may simply reflect a more complicated sample behavior than that anticipated by the analyst.
simple exothermic phenomenon of pure metal crystallization in this study does not limit the applicability of the conclusions. The instrument simply measures the heat flow rate into or out of a sample, whether the flow of heat is a consequence of reaction or crystallization or any other sample behavior. Thus a demonstration of the instrument's correct response to one exothermic phenomenon is sufficient to establish its correctness for all others. Provided that the instrument is operated within its specified dynamic range and in accordance with recommended sampling techniques, the user can be sure that it will record exothermic and endothermic phenomena with equal fidelity. Apparent errors of substantial magnitude in the measurement of exothermic events must be due either to improper experimental techniques or errors in base-line interpolation, or
LITERATURE CITED (1) E. S. Watson, M. J. O'Neill, J. Justin, and N. Brenner, Anal. Chem., 36, 1233 (1964) (2) E. M. Barrall II and J. F. Johnson, Mol. Cryst. Liq. Cryst., 8, 27 (1969). (3) E. E. Marti, Thermochim. Acta, 5 , 173 (1972). (4) E. M. Barrall 11, M. A. Flandera, and J. A. Logan, Thermochim. Acta, 5, 415 (1973). (5) M. J. O'Neill, Anal. Chem., 36, 1236 (1964). (6)A. A. Duswalr, Thermochim. Acta, 8 , 57 (1974). (7) J. H. Flynn. "Thermal Analysis," Vol. I, H. G. Wiedemann, Ed., Birkhauser, Switzerland, 1972, p 127.
RECEIVEDfor review September 6,1974. Accepted January 2, 1975.
Simultaneous Determination of 35 Elements in Rhodium Samples by Non-Destructive Activation Analysis with 10 MeV Protons J. L. Debrun, J.
N. Barrandon, P. Benaben, and Ch.Rouxel
Groupe d'Application des Reactions Nucleaires a I'Analyse Chimiuue, Centre National de la Recherche Scientifique, Service du Cyclotron, 45045 Orleans-Cedex, France
The performances of activation analysis with 10-MeV protons from a cyclotron were tested in the case of Industrial rhodium samples. ?-Ray spectrometry with a Ge-Li detector was performed directly on the activated samples, after irradiation. Ti, Zn, Cd, Sn, and Sb were present at the partper-million level: the concentration of Ca, Cr, Fe, Cu, Br, and Ru ranged from -10 ppm to -80 ppm, while Ir and,Pt were present at concentrations of several hundreds of parts per million. Upper limits of concentration were calculated for 22 other undetected elements; most of these limits range from several tenths of ppm to several ppm.
Rhodium is very important for modern industry where it is used pure or alloyed. The purity needed varies with the type of industrial application, and must therefore be known. For instance, in the making of thermocouples or wire gauzes for catalysis, the purity must be as high as possible because the alloy Rh/Pt is fragile when impure rhodium is used. As explained in an article by Gijbels and Hoste (1 ), the control of the purity of rhodium is a difficult problem because of the lack of sensitivity or the lack of precision of the methods generally used. It might be added also that many analytical procedures necessitate a liquid sample, and it is well known that rhodium is very difficult to dissolve. It is partially dissolved by a lengthy treatment with hot sulfuric acid or dissolved in a basic medium, and in both cases there are many possibilities of contamination. Several researchers (1, 2 ) have used thermal neutron activation analysis to determine iridium in rhodium. This method, usually very sensitive, is here nondestructive because short-lived radioisotopes only are produced by (n,?) reactions on lo3Rh, the stable isotope of rhodium. But the analysis of rhodium by activation with thermal neutrons is rather limited, for the two following reasons.
First, the cross-section of rhodium for thermal neutrons is high, and this leads to difficult corrections or to tedious standardizations. An alternative is to irradiate small samples, but this decreases the experimental sensitivities. Second, iridium is a major impurity of rhodium and it is well known that this element becomes very radioactive. The presence of lg41r and of lg21r,the last one being longlived, leads to poor detection limits for many elements. In this work, we intend to show that nondestructive analysis by activation with 10-MeV protons, can be used to determine many elements simultaneously, under good conditions of sensitivity and precision. A comparison will be made on the same sample between neutron activation and proton activation.
EXPERIMENTAL Irradiations with Protons. The variable energy cyclotron of the "Service Hospitalier F. Joliot" at Orsay, was used for these experiments. As shown in Figure 1, the irradiations take place in the air. The energy of the protons delivered by the cyclotron is 11 MeV, energy that is reduced to 10 MeV a t the surface of the sample because the particles have to pass through 3 metallic foils. The titanium window separates the vacuum of the cyclotron from the atmosphere. The Havar (alloy containing mainly Fe, Co, Ni, Cr, W) foil is used as a flux monitor by means of the reaction 56Fe(p,n)56Co.The aluminum foil prevents a direct contact between the sample and the monitor. Three different qualities of sintered industrial rhodium were irradiated; the irradiations lasted for 1hour with an intensity of 0.6 to 2 MAaccording to each sample. The dimensions of these samples were 12.5 X 12.5 mm, the thickness was equal to 1 mm. A surface of 0.5 cm2 only was irradiated, and this corresponds to an actual sample of -130 mg, since the range of 10-MeV protons is equal to 261.5 mg/cm2. This range corresponds to 210 microns and, consequently, the samples could have been thinner, e.g. 300 microns. Irradiations with T h e r m a l Neutrons. Rhodium number 2 was irradiated with thermal neutrons in order to make a comparison between neutron activation and proton activation on the same sample. The errors due to the absorption of neutrons were made ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
637
c ollim ator
I / Rh
+
p -“’Pd
-Ep =11MeV, -
n ,f
vacuum
I-.
AI
sample
havar
Flgure 1. Scheme representing the irradiation of a sample
Rh + p+’‘’Rh
0
I
I
I
5
10
15
Ep
MeV
*
Figure 3. Activation cross-section for the y-ray at 497 KeV of Io3Pd Eth = threshold for the (p,n) reaction
Table I. Experimental Limits of Detection for Rhodium 1 and Rhodium 2a
l
Elements
Rhodium
KO 1
Rhodium K O 2
e
Eth, Eth, I
1
5
1
I
/*
1
10
I ,
15
Ep
MeV Figure 2. Activation cross-section for the y-ray at 475 KeV of lo2Rh Here, UA = u ( % y) * ( % Isotope), where u = usual cross-section, y % = abundance of the y-ray at 475 KeV. % Isotope = abundance of lo3Rh (100%). Ethr = threshold for the (p.d) reaction and Ethp = threshold for the (p.pn) reaction
minimal by irradiating 2.5 mg of rhodium only. The first irradiation in the graphite reflector of the heavy water reactor El3 at Saclay, confirmed that the iridium content was high, and, for this reason, the others were limited t o 30 minutes with a flux of 3.10” n/cm2/sec. Measurement of Radioactivity. Direct gamma-ray spectrometry was performed on all samples, after a very light etching with concentrated sulfuric acid. The characteristics of the Ge-Li detector were as follows, relative to the 1332,5-KeV y-ray of 6oCo: resolution, 2.7 KeV; peak-to-Compton ratio, 34; efficiency, 22%; compared to an INa detector under the usual standard conditions. This detector was coupled to a 4000 channel analyzer; the data were stored on a magnetic tape. Because of the existence of an “irradiated side” and of a “nonirradiated side,” due to the limited range of the charged particles in matter, some precautions are to be taken during the measurement of the radioactivity ( 3 ) . Calculations. The spectra stored on magnetic tape were automatically processed a t the Computer Center of the C.N.R.S. at Orsay. The computer code used is a version of SAMPO ( 4 ) modified in our laboratory. At the present time, only the areas and the energies of the photoelectric peaks are automatically obtained. Automatic identification of the radioisotopes and calculation of the concentrations will soon be possible. Concentrations are calculated according to the method described by Hahn and Ricci ( 5 ) : Xppm
= ;‘B 4 x st
638
Rs, x
106
Rs
ANALYTICAL CHEMISTRY, VOL. 41, NO. 4 , APRiL 1975
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