Determination of Mercury and Selenium in Coal by Neutron Activation Analysis

Jack N. Weaver

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

In recent years, with the close attention that atmo- spheric emissions have received, there has arisen a critical need for analytical instruments and techniques which can provide the necessary precision, accuracy, and sensitivity for a variety of matrices. Chief among these matrices has been coal such as is used for electric power plant opera- tion; and the primary element of interest has been mercu- ry. Many current analytical methods such as atomic ab- sorption by Kalb (1 ) and neutron activation analysis with radiochemistry by Pillay et al. (2) have been tried on ma- trices similar to coal, but these fall short in many in- stances because of poor precision, sensitivity, or elaborate chemical separations.

The procedure described here is an optimization of the technique of neutron activation analysis (NAA) and the utilization of a newly developed nuclear detector known as the Low Energy Photon Detector (LEPD) (3 ) . Although originally intended for charged particle analysis, research shows this LEPD to be quite promising as a partner with instrumental NAA based on X-ray identification of cer- tain trace metals in irradiated materials. The identifica- tion of mercury is especially attractive because of the high cross section for production of the 197Hg isotope, its 65-hr half-life, and the strong intensity of several X-rays and a low energy y associated with its decay. Likewise, selenium (75Se) with a 120-day half-life is easy to identify using two low-energy y-rays.

The method ( 4 ) described for the determination of mer- cury and selenium in a variety of matrices including coal and fly ash achieves three significant improvements over currently used procedures. The procedure has no chemical manipulations, thereby eliminating technique-related er- rors. The analysis is strictly instrumental, thus eliminat- ing unnecessary errors involved in the determination of chemical yields. The procedure is based on the direct measurement of the X-rays from irradiated matrices, thus allowing the same technique to be used on a wide variety of matrices.

EXPERIMENTAL Apparatus. A 16-mm Ortec (LEPD) Low Energy Photon De-

tector and an ND 2200 Multichannel Analyzer were used. The LEPD plus dewar is very similar in size and shape to the stan- dard Ge(Li) detectors commonly marketed today. The LEPD unit

uid nitrogen dewar. The detector has a standard end cap window of a 5-mm thickness of beryllium. Such a detector, when coupled with an analyzer with 1024 or more channels, has a useful range from 3 to approximately 600 keV. At above 300 keV, the relative photopeak efficiency begins to drop off; but, resolution wise, it can still separate photopeaks a s close as 3 keV a t 500 keV. At 270 keV, photopeaks as close as 750 eV can be separated. Typical res- olution a t 5.9 keV is 225 eV, and a t 122 keV, it is 600 eV.

Reagent Preparation. Weigh approximately 1 g of mercuric oxide (Dried-Hi-purity Grade), dissolve it in approximately 20 ml of concentrated nitric acid in a 1-1. volumetric flask, and dilute to 1.0 1.

Weigh approximately 1 g of selenium powder (99.99% pure), dissolve it in approximately 20 ml of concentrated nitric acid in a 1-1. volumetric flask, and dilute to 1.0 1.

Procedure. Heat-seal 0.5 g of coal or fly ash (after freeze drying for 16 hr) in Amersil hi-strength silica tubing. One milliliter of the standard solution is encapsulated in like manner. Samples are irradiated in a nuclear reactor for a total of 14 hr at a flux of 3 X 1013 n/cmZ sec. Flux monitors are attached and counted for each sample.

After a decay time of approximately 3 days for 197Hg and 14 days for 7JSe, the quartz tubes are washed in 1:l nitric acid and rinsed in distilled water, cooled in liquid nitrogen, and opened approximately 5 mm from the top. Samples are then weighed intc preweighed, 4 cm in diameter by 1 cm high, flat plastic snap top counting containers which contain a very thin wiping of Vaseline as in Figure 1. The Vaseline provides a tacky surface for even dis. tribution of the coal sample for counting.

The coal sample is placed on the 16-mm Ortec LEPD, and the X-rays from the sample are counted for 15 min for Hg and 60 min for '5Se at a calibration of approximately 0 to 175 keV to obtain a spectrum as shown in Figures 2 and 3.

The quartz tube containing the irradiated standard frozen witb liquid nitrogen is broken into a beaker containing 10 ml of con. centrated nitric acid, is allowed to melt, and is then decanted into a 1-1. volumetric flask. The broken quartz vial and the bea. ker are washed with six 5-ml portions of distilled water and the final volume is adjusted to 1.0 1. One-tenth milliliter of the dilut- ed solution is pipetted into a similar, 4 cm in diameter by 1 crr high, flat plastic container which has a 0.5-g sample of nonirrad, iated coal imbedded in the Vaseline layer.

As was for the unknown, the standard is counted for 20 minutes on a 16 mm Ortec LEPD. A spectrum is obtained as shown ir. Figures 4 and 3. Either the area under the 68.79 keV or the aree under the 77.97 keV X-rays from 197Hg ( 5 ) and the 121.1 or 136.C keV photopeaks of 75Se are integrated using, for example, tht method of Covell (6).

The area for the sample is then compared with the area of thc standard and the parts per million of the mercury or selenium ir the sample is calculated using the equation

(Activity in sample)(pg Hg or S e in s tandard)(s tandard flux monitor count) (Activity in s tandard)(wt of sample in g)(sample flux monitor count)

p g / g Hg or Se =

is constructed of a virtually windowless (less than 1 Fm Ge) lithi- um drifted germanium crystal wafer and is maintained a t liquid nitrogen temperatures by use of cryosorption pumping and a liq- ( 1 ) G . W. Kalb, "The Determination of Mercury in Coai by Flameless

Atomic Absorption." 163rd National Meeting of the American Chemi- cal Society, Boston. Mass.. April 1972.

( 2 ) K. K S. Pillay, C. C. Thomas, J. A . Sondel. C. M . Hyche. Anal. Chern.. 43, 1419 (1971).

(3) Ortec. Inc., Technical Data Sheet, "8000 Series Ge(Li) Low Energy Photon Detector," Ortec. Inc.. Oak Ridge, Tenn.. Dec 1971.

(4) J. N. Weaver and D. Von Lehmden, 163rd National Meeting of the American Chemical Society, Boston, Mass., April 1972.

RESULTS In order to check the accuracy of the LEPD results,

these were compared as shown in Table I to the results from the other laboratories participating in the round- robin. Their techniques consisted of flameless atomic ab- sorption and neutron activation analysis with radiochem-

(5) C. M. Lederer. J, M . Hollander. and I . Perlman. "Table of Isotopes,' 6 th ed. Wiley. NewYork. N . Y . . 1968.

(6) D. F. Covell. Ana/ . Chem.. 31. 1785 (1959).

1950 ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973

Table I. U.S. Bureau of Mines Round-Robin-Trace Metals in Coals Average of

using N A A Instrurnen- laboratories Instrurnen-

using LEPD, and A A , using LEPD, tal N A A u tal N A A a

Sample Geological location ppm Hg PPm Hg P P ~ Se DRB-A Belmont Co., Ohio 0.16 0.14 4.34 f 0.07 DRB-B Harrison Co., Ohio 0.43 0.44 10.74 f 0.02 DRB-C Jefferson Co., Ohio 0.27 0.24 4.37 f 0.07 DRB-D Kanawha Co., W. Va. 0.09 0.07 5.07 f 0.06 DRB-E Washington Co., Pa. 0.12 0.13 2.18 f 0.09 G-1 Clay Co., Ind. 0.10 0.07 3.19 f 0.08 P- 1 Muhlenberg Co., Mont. 0.17 0.17 4.78 f 0.07 P-2 Rosebud Co., Mont. 0.07 0.06 1.87 f 0.10

P-4 Montrose Co., Colo. 0.07 0.05 4.67 f 0.07 P-5 Navajo Co., Ariz. 0.08 0.06 6.11 f 0.05

P-3 Henry Co., Mo. 0.17 0.15 2.21 f 0.09

a Average of three determinations.

PLASTIC COUNTING CONTAINER,

LVASOLINE M E D I U M W I T H TRAPPED COAL SAMPLE

Figure 1. Sample mounting for optimization of X-ray counting

0

GAMMA ENERGY, Lev

Figure 3. X-Ray spectrum of a selenium standard after a 14-hr irradiation at 1013 n/cm2 sec, a 14-day decay, and a 20-min count on a 16-mm Ortec LEPD

I o 100 Lev

GAMMA ENERGY I N k t V

Figure 2. X-Ray spectrum Of a l g 7 Hg standard after a 14-hr irra- diation at 1013 n/cm2 sec, 5-day decay, and 15-min count on a 16-mm Ortec LEPD

istry and counting on NaI detectors. If the averages of the other laboratory results are taken as the true value of mercury present in the coal, LEPD values of mercury from 0.05 to 0.15 ppm range f l8%, from 0.16 to 0.25 ppm f5%, and from 0.26 to 0.50 ppm f2%.

Also shown in Table I are results for selenium in coal, although at this time selenium was not part of the round- robin analysis.

DISCUSSION Since the research effort was slanted to develop a rapid,

accurate procedure for routine coal analysis on a large number of samples, simplicity of this technique was com- bined with thorough efforts to maintain duplicate condi- tions for both standards and unknowns. In view of this, instrumental NAA using the LEPD system has many ad- vantages over conventional NAA and radio-chemistry and flameless atomic absorption. The LEPD method com- pletely removes the problem of having to dissolve the coal and perform time-consuming radio-chemistry. The associ- ated contamination and recovery error problems are also

I

1

O I( X-RAY ENERGY I I V

Figure 4. X-Ray spectrum of a typical coal sample after a 14-hr irradiation at 1013 n/cm2 sec, 5-day decay, and 15-min count on a 16-mm Ortec LEPD

eliminated. No glassware is necessary and only a small plastic bottle or flat sealable plastic disk is needed for sample containment while counting.

In a typical coal matrix, the sensitivity of the LEPD method, when coupled with a 10 to 14 hr 1013 n/cmz sec irradiation, borders around 50 ng of mercury per gram of coal. Precision at that level is on the order of *lo% a t 100

ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973 1951

ppb, *5% a t 200 ppb, and *2% at 500 ppb. For selenium, the sensitivity is approximately 150 ng per gram of coal with precision values of &15% a t 250 ppb, *lo% at 500 ppb, and f5% a t 1 ppm.

Preservation and duplication of the counting geometry are two very important aspects of utilizing a low energy photon detector in that (1) the crystal itself is wafer thin, therefore, the sample must cover the surface area of the crystal, and (2) the soft photons or X-rays emitted from the sample are easily absorbed by the matrix of the sam- ple itself; therefore, the sample and standard must be as thin as possible for maximum sensitivity. Using a point X-ray source of 57C0, a series of thirty separate counts were taken over the 201-mm2 area of the LEPD window, and the largest variation registered was *4.5%. This indi- cates that the counting geometry within this area is rela- tively uniform, thus providing accurate measurement of the mercury X-rays at any point in the coal sample. This is an especially important feature that enhances the ease of mercury analysis in coal using the LEPD.

Use of the Low Energy Photon Detector, as applied to coal analysis can be optimized, sensitivity-wise, in several ways as follows.

1. An irradiation at a 1014 n/cmz sec flux could be used. 2. The detector cover cap could be designed such that

the sample being counted would be only several millime- ters from the beryllium window.

3. A flat plastic sample holder could be devised with a thin bottom which would reduce the 20% loss of mercury X-rays due to attenuation as was experienced with our sample holder. 4. The large diameter 25-mm Ortec LEPD could be

used.

ACKNOWLEDGMENT The author wishes to acknowledge the assistance of M.

0. Schlesinger, U.S. Bureau of Mines, for supplying the coal samples used in this analysis.

Received for review February 16, 1973. Accepted April 11, 1973. Presented at the 163rd National Meeting of the American Chemical Society in Boston, Mass., April 1972. This research was supported by the Source Sample and Fuels Analysis Branch of the Environmental Protection Agency.

Chemical Ionization Mass Spectrometry of Complex Molecules: Biogenic Amines

G. W. A. Milne, H. M. Fales, and R. W. Colburn

National Heart and Lung Institute, National lnstitutes of Health and National lnstitute of Mental Health, Bethesda. Md. 20074

It is now well-known ( I ) that a group of compounds known as “biologically important amines” or “biogenic amines” is involved in the transmission of signals in the nervous system. The complex modes of action of these compounds are poorly understood and studies on this problem have been impeded by the lack of specific and sensitive analytical methods for the detection and quanti- tation of these compounds in biological preparations.

These biologically important amines are widely distrib- uted in many tissues but are present only at extremely low concentrations (nanograms per milliliter) (2). To be useful for their detection and quantitation, any analytical method, as well as being specific, must therefore be appli- cable a t the nanogram-picogram range. Many otherwise useful analytical techniques are not adequately sensitive to deal with this problem and, in recent years, the two most useful methods to emerge have been fluorescence spectrophotometry (3-5) and mass spectrometry (6, 7).

The majority of the compounds so far identified as pu- tative transmitters are decarboxylation products of aro- matic amino acids such as tyrosine, tryptophan, histidine, and 3,4-dihydroxyphenylalanine (dopa). These compounds

(1) P. B. Molinoff and J. Axelrod. Ann. Rev. Biochem.. 40, 465 (1971). (2) L. Bertilsson and L. Palmer, Science. 177, 74 (1972). (3) A. Anton and D. Sayre. J. Pharmacol. Exp. Ther.. 138, 360 (1962). (4) E. G. McGeer and P L. McGeer, Can. J. Biochem. Physiol.. 40.

1141 (1962). (5) D. M. Shaw, A. Malleson, E. Eccleston, and D. Gundy. Brit. J. Psy-

chiat.. 111, 993 (1965). (6) A. A. Boulton and J. R. Majer. J . Chromatogr., 48, 322 (1970). (7) L. Bertilsson. A. J. Atkinson, J. R. Althaus, A. Harfast, J. E. Lind-

gren, and E. Holmstedt. Anal. Chem., 44, 1434 (1972).

are well-known (8) to be particularly labile at the carbon- carbon bond of the side chain toward electron impact (EI). The reason for this is, of course, the fact that the odd-electron molecular ion can cleave to yield either of the very stable ions I or I1 and the corresponding radical 111 which is itself stabilized by resonance with the aromat- ic ring. In the case of histamine, cleavage of the side chain is so facile that the resulting ion gives rise to the base peak of the El mass spectrum at m / e 82 while the relative abundance of the molecular ion is under 5%. Similarly, the molecular ions of tryptamine, tyramine, and dop- amine have very low relative intensities, the fragment ion at the appropriate m / e value being 5-30 times more in- tense.

\ III

+

I1

(8) K Biemann. “Mass Spectrometry,” McGraw-Hill. New York. N. Y . , 1962, p 90.

1952 ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973