New developments in hydride generation - atomic spectrometry

Hiroaki Tao1; Taketoshi Nakahara2    1 - National Institute of Advanced Industrial Science and Technology, 16-1, Onogawa, Tsukuba, Ibaraki, 305-8569, JAPAN,
2 - Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, JAPAN.

1 INTRODUCTION

For some time, the generation of volatile hydrides of As, Sb, Bi, Se, Te, Ge, Sn and Pb has been used for sample introduction in atomic spectrometry. Since the work by Holak [1], numerous investigations have been carried out, and the hydride generation techniques may be considered to be mature. However, in recent years, there has been more fundamental research on new hydride generation methods, such as electrochemical hydride generation, vesicular hydride generation, and the production of unstable hydrides such as those of cadmium and copper. Mechanisms of chemical interferences in solution, and atomization interferences in flames, have been studied in depth, and methods to overcome these interferences have been proposed. For application to biological and clinical samples, elemental speciation has become a steady trend since it is essential to identify and quantify individual chemical species, in order to know the biological and toxicological effects of the elements. Hydride generation has been used for improving the detection limits of hyphenated methods, which combine separation methods, such as high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE), with element-specific detection methods such as atomic absorption spectrometry (AAS) and inductively coupled plasma mass spectrometry (ICP-MS). Many investigations concerning the on-line photooxidation or microwave-assisted digestion of organoarsenic or organoselenium compounds prior to hydride generation have expanded the applicability of hydride generation to a variety of compounds. Since 1993 the number of papers on hydride generation in atomic spectrometry has exceeded 600. Hence, it seems almost impossible to cover all aspects of hydride generation. For more comprehensive information, the following recent reviews and books are available: Dĕdina and Tsalev [2], Tsalev [3], Yan and Ni [4], Nakahara [5] and Matusiewicz and Sturgeon [6] for hydride generation, Caroli [7], Welz [8], Burguera and Burguera [9], Szpunar-Łobińska et al. [10], Muñoz et al. [11] and Dauchy et al. [12] for speciation.

In this chapter, a presentation is made of novel developments in hydride generation, chemical intereferences and their elimination, instrumental developments for elemental speciation, and practical applications to biological and clinical samples, all of which have occurred since a previous review in this series [13].

2 NOVEL HYDRIDE GENERATION

2.1 Electrochemical hydride generation

One major shortcoming associated with use of the sodium borohydride (NaBH4)–acid reduction technique for hydride generation (HG) is its susceptibility to interferences from transition metals. This system is dependent on the oxidation state of the analyte, and NaBH4 is also a potential source of contamination, which may limit detection power. It is expensive and is unstable in solution form, which generally necessitates daily preparation of fresh reagent. Electrochemical HG (EcHG) is a possible alternative to the NaBH4 reaction. It offers the potential advantage of requiring fewer reagents, therefore less chance exists for sample contamination by traces of analyte in the reagents. The generation of arsenic hydride by electrochemical reduction was first reported by Sand and Hackford in 1904 [14]. Rigin and coworkers demonstrated the use of a batch type EcHG for the determination of As and Sn, and found this method to be remarkably free from chemical interferences [15,16]. In recent years, a number of investigations on a new flow type EcHG have been done to realize the above-mentioned potential advantage of EcHG. Selected examples of recent research on EcHG are given in Table 1.