ashed and aliquots of ash containing about 3 grams of Ca are dissolved in HCl. Strontium carrier (20 mg Sr*+)is added to all samples. When 89Sr measurement is not required, the amount of strontium is increased to 40 mg in food and bone ash samples. Precipitate the insoluble carbonates from water and soil samples with Na2C03. Collect on a glass fiber filter. Dry by washing with methanol and by placing in the oven at 110 OC for a few minutes. Precipitate the insoluble phosphates from food and bone ash by adding H 3 P 0 4and then N H 4 0 H until alkaline. Collect by centrifugation and dry by first dissolving in 20 ml of 70 % H N 0 3and then evaporating to dryness. Add 40 ml of 70% HNOB to the dry carbonate or phosphate. Digest, cool, and filter through a glass fiber filter. Use an additional 40 ml of 70% HNO, to transfer and wash the precipitate. Discard the filtrate. Dissolve the precipitate in 100 ml of water and sorb on the column. Discard the effluent. Elute with 100 ml of 1.5M ammonium lactate, pH 7.0, at a flow rate of about 1 ml per minute. Test the first 15 to 35 ml of eluate for Ca with oxalate solution to determine when all the calcium is off the column. Discard these fractions. Collect the remaining eluate and precipitate SrC03 with ammonium carbonate. Collect and weigh for recovery. Procedure 11. ZlOPb, 90Sr, AND 22eRa IN BONE. Dissolve 10 grams of bone ash in HC1. Add 40 mg Pb 2+, 40 mg Sr 2f, and 5 mg Ba2+carriers. Proceed with the phosphate precipitation, nitric acid separation, and column sorption as in Procedure I. Elute lead with 40 ml of 1 M ammonium acetate, pH 6.2. Precipitate lead with NaHCO,. Collect and weigh as PbC03. Elute and recover strontium as in Procedure I. Elute barium and radium together with 35 ml of 0.25M EDTA, pH 10. Coprecipitate Ba/RaS04 by adding S042- and demasking with acetic acid. All elutions are at a flow rate of about 1 ml per minute. Ion exchange columns are reused repeatedly. Prepare for reuse by first changing to the H+ form with HC1 and then changing to the NH4+ form with ammonium lactate. Wash and backwash with water. Measurement of Radioactivity. Lead-210 is usually determined by measuring the alpha activity of daughter 210P0, separated by spontaneous deposition on silver, after a suitable period for ingrowth. In more active samples, PbC03 may be beta-counted for 210Biafter a shorter period. Strontium-89 is determined by beta-counting SrCO, without delay. Yttrium-90 is then separated after a suitable ingrowth period and beta-counted for 9OSr assessment. Radium-226 is determined by alpha-counting Ba/RaS04 after about four weeks when radium and the three alphaemitting daughters are in equilibrium, thus attaining the counting advantage of 4 alphas per 226Radisintegration.
RESULTS AND DISCUSSION The method outlined in procedure I has been in routine use here for about two years and has been used for the evaluation of international intercomparison samples on several occasions. Strontium recovery is typically 90-95 %. Procedure I1 has been used routinely on human bone samples for about one year. Lead recovery is typically 85%, some lead being lost to the nitric acid filtrate. Strontium recoveries are again over 90 %. Invariably the barium recovery apparently exceeds 100% because of the small amounts of stable barium in environmental samples. Experiments with spiked samples show that radium is quantitatively recovered. Tracer studies show that more than 90% of any 210Poin the ash is removed with the nitric acid filtrate, and that the separated lead fraction is uncontaminated with zlOPo. Ingrowth factors based on pure *lOPbare therefore valid. Experiments also confirm that lead does not interfere in the subsequent deposition of zlOPo on silver. Experiments with heavily spiked samples followed by blank samples confirm that no cross contamination occurs. Procedure I1 is applicable to bone samples because there is no interference by sulfate. For other types of samples, interference may occur through premature precipitation of some BaS04 on addition of barium carrier. The procedure may be extended to such samples by collecting any insoluble BaS04, dissolving in alkaline EDTA, and adding to the EDTA eluate before demasking. Alternatively, alkaline carbonate fusion of the sample ash, followed by recovery and solution of the insoluble carbonates, removes the cause of inter fer ence. Our experience confirms the value of 70 HNO, not only for the separation of strontium from calcium, but also for the separation of lead, barium, and radium. Its use thus allows the subsequent sequential separation of three major long-lived bone seeking radionuclides by simple fractional elution. LLOYDP. GREGORY National Radiation Laboratory Department of Health P.O. BOX25-099 Victoria Street Christchurch, New Zealand RECEIVED for review December 3, 1971. Accepted May 22, 1972. Published with the Authority of The Director-General of Health.
Ultratrace Level Detection of Mercury by an X-Ray Excited Optical Fluorescence Technique SIR: The extensive documentation on the toxic effects of trace levels of Hg present in environmental samples has stimulated interest in the development of analytical techniques for the quantitative determination of Hg at part per billion (1 in 109) levels (I). We report here a promising new technique capable of achieving this objective.
Our studies on X-ray excited optical fluorescence of gases (2) indicated that the optical spectrum of Hg can be effectively excited on X-ray irradiation of Hg vapors in argoc. Further investigations indicated that the intensity of Hg 2537 A was greatly enhanced in a Ar-1 NP (molar %) gas mixture. In addition, intense emission from several molecular Nf band systems was also observed. The most prominent of these
(1) R. A. Wallace, W. Fulkerson, W. D. Shults, and W. S. Lyon,
“Mercury in the Environment-The Human Element,” ORNL NSF-EP-1, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1971.
(2) A. P. D’Silva, E. L. DeKalb, and V. A. Fassel, Pacific Conference on Chemistry and Spectroscopy, Anaheim, Calif., October 1969.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
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I
X - R A Y TUBE
1
O 9 I
H q 2537
a
SPECTROMETER
Hp GEUERATION FLASK
Figure 1. System for generation and X-ray excitation of Hg fluorescence
08
-
06
HQ 2537 d l100ppb)
Figure 2. Effect of He, Nz, Ar, and Ar fluorescence
+ 1 ZNp gases on Hg
were the second positive bands (Cm, + Bang)in the 310-400 nm region, the first positive bands Bang-+ AaZ+,) in the near infrared and the NO y bands in the ultraviolet. The emission spectrum in the 700-900 nm region consisted of relatively intense emission from argon lines. The analytical applications of these observations were evaluated utilizing the experimental arrangement shown in Figure 1. A Hg vapor generation system similar to that used in atomic absorption analysis (3) was continuously flushed by an Ar-1% N Z gas mixture. The Hg vapor released into this gas was fed to a 2-mm i.d. quartz tube placed in a Pb-shielded enclosure. The wide open end of the discharge tube at the top was sealed with a Mylar film to facilitate X-ray irradiation of the gases. A tungsten target X-ray tube (OEG-50, Machlett Laboratories, Springdale, Conn.) operated at varying power levels was used in these experiments. A 5-cm length of the 2-mm i.d. discharge tube was focused on the slit of a 0.25-meter Jarrel-Ash grating spectrometer. The spectral features were recorded using instrumentation already described ( 4 ) . The following observations were made when ultratrace amounts of the Hg present in 2 ml of 10% HC1 solution passed through the discharge tube. In Figure 2 , the fluorescence of Hg at 2537 A observed with He, Ar, Nz, and an Ar-1% Nz gas mixture is shown. The striking enhancement in the presence of Ar-1 % N2 gas mixture is readily observed. The quantitative nature of the fluorescence is shown in Figure 3. The fluorescent signals (3) W. R . Hatch and W. L. Ott, ANAL.CHEM., 40, 2085 (1968). (4) E. L. DeKalb, V. A. Fassel. T. Taniguchi. and T. R. Saranathan, ibid., p 2082.
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Figure 3. Quantitative nature of Hg fluorescence at ppb levels obtained from the three different 10-ppb level (10 ng/ml) experiments are an indication of the reproducibility of the method. The detection limit of this technique is expected to be at the fractional ppb level since a number of experimental variables remain to be optimized. From experimental observations made in our laboratory and extensive information available on the Ar-N2 system (5), a tentative excitation mechanism can be postulated. In an Ar-N2 system, collisions of the second kind between metastable Ar( aPo,2)atoms generated on X-ray irradiation and N2 molecules result in the excitation of the second positive NZsystem (6). Energy transfer by a cascade process results in the subsequent excitation of the first positive and VegardKaplan Nz band systems. The energy transfer mechanism may be indicated as follows: Secpnd positive
-First positive
Ar 3P2+.N 2C311, +NzB311g N2A3Z',
Vegard-Kaplan bands
NzX'Z'g
The NZA3Zfgis metastable and excitation of Hg by energy transfer is known to occur (7,8) as follows:
+ Hg'So +NzX'Z'g + Hg3P1 Hg3P1+HgSo + hv (2537 A)
N2A38+,
It is of interest to note that experimental observations of the selective energy transfer process described above were reported as early as 1911 (9). ARTHURP. D'SILVA VELMER A. FASSEL Ames Laboratory-USAEC and Department of Chemistry Iowa State University Ames. Iowa 50010 RECEIVED for review May 4, 1972. Accepted June 30, 1972. ( 5 ) E. S. Fishburne, J . Clwrn. Plzys., 47, 58 (1967). (6) B. Brocklehurst, Atomic Energy Research Establishment, Harwell, C/R 2669, 1963. (7) W. R. Brennen and G. B. Kistiakowsky, J . Chrm. Pliys., 44, 2695 (1966). (8) R . A. Young and G. A . St. John, ibid., 48, 2572 (1968). (9) R. J. Strutt, Proc. Roy. Soc., Ser. A , 85, 1219 (1911).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
