Anal. Chem. 1980. 52, 1999-2000
Table 11. Reproducibility of Analysis of Inorganic Peroxide Solutions by Chromite Method per0 x ide
test
present
Sample
solution
A B
c
D
H,O, wt fractiona for trial
in
HZO, HP, H,O, Na,O,
1
0.0030 0.0117
0.0175 0.0170
a Calculated from eq 4. peroxide.
2
3
0.0031 0.0030 0.0118 0.0174 0.0175 0.0171
av 0.0030 0.0118 0.0175 0.0170
Analyzed as hydrogen
acid-phosphoric acid mixture, resulting in a color change to the characteristic yellow-gold of dichromate. After the acid additions, the solution was immediately titrated with ferrous ammonium sulfate solution t o a clear green end point.
RESULTS AND DISCUSSION T h e weight fraction of hydrogen peroxide in A, calculated by eq 4, was 0.0030, which compared exactly with the weight fraction of hydrogen peroxide found by the permanganate method. The new method works equally well at higher weight fractions of hydrogen peroxide in alkaline solution, as well as for solutions of alkali and other soluble inorganic peroxides (Table I). A comparison of results for several different peroxide concentrations shows the values for the weight fraction of hydrogen peroxide in all samples were within 1% of the values obtained by the conventional permanganate analysis,
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except for D, the sodium peroxide solution. In this case, the value obtained for the weight fraction of peroxide was significantly greater than the value obtained from the permanganate analysis. Apparently, in spite of all precautions, the acidification step in t h e permanganate analysis caused some decomposition of the relatively unstable sodium peroxide. Reproducibility of results for the new "chromite" method of analysis appears good (Table 11). In another series of six trials on a typical alkaline hydrogen peroxide solution (not detailed here), all values for weight fraction hydrogen peroxide were in the range 0.0305 A 0.0002 (6). Interferences with this method include the presence of reducing species in the 50% NaOH used in the analysis. For best results, the mixture of 20 mL of 50% NaOH, 5 m L of chromic chloride solution, and 5 mL of deionized water used in the determination should be prepared fresh for each sample run and used immediately. LITERATURE CITED (1) Schumb, W. C.; Satterfield, C. N.; Wentworth, R. L. "Hydrogen Peroxide"; Reinhold: NY, 1975: pp 553-560. (2) Pierson, R. H. "Standard Methods of Chemical Analysis"; Welcher, F. J., Ed.; Drieger: Huntington, NY, 1975; Vol. 2, p 1318. (3) Melior, J. W. "A Comprehensive Treatise on Inorganic and Theoretical Chemistry"; Longman, Green and Co.: New York. 1952; Vol. I , pp 944-945. (4) Lynch, R. W. Anal. Cbem. 1980, 52, 348-349. (5) Schreyer. J. M.; Thompson, C. W.; Ockerman, L. T. Anal. Cbem. 1950, 22, 1426-1427. ( 6 ) Lynch, R. W.; Smith, M. R.. unpublished results; Charleston, TN, 1980.
RECEIVED for review May 1, 1980. Accepted July 28, 1980.
Minimization of Nuclear Magnetic Resonance Spinning Sidebands by Spinning Rate Modulation Brad Bammel and Ronald F. Evilia" Department of Chemistry, University of New Orleans, Lakefront, New Orleans. Louisiana 70 122
Spinning sidebands are well-known unwanted peaks which are observed in nuclear magnetic resonance spectra on either side of a strong resonance a t integer values of the spinning rate ( 1 , 2 ) . Modern, high-quality magnets and NMR sample tubes can reduce these sidebands to negligable (- 1'70of main resonance intensity) levels for most applications. Nevertheless, occasions do arise when the residual sidebands of even a high-quality instrument are unacceptable. For example, if one is trying to identify resonances of a dilute sample in the vicinity of a very strong (e.g., solvent) resonance. Also, if one wishes to remove unwanted peaks by spectral subtraction procedures, the presence of spinning sidebands can frustrate these efforts as they are not likely to be exactly reproducible. Elimination or reduction of the solvent or other large resonances may be possible in some cases by isotopic substitution, but this can be quite expensive. Another approach to elimination of spinning sidebands involves time averaging several spectra a t different spinning rates ( 3 ) . Since the positions of the spinning sidebands are determined by the spinning rate, they will average to a broad distribution while the true absorption peak will be unaffected. The availability of pulsed Fourier transform instruments makes this latter approach possible in a short time period. This paper describes the construction and evaluation of a 0003-2700/80/0352-1999$01.OO/O
simple device for spinning rate variation during pulsed Fourier transform NMR operation which effectively removes spinning sidebands in a short time without degradation of the magnetic field homogeneity necessary for good spectral resolution.
EXPERIMENTAL SECTION The spinning rate modulator consists of a solenoid valve which is inserted in the spinner air line and an electronic timer. The modulator works by shutting off the spinning air and allowing the spinning rate to decrease to about 10 Hz, in a time determined by the momentum of the spinner turbine, but not to stop completely. Following this slow down period, the solenoid is opened to allow air to hit the turbine for a time sufficient to reach a rotation rate of approximately 40 Hz. Thus, during the time that the modulator is turned on, the spinning rate is continually changing over a range of about 30 Hz. A 555 timer operating as an astable multivibrator serves as the electronic timing device. The output of the 555 turns a transistor switch in series with a 12-V dc relay on and off. The acceleration and deceleration rates of the spinner used in this study were such that close to equal on and off times were necessary. This was accomplished by making one of the voltage dropping resistors in the standard astable configuration much larger (and variable) than the other. A more sophisticated circuit giving truely independent on and off time adjustment would be somewhat more convenient but did not appear to be necessary in this particular case. On and off C 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
Flgure 1. Effect of spinning rate modulation on spinning sidebands as a function of the number of scans: (a)single scan, no modulation; (b) single scan with modulation: (c) 2 scans with modulation; (d) 4 scans with modulation: (e) 8 scans with modulation: ( f ) 64 scans with modulation; (9) 64 scans without modulation gain X 10; (h) 64 scans with modulation gain X 10.
times of approximately 10 s gave the desired behavior. All spectra reported were obtained on a JOEL FX90Q spectrometer equipped with the modulator described above. All spectra were obtained with 10-nim sample tubes. The 2 axis was slightly detuned to generate larger than normal spinning sidebands.
RESULTS AND DISCUSSION Figure 1 shows the effect of modulation on the sidebands as a function of the number of scans. Incomplete removal of the sidebands is obtained in a single shot experiment, but extremely long time averaging is not required. Essentially maximum removal of the sidebands is observed in approximately eight scans. Since this technique is most needed in those cases where dilute solutions are being studied, one will usually be applying more than a single pulse anyway and, therefore, spinning rate modulation will not, in general, increase the observation time. Figure 2 shows the detection of approximately 10% ethyl acetate in ethanol solvent. Without spinning rate modulation the downfield quartet from the ethyl acetate is obscured by overlap with the sidebands. The ethyl acetage quartet is easily identified, however, in the spectrum acquired with spinning rate modulation. There is no great trick to construction and operation of this device. If the spinning rate gets below approximately 7 Hz, anomalies are observed, possibly due to incomplete averaging of field inhomogeneities or vibration. If the average spinning
Flgure 2. NMR spectra of 10% ethyl acetate (b) with modulation (32 scans).
in ethanol: (a) without
modulation:
rate is too low, a significant increase in the magnitude of the sidebands is observed, because of the spinning rate dependence of the magnitude of the sidebands ( 1 , 2 ) ,although they are broadened and easily recognized. I t is also important that the spinning rate not remain a t any single frequency for any length of time. Thus, the on time of the solenoid is adjusted so that the maximum spinning rate possible is not quite reached. When properly adjusted, the spinning rate is constantly changing over the range of 10 to -40 Hz. A higher maximum speed may be preferable but was not possible with the standard FXSOQ spinner system. An alternative modulator design, in which a peristaltic pump head and slowly turning motor varied the air turbine pressure, was found not to work as well as this design and was considerably more difficult to adjust ( 3 ) . This design might, however, be superior for use on instruments in which the sample tube rides on a mechanical bearing rather than on an air bearing as in the FXSOQ.
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LITERATURE CITED (1) Williams, G. A , ; Gutowsky, H. S. Phys. Rev. 1956, 104, 278-283. (2) Halbach, K. Helv. Pbys. Acta 1958, 29,37-46. (3) Ernst, L.; Wray, V.; Lincoln, D. German Patent 28 16 225, 1979.
RECEIVED for review May 30, 1980. Accepted July 25, 1980. We thank the National Science Foundation for Grant CHE78-02081 which provided matching funds for the purchase of the NMR spectrometer. R.F.E. also thanks the Merck Company Foundation for a faculty development grant.
