An Inexpensive Way To Do Flash Chromatography Barry M. Jacob? Barnard College, New York, NY 10027 Flash chromatography1 has rapidly berump a standard mrthod for the isolation of nornwlatile rompuunds in organic synthesis because o i its spred, generality, and relarivrl? low cost. Its rapidity particularly recommends it for instrustional lahs because it can he inteeratrd with svnthetic experimenu withuut substantially lengthening them. H o w . er. thoueh inexoensivr in the research lahorarors, the initial investment for a teaching lab is substantial, principally because the oressure control valve required is costlv. Usually, new columns must also be purchased because the ones uitd in most teachlnp lahs do not have a around win1 for attachment to the pressure control valve. Typically, the standard glassware for a 10-mm-diameter column setup costs about $100, and more for a larger one.2 When the source of the pressure used during chromatography is a "house" air line, this special glassware is essential because the full pressure of such lines is both too high for good separation and a safety hazard with glass columns. I t is possible to set up low-pressure regulators and manifolds for house air lines or compressed gas tanks, hut that too has a high price, as aregulator would be needed for each outlet and a needle valve for each column. An inexpensive substitute for a compressed air line or tank is an aquarium diaphragm or "vibrator" air pump, sold in any tropical fish pet shop for $6.00 to $10.00. An even cheaper valve (usually $1.00 to $2.00) that substitutes for the pressure controller is available from the same sources (the valves are normally used to divide the output for aquarium aerators and filters). Still et al. noted' that flash chrumatogra~~hy was an exrension of the short-column mrthod of Hunt and H i c b ~ . ~ ' C h e of their latter used an aquarium air pump to speid columns, hut, because they were using very fine mesh packings, settling required an hour. They also used moderate piessures, larger than these pumps can deliver, during elution. The diaphragm pumps typically deliver air a t under 6cm-Hg pressure and can be used safely (thoughnot efficiently) without any need for a pressure relief or control valve. Precise control of the pressure so that flow rates can be adjusted is essential to the flash technique hut easily accomplished if the output of the p u m p i s attached to a T tube, with one arm going to the column and the other to the
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inexoensive brass or olastic air valve mentioned above The air hbse from the T t i b e can be a~rachedto the column using a soft, one-hole rubber stopper. No clamp is needed because the pressure exerted by the pump is not sufficient to cause eiection of the stopoer from the column, even a t full pressire, unless the ouiiet of the column is closed. One Change in the standard flash procedure1 is required. The pressure from the pump is sometimes not high enough to drive all the air from a dry-packed column. Instead, Hunt and Rigby's method of pouring in a slurry and then applying the pump a t full pressure (with the column outlet open) to settle the nackine is used. Since the silica normallv used in flash columns settles rapidly, this produces a column ready for samnle aoolication in about the same amcunt of time that ~ t i i i ' spidcedure takes. The valve is then reset to give the standard 2-iu./min eluant fall. Using this equipment, students in my lab have separated mixturr; of flunrtne and fluorenonel and uf fcrrocene and acetylferrocene5 in a little over a half hour, timed from the beginning of packing of the column to completion of the elution. One minor orecaution is needed. Ooeratine the pumps with both the Lalve and column closed for prolonged neriods can damaee the oumn . . diaohraem. . - However, replacement diaphragms cost only a few cents. With slightly stroneer (but still low-oressure) . Dumps . it is also possible to have more than onestudent pcr pump, using additional Ttuhes to di\,idr the w t v u t , hut this requires carerul coordinatiol~by the students in setting the vafve and eluting their colum& together.
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' Still, W. C.; Kahn, M.: Mitra, A. J. Org. Chem. 1978, 43, 29237975
A column, controller, and clamp are listed for $102 in the Ace Glass. Inc.. catalog 900. Many supply houses sell J. T. Baker prepacked columns starting at $155. Hunt, B. J.: Rigby, W. Chem. Ind. 1967, 1868-1869. Fieser, L. F.: Williamson, K. L. Organic Experiments; Heath: Lexington. MA, 1983: pp 163-164. Roberts. R. M.: Gilbert. J. C.: Rodewarld, L. B.: Wingrove. A. C. An introductionn to Modern Experimental Organic Chemistry. 3rd ed.: Holt. Rinehart 8 Winston: New York. 1979; p 396.
Volume 65
Number 5
May 1988
459
