Anal. Chem. 1996, 68, 1401-1407
Quantitative Analysis of Solids by High-Resolution 1H NMR Changhua C. Liu† and Gary E. Maciel*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
1H
CRAMPS and MAS-only NMR experiments have been examined systematically from a quantitative point of view. Silicone rubber was characterized as a proton NMR intensity standard and found to be successful in quantitative analysis for typical solids. Important parameter variations involved in the CRAMPS experiment, especially for quantitative analysis, are discussed. The application of the silicone rubber intensity standard for quantitative 1H NMR analysis (spin-counting) to silica is described, demonstrating the utility and power of 1H NMR for quantitative analysis. The combination of magic-angle spinning (MAS)1 and multiplepulse line-narrowing,2,3 referred to as CRAMPS (combined rotation and multiple-pulse spectroscopy),4-7 is capable of providing highresolution 1H NMR spectra that can be extremely useful for the analysis of solids. In general, CRAMPS techniques are normally utilized to eliminate the potentially severe line-broadening effects due to homonuclear dipole-dipole interactions (via a multiplepulse sequence) and chemical shift anisotropy (via MAS). However, 1H CRAMPS has been among the most technically demanding solid-state experiments, in which there have often been difficulties associated with the strict requirements on the widths, shapes, phases, and intervals of the high-power radio frequency (rf) pulses employed. Because of the nontrivial tuning of a spectrometer involved in the CRAMPS experiment, a detailed assessment of the quantitativeness of CRAMPS would seem to be an important task to fulfill, and yet no published results are known. Some promising preliminary results had been obtained in our research group on mixtures of Delrin with polystyrene, demonstrating the viability of spin-counting in 1H CRAMPS experiments.5 The spectral intensities of CRAMPS peaks are usually presented and interpreted in a relative sense, i.e., in terms of ratios of peak intensities within a spectrum. However, in order to maximize the value of any analytical technique, it is necessary to relate intensities to the quantity of analyte in the absolute sense. † Current address: Pharm-Eco Laboratories, Inc., Lexington, MA 02173. (1) Andrew, E. R. Phil. Trans. R. Soc. (London) 1981, A299, 505. (2) Waugh, J. S.; Huber, L. M.; Haeberlen, U. Phys. Rev. Lett. 1968, 20, 180. (3) Burum, D. P.; Rhim, W. K. J. Chem. Phys. 1979, 71, 944. (4) Gerstein, B. C.; Pembleton, R. G.; Wilson, R. C.; Ryan, L. J. Chem. Phys. 1977, 66, 361. (5) Bronnimann, C. E.; Hawkins, B. L.; Zhang, M.; Maciel, G. E. Anal. Chem. 1988, 60, 1743. (6) Maciel, G. E.; Bronnimann, C. E.; Hawkins, B. L. In Advances in Magnetic Resonance: The Waugh Symposium; Warren, W. S., Ed.; Academic Press: San Diego, CA, 1990; Vol. 14, pp 125-150. (7) Gerstein, B. C.; Dybowski, C. R. Transient Techniques in NMR of Solids; Academic Press Inc.: Orlando, FL, 1985.
0003-2700/96/0368-1401$12.00/0
© 1996 American Chemical Society
This usually requires the use of an absolute intensity standard, which provides a peak in the spectrum for which the intensity has an absolute meaning, which in NMR represents a known number of nuclei. An often used alternative to CRAMPS for 1H NMR experiments on solid samples is MAS-only, ideally employing high-speed spinning.8 However, it is well known that for many types of samples, e.g., typical organic solids, the MAS speeds that are commonly available are insufficient for effective averaging of the 1H-1H dipolar interactions. To be a reliable intensity standard for NMR experiments, a substance should meet the following criteria: (1) chemical inertness and stability; (2) suitable chemical shift in a vacant region of the spectrum (to avoid peak overlaps); (3) small, but not too small, line width so that a small amount of the intensity standard yields a substantial integrated intensity, for convenience and accuracy of measurement; and (4) relaxation behavior that permits accurate absolute intensity measurements under the conditions of the NMR experiments of interest. EXPERIMENTAL SECTION NMR Experiments. Proton spectra obtained by the CRAMPS technique and by single-pulse (MAS-only) experiments were performed at 360 MHz on a severely modified Nicolet NT-360 spectrometer. The CRAMPS experiments used the BR-24 pulse sequence,3 with a 90° pulse length of 1.1-1.2 µs and a pulse spacing of 3.0 µs. Magic-angle spinning in the CRAMPS experiments employed a spinner based on the design of Gay,9 operated at speeds of 1.5-2.0 kHz. To provide the capability of high-speed MAS for proton detection, a high-speed spinner system (up to 12 kHz) and probe were built, based on the design of a high-speed/variable-temperature MAS system, using a 4-mm pencil-type zirconia rotor developed by Chemagnetics. Kel-F was chosen to be the stator material to minimize the proton background signal. Chemical shifts were determined by referencing, via sample substitution, to the 1H peak of tetrakis(trimethylsilyl)methane (TTMSM) and are reported here relative to tetramethylsilane (TMS). Samples. Silicone rubber was obtained from Alltech. Adipic acid and monoethyl fumarate (MEF) were obtained from Eastman Kodak. Poly(methyl methacrylate) (PMMA) was obtained from Polyscience, Inc. The Cab-O-Sil fumed silica was grade HS-5, obtained from the Cabot Corp. The silica gel was Fisher S-679. (8) Dec, S. F.; Bronnimann, C. E.; Wind, R. A.; Maciel, G. E. J. Magn. Reson. 1989, 82, 454. (9) Gay, I. D. J. Magn. Reson. 1984, 58, 413.
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RESULTS AND DISCUSSION Intensity Standard. Silicone rubber, poly(dimethylsiloxane) (PDMS), [-Si(CH3)2-O-]n, was examined for use as an intensity standard for 1H NMR. This material is, to a substantial degree, chemically inert and stable, and its 1H NMR spectrum contains only one peak. Furthermore, the methyl group chemical shift is on the extreme high-shielding side of the usual 1H chemical shift scale for diamagnetic samples. This chemical shift was determined to be almost 0 ppm (0.045 ppm) relative to TMS, both by sample substitution and by dissolving the silicone rubber in chloroform. The 1H spin-lattice relaxation time of silicone rubber was determined by a 1H CRAMPS inversion recovery experiment10 to be about 800 ms, which is short enough to be useful and practical. In spite of all of the advantages of silicone rubber that are mentioned above, one characteristic of silicone rubber that, a priori, could cause it to fail as a standard for 1H CRAMPS or MASonly experiments is that it is a rather mobile polymer, and molecular motion can destructively interfere with line-narrowing by multiple-pulse or MAS techniques.6 If the correlation time describing molecular motion (or proton exchange) of a given molecule is similar to the cycle time of the coherent line narrowing method (e.g., multiple-pulse cycle time, tc), the effectiveness of the method in averaging the 1H-1H dipolar interaction will be reduced.6,10 For typical 1H CRAMPS experiments, the relevant motional correlation times that could lead to interference with linenarrowing by coherent averaging techniques are in the range 0.01-0.1 ms. An unaveraged dipolar interaction in an organic solid would broaden an NMR peak over many ppm, perhaps beyond convenient detection. With an extremely large line width, the intensity of a CRAMPS peak could be virtually indistinguishable from baseline noise or pedestal effects.6 Of course, if isotropic motion were to occur at rates (in hertz) that exceed the magnitude of the dipolar interactions, then the motion would average the interactions to zero without regard for the presence or absence of multiple-pulse and/or MAS averaging. The only motional regime for which a properly designed and executed CRAMPS experiment cannot produce a peak that is free of significant homonuclear dipolar broadening is the regime in which the correlation time of the motion, τc, is comparable to the cycle time of the multiple-pulse averaging, tc, i.e., | gHD/h| g τc-1 ≈ tc-1, where |HD/h| is the magnitude of the dipolar interaction in hertz. To assess any molecular motional interference with CRAMPS line-narrowing, a series of CRAMPS experiments was carried out on silicone rubber as a function of the multiple-pulse cycle time, tc, as determined by the pulse spacing period (τ); the resulting spectra are shown in Figure 1. Normally, a 3.0-µs pulse spacing time is used in our BR-24 experiments. To cover a range of cycle times (36τ), τ was varied from 2.4 to 4.0 µs. One can see from Figure 1 that, for 3.0-4.0 µs, the 1H CRAMPS spectra of silicone rubber look very similar, and the integrated areas are very close to each other. Of course, with large pulse spacings, the spectrum will show spectral distortion due to phenomena other than (or in addition to) molecular motion, i.e., the intrinsic inability of the multiple-pulse sequence to average even a static dipolar interaction. One can expect that, as the cycle time in a CRAMPS experiment is increased beyond some threshold value related to the strengths of the dipolar interactions one is trying to average, (10) Bronnimann, C. E.; Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1988, 110, 2023.
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76
93
114
127
140
165
Figure 1. 1H CRAMPS spectra of silicone rubber as a function of the multiple-pulse spacing, τ.
the effectiveness of the multiple-pulse sequence for line-narrowing is diminished. Below we see resolution degradation in rigid samples when τ values larger than 3.0 µs are used. One sees in Figure 1 that when τ is decreased below 3.0 µs, the peak line width is increased, along with a decrease in the integrated intensity. Ideally, one would expect the overall performance of the multiple-pulse sequence to improve as τ is decreased or remain constant when a sufficiently small τ value is reached.6 This expectation is based on the unachievable ideal of perfect pulses and electronics. The observed pattern was initially worrisome because it could indicate that molecular motion of silicone rubber interferes with CRAMPS line-narrowing. However, when CRAMPS experiments were carried out as a function of the pulse spacing time on other, much more rigid samples (vide infra), the same kind of line width and intensity behavior were observed; hence, we concluded that the degradation of signal in the CRAMPS experiment when the multiple-pulse spacing parameter is decreased below 3.0 µs is an underlying characteristic of the CRAMPS technique (at least in our laboratory). Analogous experimental results on monoethyl fumarate, adipic acid, and poly(methyl methacrylate) are shown in Figure 2. All the spectra for each sample are plotted on a fixed absolute intensity scale. One can see from Figure 2 that the integrated intensities of all three samples are dependent on the multiple-pulse spacing time. Figure 3 summarizes the dependence of the total integrated intensity of each spectrum vs τ for all samples, including silicone rubber. Consistently, the integrated intensity for each sample is maximal for τ ) 3.0-3.4 µs and decreases when τ is reduced below 3.0 µs. In general, in our laboratory, 3.0 µs seems to give the best BR-24 CRAMPS signal-to-noise ratio; this is the value that we have used in most CRAMPS experiments. With our 360-MHz spectrometer, a 2.4-2.8-µs τ value is probably not long enough to allow maximum receiver recovery before sampling, resulting in some signal distortion, including a reduced signal intensity.
a b
c
Figure 2.
1H
CRAMPS spectra of (a) MEF, (b) adipic acid, and (c) PMMA as a function of the multiple-pulse spacing, τ.
Figure 3. Plots of CRAMPS integral vs multiple-pulse spacing for (a) silicone rubber, (b) adipic acid, (c) MEF, and (d) PMMA. The three sets of data points for each figure represent three individual experiments, i.e., on three completely different samples in each case.
Thus, while a shorter cycle time in multiple-pulse experiments can reduce the effects of higher-order terms on line width, this may be achieved, largely for reasons of instrumental limitations, at the expense of sensitivity. With multiple-pulse averaging of dipolar interactions, the cycle time of interest is not necessarily the cycle time of the entire sequence, but rather the period of the smallest subcycle within
the sequence that eliminates the dominant terms (HD(0)) of the dipolar Hamiltonian.3 The BR-24 pulse sequence is based upon six four-pulse subcycles, each of which averages HD(0) to zero over the course of the subcycle time (tc′) composed of six τ periods. With the 3.0-µs τ period used in these experiments, the resultant tc′ is 18 µs, and 1/tc′ is 56 kHz. Since τ ) 3.0 µs yields optimized rather than degraded CRAMPS performance, we can conclude that motion in our silicone rubber standard does not manifest a substantial component with a correlation time near 18 µs. From the patterns displayed in Figure 3, one can draw the same conclusion (not surprisingly) for the other three samples studied. In the case of MAS-only experiments, the relevant cycle time is a rotor period, (νrot)-1. An intrinsic time dependence in the sample with a correlation time near (νrot)-1 can lead to linebroadening.11 Hence, 1H MAS-only NMR spectra of silicone rubber were obtained as a function of spinning speed from 2.0 to 12 kHz; the resulting spectra (not shown here) are nearly the same for all MAS speeds, with a nearly constant line width of about 0.1 ppm. There is hardly any difference from one spectrum to another. Hence, there is no substantial molecular motion of silicone rubber with correlation times in the range 0.08-0.5 ms; i.e., molecular motion does not interfere with MAS line-narrowing for this sample in this study. From the 1H CRAMPS and MAS-only results on silicone rubber described above, we have established suitable ranges of experimental parameters that allow us to eliminate the possibility of motional interference with these two line-narrowing techniques, thus setting the stage for the use of this material as a solid-sample 1H NMR intensity reference. To examine prospects for quantitative applications of this intensity reference in solid-state 1H NMR (11) Sewelack, D.; Rothwell, W. P.; Waugh, J. S. J. Chem. Phys. 1980, 73, 2559.
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Figure 4. 1H CRAMPS spectra of a mixture of silicone rubber with MEF (2.7 mg, 25.5 mg) as a function of multiple-pulse spacing, τ, showing integrated intensities (arbitrary units).
to complex samples or mixtures of materials, the experiments described below on known mixtures were carried out. Quantitative Analysis by 1H CRAMPS. A series of samples was prepared, each consisting of a known amount of silicone rubber mixed with a known amount of a given powdered compound or polymer. Although we have obtained consistent results on mixture samples in which the intensity reference is physically separated from the analyte, the results shown here were based on physically mixed samples. A mixture of MEF and silicone rubber, prepared in the hydrogen weight ratio of 6.7:1, was examined. 1H CRAMPS spectra of this sample, obtained as a function of multiple-pulse spacing time, are shown in Figure 4. Since there is no spectral overlap between MEF and silicone rubber signals, quantitation can be assessed by a simple integration approach. The result obtained for the 1H integral ratio of MEF to silicone rubber determined by three experiments, with 3.0-µs pulse spacing time (τ), is 6.3 ( 0.3. Triple determinations by an analogous procedure with τ ) 3.0 µs on a 7.3:1 mixture of adipic acid and silicone rubber (again, no peak overlaps) yielded the corresponding integral ratio of 7.5 ( 0.4 after correction for effects of 1H T1 (240 s for adipic acid). For a mixture of PMMA and silicone rubber, there is peak overlap in the spectra, but it is not severe, as can be seen in Figure 5. Deconvolution was carried out to properly evaluate the intensities for each component in the mixture. In this case, a value of 8.7 ( 0.4 (τ ) 3.0 µs) was obtained from three separate determinations on a 8.7:1 mixture of PMMA and silicone rubber. Integrated intensity ratios determined on these three mixtures at other τ values are somewhat different from those obtained at 3.0 µs. Plots of these ratios vs τ for the three mixtures are shown in Figure 6. These plots show that the intensity ratios are not highly sensitive to τ variation in the region of τ ) 3.0 µs. 1404 Analytical Chemistry, Vol. 68, No. 8, April 15, 1996
Figure 5. 1H CRAMPS spectra of a mixture of silicone rubber with PMMA (2.2 mg, 19.1 mg) as a function of multiple-pulse spacing, τ, showing integrated intensities (arbitrary units).
Figure 6. Plots of the integral ratio of (a) MEF, (b) adipic acid, and (c) PMMA to silicone rubber in each of the mixtures of the types represented in Figures 4 and 5 vs multiple-pulse spacing, τ.
The positive results shown above indicate that silicone rubber is useful as a spin-counting reference for quantitation in 1H CRAMPS experiments for those cases in which its resonance does not interfere with the spectral region of interest. Important Operating Parameters for Quantitative CRAMPS. The CRAMPS experiment involves important variation and adjustment of critical experimental parameters. Preparation pulses, pulse phases, receiver phase, pulse width, cycle time, offset, and MAS rate should all be treated as parameters that can be varied for optimization of the CRAMPS spectrum. To achieve
quantitatively reproducible results from CRAMPS experiments, a number of things should be kept in mind. First, the proper selection of receiver phase and the phase and width of the preparation pulse is important to avoid debilitating pedestal signals that arise from magnetization that is prepared parallel to the effective field of the multiple-pulse train.6,12 If the pedestal is broad, as it can be if there is strong homogeneous broadening, it can interfere with the spectrum and destroy quantitation. Although the integral ratio for a macroscopically uniform mixture is not markedly affected by the receiver phase, this parameter does have a significant effect on the absolute intensities in the CRAMPS spectrum. This is very important when the standard is physically separated from the analyte (e.g., in a separate capillary), because then one needs comparably scaled absolute intensities for both components (standard and sample). Second, a severe distortion can result if there is overlap of one or more peaks in the spectrum with rotor frequency lines,13,14 which are caused by coupling between magic-angle spinning and pulse imperfections, e.g., finite (nonzero) pulse width, pulse droop, and phase errors. The MAS speed should be carefully adjusted to eliminate or move rotor lines from the spectral range of interest. We have already discussed the interferences of sample dynamics associated with motion and/or chemical reactions with the multiple-pulse averaging of 1H-1H dipolar interactions, if the correlation time constant of the dynamics is comparable to the cycle time (tc) of the multiple-pulse sequence. An analogous problem can also arise with magic-angle spinning.11 In order for both types of averaging (multiple-pulse and MAS) to be effective, the cycle times of these two periodic averaging processes should be very different. A previous report indicated that 1.5-2.0-kHz MAS speeds seem to minimize MAS interference with the BR-24 multiple-pulse sequence with a τ value of 3.0 µs.6 Finally, for both the CRAMPS and the MAS-only experiments, proton spin-lattice relaxation has to be considered when quantitation is required. For many pure, powdered substances, especially crystalline samples without methyl groups, the 1H T1 value can be extremely large; e.g., the 1H T1 of adipic acid is about 240 s. Quantitative Analysis by 1H MAS-Only. As mentioned above, even very modest MAS speeds are sufficient to average the residual 1H-1H dipolar interactions of the highly mobile silicone rubber structure in MAS-only experiments. Unfortunately, a quantitative 1H NMR approach analogous to that described above for 1H CRAMPS is not very attractive for MAS-only experiments on nonmobile systems like MEF, adipic acid, and PMMA with the equipment employed in this study, with a maximum MAS spinning speed of 12 kHz, because this is far too small to average the 1H-1H homonuclear dipolar interaction in these rigid or largely rigid, proton-rich samples.8 Hence, MAS-only 1H NMR experiments with MAS speeds ranging up to 12 kHz result in very broad peaks, with spinning sidebands that cover a range of a few hundred ppm, as shown in Figure 7 for the case of MEF. Similar results, with very broad bands at low MAS speeds giving way to spinning sidebands at high MAS speeds, were obtained on adipic acid and PMMA. The total integrated intensities of the type of spectra shown in Figure 7 (also for adipic acid and PMMA) were (12) Haeberlen, U. High Resolution NMR in Solids; Academic: New York, 1976; p 149. (13) Vega, S.; Olejniczak, E. T.; Griffin, R. G. J. Chem. Phys. 1984, 80, 4832. (14) Olejniczak, E. T.; Roberts, J. E.; Vega, S.; Griffin, R. G. J. Magn. Reson. 1984, 56, 156.
Figure 7. speed.
1H
MAS-only spectra of MEF as a function of spinning
Table 1. Spin-Counting Results (Integrated Intensities)a from the MAS-Only Experiments on MEF, Adipic Acid, and PMMA, Compared to Silicone Rubber as an Intensity Standard MAS rate (kHz)
silicone rubber MEF adipic acid PMMA
expt
0
3.0 5.0 7.0 9.0 12
avb
avc
1 2 3 1 2 3 1 2 3 1 2 3
45 46 45 41 40 39 44 45 43 42 39 40
44 43 42 42 42 41 46 43 45 44 41 38
44.2 43.0 41.0 40.3 39.5 44.6 44.8 43.6 42.2 40.3 40.0
43.6
45 45 44 42 39 39 45f 46f 44f 41 42 40
42 45 41 41 40 39
42 44 42 40e 41e 38e 44 45 41 41 42 40 38 42 41
43 42 44 40 40 41 44 45 45 43 42 39
ratiod
40.3
0.92
44.3
1.0
40.8
0.94
a Numbers are values of the total integral (in arbitrary units) of each spectrum of the type presented in Figure 7 after correction for sample weights. b Average of results obtained from different spinning speeds; estimated error, (2-3%. c Average of results obtained from three individual sets of experiments; estimated error, (3%. d Integral ratios of MEF, adipic acid, or PMMA to silicone rubber after normalization in terms of each sample weight and chemical composition; estimated error, (4-6%. e Obtained at 10 kHz. f Obtained at 6 kHz.
referenced to the silicone rubber standard via sample substitution back and forth. The resulting data, based on three separate experiments on each sample, are given in Table 1. The total integrated intensities of each sample are seen to be essentially independent of the MAS spinning speeds utilized in the experiments. Table 1 also shows the integrated intensity ratio of the silicone rubber signal to that of each sample (MEF, adipic acid, and PMMA) after normalization with respect to sample weights. One can see that the total integrated intensity ratio for each sample and silicone rubber very closely reflects the proton ratio calculated from the weight and elemental composition of the samples. Spin-Counting Experiments on Silica. As examples of the quantitative application of solid-state 1H NMR techniques based on the silicone rubber intensity standard, we present spin-counting results on two types of amorphous silicas, fumed silica (Cab-OSil) and silica gel. All the peaks in the 1H spectra of silica surfaces Analytical Chemistry, Vol. 68, No. 8, April 15, 1996
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Table 2. 1H Integral Ratios of Silica (Silica Gel and Cab-O-Sil) and Silicone Rubber after Correction for Sample Weights,a Obtained from CRAMPS and MAS-Only Experiments silica gelb CRAMPS MAS-only speed (kHz)
12 10 8.0 6.0 4.0 2.0
Cab-O-Silc
11
4.9
11 12 11 9.8 8.1 6.6
4.5 4.4 4.2 3.8 3.3 2.9
a Numbers in this table are obtained by the following formula: (integral of silica/weight of silica)/(integral of silicone rubber/weight of silicone rubber). b Estimated error, (3%. c Estimated error, (4%.
Table 3. Relative Population of Each Proton Species on Silica Gel and Cab-O-Sil Detected by CRAMPS and MAS-Only Experimentsa Figure 8. 1H CRAMPS spectra (top) and MAS-only with 12-kHz sample rotation (bottom) spectra of untreated (a) silica gel and (b) HS-5 Cab-O-Sil, mixed with silicone rubber (53.1 mg of silica gel mixed with 1.1 mg of silicone rubber; 29.6 mg of Cab-O-Sil mixed with 0.30 mg of silicone rubber).
are located at least 1 ppm higher in chemical shift (1 ppm lower in shielding) than the silicone rubber proton peak;10,15,16 thus, there are no peak overlaps that would prevent use of this intensity standard. Because of its far superior performance in detecting protons in rigid environments, the CRAMPS approach is expected to be more effective generally than MAS-only, unless the characteristic time of motion present in a sample is similar to a critical cycle time of the CRAMPS experiment. The less effective detection of rigid or less mobile species by the MAS-only approach would result in an apparent loss of intensity due to the incomplete averaging of the homonuclear dipolar interactions if the MAS speed is not fast enough. Both CRAMPS and MAS-only techniques have been used in the study of the silica gel and fumed silica surfaces by 1H NMR.10,15,16 Application of the quantitative approaches described above, based on the two line-narrowing techniques and the silicone rubber intensity standard, is described here. Figure 8, parts a and b, shows the spectra of mixtures of the intensity standard with untreated silica gel and with untreated CabO-Sil fumed silica, respectively, obtained by CRAMPS (top spectra) and MAS-only with 12-kHz spinning speed (bottom spectra). The peaks at 0 ppm in these spectra come from the internal standard, silicon rubber. For both silicas, the relatively narrow peak in the center, around 3.5 ppm, is due to water molecules that are physically adsorbed on the silica surface; the broad peak on the lower shielding side of the spectrum is attributed to the hydrogenbonded silanol protons, and the isolated (non-hydrogen-bonded) silanol protons on the untreated Cab-O-Sil silica give rise to a peak at 2.0 ppm. Data for each spectrum in Figure 8 were obtained in the following two ways: (1) total spectral integration and (2) integration of individual peaks obtained by spectral deconvolution/ (15) Kinney, D. R.; Chuang, I-S.; Maciel, G. E. J. Am. Chem. Soc. 1993, 115, 6786. (16) Liu, C. C.; Maciel, G. E. J. Am. Chem. Soc., submitted.
1406 Analytical Chemistry, Vol. 68, No. 8, April 15, 1996
relative population (%)b sample silica gel Cab-O-Sil
b
line-narrowing technique
H-bonded SiOH
physisorbed water
isolated SiOH
CRAMPS MAS-onlyc CRAMPS MAS-onlyc
34 36 31 35
66 64 63 58
0 0 6 7
a Obtained from computer deconvolution/simulations of spectra. Estimated error, (5%. c Obtained with 12-kHz MAS spinning speed.
simulation via computer. Summaries of these two types of results are given in Tables 2 and 3. One can see from Figure 8 and Table 2 that, after correction for the weights of silica and silicone rubber used, the MAS-only technique with 12-kHz spinning rate is able to detect essentially the same amount of protons in both Cab-OSil silica and in silica gel as does the CRAMPS experiment. To inquire whether the MAS-only experiment generates additional intensity in spinning sidebands that are outside the 20-kHz bandwidth used in the experiments represented in Figure 8, a 1H MAS-only spectrum was also obtained on untreated Cab-O-Sil with a spectral width of 100 kHz. The resulting spectrum is shown in Figure 9. The fact that the very broad apparent band underneath all three obvious peaks is not due to a poorly narrowed resonance of silanols with very strong hydrogen bonds (thus, with very strong dipolar interactions) can be seen by subtracting the spectrum of an empty rotor (Figure 9b) from that of the samplefilled rotor (Figure 9a). Subtraction of the two spectra yields a resulting spectrum (Figure 9c) in which the original very broad band of Figure 9a is absent. Apparently, 12-kHz MAS is enough to average all of the 1H-1H dipolar interactions of the hydrogenbonded silanols of Cab-O-Sil silica, with essentially undetectable residual spinning sidebands. Table 2 also includes the results of 1H MAS-only experiments carried out as a function of spinning speed. The lesser amounts of protons detected using low spinning speeds (i.e., 2-6 kHz) than at high spinning speeds (i.e., 8-12 kHz) reflect the unaveraged 1H-1H dipolar interaction of hydrogen-bonded silanols at lower MAS spinning speeds. For the 12-kHz spectrum, the result is within experimental error of the CRAMPS result. The summary given in Table 3 shows that CRAMPS and MAS-only with
density on silica surfaces from the integrated intensities of the NMR signals for a known weight of silica. Based on information in Tables 2 and 3 on the integral ratios of Cab-O-Sil silica and silica gel and from the known surface areas of these samples (325 m2/g for Cab-O-Sil and 666 m2/g for silica gel), the concentrations of silanol groups on the untreated samples are found to be 2.8 ( 0.3 and 6.3 ( 0.3 OH/(nm)2 respectively, in good agreement with results obtained by other techniques.17-20
Figure 9. 1H MAS-only (12 kHz) spectra of (a) untreated Cab-OSil, (b) empty spinner (each obtained with a 100-kHz spectral width), and (c) result of subtraction of spectrum b from spectrum a.
sufficiently high MAS speed give very similar results for the relative amounts of each kind of proton on the silica surface. The concentration of silanol groups on the silica surface has been studied vigorously since the 1960s, based on a variety of methods.17-20 The 1H NMR intensity standard used in this work provides another very useful way of calculating the hydroxyl (17) Zhdanov, S. P.; Kiselev, A. W. Zhur. Fiz. Khim. 1957, 31, 2213. (18) Legrand, A. P.; Hommel, H.; Tuel, A.; Vidal, A.; Balard, H.; Papirer, E.; Levitz, P.; Czernichowski, M.; Erre, R.; Damme, V.; Gallas, J. P.; Hemidy, J. F.; Lavalley, J. C.; Barres, O.; Burneau, A.; Grillet, Y. Adv. Coll. Surf. Sci. 1990, 33, 91. (19) Morrow, B. A.; McFarlan, A. J. Langmuir 1991, 7, 1695. (20) Zhuravlev, L. T. Langmuir 1987, 3, 316.
CONCLUSIONS Silicone rubber is a useful intensity standard for 1H NMR of solids, for both CRAMPS and MAS-only techniques. Using physical mixtures of the analyte and the intensity standard, CRAMPS and MAS-only 1H NMR can provide absolute intensities reliable to within about 5%. Of course, each laboratory should calibrate the particular silicone rubber sample to be used as an intensity standard, as different preparations will yield samples with different properties. CRAMPS experiments are technically more difficult to perform in terms of optimizing operating parameters, but quantitative analysis is achievable. MAS-only experiments are relatively easier to implement than CRAMPS, but quantitative results can be obtained only if dipolar interactions are small enough to be averaged well with the MAS speed available (so peak overlaps can be avoided), unless spectral detail (e.g., chemical shift resolution) is unnecessary and one uses intensity referencing by sample substitution. ACKNOWLEDGMENT The authors thank Dr. I-Ssuer Chuang and Mr. Jincheng Xiong for helpful discussions and gratefully acknowledge partial support of this research by National Science Foundation, Grant No. CHE9021003, and Grant No. F49620-95-1-0192 from the U.S. Air Force Office of Scientific Research. Received for review October 25, 1995. Accepted February 1, 1996.X AC9510696 X
Abstract published in Advance ACS Abstracts, March 1, 1996.
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