194.93779. T h a t formula is H4C6NCls which is correct €or the sample, trichloroaniline. Peaks can be observed a t 159.96703 corresponding to p-C1; a t 125.00142 corresponding to p-2C1; and a t 90.03779 corresponding to p-3C1. The structure of the compound is well represented in the formulas derived as a final result of the data reduction.
abundance for any one permutation formula is larger than that for the original formula, the mass of the permutation formula is computed, and a check is made to determine the existence of an ion corresponding to t h a t mass in the spectrum. If the ion corresponding to the more abundant permutation formula is not found, the less abundant original formula is rejected. In other words, if any one of the permutations which are more abundant is not found, the formula is unsatisfactory. The final printed output is shown in Table V for the same trichloroaniline spectrum as in Tables I and 111. The columns give the observed mass of the ion, its intensity in per cent of tallest peak, the number of rings and double bonds for the formula, the ppm error between the observed mass and the mass calculated for the formula, and the formula. The ppm error averages 5-10 ppm, but worst-case error runs as high as 30-40 ppm. It is expected that this will improve when vibration is eliminated from the instrument, b u t a tolerance of 50 ppm between the formula and the observed mass is being used until then. It should be noted t h a t even with a 50-ppm tolerance in the mass, most of the ions correspond to only one formula. Only one formula is given for the molecular ion a t mass
ACKNOWLEDGMENT We wish to thank Paul Bender and his staff for their efforts and cooperation in the development of this work and Milt Levenberg of Abbott Laboratories, Chicago, Ill., for his valuable advice and comments on various aspects of the approach to obtaining meaningful data. Received for review December 22, 1972. Accepted February 26, 1973. This research was supported by the Air Force Office of Scientific Research under AFOSR 69-1725, by the Wisconsin Alumni Research Foundation, and by the National Science Foundation under GP-36236X. The AEI MS-902C mass spectrometer and the Raytheon 706 computer system were purchased in part by funds supplied by the National Science Foundation.
Application of Chelating Ion Exchange Resins for Trace Element Analysis of Geological Samples Using X-Ray Fluorescence C. W. Blount Department of Geology, University of Georgia, Athens, Ga. 30602
D. E. Leyden, T. L. Thomas, and S. M. Guill Department of Chemistry, University of Georgia, Athens, Ga. 30602
The use of chelating ion exchange resins for the quantitative batch extraction of ions of trace elements and as a matrix for the determination of the elements using X-ray fluorescence is described. Chelex-1 00, an ion exchange resin containing iminodiacetic acid functional groups, is used for the determination of cobalt and nickel in U.S. Geological Survey samples. The selective extraction of bismuth is achieved by pH control, and bismuth was determined in several geochemical standard samples. N M R R , a chelating resin highly selective for gold and platinum metals, is used for the specific extraction of gold. In all cases, the determination is performed by pressing the resin into pellets and using these pellets as samples for X-ray fluorescence. Samples containing as little as 0.04 ppm gold, 0.2 ppm bismuth, and 15 pprn cobalt or nickel were analyzed.
Combined applications of ion exchange materials and X-ray spectroscopy for elemental analysis have been in use some time. Campbell et a2. (1) have recently reviewed applications of ion exchange, resin loaded papers in this field. However, only a few types of resin loaded papers are commercially available. Batch equilibration of a n ion ex-
change resin with a solution of the ion to be determined has also been reported (2-8). Use of the latter technique provides a homogeneous distribution of the sample on the resin. However, a high distribution coefficient of the ion of interest is required for quantitative removal of the ion from solution, especially if large concentrations of other ions are present. In order to further develop this technique for elemental analysis, there are several factors to consider. With regard to extraction methods, the use of column techniques has some advantages over batch equilibrations for the more common cation ion exchange resins ( e . g . , Dowex 50). However, the former technique usually requires a larger quantity of resin and the sample must be (1) W. J. Campbell. T. E. Green, and S. L. Law, Amer. L a b , June, 1970, p 28. (2) J. N. Van Nickerk, J. F. De Wet, and F. T. Wybenga, Anal. Chem., 33,213 (1961). (3) M. J. Miles. E. ti. Doremus, and D. Valent. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 1966, paper 236. (4) C. W. Blount, W. R. Morgan, and D. E. Leyden, Anal. Chim. Acta. 53,463, (1971). (5) C. W. Blount, R. E. Channell. and D . E. Leyden, Ana/. Chim. Acta, 56,456 (1971). (6) D . E. Leyden, R. E. Channell, and C. W. Blount, Anal. Chem., 44, 607 (1972). (7) R . L. Collin, Anal. Chem.. 33, 605 (1961). (8) A . T. Kashuba and C. R . Hine, Anal. Chem 43,1758 (1971). A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 7, JUNE 1973
1045
carefully mixed in order to obtain a homogeneous distribution of the desired element in the final sample matrix. Batch extractions generally require less resin and the sample has a homogeneous distribution of the sought element. The addition of a binder, while necessary for pelletizing some resins, is objectionable because it dilutes the sample and adds to the risk of sample contamination. It would be highly desirable if a rapid, quantitative batch extraction could be achieved with a small quantity of an ion exchange resin which is highly selective for ions of the element or elements being determined. If such selectivity is not present, the chemistry of the functional groups on the resin and the ions in solution must be manipulated to accomplish the same result. In addition, a method of pelletization without the need of a binding agent would be desired. For a few ions, sufficiently selective resins may be available. However, because a selective ion exchange resin is not available for every ion of interest, a combination of approaches is required. Of the various types of ion exchange resins, those containing chelating functional groups frequently have the greatest distribution coefficients and, occasionally, remarkable selectivity. This report deals with the applications of two chelating ion exchange resins as the extracting agent and the sample matrix for the determination of some trace elements in geological samples. These samples were chosen because of their chemical complexity and the interest in procedures for the determination of trace elements in these types of materials. Three possible approaches to the problems of selectivity of extraction are considered. The determination of Co and Ni in silicate rocks represents the least favorable case in which interfering elements are removed by a prior ion exchange column separation. The elements removed are major constituents which would load the chelating resin and prevent quantitative extraction of the trace elements sought. The determination of Bi in silicate rocks and ores illustrates the use of chemical manipulation (pH) to achieve selectivity. The determination of gold in silicate rocks illustrates the application of a resin with a high selectivity for that element. Additional specific examples of the use of chelating resins have been reported recently by us (4-6) and others (1, 9).
EXPERIMENTAL Apparatus. The analyses were performed using a Philips-Norelco P W 1410 vacuum spectrograph powered by a Philips-Norelco XRG 5000 power supply with a 100 KV option. The target source, analyzing crystal, and detector were varied to optimize the signal-to-noise ratio. The resin samples were pelletized using a $-inch diameter hardened stainless steel die, a t a pressure near 30,000 psi, which was simultaneously heated to 150-180 "C for approximately 10 minutes using a hot air gun a s previously described (5). Reagents. Chelex 100 (100 to 200 wet mesh) was obtained from Bio-Rad Laboratories. The resin was washed with a hot solution of disodium EDTA to remove any trace metal contamination, and thoroughly rinsed with distilled water. The resin was then partially dried a t a temperature below 60 "C and stored in a desiccator containing a saturated solution of NaHS04 to maintain a constant humidity as described previously (5). NMRR (16 to 50 mesh) was obtained from Ayalon Water Conditioning Co., Ltd., Haifa, Israel, ground under water (100 to 200 wet mesh), dried, and stored in the same manner as Chelex 100. All other chemicals used were of reagent grade. Procedure. Determination of Cobalt and Nickel. Four USGS standard rocks were analyzed. These rocks were designated BCR-1 (Basalt), DTS-1 (Dunite), PCC-1 (Peridotite), and AGV-1 (Andesite). The description of these samples and the sampling and processing techniques employed in obtaining these samples is covered by Flanagan 110). Accurately weighed portions of these (9) S. L. Law, Sclence. 174, 285 (1971). (10) F. Flanagan, Geochim. Cosmochim. Acta. 33, 81 (1969)
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A N A L Y T I C A L C H E M I S T R Y , VOL. 45, N O . 7, J U N E 1973
rocks were dissolved in approximately 20 ml of hydrofluoric acid (4970) and about 2 ml of concentrated nitric acid. This mixture was warmed for several hours. Subsequently the H F was evaporated to dryness. The resulting residue was dissolved in approximately 50 ml of IN HF and stirred overnight. Following the second dissolution, any remaining residues were filtered using a plastic filtering funnel with 8-cm filter paper. The residues were then washed several times with small volumes of 1N H F to remove any metal ions possibly adhering to the residue. These residues seldom exceeded 0.5 mg and were assumed to be acid resistant silicates or oxides. Since these rocks contained from 5-1070 iron, and also since the cobalt K a peaks are very close to the iron K@peaks. it was necessary to remove the iron from the samples 110). This was accomplished using the ion exchange procedure of Danielsson and Ekstrom (11). The only significant change in this procedure is that a smaller amount of resin was used in our columns. The columns were prepared from a n H F resistant material. Teflon (Du Pont) wool replaced the normal glass wool plugs. The column was packed with 0.5 gram of Dowex 50-X8. The resulting column volume was approximately 1.5 ml. A flow rate between 1.5-2.5 ml per minute was used. Although the elution curves and other data reported by Danielsson and Ekstrom appear valid, further studies were made using the sample matrices involved in our particular problem. The filtrate previously described was passed through this column and the column was washed with LV H F to remove any of the iron fluoride complex which may have remained on the column. The presence of iron in the effluent liquid was detected by using a concentrated aqueous solution of potassium thiocyanate. I t was found t h a t after 20-25 ml of LV H F was passed through the column, a negative test for iron was attained. This volume was followed by a n equal volume of the same solution. The column was then rinsed with 20 ml of distilled deionized water. The trace metals on the column were eluted with 60 ml of 41V "03, this effluent liquid was collected in a Teflon beaker. The column was again rinsed with H 2 0 and this liquid was also collected. Following elution of the metals from the column with 4 N H S 0 3 , the effluent was evaporated to dryness. The use of the residue as a sample form is not practical because of lack of homogeniety and difficulty in treatment. Therefore. the residue was dissolved in 100 ml of H20. After dissolution, there was usually less than 0.01 mg of residue remaining, which was neglected. The analysis of these sample solutions was affected by concentrating the metal ions on 200 mg of Chelex 100 in the sodium form, This provides a convenient, homogeneous sample with a reproducible matrix. This was done by tumbling the metal-containing solutions with the resin in 500-ml polyethylene bottles. The pH of the solution in contact with resin was adjusted to 9.0 f 0.1, and the bottles were rotated on a device a t approximately 55 revolutions per minute. After tumbling the mixture for three hours, the resin was collected on fine glass filter frits, dried, and subsequently pressed into small pellets. The actual analysis was done by preparing calibration curves for nickel and cobalt. This was done by adding aliquots of standard metal solutions to the resin and treating them as has been previously described. A blank sample was also prepared by simply emulating the sample dissolution, separation, and equilibration procedures for which no rock sample was used. but all reagent volumes were the same as were employed in dissolving rock samples. Determination of Bismuth. The samples used for bismuth determination are geochemical standard samples provided by Kennecott Exploration. Inc. The USGS samples used for the cobalt and nickel determination do not contain enough bismuth for testing purposes. Two procedures were used for the digestion of rock samples containing bismuth. Most samples were dissolved using a mixture of concentrated hydrofluoric and nitric acids according to procedures already well described in the literature (12). A few samples were dissolved in boiling concentrated nitric acid following the procedures described by Ward et al. (13). Samples of ground rock weighing from 1 to 20 grams were weighed into Teflon beakers, dampened with a few milliliters of water, and attacked in 40% H F containing a few milliliters of (11) L . Danielsson and T . Ekstrom, Acta Chem. Scand., 20, 2402 (1966). (12) J. Dolezal, P. Porondra, and 2 . Sulcek, "Decomposition techniques in inorganic analysis." English Translation, D. 0. Hughes Ed., American Elsevier Publishing Company, New York. N.Y., 1968. (13) F. N . Ward, H. M. Nakagawa. T. F. Harms, and G . H. Van Sickle, "Atomic Absorption Methods of Analysis Useful in Geochemical Exploration," U.S.Geol Survey Buii. No. 1289 (1969).
concentrated nitric acid. After several hours of stirring a t low heat, the solution was allowed to evaporate to dryness. The samples were fumed to near dryness several times with nitric acid and then dissolved in 2N nitric acid. Any insoluble residue was then separated by filtration, washed with hot dilute 0.01M nitric acid and discarded. The combined filtrate and washings were transferred to a polyethylene bottle, diluted 2 to 3 times with distilled water, and the p H was adjusted to 2 using sodium bicarbonate or sodium acetate. Strong bases were not used since they promote the formation of polynuclear complexes of bismuth which do not redissolve except in strong acid (14). An aliquot of ascorbic acid 3 to 5 times in excess of the iron content was added and the solution stirred to effect the reduction of the iron. A 180milligram aliquot of Chelex 100 resin in the acid form was then added, the p H rechecked, and the solution-resin slurry equilibrated for 24 hours on a sample rotator. After equilibration, the resin was recovered by filtration, dried and pelletized. A few samples were weighed into borosilicate glass beakers, heated to near boiling for 2 hours in concentrated nitric acid. The solution was then diluted with an equal amounttof distilled water and brought to boiling. The residue was removed by filtration, washed, and the filtrate treated as described above. Determination of Gold. Samples used for gold determination include some of the geochemical standards provided by Kennecott Exploration, Inc., and others provided by the U.S. Bureau of Mines. Samples or rock powders ranging from 5 to 20 grams were heated to near boiling with stirring for 2 hours in aqua regia in covered borosilicate glass beakers. After dilution with water and cooling, the solutions were filtered to remove undissolved material. This material was rinsed with hot 1 : l O diluted aqua regia. Most of the solutions were then diluted to approximately 300 ml and the p H was adjusted to between 0.5 and 1 using 8M sodium hydroxide. After p H adjustment, 150 milligrams of dried, ground NMRR resin (100-200 mesh) was added to the solutions and the slurry equilibrated on a sample rotator for nearly 24 hours. The resin was then recovered by filtration, washed with dilute HC1 and water, dried, and pelletized following the procedure used for Chelex 100. Standard gold solutions were prepared by dissolving a weighed amount of pure gold wire in aqua regia and diluting to volume using a dilute aqua regia solution. Standard samples were prepared by equilibrating aliquots of the standard gold solution with 150-milligram lots of NMRR resin at a p H of 0.5. Recovery efficiency was tested by extracting 2 micromoles of gold with the resin. The filtrate was then extracted with a second lot of resin. The second lot contained about 0.01 micromole of gold indicating a recovery of nearly 99.5% for the first equilibration. The effect of equilibration time on recovery was studied a t Y4, 1, 6, and 24 hours in dilute aqua regia solutions. The 6- and 24-hour equilibration time resulted in quantitative recovery, within the precision of the analysis. All further samples were equilibrated for 24 hours to ensure complete recovery from rock samples.
RESULTS AND DISCUSSION
PH Figure 1. Per cent metal ion extracted by Chelex 100 as a function of pH Bi, TI - - - - ; Cu, W, Mo - - - - - - - ; . . . . , . , , . . . . . . ; Cd -0-0-; Zn, Co
Fez+ a
. .- . . -
i
Fe3+ ---; La, Pb, Ni Sr, Ba - . - - .;
.
- - -;
1
I
F 0 a
I
E 1
2
3
4
5
PH
6
7
8
1
2
3
4
5
6
7
8
PH
Fraction of metal ion extracted as a function of pH for several lots of Chelex 100. (Not corrected for X-ray absorption effects at high resin loading.) A: Lots 3636, 7193, and 9155, B: Lots 10278 and 9364. Figure 2.
Bi - - - - ; 0 -;
Sr-.
La . . . . , . . . . . . . . , . ; Zn - - -; C u - - - - -
-.
- -; Cd - 0 -
-
Quantitative recovery of cations by batch extraction with Chelex 100 depends upon the p H of the solutions, equilibration time, ionic strength of the solutions, oxidation state of those cations with multiple oxidation states, the presence of complex forming substances, and the quality of the resin itself. The effect of pH on the extraction of some cations is illustrated in Figure 1. Bismuth and thallium(II1) show a strong affinity for Chelex 100 with nearly quantitative recovery achieved a t a p H near 1. Ferric iron, copper, molybdenum, and tungsten are also taken u p a t low p H values. On the other hand ferrous iron, in the presence of an excess of a reducing agent such as ascorbic acid, is not strongly complexed below pH 6. From the p H profiles it is evident t h a t selective extraction of a few elements by p H control is feasible (uiz.,Bi3+ in the presence of Fez+). For most other cations additional manipulations will be required to eliminate interference from iron or other common elements. The pH profiles in Figure 1 were obtained with lots of Chelex 100 t h a t showed optimum properties. The proce-
dure used for all elements except W and Mo was to equilibrate dilute aqueous solutions a t the designated p H for 24 hours. W and Mo were recovered from solutions containing a 100-fold excess of hydrazine by heating and stirring a t the designated p H for 1 hour. Several resin lots that showed less than optimum properties were also studied. The results are shown in Figure 2. Reaction rates between cations and Chelex 100 a t low p H values are known to be slow because the exchange is between the cation and a hydrogen ion on a weakly acidic functional site (15). The effect of p H and equilibration time for the recovery of bismuth from dilute nitric acid solutions was studied. Up to 90% of the bismuth was rapidly recovered. Complete recovery required considerably more time, e.g., 11 hours a t p H 2 and 24 hours at pH 1. In a n earlier paper (4), barium recovery at a p H of 10.5 to 11 was shown to be quantitative in 15 minutes. In the pH profile, Figure 1, barium recovery was quantitative a t a pH near 4 after 24 hours. In some samples, a low extraction pH is required. For example, ions of a number of elements ( e . g . , Bi, La, Ti, Fe, Al, T1, W, Mo) behave as cat-
(14) G. Schwartzenbach and H. Flaschka, "Cornplexornetric Titrations," M e t h u e n , London, 1969.
(15) D. E. Leyden and A. L. Underwood, J . Phys. Chem., 68 2093 (1964)
ANALYTICAL C H E M I S T R Y ; VOL. 45, NO. 7, J U N E 1973
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ions in acid solutions, but tend either to hydrolyze and precipitate, or become anionic in neutral to basic solutions. Elements such as these are best extracted a t p H 2 to 2.5. In order to carry out extractions of other ions a t higher p H values, the elements mentioned above must either be absent, removed by appropriate extractive procedures, or masked by complexing agents to avoid possible occlusion of other elements in their precipitates. In NMRR resin, the active chelating agent consists of two resonating amino groups attached to a carbon atom (16). This ligand helps to stabilize the d8 electronic configuration and produce a square planar complex (16) The resin shows a marked selectivity for elements that tend to form these complexes, such as gold and the platinum group metals. Selectivity toward other transition or heavy metal cations is generally very low, although recently NMRR resin has been used successfully for the recovery of mercury and methyl mercury (9) In our procedure, we observed that gold recovery was not complete using the coarse resin as supplied. Grinding this resin not only made pelletization easier. but also increased resin-solution reaction rates and gold recovery. Many ion exchange resins can be pelletized without a binder if they are finer than 100 mesh. The coarser resins ( e . g . , larger than 50 mesh) are very difficult to pelletize and some type of binder is generally needed. A few resins, e . g . , Bio Rex 100 or its derivatives, may be pelletized simply by pressing near 30,000 psi. The pelletizing of Chelex 100 resin presents some difficulty. It must be heated to about 150 "C a t a pressure of 30,000 psi or more, especially if coarser than 100 mesh, in order to produce cohesion between resin grains. Also, Chelex 100 does not pelletize well if perfectly dry, and such pellets have a strong tendency to absorb water and swell. On the other hand if this resin contains too much moisture so that the resin beads are soft, the resin may be squeezed out of the die or moist pellets will be produced. These lose moisture upon exposure to the air. tend to stick in the die, and usually shrink in diameter and warp so that they no longer present a flat ' surface for X-ray analysis. A study was undertaken to determine the optimum amount of resin for use with pellets of 0.5-inch diameter. The effect of resin quantity on count rates was determined by weighing out aliquots of resin ranging from 60 to 600 milligrams and equilibrating these with 5 micromoles of either barium, mercury, or cobalt. Count rates for resin pellets decreased as the quantity of resin increased. This decrease is due to absorption of X-rays by the increasing resin mass, and by partial shielding of thicker samples by part of the centering mask used in the sample holder. The decrease in count rate also depends upon the energy of the X-ray line measured. Thus, the highest energy line (barium K a ) is the least affected and the lowest energy line (barium L a ) is the most affected by increasing resin quantity. In addition, background count rates also increased with increasing resin quantity so that signal-tobackground ratios were reduced. From the data, the optim u m resin weight for fabricating 0.5-inch diameter pellets using Chelex 100 is 200 milligrams. More or less than this amount can be used; however, the same amount must be used for all samples in a given set of analyses. Pellets weighing less than 80 milligrams or over 300 milligrams were difficult to produce. If larger amounts of resin are required for analysis, a larger diameter pellet would be preferable. Determination of Cobalt a n d Nickel. The determination of cobalt and nickel in silicate rocks has been an ac(16) G. Koster and G .Schmuckler, Ana/ Ch\m Acta, 38, 179 (1967)
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tive analytical area in geochemical studies. Fleischer has summarized several methods applied to this problem which include polarography, colorimetry, radiochemical methods, atomic absorption, X-ray fluorescence, neutron activation, and emission spectrography (17, 18). Flanagan has tabulated the results of cobalt and nickel determinations on U.S. Geological Survey standard silicate rocks performed using these various techniques (10). However, most analyses were performed by spectrographic methods. The range of values reported for all samples is large, and because many of the values were reported to Flanagan by personal communication, little is known about the details of the procedures used or the precision of the methods. The mean values, in parts per million, of cobalt and nickel determinations for the four different rock samples appear in Table I. The results were obtained using four replicate samples. The mean and range values for cobalt and nickel in each type rock, as published by Flanagan, are also shown in Table I. Except in the case of PCC-1, the experimental values for cobalt obtained by us are consistently higher than the mean. of those published by Flanagan, but within the range. Also, in the nickel determinations, three of four experimental values are higher than the published means; although these values do appear to agree better than do the cobalt values. However, with the wide ranges encountered in the published values, one may question the validity of comparison of any results. The results obtained by others using X-ray fluorescence seem to have a n exceptionally wide range (1 7, 18). The advantages of the technique described here are worth noting. First, it is relatively rapid compared to several other techniques; the speed is limited by the sample digestion. Second, the precision and accuracy is comparable to that of colorimetry and neutron activation in the parts per million range. Third, the same analytical pellets may be used for both metals. Finally, the analysis can be effected by the use of a simple calibration curve rather than by the use of synthetic or .natural matrices for the calibration curve. It should be noted that other metals ( e . g . , Cu, Zn, Pb, Cd) could be treated in a similar manner using the ion exchange separation procedure described by Danielsson and Ekstrom (11) to eliminate interferences due to iron or other common elements. Determination of Bismuth. As mentioned earlier, p H adjustment can be used to achieve selectivity for a few of the many cations which coordinate with Chelex 100 resins. The analysis for bismuth is a n example of this type of procedure. The inherent selectivity of Chelex 100 for bismuth is especially great, so that quantitative recovery is possible in dilute nitric acid solutions at a p H near 1 using a 24-hour equilibration period. In more complex solutions containing other ions, a p H of 2 is required to ensure quantitative recovery. Only a few elements are capable of interfering in the analysis as shown in Figure 1. Among the major elements, only ferric iron and aluminum interfere. Reaction rates between aluminum and the resin appear to be slow in cold solutions similar to reaction rates between aluminum and EDTA (14). Since ferrous iron is not taken up a t a p H of 2 by Chelex 100, iron interference can be eliminated by reducing ferric iron with ascorbic acid. This procedure, however, does not eliminate interference from other common trace elements such as Cu and P b which are also taken up a t a p H of 2. A few of the samples analyzed in this study, GRLD 102 and 105, contained a considerable amount of copper.
(17) M . Fleischer and R . E. Stevens, Geochim. Cosmochim. Acta, 26, 525 (1962). (18) M . Fleischer, Geochim. Cosmochim. Acta. 29, 1263 (1965).
~ _ _ _ _ _ _
~
~~~
~~
Table I. Means and Ranges (ppm) for Published Values of Cobalt and Nickel in AGV-1, BRC-1, PCC-1, and DTS-1 (70) Cobalt USGS Rock Types PCC-1 DTS-1 BCR-1
AGV-1
a
X
Nickel Range
112 (107 f 2 ) a 132 (186 6 f 0 8) 35 5 (47 f 2)
80-300
15 5 ( 1 7 1 f 0 7)
X
Range
2430 (2450 f 3 ) 2330 (2306 f 2)
1750-3400
29-60
15 0
8-30
10-30
( 1 9 3 f 1 6) 17 8 (23 8 f 0 3 )
1 1-27
96-200
1770-3300
Values in parentheses from this work
Sample size had to be kept small in order to prevent the copper content of solutions from overloading the resin. In addition, the presence of copper reduced the emission of bismuth by absorption of the characteristic bismuth X-ray line being counted. The effect of copper absorption was investigated by equilibration of standard samples with known amounts of copper. Samples with a significant amount of copper were analyzed for copper and corrections made to determine the bismuth content of the sample. These corrections were as large as 20% for a few samples. Results of bismuth determinations are presented in Table 11. T h e analyses are based on repeated measurements of aliquots taken from the same rock sample and on replicate rock samples. The reproducibility of measurements from the same rock sample is better than those between separate samples. Results of standard addition samples are essentially the same as those obtained by direct analysis. Solutions obtained by dissolving the sample using HF and nitric acid agree quite well with those obtained using nitric acid alone. The d a t a for nitric acid digestion indicates t h a t a H F attack may not be necessary. T h e variation in result$ for all samples is larger than expected. However, this could be due to variation in the bism u t h content of samples or to analytical problems a t low concentration levels. For X-ray analysis under optimum conditions using a 3.5-kilowatt molybdenum tube a t full power (100 kV and 35 milliamperes), a detection limit near 0.1 microgram of bismuth was obtained. This corresponds to 0.1 ppm in 1 gram of rock. Determinations as low as 50-100 parts per billion appear possible in some silicate rocks using these procedures if 10- to 20-gram samples were used. GRLD 100 is a n essentially unmineralized granite from Alta, Utah, which contains approximately 50-200 ppb bismuth. T h e results of analyses of this rock for H F treated samples were significantly lower than for those obtained using nitric acid. In all the HF samples, the aluminum content was high enough to cause some precipitation a t a p H of 2. Apparently some bismuth was lost either by occlusion with the hydrolyzing aluminum or the resin was overloadecf by aluminum. The use of THAM (Tris(hydroxymethyl)-aminomethane) to complex and mask aluminum was studied but it was found that a n excess of THAM resulted in low bismuth recovery. Alternative procedures for eliminating the precipitation of aluminum have not as yet been studied. Determination of Gold. NMRR resin (rurmerly known by the designation SRXL) has been used by Green et al. for the determination of gold in silicate rocks using both X-ray fluorescence and neutron activation to determine
Table I I. Results of Determination of Bismuth in Geochemical Rock Samples Sample No.
No. of analyses
Digestion procedure
Average Bi. ppm
15.08 f 2.2 16.0 f l b
GRLD 102 102 102
16
105
6
HF-"03 HN03 HF-"03 +Std Add. HF-"03
105 105
2
"03
114
2
HF-"03 +Std Add. HF-"03
114
115
2 2
HN03 HF-"03
0.42 f 0.05h 0.77fO.l'
115
2
"03
116 116 100
1 2 4
HF-"03 HN03 HF-"03
0.88 f O . l h 1.05 f 0.1" 1.02 f 0.10 0.05 f 0.015
100
4
HN03
0.18 f 0.06
2 2
f0.9 f 1.5 5.60 f 0 . 2 1 4.9 f 0.5O 5.1 f 1
KEI value, ppma
12
16.5 14.0
0.51 f C.35"
7.8
