A Convenient NMR Microtechnique The law sensitivity inherent in proton magnetic resonance spectroscopy has stimulated the development of a variety of microtechniques designed to produce useful spectra fram milligram quantities a of sample.l.2 The previous techniques require precision machining, glass-blowing, or tedious resolution adjustments, and mast suffer from awkward sample manipulation in loading and cleaning. The need for an inexpensive and convenient microtechnique prompts us to report a modification of the method b of Haynes and Sazavskyl which is suitable t o routine analysis. Acceptable spectra recorded on a Varian Associates T-60 nmr spectrometer are readily achieved in a single scan with only 0.2 mg of material with molecular weights less than 300, while 1 mg samples c give excellent high resolution spectra. Similar results were also achieved on A-60 and HA-100 spectrometers. d An assembled microcell is illustrated in the figure. I t consists of a standard nmr tube (a), a melting paint capillary (c), and two pieces of common hard glass tubing ( 6 . d ) . The sample (-1 mg) is placed in the melting point capillary and dissolved in 10-15 111of solvent. The solution is shaken or centrifuged to the bottom of the capillary, leaving an 8-10 mm column of the sample. Less than 8 mm leads t o poor resolution, while more than 10 mm does not improve the spectrum. Approximately 18 em of 4 mm (od) glass tubing (hereafter refered to as a n insert) is placed inside a standard 5 mm nmr tuhe. A short (11-13 mm) section of glass rod is dropped down into the insert and the sample capillary is pushed down into the insert t o the top of the rod. A piece of tape should be wrapped around the nmr tube near the top to prevent it fram sliding through the spinner and damaging the probe. An nmr spectrum can now he recorded from this assembly in the standard fashion. The short glass rod is required to vertically center the sample in the probe coil. Initial centering is done by adjusting the depth of the nmr tube in the probe, until a maximum signal is observed. Once the height is determined, an appropriate length rod is cut, so that no further adjustments are necessary in subsequent samples. The sample capillary and short rod are readily recovered when the insert is removed from the nmr tube. If 2 or 3 mm (od) tubing is used in place of the melting point capillary, nmr spectra on material collected directly from glpc are possible. A 15-cm piece of tubing is placed in the collection port of the glpc. A droplet of compound condenses and is pushed through the tubing, coating the walls. When the droplet is about 3 cm from the end, the capillary is removed and scaled a t one end. The sample is diluted with solvent, centrifuged, and placed in an nmr tube equipped with an insert. Usually only one collection is sufficient for a reasonable spectrum, thus greatly reducing the time required to collect sufficient sample for a suitable spectrum. Sealing one end of a 3 or 4 mm insert makes a convenient "disposable nmr tuhe." Since these inserts cost less than 20 each the expense of using sealed nmr tubes is small and valuable samples can he retained easily. In addition to small sample requirements, these inserts need only small volumes of solvent. Typically 1 ml can be used for 20-40 samples, thus making it economically feasable to use expensive solvents. Another advantage of the small volume is that the field homogeneity a t the sample is increased, so that fewer spinning sidebands are observed with the inserts than with standard nmr tubes. This work was supported by the National Institutes of Health Grant No. USPHGM 12595, which is gratefully acknowledged.
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Haynes, L. V. and Sazavsky, C. D.,Appl. Sp~ctrosc.,24,452 (1970). 2Pravost, L. R., and Jardin, R. V.,d. CHEM. EDUC., 45,675 (1969).
Jerome Adams Martin Newcomh Brock Siege1 University of Illinois Urbana, GIRO1
Volume 50, Number 1 1 , November 1973
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