2004
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
in this paper were obtained were very low light levels, for those are the conditions for which photon counting has the most advantages. We performed identical experiments using much higher light levels and found the trends in the data to be the same as in the data presented here. The relative changes in the SNR were, however, smaller due to the much higher SNR to begin with. T h e specific P M T s tested here may not be optimum for use as photon counting detectors (16), but they are very commonly used PMTs. However, the data presented do seem to be generally representative of all P M T types. Each P M T tested showed optimum performance when used with a high operating voltage. Although each investigator should determine the best conditions for his experiments, it is safe to assume that operation of the P M T used a t or near the highest safe voltage will produce the best results. T h e effects of temperature on the SNR produced seem to be very much dependent on the specific P M T type used. This is in a large part due to relative changes in the cathode sensitivity with temperature. For a given P M T these changes can be wavelength dependent, especially when close to the long wavelength cutoff for the P M T (20). In general the increase in SNR obtained when the P M T is cooled is small, and may not be worth the additional expense, especially if the measurement system is very stable and long count times can be used. Also, there is a lower temperature limit after
which additional cooling no longer results in any increase in the SNR. For the P M T s tested here, only the R406 would have benefitted significantly from temperatures lower than the -23 "C reached by the thermoelectrically cooled P M T housing used here.
LITERATURE CITED Ingle, J. D.. Jr.; Crouch, S. R . Anal. Cbem. 1972, 44, 785-94. Murphy, M. K.; Clyburn. S.A.; Veilbn, C. Anal. Chem. 1973, 45,1468-73. Tull, R . G. Appl. Opt. 1968. 7 , 2023-29. Jones, R.; Oliver, C. J.; Pike, E. R. Appl. Opt. 1971, 10, 1673-80. Amoss, J.; Davidson, F. Appl. Opt. 1972, 7 1 , 1793-1800. Nakamura, J. K.; Schwarz, S. E. App. Opt. 1968, 7 , 1073-78. Enke, C. G. Anal. Cbem. 1971, 43(1), 69A-80A. Darland, E. J.; Leroi, G. E.; Enke, C. G. Anal. Cbem. 1979, 51, 240-45. Young, A. T. App. Opt. 1963, 2, 51-60. Morton, G. A. Appl. Opt. 1988, 7 , 1-10, Oliver, C. G.; Pike, E. R . J . Pbys. D . 1968, 1 , 1459-68. Gadsden, M. App. Opt. 1965, 4, 1446-52. Rodman, J. P.; Smith, H. J. App. Opt. 1963, 2, 181-86. Niemczyk, T. M.; Ettinger, D. G. ADD/. Spectrosc. 1978, 32, 450-53. Fried, D. L. Appl. Opt. 1965, 4, 79-80: Footd, R.; Jones, R.; Oliver, C. J.; Pike, E. R. Appl. Opt. 1969, 8 , 1975-89. Ingle, J. D., Jr.; Crouch, S. R . Anal. Cbem. 1972, 44, 777-84. Ozolins, A.; Lineberger. W. C.; Niles, F. E. Rev. Sci. Instrum. 1968, 39, 1039-43. Boileau, A. R.; Miller, F. D. Appl. Opt. 1967, 6 , 1179-82. Martin, H. "Electro-Optical Systems Design" 1976, 8, 16-20
RECEIVED for review April 23, 1979. Accepted July 26, 1979. The authors are pleased to acknowledge the support of the University of New Mexico Research Allocations Committee.
Trace Element Characterization of the NBS Urban Particulate Matter Standard Reference Material by Instrumental Neutron Activation Analysis Robert R. Greenberg Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234
The Urban Particulate Matter, SRM 1648, recently prepared by the National Bureau of Standards, with partial support from the Environmental Protection Agency, has been analyzed by instrumental neutron activation analysis (INAA) for 32 elements. Special attention has been given to reducing and evaluating the analytical errors. SRM 1632, Trace Elements in Coal, was also analyzed and the results were compared with literature and NBS certified values.
Well characterized reference materials have proved to be useful aids in verifying the accuracy of analytical procedures. Many analytical techniques suffer from problems due to chemical blank, interferences, losses during sample dissolution, incomplete sample dissolution, etc. Additional confidence in an analysis can be obtained if, by using the same procedures, the correct concentrations are found in a similar reference material. Among the most useful of the reference materials have been the National Bureau of Standards Standard Reference Materials (SRMs), because of the high degree of accuracy usually associated with the NBS certified concentrations. For a n element to be certified, its concentration is usually determined by two or more independent analytical This article not subject to U S Copyright
techniques, or by a definitive method ( 1 ) . The National Bureau of Standards cannot certify the concentration of every element in an SRM. However, the utility of many SRMs could be greatly increased if the concentration of additional elements were known. Perhaps one of the most widely referenced, recent papers in the field of analytical chemistry has been one by Ondov et al. ( 2 ) in which the concentrations of approximately 40 elements were determined in SRMs 1632 and 1633 (Trace Elements in Coal and Trace Elements in Coal Fly Ash). Another area in which knowledge of the concentrations of a large number of elements is particularly important is the study of atmospheric particulate matter. Attempts to resolve urban aerosols into their component sources frequently require that the concentrations of many elements be determined (3, 4).
The National Bureau of Standards, with partial support from the U.S. Environmental Protection Agency, has recently prepared an Urban Particulate Matter SRM from natural urban atmospheric particulate material which was collected in the St. Louis (Mo.) region. This SRM should be very useful to the many researchers analyzing atmospheric particulate material, or other material of a similar nature. T h e concentrations of nine elements have been certified by NBS. T o
Published 1979 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
increase the utility of this material, the concentrations of 32 elements (including five of the certified ones) have been determined in this work by instrumental neutron activation analysis (INAA). This method is resistant to common sources of systematic error and does not require the dissolution of the sample with the associated errors of chemical blank, failure t o completely dissolve the sample and loss of volatile species. Every attempt has been made t o minimize the errors associated with these analyses, and to evaluate their magnitudes. EXPERIMENTAL Standards. Since the results of an analysis can only be as good as the standards used, particular care was taken in preparing the multielement standards. All volumetric flasks and pipets were calibrated both before and after use. Stock solutions of the elements were individually prepared by dissolving ultrapure metals and compounds in NBS high purity acids (5,6) and/or NBS high purity water (6). Each compound was taken from an unopened bottle and dried to a constant weight. The radioactivable purity of each stock solution was checked by irradiating and counting an aliquot of each. This step proved to be necessary as significant 134Csactivity was observed in the irradiated Rb solution. Analysis of material from two bottles of RbCl (both from the same manufacturer) indicated a Cs content of approximately 1.4%, even though the manufacturer specified that the Cs concentration was no more than 50 ppm. Since Rb and Cs were combined in the same standard, and since much less Cs was required, use of this RbCl compound would have resulted in large Cs errors (on the order of 50%). A batch of "lower purity" Rb2C03from the same manufacturer was found to contain
