Application of Energy-Dispersive X-ray Fluorescence to Trace Metal Analysis of Natural Waters John F. Elder,' Scott K. Perry, and F. Paul Brady Crocker Nuclear Laboratory, University of California, Davis, Calif. 95616
w Energy-dispersive X-ray fluorescence is a relatively recent development in the field of X-ray spectrometry which improves capability for rapid multielement analysis. Application of the technique to analysis of dissolved trace metals in water requires transfer of the dissolved elements to a uniform target suitable for analysis. This can be accomplished by precipitating the elements with the nonspecific chelating agent, ammonium-1-pyrrolidine dithiocarbamate, and filtering through a membrane filter. The method is applicable to many types of aqueous samples and for analysis of most transition metals. Equipment and costs, as well as advantages and limitations of the method are discussed. Data from analysis of waters in the Lake Tahoe basin are presented and discussed.
The diversity of trace metal chemistry typical of natural water systems and the importance to living organisms of various trace elements ( I ) call for a method of analysis which allows rapid investigation of many elements simultaneously. The relatively new technique of energy-dispersive X-ray fluorescence (XRF) provides precisely that kind of simultaneous multielement analysis not possible with wavelength-dispersive XRF or with most of the other commonly used trace analysis techniques. The usefulness of XRF for solid samples has been reported frequently (2-4) but because of the difficulty of target preparation, it has rarely been applied to water analysis. Physical methods of target preparation, such as evaporation of the water followed by collection of the residue, tend to be plagued with problems of poor residue recovery, nonuniform deposition, and inordinate time consumption. Some methods applied by other investigators include chelation-extraction (5, 6), ion exchange (7), and coprecipitation (8). Luke ( 9 ) , working with metals and metal alloys, used a precipitationlcoprecipitation technique to prepare targets for analysis by wavelength-dispersive XRF. The energy-dispersive detection system is a quite recent development (10, 1 1 ) that has significantly increased the capabilities of X-ray spectrometry for elemental analysis over those of a wavelength-dispersive system. Wavelength dispersion 3ffers higher resolution below an energy of approximately 15 KeV but it is severely limited in its detection sensitivity and accuracy because of the high stability required. Energy-dispersive detection is accomplished by means of voltage pulses from the detector proportional to energies of incoming X rays, and it allows quantitative and simultaneous detection of all elements in the sample a t rates exceeding 6000 X-ray counts per second. To take full advantage of this wide multielement capability of the analyzer, the method of sample preparation must be nonspecific. It must also produce a uniform distribution of residue. The uniformity of residue deposition is extremely important for analytical X-ray work because of inherent attenuation (self-absorption) of the characteristic X rays within the target matrix itself. This effect is more Present address, Department of Plant Science, University of California, Riverside, Calif. 92502
pronounced for the lower energy X rays of lighter elements. A simple and reliable correction term may be applied in the analysis only when the deposition is uniform, A precipitation method similar to that described by Luke (9) and Watanabe et al. ( 1 2 ) can be applied to trace metal analysis of water by energy-dispersive XRF, and its simplicity permits processing of large numbers of samples in relatively short time. The bidentate chelating agent ammonium-1-pyrrolidine dithiocarbamate (APDC) forms insoluble coordination complexes with more than 30 transition metals which bond to the sulfur ligand atom (13).The alkali and alkaline earth metals are excluded. The high stabilities and low solubilities of the metal dithiocarbamate chelates (14, 1 5 ) are sufficient to permit quantitative recovery of many metals with little or no pretreatment of the natural water at a dithiocarbamate concentration of 10-3M. The usefulness of APDC chelation is enhanced by its effectiveness over broad pH ranges (16).
Methods A volume of water (usually 100 ml) free of particulate matter is measured into a suitable container. The pH can be adjusted, if judged necessary, by addition of a small amount of HCl or HN03. A 1.0% APDC solution is prepared fresh daily by dissolving ammonium-1-pyrrolidine dithiocarbamate in distilled water and filtering through a prewashed glass fiber filter (Whatman GF/C or equivalent). A minimum of 2 ml of this solution per 100 ml of water sample is introduced with thorough mixing, and the sample is allowed to stand at room temperature for 20 min to permit equilibration. The suspension is filtered through a prewashed membrane filter after which the filter is placed on a clean absorbent tissue and weighted around the edges with a plastic ring to prevent curling. The filter is allowed to dry for several hours then mounted on a target holder for X-ray analysis. About 30 min are required to reduce a natural water specimen to a form suitable for analysis. A portion of this time is for the separation of the particulate matter from the aqueous phase. This may be accomplished by centrifugation or by filtration through a membrane filter. If the latter method is used, the particulate matter, if not too coarse, may also be analyzed by XRF. Uniform targets prepared from water samples can be quickly and economically analyzed by energy-dispersive X-ray fluorescence. We have successfully analyzed up to 10 metals simultaneously utilizing this method. This has been accomplished by irradiating the targets with either molybdenum or silver anode X-ray beams (heavily filtered to achieve a high degree of bichromatization), which fluoresce characteristic X rays of the complexed metals. The silver K-radiation is particularly useful for exciting the K-radiation of the transition metals which are of primary interest in most aquatic studies. Characteristic radiation from a lanthanum or samarium secondary target has also been used to excite K lines of heavier elements such as cadmium and barium. For the lighter elements, a copper secondary target has been used. The instrumentation used in our laboratory is diagrammed in Figure 1. The system consists of components Volume 9, Number 12, November 1975
1039
i I
1
v L rear
CHbNNEL
Flgure 1. Block diagram of the XRF system LN2 = liquid nitrogen; P/A = preamplifier:ADC = analog-todigital converter
Table I. Evaluation of Analytical Accuracy Based on Detection of Metals Added t o Natural Water APDC precipitation was carried out at pH 2. Line d gives the actual amount (as measured by
atomic absorption analysis) of each metal added to the natural water a n d distilled water whose subsequent detected concentrations are shown in lines b a n d c. The recovery ratios show recovery efficiency in stream water (line e ) a n d recovery efficiency in distilled water standards (line f ) Concentration in ppb
Fe
cu
Zn
Hg
Pb
a. Ward Cr.
6.4
2.1
0
0
0
b. W a r d Cr.
72.0
94.6
11.7
129.8
97.1
66.5
94.1
26.6
135.6
101.5
92
120
102
water alone
water + M c. Distilled water + M d . Actual M
112
101
Recovery ratios
e. ( b - a ) / d f. c/d
Fe
cu
0.59 0.59
0.92 0.93
Zn
0.13 0.29
Hg
Pb
1.08 1.13
0.95 1.00
acquired separately and integrated into a unit of high analytical capability and versatility. The characteristic radiations from a wide range of elements are detected simultaneously with a highly collimated Si(Li) solid state detector (Kevex Model 3000-80 mm2) with an energy resolution of 190 electron volts a t 5000 cps. The tight geometry of our system allows full count rate capability (5000 cps for reasonable dead times and resolution) with only a modest X-ray generator. The pulsed optical feedback amplification system also from Kevex sends voltage signals (proportional to the X-ray energy) into an analog-to-digital converter and pulse height analyzer (Technical Measurements Corp.). This produces a spectrum of counting frequency vs. X-ray energy. The X-ray spectra are then stored on magnetic tape to be analyzed off-line by a PDP 15/40 computer. The monitor system (Figure 1)allows one to monitor the output 1040
Environmental Science & Technology
of the X-ray tube so that accurate normalization of unknown to known samples can be achieved. Typically two prominent peaks occur for each element, and this can cause interelement interferences which must be dealt with via a computer code. The computer program automatically locates, identifies, and integrates the X-ray peaks and then calculates concentrations using calibration constants arrived at by a statistical analysis of six replicate preparations of each of two National Bureau of Standards samples certified for trace content (NBS Bovine Liver SRM 1577 and NBS Orchard Leaf SRM 1571). The program strips out some interferences by fitting multiple peaks with superimposed Gaussian functions; other interferences are handled arithmetically after fitting. Normally isolated peaks of sufficient definition can be merely summed with a linear background subtraction to speed analysis. An XRF system is particularly suitable for small laboratories. The cost of a basic system including X-ray generator and tube, detector system, and pulse height analysis system with inexpensive magnetic tape readout is on the order of $25,000. Including off-line computer analysis, the cost of excitation and analysis is approximately $4.00 per sample. Sample preparation and personnel costs are additional.
Results To test the accuracy of the XRF technique for natural water samples, standard addition tests were conducted. These consisted of analyses of natural samples which contained standard additions of iron, copper, zinc, mercury, and lead and acidified to pH 2. The water was sampled from Ward Creek, Calif., a stream that flows into Lake Tahoe. The standard solution was made up from reagent grade FeS04-7H20, CuSO4, Z n S 0 ~ 7 H 2 0 , HgC12, and Pb(N03)2dissolved in quartz-distilled water and acidified with H N 0 3 to pH 2. Spiked samples were compared with natural water and with distilled water to which similar standard additions were made. The concentration of the standard itself was verified by atomic absorption analysis. The results of these tests are seen in Table I. One test of target uniformity consisted of replicate analyses of a single target, where the position of the 16-mm diameter target in the 8-mm diameter beam was shifted several millimeters with each repetition. The results of this test are presented in Table 11. The results of some analyses of blank Millipore HAWP filters are shown in Table 111. Only the three elements shown were detected in the blanks. Some natural water analyses performed by the technique described in this paper gave the results illustrated in Figures 2 and 3. Figure 2 illustrates an actual spectrum pro-
Table II. Replicate Analyses of Single Target The position of the X-ray beam was changed slightly for each repetition Concentration in ppb Replicate no.
Fe
Zn
Pb
1 2 3 4 5
7.6 8.2 8.2 9.3 8.3 8.3 0.6 7
1.8 1.8 1.7 1.8 1.8 1.8 0 0
3.8 3.4 3.6 3.9 3.7 3.7 0.04 1
Mean Std dev % Std dev
I
Table Ill. Metals Detected in Blank Millipore HAWP 02500 Filters Fe ( 7 7 )
Concentration in ppba Filter n o .
Fe
cu
Zn
1 2 3
2.2 2.0 2.0
4
2.5
1.5 1.2 1.1 1.5
0 0 0 0.7
a k e s u l t s given are not actual concentrations o f the filters but rather the blank subtractions, in parts per billion, that would be made for 1 0 0 - m l water samples converted t o targets on this type of filter.
Flgure 2. Spectrum showing results of XRF analysis of a water sam-
ple from the Sacramento-San Joaquin Estuary Concentrations of the elements in parts per billion are given by numbers in parentheses. K,, peaks are used for analysis. Unidentified peaks include the K j peaks of Fe, Cu, and Zn (channel numbers = 330, 424, and 468, respectively). Compton and coherent scatter peaks from the molybdenum anode tube are located to the right
40
t
520c
! I
Ot-
Fe N I C U Mo
LT
n , Fe N I Cu MO
wc
TR
SQ
I
Figure 3. Dissolved metals detected in water samples collected in February 1974 from Lake Tahoe (LT) and its tributaries-Ward Creek (WC), the Truckee River (TR), and Squaw Creek (Sa)
duced by XRF analysis of water from the Sacramento-San Joaquin estuary. The elements represented by the peaks are identified. Numbers in parenthesis are the concentrations of the elements (in parts per billion or micrograms per liter of water) determined bv computer analysis of the spectrum. Elements lighter than titanium were not quantitatively analyzed because of high self-absorption at the lower energy levels. Figure 3 shows the concentrations of elements found in four different water bodies in the Lake Tahoe basin: Lake Tahoe, Ward Creek, the Truckee River which drains the Lake, and Squaw Creek which drains Squaw Valley and empties into the Truckee River. All data presented in Figures 2 and 3 pertain to dissolved metals only; particulates were filtered out prior to analysis.
Discussion One of the important advantages of this technique is its multielement capability. As the results shown in Table I illustrate, however, no single analysis is equally reliable for all elements. A great deal depends upon the treatment of the sample during preparation of the target, as well as nat-
ural water interferences that are variable for different metals. Ratios in line e reveal the effects of the conditions of the preparation plus any interferences caused by the properties of the stream water, while line f ratios are dependent only upon conditions of the preparation, assuming an absence of interferences in distilled water. The two ratios are substantially different only for zinc, indicating negligible stream water interferences in the analysis of the other four metals. The incomplete recovery of iron and most of that of zinc occurred in distilled water standards as well as in stream water, demonstrating. that the conditions of the preparation were largely responsible for the low results and the method should be altered for analysis of those two metals. The most important controlled variable is pH. All targets whose analyses produced the data given in Table I were prepared a t pH 2. For most elements, the low pH is helpful for destroying naturally occurring complexation capacity, which usually is characteristic of natural waters (17, 18). When iron and zinc are of interest, however, the target preparation should be carried out a t a higher pH, and competing complexation capacity may be destroyed by other means such as the method of photooxidation of organic compounds by ultraviolet radiation described by Armstrong et al. (19). As previously emphasized, reliability of XRF analysis depends upon uniformity of the target. Uniform distribution of the residue produced by dithiocarbamate precipitation and filtration is confirmed by the high reproducibility of replicate analyses of a single target (Table 11). Filters containing dithiocarbamate precipitates were also examined by scanning electron microscopy which confirmed the uniformity of residue distribution. The experimental results presented in Table I1 are exemplary of a number of such tests all of which demonstrated a high level of internal consistency or precision in the measurements. Blank analyses of untreated membrane filters reveal some contamination of iron, copper, and zinc (Table 111). Blank values are low, however, and more importantly, they are consistent, so that reliable blank corrections can be made. The inherent detection limit of the X-ray system (as measured in terms of the actual mass of the target matrix is on the order of 1 ppm (20). This is high enough to prevent troublesome background levels, yet low enough to permit part-per-billion detection levels for water whose metals are concentrated by means of precipitation. This method of trace analysis is applicable to all types of natural water. Our work has included analysis of water from the Sacramento-San Joaquin Estuary, from which a representative spectrum (Figure 2 ) illustrates typical sensitivity and background levels. In addition, we have analyzed rainwater, city water, and the extremely dilute waters of Lake Tahoe. A test of the effects of ionic strength showed Volume 9, Number 12, November 1975
1041
that salinities up to 4.0% do not interfere, suggesting that there should be no particular problems in the analysis of seawater. Some types of biological samples may also be analyzed by this procedure. Trace analysis of waters in the Lake Tahoe basin (Figure 3) demonstrate, as expected, very low levels of metals. Most metals are present primarily in the particulate form. The lake itself is an extremely oligotrophic system, and as such, it contains very low nutrient levels. If those levels are increased by amounts which would be insignificant in most other systems, there can be quite marked effects upon the biotic community in the lake. The scarcity of dissolved iron renders this element an especially important limiting nutrient. Maintenance of the present water clarity a t Lake Tahoe is threatened by the large influxes of iron which result from erosion of devegetated soils in the watershed. It is common to see a great deal of variability in nutrient levels in Lake Tahoe and its tributaries, as exemplified by the high level of iron found in the Truckee River. This is often due to changes in periphyton growth in the streams. The periphyton strips nutrients from the water while it is growing but releases them rapidly during large death and deconiposition periods that occur during the winter. Conclusions
Energy-dispersive X-ray fluorescence has much to offer in the field of trace metal monitoring of ecological systems. Coupled with dithiocarbamate precipitation for conversion of water samples to XRF targets, it is capable of rapid, multielement analysis at relatively low cost. It is also a convenient technique for performing separate analyses on dissolved and particulate fractions. For studies requiring analyses of only one or two metals, atomic absorption spectrophotometry or colorimetric methods may be more appropriate. Studies of alkali and alkaline-earth metals, and elements lighter than titanium may also require other techniques. Where the transition metals are of interest, how-
ever, use of X-ray fluorescence for ecological studies of natural water systems has obvious merits. Acknowledgments
The authors wish to thank Charles Goldman for his support of this research and review of the manuscript. Personal consultation with Charles Nash and scanning electron micrographs provided bv Hans Paerl were also appreciated. Literature Cited (1) Goldman, C. R., Mem. 1st. Ital. Idrobiol., 18, Suppl., 121-35 (1965). (2) Murad, E., Anal. Chim. Acta, 67,37-53 (1973). (3) Leoni, L., Saitta, M., X-Ray Spectrom., 3,74-7 (1974). (4) West, N. G., Hendry, G. L., Bailey, N. T., X - R a y Spectrom., 3, 78-87 (1974). (5) Morris, A. W., Anal. Chim. Acta, 42,397-406 (1968). (6) Armitage, B., Zeitlin, H., ibid., 53,47-53 (1971). (7) Murata, M., Noguchi, M., ibid., 71,295-302 (1974). (8) Reymont, T. M., Dubois, R. J., ibid., 56, 1-6 (1971). (9) Luke, C. L., ibid., 41, 237-50 (1968). (IO) Rhodes, J. R., Amer. Lab., 5,57-73 (1973). (11) Giauque, R. D., Goulding, F. S., Jaklevic, J. M., Pehl, R. H., A n d . Chem., 45,671-81 (1973). (12) Watanabe, H., Berman, S., Russell, D. S., Talanta, 19, 136375 (1972). (13) Malissa, H., Schoffmann, E., Microchim. Acta, 1955, 187-202 (1955). (14) Hulanicki, A., Talanta, 14,1371-92 (1967). (15) Usatenko, Y. I., Barkalov, V. S., Tulyupa, F. M., Zh. Anal. Khim., 25, 1458-61 (1970, Russ.). [Transl. J. Anal. Chem. U.S.S.R.,25,1257-9 (19701.1 (16) Brooks, R. R., Presley, B. J., Kaplan, I. R., Talanta, 14,80916 (1967). (17) Chau, Y. K., J . Chromat. S a . , 11,579 (1973). (18) Kunkel. R.. Manahan, S. E., Anal. Chem., 45,1465-8 (1973). (19) Armstrong, F. A. J., Williams, P. M., Strickland, J. D. H., Nature, 2,481-2 (1966). (20) Perry, S. K., Brady, F. P., Nucl. Znst. Meth., 108, 389-96 (1973).
Received for review October 1, 1973. Accepted June 23, 1975. Work supported by the RANN Division of the National Service Foundation.
Atmospheric Fates of Halogenated Compounds Daniel Lillian,' Hanwant Bir Singh,* Alan Appleby, Leon Lobban, Robert Arnts, Ralph Gumpert, Robert Hague, John Toomey, John Kazazis, Mark Antell, David Hansen, and Barry Scott Cook College, Busch Campus, Rutgers University, New Brunswick. N.J. 08903
Based on the U.S. Tariff Commission report and the many uses of halocarbons, evidence exists that halocarbons as a group of pollutants are emitLed to the atmosphere in significant quantities. Halocarbons are used extensively, especially in developed countries, as refrigerants, aerosol propellants, solvents, cutting fluids, synthetic feedstocks, and in the production of textiles and plastics. Production figures indicate a healthy growth rate of the halocarbons industry in excess of 6% per annum. Their total production in the U S . between January and July 1973 (7 months) exceeded 10 billion pounds ( I ) , suggesting a yearly worldwide production greater than 30 billion lb. Atmospheric emission rates for CC13F alone have increased from an average of 0.14 billion lb per year between 1961 and 1965 to 0.51 billion lb per year between 1971 and 1972, with the U S . ' P r e s e n t a d d r e s s , U.S. Army Industrial Hygiene Agency, Edgewood Arsenal, Md. * Present address, Stanford Research Institute, Menlo Park, Calif. 1042
Environmental Science & Technology
and Canada accounting for 44% of the world emissions (2). Similarly, the yearly worldwide injections of CC12F2 and C2Cl4 into the troposphere in 1974 are estimated to be nearly 1 billion lb each. Because of the significant emissions of these and other halocarbons, their potential environmental impact, including the destruction of the protective stratospheric ozone layer, and their usefulness as chemical and physical tracers for understanding complex atmospheric and environmental factors, we have undertaken an extensive ongoing study to characterize the atmospheric behavior of this relatively unstudied group of air contaminants. Reported here are aerometric halocarbon and SFs data obtained during several programmed field studies initiated since March 1973 at various locations in the U S . which should be representative of a wide gamut of emission patterns and meteorological conditions. The compounds included in the present study are CCl3F, CC12F2, CH3CC13, CC12CC12,CCL, CHCICClp, CH31, SF6, CH2CHC1, CHC13,
