Determination of Iron in Glass and Cobalt via Charged Partic Ie Act ivation Analysis Dale L. Swindle, Leo R. Novak, and Emile A. Schweikert' Center for Trace Characterization, Department of Chemistry, Texas A&M University, College Station, Texas 77843
Destructive and nondestructive procedures have been developed for the determination of iron by charged particle activation analysis. Reaction yields on thick iron targets with proton, deuteron, helium-3, and helium-4 beams have been compared. The most favorable reactions for destructive and nondestructive analysis were the 54Fe(p,pn)53Feand 56Fe(p,n)56Co reactions, respectively. The nondestructive analyses were performed on glass while destructive determinations, including radiochemical separation, were applied to high purity cobalt. Yields from thick target experiments indicate that Fe determinations can be made at the 50-ppb level. Activation curves for many charged particle-induced reactions on Fe are presented along with considerations of possible interfering reactions.
Several procedures have been reported for the determination of traces of iron, some featuring detection limits as low as a few ppb (1-4). Their application to solid samples requires, however, a destructive procedure which can be hampered a t the sub-ppm level by reagent blanks. Methods inherently free from contamination effects-namely, neutron and photon activation analysis-are of limited sensitivity (-0.5 ppm) (5-7). An alternate nuclear activation technique, however, using proton bombardment was shown several years ago to provide a possibility for subppm iron determinations [uia 56Fe(p,n)56Co](8). The objective of the present work was to provide a comprehensive survey on various charged particle activation modes and their possible analytical application. Besides evaluating various procedures in terms of their sensitivity, selectivity, accuracy, and reproducibility, an additional topic of interest was also the question of whether charged particle activation analysis could be applied to the particularly difficult problem of determining traces of iron in cobalt. In the present study, the following steps are examined: the measurement of yields of activation reactions on iron and evaluation of possible limitations due to interferences; the development of nondestructive and destructive methods with two objectives, high sensitivity and/or absolute selectivity for iron us. cobalt; and application of these methods to the determination of iron in high purity samples and comparison with data obtained by other analytical methods. T o whom correspondence should be addressed. J. Roboz in "Trace Analysis: Physical Methods," G . H. Morrison, E d . , Interscience Publishers, New York, N . Y . , 1965, p 4 8 4 . J. Ramirez-Munoz in "Atomic Absorption Spectroscopy," Elsevier Publishing Co., New York, N . Y . , 1968, p 244. D. W. Fink, J. V . Pivinchy, and W. E . Ohnesorge, Anal. Chem.. 41, 833, (1969).
E . E . PicKett and S. R . Koirtyohann, Spectrochim. Acta, 2 3 8 , 235, (1968). H . P. Yule,Ana/.Chem.. 37, 129 (1965).
EXPERIMENTAL Irradiation. Irradiations were carried out at the Texas A&M University variable energy cyclotron. Beams of 13 and 30 MeV protons, 20 MeV deuterons, and 40 MeV helium-3 and helium-4 ions were used. All thick iron targets and samples to be analyzed were irradiated in a water-cooled target holder. Typical irradiation conditions were 1 to 2 kA/crnz beam intensities for periods of 10 min to 2 hours. Further details on the experimental setup have been described elsewhere (9). Samples. Samples analyzed for iron included high purity CObalt supplied by Materials Research Corporation, Orangeburg, N.Y., and high purity glass supplied by the National Bureau of Standards SRM 616. Monitors for quantitation were thin Fe foils (0.001-inch thickness) which were irradiated along with each sample. Radiochemical Procedures. Recover?, of 53Fe. The cobalt metal was dissolved in 50 ml of concentrated HXOs. Fe3. carrier ( 2 mgj was added and the sample diluted with 50 ml of water. The volume was brought to 200 ml with 6 N HC1 and the (H+FeC14-)n complex was extracted into 200 ml of diethyl ether. The ether was rinsed with a few ml of 6 N HC1 and the Fe was back extracted into HzO. Addition of sufficient KH40H precipitated Fe as Fe(OHj3. This precipitate was filtered and covered with thin plastic for y-ray counting. Chemical yields were determined later by drying the precipitate and weighing. It should be noted t h a t Fe(OH)3 precipitates as a hydrous hydroxide. The chemical yield determination was thus checked by using 59Fe tracer. Consistent agreement was found between the yields obtained in both ways. The chemical separation generally required 10 minutes including dissolution. Recouery of b6C0. After dissolution of the glass sample in H F and HC104, 10 mg of Co2+ were added as carrier, the solution was diluted, and boiled to dryness in a Teflon beaker. The residue was taken up in 3N HC1 and made basic with concentrated NHIOH. The insoluble hydroxides were centrifuged and discarded. Cobalt was precipitated as COS by addition of HzS and the precipitate centrifuged. The sulfide was dissolved in HC1, diluted to 5N, and placed on Dowex-1 ion exchange column. The Co was eluted with 20 ml of 5N HC1, made basic with ?JH4OH. and reprecipitated as COS with HzS. The sulfide precipitate was filtered onto glass fiber paper, dried, weighed, and covered with thin plastic before y-ray counting. Chemical yields were -70% with a total separation time of 3-4 hours. Counting and Quantitation. Nondestructive determinations of Fe in glass were accomplished by counting with a Ge(Li) ?-ray spectrometer. Characteristics of the detector were as follows: Resolution of 2.0 keV FWHM for the 1.332 MeV y-ray of 6oCo, photopeak efficiency of 3.05% relative to a 3 x 3 inch SaI(T1) detector. and peak to Compton ratio of 20:l. The absolute efficiency for the y-ray measured-namely, 0.378 MeV, 0.847 MeV and 1.238 MeV -was 1.20%, 0.43%, and 0.12%? respectively. A 4096 channel Canberra pulse height analyzer was used for data acquisition and readout. Radiochemically separated samples were of sufficient purity to be counted on a 3 X 3 inch NaI(T1) y-ray detector, which in this case could be used to take advantage of its high efficiency compared to the Ge(Li) spectrometer. The absence of activities other than 53Fe confirmed the integrity of the radiochemical separation for iron. Quantitation was accomplished by comparison of sample activities with the Fe monitor foils using the equivalent thickness method developed by Engelmann (10).
E . A. Schweikert and P h . Albert in "Radiochemical Methods of Analysis," International Atomic Energy Agency ed.. Vol. I , 1965, p 323. G. J. Lutz. Ana/. Chem.. 43, 93 (1971). E. A. Schweikert and P h . Albert, C. R. Acad. Sci.. 232 C, 345 (1966).
(9) H . L. Rook and E. A . Schweikert. Anal Chem.. 41, 958 (1969). (10) Ch. Engelmann in "Radiochemical Methods of Analysis," International Atomic Energy Agency ed., Vol. l , 1965, p 405.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 6, MAY 1974
655
Table I. Charged Particle Reactions in Irona Reaction
Q-Value, MeV
56Fe(p,n)56Co 56Fe (p,an)52Mn 66Fe(p,2n)55co b7Fe(p,a )QMn 54Fe(p,n)53Fe 56Fe ( d p )57Co 56Fe(d,2n)Wo 54Fe(d,n)W o 54Fe(d,dn)53Fe 56Fe (3He,2n)5iNi 54Fe (3He,p)56Co 54Fe(3He,d)55Co 54Fe(3He,a)53Fe 56Fe (3He,d)57Co 56Fe(3He,p)Wo 54Fe (a,n)57Ni 56Fe (a,d)W o
-5.36 -13.11 -15.45 +0.24 -13.62 $3.80 -7.58 +2.83 -13.62 -5.71 +7.43 -0.44 $6.96 $0.53 +6.87 -5.79 -11.48
f4Fe(a,d)j6C o 54Fe (a,dn)Wo 64Fe (a,~n)53Co
-10.93 -21.01 -13.62
r-Ray, MeV
dpa*h
Interferences
0.847 0.93!2 0.380 0,128 0.847 0,932 0.380 0.122 0.811 ‘0,128 0.811
2 . 4 x 104~ x 104 4 . 3 x 104 3 . 7 x 102 2 . 6 X lo6 3 . 8 x 103 3 . 3 x 105 3.0 X lo4 1 . 5 x 104 2 . 4 x 104 3 . 4 x 103 2 . 3 x 104 2 . 6 X lo6 1 . 2 x 103 1 . 2 x 103 9 . 3 x 103 5 . 2 x 103
6ONi (p,an)5Co 5*Cr(p,n)j2Mn 68Ni (p,a)55Co 54Cr(p,n)54Mn 55Mn(p,3n)53Fe 60Ni~,’d,an)57Co 58Ni(d,~t)~~Co &SNi(d,an)Wo 55Mn(d,4n)53Fe “Ni ( 3He, a )5’Ni 55Mn (3He,2n)W o 55Mn(3He,3n)W O 5*Cr(3He,Zn)53Fe 5jMn (3He,n)W O 59Co f3He,a)Wo 58Ni ( a , an)“Ni
0.847 0.932 0.380
3 . 4 x 103 3 . 0 x 103 8 . 2 X lo4
0.847 0.744 0,932 0.83% 0,380
2.1
0 .iaz
jQCo((~,c~n)~~Co jjMn ( a,n)5To SjMn(a,3n)Wo 55Mn r’a,4n)W o 5OCr ia,n)j3Fe
Q-Value, MeV
-11.65 -5.49 -1.35 -2.16 -23.93 -2.49 +6.51 -4.26 -26.15 $8.38 -2.88 -12.97 -5.92 +8.49 $10.11
-12.19 -10.47 -3.51 -23.46 -33.55 -5.20
*
a Using 30 MeV protons, 20 MeV deuterons. and 40 MeV helium-3 and helium-4, respectively. The yields correspond to the following conditions: irradiations of 1minute a t 1&Aon pure metal targets (natural isotopic composition) measured a t t o on the nuclide listed. Proton energy of 13 MeV.
RESULTS AND DISCUSSION
Thick Target Yields. Radioisotope yields from thick target irradiations are given in Table I along with reaction Q-values, major y-rays, and possible interferences. The energies of the incident particles included 13 and 30 MeV for protons, 20 MeV deuterons, and 40 MeV helium-3 and helium-4. No thick target yields were measured for radio-
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Figure 1. Activation curves for deuteron reactions on‘iron 656
isotopes of half-lives less than 8 minutes, as this was deemed the minimum decay time compatible with subsequent radiochemical separations. Nondestructive Iron Analysis. The most favorable reactions for nondestructive iron determinations appeared to be based on production of 5 6 C by ~ the 56Fe(p,n)56Coor the 56Fe(d,2n)56Co reactions. Indeed, 56C0 has quite suitable characteristics for nondestructive analysis, including 77 day half-life, well characterized y-decay scheme, and a limited number of reactions on other elements leading to its production. The limiting requirement of such a nondestructive analysis procedure is that the activities due to major matrix components be negligible after a decay time shorter than the half-life of 5 6 C ~ . Since 56C0 could be produced in higher yields by deuteron activation, this approach was considered first. The differential activation curves for several deuteron reactions on iron are shown in Figure 1. These curves were established by the stacked foil technique and were plotted relative to each other by taking into account the buildup factor, y-ray intensity, and detection efficiency. The absolute cross section for the 56Fe(d,2n)Wo a t 20 MeV has been reported at 300 mbarn (11). The data show that production of 5 6 C uia ~ the 56Fe(d,2n)56Coreaction would be favored a t energies greater than 20 MeV. However, an increase in bombarding energy would also increase the possible number of interfering reactions. Even a t 20 MeV, the yield of 56C0 from the 58Ni(d,a)5‘jCoreaction becomes a significant interference for iron determination. A t this bombarding energy, the relative yields of WOfrom iron and nickel, respectively, were measured to be about 5 to
ANALYTICAL CHEMISTRY, VOL. 46, NO. 6 , M A Y 1974
Because of the interference from nickel in the case of deuteron activation, attention was shifted to proton production of 56C0 uia the 56Fe(p,n)56Coreaction. Here also nickel can become an interference since 56C0 is also produced by 60Ni(p,an)56Co. However, this reaction has a Q-value of -11.65 MeV and would thus cease to become a source of s6C0 a t bombarding energies below -12-13 MeV. Figure 2 shows the differential activation curves for several reactions of protons on iron established as de(11)
w. H . Burgus, G . A . Cowan. J . w. Hedley. w. Hess, T. Shuli, M . L. Stevenson, and H . F. York. Phys. Rev.. 9 5 , 750 (1954).
Table 11. Determinations of Fe i n Glass and Cobalt Sample and matrix
NBS SRM 616 (Glass) NBS SRM 616 (Glass) NBS SRM 616 (Glass) Average VP Grade (Cobalt) VP Grade (Cobalt) VP Grade (Cobalt) VP Grade (Cobalt) Average
Proton activation, ppm
13 f 3a 12 i 3
NBS data, PPmb
"Fe
Atomic absorption, ppm
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19 21 30
0
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scribed above. Clearly 12-13 MeV provides an adequate activation cross section for the interference-free production of 56C0 from iron. In fact a maximum cross section for this reaction of -385 mbarns a t -13 MeV has been reported by Jenkins and Wain (12). In addition to nuclear interferences, y-ray interferences must also be considered in nondestructive analysis. In the case of WO,the most abundant y-ray has an energy of 847 keV. Its selective detection can be hampered by 849 keV and 851 keV y-rays from 9 4 Tand ~ 9 6 T ~respectively. , Both of these nuclides can be produced under the proton irradiation conditions outlined above uia 94Mo(p,n)94Tc or g6Mo(p,n)g6Tc.The extent to which these y-ray interferences must be considered depends upon the relative activities of 56C0 ( t l , ~77 : d ) , 9 4 T (~t l : ~4.8 : h), and 96Tc ( t 1 , z : 4.3 d ) a t the time of measurement. Contributions from 9 4 Tcan, ~ in most instances, be considered negligible because most samples irradiated with 13 MeV protons for more than a few tens of minutes will require a minimum cooling time of 1 to 2 days. The possibility of an interference from 96Tc must, however, be further considered. The ratio of the thick target yields measured on the respective y-ray peaks, 851 keV for 9 6 T and ~ 847 keV for 5 6 C ~was , determined to be 4 to 1 a t t o (end of irradiation), for equal weights of molybdenum and iron and for irradiation conditions similar to those used for iron determination (irradiation time of 2 hours). This ratio decreased to 1 to 18 when counting was made four weeks after to. As an alternate approach, the detection of 56C0 via the 1238 keV y-ray could be used. Its intensity is about half that of the 847 keV line; however, this should be compensated by the lower background in that spectral region. The nondestructive approach using the 847 keV y-ray of 5 6 C was ~ tested on high purity glass samples (NBS SRM 616). These are doped with 61 trace elements including iron and molybdenum. A four-week waiting period between irradiation and counting imposed by the possible presence of 9 6 Twas ~ found to be also largely sufficient for decay of the abundant matrix activities. The 847 KeV y-ray was carefully examined in terms of its relative intensity compared to the 1238 keV line and of its peak width. No interfering contribution from 96Tc was observed. To further test the validity of this procedure, 56C0 was radiochemically separated on one of the glass samples and its y-ray activity compared with the nondestructive measurements. Results by both procedures show good agreement (Table 11). The overall accuracy of this technique is also underlined by the agreement with data supplied by XBS. (12) I L Jenkins and A . G. Wain, J . inorg. Nuci. Chem.. 32, 1419 (1970) ( 1 3 ) Provisional Certificates of Analysis, SRM's 610-616 Doped Glasses, National Bureau of Standards, Washington, D.C. (1970).
1
c 4
da
1-
I
1
I
,
Trace Determination of Iron in Cobalt. In this case the method based on 56C0 cannot be applied. A nondestructive analysis is precluded because of the creation of some long-lived WO( t 1 , z : 71 d ) from cobalt itself. A post-irradiation chemical separation would not be suitable either. A reaction of interest not leading to the production of a cobalt isotope is J4Fe(p,pn)"Fe, which was also found to yield the highest specific activity among the different
I
54Fe ( p , pnl 53Fc
3.0
22.0
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Figure 3.
28
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Activation curve for the reaction 54Fe(p,pn)53Fe
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 6 , M A Y 1974
657
activation procedures investigated (Table I). This reaction requires a high bombarding energy due to its Q-value of -13.62 MeV. The activation curve (Figure 3) for 54Fe(p,pn)53Fewas thus measured for proton energies up to 40 MeV. At this energy, however, the 55Mn(p,3n)53Fe reaction becomes a significant interference. Indeed, a t 40 MeV the relative yield of s3Fe from manganese and iron was determined to be one to ten. This interference can be avoided by lowering the proton energy to 30 MeV where the relative production of 53Fe from manganese and iron is one to a hundred. With the bombarding conditions defined a t 30 MeV, this iron determination method was applied on high purity cobalt samples. The selective identification of the 8.5 min 53Fe isotope required a rapid post-irradiation chemical separation using the procedure described above. The results of repeated determinations of iron in cobalt are presented in Table 11. Based on these data, the average deviation at the 20 ppm level is estimated a t -&lo% for this proton activation technique. T o test its overall accuracy, iron determinations were carried out on duplicate samples using atomic absorption spectrophotometry. The proton activation method gave more consistent results than atomic absorption. The dispersion of the atomic absorption data was found to be due to both a reagent blank and a depression of the iron absorption signal by the cobalt matrix.
CONCLUSIONS This survey of various activation modes shows that the best suited method for nondestructive iron determinations is based on the reaction 56Fe(p,n)Wo. This technique can also be applied to a wide range of matrices, although iron determinations can be made only above the 0.5 ppm level. A destructive procedure using the reaction 54Fe(p,pn)53Fehas been developed for the special problem of determining iron in cobalt. The experimental data indicate that the destructive procedure is more sensitive (50 ppb) than the 56C0 method. I t is further applicable to any matrix which can also be dissolved quickly. The sensitivities given are based on the following assumptions: a minimum detectable photopeak equal to 6 u of the background in that spectral region, a beam intensity of 5 FA, and a length of irradiation of 20 min for the 53Fe method and of 5 h for the TO method. ACKNOWLEDGMENT We thank J. L. Debrun, Laboratoire Pierre Sue, Saclay, France, for helpful discussions during the initial stages of this study. The assistance of the cyclotron operations personnel is gratefully acknowledged. Received for review September 19, 1973. Accepted December 26, 1973. This work was supported by the National Science Foundation Grant GP-34877X.
Simple Apparatus for On-Site Continuous Liquid-Liquid Extraction of Organic Compounds from Natural Waters Martin Ahnoff and Bjorn Josefsson Department of Analytical Chemistry, University of Gothenburg, f a c k , S-402 20 Goteborg 5, Sweden
A continuous liquid-liquid extraction apparatus based on mixed settling is described. Simple and robust performance made it possible to use in field conditions. The organic refreshing system, normally used, is eliminated. The theory for extracting chlorinated pesticides continuously from water with a stationary immiscible solvent is discussed. In laboratory tests, the recovery of added pesticides is 83-96% for different pesticides and different pump rates. Polychlorinated biphenyls (PCB) were extracted from some hundred liters of water at the Gota river with different multichamber arrangements. Finally, the content of PCB was determined with ECD gas chromatography in the range 0.1-1.0 ng per liter of water.
A considerable number of methods [ c f . Goldberg ( I ) ] have been worked out to detect qualitatively and quantitatively the presence of organic compounds in natural waters. Direct measurements and identification of organic compounds in water are complicated because of the sensitivity and specificity requirements. Polluted water contains many and varied natural and synthetic compounds ( 1 ) " A Guide to Marine Pollution," E. D. Goldberg, Ed.. Gordon and Breach. New York, N.Y., 1972. Chapters 1, 2, and 4.
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that would seriously interfere with subsequent identification and quantification. The amount of organic substances, especially pesticides, in natural waters is usually very small. Nowadays, the separation and identification techniques are highly sophisticated when employing gas chromatography and mass spectrometry, which can produce information for identification a t high sensitivity especially when using online data acquisition systems. Even so, in trace analysis the concentration step from a large mass of water will further increase the sensitivity. Activated carbon filter has been employed for adsorption of different kinds of organics in natural waters since it was developed and introduced by the U.S. Public Health Service (2). However, the lack of adsorption and desorption control in addition to bacterial and oxidizing attack on the organics limits the method (3). Better results both in extraction and recovery from filter materials were found with reversed liquid-liquid partition using a hydrophobized carrier coated with a lipophilic stationary phase followed by recovery from the column by means of small amounts of organic solvents. Ahling and Jensen ( 4 ) ( 2 ) H. Braus, F. M . Middleton, and G. Walton. Anal. Chem., 23, 1160 (1951). ( 3 ) 0. J. Sproul and D. W. Ryckman, J . Water Pollut. Contr. Fed.. 33, 1188 (1961). (4) B. Ahling and S. Jensen, Anal. Chem., 42, 1483 (1970).
