECHNIQUES 163 288,145 counts (0.1 second)... 

Typical cold vapour generation AAS system used for mercury determination. The same system can be used with a flame in place of the Pyrex tube to allow the determination of hydride -forming elements. 

There have been a few research papers reporting the use of other vapour generation techniques to volatilize analytes that form unstable hydrides, or had previously been thought not to form vapours at room temperature. Examples include the use of sodium tetraethylborate to form volatile ethyl compounds of cadmium, lead and thallium. 

Gaseous and volatilised analytes can also be easily determined by FAAS and ETAAS. For example, the determination of several elements by the formation of covalent volatile hydrides e.g. arsenic, selenium) and cold vapour generation (mercury and cadmium) is feasible with good analytical sensitivity (see Section 1.4.1.1). 

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). 

Hydride and cold-vapour techniques represent a special combination of chemical separation and pre-enrichment with AAS determination, resulting in higher powers of detection for elements with volatile hydrides, eg, As, Bi, Se, Sb, Hg. Recent literature on vapour generation has been reviewed by Hill et al. (1991). Some examples of the use of hydride generation for the analysis of plant material are given by Muse et al. (1989), Leuka et al. (1990) and Ainsworth and Cooke (1990). Hydride generation can also be used with ICP-EAS (see below) and applications have been reviewed (Nakahara, 1991). 

Mercury and those elements (antimony, arsenic, bismuth, germanium, lead, selenium, tellurium and tin) which form volatile covalent hydrides may be separated from the matrix by vapour generation. The use of tin(II) chloride to generate elemental mercury and its subsequent aeration into a long-path absorption cell with silica windows has been described elsewhere in this book, as has the use of sodium borohydride to produce hydrides which are swept to a flame or heated tube for atomisation. This approach is far more successful for mercury than for the other elements, as the hydride generation technique is subject to interference from a large number of transition metals and oxyanions. 

The techniques discussed in this chapter vary in automatability and frequency of use. Thus, while automatic hydride and cold mercury vapour generation are implemented in laboratory-constructed or commercially available dynamic equipment that is straightforward, easy to operate and inexpensive, automating laboratory headspace modes and solid-phase microextraction is rather complicated and commercially available automated equipment for their implementation is sophisticated and expensive. Because of its fairly recent inception, analytical pervaporation lacks commercially available equipment for any type of sample however, its high potential and the interest it has aroused among manufacturers is bound to result in fast development of instrumentation for both solid and liquid samples. This technique, which is always applied under dynamic conditions, has invariably been implemented in a semi-automatic manner to date also, its complete automatization is very simple. 

The techniques dealt with in this chapter are discussed in terms of similarities. For this reason, hydride generation and cold mercury vapour generation are addressed first, notwithstanding the limited scope of this technique as regards analytes — scope that can... 

Because the hydride and cold vapour generation techniques have so far been used mainly with liquid and dissolved samples, all aspects related to the variables influencing the process and the characteristics of the ensuing methods, among others, have been established in the light of a liquid entering the separator. Most such aspects also affect solid samples and are thus worth some comment, as are those that are exclusive to them — all briefly as this technique has scarcely been applied to solid samples. 

The characteristics of the separator and its associated equipment used for hydride and cold mercury vapour generation are dictated by the state of the sample. 

Features of methods based on hydride or cold mercury vapour generation... 

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