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(308) B. J. Bowles, Electrochim. Acta, 10, 717 (1965).

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N. Mansurov. Sov. Electrochem.. 6, 18

(311) K. Schwabe and Ch. Weissmantei, Z. Phys. Chem. (Leipzig). 215, 48 (1960).

(312) F. Nagy, G. Horanyi, and J . Solt. Magy. Kem. Foly., 75, 530 (1969).

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(322) V . E. Kazarinov, ibid., 2, 1070 (1966) (323) V. E, Kazarinov and G. N. Mansurov, ibid..

p 1223. (324) N. A. Balashova and V. E. Kazarinov.

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T Gorokova Sov Electrochem 3, 583 (1 967)

(330) L A Medvedeva and Ya M Kolotyrkin Doki Phys Chem 143, 311 (1962)

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and B. N. Kabanov, "Kinetics of Electrode Process," University Press, Moscow, 1952, p 53.

(336) K. Hampartzumian and G. Raichevsky, in "Proceedings of the Third International Congress on Metallic Corrosion, Moscow 1966," Moscow, 1969, Vol. I l l , pp 421-7: distributor: Swets-Zeitlinger/Amsterdam.

(337) A. V. Byalobzhesky, ibid., Vol. IV. pp 287-94.

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(341) R. D. Srivastava and H. Gesser, ibid.. 9, 1405 (1 964).

(342) G. M. Budov and V. V. Losev. Doki. Akad. Nauk SSSR, 122, 90 (1958)

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(344) V. Kuvik, Chem. Listy. 61, 149 (1967). (345) N. A . Balashova and V. E. Kazarinov, Sov.

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(350) G. H. Cartledge, Corrosion. 11, 335t (1955).

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Cleaner Environments, Plenum, New York. N.Y., 1972.

NOTES I I

Rapid, Instrumental Neutron Activation Analysis for the Determination of Uranium in Environmental Matrices

Jack N. Weaver Nuclear Services Laboratory, Nuclear Engineering Department, North Carolina State University, Raleigh, N.C. 27607

In the past few years, there have been many methods de- veloped for the analysis of natural occurring uranium in a wide spectrum of materials. A recent article by Becker and LaFleur (1) discusses these and references the various tech- niques in a comparison of their method of NAA coupled with radiochemistry us. the other methods such as alpha counting, fission track counting, NAA, Amiel's (2) delayed neutron counting, etc. However, the point to be made is that although the Becker and LaFleur (and Amiel) meth- ods are accurate and highly selective for uranium in trace quantities below 50 ppb, they still retain either the draw- backs of radiochemical procedures (when above 50 ppb) which require extra equipment, glassware, chemical manip-

(1) D. A. Becker and P. D. LaFleur, Anal. Chem., 44, 1508 (1972) (2) S. Amiel, Anal. Chem., 34, 1683 (1962).

ulations in hoods, and a skilled chemist; or the handling of samples with significant gamma activity, the isolation and shielding of a nuclear reactor pneumatic terminal strictly for this use, and the use of a somewhat limited analytical system consisting of B'OF3 counters. As with all such tech- niques, the tendency is to use them sparingly only on im- portant samples on a limited basis.

The procedure described in this paper departs from these more detailed methods in that it represents a rapid instrumental method of neutron activation analysis for uranium in concentrations above 25 ppb, utilizing only a multichannel analyzer coupled to the Low Energy Photon Detector (LEPD). This procedure readily adapts to the typical scheme of a neutron activation analysis laboratory where irradiation, decay, and counting of the samples fit an efficient schedule, and it utilizes a detector (LEPD) which

1292 ANALYTICAL CHEMISTRY, VOL. 46, NO. 9 , AUGUST 1974

has broad analytical capabilities other than uranium analy- sis (3, 4) . The LEPD detector operates on the principle that a wafer thin crystal of lithium drifted germanium is highly sensitive to the X-rays and low energy gammas from reactor irradiated materials, and it is basically insensitive to the usual interferences from higher energy gammas that are experienced with large volume Ge(Li) detectors typical- ly used in neutron activation analysis.

Considering the analysis of uranium by NAA, one of the fission products produced from the bombardment of 235U by thermal neutrons is 1331 with an 8% fission yield. This fission fragment has a half-life of 21 hours, and X-rays and low energy gammas a t 14.4, 18.32, 21.40, 100.08, 106.80, and 109.32 keV.

The analysis of uranium in coal, bovine liver, orchard leaves, sea water, and various ores using the LEPD method is presented with significant improvements over other methods. These are: 1) No chemical dissolutions are re- quired as is the case with many other techniques when ap- plied to environmental samples. 2) Chemical separations which can give technique and recovery errors are unneces- sary. 3) Either liquids or solids can be analyzed because of direct measurement of the X-rays emitted after irradiation in the reactor. 4) The analysis can be performed by a tech- nician a t less expense rather than by a highly trained chemist.

EXPERIMENTAL Apparatus. The apparatus used consisted of a 16 mm Ortec

(LEPD) Low Energy Photon Detector and an ND2200 MCA cou- pled to a Hewlett-Packard computerized data retrieval system. This detector and its dewar are very similar in dimensions to a standard large volume Ge(Li) detector except in the actual con- struction of the germanium crystal itself. The windowless (less than 1 ym) lithium drifted germanium crystal wafer has a standard end cap window of a 5-mil thickness of beryllium. The detector is liquid nitrogen dewar cooled. The LEPD when coupled with a 1024 or greater multichannel analyzer has a useful range from 3 to ap- proximately 600 keV. However, for practical purposes sensitivity- wise, it is best below 300 keV. Typical resolution is 255 eV a t 5.9 keV, 600 eV a t 122 keV, and 750 eV a t 270 keV.

Reagent Preparation. Weigh approximately 1 gram of freeze dried uranyl nitrate in 1 liter of distilled water and 0.1N nitric acid. Micropipetting is used to reduce standards to 0.1, 1.0, and 5.0 yg dried concentrations in poly irradiation vials.

As a check on the uranium standards, three (0.10-gram) samples of an NBS Reference coal containing 1.35 ppm U were also heat- sealed in poly vials and used as uranium standards.

Procedure. After freeze drying, 0.25 gram of the solids (coals, liver, orchard leaves, etc.) were heat sealed in standard poly irra- diation vials as per the uranium standards. Likewise liquids were transferred in 0.5-ml portions to poly vials for heat sealing. Both samples and standards, with flux monitors attached, were irradiat- ed for 4 to 8 hours in a flux of 3 X l O I 3 n/cm2 sec.

The irradiated samples were then allowed to decay for 48 hours before counting the 331 activity. A decay of 40 to 50 hours provides the best decay time (especially for coals, ores, etc.) in a tradeoff of 13.31 decay us the background decay from other isotopes in the sample. As in a previous study (5 ) , the solids are prepared in a spe- cial counting tray which provides a duplicate condition for both standard and unknown, while the liquids are simply transferred to clean standard poly vials for counting; since self-attenuation of the low energy photons in solids poses more problems than that with liquids.

The standards were counted (decay monitored) on the 16-mm Ortec LEPD for approximately 400 seconds a t a calibration 0-125 keV. Figure 1 illustrates the spectra of 1331 from 236U fissioning. All six photons are clearly defined but only four are abundant enough for quantitative analysis a t ppb levels. The photopeaks used were 14.4, 18.32, 106.80, and 109.32 keV. Hence, ratios between these photopeaks were used for ruling out interferences from other iso-

(3) J . N. Weaver, Amer. Lab., March, 1973. (4) M. H. Friedman, E. Miller, and J. T. Tanner, Anal. Chem., 46, 236 (1974). (5) J. N. Weaver, Anal. Chem.. 45, 1950 (1973).

VI w c 3

I z P . VI c z

0

3K FISSION P R O D U C T

I I Z I HR T I " I I13

URANIUM STAND A R 0 ______

122 X - R A Y ENERGY ( k r V 1

Figure 1. X-Ray spectrum of 1331 (500 nanogram) standard after 4 hours of irradiation at l O I 3 n/cm2-sec., 48-hour decay, and a 400- second count on a 16-mm ORTEC LEPD

v1 w c 3

5

0 VI c z 3 0 "

C O A L - O K -

'11,

122

X - R A Y E N E R G Y I k e V )

Figure 2. X-Ray spectrum of a coal sample after a 4-hour irradiation at 1013 n/cm2-sec., 48-hour decay, and a 400-second count on a 16-mm ORTEC LEPD

topes present in the sample, thus providing a very accurate means of analysis.

The coal samples were counted for 400 seconds while the bovine liver, orchard leaves, and sea water were counted for 1000 seconds because of a lower concentration of uranium in these SRM refer- ence standards. Figure 2, an X-ray spectra of irradiated coal, clear- ly illustrates the fine resolution of the LEPD detector and the sen- sitivity with which it detects low energy photons. In the coal sam- ple, only the 106.80-keV photopeak exhibited any interferences while the other three main peaks showed the correct half-life and internal ratios as the uranium standard.

Data analysis was performed by the Covell (6) method and also by a computerized Hewlett-Packard, APT, and Nuclear Data ND2200 system.

RESULTS AND DISCUSSION Listed in Table I are the results of the analyses of vari-

ous environmental matrices for uranium. These range in substance from liquids (sea water) to solids (coals, bovine liver, and orchard leaves). An indication of the accuracy of the instrumental NAA-LEPD method is given in the com- parison of SRM 1571 Orchard leaves a t very low ppb levels and with the NBS-EPA Round Robin Coal a t ppm levels. Very good agreement is obtained at these levels.

Because of the interest expressed in the trace element concentrations in fuels such as coals, Table I1 presents the uranium concentrations found in the U.S. Bureau of Mines

(6) D. F. Covell, Anal. Chem., 31, 1785 (1959)

A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 9, A U G U S T 1974 1293

Table I. Uranium Determination in Environmental Matrices, ppm

Other analytical Sample No. I N S . N A A , " LEPD techniques

Orchard Leaves,

Beef Liver NBS Round Robin Coal Atlantic sea water Zinc ore Copper ore Mississippi mud

NBS SRM 1571 0.032 f 0.009 0.026, 0.028

0 . 0 3 <o .02 0.0014

1.37 f 0.08 1 .35 0.94 . . . 3.59 . . . 3.13 . . . 0.97 , . .

'I Average of 6 determinations on separate samples.

Round Robin Coals-1972. A t these ppm levels, the NAA- LEPD analysis is rapid, easy, and accurate to perform. The accuracy of the values obtained in Table I1 was checked against another NAA method coupled with radiochemical solvent extraction and the values were f 5 % of each ether.

As sources of error, consideration should be given to two possible problems-that of matrix absorption of low energy photons and the diffusion of the fission fragment lS31 through the walls of the polyethylene vials used in the irra- diation. The first problem can easily be solved by using, where possible, standards which duplicate in size, volume, and composition the sample matrix. Solids can best be counted as stable, thin layers distributed on the bottom of a thin plastic container covering the active diameter of the detector. The possible problem of 1331 loss due to diffusion through the plastic vials did not appear in our research. This probably was due to the chemical absorption of the 1331 on the coal; and to a lesser extent in the other samples, the competition of a very small concentration of iodine us. the chemical binding possibilities of a much larger matrix. At higher lS31 levels, this might not hold true, thus necessi- tating appropriate corrections to data.

There are several other points in favor of the LEPD em- phasized by the analysis of the various materials in Table I. These were: 1) The analysis of uranium in sea water which is noted for its 24Na gamma activity interference (upon re- actor irradiation) with standard GeLi and NaI detectors was not hampered by using the LEPD detector. 2) Four abundant X-rays and low energy gammas from 1331 decay

Table 11. Uranium Content of U.S. Coal, U.S. Bureau of Mines

Sample Geological location U , n ppm

DRB-A Belmont Co., Ohio 1.62 f 0.09 DRB-B Harrison Co., Ohio 0.95 f 0.05 DRB-C Jefferson Co., Ohio 1.28 f 0.07 DRB-D Kanawha Co., W. Va. 1 .07 f 0.10 DRB-E Washington Co., Pa. 1.28 f 0.01 G-1 Clay Co., Ind. 3.44 & 0.15 P- 1 Muhlenberg Co., Mont. 2.37 f 0.09 P-2 Rosebud Co., Mont. 1.56 f 0.01 P-3 Henry Co., Mo. 7.55 =k 0.28 P-4 Montrose Co., Colo. 1.92 j, 0.07 P-5 Navajo Co., Ariz. 0.70 + 0.01 NBS Round Robin Mixture, 1972 1 .35 i 0.07 Average of three determinations on separate samples. =k Denotes range

of values for three samples.

enable a rapid accuracy check for possible matrix interfer- ences as can occur in a matrix as rich in trace elements as coal. 3) Sensitivity as shown by the orchard leaves is ap- proximately 25 ppb and the precision averages 10% a t 100 ppb and 5% a t 1 ppm. Careful techniques will improve these typical precision values.

CONCLUSION The use of instrumental NAA coupled with the Low En-

ergy Photon Detector to determine uranium in environ- mental materials a t concentrations above 50 ppb is rapid in relation to typical NAA programs, accurate, and inexpen- sive with only the addition of an LEPD detector to the typ- ical neutron activation analysis laboratory.

ACKNOWLEDGMENT

The author acknowledges the assistance of the U.S. Bu- reau of Mines for supplying the coal samples and the Envi- ronmental Protection Agency for the ore and sediment samples.

Received for review January 10, 1974. Accepted April 12, 1974. Research was performed under EPA contract No. 68-02-0928.

Method for Determination of Selenium, Arsenic, Zinc, Cadmium, and Mercury in Environmental Matrices by Neutron Activation Analysis

Edoardo Orvini,' Thomas E. Gills, and Philip D. LaFleur

Activation Analysis Section, Analybcal Chemistry Division. National Bureau of Standards. Washington, D.C. 20234

Recently, heavy metal contamination of the environ- ment has been recognized as a serious pollution problem. Among these metals, particular attention is being given to Se, As, Zn, Cd, and Hg.

Neutron activation analysis is capable of high sensitivi- ty for the determination of these elements. However, the

'Guest Worker from the University of Pavia, Centro di Ra- diochimica e Analisi per Attivazione del C.N.R., Italy.

most sensitive and useful (n,?) reactions involve short half-lived isotopes of As, Zn, Cd, and Hg, thus requiring chemical separation of these elements from the activated matrix.

Most separation methods reported in the literature in- volve dissolution of the matrix, often requiring different methodology for each different matrix. Moreover, most of the techniques developed for the separation of these ele-

1294 ANALYTICAL CHEMISTRY, VOL. 46, NO. 9 , AUGUST 1974