In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp SUNY-Cortland Cortland, NY 13045
From GC to the NMR: A Simple Semipreparative Gas Chromatography Collection Method Using NMR Tubes Andrew R. Bressette Department of Chemistry, Berry College, Mt. Berry, GA 30149-5016;
[email protected] Semipreparative gas chromatography, using a gas chromatograph equipped with 1⁄4-inch packed columns and thermal conductivity detectors, is an excellent technique for isolating small amounts of volatile compounds from mixtures. The high purity of these isolated samples makes them well suited for analysis by NMR or IR spectroscopy. Many recent microscale organic laboratory manuals discuss the application of this technique and the use of commercially available GC collection tubes (1). Additionally, several articles have been published in this Journal offering modifications that facilitate the collection process (2, 3). Once a pure compound has been isolated, NMR spectroscopy is a fast, nondestructive method for structure determination. The integration of NMR spectroscopy into the laboratory curriculum, once limited to larger schools with powerful superconducting magnets, has been facilitated by a versatile upgrade developed to give new life to the multitude of 60- and 90-MHz NMR instruments prevalent in smaller schools.1 Indeed, the routine acquisition and interpretation of NMR spectra has become ubiquitous in the undergraduate laboratory experience. Although many deluxe microscale glassware kits include two GC collection tubes that permit the collection of tens of milligrams of pure compounds, several basic glassware kits include only one or no collection tubes, making it difficult for each student to isolate multiple components of a mixture. The cost of purchasing these tubes individually, either to equip a lab initially or to replace equipment lost through breakage is also quite high.2 One further limitation of these commercially available collection tubes is the small volume of purified compound one is able to collect. While excellent NMR spectra can be easily obtained by high-field instruments with only tens of milligrams of material, 60- and 90-MHz instruments often require more concentrated samples. This is especially true if students are to acquire 13C NMR spectra. More concentrated samples allow students to acquire reasonable 13C NMR spectra often with only 32 scans—an important consideration, in view of the large number of students needing instrument time during a typical laboratory experiment. Design and Use of the Collection Tube In light of these limitations, a new GC collection tube was developed utilizing a small piece of 3-mm i.d. Tygon tubing, a short piece of glass tubing (4 mm o.d., 2 mm i.d.), and a standard 5-mm NMR tube (Fig. 1). This inexpensive collection apparatus is easy to make from materials readily available in most chemistry stockrooms and avoids all the disadvantages of the commercially available collection tubes. 366
Connection for GC outlet port Tygon tubing
Glass tubing with capillary drawn end NMR tube
Figure 1. A semipreparative GC collection tube.
As an added benefit, this collection apparatus allows pure compounds to be condensed directly into NMR tubes, obviating the need to transfer the purified compound from the collection tube to the NMR tube. To assemble the collection apparatus, a short piece of glass tubing (about 30 cm) was heated in a Bunsen burner and then pulled so that the tubing had a tapered end resembling that of a glass Pasteur pipet. Once the tubing had cooled, a 90° bend was made in the nontapered end to facilitate connection to the instrument while keeping the NMR tube in a vertical position. A short piece (3 cm) of Tygon tubing was then attached to the collection tube by sliding it about halfway onto the glass tubing. Although this tube was designed for use with the horizontal outlet port of the GOW-MAC Series 350 gas chromatograph, it can be easily modified to fit the needs of other instruments. Although a Pasteur pipet could be used in place of the bent, tapered glass tubing, it is not ideal. The body of the Pasteur pipet does not fit in the NMR tube and creates an apparatus that is especially top-heavy and likely to result in breakage of the NMR tube through student handling. The apparatus connects quickly and easily to the outlet port of the GC with the Tygon tubing so that the glass tube comes in direct contact with the outlet port. This simple connector allows students to readily change collection tubes during GC runs so that multiple compounds can be easily isolated from a single injection. The tapered end of the glass tube causes any condensate to be forced into the NMR tube by the carrier gas. Any remaining residue in the collection tube can be easily rinsed into the NMR tube with the desired NMR solvent. For most compounds, simply placing the NMR tube equipped with the tapered glass tube into a wide-mouthed
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
In the Laboratory
test tube filled with an ice–water bath was sufficient to permit excellent recovery of the injected compound.3 Typically, a 100-µL injection resulted in a recovery of 60–70 µL of the compound in the NMR tube. Large quantities of purified compounds suitable for 13C NMR analysis were easily obtained with these collection tubes by performing two or three sequential 100-µL injections and isolating the various components of the mixture into separate NMR tubes. These new sample collection tubes greatly facilitate the isolation and further analysis of volatile compounds. The ease with which they can be put together and used coupled with the low cost of the materials needed makes them ideally suited for the undergraduate laboratory. Notes 1. Upgrade developed and distributed by Anasazi Instruments, 401 Cashard Ave. #103, Indianapolis, IN 46203; 317/783-4126. 2. Commercial GC collection tubes cost $18 each as listed in the Aldrich Catalog, 2000.
3. In determining the percent recovery, samples were collected on a GOW-MAC Series 350 gas chromatograph equipped with a thermal conductivity detector under the following conditions: flow rate 30 mL/min; injector port 140 °C; column 120 °C; detector 180 °C. Two different 1/4 -in. packed columns were used and both gave excellent results: (i) 8-ft 20% DC 710 on Chrom P (80/100 mesh), (ii) 8-ft 20% Carbowax 20M on Chrom P (80/100 mesh).
Literature Cited 1. Schoffstall, A. M.; Gaddis, B. A.; Druelinger, M. L. Microscale and Miniscale Organic Chemistry Laboratory Experiments; McGraw-Hill: Boston, 2000. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques; Saunders: Fort Worth, 1998. Mayo, D.; Pike, R.; Butcher, S. Microscale Organic Laboratory; Wiley: New York, 1994. Mayo, D.; Pike, R.; Butcher, S.; Trumper, P. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991. 2. Hulce, M.; Lee, S. J. Chem. Educ. 1991, 68, A142. 3. Toler, J. R.; Friedman, H. S. J. Chem. Educ. 1978, 55, 375.
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