Rapid and Nondestructive Analysis of Sulfur and Calcium by Radioactivation and Photoneutron Counting SAADIA AMlEL and J.
0.JULIANO'
Nuclear Chemistry Department, Soreq Research Establishment, Atomic Energy Commission, Yavne, lsrael
b A method is described for the activation analysis of sulfur and calcium by the assay of 5.1-minute sulfur-37 and 8.8-minute calcium-49 using photoneutron counting. Both sulfur-37 and calcium-49, formed by neutron capture in sulfur and calcium, respectively, decay with the emission of high energy (>3 m.e.v.) gamma rays which cause emission of photoneutrons from a D20 reservoir incorporated in a neutron counting assembly. The photoneutron activity is proportional to the content of the sulfur-37 or calcium-49 in the sample, the activity being -20 c.p.m. per mg. of sulfur and - 1 100 c.p.m. per mg. of calcium, when the sample is irradiated to saturation a t a flux of 1 013neutrons/cm.2-second. Sensitivities of -0.5 mg. for sulfur and -10 pg, for calcium were obtained. Sulfur and calcium in samples containing various proportions of these elements were analyzed with satisfactory precision. The use of the method for the rapid determination of sulfur and calcium in complex matrices, without any chemical processing and by a simple and versatile apparatus, is discussed.
R
of sulfur is generally based on the formation of either S35 or P32 by irradiation with reactor neutrons (4,6,10). These nuclides are pure 8-ray emitters and their assay is generally possible only after radiochemical processing of the irradiated sample and preparation of the purified fraction in a form suitable for P-ray counting. Only in limited cases, for example in organic substances, where the main matrix (C, H, 0, and K) does not become activated, can P3* be determined without separation, provided that the radioactivation of other components is a t a very low level. With S35 a direct method cannot be used a t all because of the low energy of its beta rays. For these reasons the methods for analysis of sulfur have been described as unsatisfactory (6). The same applies to the radioactivation analysis of calcium. The main ADIOACTIVATION ANALYSIS
1 Visiting scientist from the Philippine Atomic Research Center, Quezon City, Philippineb.
Table
I.
Reactor Induced Reactions and Radioactivities in Sulfur and Calcium
Abundance of target nuclide, Reaction Sulfur Syn,y)Sa5
s3y 12,y)S37
S32(n,p)P32 S33(n,p)P33 S34(n,p)P34 S34(n,a)Si31
Calcium Ca44(n,y)Ca45 Ca4B(n, y)Ca4' Ca48(n,2n)Ca47 Ca48(n,~)Ca49 Ca44(n,p)K44
yo ( I S ) 4.22 0.0136 95.0
Cross section mb.5
Properties of product ( I S ) Ti/* Radiations, m.e.v. 86.7-day 5.1-min. 14.5-day 25-day 12.4-sec.
p- 0.167; no
4.22
260 140 60 (65)b (0.63)b
4.22
2.5b
2.64-hour
p- 1 . 4 8 ;
165-day 4,7-day
8 - 0.254; no y 0 - 2 . 0 , 0.69, 7 0 . 5 , 0.81, 1.30 8 - 2.0. 1 . 0 . 0 . 5 : y 3.1,4 0,4 7 8 - 4 91, 2 71, 2 44,
0.76
2.06 0,0033 0.185 0.185
2.06
700
300 (0.25)b 1100
)
(0.08)b
8.8-min.
1 /
22 3-min.
y
p - 4 . 3 , 1 . 6 ; y 3.09 p- 1.71; no y p-0.238; no y 8 - 5 . 1 , 3.2, 1 . 0 ; y 2.1, 4.0
{
y
1.26
0 7
i : i 3 , 1.74, 2.07, 2.48, 3 . 6 Ca40(n,~)Ar37 96.97 (6.7)b 34.3-day K capture; no Ca4'(n,a)Ar4I 2.06 (0.035)b 1.83-hour p - 2 . 5 , 1 . 2 ; 1 . 3 a When not inarked cross sections are given for thermal neutrons. Effective cross sections with fission spectrum neutrons according to ref. (11). Yalues in parentheses are only estimated.
radioactive nuclide produced upon neutron irradiation is Ca45which is also a pure soft @-ray emitter, and its assay is possible only after chemical separation (IO). Table I lists the reactions relevant to activation analysis and the properties of the radioactivities induced by reactor neutrons in sulfur and calcium. The table clearly shows up the difficulties which might arise in analyzing complex samples for their sulfur and calcium content. A major source of interference in the analysis of sulfur via P32 is the formation of P32 from chlorine by the reaction Cl35 (n, C Y ) Pand ~ ~ from phosphorus by Psi (n,y)P32(IO). While the contribution from phosphorus, which has a large neutron capture cross section (200 nib.) may be partly discriminated against by the use of a thermal neutron absorber ( I d ) , that arising from chlorine cannot be eliminated. Chlorine must therefore be determined independently and the corresponding interference subtracted. When S35 is used for the analysis of Sulfur, chlorine might inter(n, p)S35. fere by the reaction Duplicate irradiations of sample and
standards, with and without thermal neutron absorbers-e.g., cadmium-are necessary for the evaluation and subtraction of the chlorine contribution, and the possibility of introducing serious errors is considerable. Other problems are the prolonged irradiations needed (>> 1 hour, because of the long half lives), radiochemical separation, and the long counting period necessary for the assay of S35and P32. Other nuclides, except S37which will be discussed below in detail, are not suitable for the analysis of sulfur, because of low activation and absence of gamma rays (P33), very short half life (Pa4),or excessive interference (Si31-produced in a larger proportion from silicon). The use of S37for the analysis of sulfur appears promising a t first sight since the S37gamma ray has an outstandingly high energy, 3.1 m.e.v., which should make it easy to resolve it from lower energy gamma rays by using a y-ray spectrometer. However, this method is handicapned by relatively poor sensitivity due to the very lon isotopic abundance of S36and its small neutron capture cross section. Analysis by S3' has been reported only in cases VOL. 37,
NO. 3, MARCH 1965
a
343
where the sulfur content is high and other 7-ray emitters are absent (7). hlthough other activities, if present, usually emit low energy gamma rays which can be discriminated against electronically, the 3.1-m.e.v. peak of S37 can only be resolved when the other activities are not more than about 10 times more intense than Sn; otherwise, sum peaks may mask the 3.1-m.e.v. peak, or the high background might overload the detector and paralyze it for a considerable fraction of the counting time. Another drawback in using S37 is the likelihood of strong interference from chlorine by the reaction C1"-
Table II.
Production and Properties of s37
Half life: 5.1 min. -,-ray energy: 3.09 m.e.v.; abundance: 90% of disintegrations Abundance Cross of target sections,
Reactions nuclide, % 0,0136 S"(n, y )S37 C137(n,p)S37 24.47 Ar40(n,a)F7
99.60
mb.
140 0. 245 (0.56)'
a Effective cross section with fission spectrum neutrons (11). Value in parenthesis is estimated (11).
(nJ
The difficulties encountered in the activation analysis of calcium can be inferred from Table I. Although production of Ca45 by a neutron flux takes place exclusively from calcium, cannot be used conveniently for the analysis (6) because of its unfavorable nuclear properties, via. the absence of gamma rays, decay by low energy beta rays, long half life and low production rate. The other neutron-produced radioactive isotopes of calcium are Ca41, Ca47, and Ca49. Ca4I is practically useless for activation analysis, being of -105-year half life and decaying only by K-capture. Its production is thus extremely low and its detection difficult. Ca47 has favorable nuclear properties but the extremely low isotopic abundance of Ca46 makes it practically impossible to detect it. Ca49 decays with the emission of high energy gamma rays which should make it easy to detect and identify it with the aid of a discriminator or a pulse height analyzer; however, it is produced from Ca@ which has a low isotopic abundance, so that in complex samples where calcium is a minor constituent the Ca49 peak may be masked by sum peaks or there may be overloading and partial paralysis of the detectors. A possibility of overcoming interferences in the assay of S3' and Ca49is to use radiochemical separation. However, this is difficult because of the short lives of these nuclides. A !recent report (6) quotes a separation taking 22 minutes prior to the assay of Ca49or its daughter 57.5-minute SC'~. The circumstances discussed above suggest that most of the problems can be solved by using a y-ray detector insensitive to energies below the range in question, viz. -3 m.e.v. One possibility is to use a pair-spectrometer, which responds to gamma rays above the threshold for pair production, via. 1.02 m.e.v. However, because of the possible presence of many nuclides emitting gamma rays with energies above 1.02 m.e.v., this detector will not have a high degree of specificity for complex samples, and additional assignment by half life might be cumber344
ANALYTICAL CHEMISTRY
some. hnother possibility is to count the photoneutron emission caused by the gamma rays in beryllium or deuterium, the thresholds for this process being 1.67 and 2.23 m.e.v., respectively. Here there are far fewer sources of interference, especially in the case of deuterium, since the number of radionuclides which can be produced by neutron irradiation and which emit gamma rays of energies above 2.23 m.e.v. is relatively small. Further, only nuclides with half lives in the range 10 times shorter or 10 times longer than those of S37and Ca49need be of concern, which eliminates most nuclides except a few fission products. Thus photoneutron counting is highly specific, and in conjunction with decay rate measurements it can be used for the nondestructive activation analysis of sulfur and calcium. The present work is based on the counting of photoneutrons in deuterium. This method was previously applied to the analysis of sodium (2) and is presently also being developed for analysis of manganese (3). Detector. T h e most suitable apparatus for detecting the high energy gamma rays is a neutron counting assembly embedded in a heavy water matrix in which the photoneutron emission takes place and which also serves to moderate the neutrons for efficient detection. Sample. T h e only condition required of the sample is that i t should not be too bulky and should not contain large amounts of neutron absorbing materials, so as to minimize systematic errors due to self-shielding. T h e presence of neutron emitters in the sample, or their formation during the irradiation, would give results which are too high and attention should be given to eliminating them before the analysis if possible, although both types of interferences can be checked experimentally and corrected for. Irradiation Conditions. During the neutron irradiation S37 can also be formed from chlorine and argon, ac-
cording to the reactions given in Table 11. Argon, a gas and a rare element, should not cause any interference in practice. Chlorine, on the other hand, is very abundant, and a well thermalized neutron flux is important when small amounts of sulfur are assayed in a sample with a large amount of chlorine. Alternatively, thermal neutron absorbers can be used in duplicate irradiations to obtain the net thermal neutron effect. I n the analysis of calcium by Ca49the problem of a foreign source of this nuclide does not arise. The duration of the irradiation should be adjusted to the approximate sulfur or calcium content. I n general, irradiations longer than 2 half lives-i.e., 10 minutes for sulfur and 17 minutes for calcium-will not add significantly to the sensitivity and accuracy of the analysis, and will only increase the probability of interferences from longlived photoneutron sources like 2.58hour MnH or 15-hour Na24. Counting Conditions. T h e yield of photoneutrons is highest when the sample is placed a t the center of a large D 2 0 tank. Efficient neutron detection is then possible by surrounding the sample with a ring of thermal neutron detectors immersed in the heavy water. When the only photoneutron sources are F7or Ca49, an integral count over one or two half lives (depending on the background level) will give the highest sensitivity and precision. If more than one component is present the decay must be followed long enough to permit the subtraction of the other components, which are most probably long lived-e.g., MnMand ?u'aZ4. EXPERIMENTAL
Samples were weighed, sealed in small (-2 cc.) polyethylene vials, and positioned in a reproducible geometry in the standard pneumatic tube irradiation containers ("rabbits"). The samples were irradiated for 10 minutes in the pneumatic tube facility of the Israel Research Reactor IRR-1 at a thermal neutron flux of 7.4 X 10l2 neutrons/ cm.2-second. The fast fission flux (fission spectrum neutrons) was 2.4 X 10l2 Counting neutrons/cm.2 - second. started about 1 minute after the end of irradiation. The irradiated sample was placed at the center of a 1-liter polyethylene bottle containing heavy water (>98y0 D20) and surrounded by a ring of six 1310Fa proportional counters. This assembly was embedded in a 40- X 40x 40-cm. paraffin block shielded externally by a cadmium screen, a layer of lead, and borated paraffin bricks to reduce the background. The pulses from the counters were amplified and fed to a recording electronic scaler. The ?-ray effect in the 13F8 counters, resulting in low energy pulses, was discriminated against by introducing
Table 111.
Production and Properties of C a 4 9
Half life: 8.8 minutes ?-rays : Abundance Energy, ( % of m.e.v. disintegrations) 89 10 0.6
3.10 4.05 4.68
Abundance Reaction of parent Cadg(n,~)Ca4~ Ca480 . 185Y0
Cross section 1100 mb.
a n electronic bias. The background of the system during the experiments was about 10 c.p.m. A more detailed description of the apparatus and other pertinent data on the operational and checking procedure were previously published (2). RESULTS
The photoneutron activity of the irradiated samples was followed until it had decayed completely. The half lives observed were in agreement with the published values, viz. 5.1 and 8.8 minutes for S" and Ca49, respectively (Tables I1 and 111). The activity of the 57.5-minute Sc4Qdaughter of Ca49 was not observed even when large activities of Cad9 were present in the sample. This was expected since S C ' ~ does not emit photons of energies greater than 2.23 m.e.v. A series of calibration measurements for sulfur analysis was made by irradiating varying amounts of elemental sulfur, in the range 15 to 115 mg. The photoneutron activity was found to be proportional to the sulfur content. The activity was 11.20 f 0.32 c.p.m. per mg. of sulfur irradiated for 10 minutes at a neutron flux of 7.4 X 10l2 neutrons/cm.Z-second (Table IV). The corresponding saturation activity a t 1013 neutrons/cm.2-second was 19.9 c.p.m. Similarly, different amounts of C a C 0 3 and CaHPOa containing 0.3 to 30 mg. of calcium were irradiated and counted. The photoneutron activity was proportional to the calcium content. The activity was 449.64 f 10.4 c.p.m. per mg. of calcium at a flux of 7.4 X 1012 neutrons/cm.2-second (Table V). The corresponding value at saturation, using a flux of 1013 neutrons/cm.2second was 1124.7 c.p.m I n no case was self shielding or flux depression observed in the samples irradiated. The detection efficiency of the counting, system-Le., the number of disintegrations of 53' or Cad9necessary to produce a photoneutron pulse was measured using Sa24 as an internal standard. The latter has a detection efficiency of one neutron count for every 2.447 X lo4 disintegrations ( 2 ) .
Table IV.
Calibration of Photoneutron Count Rate for Sulfur Analysis
Wt. of S (mg.)
Observed neutron counta
Neutron count per mg. S
14.650 36.225 52.050 65,885 81.275 102.095 105.000 114.600
469 1120 1701 2008 2500 3241 3463 3657
32.01 30.92 32.68 30.48 30,76 31.74 32.98 31.91
Count rate per mg. S a t end of 10-min. irradiation (c.p.m.)
11.33 10.93 11.55 10.76 10.86 11.22 11.65 11.27 Mean: 11.20 Std. dev.: f 0 . 3 2 (f2.57?0) a A (j-minute integral count starting 1 minute after a 10-minute irradiation a t a flux of 7.4 x 1012 nlcm.2-sec.
Known amounts of sulfur or calcium were mixed with known amounts of sodium and irradiated in the pneumatic tube. The photoneutron counts due to NaZ4and Ca49or S37were obtained from the decay curve. Csing the known detection efficiency of Na24 the amount of Ca49 or S3' was calculated. The photoneutron counting efficiencies of Sn and Ca4gwere 1.07 X lo4and 1.66 X lo4 disintegrations per photoneutron counted, respectively (Table VI). These values were in general agreement with the estimated photoneutron yield based on the D(y, n)H excitation function (8). Samples of various amounts of S, Ca, and their mixtures were prepared, irradiated for 10 minutes, and counted. The results of the sulfur and calcium determinations are given in Tables VII, VIII, and IX. Photoneutron activities from samples containing S, Ca, Na, Mn, or natural uranium are shown in Figure 1. This method of analysis was also applied to soils, plants, rubber, and mineral oils and yielded satisfactory results. As a n example, results of the sulfur analysis of mineral oils are given in Table X. I n this particular case a large weight of oil was used, about 9 grams, to compensate for the low sulfur content of the oil. Internal standards
Table VI.
Detection Efficiency of
Sample composition, mg. Na Ca S 2.965 17.710 ... 2.705 2.821 2.738 2.184 0,970 0.630 0.440
74.525 80.620 90.534
... ...
...
145:660 156.685 160.002 158.325
...
... ...
Table V. Calibration of Photoneutron Count Rate for Calcium Analysis
Wt. of Ca (mg.) CaC03 0.602 1.120 7,250 10.171
Observed count rate per mg. Ca at end of 10-min. irradiation (c.p.m.)
268 504 3314 4651
445.18 450.00 457.10 457.28
CaHPOl 0.290 0.651 1.290 4.809 9.574 14.528 21.609 30.353
122 292 593 2175 4362 6550 9528 13615
450.52 448.54 459.69 452 28 455,60 450 85 440.92 448.55 Mean : 449.644 Std. dev.: 10.4(2.30/0)
("spikes") of sulfur were used t o calibrate the measurements. Measurements of the interference of the chlorine and argon are given in Table XI. The experimental values do not agree with the calculated ones, indicating the need for re-evaluation of the corresponding cross sections (9).
S3' and
C a 4 9by Photoneutron Counting
Neutron counts per min. per mg. at end of 10-min. irradiation Na Ca S 892 880 849 860 859 845 870 851
Observed neutron count rate a t end of 10-min. irradiation (c.p.m.)
210 219 226 221
...
.. .. , ... , . . ,
Ratio disintegrations per neutron count" Ca 5 1 . 7 3 x 104 1.660 X lo4 1.609 X lo4 1.645 X lo4
4.96 5.44 5.80 5.75 Mean 1.662 ( f O . 052) X lo4; ... . . . .. .
...
1.17d x 1.074 X 1.007 X ... 1.016 X 1.069 ( f 0 .078) x ... ... ...
104 104 104 104 lo4
a Absolute disintegration rate of S3' and Ca4gcalcd. using value 2.447 X 10' disintegration/photoneutron count for NaZ4(2).
VOL. 37, NO. 3, MARCH 1965
a
345
DISCUSSION
Precision and Sensitivity. It has already been mentioned (1-3) that the
main source of error in nondestructive activation analysis using photoneutron or delayed neutron counting is irradiated flux variations. The reactor flux itself may vary, and there may be differences between the irradiation fluxes through the sample and the standard with which it is compared. Errors due to weighing of samples and instability of the counting assembly are minor when care is paid to precision. Errors due to statistics depend on the amount of the
Table
VII.
Some Results of Determinations
Sulfur content, mg. Present Founda 351.35 246.28 201- 38 __ 150 1 1 101 26 50 32 25.61 10.87 5.23 2.66 1.22 0.54
-
Sulfur
Error (mg.) (70)
+4.39 -I-1.25 $3.47 +1.41 - 1 6.5 -0 82 - 3 80 -2 53 - 3 46 - 3 42 +3 09 +6 14 -0.31 -1.21 +0.56 +5.15 -0.06 -1.15 -0.17 -6.39 +0.12 +9.84 + 0 . 1 4 +25.93 Samples irradiated for 10 minutes and counted 1 minute after irradiation for 5 minutes each in a single determination. A value of 31.7 counts/mg. obtained from calibration measurement (of 3 standards)
355.74 294.75 199 73 146 31 97 80 53 41 25.30 11.43 5.17 2.49 1.34 0.68
was used to calculate the results.
Table
VIII.
Some Results of Calcium Determinations
Calcium content, mg. Present Found4 50.352 28.633 15.282 5.215 1.200 0,832 0.321 0.125 0,082
50.592 28.979 15.229 5.257 1.217 0,814 0.288 0.151 0,070
Error (mg.) (70) +0.267 +0.346 -0.053 +0.042 +0.017 -0,018 -0.033 +0.026 -0.012
+0.53 +1.21 -0.35 +0.81 +1.42 -2.16 -1.03 +2.08 -14.63
a Conditions same as Table VI1 except a value of 1396 counts/mg. obtained and
used to calculate results.
Table IX.
Sample composition, mg. S Ca 100.343 50.888 24.955 9.853 0.999 0 998 0,999
1,034 0.897 0.979 0.999 0.995 5.385 10.928
element sought that is present in the sample, on the intensity of the irradiation flux, and on the background of the counting assembly. The precision obtained under present conditions was -2.5% for both sulfur and calcium. This can be regarded as satisfactory for activation analysis, for which a precision of 2 to 5% is expected. The precisions reported for Na ( 2 ) and M n (3) using the same method were -2% and -I%, respectively, but these cases are notable for their high photoneutron yield and long half lives, which result in improved statistics. The detectabilities-Le., the standard deviation when the sample count equals 660 fig. for the background-were sulfur and 16.5 pg. for calcium, when a single 10-minute irradiation was made at a flux of 1013 neutrons/cm.*-second and the counting was done at a background of 10 c.p.m. It is more practical to state a sensitivity level when the relative standard deviation is 10%. The corresponding values are 7.7 mg. for sulfur and 190 fig. for calcium when the irradiation time is 10 minutes. These figures refer to the statistical counting error using a single photoneutron count taken for 1 minute. By integrating the count over an extended period, the precision and sensitivity increase correspondingly. I n practice an increase of a factor of 10 is attainable for both calcium and sulfur. Further improvements are possible using higher neutron fluxes, or repeating the irradiation and counting several times. The use of a larger D20 container will increase the photoneutron yield, and a more efficient neutron detector array can be used; both will result in better sensitivity and precision. Accuracy. T h e tests of accuracy shown in Tables VI1 and VI11 indicate a general agreement with the precision of the method (given in Tables I V a n d V). Systematic errors due to self-shielding were not studied, b u t such effects can be dealt with by the use of internal standards. as pointed out previously (2). Interferences. Spurious neutron counts might arise from several different origins. Photoneutron emission
Analyses of Mixtures of Sulfur and Calcium
S/Ca ratio 97.044 56.731 25.490 9.863 1.004 0.1853 0,09142
Determination by photoneutron counting S Ca (mg.) % error (mg.) % error 95.828 55.061 21.636
-4.5 +8.2 -13.3
...
... ...
...
Resolution of sulfur from decay curve was not possible.
346
ANALYTICAL CHEMISTRY
1.164 0.982 0.893 1.065 0.943 5.207 11.267
+12.6 +9.5 -8.8
+6.6 -5.2 -3.3 +3.1
4- \
\
\
30
X)WS
10
“0
6
12 I8 24 M 36 42 48 54 60 66 72 78 84 MiUtes alter Irradidion
Figure 1 . Comparison of photoneutron count rates from different sources after a 10-minute irradiation
caused by high energy 7-ray sources like MnM, ?rTa24,and fission products, may be subtracted by following the decay and stripping ‘the decay curve. Figure 1 gives an idea of the comparative interferences of this kind. Beryllium, if present in substantial amounts, might interfere by undergoing photodisintegration with gamma rays of 2 1 . 6 7 m.e.v. emitted by active sample components. The interference of this source as well as (CY, n) reactions and spontaneous fission can be deter-
Table X.
Determination of Sulfur in Mineral Oil
Wt. of sample (g.) 8.850495 8.492900 8.540710 8.440600 8.557755 9.142090 8.275275 8.516755 8.879660 8,484135
Sulfur content (%) Activation Conven- analysis and tional photoneutron analysis0 counting6 0.067 0.063 0.043 0,200 0.19 0.23 0.19 0.21 0.22 0,19
0,052, 0,057 0.047, 0 , 0 4 9 0.044, 0,042 0,190; 0.203 0.211, 0.225 0.203, 0.205 0.202, 0.205 0.214, 0.216 0.231, 0.223 0.212, 0.205 0.218, 0.211
a Analyses performed by Chemical Testing Laboratory, Technion Research and Development Foundation Ltd., using I.P. 61/61 procedure of the Institute of Petroleum, in which sulfur is converted by combustion in a bomb t o SOa-2 and weighed as BaS04. The two values given represent independent detns. Each sample was irradiated for 10 minutes at a neutron flux of -7.4 X 1012 n/cm.2-sec. and counted for 5 minutes. A calibration value of 28.0 counts/mg. sulfur was used to calculate the results. A relative error of 10% was obtained in each detn.
mined from their different rates of decay and subtracted, or, more efficiently, the same sample can be irradiated again and counted in the neutron counting assembly in which the DzO has been replaced by natural water. Delayed neutron emission, due to its short half life (< 1 minute) is of no practical concern, since a -5-minute delay will result in the complete decay of this source. If a delay in counting cannot be permitted the delayed neutron effect can be subtracted by counting with natural water, as mentioned above. The extent of mutual interference of calcium and sulfur in samples containing both elements is seen in Table IX. Xnother possible source of interference in the analysis of sulfur is chlorine, and to a much lesser degree, argon. Table XI shows the extent of formation of S3’ from these sources. Since chlorine might be more abundant than sulfur in many types of samples, its effect would mask that of sulfur. The use of a well thermalized neutron flux is then important. Xnother possibility is to irradiate the sample behind a thermal neutron absorber-e.g., cadmium-and the difference will represent the net thermal effect due to neutron capture in sulfur. A minor source of inter-
ference is the high energy gamma ray of 37.3-minute C138,of -10-2~oabundance, which may give rise to photoneutrons (9), but because of the large difference in half lives, it can be corrected for by following the decay. The interference of argon is small as can be seen from the fact that 10 cc. of air present in the sample contribute only -1 c.p.m., when irradiated for 10 minutes at a flux of lOI3 neutrons/cm. l-second. These problems do not exist for Ca49 which is formed exclusively from calcium. LITERATURE CITED
Table XI. Formation of S3’from Different Sources [Using Mixed Neutron Flux of 7.4 X 10l2Thermal and 2.4 X 1 O I 2 Fission Spectrum Neutrons per c m 2 per sec.]
Photoneutrons/min./mg. parent after 10-min. irradiation Reaction Calcd. Exptl. SYn,r)S37 11.1 11.2 f 0 . 3 ClYn,p)S37 10.2‘ 3.4 f0.3 6.0 f 0.5 Arao(n,a)S37 84.00 Based on estimated cross sections quoted in Table I1 according to (11).
(1) Amiel, S., ANAL. CHEM. 34, 1683
(1962). (2) Amiel, S., Peisach, M., Ibid., 35, 1072 (1963). (3) Amiel, S., Stuhl, Z., Zsrael At. Energy Comm. Rept. IA-932 (1964). (4) Bouten, P., Hoste, J., Anal. Chim. Acta 27, 315 (1962). (5) Bowen, H. J. hl., Cawse, P. A., U . K .
At. Energy Authority Rept. AERER4309 (1963). (6)‘Bowen, H. J. hl., Gibbons, D.,
Radioactivation Analysis,’ ’ Oxford University Press, London, 1963. (7) Chingala, B., Cinffolotti, L., Malvano, R., Energia Xucleare 10 (7) 389 (1963). (8) Handbuch de Physik, XLII, p. 325, Springer Verlag, Berlin, 1957. (9) Juliano, J. O., Amiel, S., Atomic Energy Commission, Yavne, Israel, unpublished data (1964).
(10) Koch, R. C., “Activation Analysis Handbook,” Academic Press, New York, 1960. (11) Roy, J. C., Hawton, J. J., At. Energy of
Canada Rept. AECL-1181
(1960). (12) Simpson, H., Gibbons, D., “Radioisotopes in the Physical Sciences and Industry, ’ I International Atomic Energy Agency, Vol. 11, p. 269, Vienna, 1960. (13) Way,, K., et al. “Nuclear Data Sheets, National Academy of Sciences, National Research Council, Washington, D. C. RECEIVEDfor review June 18, 1964. Accepted December 15, 1964. One of us (J. 0. J.) thanks the Israel Atomic Energy Commission for a fellowship.
Application of Galvanic Cell to Measurement of Oxygen-Consuming Enzyme Systems HARRY LIPNER, L. R. WITHERSPOON,l and ANNE WAHLBORG Departments o f 6iological Science and Chemistry, Florida State University, Tallahassee, Fla.
b The application of a galvanic cell oxygen analyzer to the study of oxidative enzyme systems is discussed. Comparisons are made of results obtained on three enzyme-substrate systems using the Warburg manometric technique, spectrophotometry, and the galvanic cell with a bucking potential circuit. Agreement of kinetic data, high sensitivity, and versatility suggest that the galvanic cell is applicable in the analysis of oxidative systems. Data are presented which demonstrate that the galvanic cell can b e used to measure changes in oxidative systems which consume less oxygen than can b e measured b y conventional manometry or which involve no change in the ultraviolet or visible spectrum of reactants or products.
T
most commonly used in the study of oxidative reactions is the Warburg manometric technique (8). The spectrophotometer has also HE METHOD
been used in studies of oxidative reactions when the solutions are translucent and there are changes in either the visible or ultraviolet spectra (6). Polarographic techniques in which the reduction of oxygen is measured when proper potentials are applied are also used to measure oxidative reactions. The dropping mercury electrode (2) is the system usually associated with the term polarogaphy, however, the oxygen electrode (1) and the galvanic cell oxygen analyzer (3-5) are variants of the polarographic technique. Mancy (4) described a galvanic cell which utilized a membrane enclosed chamber containing electrodes and supporting electrolyte. Lipner ( 3 ) described a bucking potential circuit which resulted in a marked increase in sensitivity when used with the Mancy galvanic cell. The purpose of this study was to investigate the applicability of the galvanic cell to oxidative enzyme systems and to establish the limits of
accuracy, sensitivity, and the reproducibility of this apparatus. (The galvanic cells were constructed by E. J. Highsmith and the bucking potential circuits by F. Jordan of the Department of Biological Science, Florida State University, Tallahassee, Fla.) Oxidative systems chosen for study were ones which allowed direct comparison of data obtained with the galvanic cell, Warburg, and spectrophotometer. The tyrosinase oxidation of tyrosine to dopachrome was applicable for study with all three instruments. The tyrosinase oxidation of 3,4-dihydroxyphenylalanine (DOPA) to dopachrome was too rapid for accurate manometry and was measured only with the galvanic cell and spectrophotometer. The reaction catalyzed by snake venom L-leucine oxidase to the corresponding alpha keto acid was measured with the galvanic cell and the Warburg but 1 Present address, University of Wisconsin Medical School, AIadison, Wis
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