Anal. Chem. 1987, 59,891-896
89 1
Kinetic Control of Peak Shapes in Atomic Absorption Arsenic Determinations by Arsine Generation Stan Van Wagenen a n d Dean E. C a r t e r Department of Pharmacology a n d Toxicology, College of Pharmacy, University of Arizona, Tucson, Arizona 85721
A. G. Ragheb' a n d Q u i n t u s Fernando* Department of Chemistry, University of Arizona, Tucson, Arizona 85721
The rate processes that control the absorbance vs. time proflies that are obtained in the atomic absorption determlnation of arsenic( 111) by the sodlum borohydride reduction to arsine have been identified. The indlvldual rate constants for each of these rate processes have been calculated from the theoretical curves that give a best fit with the experimental peaks. The peak shapes are determlned to a great extent by the rate of hydrogen gas evolutlon and by the rate of removal of dlssoived arsine. Several suggestions have been made for improving the sensitivity of the determinatlon of arsenic(II1). The kinetic factors that control the peak shapes can be employed to optimize any conflguratlon of a hydride generator that Is used In atomic absorption spectrophotometry.
The generation of a gaseous hydride followed by atomization and determination of the hydride element is an established analytical technique that has been in use for more than a decade for the determination of elements such as arsenic, antimony, bismuth, germanium, lead, selenium, tellurium, and tin (1-5). The hydride is usually generated by the use of sodium borohydride (NaJ3HJ as a reducing agent; the gaseous hydride is either conveyed continuously into the atomizer (a flame, plasma, or heated quartz tube) (6) or collected in the course of the reduction process and subsequently introduced into the atomizer (7). Nanogram levels of arsenic have been determined by several variations of this technique but no attempt has been made to understand the kinetic factors that control the reduction process and the transport of the gaseous hydride, arsine, into the atomizer. These kinetic factors govern the absorbance vs. time profiles and can be exploited to determine the various forms of arsenic that are found in biological materials. The arsenic species that are usually present in biological materials are arsenic(III), arsenic(V), and the metabolites, monomethylarsonic acid and dimethylarsinic acid. In an acid solution, the rate of reduction of the two metabolites, by sodium borohydride, is extremely slow; the arsenic(III), however, is reduced very rapidly and the arsenic(V) is reduced a t a somewhat slower rate. These qualitative observations suggest that it may be p,ossible,at least in principle, to determine each of these four components by a kinetically controlled hydride generation and atomization. Several factors influence the absorbance vs. time profiles in a determination that involves a hydride generation and atomization. These factors also determine whether the peak height or the peak area is proportional to the concentration of the hydride element over a wide range of concentrations. The chemical reduction that results in hydride formation, the Present address: Department of Chemistry, Faculty of Petroleum and Mining Engineering, Suez Canal University, Suez, Egypt. 0003-2700/87/0359-0891$01.50/0
configuration of the hydride generator, the rate at which the hydride is transported into the atomizer, and the atomization process are the more obvious variables that have a profound effect on the peak shape. Factors such as the stability of the gaseous hydride, solubility of the hydride under various conditions of acidity in the solution in the hydride generator, and the temperature in the hydride generator and in the atomizer are additional factors that affect the peak shapes. For any configuration of a hydride generation-atomization system, the maximum sensitivity can be achieved by optimization of all the variables that govern the absorbance-time profiles. We have employed a widely used hydride generation-atomization system for the sodium borohydride reduction of arsenic(II1) in acid solution and we have recorded the absorbance vs. time profiles that were obtained in the determination of 50-500 ng of arsenic(II1) in 10 mL of solution. We have developed a kinetic model that fits these absorbance-time profiles and we have identified the rate processes that govern the peak shapes. The manner in which we have utilized this model to achieve a maximum sensitivity by optimizing the system variables is described below. EXPERIMENTAL SECTION Arsine was generated in a conventional hydride generator assembled from a three-neck round-bottom flask. The apparatus has been described in detail previously (8). A standard stock solution of As(II1) was prepared by dissolving As203in 0.2 M NaOH. A 6% solution of NaBH4 was freshly prepared daily by dissolving 6 g of NaBH, in 100 mL of 10% NaOH solution. The NaBH4 solution was injected into the bottom of the solution in the round-bottom flask with a syringe. Nitrogen gas was bubbled continuously through the solution and the flow rate of the nitrogen was controlled by using a Matheson 603 flowmeter. A nitrogen-hydrogen diffusion flame was used with the burner in the atomic absorption spectrophotometer (Instrumentation laboratories Model 251), and the gas flow rates were controlled with a Matheson K7630 flowmeter. The arsenic hollow cathode lamp was operated at 4.5 mA and the transmitted light was monitored at 193.7 nm at a slit width of 2.0 nm. The single beam mode was used with deuterium background correction. A typical absorbance vs. time profile is shown in Figure 1. The data acquisition system consisted of an IBM-PC which was modified by the addition of a Data Translation DT 2805 analog-to-digital (A/D) converter and a DT 707 terminal board that interfaced the A/D converter and the atomic absorption spectrophotometer. A monochrome graphics card and an 8087 math coprocessor integrated circuit chip were also added and the random access memory of the computer was expanded to 544K. All calculations, as well as the data acquisition, were performed with the aid of the software package ASYST (McMillan Software Co., New York, NY). CALCULATIONS Kinetic F a c t o r s T h a t Govern t h e P e a k Shape. The following rate processes govern the peak shapes (absorbance vs. time profiles) that are obtained in the determination of arsenic(II1) by the NaBH, reduction to AsH3 and the subse0 1987 American Chemical Society
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