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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
Thorium-229 as an Isotopic Tracer for the Radiochemical Determination of Thorium Isotopes in Biological Samples McDonald E. Wrenn, Narayani P. Singh," Shawki A. Ibrahim, and Norman Cohen Institute of Environmental Medicine, New York University Medical Center, 550 First Avenue, New York, New York 10016
Thorium-234 has traditionally been employed as an isotopic tracer to determine t h e extraction efficiency and the total analytical yield for t h e radiochemical determination of "natural" thorium (1-3). Accordingly, a number of methods have been reported for its preparation (4+). Since this nuclide has a relatively short half-life of 24.1 days, i t is necessary t o prepare new batches frequently. Although '%Th tracer may be prepared without extensive difficulty, and with good assurance of isotopic purity, determinations of its chemical and isotopic purity must be made for 234Thseparated from its uranium precursor. Furthermore, since 234This a /3 emitter, whereas most other "natural" thorium nuclides emit a particles, /3 or y measurements are required in addition t o low background a spectrometry. By contrast, in addition to being a n cy emitter, 229Thhas a long radioactive half-life of 7340 years. Accordingly, the tracer ("'Th) and t h e nuclides being determined (2329230~228Th) are deposited in identical fashion together on a single planchet (Le., same thickness, nonuniformity of deposit, etc.), counted simultaneously with the same detector over exactly the same counting period, and accordingly t h e effects of counting times and different relative detector efficiencies cancel out. With 234Thtracer, small changes in distribution can produce profound changes in counting efficiency and subsequent accuracy ( 3 ) ,so that great attention to these details is required to utilize that technique properly. T h e increase in accuracy, simplified counting procedures, a n d freedom from the need t o make decay corrections and prepare fresh tracer periodically, in order of importance, are reasons why '"rh is an improved tracer for most applications. For t h e above reasons, studies have been undertaken using 229Thas a n isotopic tracer for thorium. Thorium-227, another cy-emitting radionuclide, was also considered as a possible isotopic tracer. Since L27This a decay product of 235U,however, *"Th should be present naturally in some biological samples. More importantly, in addition t o its having a relatively short half-life (18.2 days), its a energies (Table I) are too close to the decay products of "8Th (224Ra,cy = 5.68 MeV and "'Bi, a = 6.05 MeV) to be easily resolved. Accordingly 227This not suitable for a thorium tracer.
EXPERIMENTAL Materials. The isotopes ' T ' h and 229Thwere obtained from the Oak Ridge National Laboratory, and 23"Th from Isotope Laboratory Products. Trilaurylamine (TLA) was purchased from Matheson Coleman and Bell Manufacturing Chemicals (MC/B); a 25% solution was prepared in xylene and shaken for 10 min before use with 4 M HNO, in the ratio of 3:l. Other reagents and equipment employed included: methyl red indicator, platinum electrodes and planchets, nickel disks, electrolytic cells (7), and electrolytic analyzer (Sargent Welch). Procedures. Individual a spectra for the isotopes ' T h , "Vh, and 23aThwere obtained from the standard solutions, both by direct transfer and evaporation onto platinum planchets and by electrodeposition. In addition, (Y spectra were obtained for mixtures of these isotopes prepared in aqueous solution and from a pre-spiked 200-g sample of beef liver which had been digested with HN03 followed by a mixture of HNOl and H,SO,. In this latter determination, thorium was extracted into 2 5 7 ~TLA in xylene, and back-extracted with 10 M HC1. converted to sulfate with H2S0, and finally electrodeposited onto a platinum planchet. 0003-2700/78/0350-1712$01 0010
_ -.I.
Table I. Half-Lives and Energies of Thorium Isotopes of Interest thorium isotopes of interest
half-life
Z27Th
18.2 d
'lDTh
1.910 y
229Th
7340 y
230Th
8.0 x 10" y
232Th
1 . 4 1 x 10'" y
234Th
24.1 d
n
energy
6.04 (23%) 5.98 (24%) 5.16 ( 2 1 % j 5 . 7 2 (14%) 5.43 (71%) 5.34 ( 2 8 % ) 5.05 ( 7 % ) 4.97 (10%) 4 . 9 0 (11%) 4.84 (58%) 4.81 (11%) 4.68 (76%) 4.62 (24%) 4.01 ( 1 6 % ) 3.95 (24%) . . D-
-~
The details for this chemical extraction have been previously reported (8).
RESULTS AND DISCUSSION T h e energies of the thorium isotopes ( 2 3 2 , 230, 229, and 228 are listed in Table I. Since the resolution of our silicon surface barrier detector was 45-50 keV a t a source to detector distance of 1 mm, inspection of the a energies shows that all four isotopes ought t o be resolved without any appreciable interference. Both ' T h and ' T h have short-lived daughters which emit a particles, but the energies of all their daughters' a are higher than 5.68 MeV, so that no spectral interference occurs from ingrowth of daughters of either T ' h or 22!'Th. Figure 1, which shows the cy spectrum obtained from a mixture of n s , z B ~ electrodeposited ~h on a platinum planchct,, demonstrates that, as expected: the a energy peaks are well resolved for these three isotopes of thorium. The radiochemical purity of 229Th is not as good as we would desire, since that obtained from Oak Ridge National Lahoratory contained about 8 % , by activity, ' q h , and 0.3% "'Th. T h e cy spectrum of this tracer is shown in Figure 2. In spite of the presence of " T h , the tracer is still useful provided that care is taken in the addition of 229Thto be sure that the adventitiously added 22RThdoes not exceed the 228'1'hin the sample to be measured. The 2 z T hcontamination in the tracer limits the degree to which the error in yield can tie minimized. However, in practice, this limitation has not heen found to be significant when dealing with human or snimal tissues. A number of human tissues, including lung, liver, hone, kidney, lymph nodes, and spleen have been analyzed f o r their isotopic composition of thorium, using 22'3Thas an isotopic tracer. A typical cy spectrum of thorium in the human lung is given in Figure 4. T h e spectra in Figures 1 and 2 were obtained a t 6.7';iC counting efficiency. T h e counting efficiencl, was incrra,.ed to 14070 by appropriately reducing the distance between the diode and the sample. Even at this separation of diode and the source, the resolution does not change appreriahl maximum (Y energy peaks were well separated (Figure further increase in the counting efficiency tc~24% did i i t ~ t cause any resolution problem (Figure 4). '2 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
Y
3280
3340
1713
3460
340CI
Channel NumDer
F w e 3. CY spectra of a mixture of m.m.a% from beef liver. Counting efficiency of the counter = 14%
1 -
I E40
Channel Number
Figure 1. CY spectra of a mixture of 228,229,23qh electrodeposited from an aqueous solution. Counting efficiency of the counter = 6.7% 1400
1
c
i
30 t
1
'"Th
hi,
'"Th 5 43
'"Th
"'Th
468
I
7 -
, 0
J) 7n_, -_,
,
60
160
240
320
, 400
*"Po
_ ,,
.~
480
Chonnel Number
Figure 2. a spectra of thorium-229 tracer. Thorium-229 extracted into trilaurylamine, backextracted with 10 M HCI and electrodeposited on a platinum planchet. Counting time = 985 rnin. Total counts for 22qh = 11 962. Total counts for 228Th= 974. Total counts for 230Th
= 38
A t this time only two disadvantages exist for the use of 229Th. (In the future ' T h might be found as a contaminant around thorium fuel reprocessing plants.) T h e first is contamination of available "'Th with approximately 8% 228Th which results in a small increase in the lower limit of detection for this isotope. Second, there is a recoil problem resulting in contamination of the diode after continuous operation. This contamination, however, is in the energy region of 5.68 to 8.78 MeV and does not interfere with the spectra of the thorium isotopes under consideration (Table I). Furthermore, this recoil contamination problem can be reduced by using the techniques reported by Sill and Olson (9) and also by good technique, namely by removing any source from the chamber while not counting, and avoiding the unnecessary use of overly active sources. Our experience after six months of utilization is that no contamination which would interfer with "natural" thorium measurements is evident. T h e disadvantages include (1) contamination of current stock 229Thwith about 6% zz8Th. ( 2 ) Buildup of decay products of " T h and 2BTh on the diode in the higher energy regions, which do not interfere with the thorium isotopic analyses, nor with the energy regions of 21"Po, 241,243Am, 238,239,240Pu, and 234,235.?38U. T h e daughters could interfere if t h e diode were used for other 01 energies. This is only a difficulty if the diode must be used for other purposes than the detection of the isotopes plus those identified above. (3) In the thorium fuel cycle *'Th is a decay product of 233Uand, in situtations where man-made 22?I'h may occur, the potential for its presence should be recognized. Care should be exercised, therefore, in situations in which the magnitude of this source may be significant. In conclusion, the advantages of ''Th as an isotopic tracer
L-
220
260
300
31C
58C
420
Chonnel Number
Figure 4. a spectra of a human lung containing 1.1 pCi 228Th1.4 pCi 230Th, and 1.3 pCi 23?h and added 0.8151 dpm *?h tracer. Counting efficiency of the counter = 23.75%. Mass of the lung = 330 g. Counting time = 1448 min. Radiochemical recovery = 47.3%
for the radiochemical measurement of other isotopes of thorium are the following. (1)The increase in accuracy associated with simultaneous a spectrometric measurement of the tracer (2:i9Th)and t h e nuclides being determined (252,230~228'Th). (2) T h has a long half-life, 7340 years, and therefore unlike '34Th requires neither repeated preparation and the associated time-consuming processes of purification and standardization, nor decay corrections. (3) 229This an CY emitter as are the naturally-occurring isotopes of thorium which are of interest; accordingly, the u spectrum provides, as part of the measurement, the determination of yield, thereby saving time and minimizing handling and separate measuring techniques.
ACKNOWLEDGMENT We thank Claude Sill for a number of constructive observations which significantly increased the clarity of the paper.
LITERATURE CITED (1) D. R. Percival and D.B. Martin, Anal. Chem., 46, 1742 (1974). (2) C. W. Sill, K. W. Puphal. and F. W. Hindiman. Anal. Chem., 46, 1725 (1974). (3) C. W. Sill, Anal. Chem., 49, 618 (1977). (4) S. S. Berman, L. E. McKinney, and M. E. Bednas, Talanta, 4, 153 (1960). (5) C. W. Sill, Anal. Chem., 36, 675 (1964). (6) C. W. Sill, Anal. Chem., 46, 1426 (1974). (7) U S . Health and Safety Laboratory Manual, 1976. (8) N. P.Singh, S.A . Ibrahim, N. Cohen, and M. E. Wrenn, 23rd Bioassay Meeting Abstract, 1977, p 27. (9) C. W. Sill and D. G. Olson, Anal. Chem , 42, 1596 (1970).
RECEIVED for review March 30,1978. Accepted June 12, 1978.
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ANALYTICAL CHEMISTRY, VOL. 50,
NO. 12, OCTOBER 1978
This research was supported by Contract No. AT(49-34) 0358 Nuclear Regulatory Commission, Contract No. from the U.S. EY-7643-02-2968 from the US.Department of Energy and
is part of a center program supported by Grant No. ES 00260 from the National Institute of Environmental Health Sciences and Grant No. CA 13343 from the National Cancer Institute.
Off-Axis Imaging for Improved Resolution and Spectral Intensities S. G. Salmon and J. A. Holcombe" Department of Chemistry, The University of Texas at Austin, Austin, Texas 78772
Optical systems for image transfer from source to spectrometer often employ on-axis refractive optics, i.e., lenses, because of the apparent ease in alignment and lens availability. Unfortunately, chromatic aberrations are inherent in any lens system, which can present several problems if image fidelity is required at different wavelengths. T h e focal length of a lens is generally specified by the manufacturer for the Na D line a t 589 nm. Using first-order approximations, the Gaussian formula readily shows the functional dependence of focal length on wavelength:
where f x is t h e focal length and nh is the index of refraction a t a specified wavelength, and rl and r2 are the radii of curvature of the respective surfaces. It follows that the focal length of a lens at different wavelengths can be determined by t h e ratio:
T h e implications on focusing are readily apparent. For example, a fused quartz lens with a nominal focal length of 15.0 cm will have a n actual focal length of 14.35 cm for t h e Cu resonance line a t 325 n m and 13.8 cm for the Zn resonance line a t 214 nm. It can be easily demonstrated that a system which is aligned to project an image on the spectrometer slit with unit magnification at 589 n m will result in grossly defocused images of the source at 325 and 214 nm. The lens could be moved to provide focused images on the entrance slit for each new wavelength, but the lateral magnification will not remain constant. For example, at 325 nm, a magnification of either 1.58 or 0.63 is available, while a t 214 nm, a magnification of 2.22 or 0.45 is obtainable by repositioning the lens t o place a focused image on the entrance slit. It should be recognized that the attainment of a focused image with unit magnification at any given wavelength can be achieved only by moving both the source and the lens, a task which is either difficult or impossible in most instances. T h e preceding discussion assumes the use of simple spherical lenses and is not intended to reflect the more expensive and complex optical configurations available which employ compound lens systems or achromats. Another difficulty encountered with refractive optical components is flare or internal reflection. While this can be minimized through use of coated lenses, the process is often expensive. Problems associated with chromatic aberrations and flare can be avoided by using reflective optics. However, this generally requires off-axis illumination which results in the appearance of higher order aberrations. For the application under discussion, the aberrations of astigmatism and coma are the main distortions t h a t need to be considered. Astigmatism, a higher order aberration, should be of particular 0003-2700/78/0350-17 14$0 1 .OO/O
concern and recently has been treated in excellent detail by Goldstein and Walters (I). When a spherical mirror is illuminated off-axis, two astigmatic images are formed. Closest to the mirror there is a line image perpendicular to the plane defined by t h e point object and the mirror normal which is referred to as the tangential image. The image farthest from t h e mirror is a line parallel to the incident plane and perpendicular to the tangential image which is referred to as the sagittal image. Located between the tangential and the sagittal images is the best approximation of a point image which is often referred to as the circle of least confusion. Mathematical equations for determining t h e positions and heights of the astigmatic images have been reported ( I ) . Pairs of mirrors have been used in various configurations to compensate for the off-axis aberrations and improve the image quality for spectrographic applications (I+). Astigmatic imaging through the use of an over-and-under, symmetric arm arrangement can be used t o an advantage when high quality spatial information is sought using photoelectric detection. An "over-and-under" configuration consists of the object point and the mirror normals lying in a vertical plane and results in a tangential image which is a horizontal line and a sagittal image which is a vertical line. A symmetric arm configuration consists of two mirrors being illuminated a t the same off-axis angle and corrects for coma. A diagram of the over-and-under, symmetric arm configuration showing the locations of the astigmatic images is given in Figure 1. T h e optical arrangement discussed in the paper is capable of optimizing both spatial resolution of the source and the detected spectral intensity. In addition, the chromatic aberration associated with refractive optics is eliminated. This is accomplished by employing the tangential and sagittal image planes to define the optimal vertical and horizontal foci, respectively in a symmetric arm, over-and-under, two-mirror system. EXPERIMENTAL Apparatus. A Jarrell-Ash 0.5-m Ebert monochromator equipped with a 1P28 multiplier phototube, a 100-pm entrance slit and 150-pm exit slit was used throughout. The photomultiplier tube output was capacitively filtered and measured with a digital multimeter. Spherical concave mirrors with approximate focal length of 800 mm were used in an over-and-under configuration in the foreoptics. Both a point light source (Oriel Corp.) and a 25-pm slit backlighted with a high intensity lamp were used in the object plane as test sources. Scanning slits of 25+m and 10-pm widths were used. Procedure. The mirrors were arranged in an over-and-under, symmetric arm configuration. The distance between the mirrors was 470 mm and the off-axis angle of illumination, Le., the angle between the incident beam and the mirror normal, was 5" for both mirrors. By placing the point source at the focal point of the first mirror, the reflected light was collimated and fully illuminated the second mirror. Under these conditions a separation of approximately 12 mm between the tangential and sagittal images was obtained. The distance from the second mirror to the monochromator entrance slit was approximately equal to the focal 1978 American Chemical Society
