Quantitative Analysis by Nuclear Magnetic Resonance Spectrometry Using Precision Coaxial Tubing Koichi Hatada, Yoshio Terawaki, Hiroshi Okuda, Kazuhiko Nagata, and Heimei Yuki Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, Japan NUCLEAR MAGNETIC RESONANCE (NMR) spectrometry has been applied to a wide variety of quantitative organic problems, but an intensity standard is required to obtain higher precision ( I ) . The standard compound can be added to each sample as an internal standard or in a separate tube as an external standard. In the latter case there is a possibility of instrumental variation during the interval between the time when the spectra of the sample and the standard are obtained, while in the case of an internal standard the contamination of a sample by the reference compound cannot be avoided. We have developed an analytical technique for the determination of a molecular species in a mixture using an external standard in a precision coaxial tubing (Wilmad Glass Co., inside diameter: inner tube 1.3 mm, outer tube 4.2 mm.). The method has also been used for the elemental determination of hydrogen. In addition, we have developed an analytical method for Some binary mixtures by the measurement of chemical shift using this precision coaxial tubing.
5 0 Hz I
EXPERIMENTAL
The NMR spectra were taken on a JNM-4H-100 spectrometer at 22.5 =t0.2 "C with an internal lock system at 100 MHz. The signal intensity was determined by a JES-ID-2 electronic integrator. Five determinations were made on each sample, and the results were averaged. The conditions of measurements were carefully chosen to minimize the errors arising from the effects of saturation. The chemical shifts were calibrated at each measurement by means of the built-in frequency counter. The precision of the measurement was 1 0 . 1 Hz. All the organic chemicals were obtained from Nakarai Chemicals CO., and were purified by the published methods. The deutero compounds were also obtained from Nakarai Chemicals Co. and used without further purification. RESULTS AND DISCUSSION
Analysis by Intensity Measurement. In a typical experiment the determination of water in deuterium oxide is described. A mixture of water and acetic acid is placed in the central capillary of the coaxial cell and deuterium oxide containing water is placed in the surrounding annulus. The measurement is done using the methyl signal of acetic acid as a lock signal. Figure 1 shows that the signal caused by the water in the central capillary ( A ) is slightly shifted to a lower field than that of the water in the surrounding annulus ( B ) (about 55 Hz at 24.2 mole of acetic acid). The intensity ratio of signal B and signal A can be determined very precisely since the positions of these two peaks can be adjusted by the concentration of acetic acid so as to be close together without overlapping, which is one of the features of this technique. After the additions of known amounts of water (from 0.5 to 10 wt to the commercial deuterium oxide in the outer cell, the intensity ratios are measured. A linear relationship with a slope of 1.13 l/wtx H?O is obtained between the intensity ratio and the amount of added water. Extrapolation of the
z
z)
(1) P. J. Paulsen and W. D. Cooke, ANAL. CHEM., 36, 1713 (1964).
1518
ANALYTICAL CHEMISTRY
Figure 1. NMR spectrum of water in precision coaxial tubing Inner tube (A). 75.8 mole % of H 2 0 in acetic acid Outer tube ( B ) . 5.0 wt of H20 in D20
plots to zero intensity ratio gave a water content of 0.4 w t z in the original deuterium oxide. By using this plot as a calibration curve, one can determine the amount of water in deuterium oxide with an absolute error of about &O.lx. Similarly, the amounts of t-butyl vinyl ether (t-BVE) in CC14 were determined by the use of this technique (Table I). In this case t-butyl vinyl ether containing hexamethyldisilane (HMDS) was placed in the central capillary and HMDS signal was used as an internal lock. The difference in the chemical shift between the signal of the sample in the surrounding annulus and that due to the central capillary was adjusted by changing the amount of HMDS in order to obtain an accurate measurement of intensities. In this procedure all calibration data and measurements have to be prepared using the same coaxial tubing, since the
Table I. Determination of t-BVE in CCla t-BVE (mole fraction)
Taken
Found
0.041 0.031 0.021 0.007
0.042 0.031 0.020 0.006
Error 0.001
O.Oo0 -0.001 -0.001
~~
Table 11. Results of Hydrogen Analysis NMR analysis Compound Polystyrene PBVEa P (p-Me-BVE)* Methanol DMFc
Calcd.
Found
Error
7.72 7.50 8.16 12.58 9.65
7.60 7.67 8.03 12.51 9.32
-0.12 0.17 -0.13 -0.07 -0.33
a
Poly(benzy1vinyl ether).
c
Dimethylformamide.
Elementary analysis Found Error 7.80 7.30 8.12
0.08 -0.20 -0.04
... ...
... ...
* Poly(p-methyl benzyl vinyl ether).
area of both inner and outer tubes varies slightly from one tube to the next. A minimum sample size required for the analysis is about 0.3 ml. With the procedures described here, one can avoid large errors due to the instrumental variation during the switching of sample tubes and the contamination of sample due to an internal standard. Furthermore the sample of interest itself is used as the intensity standard in this technique. A further advantage of this technique is the added convenience of being able to prepare an intensity standard in the inner tube whose concentration is close to that of the sample to be analyzed. This increases the accuracy of the area measurement, but is often difficult to do by addition of the standard directly to the sample solution, especially in the case of small or very dilute samples. By using our technique a dilute standard solution could be accurately prepared on a larger scale and an aliquot could be added to the inner tube. Williams ( 2 ) has described the procedure for the determination of per cent hydrogen in organic compound using n-octane as an external standard and obtained accurate results as good as in the case of combustion analysis. We have slightly modified the above technique and applied it to the hydrogen analysis. Trifluoroacetic acid was placed in the central capillary of the precision coaxial tubing, and tetramethylsilane was added as a reference for internal lock. About 10 w t z solutions of several organic compounds (chloroform, methylene dichloride, benzophenone, benzene, methanol, ethanol, cyclohexane, and n-hexane) in CCl4 were placed in the surrounding annulus. The intensity ratio of the total signals of the organic compound in cc14 and the signal arising from the carboxylic hydrogen of trifluoroacetic acid were determined. A plot of the intensity ratio against the calculated hydrogen content in the CC14solution yielded a linear relationship with a slope of 0.730 l/g-atom hydrogen. Using this as a calibration curve the rapid analysis of hydrogen could be done accurately. Some typical results are given in Table 11. They compare favorably with the results from the combustion method of elementary analysis and are much easier to obtain. Trifluoroacetic acid is a useful intensity standard, since its hydrogen signal does not interfere with the NMR signals of most organic compounds. Analysis of Some Binary Mixtures by Chemical Shift Measurement. The method can be used to analyze the benzene content of a benzene-CC14 mixture. In a typical experiment benzene containing 29.5 m o l z of the reference compound, HMDS, was placed in the central capillary of a preci-
(2) R. B. Williams, “Conference on Molecular Spectroscopy,” Pergamon Press, New York, 1959, p 26.
30
zz
20
Lo
10
-
0
30 -
40
2 Lo
. 10 . 20
0 I
.
.
.
.
0.5
0
,
1.0
Mole F r a c t i o n of t-BVE
Figure 2. Calibration curve for analysis of HB \ /Hc t-BVE C=C -CC14 system
/
\
HA sion coaxial tubing and a mixture of a known molar ratio of benzene and CC14 was put in the surrounding annulus. The NMR spectrum consisted of two chemically shifted sharp absorptions from the benzenes in both the containers. The chemical shift separation between the two signals, 6 = 6(su,,, 6(,,t,~, was linearly dependent upon the mole fraction of benzene in the outer mixture. Using this linear relationship one can rapidly determine the benzene content within an absolute error of about 10.5m o l e z . The same procedure was applied to analyzing t-butyl vinyl ether (t-BVE) in cc14. Good correlations were obtained between the chemical shift separation 6 and the mole fraction of t-BVE for all hydrogen signals in the compound as shown in Figure 2. Using these four curves, the determination of t-BVE content in a given mixture with CC14 could be carried out very precisely. Nonlinearity of these curves above 50 m o l e z of t-BVE may be partly due to the intermolecular interaction among t-BVE molecules themselves. Such interaction would be common in concentrated solutions, especially in systems containing polar functional groups and one should never assume a linear calibration over a large concentration VOL. 41, NO. 11, SEPTEMBER 1969
0
1519
System t-BVE-toluene i-PVE-CClr
a
Table 111. Analyses of t-BVE-Toluene and i-PVE-CClr Systems Mole fraction Taken Found t-BVE 0.651 0.653 (0.651)” CPVE
Error
0.402 0.192
0.395 (0.396)O 0.190 (0.198)&
0.002 (0.000)5 -0.007 (-0.006)” -0.002 (0.006)“
0.820 0.411 0.228
0.810 0.407 0.240
-0.010 -0.004 0.012
Determined from the signals of toluene.
System Benzene-toluene Hz0-Dz0
Table IV. Analyses of Benzene-Toluene and H20-D20 Systems Mole fraction Taken Found Benzene 0.788 0.795 HzO
0.490 0.106
0.485 0.106
0.772 0.380 0.095
0.750 0.400 0.105
Error 0.007 -0.005
O.Oo0 -0.022 0.020 0.010
range. So it is necessary to prepare a calibration curve over the range to be studied, and use of a two-point calibration or extrapolation beyond the calibrated range should be avoided. The analyses of t-BVE-toluene and isopropyl vinyl ether (i-PVE)-CC14 systems were carried out in similar fashion. The results are summarized in Table 111. The magnitude of the change in 6 value mainly depends on the change in the volume susceptibility x u of the sample solution along with the variation of its composition. With a larger difference in x V value between each component in the mixture the magnitude of the change in 6 will generally increase and a more precise determination can be expected. However, as shown in Table IV, the procedure gave good results when applied t o benzene-toluene and water-deuterium oxide systems where the x C values of the components are very close together -xu x 106 (20 “C);benzene 0.611, toluene 0.618, water 0.719, deuterium oxide 0.715 ( 3 , 4 ) .
The analytical method described here requires only a few minutes per determination once the spectrometer is set up. As the technique does not involve intensity measurements, the stability and linearity of the amplifiers have no effect on the results. It is also necessary to use the same sample tube throughout preparation of the calibration curve and in the analysis. The method is generally limited to a two-component system. The presence of a small amount of third component could have an effect on the measured 6 value and thus cause an error in the analysis. Particularly when the x u of the components are similar or an impurity with a dissimilar xuis present, the effect would become large. So the presence of any other compound should be avoided.
(3) J. W. Emsley, J. Feeney, and L. H. Sutcliffe, “High Resolution
The authors are grateful to H. Sakurai for a supply of hexamethyldisilane.
Nuclear Magnetic Resonance Spectroscopy,” Pergamon Press, 1965, p 605. (4) C. D. Hodgman, R. C. Weast, R. S. Shankland, and S. M. Selby, “Hand Book of Chemical and Physics,” 44th ed., The Chemical Rubber Publishing Co., 1962, p 681, 2744.
1520
ANALYTICAL CHEMISTRY
ACKNOWLEDGMENT
RECEIVED for review April 7, 1969. Accepted June 13, 1969.
