Instrumental Neutron Activation Analysis of Standard Reference Material 1941,

Organics in Marine Sediment

Element Content and Homogeneity

S U S A N F. S T O N E , * BARBARA J. K O S T E R , AND R O L F ZEISLER

Center for Analytical Chemistry, National Institute of Standards and Technology, Gaithersburg, MD 20899

Received June 8, 1989; Accepted December 11, 1989

ABSTRACT

The National Institute of Standards and Technology has devel- oped a new Standard Reference Material 1941, "Organics in Marine Sediment." In addition to the organic constituents, over 30 elements have been determined by instrumental neutron activation analysis and prompt-gamma activation analysis. The homogeneity of the ma- terial was investigated and relative standard deviations of single- element concentrations in 250-mg samples were found to be 1% or less with regard to major inorganic constituents and rare earth ele- ments. A slightly higher relative SD was found for elements that may stem from biological or anthropogenic input. The element concentra- tions determined in this work are discussed in comparison to concen- trations in other similar reference materials. Concentrations for 31 elements will be included for information on the certificate.

Index Entries: Elemental analysis; environmental monitoring; gamma spectrometry; instrumental neutron activation analysis (INAA); prompt gamma activation analysis (PGAA); sediment; stan- dard reference material.

*Author to whom all correspondence and reprint requests should be addressed.

Biological Trace Element Research Editor: G. N. Schrauzer �9 1990 by The Humana Press Inc.

579

580 Stone, Koster, and Zeisler

INTRODUCTION

Certified reference materials (CRMs) are widely used to validate analytical methods and to assess the quality of data generated in pro- grams in which many laboratories and individuals are determining the analytes of interest. For example, the National Oceanic and Atmospheric Administration's (NOAA) National Status and Trends (NS&T) program collects data and analytical results from more than 200 sites in US coastal and estuarine waters (1). These data are generated by 10 participating laboratories, and hence need a common base for comparison. Natural matrix certified reference materials (CRMs) have been available since 1971 (2), preceded only by such intercomparison materials as Bowen's Kale (3). However, these CRMs had been characterized for inorganic trace constituents only. During the past decade, the advent of reliable analytical techniques for the determination of organic trace constituents, particularly those of environmental concern, has led to programs in the CRM-issuing agencies to develop and certify a number of natural mate- rials for organic trace constituents. In the area of marine sediments, the International Atomic Energy Agency (IAEA), Bureau of Reference Com- munities (BCR), and the National Research Council of Canada (NRCC) have issued several materials. However, for each material, only a limited number of analytes or groups of analytes are normally determined. No single sediment material has been prepared and characterized for poly- chlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), chlorinated pesticides, and inorganic constituents.

To meet the need for a single CRM containing all analytes of interest to marine monitoring programs, the National Institute of Standards and Technology (NIST, formerly the National Bureau of Standards) has de- veloped Standard Reference Material (SRM) 1941, Organics in Marine Sediment (4). The certificate will provide certified concentrations for 11 PAHs, and non-certified concentrations for 24 PAHs, 15 PCB congeners, and seven chlorinated pesticides. Since trace elements may be correlated with organic pollutant levels and vice versa, it may be useful to know the inorganic constituents in this material. Uncharacterized constituents may also influence results of analytical procedures, depending on their con- centration in the sample. In addition, certification-quality inorganic data on this SRM can provide the user with another benchmark sample for monitoring inorganic constituents in a sediment matrix. Also, the homo- geneity of the material can be assessed easily with an analytical technique that requires only small samples and virtually no sample preparation. Hence, an NAA approach was used to determine the concentrations of 30 major and trace elements, combining the methods of neutron-capture PGAA and INAA to determine as many elements as possible in a single aliquot.

Element Content and Homogeneity of S1921 1941 581

MATER/ALS AND METHODS

SRIq 1941 Sample Collection and Preparation

The sediment used for the preparation of SRM 1941 was collected in the Chesapeake Bay, at the mouth of Baltimore (MD) Harbor. The details of the collection and preparation are described by Schantz et al. (4). The SRM is provided as a dried, homogenized material. In addition to the SRM 1941 samples, various control materials were included in the differ- ent analyses for quality assessment. These included SRM 1645, River Sediment; SRM 1646, Estuarine Sediment; SRM 2704, Buffalo River Sedi- ment; and Certified Reference Material SD--N-115-1/2, Sediment, from the IAEA. Aliquots of 200-300 mg from two bottles of the reference material were used, as received, for the analyses. Samples were pel- letized in a KBr pellet press and packaged in Teflon1" film for PGAA or in linear polyethylene film for INAA.

Prompt-Gamma Activation Analysis The PGAA was performed at the National Bureau of Standards

Reactor facility (5). The neutron fluence rate at 20 MW reactor power is 3.5. 1012n �9 m -2- s-1. The pelletized samples were positioned so that they were at 45 ~ with reference to the neutron beam. Spectral data were collected with a gamma spectrometer (relative efficiency of 25%; resolu- tion of 2.0 keV full width at half maximum [FWHM] at 1332 keV) com- bined with a split annulus NaI(T1) detector. Counting times ranged from 8 to 16 h/sample. Spectral data were collected in the Compton sup- pressed mode with electronics as described previously (5) and were transferred through a Nuclear Data (ND) micro multichannel analyzer (~MCA) interfaced to a VAX 730. Data were stored on disk, and peak searches were done on each spectrum using ND software (6), with additional fits using a channel summing program implemented on the VAX.

For quantitation of elemental concentrations, monitor activation analysis was used, with titanium foils counted between samples as flux monitors. Sensitivities for pertinent energy lines of elements (counts �9 s -1 . mg- i ) were tabulated in a spreadsheet, along with corrections for interferences, background, and enhancement effects (7). Peak areas, sample mass, counting time, and appropriate titanium-foil count rates were entered into the spreadsheet, and the concentrations for the listed elements were calculated.

Instrumental Neutron Activation Analysis

The INAA procedure consisted of two irradiations followed by gam- ma counting after appropriate decay times. The first irradiation was 30 s

582 Stone, Koster, and Zeisler

at a fluence rate of 2 �9 1017n �9 m - 2 �9 s - 1 in the RT-4 pneumatic irradiation facility at 15 MW reactor power (8). Each irradiation was carried out on a single pellet of SRM 1941, together with one of the standards. All irradia- tions were completed within a 6-h period with the reactor in thermal equilibrium and no major changes in control settings. Thus, possible fluence rate changes were expected to be much smaller than 1%. Com- parison of specific activities in the standards irradiated during this run confirmed the expected stability. This was followed by a 16-h irradiation in the same irradiation facility. Samples, control materials, and standards were irradiated in two sets, packaged in the center of each irradiation container. To compensate for the linear neutron flux gradient along the axis of the container, the irradiation was performed in two equal time intervals of 8 h each, with an 180 ~ inversion of the rabbit at the midpoint of irradiation.

The gamma spectrometry was based on high-resolution germanium spectrometers linked to ND micro multichannel analyzers (laMCAs)/VAX 730 or ND6620 MCAs. Performance criteria of the spectrometers were selected to match the requirements of the complex gamma spectra of the sediments. For the assay of short-lived nuclides, an intrinsic germanium detector (25% relative efficiency and 1.72 FWHM at 1332 keV) was used coupled to an Ortec 572 amplifier and the ND581/ND599 ADC and loss- free counting module, utilizing Westphal's principle of loss correction for a decaying source (9). An Ortec 644 fast busy delay aided in the timing for the ND599 module. Count rates up to 70,000 s -1 can be handled by the system with a typical detector resolution of 1.95 keV FWHM for 1332 keV. Sample assays were started after the count of the standards, ap- proximately 500 s decay. Counting time for standards and samples was 300 s. The samples were counted twice after the long irradiation: the first count after 5-8 d, and the second count after 7-8 wk. The spectra from the first counts were collected with an intrinsic germanium detector (relative efficiency 28.5% and 1.8 keV FWHM resolution). A Tennelec 244 amplifier with built-in pile-up rejector made the system capable of han- dling 20000 cts/s without degradation in resolution. This allowed for a high sample throughput using a 4-h counting time for each sample with the help of a sample changer. A Gamma-X spectrometer with a relative efficiency of 7% and 1.65 keV FWHM resolution connected to a ND 6600 system was used for the second set of counts. Additional counts during the first counting cycle were also done using this higher resolution spectrometer to better resolve the complex spectra and to provide control for the quantitation of the shorter lived nuclides.

Quantitative evaluation was accomplished using the comparator method. Comparator standards were prepared from primary solutions of high-purity metals or compounds in ultrapure acids and water. These standards were combined into multielement standards, which were then pipeted onto Whatman filter papers. These were air-dried and then

Element Content and Homogeneity of S I ~ 1941 583

formed into pellets. The VAX computer or ND6600 (as appropriate) was used for data storage and processing, with ND software for peak searches, nuclide identification, decay corrections (using data from ref. 10), and concentration calculations (7,11). Spectral interference correc- tions were calculated directly for the specific detectors used in counting, whenever possible. Recent work by James (12) was used to confirm the best selection of energies for elemental determination.

RESULTS AND DISCUSSION

For the INAA assay of the sediment samples, the optimum perfor- mance of the gamma spectrometers was essential. The gamma spec- trometers had to have high count-rate capabilities, while maintaining excellent resolution. As an example, the resolution obtained with the Gamma-X spectrometer in the second count of the lS2Eu, 6SZn, 46Sc triplet at 1112, 1115.4, and 1120 keV is illustrated in Fig. 1. High count rates caused by major activities from certain nuclides had to be tolerated in each of the counts in order to quantitatively determine elements with lower intensity nuclides of similar or shorter half-lives. An additional advantage of utilizing the high count-rate capabilities of the described system was a considerable time saving in counting, allowing a rapid throughput on the sample changer. This was particularly advantageous

f 76 re It d 1 a for the determination of As, since the 26.3-h half-life o As su e "n significant loss of activity during the first series of counts after the long irradiation. Also, since a second count of the samples for shortqived nuclides, after the short irradiation, would not have yielded significantly improved results for Mn and K, it was worthwhile to handle the high count rate with the loss-free system. However, some additional uncer- tainty is introduced by the loss-free mode, which has been included in Table 1.

The recommended element concentrations in SRM 1941 were deter- mined by this NAA procedure and are listed in Table 1. Uncertainties represent observed standard deviations of the calculated means. Includ- ed are the results for one control material, IAEA-SD-N-1/2, the certified values and confidence intervals of which have been published in the literature (13). No significant deviations have been observed for this material, except for Ce, Ag, and Th, which are 5-10% lower, and Mn, which is about 10% higher than the upper limit of the reported confi- dence interval of the certificate. The result for Se is 15% lower than the lower limit of the reported noncertified confidence interval; however, no such bias was apparent in the other reference materials that were an- alyzed.

The homogeneity of this material, as determined for inorganic ele- ments, was very good; observed SD of 1% were found for the major

5 8 4

W

0 0

250000

9000

0

Stone, Koster, and Zeisler

A

1100 1110 1120 Energy (keV)

k

9000

_.= c :3 0 o

7000

5000

3000

1000

B

}J J 1100 1110 1120

Energy (keV)

Fig. 1. A: Full scale (linear) view of a spectrum region from a SRM 1941 sample showing the Eu-Zn-Sc triplet at 1112-1120 keV. B: Expanded view of the same region showing the resolution between the three peaks.

constituents, such as Fe and Cr, for Sc, and for most of the rare earth elements (REE), in aliquots of 200-250 mg. For example, the range of values for Sc was 32.56-33.57 mg/kg, with an observed SD of 0.37 (1.1%) and a reduced chi-squared (X 2) of 0.928. This indicates that most of the variation was caused by the count ing error. The ranges for As and Sb were slightly larger. With similar count ing statistics, the reduced X 2 for As and Sb were 9.4 and 12.0, respectively. This indicates probable in- homogenei ty . These elements and others, such as Se and Ag, which show larger uncertainties, are characteristic of biological and anthro-

Element Content and Homogeneity of SRPl 1941

Table 1 Ins t rumen ta l N e u t r o n Act iva t ion Analys i s Results.

E lement Concen t ra t ions in mg/kg, Dry We igh t Basis

585

Gamma Energy SRM 1941 IAEA-SD-N-1/2 Element Used Mean Uncertainty 2 Value Uncertainty .~

B 1 477 75.5 1.7 87.7 1.5 Na 1368, 2754 12910 290 10740 120 AI 1778 64800 2400 37290 650 Si: 3539, 4933 222000 8000 272000 11000 S 1 841 16410 840 3000 I000 (21 1642, 2167 16400 400 9460 120 K 1 770 15800 120 15200 580 Se 889 34.34 0.42 6.913 0.060 Ti ] 320 17220 310 2812 93 V 1434 812 31 79.0 1.3 Cr 320 634.5 9.7 150.8 2.4 Mn 846 787.8 9.5 856.1 5.7 Fe 1099,1292 105400 1000 36230 380 Co 1173,1332 27.48 0.14 11.455 0.074 Zn I 115 1012 434 437.6 6.5 As 559 75.4 4.0 56.30 0.44 Se 264 10.11 0.50 1.76 0.27 Rb 1076 92.1 1.3 69.6 2.2 Ag 657 1.24 0.47 1.54 0.25 Sb 1691 15.20 0.42 3.55 0.061 Cd I 558 2.32 0.29 12.30 0.52 Cs 795 4.78 0.13 4.36 0.10 La 1596 359 12 29.11 0.29 Ce 145 271.8 3.6 54.77 0.77 Sm 1 333 25.69 0.40 5.52 0.20 Eu 778 2.184 0.060 1.065 0.042 Tb 298 2.15 0.14 0.669 0.0010 Gd 1 182 15.16 0.37 5.43 0.35 Hf 482 22.43 0.29 8.48 0.12 Ta 1221 16.43 0.45 0.990 0.12 Th 311 25.63 0.25 6.73 0.10 U (1596) (22) .g6.6

~PGAA. 2ts/N/n for P = 0.05. 31s counting statistics. qncludes an allowance for uncertainties in peak integration.

pogenic sources. Such elements as As and Se may occur with the organic compounds in the sediment, hence the observed variability may be an indication of the homogeneity of the organic compounds.

The composition of this material appears unusual in many respects, and is discussed in detail elsewhere (4). The material contains high levels of S, Fe, Ti, Sc, and many of the REE, compared to other sediment reference materials; e.g., many of the REE had concentrations higher than other similar reference materials by a factor of 10 or more. This unusual composition in addition to the normal complexity of a sediment spectra meant that great care was necessary both during the analysis and when considering interferences, especially those involving the REE.

586 Stone, Koster, and Zeisler

Some elements, such as Na, C1, K, Ti, Cd, and Sm, could be ob- tained by both INAA and PGAA. The results for Na, C1, Ti, and Sm were comparable, but again, because of the complex matrix, one technique generally obtained more precise results than the other. The more precise results were reported in Table 1. For K, Cd, and Sin, the PGAA results were used.

The characterization of inorganic constituents in SRM 1941, Organics in Marine Sediment, has proven to be useful, both for elemental an- alyses, and for added information that has proven to be helpful for the organic analyses. For example, knowing about the high sulfur content of SRM 1941 resulted in the development of an additional clean-up pro- cedure to accomplish the organic analyses. The material will also be a useful control material in the inorganic analyses of samples from envi- ronmental monitoring programs, providing a probable upper boundary for such samples.

ACKNOWLEDGMENTS

The authors acknowledge the assistance of D. L. Anderson from the Food and Drug Administration for his advice in the analysis of the PGAA data. Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

REFERENCES

1. G. G. Lauenstein and J. A. Calder, Progress in Environmental Specimen Bank- ing, S. A. Wise, R. Zeisler, and G. M. Goldstein, eds. NBS Spec. Publ. 740, US Government Printing Office, Washington, DC 1988, 19-30.

2. SRM 1571, Orchard Leaves, Certificate, National Bureau of Standards, Gai- thersburg, MD 20899 (1981).

3. H. J. M. Bowen, Proceedings of the Society, Analytical Chemistry Conference, Nottingham, P. W. Shallis, ed; W. Heifer, Cambridge, UK 1965, 25.

4. M. M. Schantz, B. A. Benner, Jr., S. N. Chesler, B. J. Koster, S. F. Stone, R. Zeisler, and S. A. Wise, submitted to Fresenius Z. Anal. Chem.

5. D. L. Anderson, M. P. Failey, W. H. Zoller, W. B. Waiters, G. E. Gordon, and R. M. Lindstrom, J. Radioanal. Nucl. Chem. 63, 97 (1981).

6. VAX/VMS NAA package, Nuclear Data, Jan. 1985 with updates through Aug. 1988.

7. D. L. Anderson, Y. Sun, M. Failey, and W. H. Zoller, Geostandards Newsletter 9, 219 (1985).

8. D. A. Becker, J. Radioanal. Nucl. Chem. 110, 393 (1987). 9. G. P. Westpahl, J. Radioanal. Chem. 70, 387 (1982); Austrian Patent 368,291;

US Patent 4,476,384.

Element Content and Homogeneity of SR/~ 1941 587

10. From Nuclear Properties spreadsheet by C. A. Stone, using data extracted from the NUDAT database at the National Data Center at Brookhaven National Laboratory (Feb. 1988).

11. T. Meyers, Neutron Activation Analysis Package Operational Instruction Docu- mentation, Nuclear Data, 1978.

12. D. J. James and P. N. Boothe, J. Radioanal. Nucl. Chem. 123, 295 (1988). 13. Certificate for IAEA/Sediment SD-N-1/2, International Atomic Energy Agency,

International Laboratory of Marine Radioactivity, Musee Oceanographique, 98000, Monaco, 1985.