Development of a Simple System for the Determination of Arsenic after Hydride Generation Atomic Absorption Spectrophotometry

A simple and inexpensive hydride generation system that uses a pyrex boiling tube as a reduction vessel to hold the acidified sample into which 2cm of 100-300mg sodium tetrahydroborate (III) solution is injected such that the excess molecular hydrogen evolved carries the generated arsine directly through the atomic absorption spectrophotometer nebuliser to a nitrogen-hydrogen entrained air flame for atomic absorption measurement is described. The average absorption by arsenic atoms generated from the total arsine evolved was measured by integrating the absorbance for 16 seconds. The use of the absorption signal integration mode was found to free the technique from most kinetic interferences, while the stripping of the arsine from solution reduces the potential for chemical interferences in the solution phase. Gas flow rates affected sensitivity markedly, but the optimal flow rates for nitrogen and hydrogen were standardised on 90cms and 30cms respectively as a compromise between lower flow-rates giving longer arsenic residence times and higher sensitivity, and higher flowrates giving a stiffer, less draught-sensitive flame. An optimum flame height of 6mm above the burner produced sufficient atomization and low background absorption. Generally, sample volumes between 5-20cm could be used in different acidic media within the concentration range 10% (v/v) – 50% (v/v). The proposed system produced a significantly improved atomic absorption sensitivity of 25ng and a detection limit of (95% confidence) of 5.2ng for arsenic. Both performance characteristics were found to be better by a factor ≥ 12 and ≥ 27 respectively than achieved with air-acetylene flames. The method is highly reproducible at trace levels having an overall precision of 1% for 0.5μg of arsenic. The upper limit of linearity of the response for atomic absorption measurements is about 0.7μg. In use, the apparatus is fast and convenient to operate, enabling about forty determinations per hour. @ JASEM Braman et al, (1972) introduced the sodium tetrahydroborate (III) acid reaction (NaBH4/acid) system as a better alternative to the first most frequently used metal/acid reaction of Holak (1969) to quantitatively generate volatile covalent hydrides for Atomic Absorption Spectrophotometric (AAS) measurement. Since then several workers have used the method of Braman et al, (1972) to develop expensive Hydride Generation (HG) accessory currently in the market. Inspite of the progress made in the development of the HG system and its advantages over the conventional method (by solution aspiration into the air-acetylene flame), many analytical/quality control laboratories are yet to adopt the HG system for hydride forming elements of Groups IV, V, and VI either due to cost implications, lack of expertise, or more importantly, the inability of analysts to take cognisance of the serious limitations of the airacetylene system in chemical analysis. These limitations include: poor atomic absorption sensitivity; poor detection limit; poor atomization efficiency; significant chemical interferences and severe flame absorption in the low ultraviolet region of the spectrum where most of the absorbance lines of hydride forming elements are located. These limitations all combine to introduce errors particularly at trace levels among which is a risk of concluding that a determinant is present when it is not or a risk of concluding that a determinant is absent when infact it is not. This explains why the quality of analytical data generated for covalent hydride elements using the air – acetylene flame method at trace levels have become suspect, inaccurate and unreliable. Consequently, we were driven by the need to develop an operationally simpler, inexpensive and rapid HG system which can be used to minimize or improve on the serious limitations associated with the air-acetylene flame atomization technique for arsenic.In this study, arsenic was selected as the analyte considering its wide distribution throughout the biosphere and its significance as an environmental pollutant with no known beneficial biological function. Furthermore, because it is both toxic and carcinogenic (Hernberg, 1977) with adverse effects on the nervous system, cardiovascular system and the genetic system in man (Fowler et al 1979), the need for a simple and sensitive analytical system for detection of low level arsenic in a wide range of environmental and biological samples is axiomatic.It is hoped that chemists and analysts will take advantage of this simple HG accessory to improve on the quality of analytical output for arsenic in the course of their investigations. J. Appl. Sci. Environ. Mgt. 2001