Simple, Inexpensive Apparatus for Sample Concentration Thomas J. Bruno Thermophysics Division, National Institute of Standards and Technology, Boulder, CO 80303
I n many synthetic and analytical procedures, it is often necessary to increase the concentration of a solute in a solution by the controlled evaporation of the solvent. This is achieved most easily using a concentrator such a s the micro or Kuderna Danish designs ( I ) . These are glass flasks consisting of a large volume (10-100 mL) solution vessel placed above a much smaller (0.5-1 mL) cold finger trap. A reflux condenser fits on top of the concentrator flask via a mound " elass "ioint. A soecial electric heating unit can he purchased commercially to use these concentrators. They usually consist of a n aluminum block with a large drilled hole that accommodates the solution vessel section of the concentrator, leaving the cold finger exposed to ambient air. The solution vessei is warmed by theblock to provide controlled evaporation. There are a numher of disadvantages to this apparatus and method. The electrically heated hlock poses some hazard, especially when the solvent being driven off is flammable and has a vapor density greater than that of air (e.g., diethyl ether, carbon disulfide). Vapors can accumulate a t the base of the unit and can be ignited by arcing in the controller or switch. Such a unit would, in fact, not be legally usable in a n explosion-proof laboratory (Class 1, Grouos A and B of the National Electrical Code). There ure also u numhcr of practical disadvantages aside from the safety issue. The cold finger trap in ambient air is wr~rmedsipificnntly through conduction from the hcating hlock. This is undctsirahle, since thls section should he kept cod to minimize bumping and potential product or solute decomposition. In addition, it is difficult 11)ohserve the liquid in the solution vessel section while it is in the hvatinr" block. Moreover, the commercial heating units are also very expensive. The apparatus of Figure 1provides a good alternative to the commercial units. The device consists of two ioined chambers made from polycarhonate tubing and platk. The u m e r chamber contains the solution vessel of the concent;&r, and the lower chamber contains the cold finger. The reflux tuhe is placed on the concentrator flask, and retaining springs hold the entire apparatus in place. The upper chamber also contains a provision for a thermometer or thermocouple. The upper chamber is then heated, and the lower chamber cooled, using the hot and cold outlet streams from a Ranque-Hilsch vortex tuhe. The vortex tube is a unique and useful heating and cooling device that has no movine Darts. and ouerates from a s u o ~ l vof compressed air (2:f). vortex tudes are available cbGiercially in a numher of sizes a t a relativelv modest cost. and have been applied in many laboratory instruments in recent
years (8-13,. Since the operational theory ofthe device has been described in dctail in thrs J o u r n a l ,101and elsewhere. this note will discuss only the application. A schematic diagram of a vortex tube is sho-in Figure l b . A source of comoressed air is connected to the inlet. and two outlet streams are produced, one hot and the other cold. The cold air outlet can reach temperuture.i as low a s 4 0 'C, while the hot air outlet can reach 200 'C. It is oossihle to "tune" the hot and cold outlet temperatures to obtain almost any desired temperature difference between the above mentioned limits. This is done by adjusting the needle valve a t the hot outlet, and by using different vortex generator inserts. A numher of s i c h in'erts are supplied with the vortex tuhe. Usuallv. .. i t is more imoortant to control the temp e r a t u r e of t h e u p p e r chamber, because t h i s will determine the solvent evaporation rate. For this reason, this chamber contains a thermometer. The lower chamber cooling prevents bumping and helps to induce a concentration gradient driving the solute to the cold finger. I t also
Contribution of the United States Government. Paper not subject to copyright in the United States.
Figure 1. A schematic diagram of the vortex heatingicooling sample concentration apparatus.
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Vortex Generator
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Service Alr
Control Needle Valve Nozzle
Cold Outlet
Figure 2. A schematic diagram of a vortex tube helps prevent decomposition of sensitive solutes. An exact measurement of the cold finger temperature is usually unnecessary; touching the lower chamber with the hand to verify cooling will suffice. The exhaust air streams from the upper (solution vessel) chamber and the lower (cold finger) chamber are joined a t two fittings placed opposite from the inlet fittings. This combined flow stream is directed into a commercially available baffle muffler that serves to reduce the sound of escaping air to a comfortable level. When used in a fume hood, the noise of the escaping air is hardly noticeable. The air supply used to service the vortex tube should be relatively clean, and be free of gross oil droplets, rust particles, and liquid water. The presence of water vapor in the air is less of a problem in this application than in situations where lower temperatures are desired from the cold outlet of the vortex tube. A simple and inexpensive particulate filter, readily available for a few dollars, has proven to be an adequate air handling precaution for this application. The concentrator apparatus has been applied in work on natural products such as bcarotene and taxol and has proven to be convenient and reliable (14).The safety features mentioned above make i t es~eciallvattractive for use by students in teaching labs. The concentrator described in this note is very economical to build. The parts for the unit of Fieure 1. includine the vortex tube and muffler. are about five times less than the cost of the commercial electric concentrator unit. The construction is simple, requiring about 1- 2 h in a rudimentary machine shop, and modifications for multiple sample tubes can be incorporated easily. The use of such a device should be considered for all routine sample concentration procedures.
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Sources of Vortex Tubes and Mufflers Vartec Corporation, 10125 Carver Road, Cincinnati, OH 45242, (513) 891-7474, Technical Contact: Karen Thee Exxair Corporation, 419 Findlay Street, Cincinnati, OH 45214, (513) 381-4388, Technical Contact: Edward Crotty Transonix Corporation, 44 Steadman Street, Lowell, MA 01851, (508)453-7860, Technical Contact: Jaques Abrams Certain commercial equipment, instruments or materials are identified in this paper in order to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. Acknowledgment The fmancial support of the Gas Research Institute is gratefully acknowledged. Literature Cited 1. Shugar, G. J.; Shugar, R. A.; Bauman, L.; Baumsn, R. S. Chernlcd lkhnlcions R-dy RefemnceHondbooh,2nd ed., Maraw-Ha: New Ymk, 1981. 2. Ranquo, M. G. J. P k y s Rad. 1933.4.112. 3. Hilsch. R R o u Sci. Inalr. 1947.18(2). 108. 4. Blakr, M. P. J. S c i Instr. 19M.27181, 1968. 5. Blatt, T A,: k c h , R.B. ASME Papr 62-WA-200,1963. 6. Jordan,R. C.; %eater, G. B. Refrigemlion a n d f i r Conditioning, 2nd ed., PrentiHall: Englewood CliUa,1948. 7. Abbott, M. M.; VanNess, H. C. Thomodyurnics, (Schaum's Outline Series) MeGrsw-Hill: New York, 1972, p 226. 8. Bruno, T J. Lrg. Chromologr HPLCMng. 1985.412). U4. 9. B m o , T J . h I Chern. 1988.58, 1596. 10. Bruno, T. J. J. ChemEduc 1987, M111),987. 11. Bruno, T. L; Svomnos. P D. N. CRCHondbook ofBosie TobleosforChemiml Amlysin; CRCPress: Boca Raton, 1989. 12. B m a , T J.: Svoronos, P. D. N. NBS TrchnicalNofoNo. 1096,1986. 13. Smith,D. R. S u p s x ~ i t l aRuid l Chmrnotopphy, Royal Society, CRC Re-: Iandon, 1989. 14. Bruno, T. J. I n Bruno, T J.: Ety, J.F., Eda. Svppreritlal Ruid lbhnology:Reuiews in M&m Theory andAppiimtions: CRC Press:Roea Raton, 1991.
