Anal. Chem. 1989, 6 1 , 1834-1836
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Determination of Silicon in National Institute of Standards and Technology Biological Standard Reference Materials by Instrumental Epithermal Neutron Activation and X-ray Fluorescence Spectrometry Ernest S. Gladney* Health and Environmental Chemistry, Group HSE-9, M S K-484, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Peter E. Neifert and Nathan W. Bower Chemistry Department, Colorado College, Colorado Springs, Colorado 80903
The measurement of silicon in biological standard reference materials has been investigated by both boron-filtered eplthermal neutron activation analysis and wavelength dispersive X-ray fluorescence. Good agreement between the methods was obtained for 11 different materials ranging from the methods’ detectlon limits of about 100 pg/g to the highest concentration In these samples of 12 000 pg/g silicon.
INTRODUCT10N Silicon has been recognized as an essential micronutrient since 1972 ( I ) . It plays an important role in the development of young bones and is also important in some plants. However, rapid and accurate methods for the determination of silicon in biological materials have proven to be difficult to develop. This is evidenced by the general scarcity of silicon data on biological reference materials in the literature. We have located only 27 reported values for the ll different biological standard reference materials (SRMs) that the U.S. National Institute of Standards and Technology (NIST-the new name for the former National Bureau of Standards) has produced since 1972. Although silicon has traditionally been measured by gravimetric or colorimetric methods in many environmental matrices, the concentrations are too low for gravimetric work, and interferences from other elements (especially phosphorus) are too severe for colorimetry in biologicals. Thermal, epithermal, 14-MeV, and thermal neutron capture prompt y-ray neutron activation methods have all been applied to the problem with widely varying results. Similarly, energy dispersive, wavelength dispersive, and charged-particle X-ray fluorescence have produced divergent data, though the reasons for this are unclear. Inductively coupled plasma atomic emission spectrometry and spark-source mass spectrometry have seem limited application, although the sensitivity of the latter is competitive with that of radiochemical neutron activation for silicon in biological samples. Atomic absorption has been utilized, but with only limited success and sensitivity. High background for all three stable silicon masses from molecular ions probably precludes the trace determination of this element by inductively coupled plasma mass spectrometry. An excellent paper by Jones et al. (2) has described the nuclear basis of silicon determination via epithermal neutron activation analysis. The displacement of a proton by a high-energy neutron on the two most abundant isotopes of
* Author
to whom correspondence should be addressed.
silicon produces radioactive aluminum isotopes with useful half-lives, abundances, and y-ray decay energies. The first of these is the 28Si(n,p) %A1reaction. Unfortunately, 28A1is also produced by capture of slower neutrons by stable 27Al via the (n,y) %AI reaction and by the fast neutron 31P( n p ) 28A1reaction on stable phosphorus. Aluminum is not particularly abundant in most biological materials, but phosphorus frequently occurs in concentrations of 0.1-1 % or more. There are no nuclear reaction interferences with the ?5i (n,p) 29Alreaction, but the overall sensitivity for silicon determination with this reaction is limited by the low 29Sinatural abundance. High-resolution y-ray detectors are required to separate the 1273-keV 29Aly-ray from the 1268-keV single escape line of the more intense 1779-keV y-ray of However, detectors with resolutions of less than 2.0 keV fwhm are generally adequate to achieve base-line separation between these two transitions. Therefore, the preferred method of analysis would be to use the %i (n,p) 29Alreaction, although the silicon concentration may also be determined from the 28Si(n,p) 28A1 reaction if the phosphorus and aluminum concentrations are first determined. A number of methods would be applicable for the determination of phosphorus and aluminum, including neutron activation analysis and colorimetry. Wavelength dispersive X-ray fluorescence (XRF) readily resolves the silicon peak from both the aluminum and the much larger phosphorus peak usually found in biological materials. In fact, there are few interferences expected, other than the general scatter background usually found for particulate samples. The XRF analysis of light elements in plant matrices has been adequately demonstrated elsewhere ( 3 , 4 ) . However, a simplified procedure that requires less handling and equipment and no cellulose binder or diluent is described here.
EXPERIMENTAL SECTION Instrumental Epithermal Neutron Activation. The boron-filtered facility at the Los Alamos Omega West Reactor has been described in detail previously (5). Samples of the various NIST SRMs weighing approximately 0.1 g were packaged in small polyethylene snap-capvials and irradiated in plastic rabbits inside the ‘OBenriched boron-shielded epithermal neutron facility fixed near the core of the reactor. Following a 5-min irradiation, the samples are extracted pneumatically. After decaying for 5 min the samples are counted for 5-10 min at the face of a large intrinsic Ge y-ray detector coupled to a 4096-channel pulse-height analyzer. The y-ray spectrum may be transferred to magnetic tape for off-line data reduction or digitally integrated directly on the cathode ray tube display of the multichannel analyzer. The analyses were standardized against 500-pg quantities of silicon
0003-2700/89/0361-1834$01.50/0Q 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
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Table I. Results of the Analysis of Silicon in Biological NIST Standard Reference Materials (Concentration in pg/g) This Study method 1549 %A1
1566
1567
1570
1569
1568
11000 f 1000 28A1
