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Anal. Chem. 1980, 52, 240-245
Concentrations of Elements in the National Bureau of Standards’ Bituminous and Subbituminous Coal Standard Reference Materials M. S. Germani, Inci Gokmen, A. C. Sigleo, G. S. Kowalczyk,’ Ilhan Olmer,’ A. M. Small,3 D. L. Anderson, M. P. F a i l e ~M. , ~ C. Gulovali,‘ C. E. Choquette,’ E. A. Lepel,’ G. E. Gordon,’ and W. H. Zoller Department of Chemistry, University of Maryland, College Park, Maryland, 20742
Concentrations of 51 elements in the recently issued NBS Bituminous Coal (SRM 1632a) and 43 elements in Subbituminous Coal (SRM 1635) have been measured mainly with the use of Instrumental neutron activation analysis and neutroncapture prompt y-ray activation analysis. The results are In good agreement with values reported for some of the elements In the standards by NBS. Concentration values for many elements in addition to those given by NBS are presented for comparison with results that may be obtained by other laboratories.
Many analytical laboratories are called upon to analyze complex materials from the environment for a wide range of elements, especially toxic elements that occur in parts per million (ppm) or smaller concentrations. Even with the best state-of-the-art methods, these analyses are difficult to perform because of the complexity of samples (e.g., rocks, soil, ores, coal, atmospheric particulate matter) and the low levels a t which many elements of interest occur in them. Thus, it is vitally important that laboratories working in this field frequently check their analytical procedures by analyzing standards of complex materials in which concentrations of the elements are well known. T h e National Bureau of Standards (NBS) in cooperation with the Environmental Protection Agency (EPA) provided a valuable service to analytical laboratories by developing a set of environmental Standard Reference Materials (SRMs) for coal, fly ash, fuel oil, and gasoline in 1972. These materials were distributed to 85 laboratories for “blind” round-robin analyses for several elements and, simultaneously, NBS did analyses for certification. About 50 laboratories turned in results, which were discussed and compared with the NBS values a t a meeting held a t the National Environmental Research Center, Research Triangle Park, N.C. in May 1973. Some results of the inter-laboratory comparisons have been discussed by Ondov et al. (1). Briefly, many laboratories were quite surprised to find rather large deviations of their reported results from the NBS values. With the funds available for development of the standards, NBS was able to certify the coal standard (SRM 1632) for only 14 elements (plus information values for 7 others) and the fly ash (SRM 1633) for 13 elements (with information values for 6 others) (2). These were very useful standards for laboratories Present address: Northeast Utilities, P.O. Box 270, Hartford,
Conn. 06101.
Present address: Ankara Nuclear Research Center, Ankara, Turkey. 3Present address: Stark’s CP. Inc., Silver Sprin , Md. 20910. 4Present address: Babcock and Wilcox, Lynchiurg Research Center, P.O. Box 1260, Lynchburg, Va. 24505. 5Presentaddress: Sigma Data Corporation, Goddard Space Flight Center, Greenbelt, Md. Present address: Division of Physical Science, Pacific Northwest Laboratory, Battelle Memorial Institute, Richland, Wash. 99352. 0003-2700/80/0352-0240$01 OO/O
studying these kinds of materials; however, some laboratories have need of elemental concentrations for many elements beyond those listed by NBS. For this reason, four laboratories that had used nuclear analytical methods (University of Maryland, Battelle Pacific Northwest Laboratory, Washington State University, and Lawrence Livermore Laboratory) published a joint paper in which they gave their combined results from the round-robin experiment for about 40 elements in SRMs 1632 and 1633 (3). Although these numbers did not carry the weight of NBS certification, it was hoped that other laboratories would publish their values for these elements in order to build some consensus on the concentrations. In fact, this has happened: many laboratories have reported their values for one or both standards (for example, Refs. 4-11), much as has been done for the U S . Geological Survey’s standard rock samples. With the exception of one element (In, discussed below), the values reported by the four laboratories have held up well in the light of many comparisons. The analytical community has found the coal and fly ash standards so valuable t h a t the supplies are exhausted. For that reason, NBS recently released two new coal standards, SRM 1632a, a bituminous (Pennsylvania) coal, and SRM 1635, a subbituminous (Colorado) coal (12), and a new fly ash standard (SRM 1633a) is under development. Because of the value of the previous data given on SRMs 1632 and 1633 and as an exercise in testing our own procedures, we have analyzed the new coal samples for as many elements as can be reliably measured by several instrumental nuclear analytical methods. Seven individuals and one pair of participants analyzed the standards independently by instrumental neutron activation analysis (INAA). Another pair analyzed the standards by a new method, neutron-capture prompt y-ray activation analysis (PGAA). Some elements were measured by instrumental photon activation analysis (IPAA),and activation by 14-MeV neutrons from a generator or high energy neutrons from a reaction induced a t the Maryland cyclotron.
EXPERIMENTAL INAA. Samples for INAA and the other methods were freeze-dried for 24 h and kept over P205before weighing. This is especially important for SRM 1635, as it contains about 20% H 2 0by weight. The general procedures used were similar to those previously described (3, 13, 14). Samples were irradiated in the NBS reactor at a flux of 2 to 5 X l O I 3 n/cm’-s. Irradiations of 5 min were used to observe elements with short-lived products: Mg, Al, C1, Ca, Ti, V, Mn, Br, Sr, In, I, and Dy. Four-hour irradiations were used for elements with long-lived products: Sc, Cr, Fe, Co, Zn, Se, Rb, Zr, Sb, Cs, La, Ce, Nd, Eu, Tb, Yb, Lu, Hf, Ta, Th, and U. Species of intermediate half-life or with both short- and long-lived products could be determined in either or both types of irradiation: Na, K, Ga, As, Ba, Sm, and W. Details of the procedure varied somewhat from one person t o another but, in general, samples of 25-100 mg were used for short irradiations and of 5Ck-130 mg for long irradiations. These are smaller than the 250-mg minimum recommended by NBS to guarantee homogeneity (22),but we saw no evidence for inhomogeneities in this size range. Samples were sealed in polybags and polyvials C 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
and placed in an irradiation container (“rabbit”) along with Ni flux and/or elemental monitors. Corrections were made for the elemental contents of filter papers and polyethylene packaging materials. All handling of standards, preparation of monitors, and packaging of samples for irradiations were done in a Class 100 clean room a t the University of Maryland. Elemental monitors were made from the highest purity metals or salts available. Each high purity material was first analyzed by INAA to check on its purity prior to use. Then the metals or salts were dissolved in high purity acids and diluted to the desired volume with triple-distilled water. Less concentrated multielement standard solutions were made from aliquots of the appropriate elemental stock solutions and further diluted with distilled water. In general, we use several different multielement solutions: some containing elements observable in 5-min irradiations and others containing those seen in long irradiations. The relative amounts of the various elements used in the monitors are usually about the same as they occur in the sample being analyzed so that the spectral features are quite similar between the monitor and sample spectra to minimize errors in comparisons of peak areas. The multielement solutions are pipetted dropwise over large areas of Whatman 541 filter papers, which are dried, folded, and pressed into pellets of 1-cm diam in a Nylon-coated Ni die. These monitor pellets are prepared in batches of about 100 and several are checked against older NBS standards (mainly SRM 1632 and 1633) prior to use. Qpically, spectra of samples from 5-min irradiations were taken a t about 6-12 min and 15-20 min after irradiation using large (- 12% efficient), high-resolution Ge(Li) detectors coupled to 4096-channel analog-to-digital convertors and a computer-based data system (Tennecomp TP-5000). Spectra from the 4-h irradiations were taken for -4 h at 3-5 days and for 8 to 24 h at 2@30 days after irradiation. Either the major lines or the only lines of some irradiation products are in the very complex low energy (
