ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979 (4)
less than from air. I wonder whether the results of Edmonds, Henslee, and Guerra relating to particle size effects indicate true particle size-XRD effects or rather indicate (at least in part) relative filter efficiencies for the different particle sizes coupled with a particle size effect on diffraction peak height. Regardless of the validity or otherwise of such speculation, I feel it is poor analytical chemistry practice for standards to be prepared by water filtration when field samples are collected by airborne filtration.
(5) (6) (7) (8)
(1) J. W. Edmonds, W. W. Henslee, and R . E. Guerra, Anal. Chem., 4 9 , 2196-2203 (1977). (2) G. Heidermanns, Staub-Reinhalt. Luff (fng.e d . ) , 34, 207-21 1 (1974). (3) S. Altree-Williams, Anal. Chem., 4 9 , 429-432 (1977).
RECEIVED for review June 2, 1978. Accepted November 6, 1978.
S i r , Altree-Williams ( I ) has ignored the primary data supporting cy-quartz preferred orientation on sampling membranes and casually applied the relationship between intensity and volume of sample irradiated for a normal (fixed divergence slit) diffractometer to data obtained on a diffractometer fitted with a variable divergence (8-compensating) slit. His failure to use the equations as they apply to the latter instrument compounds his misinterpretation and disguises some interesting facts which subsequently reinforce the original conclusions (2). The intensity of a given ( h h l ) for a flat sample whose surface is a t a parafocusing diffractometer center is as follows ( 3 )
and
d
ohk1
Evaluation of Equation 1 yields
where k h k i includes terms which are constant a t 8)Ikl. limiting values of d , one obtains and
d
-
-
*; I h k l
0; Ihkl
a khkl/p*
a 2 d k h k l COSeC e h k !
in agreement with Altree-Williams ( 1 ) . For an instrument with a variable divergence slit (0compensating slit), however, the relationships d o n o t simply include another term, but change drastically. Since the slit size is chosen to always irradiate the same sample surface area, 1 becomes a constant and Equation 1 leads to Y
For limiting values of d , one obtains 0003-2700~79/0351-0305$01,00/0
-+
0; I ' h f i , =
k'hkid
(8)
indicating that for thin samples a constant volume is irradiated. Therefore, the t h e relative intensitips f o r a t h i n s a m p l e ( d nt o be e x p e r i r n e n t a l l ~e~q u i i , a l ~ n (2). t In light of the magnitude of effect that particle orientation has upon the calibration curves for standards prepared from liquid filtered suspensions, use of calibration curves obtained in this manner n i t h a properly sized standard contribute less error to the analyses than does sample orientation (particle shape). over which the analyst has no control.
ACKNOWLEDGMENT T h e author thanks Ludo K. Frevel for reviewing t h e manuscript. LITERATURE CITED (1) S. Altree-Williams, Anal. Chem., preceding comment in this issue (2) J. W. Edmonds, W. W. Henslee. and R. E. Guerra. Ana/. Chem., 49, 2196-2203 (1977). (3) A. J. C. Wilson, J . Sci. Instrum.. 27. 321-325 (1950)
J. W. E d m o n d s Analytical Laboratories, Bldg. 574 Dow Chemical Co. Midland, Michigan 48640
RECEIVED for review October 6, 1978. Accepted November 6, 1978.
AIDS FOR ANALYTICAL CHEMISTS Determination of the Natural Abundance of Iron-58 by Neutron Activation Analysis P. F. Schmidt" 18 103
Bell Telephone Laboratories, Incorporated, Allentown, Pennsylvania
J. E. Riley, Jr. Bell Telephone Laborafories, Incorporated, Murray Hill, New Jersey
07974
T h e natural abundance of iron-58 is very low and has been reported by various investigators in the range from 0.29 t o 0.33%. With F e 2 0 3samples highly enriched in 5RFebeing available from Oak Ridge National Laboratory (ORNL), a straightforward determination of the natural abundance of 58Fe is possible by co-irradiation of the enriched and natural material, a n d comparison of t h e 59Fephotopeak intensities. T h i s experiment was performed with an Fe,O, sample 82.48% enriched in 5sFe, a n d a natural iron foil, the iron content of which had been determined to he 99.63% by mass spectroscopic examination. T h e relevant data and an outline of t h e calculations are given in t h e Appendix. T h e natural abundance of jsFe was found to he 0.283% h 0.010%, in good agreement with a recent recommendation by Holden ( 1 )based on older mass spectroscopic data ( 2 ) . T h e presently accepted thermal cross section for 58Fe is 1.14 b; in establishing this cross section, the natural abundance of 58Fewas assumed to be 0.33% (3). Since t h e natural abundance enters the calculation of necessity, the thermal cross section of 5sFe should be higher by t h e ratio 0.33 X 1.14 = 1.33 b 0.283 APPENDIX D e s c r i p t i o n of the M e a s u r e m e n t s a n d C a l c u l a t i o n s to D e t e r m i n e t h e N a t u r a l A b u n d a n c e of 58Fe f r o m t h e 0003-2700/79/035 1-0306$01 .OO/O
C o - I r r a d i a t i o n of a 99.63% P u r e N a t u r a l I r o n Foil w i t h a n Fe203S a m p l e 82.48% E n r i c h e d i n 5sFe None of the trace impurities in the iron foil have large resonance integrals which could produce a measurable selfshielding effect. T h e purities of both the iron foil and of t h e Fe203sample were established by mass spectrometry, for the iron sample by t h e Analytical Chemistry Department a t Murray Hill, for the Fe,O, sample by ORNL (no impurities were detected in t h e latter case).
Materials Csed. Fe2*03,2.46 mg, 82.48% enriched in 58Fe, obtained from ORNL. T h e isotopic analysis of this material is given as follows: at.%
' 4Fe "Fe "Fo
"Fe
precision
0.46 15.57
t 0.05 to.10
1.48 82.48
*0.06
io.10
The symbol Fe* is used hereafter to refer to iron of the above isotopic composition.
21.93 mg F e foil, 99.63% p u r e = 21.85 m g p u r e iron Both the F e 2 * 0 3and the Fe foil were co-irradiated for 60 s in the "front row position" of the University of Missouri C 1979 American Chemical Society
