however, some other important relations which should be considered more closely. In thin-layer chromatography, one obtains more concentrated spots than on paper. This is important for the lowlevel detection limit. The resulting number of grains per unit area to a first approximation is inversely proportional to the spot area. However, the number of grains itself may in addition increase when the area containing the activity is decreased, due to the more effective production of nucleation centers. What is of importance for their establishment is the concentration of photons both in time and space ( c f . the above mentioned sensitive volume element). The exposure of the film can be made with a mirror placed below the chromatogram to increase the photon flux reaching the film. Wilson and Spedding found a slight improvement (8) while Contractor and Shane did not (26). For photographic plates with emulsion on both sides, the one not being employed in the detection should preferably be covered water tight during the development so that the following fixation of both sides can reduce the fog background by 50% (8, 10). Photographic sensitization or image intensification can also be employed ( I O , 16, 19, 20). The optimum amount of scintillator should be used, and this is more for radiocarbon than for tritium. Because of overlap between the absorption and the emission spectra of the scintillators, they exhibit self-absorption when applied in too large amounts. This is a well-known phenomenon for anthracene ( 2 4 ) and has also been shown by Randerath to occur for PPO ( 1 0 ) . One might further suggest that the fluorographic efficiency could depend upon the lifetime of the excited states of the scintillator. As shown in Figure 1, processes 4, 5 , 9, (24) E Schram. 'Organic Scintillator Detector," Elsevier, Amsterdam/ London/New York, 1963
and 11 all compete for the available free electrons. Here 9 and 11 can only occur after the diffusion of one, and two, Ag+ ions, respectively, into the center, and shallow traps are possibly not stable enough to wait for electrons produced by the next scintillation. Thus it might be that process 4 competes more favorably for very short scintillations than for more long-lived ones releasing the same number of photons. The possibility of increasing the efficiency by choosing scintillators with other lifetimes should be further investigated. In connection with the temperature effect, it should be stated that different films show different behaviors and have their own particular optimum temperatures (IO). For most scintillators, the scintillator efficiencies vary only very slightly, and one should not expect the cooling of the pertinent solid scintillators to produce more than about a 5-1070 increase. On the other hand, frozen benzene shows a drastic increase in efficiency upon cooling ( 1 1 ) . The use of a high-efficiency scintillator, although important, is not in itself sufficient. One should also attempt to choose one whose emission spectrum best fits the spectral sensitivity range of the film material to be used (25). When Randerath (10) states that the green fluorescence indicator contained in Merck F-254 thin layers (ZnzSi04:Mn) doesn't increase the sensitivity of fluorography detection on his film material, this might possibly be a matter of poor spectral fit. The emission of ZnzSi04:Mn is peaked a t 530 nm (25). Received for review August 28, 1972. Accepted March 6, 1973. (25) S. Prydz, T. Seim, and J. F. Koren, Physica Norvegica. 4, 247 (1970)
Determination of Zinc and Nickel by Charged Particle Activation Analysis Dale L. Swindle and Emile A. Schweikert' Center for Trace Characterization, Chemistry Department, Texas A&M University, Coilege Station, Texas 77843
Procedures for the sub-ppm determination of zinc and nickel have been developed using charged particle activation analysis. Reactions with protons, deuterons, helium-3, and helium-4 beams on thick Zn and N i targets have been investigated. The most favorable reactions for Zn and N i analysis were the 66Zn(p,n)66Ga and the 58Ni(p.pn)57Nireactions, respectively. A series of samples was analyzed with zinc or nickel contents ranging from 500 to less than 1 ppm, utilizing a post-irradiation chemical separation of the product nuclides. Results from thick target experiments indicate that both Zn and Ni determinations can be made at the ppb level. Activation curves (relative excitation functions) for the reactions used for analysis, as well as possible interfering reactions, are presented and discussed.
Charged particle activation analysis has so far been mostly applied for the determination of light elements (2
1 hour; therefore, only reactions resulting in p r o d u c t isotopes of t1:2 > 1 h o u r were considered. A m o n g t h e reactions surveyed, 66Zn(p,n)G6Ga a p p e a r e d to b e t h e m o s t favorable, featuring: (a) a high yield; (b) the increased p e n e t r a t i o n of p r o t o n s over o t h e r c h a r g e d p a r t i cles which results i n larger s a m p l e v o l u m e s s u b j e c t e d t o analysis; ( c ) a p r o d u c t n u c l i d e w i t h a half-life (9.5 hr) conv e n i e n t for both, sufficient b u i l d - u p i n a 2- to 3-hour irr a d i a t i o n , and t h e t i m e necessary for post-irradiation c h e m ical s e p a r a t i o n ; and (d) a m i n i m u m of interfering reactions yielding 66Ga. Furthermore, s i m p l e , highly selective procedures a r e available for t h e recovery of gallium. T h e m a g n i t u d e of a possible interfering reaction, 70Ge(p,cun)66Ga w a s e v a l u a t e d b y c o m p a r i n g the 66Ga production from t h i c k t a r g e t s of G e and Z n , respectively. Results i n d i c a t e d only a 3% contribution f r o m the reaction o n g e r m a n i u m t o w a r d production of 66Ga, a s s u m i n g equal concentrations of g e r m a n i u m and zinc i n the sample. Other interferences s u c h as t h o s e d u e t o 72Ge(p,a3n)"Ga and 69Ga(p,p3n)66Ga c a n b e totally avoided w i t h p r o t o n i r r a d i a t i o n energies of 20 M e V , since their Q-values a r e -28.32 M e V and -29.93 M e V , respectively. T h e differential a c t i v a t i o n curve for t h e 66Zn(p,n)'jeGa reaction is shown in Figure 1. T h e data were o b t a i n e d b y the s t a c k e d foil t e c h n i q u e and e a c h foil w a s normalized t o t h e foil i r r a d i a t e d at 20.0 M e V . It s h o u l d be n o t e d that Blosser e t al. (19) h a v e m e a s u r e d t h e a b s o l u t e cross section for t h e 66Zn(p,n)66Ga reaction t o b e 585 m b a r n s at 1 2 MeV. H i g h p u r i t y a l u m i n u m and a series of e l e m e n t a l l y (17) Radiochemistry of Nickel, U.S. Rep. NAS-NS 3051. (18) Ch. Engelmann, "Radiochemical Methods of Analysis," Vol. 1 , International Atomic Energy Agency, Vienna, 1964,p 405. (19) M . G.Blosser a n d T. t i H a n d l e y , Phys. Rev.. 100, 1340 (1955).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 12. OCTOBER 1973
Table I. Charged Particle Reactions on Zinc Reaction
y-Ray, MeV
dpsa
Interference
- 5.96 - 1.78 0.86
1.039 0.185 0.283
1 . 2 x 106 7.0 x 104 3.8 x 105
-I- 3.05
0.185 1.039 0.383 1.039
6.6 X 1.2 x 1.1 x 8.9 x
70Ge(p,otn)66Ga 70Ge(p, 61Ni(p,n)61Cu 62Ni ( ~ , 2 n ) ~ ' C u 'OGe(d,m~)~~Ga
0.185
2.4
x 104
+
1.107
6.7 x 103
+
0.383 1.107 0.185 1.039 1.078
7.2 x 4.0 x 2.2 x 4.4 x 8.2 x
Q-value, MeV
6 6 Z n ( p , n )66Ga 6 7 Z n ( p , n )67Ga 64Zn ( p ,C U ) ~ ' C U
+
66Zn ( d , n )67Ga 66Zn ( d , 2 n )66Ga 64Zn (3He,n)66Ge 64Zn(3He,p)66Ga
-
+
+
8.18 1.60 5.35b
-
66Zn (3He,d)67Ga 67Zn(3He,n)69Ge f 68Zn (3He,2n)69Ge 64Zn (cu,2n)66Ge 66Zn ( ~ x n ) ~ ~ G e 6 4 Z n (a , ~ ) ~ ~ G a 6 4 Z n (cu,d)66Ga 6 6 Z n (cu,d)68Ga
0.22 6.10 - 4.10 -18.98 - 7.43 - 3.99 -13.00b - 12.52
+
Q-value, MeV
-10.06 1.17 - 3.02 - 13.62 - 1.05
+
+
lo4 105
104 104
- 4.77 4-13.05 6.46
104 104 104
70Ge(a,m)69Ge
105
65Cu(cu,2n)67Ga 63Cu(cu,n)66Ga
104
69Ga(cu,an)68Ga
9.05
-11.53 -14.12 - 7.52 -10.32
"The yields correspond to the following conditions: irradiations of 1 minute at 1 pA on pure metal targets (natural isotopic composition) measured at to on the nuclide listed. *The yields measured for 66Gaobtained from 64Zn(3He,p)66Ga or 6 4 Z n ( ~ , d ) 6 6 G were a corrected to avoid interfering contributions from 64Zn(3He,n)66Ge
5 66Ga+ 66Zn(3He.3n)66Ge 5 66Gaor64Zn(a,2n)66Ge
66Gaf 66Zn(a,4n)66Ge
66Ga.
-
40
Table I I. Results of Zinc Analysis Sample
Glass SRM 610
Average Glass SRM 612
Average Glass SRM 616
Average
A l u m i n u m MAR2 Average
Proton activation, ppm
463 452 421 444 f 24 72.9 62.9 57.5 64.4 f 8 3.5 3.2 3.1 3.3 f 0.2 0.9 1 .o 1.1 f 0.2
Other methods ppm
z
0 g30 VI
433a
VI VI
0 D: V
20 W
58 & 3 b
> + _1
w
C Y
:
I O -
3.0 f 0.3b
1 C
:0
N.Y.
8
6
Atomic absorption, this iaboratory.
