Simple Automatic Pipet. William Nye, Department of Chemistry, Stanford University, Calif. increasing number of biochemical methods employ autoAlthough two types are now on the market, the pipet described can be made from readily available materials, exposes only glass and mercury to the liquid pipetted, and has very few moving parts, and the electrical portion can serve a number of glass pipets by transferring the solenoid coil. The flow of liquid is controlled by a double glass valve with an iron core, When the current flows through the solenoid coil, the lower valve opens and the upper valve closes. When no current flows, the top valve is open and the lower is closed. The fluid flowing in the upper valve displaces mercury until the current shuts off, then flows out through the lower valve. N
A matic pipets.
Figure 1 illustrates the principal features. The magnetic valve assembly waa made by sealing 3-mm. capillary tubing onto each end of a 12-cm. length of 12-mm. borosilicate glass tubing, and a side arm of 3-mm. capillary tubing (used to give added strength for support of the mercury-filled U-tube) was sealed on a t an angle, which allows air to be displaced from the U-tube during fillin . Then the 12-mm. tubing is cut just above the side arm. #he magnetic valve is made by sealing a 6-mm. rod in one end of a length of '/,-inch iron rod, constricting the tubing just above the iron, and sealing a 6 m m . borosilicate glas8 rod The rod a t either end is drawn into the other end of the tubin to a point so that the assembfq will move freely for 2 to 5 mm. along the iength of the cylinder. Each end is seated to a watertight fit in ita end of the cylinder by grinding with fine emery dust; very little grinding is neceasrtr , if the ti s of the rods are radially symmetrical. The valve is tien place! inside the cylinder and
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the cylinder is sealed. It is important that this assembly be kept straight and that a 12-mm. tubing fit into the llrinch brass tubing core of the solenoid coil. The diameter of bulb A should be relatively large, in order to reduce the pressure necessary to operate the pipet, and the top of the bulb should be located approximately on the same level as the center contact, b. The volume of B is about 1ml. less than the volume to be pipetted and B may be eliminated for small volumes if the proper size of glaas tubing is chosen. The volume delivered by the. pipet is determined by the volume between a and b, which are pieces of tungsten wire sealed into tubing. The solenoid coil consists of 0.5 pound of No. 34 enameled copper wire wound on a spool made with a '/$-inch brass tube 2 inches long and l/a2 inch in wall thickness. Such a coil supplies force to keep the upper valve closed when the rectified current from a 110-volt alternating current course is put through it. All exposed wiring should be covered by tape, rubber tubing, or some other suitable insulation. Filliig and Operation. The U-tube is filled with mercury t o a level just below b. The fluid to be pipetted may be placed in a reservoir a t sufficient height to raise mercury in the outer side of the U-tube slightly above a, or this pressure may be attained by using a closed reservoir under proper gas or air pressure. The pipet is filled with fluid by alternately turning the two-way stopcock to the reservoir and to the air until all air in the pipet is displaced by fluid. If the stopcock is left open to the reservoir and the electrical circuit is complete, the pipet will repeatedly deliver fluid equal to the volume between a and b. Accuracy. A 1-ml. pipet delivered the desired volume a t 4-second intervals with a maximum difference of 0.004ml. (0.4%). A 5ml. pipet delivered a t 4-second intervals with a maximum difference of 0.030 ml. (0.6$&).The maximum difference may be decreased by decreasing the rate of filling and/or.emptying. This work was done during a research assistantship on a project supported by the United States Public Health Service. Dispersion of F i e Particles for Electron Microscopy. Vincent Salines, Interchemical Corporation, New York, N. Y. N the electron microscopy of pigment particles, it is often difficult to make very fine dispersions. The author has found the following method generally satisfactory. The pigment powdemarbon black, phthalocyanine, bensidine yellow, etc:-is dried in an oven at low heat, usually 40" to 80" C., to remove as much moisture as possible without changing its article eharacteristics. &ith the tip of a stiff spatula, 0.10 to 0.25 gram of the pigment is transferred to a white Carrara glass slab (ground with 320mesh abrasive to impart "tooth"). On this small mound of pigment is dropped 1 to 2 ml. of a solution containing a p roxi mately 10% of nitrocellulose in Cellosolve acetate (nitroceh'ulosi in stick form offered by Mallinckrodt Chemical Company under the name Parlodion has been found satisfactory). By means of the spatula, the mixture of pigment and solution is rubbed out; as the solvent is lost and dryness approaches, the rubbing is stepped up in vigor and speed, in order to obtain the maximum shearing effect and dispersion of the pigment in the binder. When the mixture appears dry, a few drops of distilled octyl acetate (2-ethylhexyl acetate) are added and the mixture is covered with a Petri dish. I n a short time, the mas8 will be softened by the solvent action of the octyl acetate. A few more drops of solvent are added and the mixture is again worked vigorously with the spatula until it appears to be fully homogeneous. One small drop of it is then allowed to fall on the swept surface of distilled water in a paraffin-coated dish. It will spread out into a very thin film. Evaporation of the octyl acetate may be hastened by use of an infrared lamp. The period of evaporation, using the infrared lamp, is between 2 and 3 minutes. Lectromesh (200mesh) disks (C. 0. Jelliff Manufacturing Company) are dropped shiny side down onto the film and picked up in the usual manner to be dried before insertion into the electron microscope. All solvents must be freshly distilled, because water and other contaminants cause the film to be fragile and lacy. The Parlodion solution must contain no plasticizer, as this produces films which creep in the electron beam.
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8 MM. TUBING
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Figure 1. Automatic Pipet Re. Relay loo0 ohmnor more, S.P.S.T. normallyopan. Sa. Selenium rectifier. Seletrun SM1.. R. 25-ohm 2-watt reaistor. C. 29. mfd. 300-volt deotmlytic condenwr
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