Isotopic Analysis of Chromium in Lunar Materials by Mass Spectrometry of the Trifluoroacetylacetonate N. M. Frew,’ J. J. Leary, and T. L. Isenhour Department of Chemistry, University of North Carolina, Chapel Hill, N.C. 27514
A microtechnique for analysis of chromium in small samples of geological materials has been developed. Following acid dissolution of 1- to 2-mg samples of silicate rocks, chromium is converted to a volatile chelate by reaction with l,l,l-trifluoro-2,4-pentanedione (Htfa) in a sealed tube and extracted into hexane for subsequent analysis by isotope dilution and electron impact mass spectrometry. The chromium contents of USGS silicate rock standards PCC-1 (peridotite) and DTS-1 (dunite) and two Apollo 12 lunar samples (12070) and (12002) have been determined and are in good agreement with other reported values. In addition, the method has allowed determination of chromium isotope distributions in Apollo 11 fines (10084) and in Apollo 12 fines (12070), and four crystalline rocks (12002, 12038, 12052, and 12063), with uncertainties of 1%. The lunar chromium distributions are in agreement with terrestrial values and no significant variations were observed among the crystalline rocks or the fine materials. CLASSICALAPPLICATIONS of chelating agents in analytical chemistry depend primarily on the light absorbing properties of the metal chelate, which allow detection by conventional visible and ultraviolet spectrophotometry. In addition to high absorptivity coefficients, other properties such as selectivity, large stability constants, and favorable distribution coefficients for various solvent systems are used to advantage in extraction and preconcentration procedures antecedent to general instrumental methods, notably atomic absorption spectrophotometry. Interest in various fluorinated P-diketones has grown because of the excellent volatility and thermal stability characteristics of the complexes with a variety of metals. The enhanced volatility of the fluorinated derivatives makes them suitable for use with two additional methods of detection the gas chromatograph and the mass spectrometer. A number of applications to trace analysis of metals by both gas chromatography and mass spectrometry have been described, including analysis of steels, nonferrous alloys, ores, blood plasma, urine, and other biological materials (1-8). The two techniques compare very favorably in terms of sensitivity, both showing absolute detection limits for many metals in the Present address, Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543 (1) R. W. Moshier and R. E. Sievers, “Gas Chromatography of Metal Chelates,” Pergamon Press, Oxford, 1965. (2) W. D. Ross and R. E. Sievers, ANAL. CHEM., 41, 1109 (1969). (3) J. L. Booker, T. L. Isenhour, and R. E. Sievers, ibid., p 1705. (4) R. E. Sievers, J. W. Connolly, and W. D. Ross, J. Gas Chromatogr., 5 , 241 (1967). ( 5 ) M. L. Taylor, E. L. Arnold, and R. E. Sievers, Anal. Lett., 1, 735 (1968). ( 6 ) W. D. Ross and R. E. Sievers, Talanta, 15, 87 (1968). (7) J. Savory, P. Mushak, F. W. Sunderman, Jr., R. H. Estes, and N. 0. Roszel, ANAL.CHEM., 42, 294 (1970). (8) G. H. Booth, Jr. and W. J. Darby, ibid., 43, 831 (1971).
picogram range or less (9-14). The mass spectrometric method, however, offers advantages over gas chromatography in that interferences are less likely because of the greater attainable resolution and that isotopic analysis of metals is possible. This paper extends the use of fluorinated P-diketones to the analysis of geological materials. A specialized microtechnique is presented for isotopic analysis of chromium in small samples of geological materials by conversion of the chromium to the trifluoroacetylacetonate and subsequent analysis by mass spectrometry. The method was developed as part of the Apollo program, in order to obtain possible information on high energy spallation reactions during nucleosynthesis in the early history of the solar system. The stable isotope S4Cris an excellent spallation indicator, being unshielded with respect to decay of the short-lived isobars 54V and S4Mn. Coupled with computerized data acquisition, the analysis described has allowed measurement of chromium isotope distributions in lunar materials with uncertainties of 1 % or less. In addition, the chromium content of several USGS rock standards and lunar samples has been measured by stable isotope dilution. Errors due to complex matrix effects inherent in other instrumental methods (emission spectrography, spark source mass spectrometry, X-ray fluorescence, and neutron activation analysis) are avoided, and the use of extremely small samples (1 to 2 mg) compared to older wet methods is possible, with accuracies and precisions in the 1 range. EXPERIMENTAL
Measurements were made by electron impact mass spectrometry after dissolution of the geological samples and conversion of the chromium to chromium(II1)-trifluoroacetylacetonate. Reagents. All chemicals were of reagent grade quality and were used without further purification with exceptions as follows: H(tfa) (l,l,l-trifluoro-2,4-pentanedione; mol wt 154.09, bp 107 O C), obtainable from Pierce Chemical Co.; Standard Reference Material 979 (15), a chromium isotope standard available as Cr(N03).9H20from the National Bureau of Standards (absolute abundance ratios : J°Cr/s*Cr, 0.05186 =t 0.00010; 53Cr/52Cr,0.11339 f 0.00015; and 54Cr52Cr,0.02822 =t 0.00006);5Fr tracer available as 5OC1-203 (9) L. C. Hansen, W. G. Scribner, T. W. Gilbert, and R. E. Sievers, ibid., p 349. (10) B. R. Kowalski. T. L. Isenhour. and R. E. Sievers. ibid.., 41., 998(1969). (11) W. D. Ross and R. E. Sievers, Decelop. Appl. Spectrosc., 8 , 181 (1970). (12) R . Belcher, J. R. Majer, R. Perry, and W. I. Stephen, Anal. Chim. Acta, 43, 451 (1968). (13) J. K . Terlouw, J. J. deRidder, W. Heerma, and G. Dijkstra, 2. Anal. Chem., 249,296 (1970). (14) J. K. Terlouw and J . J. deRidder, ibid., 250, 166 (1970). (15) W. R. Shields, T. J. Murphy, E. J. Catanzaro, and E. L. Garner, J. Res. Nut. Bur. Stand., 70A, 193 (1966). ~
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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Table I. Standard Instrument Operating Conditions for Isotope Ratio Measurements Using Cr(tfa), Ion chamber temperature 50-70 "C Electron accelerating voltage 70 V Ionizing current 100 pA Ion accelerating voltage 8 kV Electron multiplier 1.4-1.8 kV Magnet current setting - 45 Resolving power lo00 Bandwidth 10 Hz Peakwidth minimum Mass ratios as xx vv XXCr(tla)z+/Y~Cr(tm)z+ 52 50 1.005603 53 52 1,002793 54 52 1.005582 Interface threshold ov Bias ov 4.5 kHz Sample frequency
__
Table 11. Comparison of Cr Isotope Ratios Observed for Cr(tfa),+ Ion with True Cr Isotope Ratios 5OCr/Wr 53Cr/52Cr 5 'Cr/5 2Cr 0,0520" 0.11350 0,0284" 5oCr(tfa)z+/52Cr(tfa)z+ 53Cr(tfa),+/Wr(tfa),+ s4Cr(tfa)z+/52Cr(tfa)2+ 0.0518 0.2256 0.0547 a Based on values of Svec et al. (20) for terrestrial.
from Oak Ridge National Laboratory (isotope assay: 95.9% 6oCr,3.76% 52Cr,0.26% 53Cr,0.05% 54Cr). A chromium isotope standard solution was prepared by dissolving 19.3 mg of the Cr(N03).9Hz0 in 10 ml of distilled water to give a concentration of 0.251 pg of Cr/pl, A 6oCr tracer solution was prepared by fusion of 16.2 mg of 5oCr2O3 in NazC03/Na202;the fused cake was dissolved in dilute H N 0 3 and diluted to an approximate concentration of 1 pg of Cr/pl; the exact concentration of the tracer was 'subsequently established by a back-calibration using a standard Cr(V1) solution prepared by dissolving 275.792 mg of twice recrystallized KzCrz03in 100 ml of distilled water to give a final concentration of 0.9749 p1 of Cr/mg solution at 25 "C. Samples of USGS BCR-1, PCC-1, and DTS-1 were kindly provided by Francis J. Flanagan of the U.S. Geological Survey. For determination of chromium content, the USGS samples were dried at 110 "C for 1 hour. The lunar samples received from NASA for study included four crystalline rocks, 12002,141; 12038,60; 12052,46; 12063,68; and a sample of fine material, 12070,8, from the Apollo 12 landing site; a sample of fine materials, 100084,145, taken from the Apollo 11 site was also examined. The crystalline rock samples were crushed to a workable size in a Plattner diamond mortar and analyzed without further homogenization or drying. Procedure. Portions of the lunar samples ranging from 1 to 2 mg were weighed into 2-inch sections of low density polyethylene tubing (0.106 x 0.138 in.) sealed at one end. If the chromium content was to be determined, an amount of the 50Cr tracer solution containing approximately the same weight of chromium estimated to be in the sample was also weighed into the tube, using a microcapillary pipet. Twenty microliters of concentrated H F (48 %) were added, the tube was heat-sealed, and placed in an oven at 80 "C for 2 hours. The solution products were transferred to a borosilicate glass tube (14 mm X 7 mm) using 200 pl of HC104 (72 %) or a 2 :1 mixture of H3P04/HC10ain three rinses; the tube was sealed, and the contents were oxidized during a second heating for 2-8 hours at 215 "C. After being cooled, the tube was opened and the contents were buffered to pH 5 with sodium acetate; approximately 200 pl of H(tfa) were then added, and the tube was resealed for heating at 110 "C for 2 hours. The Cr(tfa), produced was then taken up in 0.5 to 1 ml of hexane 666
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
and washed with 10 ml of 1N NaOH, shaking vigorously for 2-3 minutes. The organic layer was drawn off, and aliquots were introduced to the mass spectrometer. The instrument used was an Associated Electronics Industries MS-902 high resolution mass spectrometer equipped with a direct insertion probe. Because of various effects leading to signal irreproducibility, e . g . , sample volatilization rate, instrument noise and drift, etc., absolute abundance measurements were not attempted ; instead, isotope ratios were measured using the peak-switching circuitry of the MS902, which alternately scans two different mass positions by means of rapid voltage switching. The mass spectrometer is coupled to a Digital Equipment Corporation PDP-8 computer and analog-to-digital converter through an Applied Data Research interface. As described elsewhere, slight modification of this system, using the peak-switching circuitry of the mass spectrometer and data collection and integration programs produces direct measurement of peak area ratios to a relative standard deviation of 0.1 to 1 % (16). Standard instrument operating conditions were as listed in Table I. Aliquots of the hexane solution of Cr(tfa)3 were evaporated onto the tip of the direct insertion probe and, with the tip retracted, the probe was inserted into the source. With the accelerating voltage off and the filament current at zero, the output of the oscilloscope amplifier was adjusted such that sweep integrals of approximately zero for no signal input were obtained; the accelerating voltage and filament current were then switched on, and the magnet current was adjusted to focus the appropriate mass fragment on the collector slit. All measurements were made on the Cr(tfa)2f fragment since this peak is the most intense in the spectrum and also appears in a mass region (m/e 356-360) which is virtually free of independent organic interferences. Because of the occurrence of I3C and I8O in the Cr(tfa)S+ ion, the chromium isotope distribution is significantly modified to give an "apparent" isotope distribution; the approximate equations relating the true and apparent distributions (ignoring low abundances of 2H, I7O,and 14C)are given by A'(Wr) = A(50Cr)P(12Clo1604) (14
= 0,886 A ( W r )
A'(62Cr)
=
+
A(52Cr)P(12Clo1B04) A( W r ) [ P (12C1
+ P( 2Cn 3C2
l60 4)1
+ 0.0121 A(Wr) (1b) A'(63Cr) = A(S3Cr)P(12Cln1604)+ A(52Cr)P(12C913C11B04) = 0.886 A(53Cr) + 0.0994 A ( W r ) (1c) A'(54Cr) = A(64Cr)P(12Clo1604)+ A(53Cr)P(12C913C11B04) + = 0.886 A(S2Cr)
+ + 0.0994 A(Wr) +
A(62Cr)[P(12Cl~'6031801) P(12C~13C~1604)] = 0.886 A(54Cr)
0.0121 A(32Cr)
(1d)
where A and A' represent the true and apparent relative abundances, respectively, for each isotope, and P represents the probability of occurrence of the indicated combination of 13C and 1 8 0 . Table I1 compares the actual and calculated apparent chromium isotope ratios for the Cr(tfa)*+fragment. The use of a chromium isotope standard in the form of Cr(tfa)s allows direct comparison of the measured ratios between terrestrial and lunar samples, since the corrections are identical. Standard terrestrial samples were prepared using milligram quantities of USGS BCR-1 (natural chromium content approximately 20 ppm), which were spiked with 3.5 pg of chromium prepared from NBS Reference Material 979; use of the standard terrestrial samples also served to calibrate the instrument for discrimination effects due primarily to the (16) N. M. Frew and T.L. Isenhour, ANAL.CHEM., 44,659 (1972)
voltage switching, and for daily variations in instrumental parameters. A series of 100-250 sweep integrals per peak were collected alternately as the chromium chelate evaporated from the probe and were averaged in groups of ten to give 10-25 values for the ratio being measured; the standard deviation for such a series of averaged values was typically 0.5 after rejection of any erratic results. Widely deviating values traceable to momentary disturbances in the instrument electronics, were rejected if their deviation from the series mean exceeded f0.05 times the standard deviation of the series. For quantitative determinations of chromium content, a slightly different procedure was followed in that the individual sweep integrals for each mass were punched onto paper tape for processing on a larger computer. The data points corresponding to the low mass and high mass envelopes were treated by threepoint and two-point smoothing, respectively, to compensate for the time offset between measurements of the low and high mass sweep integrals, and for the unevenness of the evaporation process (Figure 1). This procedure is necessary for samples which evaporate rapidly from the probe and considerably reduces the spread of the measurements. Resulting ratios were then averaged in groups of ten as before. A series of quantitative blank determinations yielded a mean of 0.043 pg of Cr; corrections applied for this level of contamination were generally less than 1 %. RESULTS AND DISCUSSION
Chemical Considerations. Chromium occurs in terrestrial rocks and in the lunar samples primarily as chromite (FeO Crz03),picotite, and other spinels imbedded in a matrix of silicate minerals. The acid dissolution described has been shown to be effective on a variety of silicates and other minerals; the silicate matrix is easily decomposed within 30 to 120 minutes using concentrated HF, leaving an opaque fraction containing chromite; the chromite is then brought into solution using a concentrated perchloric/phosphoric acid mixture as a solvent (17), prior to reaction with the chelating agent. Dissolution using perchloric acid alone is incomplete. Of the fluorinated 0-diketones, H(tfa), H(hfa) (1,1,1,5,5,5hexafluoro-2,4-pentanedione), and H(fod) (1,1,1,2,2,3,3-hepe
(17) J. A. Maxwell, “Rock and Mineral Analysis,” John Wiley & Sons, New York, N.Y., 1968, pp 94-95.
0
10
40
60
80
120
100
TIME
140
160
180
200
220
(SECONDS)
Figure 1. Time-resolved trace of 52Cr(tfa)z+(m/e 358) and 5*Cr(tfa)%+ fragments as Cr(tfa), evaporates from insertion probe tip at 50 “C
tafluoro-7,7-dimethyl-4,6-octanedione)were considered for use; the choice of H(tfa) was based on several considerations: intermediate volatility ; intermediate stability; and freedom from undesirable side reactions during the synthesis. While Cr(hfa), is considerably more volatile, and would enhance detection limits, Cr(tfa), is superior for precise determination of isotope ratios; Cr(tfa), evaporates slowly from the tip of the insertion probe at 50 “C and provides a relatively steady ion current over a period of 10-15 minutes for samples as small as 5-10 pg. Since the iron content of the samples is high, 54Feacts as an interference in the measurement of the 54Cr/52Crratio and must be removed. The chromium complex is kinetically inert to ligand exchange, and removal of the iron can be accomplished by back extraction into the aqueous phase with a quick washing of sodium hydroxide; such a procedure is ineffective with Fe(fod), since its stability is enhanced relative to Fe(tfa), by the inductive effect of the rerr-butyl group of the ligand. In addition, H(fod) is unsuited because of its tendency to form soap-like salts of sodium and potassium when large amounts of these elements are present. H(hfa) is troublesome due to a tendency to react with water to
xl
I c:P
Cr(TFA)j 511
*
t v)
5c E
x 10
: IO
250
300
350
do
4jo
560
m/e
Figure 2a. Mass spectrum of pure chromium trifluoroacetylacetonate, Cr(tfa), ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
667
xl
Fe Fe
Cr
Fe Fe
AI I
1 1
300
250
350
,
I . .
1
I
1
400
450
500
m/e
Figure 2b. Mass spectrum of hexane extract from lunar fines 12070 before removal of iron by back-extraction with 1N NaOH
Xl
Cr
Cr
:r 1
I
"I.
Cr 1
r..l
1
I.
11.
AI
1.1
I.
, I
x 10
I
form the tetrahydroxy addition compound in aqueous solution (18). The reaction using H(tfa), however, produces a clean solution of Cr(tfa)s, Al(tfa)s, and Fe(tfa)a in the excess H(tfa), leaving a clear aqueous phase and hydrous silica; a washing with sodium hydroxide then easily removes the Fe(tfa)s as well as excess ligand. The spectra of Cr(tfa)a and of a hexane extract derived from a sample of lunar fines 12070 are shown in Figures 2a and 26, respectively. The isotope grouping of chromium is easily recognizable for many of the fragments; several of these were peak-matched to confirm their identities as fragments of Cr(tfa)s; other identifiable peaks include fragments of Fe(tfa)s and Al(tfa)S. Figure 2c (18) B. G.Schultz and E. M. Larsen, J. Amer. Chem. Soc., 71, 3250 (1949).
668
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
.
I.
I
shows a spectrum of the same lunar sample after the sodium hydroxide washing; the resulting extract contains the chromium chelate along with a small amount of the aluminum chelate; no peaks due to the iron compound are discernible. Other metals present in 12070 in significant quantities and which have isotopes which might contribute as interferences ( e x . , titanium) do not appear t o form chelates under the conditions used. A small unidentified peak is visible at m/e 355;however, the m 1 peak would contribute less than 1 to the W r peak at m/e 356,assuming approximately the same number of carbons in both fragments. The simplicity of the spectrum in Figure 2c allowed the procedure to be used with confidence for the measurement of chromium isotope ratios in the lunar materials. Chromium Isotope Ratios in Lunar Samples. The results of repeated measurements of chromium isotope ratios in the four
+
Table 111. Summary of Chromium Isotope Ratios in Terrestrial, Meteoritic, and Lunar Materials Sample type “Cr/62Cr 5 3Cr / 6 *Cr s°Cr/52Cr NBS Isotope Ref. 979” 0.02822 0.11339 0.05186 Terrestrialb 0.0284 0.1135 0.0520 Meteoriticc (stone) 0.0282 0.1135 0.0519 (iron) 0.0280 0.1138 0.0522 (chondrules) 0.0282 0.1136 0.0517 12002,141 (cryst. rock) 0.0283 f 0.0002 0.1133 f 0.0006 0.0521 f 0.0014 12038,60 (cryst. rock) 0.0281 f 0.0003 0.1136 f: 0.0004 0.0514 f 0.0004 12052,46 (cryst. rock) 0.0278 =t 0.0003 0.1120 i. 0.0008 0.0510 rt 0.0009 12063,68 (cryst. rock) 0.0281 =I= 0.0003 0.1125 =t 0.0007 0.0520 5 0.0011 12070,8 (fines) 0.0283 f 0.0003 0.1135 f 0.0007 0.0523 f 0.0012 10084,145 (fines) 0.0288 5 0.0007 0.1138 rt 0.0013 0.0521 i.0.0005 a Shields et al. (15). * Svec et al. (20). Murthy and Sandoval (21).
crystalline rock chips and two fine samples are given in Table 111, along with a summary of terrestrial and meteoritic ratios measured to date. The standard deviations of the lunar ratios (measured as S4Cr/Wr, 58Cr/52Cr,and 60Cr/52Cr) are generally 1 or less, except for a few cases where spurious background contamination raised the noise level. As with both terrestrial and meteoritic materials, some variations occur in the lunar samples; however, the variations are generally of the same order as the precision of the measurements and unequivocal conclusions as to the meaning of the observed differences are not possible. There appedr to be no significant differences between the crystalline rocks and the fine materials; further, the lunar materials are isotopically very similar to both terrestrial and meteoritic materials (19-21). The lunar distributions generally agree with suggested terrestrial values within estimated error limits (1 u). Of possible significance is the 54Cr/5ZCrratio observed for the Apollo 11 fines (10084), which is about 1,SZ higher than the average terrestrial value; however, the variance for this series is also high, and a meaningful estimate for a particle flux differential cannot be made. The deviations in the 54Cr/52Crratio expected on the basis of certain nucelosynthesis models are much larger. Following considerations analogous to those of Murthy and Sandoval (21), an orderof-magnitude estimate of 54Crproduction for the high proton flux postulated by Fowler et al. (22), is roughly 2000-3000 ratio would thus ppm. The expected value for the 64Cr/52Cr be in the neighborhood of unity, measurably different from terrestrial values. The observed uniformity of the results leads one to conclude that either the samples of the lunar regolith studied do not constitute primordial matter and have undergone the same isotopic homogenization as that theorized for terrestrial and meteroritic samples; or the high energy particle flux intensity during the early formative stages of the solar system was much lower than has been estimated. Chromium Content by Isotope Dilution. Stable isotope dilution has been applied to chromium content determinations in the lunar samples with all of the advantages of an internal standard method. Chelates of acetylacetonate and dibenzoylmethane have been used previously for the deter-
Table IV. Error Magnification in Determination of Chromium for Various Values of R, Weight of sample chromium x in terms of weight of tracer 62Cr/S0Cr Error in x for 1 y used isotope ratio, R, error in R,, 1.ooy 0.873 1,09 (minimum) 1 . 7 7 ~0.560 ; y 1.47;0.515 1.11 4 . 0 4 ~ 0.243 ; y 3.02; 0.249 1.2 10.73 y; 0.092 y 6.35; 0.119 1.5 2 1 . 8 6 ~ 0; . 0 4 6 ~ 9.65; 0.079 2 84.73 y; 0.011 y 15.4; 0.049 5 180 y; 0.005 y 17.3; 0.044 10
mination by isotope dilution of copper and iron in artificial mixtures prepared from the pure chelates (23). This section demonstrates the applicability of the method using trifluoroacetylacetone to analysis of a difficult sample type, namely, silicate rocks. The general expression for a weight x of an element having n isotopes, to which a weight y of the tracer has been added is
i=l
where A,, Re, and R, are measured ratios for isotopes i and k in the normal sample, the enriched tracer, and the mixture, respectively; the At and Bi are the abundances of the isotope i in the natural and enriched samples, and Mi is the molecular weight for isotope i. Of the three possible isotope pairs for chromium, the 52Cr/60Cr pair was selected since 5 0 is of naturally low abundance and can be obtained as a tracer in high abundance; also the corrections for I T , l*O, etc., are the simplest for this pair. The accuracy of the analysis is enhanced by consideration of possible errors resulting from the use of Equation 2, which simplifies to
(3) (19) G. D. Flesch, H. J. Svec, and H. C. Staley, Geochim. Cosmochim. Acta, 20, 300 (1960). (20) H. J. Svec, G. D. Flesch, and J. Capellen, ihid., 26, 1351 (1962). (21) V. R. Murthy and P. Sandoval, J. Geophys. Res., 70, 4379 (1965). (22) W. A. Fowler, J. Greenstein, and F. Hoyle, Geophys. J., 6 , 148 (1962).
if errors in Rm only are considered, it can be shown (24),that (23) J. K. Terlouw and G. Dijkstra, Preprint of International Conference on Mass Spectrometry, Brussels, August 1970. (24) R. K. Webster, “Mass Spectrometric Isotope Dilution Analysis” in “Methods in Geochemistry,” A. A. Smales and L. R. Wagner, Ed., Interscience, New York, N.Y., 1960. ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
669
Table V. Individual Determinations of Cr Concentration for Standard Dichromate Solution (True Concentration 975 ppm) Amount Cr, pg Used Found 10.325 10.20 9.848 9.84 7.646 7.72 4,468 4.45 3.692 3.73 2.972 2.90 2.607 2.61 0.855 0.840 0.812 0.823 0.813 0.811
Sample 1 2 3 4 5
6 7 8 9 10
Re1 error,
z
-1.2 -0.1 +0.9 -0.3 +0.9 -2.3 +0.2 -1.7 +1.4 -0.2
Equiv Ratio, concn, Rm PPm 1.444 963 1.023 974 984 0.890 972 0.812 984 0.848 0.760 952 1.116 977 0.223 958 987 0.231 973 0.203
the magnification of errors in the measurement of R, is a minimum for Rm = (Rn X (4) Table IV lists ranges of 52Cr/50Crratios and the minimum error incurred at the limits of these ranges for a 1% error in the measurement of R,; the minimum error is 1.09% for R, = 0.873, and all measurements were made as close to this optimum ratio as possible; in the case of chromium this is convenient since equal amounts of sample and tracer chromium yield a ratio of 0.873, identical to the optimum value. For accurate determinations, a back-calibration or reverse isotope dilution must be performed to correct for an inaccurate knowledge of the tracer concentration and for instrumental discrimination ; the latter is particularly important in this case, since the rapid voltage switching used has been shown to cause discrimination on the order of 3-4z.The apparent concentration of the tracer is determined using a standard solution of high purity KzCr20,; the quantity of chromium in the sample can then be related back to the known concentration of the standard dichromate solution as follows: x = =
Yappar
Xatd
x
X
D[R,(sam) - Re1 DlRn - Rm(sam>l
D[Rn - Rm(~td)l D[R,(sam) - Re] X D[Rn - R,(sam)l D[R,(std) - Re]
(5)
D is a discrimination factor introduced by the instrumental measurement. Equation 5 assumes that Rm, Rn, and Re are all measured experimentally and provides for exact cancellation of systematic errors. A practical difficulty is that measurement of each of the three ratios introduces random error on the order of 1 %; however, since the values of Rn and Re were well established in this case, it was possibe to measure R, only and thus incur a smaller error. Equation 5 is altered to give x =
Xatd
x
DR,(sam) - Re Rn - DR,(std) X Rn - DR,(sam) DR,(std) - Re
The error due to non-exact cancellation is less than 0.5% for values of R, in the range 0.25 5 Rm 5 3.7 (D = 1.035), and is exactly zero for R,(sam) = R,(std). The expected error is thus held to the combined errors of the tracer calibration and the measurement of R,, i.e., to 1 or 2 %, well within the expected inhomogeneity of the geological samples. The degree of accuracy attainable is illustrated in Table V, which lists the results of 10 separate determinations of chromium in samples of the standard dichromate solution; the samples contained from 1-10 pg of chromium, the approximate range expected in the lunar samples, assuming a sample size of 1-3 mg. The relative error in absolute amount of chromium ranges from 0.1 to 2 . 3 z . Values for the equivalent concentration of the standard solution (Le., corrected for dilutions), show a mean of 972 ppm us. the true value of 975 ppm (- 0.3 % error), with a standard deviation among the ten samples of 12 ppm (1.2%). Since additional errors may be inherent in an actual rock analysis due to chemical differentiation, the method was tested on two U S Geological survey standards, PCC-1 (peridotite) and DTS-1 (dunite), which have chromium contents similar to that of the lunar materials. The results of multiplicate analyses are reported in Table VI and are compared with data already compiled for these standards by other methods. For DTS-I, the agreement with most of the other values reported is excellent. Less agreement exists among the values reported for the chromium content of PCC-1; the mean value obtained in this work agrees within the estimated error limits with that reported by Sievers et al. for the same sample split/position, using electron capture gas chromatography
Table VI. Chromium Contents of USGS Silicate Rock Standards PCC-1 and DTS-1 (weight per cent) Sample DTS-1 (dunite) PCC-1 (peridotite) Analytical method 2716 Split/position" 8/26 mass spec,; vol. chelate 0.266 i 0.010 0.401 =k 0.006 This work gas chrom; vol. chelate 0.273 f 0.008 0.394 ZIZ 0.017 1 emission spectrography 0.285 0.397 2 X-ray fluorescence 0.255 0.406 3 emission spectrography 0.276 0.400 4 emission spectrography 0.300 0.420 5 emission spectrography 0.215 0.301 6 a
Sample split/position for this work and laboratory 1 only.
Sample This work 1 2
3 4 5
670
0
Table VII. Chromium Contents of Apollo 12 Samples (weight per cent) 12002 Analytical method 12070 0.599 f 0.024 mass spec. ; vol. chelate 0.270 f 0.014 0.621 i 0.031 gas chrom.; vol. chelate 0.270 f 0.016 0.28 spark source mass spec. 0.28, 0.303 emission spectrography 0.227, 0.243,0.248 0.56 activation analysis 0.26,0.28,0.29,0.30 0.540.67 0.208,0.29 X-ray fluorescence
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(6)
(25). Results for PCC-1 obtained by emission spectrography show considerable variations. The results of chromium analyses on a sample of Apollo 12 fines (12070,8) and a crystalline rock (12002,145) are shown in Table VII. Excellent agreement with other reported values which show a mean value of 0.267% chromium is obtained for the fine materials (26); the natural variation in the content of the fines appears to be about 5% based on the standard deviations of the results. As expected, inhomogeneities in the crystalline rock are more pronounced; a 0.024% chromium was obtained, as comvalue of 0.599 pared with the results of Sievers et a f . (0.621 =t 0.031%), Goles (0.56%), and Brunfelt (0.54-0.65 %) (25, 27, 28). The chromium content of 12002 is apparently the highest of
the Apollo 11 and 12 samples. Results for crystalline rocks 12004 and 12009 obtained by LSPET (29) also show high chromium contents (0.58 % and 0.52 %, respectively); these samples were gathered from the same vicinity on the lunar surface as rock 12002, and it has been suggested that their similarly high chromium contents may indicate a common origin for these rocks (25).
(25) R. E. Sievers, K. J. Eisentraut, D. J. Griest, M. F. Richardson, W. R. Wolf, W. D. Ross, N. M. Frew, and T. L. Isenhour, Proceedings of the Apollo 12 Lunar Science Conference, MIT Press, Cambridge, Mass., in press. (26) G. H. Morrison, ANAL.CHEM.,43 (7), 22A (1971). (27) G. G. Goles, A. R. Duncan, M. Osawa, M. R. Martin, R. L. Beyer, D. J. Lindstrom, and K. Randle, Proceedings of the Apollo 12 Lunar Science Conference, MIT Press, Cambridge, Mass., in press. (28) A. 0. Brunfelt, K. S. Heier, and E. Steinnes, Proceedings of the Apollo 12 Lunar Science Conference, MIT Press, Cambridge, Mass., in press.
RECEIVED for review June 21, 1971. Accepted November 24, 1971. Work was supported by the Materials Research Center, University of North Carolina, under contract DAHC 15-67-C-0223 with the Advanced Research Projects Agency. We also acknowledge support from the Biotechnology Resources Branch of the Division of Research Resources, NIH, under Grant PR-330.
*
ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of L. E. Wangen in the experimental work. We also thank David Rosenthal for his cooperation in our use of the mass spectrometer facilities of the Center for Mass Spectrometry, Research Triangle Institute, Durham, N.C.
(29) LSPET (Lunar Sample Preliminary Examination Team), Science, 167, 1325 (1970).
Establishing Water Contents of Hydrogen Form Resins by Heats of Immersion Merlin D. Grieser, Alan D. Wilks,’ and Donald J. Pietrzyk University of Iowa, Department of Chemistry, Iowa City, Iowa 52240 Heats of immersion in water and several organic solvents were determined by calorimetry. Various experimental steps in handling the cation resins and solvents were evaluated. Water contents of the resin were determined by Karl Fischer titration and weight loss. Comparison of the two methods established that the former method is unsuitable for the determination of low levels of water. Procedures for the preparation and storing of “reproducibly dry” cation resin and for the determination of low water levels based on heat of immersion are described and evaluated.
CHARACTERISTICS OF ION EXCHANGE RESINS must be established if fundamental investigations of their properties are to be meaningful. Thus, homogeneity, amount of cross-linking, porosity, surface area, capacity, water content, and others must be, if possible, quantitatively described (1, 2). One property which has been very difficult to show experimentally is the water content or the degree of dryness of ion exchange resins, The two most popular methods proposed for the determination of water contents of ion exchange resins are Karl Fischer titration (2-4) and weight difference between wet and Present address, Universal Oil Products, 30 Algonquin Rd., Des Plaines, Ill. 60016 (1) J. A. Marinsky, Ed., “Ion Exchange” Vol. 1-2, Marcel Dekker, New York, N. Y., 1969. (2) F. Helfferich, “Ion Exchange,” McGraw-Hill, New York, N. Y . . 1962.
“dry” resin (2, 4-7). These latter procedures involve elevated temperature, vacuum, and presence of a desiccant (P20,) or some combination of these. More recently, the results obtained by the two methods have been compared (3, 8-10). This has been done for strongly acidic cation resins of the gel (microreticular) and porous (macroreticular) variety as well as for anion resins. Poor results were obtained at all water levels using the Karl Fischer titration (10) while others have reported that the same method yielded poor results only at low water levels (3,8,9). Other suggested methods are NMR (11, 12), azeotropic distillation (3),increased indicator concentration (3,and tri(3) F. X. Pollio, ANAL.CHEM.,35, 2164 (1963). (4) A. Dickel and J. W. Hartman, Z . Phys. Chem. (Frankfurt am Main), 23, 1 (1960). ( 5 ) K. W. Pepper, D. Reichenberg, and D. K. Hale, J. Chem. Soc., 1952, 3129. (6) G. Scatchard and N. J. Anderson, J. Phys. Chem., 65, 1536 ( 1961). (7) W. R. Heumann and F. D. Rochon, Can. J. Chem., 43, 3483 ( 1965). ( 8 ) H. D. Sharma and N. Subramanian, ANAL.CHEM.,41, 2063 (1969). (9) Ibid., 42, 1287 (1970). (10) W. R. Heumann and F. D. Rochon, ibid., 38,639 (1966). (11) J. P. devilliers and J. R. Parrish, J. Polym. Sci., Part A, 2,1331 (1964). (12) R. H. Dinius, M. T. Emerson, and G. R. Choppin, J. Phys. Chem., 67, 1178 (1963). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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