Waste
Figure 5.
Proposed flow diagram for automated tRNA assay
r CY and znsitivity, the dialysis method has the advantage of providing undenatured aminoacyl-tRNA which can be studied further. It seems reasonable to believe that the same principle can be applied to the assays required in the study of enzymatic polymerization such as with DNA polymerase, RNA polymerase, or RNA phosphorylase as well as in protein synthesis.
RECEIVEDNovember 12, 1970. Accepted April 1, 1971. Investigation supported in part by a grant from the National Institutes of Health No. A.M. 02493. A part of this work has been presented at the 158th National Meeting of the American Chemical Society, New York, N. Y . , September 10, 1969, Abstract No. Biol. 156.
ctivatisn Analysis of Sediments for Halogens Using Sziiarb-Chalmers Reactions L. J. Walters, Jr.,l and J. W. Winchester* Department of Geology and Geophysics, Massachusetts Institute of Technology, Cambridge, Mass.
Nuclear recoil reactions following neutron capture are used to determine the character of binding of chlorine, bromine, and iodine in sedimentary minerals. After neutron irradiation of thoroughly-washed clays, a substantial fraction of induced iodine radioactivity is removed by a water leach, suggesting a predominantly water-insoluble surface binding of natural iodine t o particle surfaces. A smaller fraction of bromine and very little chlorine appear to be surface-bound. Preirradiation removal of sedimentary interstitial water by squeezing and ion-exchange replacement by washing with aqueous ammonium nitrate operationally distinguish soluble from surface-adsorbed states; postirradiation fusion and analysis following the water leach distinguishes internally-bound from surfaceinsoluble states. An analytical procedure for distinguishing the four degrees of association of halogens in sediments has been tested by radiotracers and by activation analysis of standard samples, and has been applied to studies of geological samples of recent marine sediments and ancient sedimentary rocks. 1 Present address, Department of Geology, Bowling Green State University, Bowling Green, Ohio, 43403.
2 Present address, Department of Oceanography, Florida State University, Tallahassee, Fla., 32306.
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
PREVIOUS RESEARCH has suggested that iodine and to some extent bromine are concentrated in sediments in a surface layer around individual sediment particles, while chlorine is found predominantly in the water phase of sediments or substituted for hydroxyl ions within the mineral grains (1-5). Nuclear recoil reactions following charged particle activation have been applied to surface analysis by a number of investigators (e.g., 6, 7). In the present work we have used neutron activation analysis and nuclear recoil to distinguish surface-bound iodine, bromine, and chlorine from halogens located within the bulk material and tested the technique with marine sediments and sedimentary rocks.
(1) L. A. Gulyayeva and E. S . Itkina, Geokhimiya, 524 (1962). ( 2 ) A. I. Mun and Z. A. Bazilevich, ibid., 175 (1962). (3) A. G. Collins and G. C. Egleson, Science, 156, 934 (1967). (4) L. Greenland and J. F. Lovering, Geochim. Cosmochim. Acta, 30, 963 (1966). ( 5 ) M. Gillberg, ibid., 28,495 (1964). (6) J. F. Lamb,D. M. Lee, and S. S. Markowitz, ANAL. CHEM., 42, 212 (1970). (7) W. D. Mackintosh and J. A. Davies, ibid., 41 (4), 26A (1969).
An analytical scheme was developed by which four hypothetical degrees of binding were defined. Water-soluble halogens were separated into two fractions. The first fraction included the halogens in the interstitial water and could be removed with the water mechanically by centrifugation or filter pressing. The second fraction contained water-soluble halogens bound t o the sediment particles through a n anionexchange mechanism. An ammonium nitrate solution, with which the samples were washed, displaced these ionicallybound halogens. The remaining halogens on the surface of the sediment were insoluble in the ammonium nitrate wash solution. They were divided into two more fractions depending o n whether or not they were water-soluble following neutron capture. The third fraction contained “surface-bound” halogens which were removed by a rapid water leach following neutron irradiation. The fourth fraction (“internally-bound”) contained those halogens that were not water-soluble after neutron capture, but were released by total fusion of the sample. There is no sharp distinction between the surface-bound and internally-bound halogens. Following neutron capture, the recoil of active atoms from the surface film may be in any direction, and, for geometrical reasons, the retention of activity within the surface film may be considerable. The solubility of the atoms going toward the grains in the leach solution may be affected by the hydrophobic or hydrophilic nature of the organic film around the grains. It has long been believed that most iodine and bromine in sediments enter as organic compounds which are contained in dead marine organisms and plants (8). A chemical investigation of the halogens in the sedimentary environment must take into account the interaction between water and mineral phases as well as the organic material and its state of preservation. ‘The four hypothetical degrees of binding of the halogens defined above represent an attempt to establish the location, amount, and type of bonding of iodine, bromine, and chlorine in sediments. As examples of expected behavior, water-soluble organic molecules as large as 3,5-diiodotyrosine may be primarily in the first (mechanically removable) fraction. Halogens associated with the organic material in sediments through an anion-exchange mechanism, such as binding to quaternary nitrogen, should be removed by the NHdN03 wash solution, depending on the OH-/X- and NOs-/X- selectivity ratios. If these selectivity ratios are very high, these halogens may remain attached to the surface of the sediment particles, but easily-exchangeable halogens will be found in the second (slowly water-soluble) fraction. Tightly-bound halogens should be found in the “surface-bound” or “internallybound” fractions. Recoil of iodine, bromine, and chlorine following neutron capture is great enough to break any covalent carbon-halogen bond and should remove adsorbed halogens from the particle surfaces providing that recombination does not occur.
average halogen content of the wash solution was 0.10 ppm C1, 0.002 ppm Br, and 0.0004 pprn I. Interstitial Halogens. The natural interstitial water, containing soluble salts, can be removed from sediment that will undergo plastic flow under pressure by filter pressing (9) or by centrifugation and decanting the supernatant liquid. This was performed in a few samples, although for the majority only the N H 4 N 0 3 washing was carried out, as described in the following paragraph. Pre-Irradiation Removal of Water-Soluble Halogens. Complete removal of all water-soluble halogens was assured by washing the sediment samples step-wise eight times with 0.025M N H 4 N 0 3 . U p to 1.0 gram of powdered sediment (less than 100 mesh) was placed into a weighed borosilicate glass centrifuge tube; 25 ml of the wash solution was added and stirred using a Teflon (Du Pont) coated stirring bar. The mixture was boiled gently over a small flame for one minute and then cooled in cold water. The first washing step included ultrasonic agitation for two hours, after which the suspensions were centrifuged and the water decanted. The samples were weighed both before and after decanting, which allowed the weight of solution decanted and the aqueous holdup to be calculated. The above washing procedure (heat, cool, centrifuge, and decant) was repeated seven more times, but without ultrasonic treatment. Neutron Activation Analysis of Iodine, Bromine, and Chlorine. Samples of water wash, interstitial water, or washed sediment as a slurry (80 mg of sediment and 0.25 ml of water) were irradiated in the pneumatic-tube facility a t the M I T reactor for twenty minutes in a thermal-neutron flux of 2.3 x 1013 n/cm2/sec. The solvent-extraction technique for halogen analysis (10, 11) required the samples to be in the form of a water solution. Because the presence of sedimentary particles in the samples could result in the contamination of the AgX precipitates with 24Na, a Fe (OH)r scavenge step (12) and filtering through a 0.45-p Millipore filter was included prior to the chemical separation of the halogens. The extraction of It into CCla was done twice to ensure purity of the resulting AgI precipitate. Since sediment samples may contain a large amount of iodine and bromine relative to chlorine, the chlorine analysis was modified t o include removal of Iz and Brz by distillation under slightly acid and strongly acid conditions, respectively, using “ 0 3 for the oxidation. This effectively removed 9 5 % of any 12sIor 80--82Bractivity. A selective absorber was used t o further discriminate against 1281 and 80-82Bractivity in the AgCl precipitate by placing aluminum disks l/S-inch thick (857 mg/cm2) over the AgCl precipitate for /3 counting of T I . This totally removed laIp activity and reduced the s0-s2Br/~Clactivity ratio by a factor of 41. When these precautions were used, the resultant half-lives of the various radio-nuclides agreed with the known values (13). The beta activity was measured with a Widebeta automatic counter. Post-Irradiation Leach. The measurement of surfacebound exchangeable halogens from a n irradiated solid sample requires that the solid material be equilibrated with a water solution in such a manner that active halogen atoms o n the surface of the particles be exchanged with inactive atoms in the leach solution. To fulfill this condition, a leach solution
EXPERIMENTAL
Reagents. A 0.025M N H 4 N 0 3 wash solution of low halogen content was prepared using distilled demineralized water, reagent grade H N 0 3 , and N H 4 0 H prepared by distilling N H 3 from reagent-grade N H a O H into distilled demineralized water and adjusting to a p H of 7.8-8.0. The (8) A. P. Vinogradov, “The Elementary Chemical Composition of Marine Organisms,” (Trawl. by J. Efron and J. K. Setlow), Yale University, New Haven, Conn. 1953.
(9) F. T. Manheim, U.S . Geol. Surv. Profess. Pap., 55O-C, C256, ( 1966). (10) R. A. Duce and J. W. Winchester, Radiochim. Acta, 4, 100 (1965). (11) K. W. Liekrman, Ph.D. Thesis, University of Kentucky, Lexington, Ky., 1966. (12) L. J. Walters, Jr., Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass, 1967. (13) C. M. Lederer, J. M. Hollander, and I. Perlman, “Table of Isotopes,” 6th ed., John Wiley and Sons, New York, N. Y., 1967. ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
e
1021
1
Wash number
Figure 1. Concentration of water-soluble iodine (A), bromine (o), and chlorine (m) for a Gulf Coast sediment was used containing 3.8 mg C1, 3.7 mg Br, and 3.9 mg I, 180 mg NaN03, and 0.1 pCi of 22Nadissolved in 15 ml of water. The 22Na tracer was included in the leach solution to measure the addition of water which occurred in the transfer of the sediment slurry from the irradiation vial and the loss of water during the boiling step that followed. The large amount of N a N 0 3 carrier was necessary to minimize 2 2 N a exchange with the 23Na of the clays, and 22Na was chosen because it would not cause counting interference in the silver halide precipitates of the iodide, bromide, or chloride fractions. After irradiation, the slurried sample was washed from the irradiation container into a n Erlenmeyer flask containing the leach solution, using a stream of water. The mixture was then boiled for one minute, cooled, centrifuged, and decanted. Aliquots were taken for chlorine, iodine, bromine, and 22Nadilution determinations. Fusion of the Solid Sample. I n cases where the irradiated mineral material was to be fused, the residue from the above procedure was transferred to a platinum crucible, carriers and K O H were added, and the mixture was fused. The fusion mixture contained four pellets of KOH, 1 ml of concentrated carrier solution (33.7 mg.Cl, 48.6 mg Br, and 58.7 mg I), and the sediment to be fused (about 80 mg). The fusions were carried out in a 20-1111 platinum crucible, kept covered during the period of intense heating. A low flame was used to evaporate the water; then the temperature was increased to effect dissolution of the sample by fusing at a dull red heat for 1-3 minutes, After fusing, the crucible was cooled; the cake was taken up in water, and the crucible rinsed twice with water and then to remove the activity. The fused mixture with 8N " 0 3 was partially neutralized with NaOH, then made basic with N H 4 0 H to a phenolphthalein end point. The hydroxides were centrifuged and the clear supernatant solution decanted. Portions of this solution were taken for chloride, iodide, and bromide determinations. Procedure for Tracer Experiments. Five samples of powdered shale were fused according to the procedure above. In addition to the normal reagents and carriers present, 10 pCi of I 3 l I - and 82Br- were also present. After fusing, the samples were diluted t o 25.0 ml, and portions were taken for gamma counting and to test the iodine and bromine chemical separations. The samples were gamma counted twice (3 X 3-inch NaI(T1) detector and 400-channel pulse-height analyzer), first for E2Br activity, using a gain base-line setting 1022
ANALYTICAL C H E M I S T R Y , VOL. 43, NO. 8, JULY 1971
2
3 4 5 Wash number
6
7
8
Figure 2. Relative concentration of l 3 I I and *2Br as a function of wash number. Sample VF was used for iodine data and sample WB for bromine that eliminated l 3 I I activity and the low-energy 82Br activity. TWOweeks later, after '12Br had decayed out, the low-energy I 3 ' I was counted. The samples of separated iodine and bromine were both beta counted and weighed. The average chemical yields from the gamma counting were: 1311 98.0 + 2 . 0 z , 8 2 B r97.7 k 2.27& and the ratio of 1311/E2Brwas 1.003 0.018. The beta counting of separated iodine and bromine fractions had the following ratios of activity/z weight AgX: 13'1 1.007 f 0.014 and 82Br 1.053 f 0.092. N o significant loss occurred during the fusion of the sediment samples, and the minor losses of the fusion step did not result in fractionation of activity and carrier. The fusion conditions essentially destroy any organic compounds or minerals, which contain halogen atoms, to yield I-, Br-, and Cl-, thus effecting adequate mixing between carrier and activity during a n actual analysis. Retention of Known Compounds. Known amounts of organic halides dissolved in benzene were added to sediments to test their behavior in the analysis scheme. After the evaporation of the benzene, the sediment samples were washed using the water-wash procedure above. Following irradiation, the amount of surface-bound organic halide remaining was determined using the post-irradiation leach procedure.
*
z
RESULTS AND DISCUSSION
Investigation of the Washing Process. The existence of two degrees of binding (easily water-soluble and slowly watersoluble) is indicated by the analyses of the pre-irradiation water washes. Figure 1 shows the concentration of water soluble iodine, bromine, and chlorine as a function of wash number. The initial slope is essentially due t o dilution of the interstitial salt in the sediment, and correlates with the measured holdup volumes for the washes. After the interstitial halogens have been reduced t o less than 0.1 ppm, the presence of slowly-soluble halogens is evident. This is shown by the relatively constant values for iodine and bromine in washes 4-8 (Figure 1). The values for chlorine in these washes could not be distinguished from the blank. However, in other samples for which only 2 or 3 washes were analyzed, chlorine also showed a constant value above the blank. Here the three halogens were measured in wash numbers 1,5, and 8. In order to examine the questions of blank interference, the washing process was tested using radioactive I31I and a2Br tracers; the results are shown in Figure 2. These results
Figure Relation between water-soluble, surface-bound, and internally-bound chlorine for 30 samples
/‘
e.
4
/c
2
Surface bound
were obtained by equilibrating the Volden Fjord sediment with 1 3 1 1 and the Wilkinson Basin sediment with a2Br, then washing according to the above procedure. The same general pattern is shown in Figure 2 as was observed for natural halogens in Figure 1. Concentrations in washes 1, 2, and 3 follow a dilution principle as the interstitial salt or activity is removed by batch washing. The relatively constant concentrations in washes 4-8 represent a slow transfer of halogens from the solid particles to the wash solution. A difference was observed for the relative amounts of iodine, bromine, and chlorine in interstitial waters removed by mechanical squeezing, and the halogens removed by washing with the N H 4 N 0 3solution. Two shallow Atlantic Samples, WB (Wilkinson Basin off Cape Cod) and VF (Volden Fjord off Norway), were analyzed both ways, and the results are shown in Table I. The Br/Cl and I/C1 ratios are significantly lower for the mechanically-expressed interstitial water, indicating that bromine and iodine are preferentially attached to the sediment particles. This same effect, observed in several other recent sediments, appears to be greater for clay sediments than for carbonate-rich sediments. The Br/Cl ratios in the wash solution are approximately the same as for sea water, indicating iodine enrichment in sediments. The exceptions were sedimentary rocks containing relatively fresh water. Surface-Bound Halogens. Collins and Egleson (3) suggest that iodine and bromine are bound to the surface of sediment particles as organic compounds. As a test of this hypothesis, a sample of the Volden Fjord sediment, that had been washed to remove water-soluble halogens, was successfully extracted with benzene, acetone, and dimethylsulfoxide in a Soxhlet apparatus (24 hours extraction time for each solvent). The organic extractions removed 27 % CI, 63 % Br, and 23 % I from the surface-bound fraction. However, in analyzing the individual extracts after evaporation to small volume, only 10% of the amount removed from the sediment surface was recovered, suggesting volatility loss during the evaporation of the solvent. Kinetic Study of Surface Exchange. The rate of transfer from the solid phase t o a 0.025M KBr solution was investigated with 82Brtracer. Approximately 10 mCi of 82Br,and 48 mg NHlBr in 15.0 ml of water, was contacted with 0.44 gram of the Volden Fjord recent sediment. After 27 hours the water-soluble activity was removed by washing eight times with a 0.025M N H a N 0 3solution, removing all but 0.41 % of the total activity from the sediment. The growth of activity
Sample WB VF
Sea water
80
60
40
20
Interrally band
Table I. Comparison of Interstitial and Water-Soluble Iodine and Bromine Interstitial water Wash solution I/Cl Br/CI I/C1 Br/CI 0.6 x 2 . 1 x lea 24 x 3.7 x lo-’ 4.3 X 2.1 x 51 x 10-6 3.5 x 10-3 3.3 X lo-‘ 3.4 x l e a
Table 11. Analyses of Known Compounds Amount Amount added, found, Found, Compound PpmX PpmX 1-1odohexane 1160.0 0.49 0.042 1-1ododecane 1390.0 30.0 2.16 1-1odododecane 1020.0 478.0 46.9 1-Bromododecane 581 .O 218.0 37.5 1-Chlorododecane 613.0 230.0 37.5 2,&Dibromophenol 1070.0 35.3 3.30 FeCls-Protoporphyrin I X Complex 910.0 48.0 5.27
z
in the 0.025MKBr solution was then followed at 24 and 58 “C. The rate of increase of total activity in the aqueous phase was constant with time, Le., d*Br-/dt = k , and the activation energy for the process was estimated to be 16-19 kcal/mole. Studies with Known Compounds. Several known halogenbearing compounds were added t o a sample of Gulf Coast sediment samples, Table 11, in order t o determine what range of compounds are detected by the post-irradiation leach. The recovery values of the normal halides indicate that compounds having a t least twelve carbon atoms will remain attached t o the sediment particles during the washing process and subsequent drying of the sediment. Dibromophenols are of interest because they have been isolated from red algae (14) and marine hemicordate worms (15). However, most of the 2,6-dibromophenol was lost because of its high vapor pressure. Porphyrin complexes are important in sediments and crude oils (16), but most likely the halogen is watersoluble or exchangeable, as indicated by the low recovery value. Based on analyses of these compounds, 3,5-diiodo(14) J. S.Craigie and D. E. Gruenig, Science, 157, 1058 (1967). (15) R. B. Ashworth and M. J. Cormier, ibid.,155,1558 (1967). (16) H. N. Dunning, Chap. 9 in “Organic Geochemistry,” The Macmillan Company, New York, N. Y.,1963, p 367. ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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Water soluble
Water soluble
//
/ ?-
BROMINE
IODINE
80
m
/
.
I*
v
Surface bound
v 80
v
v 60
.' ' . - . v
v 40
v
v 20
v
\
Internally bwnd
Surface bound
80
60
40
20
Internally b a n d
Figure 4. Relation between water-soluble, surface-bound, and internally-bound bromine for 30 samples. Lithified sedimentary rocks (m), Gulf Coast Tertiary sediments (A), Recent marine sediments ( 0 )
Figure 5. Relation between water-soluble, surface-bound, and internally-bound iodine for 30 samples. Lithified sedimentary rocks (m), Gulf Coast Tertiary sediments (A), Recent marine sediments ( 0 )
tyrosine probably would not be detected by this procedure because of its high solubility in water. Distribution of Chlorine, Bromine, and Iodine in Sediments. The value of the analytical technique can be shown by inspection of some of the results obtained. Figures 3-5 show the relative amounts of hater-soluble, surface-bound, and internally-bound chlorine, bromine, and iodine in sediments and sedimentary rocks and further define the operational definitions of three degrees of binding. Chlorine, Figure 3, is contained primarily in the watersoluble fraction released by pre-irradiation aqueous NH4N03 wash, as expected considering that CI- ion is a major component of most formation waters. The distribution of chlorine within the sediment after the water-soluble salt has been removed shows the effects of mineralogical substitution and the extent of ion exchange on a surface film. There is an apparent consistency in the ratio of surface-bound to internally-bound chlorine, the average value being 0.58. Thus, more than twice as much chlorine is located in the interior of sediment particles than on the surface. In order to determine whether the surface-bound chlorine comes from a surface film or recoils from the outer layer of uncoated mineral grain, a sample of the mineral, chlorapatite, containing 3.9% C1, was analyzed in the same way as the sediment samples. This sample had a ratio of surface-bound to internally-bound chlorine of 0.0033. This low ratio for chlorapatite indicates that the amount of chlorine found in the surface-bound fraction of sediments is too great (ratio 0.58) to have originated entirely from the outer layer of the mineral grains. Instead, a small fraction of chlorine appears to be bound to a film coating the sediment particles. Figure 4 shows that bromine in sediments is divided between the water-soluble form and that bound to the sediment particle surfaces, with very little additional released by fusion of the sample. This represents an intermediate chemical behavior between bromide ion in the interstitial water and bromine associated with the sediment particles either as Brheld by anion exchange or possibly by covalent bonds with carbon. Iodine is associated mainly with the surface-bound fraction, Figure 5 , although the contribution of the internally-bound fraction is somewhat greater for iodine than bromine. Recombination of iodine with a surface film may proceed at a
faster rate than for bromine, but, by our operational definition, this iodine is internally bound if recombination with a surface film has rendered the radioactive atoms insoluble in the post-irradiation leach. Szilard and Chalmers (17) have shown that an appreciable fraction of the iodine released during neutron capture by recoil resulting from gamma emission is retained in the bulk organic phase of halohydrocarbons. Libby (18) has attributed this retention to a cage effect on the hot atom. However, Cohen and Trumbore (19) show that the retention of the radioactive atoms in ethyl iodine is due to a recombination or exchange of the radioactive atom with the bulk phase. This reaction is of zero order with respect to iodine concentration, and the apparent isotopic exchange is very rapid at low concentrations of iodine. The dispersed state of sediments would cause a lower rate of exchange, although recombination still would have an effect in terms of the retention of radioactivity, thereby giving rise to a high amount of activity in the internallybound fraction. The effect of recombination was observed in the case of SaBr in the Volden Fjord sediment. Upon standing 6 hours following irradiation, the ratio of surfacebound/internally-bound bromine was 0.72, as compared to 25 immediately after irradiation. The highest proportion of internally-bound iodine, Figure 5 , occurs for sedimentary rocks that contain mature organic material and are associated geologically with sub-bituminous to bituminous coal (12). The maturation process for organic material in sediments results in a kerogen being formed that is progressively more hydrophobic, and the hydrophobic/ hydrophilic nature of the sediment surfaces will limit the effectiveness of the post-irradiation leach and enhance the recombination of the activated atoms.
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
CONCLUSIONS
A neutron activation analysis technique involving SzilardChalmers effects has been shown to be useful in determining the partition of iodine, bromine, and chlorine between the water-soluble fractions of sediments. The relative pro(17)L.Szilard and T. A. Chalmers, Nurure, 134, 462 (1934) (18) W.F.Libby, Jr., J . Amer. Chem. SOC.,69,2523 (1949). (19) E.D. Cohen and C. N. Trumbore, Science, 148,1460(1965).
portions of surface-bound halogens found by the technique are I > Br > CI,an order consistent either with an attachment of these elements to organic matter in sediments through a n anion-exchange mechanism or covalent bonding to carbon; the results of the NHINOl pre-irradiation washes tend t o support the former interpretation. The results of this investigation reveal that real differences of binding of chlorine, bromine, and iodine to sediments exist and that neutron activation and nuclear recoil separation is an effective means to measure these differences in geochemical studies.
ACKNOWLEDCMENT The authors thank W. H. Zoller for his assistance in the experiments and critical comments on the manuscript, and J. W. Irvine for many helpful suggestions during the course of this work.
RECEIVED for review November 4, 1970. Accepted March 22, 1971. This work was supported in part by the Office of Naval Research under contract Nonr 1841 (74) at the Massachusetts Institute of Technology.
Analytical Significance of Peaks and Peak Ratios in X-Ray Fluorescen,ceAnalysis Using a High Resolution Semiconductor Detector Analysis of Uranium Solutions by X-Ray Spectrometry Cesia Shenberg and Saadia Amiel Nuclear Chemistry Department, Soreq Nuclear Research Centre, Yavne, Israel
L X-ray peaks detected in the X-ray fluorescence spectrum of uranium in solution and the respective peak ratios were tested for proportionality to uranium content over a wide range of concentrations. The uranium L X-rays were induced using an 241Am-iodine source-target assembly. The direct comparison method of analysis was compared with the peak ratio method. Individual peaks of L, Lg, and L, lines were checked as well as their ratios to the backscattered I K, and I Kg lines. It was found that the relative intensities of the U L X-ray lines vary with the U concentration in a nonlinear manner. The ratio U L/I K a i n e o h e r e n t is the one least influenced by self-absorption, which makes it most suitable for the quantitative determination of uranium. The range of 0.12-500 mg U/ml was studied and satisfactory results were obtained. X-RAYFLUORESCENCE analysis is based on the use of one of the induced X-rays from the element in question. Since several X-ray peaks are induced from each element, especially when L X-rays are used for analysis, one usually selects the most outstanding and the best resolved X-ray peak for assaying the element. Systematic errors due to matrix effects may be accounted for and eliminated by taking peak ratios. A common analytical technique, the direct comparison method, involves comparing the intensity of an X-ray line with standards having the same geometry and approximately the same composition. Another technique, e.g., as demonstrated by Giauque (I) employing a source-target assembly, utilizes the characteristic X-rays induced in a target by the primary source t o excite X-rays in the sample. Such an analysis is facilitated by the use of a high resolution solid state Si(Li) spectrometer (2). This technique, using a source (1) R. Giauque, ANAL.CHEM., 40, 2075 (1968). (2) H. R. Bowman, E. K. Hyde, S.G. Thompson, and R. C. Jared, Science, 151, 562 (1966).
target assembly, offers two advantages; the use of the target X-rays backscattered from the sample as a reference; and selection of the target element to promote high sensitivity of the induced X-rays of the element sought in the sample (i.e., the target K X-rays produced are just aboce the absorption edge of the element X-ray line in question). Thus, the energy of the backscattered X-rays is near that of the excited X-rays in the sample. As a result, counting efficiency and transmission of both the backscattered X-rays and fluorescent X-rays are very similar, permitting the use of the ratio between them for quantitative analysis. The use of the backscattered peak of the target X-rays as a reference minimizes matrix effects and increases the accuracy of the analysis. The purpose of this work was to study the variation and analytical significance of the peak intensities, peak ratios of uranium L X-rays, and the ratios of different U L X-rays to backscattered target K X-rays. The ultimate aim was to obtain a suitable method for determining the uranium content in solutions over a wide range of concentrations. To date, X-ray fluorescence analysis of uranium, using L X-rays, has been reported by Klecka (3) who used an lZ6Isource with a Si spectrometer. The analyses were based on the U La peak and the calculation was performed by the direct comparison method. A linear variation in the range 0.1-2.6 mg U/ml solution was reported. Karttunen and Harmon (4) reported similar results in the range 8-20 mg U/ml. They used a 109Cd source for excitation and a proportional counter for detecting the emerging X-rays.
(3) J. F. Klecka, University of California, Lawrence Radiation
Laboratory Report, UCRL-17144 Rev. October 1966. (4) J. 0. Karttunen and H. R. Harmon, Spectrochim. Acta, 24B,
301 (1969). ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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