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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
radioactive source, in which recombination occurs. For a p of 0.07, k , is 7.1 h 3.1 X lo-" cm3 molecule-' s-l which lies close to the swarm beam data. For a /3 of 0.09, cu(E/p)is 3.41 cm-' Torr-' in agreement with temperature corrected values reduced from swarm beam data. Christophorou et al. indicate that a ( E / p )is independent of total pressure variations ( I O ) . Assuming that ion densities increase linearly over the radioactive source region, which occupies one sixth the length of the reaction region, these values for /3 bracket the expected value of 0.08 if the /3 of one sixth is reduced by a factor of two to correct maximum measured ion densities for average densities within the region. The 44% error in k , may mask temperature related effects in the plasma chromatographic results (28). A concluding remark is that the product of the ionization efficiency and the carrier gas flow rate is a constant for plasma chromatography. For chlorobenzene, this constant is 3.4 x cm3 (200 "C)/s for the reaction region before attenuation is imposed by the transmittance of the grid structures. For the operating parameters outlined in Table I, the constant is 7.7 x lo-* cm3 (200 OC)/s for the overall plasma chromatographic system.
F. W. Karasek, 0. S. Tatone, and D. M. Kane, Anal. Chem., 45, 1210
(1973).
F. W. Karasek and D. M. Kane,
J . Chromatogr., 93, 129 (1974). L. G. Christophorou, R. N. Compton, G. S. Hurst, and P. W. Reinhardt, J . Chem. Phys., 45, 536 (1966). R. N. Comoton. L. G. ChristoDhorou. and R. H. Huebner. Phvs. Let?.. 23 (ll),656 (1966). R. N. Compton, R. H Huebner, P W Reinhardt, and L. G. Christophorou, J . Chem. Phys., 48, 901 (1968). J. E Lovelock and N. L. Greaorv. 3rd I n t . Svmo. Gas Chromatour.. Academic Press, New York, London, 1962, 2i9. J. E. Lovelock, Nafure (London), 189. 729 (1961). J. E. Lovelock, P. G. Simmonds, and W. J. A. VandenHeuvel, Nature (London). 197,249 (1963). J. E. Lovelock. Phvs. Proc. Radiat. Biol.. Proc. Int. Svmo.. 1963 (Pub. 1964),p 183 . A Zlatkis and J E Lovelock, Clm Chem ( Wmston-Salem, N C.), f l ,
6
1
.
259 (19651 G. Castello and G. D'Amato, J , Chromatogr.. 54, 157 (1971). E. W. McDaniel, "Collision Phenomona in Ionized Gases", Wiley, New York, N.Y., 1984, p 536. P. A. Lawless and G. E. Spangler, Rev. Sci. Instrum.. 48, 240 (1977). I . R. Gatland, Case Stud. At. Phys., 4 (6).371-437 (1974). H. S.Carsbw and J. C. Jaeger, "Conduction of Heat in Solls", Cbrendon Press, Oxford, 1973,pp 196-201. J. E. Lovelock. "Gas Chromatography", R. P. W. Scott, Ed., Butterworths, Inc., Washington, D.C., 1960,p 26. A. Fontijn, A. J. Sabadell, and R. Ronco, Anal. Chem., 42,575 (1970). J. J. Ritter and N. K. Adams, Anal. Chem., 48, 612 (1976). J. M. Sedlak and K. F. Blurton, Anal. Chem., 48, 2020 (1976). L. G. H. Huxley and R. W. Crompton, "The Diffusion and Drift of Electrons in Gases", Wiley. New York, N.Y., 1974,Chap. 14. E. W. McDaniel, "Collision Phenomena in Ionized Gases", Wiley, New York, N.Y., 1964,Chap. 12.
LITERATURE CITED (1) F. W. Karasek, Anal. Chem., 48, 710A (1974). (2) R. A. Keller and M. M. Metro, Sep. Purif. Methods 3 (l),207 (1974). (3) E. W. McDaniei and E. A. Mason, "The Mobility and Diffusion of Ions in Gases", Wiley, New York, N.Y., 1973. (4) G. E. Spangier and C. I. Collins, Anal. Chem., 47, 393 (1975). (5) R. A. Keller and M. M. Metro, J . Chromatogr. Sci., 12, 673 (1974). (6) G. E. Spangler and C. I. Collins, Anal. Chem., 47, 403 (1975). (7) F. W. Karasek and 0. S. Tatone, Anal. Chem., 44, 1758 (1972).
RECEIVED for review July 21, 1977. Accepted November 1, 1977. This research was conducted under the authority of the D e p a r t m e n t of Army Projects lT161102AH51, lT161101A91.4, and lW762712AH66, Fort Belvoir, Va.
Determination of Molybdenum by Neutron Activation and Srafion NMRR Ion Exchange Resin Separation R. A. Nadkarni and G. H. Morrison* Department of Chemistry, Cornell University, Ithaca, New York 14853
Srafion NMRR ion exchange resin has been used to separate molybdenum after neutron activation. The method is used to determine the trace amounts of molybdenum in steels, and geological, biological, and environmental standard reference materials.
Molybdenum is an essential trace element in biological systems, and in addition has important consequences in metallurgical and geological systems. Because of the low concentration levels involved, neutron activation methods of analysis appear to be most applicable. Direct determination of Mo in complex natural samples by instrumental neutron activation analysis (INAA) is difficult because of its low level in these materials. In addition, the low energy gamma peak of "Mo is masked by the radiation produced by the large amounts of 24Na,82Br, 32P,etc. upon neutron irradiation. Therefore, radiochemical separations are required for the accurate determination of trace amounts of Mo. An ion exchange resin that has been shown to be very effective in radiochemical separations is Srafion NMRR. I t has been used to quantitatively separate trace noble metals 0003-2700/78/0350-0294501.OO/O
including Au and Ag (1-91, and Hg (9-13) and Cu (14) from geological, biological, and environmental materials. The resin binds ions with the ds electronic configuration forming square planar complexes; hence its selectivity for noble metals. Recently Nadkarni and Morrison (8) and Muzzarelli and Rocchetti ( 2 5 ) studied the behavior of several common transition metals with Srafion NMRR resin, and only Mo was found to be quantitatively retained on the resin. The present study describes the use of this resin to separate Mo during neutron activation analysis of standard samples of steel, rock, coal, and plant and animal material. The results are in good agreement with the certified or literature values.
EXPERIMENTAL Samples and Reagents. The following standard samples were analyzed for their Mo content: NBS Stainless Steel 121d, NBS Low Alloy Steel 661, USGS Basalt BCR-1, NBS Coal SRM-1632, NSB Orchard Leaves SRM-1571, NBS Bovine Liver SRM-1577, and Bowen's Kale. These materials were dried as specified by the suppliers. Srafion NhlRR ion exchange resin was obtained from Polycycle, Inc., Palo Alto, Calif. The resin was equilibrated with 0.5 N HCl for several hours before use. The resin was then packed in glass columns of 1.5-cm i.d. and 15-crn length. Irradiation standards 'C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
Table I. Absorption of 9 9 Mon ~ Srafion NMRR Resin Experiment Absorption, % Elution, % pH 1.05 pH 1.95 pH 3.15 pH 4.10 pH 5.65 pH 6.05 0.5 N HCI 2 N HC1 8 N HC1 4 N ",OH 1 M KCNS 0.5 M EDTA
99.8 99.9 100
100 100 100
6.0 11.2 7.5 21.6 62.4 0.35
were prepared from diluted aliquots of molybdenum solution prepared with "Specpure" Moo3. "Mo tracer was prepared by irradiating "Specpure" Moo3. Irradiation. Samples, 200-400 mg, in sets of four and a standard containing -40 Fg Mo were sealed in high purity quartz vials and were irradiated in the central thimble facility of the Cornel1 TRIGA Mark I1 reactor at a thermal neutron flux of 3 X 10" n cm-2 s-] for periods of 22 to 35 h. Radiochemistry. The samples were allowed to cool for -60 h before separating molybdenum. This considerably reduces the activity of other short-lived major nuclides such as 24Na,42K,etc. The geological and coal samples were dissolved by alkali peroxide fusion; the biological samples were dissolved in an HNOB+ HC104 mixture (1.5:l);and the steel samples were dissolved in aqua regia, all in the presence of 2 mg Mo6+carrier. The pH of the solutions were adjusted to -1 and then passed over Srafion NMRR columns at the rate of 1 mL/min. The resin was washed 10 times with 10-mL portions of distilled water. The washed resins were transferred to Pyrex Erlenmeyer counting flasks and made up to 25 mL with water. The standards were treated identically. Counting. The samples and standards were counted using a coaxial 56 cm3Ge(Li) detector and a 4096 channel analyzer. The system had a 1.96-keV (FWHM) resolution, a peak-to-Compton ratio of 37:l and the counting efficiency of 12.9%. The data were computed on a PDP-11/20 computer. No variations in the intensity of the 140-keV radiation resulting from different thicknesses of the flasks used were observed.
RESULTS AND DISCUSSION Srafion NMRR was tested for its absorption behavior towards molybdenum by passing solutions containing 2 mg of Mo6+carrier and a fixed quantity of %Mo tracer a t varying p H over the resin column, and counting both the resin and the effluent phases. T h e results in Table I show that at least u p to p H 6, Mo6' is quantitatively absorbed. Absorption behavior above p H 6 was not checked, since well below p H 6, other metals will start precipitating as their hydroxides. Several reagents were used to try to elute the absorbed Mo6+ from the resin. However, in no case was complete elution obtained. Maxium elution (62%) was achieved with 1 M KCNS (Table I). However, in view of quantitative absorption, it is unnecessary to elute M o from the resin, and the whole resin phase is used for gamma spectroscopy. In another experiment, 350 mg of unirradiated BCR-1 rock was mixed with 99M0 tracer and run through the entire procedure. The absorption of %Mo on the resin was complete even in this case. T h e only useful isotope of molybdenum for radiochemical work is %Mo (66 h half-life) with a 140-keV y-ray peak. *Mo decays to its daughter 9 9 m T(6.05 ~ h) and both have to be allowed to equilibrate before counting (40 h). T h e only interfering nuclear reactions in the molybdenum determination and 235U(n,F)99Mo.However, the amount are 102Ru(n,a)99Mo, of ruthenium in most materials is so low that the contribution from its reaction towards molybdenum activity is negligible. T h e 235U(n,F)99Moreaction is important in most geological silicate samples where the ratio of Mo:U is unity or less. Under
295
Table 11. Determination of Molybdenum in Standard Reference Materials Material Present, ppma Found, ppmC NBS Stainless Steel 121d NBS Low Alloy Steel 661 USGS Basalt BCR-1 NBS Coal SRM-1632 NBS Orchard Leaves SRM-1571 NBS Bovine Liver SRM-1577
0.165%b
0.167
t
0.007%
0.19%b
0.191
i
0.003%
1.5 ( 1 6 ) ;1.15 ( 1 7 )
1.39
i-
0.21
0.2 ( 1 8 ) ; 3.08 (19); 3.14 3.4 ( 2 0 ) ; : ( 2 1 ) 0.3 = 0.1 0.24
t
0.28
i
0.02
3.2b; 3.04 ( 2 2 ) ; 3.12 * 0.26 3.33 ( 2 3 ) ;3.19 ( 2 4 ) ; 3.5 ( 2 5 ) Bowen's Kale 2.28 ( 2 6 ) 2.13 i 0.12 a Reference numbers in parentheses. NBS Certificates of Analysis. Mean value i standard deviation.
our irradiation conditions, 1 pg of U produced 0.82 pg of apparent "Mo. In biological and environmental analyses, where the Mo:U ratio is very large, e.g. Bovine liver, 4000; Kale, 228; Orchard Leaves, 10.3; the U contribution can be safely ignored. The proposed method was applied to several standard reference materials of metallurgical, geological, biological, and environmental nature. T h e results are given in Table 11. In each case, four replicate analyses were carried out. The results are in excellent agreement with the certified and/or literature values where available. 59Feproduced from steel samples also has a y-ray peak a t 143 keV, very near the 140 keV peak of 99Mo;however, Srafion NMRR resin has been shown not to absorb any Fe3' (8). Flanagan (16) gives 1.1 ppm as "magnitude" molybdenum value in the USGS standard basalt BCR-1; other workers have found 1.15 ppm by epithermal NAA (17). In silicate rocks where uranium content is high, the contribution from 235U(n,F)99Mo has to be corrected by irradiating a separate aliquot of uranium. Among biological and environmental standard samples, NBS has not certified the molybdenum value for coal: SRM-1632; however, literature values range from 0.2 to 5 ppm, obtained by the techniques of photon activation analysis (It?), epithermal NAA (191, and INAA (20, 21). The true value is probably around 3-4 ppm. Our value for NBS Orchard Leaves, SRM-1571, is in good agreement with the certified value by NBS. For Bovine Liver: SRM-1577, NBS has given 3.2 ppm as "information" value. Other workers also have found molybdenum close to this value by t h e techniques of radiochemical NAA (22-24) and proton induced x-ray emission (25). The value for molybdenum in Bowen's Kale is not certified, but Bowen (26) gives 2.28 ppm as a grand mean of molybdenum results by colorimetry and NAA. I n all cases our results are in good agreement with the recommended values. T h e accuracy of the results is better than 1%for steels,
