Environ. Scl. Technol. 1988,22, 527-531
Rainwater Analysis: A Comparison between Proton-Induced X-ray Emission and Graphite Furnace Atomic Absorption Spectroscopyt Hans-Christen Hansson,* A * Ann-Kristin P. Ekhoim,$and Howard B. Ross*
Department of Nuclear Physics, University of Lund, Solveg. 14,S-22 362 Lund, Sweden, and Department of Meteorology, University of Stockholm Arrhenius Laboratory, S-106 9 1 Stockholm, Sweden
rn Rainwater was collected and analyzed for trace metals by graphite furnace atomic absorption spectroscopy (GFAAS) and proton-induced X-ray emission (PME). For the PIXE analysis, a nonselective preconcentration technique was used to dry the samples onto polystyrene films. Good agreement was found in the concentrations for the elements determined in common (Cu, Fe, Mn, Pb, and Zn), even though the PIXE analysis had some problems with sample contamination. An advantage of the PIXE analysis is that 26 elements can be determined simultaneously. The elements P, S, K, Ca, V, Mn, Fe, Ni, Zn, As, Br, and P b had concentrations significantly higher than the cheniical blank and detection limits and thus could be routinely analyzed by PIXE. The technique is promising for the other elements Ti, Cr, Co, Cu, Ga, Ge, Se, Rb, and Sr because considerable improvements in detection limits can be achieved by lowering the content of the blank and by increasing the analysis time. Introduction With the advent of industrialization, the atmosphere, via wet and dry deposition, has become a major source of trace metals to ecosystems in and around industrial areas. Trace metals such as Cd, Hg, As, and Pb, which are highly toxic even at low levels, are known to be accumulating in the biosphere and are to a great extent cycled through the environment by atmospheric transport (1-5). In addition, Cd and Pb even have been investigated as possible causes of forest decline in Europe and the United States (6). Around anthropogenic point sources such as smelters, atmospheric deposition of Cr, Cu, Ni, and Zn has led to adverse affects to forest ecosystems (7,8). It is primarily for these toxicological reasons that interest has grown in monitoring trace metal concentrations iq atmospheric precipitation. By far the most popular instrumental method for determining trace metals in atmospheric precipitation and even other natural water samples is graphite furnace atomic absorption spectroscopy (GFAAS) (9). Many trace metals in atmospheric precipitation can be analyzed without preconcentration. With the advent of computer-controlled autosamplers, preconcentration is even less of a problem and can now be performed directly in the furnace. This saves much time. Still, the major drawback with most commercial GFAAS instruments is that only one element at a time can be analyzed. Another spectroscopic technique, inductively coupled plasma (ICP), is multielement but sensitivities are generally too low for natural water sample determinations (10). Therefore, samples may need to be preconcentrated. Recent advances in the interfacing of mass spectrometers with ICP and graphite furnaces offel: promising multiele'This paper is Contribution No. 578 from the University of Stockholm Arrhenius Laboratory. 4 University of Lund. University of Stockholm Arrhenius Laboratory.
*
0013-936X/88/0922-0527$01.50/0
mental techniques with very low detection limits (11,12). However, commercial availability of such instrumentation is in its infancy. Techniques such as X-ray fluorescence, instrumental neutron activation analysis (INAA), and proton-induced X-ray emission (PIXE) can simultaneously determine a large number of trace metals in a single analysis. These techniques have been used extensively in determiningtrace metal concentrations in atmospheric aerosol samples (13-20). INAA has also been used to determine trace metal concentrations in natural water samples; the analysis, however, requires preconcentration of the sample onto a solid substrate such as activated carbon or filter paper (21-23). In PIXE analysis as well as with INAA, one can not directly analyze water samples but must first preconcentrate the sample onto a solid substrate (24). Recently, we reported a nonselective preconcentration technique for P M E analysis of water.samples, which is simple and quick (25). The liquid is forced through a nebulizer and pressurized air into a dry clean airstream where the droplets evaporate. The solid residual aerosol is collected by an impactor on a thin polystyrene film. The film is then analyzed by PIXE. This technique is similar to the one used hy Kellogg et al. (26)for the analysis of solutions with X-ray fluorescence analysis. In this paper, we report a comparison between GFAAS and PIXE using this preconcentration technique for the analysis of rainwater. The two laboratories involved in this study have much experience and expertise in the analytical methods they use respectively. Experimental Section In October 1984, monthly precipitation samples for trace metal analysis were collected at four sites in Sweden (Figure 1) as part of the SNV-PMK (The National Swedish Environmental Protection Board-Environmental Monitoring Program). At each site there were five acidwashed rain collectors, Rain collectors were of funnel and bottle type and are constructed out of conventional polyethylene. They were placed 1m from the ground and in open fields, at least 10 m from the nearest tree. The maximum distance between collectors did not exceed 40 m. Sampling locations were at least 500 m from the nearest road and 290 m from the nearest building (27-29). For GFAGS analysis, the samples arrived at the University of Stockholm Rain Chemistry Laboratory and were acidified to pH =l.O with concentrated HNO, (supra-pur, Merck A.G.) to prevent adsorption onto the container walls. Cd, Cu, Fe, Mn, Pb, and Zn were then determined by GFAAS. A detailed description of collection and analytical methodologies to avoid sample contaminations is given by Ross (27-29). For PIXE analysis, 50 mL of the samples was poured into ultraclean conventional polyethylene bottles ' in a metal-free clean room. Previous studies indicate that these bottles do not appreciably effect metal concentrations. Samples were then sealed in two plastic bags and sent to
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 5, 1988 527
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Table I. Arithmetic Mean Trace Metal Concentrations (fig L-') for Each Sampling Site and Associated Standard Deviations Determined with GFAAS" sampling site Arup Bredkalen
ARpvreten Mn Fe Cu Zn Cd Pb
3.8 f 0.5 1 9 f
