A New Microcell Technique for NMR Analysis Sophia J. Yu' Northeast Missouri State University. Kirksville, MO 63501 Small sample analysis is not unusual for routine organic chemistry research. Techniques for separation and identification of small samples are important in terms of time, manipulation, and cost. Usually a few milligrams of sample can be separated easily and quickly by simple methods such as thick-layer chromatography. With this amount of compound, all-glans microcells are commonly applied to NMR analysis. Several problems arise with these microcells. Samples are difficult to recycle, if recycling is required. Cell cleanine creates the risk of contamination. I n addition, the micnrrlls are ratht;r rxpensivr; each normally costi ahout ,517.00 ' Due ru the difficlllties of handling, more than a feu microcells may be needed if numerous samples are run in the same period of time. Anew technique resolviugmost of these problems was invented recently in our Lab. By using this technique, the expensive microcells are eliminated and replaced with disposable units a t minimum cost. The microcells used in the new technique are constructed for each experiment. The assembly is very simple. As shown in Figure 1,a round-bottom NMR tuhe and a melting point capillary (sealed a t one end) are required. A hydrogen-free suspending solution is selected; carbon tetrachloride is the preferred choice. Approximately 10% of the sample is dissolved in the desired solvent (usually CDC13 and TMS) and introduced into the capillary with a microsyringe. The 12solving limit is 1.5-4.0 mg, depending on the complexity of the spectrum. More complex and detailed sample spectra require a larger sample size. Therefore, 15-40 pL of the 10% sample is used in the capillary. This volume cannot be reduced further because it centers the capillary in the suspending solution for the sake of increasing sensitivity, as does the round-bottom of the NMR tuhe. The capillary can he either open or sealed. Recycling is accomplished easily by withdrawing the caoillarv and breakina the two ends to intruduce the sample to, fur examplr. a K H r pellet. \'irtuall\. no rleaninr is nwded hetuevn cach ex~erimtx1, and the rhances ot'ron;alailli~tionare minimizrd &cause no direct contact occ~trsbetween the sample and thc N M R
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Present address: Chemistry Department, University of Nebrash, Lincoln, NE 68588. For example the Wilmad 508-CP at $16.55 each.
812
Journal of Chemical Education
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NMR t u b e
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Figure 1. The selting of the new NMR microcell.
tube. Easy preparation is convenient for the analysis of a large number of samnles, and the cost for each experiment is only about $0.005. The efficiency and significance of the new technique have been tested with a series of experimental analyses. In each analysis, a compound representing a specific type of spinning pattern was chosen. The spectra of macro- and microsamples were compared for each compound, and the latter
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were made by using the new technique. With 1.5-4.0mg of material used and-with spectra ranging from the simple AzXz to the complex ABC type, the sensitivity obtained from the microtechnique was above 19:1.3,4 For example, acrylonitrile was selected as representative of the ABC type of spectrum (Fig. 2). Comparing the spectra of the microsample (h) and the macrosample (a), the resolution of the microsam~le is not as eood as that of the macro. svectrum . ii~mplt.But wirh 2.3 mg of sample, the sensitivity of the .Dectrurn satisfied the essential reuuirementi for the idenrification. Thirteen peaks are recognized in the microsample spectrum as compared with 14 in the macrosample spectrum. No other special operational technique is required to work with the new microcell. In switching from macrosample to microsample, only two parameters, the spinning speed and the filter, need to be changed. They are adjusted until the sensitivity is satisfied for the complexity and the quality of the material used. It is necessary to adjust the spinning speed until the microcell centers in the vortex of the suspending solution, similar to the way external standard capillaries are used. The TMS standard is internal as ordinary CDCI3 with 1%TMS is used as solvent. The technique was not tested with thick-walled tubes. The new technique has the benefits and the capability for small sample analysis. Experiments can be set up economically with all essential equipment for NMR analysis, even class assignments for undergraduate students. Also, samples can he recycled with virtually no danger of destructive interference such as contamination, and proper handling eliminates the burden of cell cleaning and compound loss. The technique is recommended to the chemists engaged in both academic and industrial research. I thank K. R. Fountain for suggesting this research and the admimistration of NMSU for support and encouragement. Our ~~
Instrument was the Varian EM 360 ~~
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Figure 2. Macrosample (top) and microsample (bottom) of acrylonitrile.
sample of this concentration. Thls parameter is a function of how many molec,les one gels nto the magnetic field in the resonance cavity so a cell such as Wilmad 508-CP would have some advantage.
Volume 64
Number 9
September 1987
813
