A Simple Method for Drying an NMR Spinner Air supply^
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t o air Many NMR systems such as the Varian 360 instruNMR ment in our laboratory use a compression pump to supply air for sample spinning. If the laboratory air is relatively input from humid, compression to a typical pressure of 2 atm can hose \ \ compressor easily raise the partial pressure of gaseous H20 to the clamp point where condensation will occur. This causes ohnous problems with smooth simple spinning as well as with filter clogging. As noted in a recent report on an effective commercial drying system (Van Air Systems),' inclusion ... clamp .... .... .... .... of a drying tube in the gas line adds thejnconvenience of frequent replacement of desiccant, and we suspect that fine particles from the desiccant may cause additional i c e slush in, problems. insulated A simple alternative to a commercial dryer is a modi- c o o l e r fied cold trap shown in the figure above. The trap may he constructed from a 250-mL sepmatory funnel as shown or from an all-glass trap with a stopcock added at the hottom. Water in the com~ressedair condenses preferentially in the cwled trap, gihng a compressed airsupply with a water partial pressure closer to the vapor pressure a t 0 OC. The stopcock is left open and the hose clamp on the water drain, which must be at least partially open to avoid excessive pressure build-up, is adjusted to permit automatic steady purging of the condensed moisture. In our experience with this arrangement the ice bath is changed each day, a simple operation that requires no dismantling of the air system. (Most recently we have used a cwler with a drain, indicated by the dashed lines in the figure, to make the addition of ice and removal of excess cwling water even easier.) The air supply may not be as dry as that produced by a commercial system, but we have experienced no problems due to moisture. The long length of tubing between the cold trap and the NMR cavity permits the dried air to warm up to room temperature before entering the thermostated cavity. The glass trap is wrapped with fiber-reinforced tape to minimize the hazard from accidental breakage of the pressurized trap.
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1 Roberts,
R. Am. Lob. 1988, (Oct. News Ed.), 16. Mbhael S. Holdsn, Paul J. Ogren, and Steven D. Lalnhart Earlham College Richmond. IN 47374
Volume 67
Number 5
May 1990
445
