Quantitative Aspects of Nuclear Quadrupole Resonance Spectrometry of lnorganics and Minerals Harry D. Schultz and Clarence Karr, Jr. Morgantown Coal Research Center, Bureau of Mines, US.Department of the Interior, Morgantown, W.Va. 26505 Quantitative nuclear quadrupole resonance (NQR) spectrometric analyses of synthetic mixtures and natural ores are presented, apparently for the first time. Reproducible linear quantitative calibration curves were obtained on several inorganic compounds (Cu,O, HgCI,, BiCls), using the signals from 63Cu, 65Cu, asCI, and 209Bi. Synthetic mixtures of HgCI, in NaCI, and BiCI, in KCI, were analyzed by NQR for weight-per cent HgCI, and BiCI,, respectively, with good results. I n addition, several samples of cuprite ore were analyzed for weightper cent of Cu,O from the standard curves. Noted were the influence of various instrument parameters and the physical state of the sample on quantitative NQR spectrometry.
NUCLEAR QUADRUPOLE RESOKANCE (NQR) spectrometry is a relatively new technique for the structure determination of compounds. An NQR spectrometer detects the absorption of energy from quadrupolar nuclei that are located in nonspherical electrical fields. It is similar in many ways to nuclear magnetic resonance (NMR) spectrometry primarily in that an applied R F field is used to effect nuclear spin transitions. With either technique an operator can discern subtle properties of molecular structure, such as geometric isomers in organics or crystal structure differences in inorganics, from the resonance frequencies and intensities of the signals. The most important difference is that NQR uses the built-in electric field gradients in a crystal to observe the frequency of the nuclear spin transitions, whereas NMR requires an externally applied magnetic field. However, several conditions must be met before NQR signals can be observed. In the first place, the compound must contain at least one isotope whose nucleus has a quadrupole moment that is greater than zero (Q > 0) and a spin quantum number that is greater than one half ( I > I/$ Approximately 130 isotopes meet these requirements ( I ) . Second, samples must be solid. If the samples are not solid at ambient temperature, they must be cooled, usually with liquid nitrogen (77 OK) to make them solid. All the inorganic samples discussed in this paper were solids at ambient temperature. Virtually all minerals are solids. Although NQR spectrometry has been used to advantage in crystal structure determinations of inorganics, it has apparently never been used previously for quantitative analyses of synthetic mixtures or natural ores. In fact, the operating parameters for the instrument and the effect of the physical state of the sample for quantitative use of NQR spectrometry apparently have not been previously determined. EXPERIMENTAL
The studies were made on the Wilks Model NQR-1A nuclear quadrupole resonance spectrometer ( I ) . The Wilks spectrometer is a transistorized version of the first selfadjusting NQR instrument developed by Dr. George Peterson of Bell Telephone Laboratories in 1965 ( 2 , 3). The spectrometer includes a self-quenched oscillator employing a feedback loop. A servo motor is used to adjust the coherence, thus solving the problem of nonlinearity in the feedback system and its consequent limit of scanning band
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Figure 1. Effect of sample position in RF Coils As the spectrometer is changed in frequency, any variation in noise level is detected and compensated. The resonance signal was recorded on a Leeds & Northrup strip chart recorder. RESULTS AND DISCUSSION
Operating Parameters. Instrument operating parameters were determined first. The position of the sample in the radio frequency coil was very important for obtaining the maximum sensitivity. Figure 1 shows the top portion of a curve, probably gaussian, obtained by placing a relatively thin layer of CupO at different heights in the radio frequency coil, the resonance of the 63Cu being 26.5 MHz at room temperature. The scan rate of 4 represents 8.33 MHz per hour. Figure 1 demonstrates that the strongest resonance was obtained when the sample was positioned in the center of the coil. This is due to the nonlinearity of the RF-field intensity along the axis of the coil. The quantitative aspect of this effect is demonstrated in the S-shaped curve obtained, Figure 2. In this instance, the peak height increased nonlinearly a s the total weight of CuzO was increased, with the sample vial setting on the plate below the coil. This effect depends on changing the number of quadrupolar nuclei present. Again, the largest increases were observed for that part of the sample near the center of the coil. Consequently, for quantitative NQR analyses, only the coil region should be used, although a weak signal can be obtained on that portion of the sample along the axis immediately outside of the receiving coil. All samples must occupy the same volume within the same portion of the coil, (1) T. P. Das and E. L. Hahn, “Solid State Physics,” Supp. 1,
“Nuclear Quadrupole Resonance Spectroscopy,”Academic Press, Inc., New York, N.Y., 1958, 223 pp. (2) P. M. Bridenbaugh and G. E. Peterson, Rev. Sci. Instrum., 36, 702 (1965). (3) G. E. Peterson and P. M. Bridenbaugh, ibid., 35, 698 (1964). VOL. 41, NO. 4, APRIL 1969
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Figure 2. Effect of quantity of sample in RF Coil
Figure 3. Calibration curve for 69Cu (26.5 MHz) in Cu,O
for a given quantitative analysis. This may require dilution of the sample by thorough blending with a solid that will not react chemically under these conditions nor contain an isotope that is NQR active in the region used. The results were reproducible from one day to the next. Reproducible results could be obtained with spectrometer settings other than those shown in the figures. Sample Purity. Impurities in the crystal lattice produce line broadening and reduce a signal intensity. However, this phenomenon is not observed with mechanical mixtures of compounds. To demonstrate this with inorganic compounds, two compietely separate 35Cl NQR signals were obtained from mixtures of NaC104(at 27.8 MHz) and KC104(at 29.9 MHz). The mixtures gave signals for the NaC104 and KC104 of intensities equal to the signals obtained on the single compounds. The only reference found in the literature on NQR of mixtures was for separate signals for 35Cl in a mixture of two solid phases of hexachlorocyclopentadiene( 4 ) . N o quantitative analysis was made on this organic mixture by NQR. In most instances crystal lattice impurities are either negligible, or sufficiently consistent, so that practical calibration curves can be determined. The question of relative lattice impurity can be resolved readily by comparing samples, all of known
high purity, from different sources. The effect on the lattice of grinding the samples is shown later in this report. Calibration Curves for Pure Inorganics. Varying amounts of CuzO, 0.32 to 7.31 grams, were weighed into vials and the total volume was held constant using HgCl? as the filler material or diluent. In each instance the height of the total sample corresponded to the height of the R F coil. The height of the center line of the split resonance peak was measured in centimeters, and the relationship of peak height to weight of CuzO was plotted. Figures 3 and 4 show the results of these determinations. Two excellent linear relationships of 14 points were obtained, one for 63Cu at 26.5 MHz and one for 6Cu at 24.6 MHz. In addition, linear calibration curves were established for the two signals from s5Cl in HgC1, observed at 22.4 and 22.5 MHz using CUZOas diluent, as shown in Figure 5, and for z09Bi in BiC13 observed at 37.5 MHz using Bi203 as diluent, as shown in Figure 6. Linear calibration curves were thus obtained with four different nuclides ( W u , 6jc1.1, sjC1, ZOgBi), two different nuclides in the same environment ( W u and GjCu in CuzO), and the same nuclide in two nonequivalent environments ( W 1 in HgC12). Shoji Kojima et al. (5) studied the integrated intensity us. ( 5 ) S . Kojima, S. Ogawa, M. Minematsu, and M. Tanaka, J. Phys. SOC. Jup., 13, 446 (1955).
(4) M. Hayek and D. Gill, J . Chem. Phys., 47, 3680 (1967).
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Figure 5. Calibration curve for sjC1 in HgClz
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Table I. Quantitative NQR Analysis of Synthetic Mixtures Wt of BiC13,g, BiC13, wt ”/, Total wt of BiC13 from 37.5 MHz from 37.5 MHz mixture, g wt % calibration curve calibration curve
Wt of BiC13 in KCI, g 1.20 2.56 3.77
4.23 4.86 5.20
28.4 52.7 72.5
Wt of HgC1, in NaCl, g
Total wt of mixture, g
HgCl, wt %
1.04 2.41 3.92
3.55 4.73 5.07
29.3 51.0 77.3
1.05 2.63 3.82
1.10 2.45 3.90
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Wt of HgCl,, g, from calibration curves 22.4 MHz 22.5 MHz
Difference, wt %
1 .oo 2.30 3.87
31.0 51.8 76.9
Difference, wt ”/, 22.5 MHz
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Table 11. Quantitative NQR Analysis of Cuprite Mineral Samplesa
Sample source Butte, Montana No. 1
Weight of sample, grams
Weight of Cu,O in cuprite, grams (from 26.5 MHz)
5.01 4.58 3.07 4.57 4.03 4.58 2.29 6.89 6.89 6.79 5.70 6.79
1 .oo 1.15 1 .oo 1.70 0.90 0.65 0.35 7.00 7.05 6.95 5.60 7.05
Butte, Montana No. 2 Bisbee, Arizona Ajo, Arizona
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Cu,O in cuprite, wt % 26.5 MHz 24.6 MHz 20.0 25.1 32.6 37.1 20.9 14.2 15.3 101.6 102.3 102.4 98.2 103.8
29.5 33.9 21.2 99.1 99.4
Physical state of sample: solid chunks.
amount of p-dibromobenzene impurity in a-phase p-dichlorobenzene. They observed a decrease in integrated intensity commencing at a concentration of impurity of 0.1 %, and they plotted a linear decrease in peak area measurements up to 1 impurity. The instrument used was a continuous wave spectrometer described by Wang (6). Area measurements were taken from the recorded derivative curve. Our perimeter area measurements obtained by enclosing the split resonance signal in a hand drawn envelope gave calibration lines of less reproducibility and linearity than peak height measurements. In day-to-day work with calibration curves, it was occasionally desirable to compensate for instrumental drift by checking the signal intensity of a few standard samples and adjusting the servo reference and gain accordingly before proceeding to unknown samples. Signal intensities would then automatically fall at the appropriate point on the calibration curve. The calibration curves indicate that the quench frequency is reproducible, and therefore peak height measurement is acceptable for quantitative analyses. Considering the rather large amount of data collected to establish the calibration curves, the reproducibility of the quench frequency is shown by the excellent linearity of the lines, the reproducibility of the calibration lines, and the reproducibility of the specific points on the line. The reproducibility of the individual points on the calibration line was approximately 0.1 cm on successive scans. If the quench frequency varied randomly, these observations could not be made. Investigations of the stability of the quench frequency, using an oscilloscope, were (6) T. C. Wang, Phys. Rev., 99, 566 (1955).
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Figure 6. Calibration curve for 209Bi (37.5 MHz) in BiCI3 conducted at several resonant frequencies. The results indicated that the quench frequency was stable at a given resonant frequency as long as the quench gain and quench reference controls were not varied. Naturally, for the determination of any specific calibration curve, the quench gain and reference are held at a constant value. Synthetic mixtures of BiCI, in KCl, and HgC12 in NaC1, were analyzed using resonances of 209Bi at 37.5 MHz, and 35C1 at 22.4 and 22.5 MHz for the mercury compound. In each instance the compound under investigation and the diluent were weighed, thoroughly blended, and placed into vials. The filler material was in each instance a different compound than VOL. 41, NO. 4,APRIL 1969
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Sample source Butte, Montana No. 1 Ajo, Arizona
the one used in determining the calibration curve. This was necessary for a genuine evaluation of the accuracy of the analytical method. The total volume was held constant for each mixture. The height of the resonance signal was directly proportional to the weight of the compound in the sample. The results for six different samples are summarized in Table I. These results suggest that NQR quantitative analysis of synthetic mixtures is comparable in accuracy to other spectrometric methods such as infrared analysis. Analysis of Mineral Samples. Several different samples of the mineral cuprite were analyzed for C u 2 0 content. The results of 17 analyses of four different cuprite ores are summarized in Table 11. When two or more vials were filled to a line (representing the calibration volume) with small, solid pieces of an ore sample, and the peak height was determined as a measure of the weight of Cu20 in the cuprite ore, the analyses showed the same C u 2 0 content within a few weightper cent. In 5 representative instances, the 24.6 MHz values were also determined to demonstrate good agreement with the 26.5 MHz values. There was a decrease of the W u signal at 26.5 MHz (Table 111) as the result of grinding in a tungsten carbide vial. This is attributed to the alteration of the crystal lattice system of CUZO.The X-ray diffraction pattern of ground cuprite showed broadening in the back reflection region as contrasted to the normal X-ray diffraction pattern of unground cuprite and synthetic Cu20. The apparent reason for this broadening is slight disorder in the crystal lattice system created by the vigorous grinding with the tungsten carbide vial. A detailed grind study on minerals and inorganics is in progress, employing several different grinding methods. The conclusions will be reported in a separate publication. Preliminary results indicate no reduction in signal intensity of cuprite if an agate vial or hand grinding with mortar and pestle is employed. Obviously some ore samples must not be subjected to vigorous
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Table III. Effect of Grinding on Cuprite Weight Weight of CuzO Physical state of sample, in cuprite, grams of sample grams (from 26.5 MHz) Solid chunks 4.58 1.15 Grind 5 min. 4.15 0.25 Solid chunks 7.05 6.79 6.42 4.80 Grind 5 min. 5.96 1.75 Grind 10 min. 5.82 0.23 Grind 15 min.
Cu,O in cuprite,
wt % (from 26.5 MHz) 25.1 6.0 103.8 14.8 29.4 3.9
grinding before the analysis. This presents no analytical problems because an average value can be determined from several different selections of small pieces of ore, or a mild grinding method can be employed. This is demonstrated for five different selections of small pieces of the high-purity Ajo, Ariz., ore (Table 11). All samplings consistently gave values close to 100% Cu20. In this regard, NQR analysis is faster and much simpler than infrared analyses in which samples must be ground until reproducible absorptivities are obtained. In addition to the mineral cuprite, the resonance of 1zlSb in the mineral stibnite (Sb2SJ was detected at 45.1 MHz on two different ore samples. Pure Sb2S3of the same crystal structure as the mineral was not available for determining a calibration curve. Identity of crystal structure is shown readily by identical frequency values for the NQR signals. Work is in progress on other minerals and inorganics. The results reported here indicate that quantitative NQR analysis of ores and other natural mixtures for various specific inorganics and minerals gives reproducible results that are comparable to other spectrometric methods in accuracy and has considerable promise for future development. ACKNOWLEDGMENT
We are grateful to John J. Renton, Geology Department, West Virginia University, for providing X-ray diffraction data on samples. RECEIVED for review November 6,1968. Accepted February 6, 1969. Presented at the Division of Analytical Chemistry, 156th Meeting, ACS, Atlantic City, N.J., September 1968. The equipment is named in this report for identification only and does not necessarily imply endorsement by the Bureau of Mines.
