AIDS FOR THE ANALYST Pipet-Co ntrolling and Aspirating Device with PressureRelieving Action Robert C. Backus, Virus Laboratory, University o f California, Berkeley, Calif.
microvolumetric pipet measP urements require the use of a manual aid that combines rapid filling RECISE
and discharge rate with fine control of the position of the meniscus. The use of modified hypodermic syringes for this purpose is, in the hands of many otherwise technically competent individuals, difficult and frustrating, occasionally leading to accidental displacement of the contents of the pipet. An aspirating device has been in use in this laboratory for routine micropipetting operations that is convenient to manipulate and is precise in the control of microvolumetric pipets. It also has been found useful for a number of other applications. Construction of the device is relatively simple, without critical dimensions or tolerances.
As shown in the figure, the aspirator is composed of a length of rubber tubing held within a rigid barrel by folding the tubing ends concentrically back over the ends of the barrel. A thumb-actuated wheel, guided by a slot in the barrel, rolls longitudinally over the rubber tubing. The nTheel is retained within the slot by a transverse bar. the ends of which extend on either side of the wheel into slideways milled in the facing sides of the slot. Pipets are attached by insertion of the pipet shank into either end of the aspirator.
aspirator serving to position the meniscus and discharge the fluid. Because of the interchangeability of filled pipets, one aspirator can be used for a number of operations involving sequential additions and the holding of measured samples until mixing is desired. It is quite feasible to make transfers from one pipet to another using an aspirator in each hand, thus facilitating measurement by difference of quantities for which microvolumetric pipets are not usually calibrated. Another use for which the aspirator is particularly well suited is the mixing of microvolumes of reactants by aspiration within silanized glass tubes of small bore. A controlled to-and-fro motion is induced in the liquid column by pressing and rolling the thumb wheel of the aspirator back and forth. The wheel can be set a t will to position the liquid column a t any desired place within the mixing tube, eliminating the problem occasioned in closed-system aspiration by the creeping of fluid along the tube during the mixing operation. The filling of pipets with capacities several times larger than that of a single stroke of the aspirator can be achieved by a series of displacement strokes, momentarily releasing pressure on the thumb wheel while returning the wheel for a succeeding stroke. In this way the aspirator is used as a hand pump. The pumping action can be extended to the filling of smaller macrovolumetric pipets by attaching a check valve to the open end of the aspirator to prevent excessive loss of fluid due to the return of stir during the release of
Thermostated Microcell Adapter for Perkin-Elmer Infrared Spectrophotometers S. T. Zenchelsky and J. S. Showell’, Ralph G. Wright Laboratory, School of Chemistry, Rutgers University, New Brunswick, N. J.
T
deuterium content of organic compounds can be determined in a number of ways, but most involve burning the compound and isolating water, which is then analyzed directly for deuterium content or converted to hydrogen for analysis. The former alternative is most attractive because of the development of an elegant infrared spectrophotometric technique by Trenner, Arison, and Kallrer ( 3 ) . HE
T t J m b wheel
SIDL VIEW Re’aining bar
CROSS
sEcrio,v
Figure 1.
ilir is controllably displaced in either direction in the tubing by siniultaneously pressing and rolling the wheel along the tube. I n the released position the !Theel does not compress the rubber tube enough to prevent passage of air through the tube (see cross section). Thumb pressure on the wheel may thus be released at any point in the displacement stroke to equalize the internal pressure with that of the atmosphere. This arrangement permits removing and reaffixing a filled micropipet n-ithout displacing the fluid contents. Advantage can be taken, if desired, of capillary action in filling pipets. the
the wheel between strokes. The aspirator obviously can be scaled up in size t o accommodate pipets of large capacity. The rubber tubing may be quickly replaced in the event that corrosive or radioactive materials are draKn into it.
Partial cutaway view
Thermostatinp fluid channel, thermometer well, hose connections. clamping screw, and microcell guide bare
ACKNOWLEDGMEN’I
Their technique involves the measurement of absorbance at the 0-D stretching frequency (3.98 microns) as a function of the deuterium concentration of water. -4s the stretching frequency was found to be temperature-dependent,
The author wishes to express his appreciation to George Lauterbach for the fabrication of several niodificatiom of this device.
Present address, Chemistry Department, Columbia University, New York, N. Y . VOL. 29, NO. 1, JANUARY 1957
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