of powdered sulfur (Ss).Nor is it likely that colloidal sulfur, formed at the expense of the hydrogen sulfide, is the catalyst, for the following reason. Decomposing solutions are always clear and colorless for the first few minutes after initiation of the reaction. In those cases in which the fast reaction is observed, a transient yellow turbidity develops; in all likelihood, colloidal sulfur. But this turbidity. although it does eventually disappear, remains for some time after the fast reaction has ceased. Also, the distinct odor of hydrogen sulfide can be detected in the reaction flask during the fast reaction, but disappears as the rate decelerates. Thus, hydrogen sulfide is the probable catalyst for the fast reaction.
Both Rinker (10) and Spencer ( 5 ) interpret their results in favor of a radical mediated mechanism, while Wayman and Lem (19, 20) propose a predominantly ionic mechanism to account for their polarographic data. It is beyond the scope of this report to discuss the as yet unresolved question as to the mechanism by which dithionite decomposes. I t is to be hoped that the facts established here and the quantitative analytical method provided may be of value to those who continue to investigate this problem. Received for review August 31, 1973. Accepted November 7 , 1973. (20) W. J. Lem and M. Wayman, Can J. Chem , 48,776 (1970)
Determination of Lead Using Charged Particle Activation Analysis David C. Riddle’ and Emile A. Schweikert2 Center for Trace Characterization, Chemistry Department, Texas A&M University, College Station, Texas 77843
Trace analysis methods have been studied using the following nuclear reactions: 206,207Pb(p,~n)206Bi, 206,207,208Pb(3He,~n)207Po, and 206,207Pb(d,~n)206Bi, 2 0 6 , 2 0 7 P b ( a , ~ n ) 2 0 7InP ~addition, . a rapid, nondestructive (1112 technique based on the reaction 206Pb(d,p)207mPb = 800 msec) has been evaluated, and proton and deuteron activation have been further examined for measuring 204Pb/206Pbratios. Activation curves have been determined for the reactions investigated and results of destructive and nondestructive determinations are presented. The detection limits are estimated at 0.01 ppm for proton and deuteron activation and at 0.1 and 0.2 ppm for 3He and 4He activation, respectively.
A variety of methods have been developed for the detection of traces of lead. For elemental assays, atomic absorption. spectrophotometry, and neutron and photon activation methods have been reported with detection limits for lead ranging from 10-100 ppb (2-10). Still more sensitive techniques ( - 2 ppb) have been described recently, based on polarography ( I I ) . Isotope ratio measurements are usually carried out cia mass spectrometry (12); a few Present address, Environmental Engineering Division, TEES, Texas .4&M Lniversitp. College Station, Texas 77843. To whom correspondence should be addressed. (1) C. L. Chakrabarti, J . W . Robinson, and P. W. West, Anal. Chim. Acta. 34, 269 (1966). (2) E . A. Schweikert and P h . Albert, Radiochem. Methods Anal.. Proc. Symp., Salzburg, Austria. 1. 358 (1965). ( 3 ) C . A . Baker, A t . Energy Res. Estab. (G. Brit) Rep. R-5265 and R 5547 (1967). (4) G . J . Lutz, Ana/. Chem.. 43, 9 3 (1971). (5) D. Brune, S. Mattson. and K. Ltden, Anal. Chim. Acta, 44, 9 (1969). (6) J. S. Hisiop.Ana/yst (London).97, 78 (1971). ( 7 ) A . Chattopadhyay and R. E . Jervis. Radiochem. Radioanal. Lett., 11, 331 (1972). (8) H . R. Lukens, Jr., Radioanal. Chem.. 1 , 3 4 9 (1968). (9) M . Wiernik and S. Amiei, Radioanal. Chem., 3, 393 (1969). (10) W . E. Kuykendall and R . E Wainerdi, Proc. V l t h lnt. Symp. Microtechniques. E., 129 ( 1 970). ( 1 1 ) E . E . Buchanan, T. D. Schroeder. and B. Movosel, Ana/. Chem., 42, 370 (1970) (12) W . U . A u l t , R. G. Senechai, and E. E. Woodland, Environ. Sci. Techno/.. 4, 305 (1970).
methods based on nuclear reactions have also been proposed (13-15). The present study was undertaken to assess the possibilities for the determination of both total lead and lead isotope ratios using charged particle activation analysis. Few reports have appeared in the literature on this technique applied to lead. Work by Cobb (13) and by Peisach (16) has shown some of the possibilities offered by 4He activation. Very recently 3He activation has also been examined ( 1 7 ) . Earlier studies in this laboratory have indicated that sub-ppm determinations of lead are feasible using proton or deuteron activation (14, 18). These studies led also to a proposal that zo4Pb/206Pbisotope ratios could be determined using charged particle activation analysis ( 1 4 ) . This paper reports on further investigations of charged particle activation applied to the trace lead and lead isotope ratio analysis using proton, deuteron, and 3He activation.
EXPERIMENTAL Irradiation. Charged particle activation of samples was carried out a t the 88-inch Texas A&M University Variable Energy Cyclotron. Irradiation energies used were 20 MeV for deuterons, 20 MeV and 30 MeV for protons, 40 MeV for 3He, and 40 MeV and 50 MeV for 4He ions. Beam intensities were limited to 3-4 p A / cm2 to preserve sample integrity. Samples. Analyses were performed on high purity glasses doped with 61 trace elements issued by the National Bureau of Standards (SRM’s 610-616). High purity metals obtained from Materials Research Corporation, Orangeburg, N.Y., were also examined. Isotope ratio measurements were performed on samples with isotopic ratios certified by NBS (SRM’s 981-983). Thick targets of lead metal were used as standards. The following materials were used as flux monitors: copper foils (30 pm thick), iron foils (30 pm thick), and mica (muscovite foils -25 pm thick). Bismuth Isotope Separation. Bismuth carrier was added to the dissolved sample and the solution made basic with NaOH to (13) J. C. Cobb, J. Geophys. Res., 69,1895 (1964). 13, 58 (1970). (14) E. A. Schweikert, Trans. Amer. Nucl. SOC., (15) G . W. Reed, i . Kigoshi, and A. Turkevich, Geochim. Cosmochim. Acta, 20, 122 (1960). (16) M . Peisach. Ann. Res. Rep., Southern Universities Nuclear institute, FaureC. P., South Africa, 32 (1971). (17) 8.Parsa and S . S . Markowitz,Anal. Chem., 46, 186 (1974). (18) D. C. Riddle and E. A. Schweikert, J. Radioanal. Chem. in press (1973).
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 3, M A R C H 1 9 7 4
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'OVA /I:
Table I. Nuclear Reactions Utilized Principal
06
0
0
05t
0
10
1
20
IO
0.51
I
?-ray energies,
Reaction
M6Pb (p,n) 207Pb(p,2n) 2QsPb (p,3n) M4Pb(p,n) 2Q6Pb (p,2n) 206Pb(d,2n) 2oiP b (d,3n) 206Pb 204Pb (d,2n) *Q6Pb(3He,2n) 207Pb(3He,3n) 206Pb (4He,3n) m7Pb (?He,4n)
30
./
/
ENERGY ( M e V )
Figure 1. Activation curves for the following reactions (A) 207Pb(d.p)207mPb; (e) 206,207Pb(p,~n)206Bi;(C) 207,206Pb(d,xn)206Bi;(D) 2 0 * ~ 2 0 7 ~ 2 0 6 P b ( ~ H e , ~ n (E) )*07 207,206Pb((u,~n)207Po Po;
Half-life
KeV
6.24 d 6.24 d 6.24 d 11.3h 15.3 d 6.24 d 6.24 d 800 msec 11.3h 5.7h
803, 881 803, 881 803, 881 376 703 803, 881 803, 881 570, 1064 376 992, 743 992, 743 992, 743 992, 743
5.7h 5.7h 5.7h
56Fe(p,2n)55Co.Thin mica (muscovite) foils were used as beam monitors for 3He and 4He activation by measuring the 18F activity produced by the reactions 160(3He,p)18Fand 160(4He,pn)1MF, respectively. Fast Sample Transfer System. Detection of the 800-msec Zo-imPb isotope required a system for transferring the sample rapidly from the irradiation position to the counting position. The system used has been previously described by Debrun et al. (19). Sample transfer time from the irradiation site to the counting position was l second, for a distance of -8 m .
-
RESULTS AND DISCUSSION a phenolphthalein end point. Bi(0H)s was separated by centrifugation, after cooling, and redissolved in 6M HC1. Bi3+ was then extracted into a 1% solution of diethylammonium diethyldithiocarbamate in chloroform and back extracted into 12M HC1. The Bi3+ solution was then diluted and sufficient NaOH added to reach the phenolphthalein end point. Bi(0H)s was again separated by centrifugation after cooling. After bismuth metal was formed by adding NaHSnOz, the precipitate was separated by centrifugation and washed several times with 6M HCl. Final steps were filtration, drying, weighing for chemical yield, and mounting for counting purposes. Chemical yields obtained were on the order of 90-95%. Polonium Isotope Separation. The sample solution (15-20 ml) was diluted to -100 ml and 1 ml of 1.5M HC1 added. This solution was raised to 80 "C and a 1-cm2silver foil (25 pm thick), previously cleaned with HC1, was suspended in the sample solution for -60 minutes to allow spontaneous deposition of polonium onto silver. The foil was then dried and counted. The chemical yield was obtained by exactly duplicating these conditions for a synthetic sample solution containing a known amount of polonium tracer. Chemical yields were on the order of 35-50%. Very recently, a somewhat different procedure with chemical yields above 90% has been reported (17). In the case of the gold samples, a pre-separation of the gold was necessary because of interference with the deposition of polonium onto silver foil. This separation was accomplished by extracting the gold into diethyl ether. Quantitation. y-Ray counting for destructive determinations was performed on a 3- x 3-inch NaI(T1) detector coupled to a 400-channel analyzer. Nondestructive analyses were performed using a high resolution Ge(Li) detector (resolution: 2.0 keV FWHM for the 1.332-MeV y-ray of ' W o ; photopeak efficiency relative to 3- X 3-inch (NaI(T1): 3.05%; absolute efficiency for 1.332MeV y-ray: 0.24% a t 0 cm) coupled to a 1024-channel analyzer. The sample activities measured were compared to those of the standards irradiated under identical conditions according to the following equation: A , X lo6 X F
Weight in ppm =
YxA,xW
where AI = sample activity; F = beam intensity ratio, standard/ sample; Y = chemical yield; A2 = specific activity of standard; and W = weight of activated sample. Beam intensity corrections for deuteron irradiations were made by measuring the 64Cu or 'j5Zn activity induced in a thin copper foil by the reactions 6 3 C ~ ( d , p ) 6 4and C ~ 65Cu(d,2n)65Zn,respectively. For proton irradiation, thin iron foils were used as beam monitors, the activities measured being from the reaction 396
Total Lead Determination. The nuclear reactions studied are listed in Table I. Only those occurring on the most abundant lead isotopes were considered for this purpose, in order to minimize errors due to possible variations in the isotope abundances. It must be further noted that these reactions are interference-free for the bombarding energies indicated. A first possibility which appeared of potential interest as a very rapid nondestructive lead assay technique is that based on the reaction 206Pb(d,p)207mPb.Selective detection of 207mPb can be readily achieved by rapidly transferring the sample after bombardment to a well shielded y-ray detector. Thick target yields for this reaction were measured at varying incident deuteron energies (Figure 1A). The activities recorded a t 20 MeV indicate that an experimental detection limit of 250 pg of lead may b e ' achieved under the following conditions: a 20-pA irradiation of 1-sec duration (approximately equivalent to 10 successive irradiations of 1 sec each at 2 FA), a delay time of 1 sec, followed by a count of 8 sec yielding a minimum detectable peak equal to six standard deviations of the NaI(T1) background a t the y-ray energy of interest. The detection limit could be further affected by the activity of the major sample components. This approach appears thus only of interest for samples where lead is present as a minor element rather than a trace component. Attention was thus focused on the reactions yielding longer lived nuclides. Both proton and deuteron activation produce 206Bi. Their respective activation curves (relative excitation functions) are presented i n Figure 1, B and C. The experimental detection limits for 30-MeV protons and 20-MeV deuterons are in each case estimated a t 10 ppb for radiochemically pure samples, and based on a 12 FA-hr irradiation ( L e . , 3 hr at 4 k A ) , a delay time of 6 hours, and a minimum peak equal to six standard deviations of the NaI(T1) background at the y-ray energy of interest. Deuteron activation was further assessed in case studies on gold and glass samples. The glasses doped with 61 (19) J. L. Debrun. D. C. Riddle, and E. A . Schweikert, Anal. Chem.. 44,
ANALYTICAL CHEMISTRY, VOL. 46, NO. 3, MARCH 1974
1386 ( 1972).
Table 11. Results of Nondestructive Lead Determinations Using m7,mSPb(d,~n) m6Bi
Table IV. Results of Destructive Lead Determinations Using 3He Activation
Sample
Value found, ppm
Certified value, ppm (20)
Glass SRM 610 Glass SRM612
438 35
425 38.57
Sample
Value found, ppm
Glass SRM 614 Gold (“Marz”) Gold ( “ M a d ’ )
2.6 0.8 0.8
‘NBS reference (20).
~
Other methods
2. 34a 0.8, 0 . 9 *
0.8, 0 . g b
*Deuteron activation (see Table 111).
Table 111. Results of Destructive Lead Determinations Using Deuteron Activation Sample
Glass SRM 612 Glass SRM 612 Glass SRM 614 Glass SRM 614 Glass SRM 616 Gold (“Marz”) Gold (“Marz”)
Value, found, ppm
Other methods, ppm (20)
41 32 3 5 3 4
38 57 38 57 2 34 2 34 1 86
2 0 0 9
0 8
trace elements were chosen in particular as typical examples of high purity materials with respect to the number and concentration levels of trace impurities present. Because of the long half life of 206Bi, both nondestructive and destructive determinations appeared feasible. The nondestructive procedure was tested on two glass samples of different concentration levels. The results of the two single analyses are in good agreement with the values supplied by NBS (Table 11). The detection limit for nondestructive assays will depend on the nature of the matrix. It appears that sub-ppm levels of lead can be determined in cases where a l-week waiting period is sufficient for major matrix activities to disappear. Several analyses were also run using the post-irradiation chemical separation procedure for 206Bidescribed earlier. The results of each determination are given in Table 111. The concentration range covered and the agreement with data supplied by NBS validate the method in terms of its sensitivity and accuracy. For these analyses, the overall error is estimated t o range from *lo% a t the 20- to 50-ppm level to *20?7‘ a t the 1-to 10-ppm level., To complete a comprehensive survey, 3He and 4He activation were also considered. A host of reactions may be induced by these particles; however, only those yielding 207P0(half-life: 5.7 hr) have been investigated (Table I), because all other products have half-lives in excess of 10 days. Stacked foil experiments were carried out to establish the activation curves for 3He and 4He activation of lead, shown in Figure 1, D and E, respectively. The 3He data agree well with very recent independent measurements ( 1 7 ) ;the 4He activation curve for lead is in agreement with data previously published by John (21). These activation curves indicate that the 3He and 4He induced reactions should be less sensitive than the proton and deuteron induced reactions because of the high reaction thresholds. This leads to a reduced activated sample volume and thus to decreased sensitivity. 3He activation with subsequent radiochemical separa~ been studied on high purity samples with tion of 2 0 7 Phas lead contents ranging from a few ppm to -0.8 ppm (Table IV). In using 3He activation, a possible interference due to bismuth must be considered: 209Bi(3He, a 2 0 7 P ~The . magni~ 4 n ) ~ and ~ ~ 209Bi(3He,5n)207At P o tude of this interference has been studied recently by Parsa and Markowitz ( 1 7 ) . Their findings indicate, that a t (20) Provisional Certificates of Analysis, SRM’s 610-616, Doped Glasses, National Bureau of Standards. Washington. D.C. (1970) (21) W. John, Phys. Rev., 103, 704 (1956).
35-MeV 3He energy and for equal amounts of lead and bismuth, the 2 0 7 Pactivity ~ from bismuth is about 10% that originating from lead. In the samples examined in the present work, the bismuth levels were determined independently to be a t least an order of magnitude lower than the lead concentration (18). Consequently, the possibility of an interfering contribution due to bismuth was considered negligible in these particular materials. This is further underscored by the agreement between the value found and certified data supplied by NBS in the case of the glass sample. For the gold sample, the values obtained are in good agreement with those previously found via deuteron activation (Table 111). An additional comment must be made on the chemical separation procedure used for 207P0.Because of the length of time required to dissolve the sample (3-5 hr for the glass) compared to the half-life of the isotope measured (5.7 hr), the separation scheme for polonium, previously described, was designed to be simple and rapid, but gave only 3550% yields. In cases where the sample may be dissolved quickly, a more involved separation scheme may be used. This procedure is based on the extraction of polonium into dithizone in chloroform followed by back extraction into aqueous solution using 4M HC1 followed by coprecipitation with tellurium metal reduced in situ by SnC12. The radiochemical yield may be determined by adding a known amount of 206Potracer to the sample solution and measuring the 206Po activity in the final product. This procedure gives yields in excess of 90%. The experimental detection limit for 3He activation in a sample such as the glass is estimated a t 0.1 ppm for a 40MeV, 12 fiA-hr irradiation. The 4He activation method with radiochemical separa~ to offer possibilities similar to 3He tion of 2 0 7 P appears activation. Thick target yields a t 40 and 50 MeV indicate detection limits of 0.3 and 0.2 ppm, respectively. Isotope Ratio Measurements. This discussion is concerned with reviewing only the procedures for determining zo4Pb/206Pb.The possibilities of charged particle activation for measuring *04Pb/208Pb are somewhat limited and have been covered in earlier reports (14). The determination of the isotope ratio is made by activating a standard of known ratio to measure an experimental cross section ratio, 6204/206. This ratio is then used to calculate the 204Pb/206Pbratio in the sample as follows: Standard: (r204/206 = A204/206 X R2041206x
I x (1 - e-”)~06/204 (2) Sample: R204/206 = A204/206 x
cT206/204
xIx
(1- e
)206/204
(3)
where u = reaction cross section; A = measured activity corrected to t = end of irradiation; R = isotopic ratio; I = particle intensity; X = decay constant; and T = irradiation time. Proton activation can be used for the simultaneous detection of 204Pb and 206Pb uia zo4Pb(p,n)204Bi and zo6Pb(p,2n)205Bi,respectively (see Table V). Both reac-
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397
Table V. Reactions Used for Lead Isotope Ratio Measurements Reaction
m4Pb(p,n)20'Bi 206Pb(p,2n)zo6Bi zo4Pb(d,2n),o4Bi 206Pb(d,2n)206Bi
Threshold energy, MeV
Half-life
Principal y-rays, keV
Interfering reactions
-10 -12 -8 -8
11.2 h 15.3 d 11.2h 6.24 d
376 703 376 803, 880
zosPb(p,3n)204Biif E , 2 20 MeV zo7Pb(p,3n)205Bi if E , 2 18.5 MeV 206Pb(d,4n)20iBiif Ed 2 22 MeV 207Pb (d,3n)za6Biif E d 2 1 3 . 5 MeV
Table VI. Results of zo4Pb/m6PbIsotope Ratio Measurements Sample
Lead isotope ratio standard SRM 982 Average Lead isotope ratio standard SRM 983 Average
Activating particle
1
deuteron deuteron proton
t
deuteron proton
tions are interference-free a t irradiation energies below 18.5 MeV. This method has been tested on samples with known isotopic ratios (NBS SRM's 982 and 983). The results obtained are given in Table VI; the certified values by NBS are also listed for comparison. In deuteron activation, the pertinent reactions used for measuring 204Pb/206Pbratios are 204Pb(d,2n)204Biand 2°6Pb(d,2n)206Bi,respectively. To avoid an interference due to 207Pb(d,3n)206Bi, the bombarding energy should be kept below 13.5 MeV. In practice, it appears that energies 2 to 3 MeV higher can be used without deteriorating the present accuracy of the 206Pb measurement. This technique has also been applied on samples with known isotopic abundances (Table VI), using the method of calculation described above. The data presented in Table VI establish the feasibility of this activation technique for the simultaneous measurement of zo4Pb/206Pbratios. The small number of determinations made precludes a refined assessment of the precision of this method. In this respect, it will clearly be lim(22) Certificates of Analysis, SRM's 981-983, Lead Isotope Ratio Standards, National Bureau of Standards, Washington, D C. (1969)
Value found
Certified value ( 2 2 )
0.0208
0.0217 0,0278 0.0234 i 0.0040 0.0004
0.027219 i 0.000027
0.0003 0.0003 f 0.0001
0.000371 i 0.000020
ited in comparison to mass spectrometry. The features of this approach are its inherent accuracy (freedom from reagent blanks) and its simplicity while still providing subppm level sensitivity. CONCLUSIONS
For total lead determination, techniques with sub-ppm detection limits can be devised with each of the four different activation modes examined. Deuteron activation appears to offer the most versatility with possibilities for very rapid part-per-thousand level analyses, nondestructive or destructive sub-part-per-million determinations, and for measuring 204Pb/206Pbratios under conditions providing maximum sensitivity. ACKNOWLEDGMENT
The assistance of the cyclotron operation personnel is gratefully acknowledged. Received for review July 13, 1973. Accepted October 5 , 1973. This work was supported by the Robert A. Welch Foundation, Grant A-339.
Theory of Heat-Flow Calorimeter Satohiro Tanaka and Kazuo Amaya National Chemical Laboratory for Industry, Hon-machi, Shibuya-ku, Tokyo, Japan
Boundary value problems for generalized, spherical, and cylindrical models of a heat-flow calorimeter are solved under some ideal conditions. The proportionality relation between the heat liberated and the time integral of the temperature deviation from steady state is deduced for the spherical and cylindrical models. Optimum conditions for maximum sensitivity of the calorimeter are evaluated for the two models. The method of transforming the temperature variation, as a function of time, to obtain the thermogenesis function is investigated through solution of an integral equation relating the temperature and thermogenesis function. 398
In previous papers (I, 2), idealized one- and three-dimensional models of a heat-flow calorimeter in which the heat transfer takes place only by conduction were presented. The proportionality relation between the quantity of heat evolved or absorbed in a reaction vessel of the calorimeter and the time integral of the temperature deviation from the steady state a t any point in the thermal conductor of the calorimeter was proved under some ideal conditions for both the models. Verhoff ( 3 ) has also de(1) M. Hattori, s. Tanaka. and K. Amaya, B u / / . SOC. Chem. Jap.. 43,
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 3, M A R C H 1974
1027 (1970). (2) S. Tanaka and K. Arnaya, ibid., 43, 1032 (1970). (3) F. H. Verhoff, Ana/. Chem., 43, 183 (1971).
