COMMUNICATION
X-Ray Fluorescence Determination of Trace Toxic Elements in Water
A rapid procedure for the x-ray quantitation of As, Se, Hg, T1, Pb, and Bi in water in concentrations as low as 0.01 p.p.m. involves addition of the metals by pyrrolidine dithiocarbamate and extraction into chloroform. The chloroform is evaporated onto filter paper disks and submitted to x-ray analysis. The procedure can be expanded to include Ag, Cd, In, Sn, Cu, Zn Ni, Co, and M n in water or water solutions.
T
he quantitative microdetermination of metal ions in water has always presented problems involving concentration of the sample, specificity, and method sensitivity. Some of the transition elements and As, Se, TI, Pb, and Bi are particularly troublesome. Emission spectrographic methods (Kopp and Kroner, 1965) have been proposed, as well as atomic absorption procedures. Both methods are well adapted to automation but lack sensitivity and versatility for the multiple determination of As, Se, Hg, and TI. The x-ray fluorescence spectrograph is ideally suited to the determination of these four elements in particular and also increases the number of elements to cover almost all the periodic table, with suitable extraction and concentration techniques. Although arsenic and selenium determination was the primary purpose of this investigation, it was expanded to include Cu, Zn, Ag, Cd, Sn, Sb, Hg, TI, Pb, and Bi. Welcher (1963) outlines techniques of solvent extraction, wherein diethyl dithiocarbamate o r 4-methyl-2-pentanone will extract arsenic and selenium from aqueous solutions ; however, a different pH is required to extract each element effectively. Pyrrolidine dithiocarbamic acid (PDCH) (Lakanen, 1966) and its ammonium salt (APDC) (Mulford, 1966; Slavin, 1964) are reported to chelate and extract the following elements a t the same p H : As, Bi, Cd, Co, Cu, Ga, In, Fe, Pb, Mn, Hg, Mo, Ni, Pd, Se, Ag, T1, Sn, V, and Zn. Lakanen (1966) described the optimum pH as in the range of 4.5 to 6. Experinien tu1
The molybdenum target x-ray tube was selected for the analysis, since its K a (0.71 A.) is approximately 0.2 A. shorter than the absorption edge for As, Bi, Hg, Pb, Se and TI, making it optimum for these elements. A slight loss of efficiency had to be tolerated for the other elements. A Phillips (Norelco) constant potential vacuum spectrograph, operated a t 50 kv. and 45 ma., with scintillation counter, LiF analyzing crystal, pulse height analyzer, and an air path was used in our studies. Although a vacuum o r helium path would have increased the x-ray intensities slightly, it was found that an air 164 Environmental Science and Technology
path permitted the detection and quantitation at the required sensitivity level (0.01 p.p.m.) (Federal Register, 1962). Background and line counts were taken from the recorder scan rather than using fixed count or fixed time for each element. Adequate precision was obtained with this technique (Table I) and it shortened the analysis time considerably. In view of Mansell’s work (1963, the efficiency of recovery was checked using 500 ml. of distilled water spiked with 25 p g . of the desired elements, buffered at pH 4.8 with ammonium acetate-acetic acid buffer, and treated with increasing amounts of a 1 % APDC solution. Only arsenic showed increasing counts with increasing amounts of APDC solution. Results showed that 5 ml. of 1 2 APDC solution completely extracted 25 pg. of arsenic from 500 ml. of sample. Water normally contains less than 0.05 p.p.m. of the elements extracted by APDC, but iron, copper, and zinc may be present in higher concentrations. Therefore 10 ml. of a 2 % APDC solution per liter of water was used and found adequate for complete extraction. Should a water sample contain high concentrations of one o r more of the elements extracted by APDC, the sample should be extracted again with additional reagent, and this extract added to the original extract. The presence of high concentrations can be checked by making an x-ray scan from about 10” through 58” (all extracted elements heavier than Fe). This qualitative scan is used as the quantitative scan in the absence of high concentrations of an extracted element o r elements. Absorption and enhancement may occur in the presence of high concentrations of some of the extracted elements, notably zinc and copper which may be present in concentrations of 5 and 1 p.p.m., respectively (Federal Register, 1962). No enhancement was noted during these studies from elements present at the “maximum allowable concentration” (Federal Register, 1962). A zinc concentration of 0.2 p.p.m. showed only slight absorption of the arsenic Ka (approximately 3 x of the arsenic present), while selenium K a will tolerate up to 0.4 p.p.m. of zinc before absorption becomes apparent. A survey of drinking water samples analyzed by this laboratory during the past year indicates that over 4 0 2 of the samples contained less than 0.2 p.p.m. of zinc and over 80% contained less than 0.4 p.p.m. Absorption, when present, may be compensated for by running duplicate samples containing standards of the desired elements. Reagents
Ammonium pyrrolidine dithiocarbamate may be obtained commercially from the K and K Laboratories, 177-10 93rd Ave., Jamaica 33, N.Y., or prepared according to Slavin (1964). Pyrrolidine dithiocarbamic acid may be prepared according to Lakanen (1 966). Although both reagents worked satisfactorily, a 2 % aqueous solution of APDC was more convenient to use.
Table I. Precision of Fixed Count cs. Count Readouta
Time at 2 0 Degrees __-__
Av.
34.27 33.62 33.74 33,88 34.16 33.47 33.44 34.27 34,03 34.22
Net Sec. 44,92 45.50 44.87 45.91 44.15 45.97 45.61 44.59 44.99 45.10
Net Counts/Sec. 222.61 219.78 222.86 217.81 226.50 217.53 219.25 224.26 222,27 221.72
As Recovered, pg. 10.05 9.93 10.06 9.84 10.23 9.82 9.90 10.12 10.04 10.01
33.91
45.16
221,43
10.00
36.2'
35.4"
33.98"
73.01 71.87 70.53 72.30 71.76 71.63 71.29 71.75 71.18 71.27
85.37 86.37 86.68 87.28 84.86 87.24 86.80 85.96 86.85 87.37
71.66
86.48
Error, 0.5 0.7 0.6 1.6 2.3
1.8 1. o 1.2 0.4 0.1 1.02 -1.8 to + 2 . 3
Counts Determined from Continuous Scan at 1 O per Min., 2 per Inch, Full Scale 1000 Counts per Sec. Net As Counts/Sec. Recovered, wg. Error,
Av.
190 180 180 190 185 1so 185 185 170 175
10.43 9.89 9.89 10.43 10.16 9.89 10.16 10.16 9.34 9.62
4.3 1.1 1.1 4.3 1.6 1.1 1.6 1.6 6.6 3.8
182
10.00
2.7 - 6 . 6 to f 4 . 3
fl Average arsenic recovered considered to be 10.00 pg., since a 10-pg. standard was sample. In fixed count mode, background was obtained by taking average time accumulated at 32.6" and 35.4" and from this subtracting peak time at 33.98". Mo radiation, 50 kv. (CP) at 45 ma., LiF crystal, scintillation counter, air path.
The ammonium acetate-acetic acid buffer was prepared by adding equal volumes of 0.5N ammonium acetate and 0.5N acetic acid with the final pH adjusted to 4.8 with ammonium hydroxide o r acetic acid as necessary.
Preparation of Sample The p H of a I-liter water sample is adjusted to 4.8 =t0.2 with 10 ml. of the ammonium acetate-acetic acid buffer; the addition of acetic acid or ammonium hydroxide may be necessary to bring the p H of some waters into this range. The sample is placed in a suitable separatory funnel, 10 ml. of a 2 % aqueous APDC solution added, and the sample mixed. Allowing the sample to stand for 2 to 4 minutes at this time improved reproducibility. Ten milliliters of chloroform are added and the sample is shaken for at least 1 minute. The chloroform is allowed to separate and then drained into a 15-ml. centrifuge tube. The centrifuge tube is placed in a hot water bath and the chloroform evaporated by directing a gentle stream of air into it. The extraction with 10 ml. of chloroform is repeated two more times, combining the chloro-
form extracts. It was found that 70 to 85z of the elements were extracted the first time and better than 95 the second time; the degree of recovery is proportional to the time the chloroform is shaken with the sample. When the sample was shaken for 5 minutes in a mechanical shaker, 90 to 100% was recovered in the first extract. After the third extraction the chloroform was evaporated to between 0.25 and 0.5 ml. and then spotted o n 1;'2-inch diameter filter paper disks cut from Whatman No. 40 o r No. 41 with a cork borer. Spotting Techniques. METHOD A. The disk is placed on a 47-mm. watch glass which is heated either on a hot plate or by an infrared lamp. The chloroform extract must be added or it will be deposited on the watch glass, which necessitates redissolving and adding the solution to the disk which has been placed on a clean watch glass. METHOD B. The filter disk is attached to a 2- X 2-inch piece of Mylar film ( l : ~mil) with four small spots of colloidion placed at the edge of the disk. The disk and Mylar film are placed on the hot plate, with the temperature adjusted so that the Mylar does not wrinkle. The chloroform extract may be added faster than in Method A. With this method the disk Volume 1, Number 2, February 1967
165
may be mounted so that the sample is exposed directly to the primary x-ray beam. With both methods the centrifuge tube is washed with about 0.25 ml. of chloroform; this is added to the sample disk. Discussion
This project was initiated to develop a method for determining arsenic and selenium in drinking water, but was readily expanded to include additional elements and encompass wider applications, such as toxic elements in wet-ashed biological samples, air samples, industrial waste, and soil. By replacing the chloroform with a more suitable solvent this procedure could be adapted to atomic absorption. The procedure is applicable to emission spectroscopy (except for As, Se, and Hg) by evaporation of the extract on an electrode or wet-ashing. The limit of detection is considered to be three times the square root of the background count and generally permits quantitation down to 0.005 p.p.m, based on an original 1-liter water sample. Counts from concentrations of 0.01 p.p.m. generally ranged from 100 to 300 counts per second above background, depending on the element and conditions. In all cases the counts cs. concentration plotted as straight lines on linear paper in the concentration ranges of 0.005 to
166 Environmental Science and Technology
0.5 p.p.m. Precision and accuracy were normally within + 10 of the amount present and only on one occasion did the error run as high as +18.5%. Even with a precision and accuracy of + 2 0 x at the maximum allowable concentrations of 0.01 p.p.m., the error would be 0.002 p.p.m.-not normally significant. This value may be improved by using a fixed count technique.
x
Literature Cited Federal Register, Title 43, Public Health Service, Part 72, Interstate Quarantine Drinking Water Standards, March 6, 1962. Kopp, J. F., Kroner, R. C., Appl. Spectry. 19, No. 5 (1965). Lakanen, E., P & E Atomic Absorption Newsletter 5, No. 2 March-April 1966). Mansell, R. E., P & E Atomic Absorution Newsletter 4. No. 5 (May ‘1965). Mulford, C. E., [bid., 5, NO. 4 (July-August 1966). Slavin, W.. Ibid., 3. No. 10 (November 1964). Welcher, F. J., “Standard Methods of Chemical Analysis,” Vol. 11, Part A, Van Nostrand, New York, 1963.
Frank J. Marcie Regional Environmental Health Laboratory United States Air Force (AFLC) Kelly AFB, Tex.
