AIDS F O R THE ANALYST Water Repellency of Silicone on Microburet Tips Mary H. Pugh, National Institute of Dental Research, National Institutes of Health, U. S. Department of Health, Education, and Welfare, Bethesda, Md.
WHERE volumetric
measurements are approximated to 0.001 ml. using the Koch microburet, rigid control of tho size of the droplets is desirable. Occasionally the droplets may fail to hang freely and may tend t o form by creeping upward, away from the orifice, owing to the hydrophilic nature of glass. In such instances suspicion may be directed t o contamination or age of the standard solution or t o a newly greased stopcock. The coudition may be remedied by drying the end of the tip with cleansing tissue and immediately rubbing it with a siliconetreated tissue (Sight Savers), thus rendering the surface on the ground point of the tip hydrophobic. The silicone in the prepared tissues appears to be more effective than some other silicones. After coating the dropping tip of a Koch microburet with silicone, individual volumes of about 0.0001 ml. can be delivered easily. This degree of control may be of some consequence in such volumetric procedures as calibrating micro volumetric glassmare or determining the volume of a small, irregularly shaped, biological specimen. The application of a silicone coating to the tips of capillary burets and capillary pipets may prove useful in quantitative ultraniicroanalysis.
Filter System for Radioactive Exhaust from Flame Spectrophotometer J. H. Edgerton, H. G. Davis, 1. C. Henley, and M. T. Keliey, Analytical Chemistry Division, O a k Ridge National Laboratory, O a k Ridge, Tenn. L.um
spectrophotoniet.ric analysis of radioactive solutions is
Fhazardous because the radioactivity is Provision dispersed in the vapors for removing
of the flame and in the exhaust gases. this radioactivity is not usually made in laboratory fume hoods or on flame spectrophotometers. An effective and economical filter system for removing the radioactivity has been built which uses a special filter ( 4 ) in an auxiliary exhaust system attached t o the burner housing of the spectrophotometer. Other filter media, such as those described by Stafford (3) and by Smith (b), should prove equally satisfactory for this application.
The filter system built for use on the Beckman Model B flame spectrophotometer is shown in Figure 1. A 2.5-inch-diameter duct of l(i-gage, Type 347 stainless steel is attached to the exhaust port of the burner housing. This duct extends vertically for 24 inches above the housing and then flares into a right-angled elbow, which is connected to the filter unit by a 4-inch-diameter flexible conduit of galvanized iron. This nystem of ducts acts as a heat exchanger for the hot exhaust gases, and is designed t o facilitate replacement of the filter unit. By means of another section of flexible conduit, the filter unit is attached to a 2-inchdiameter squirrel-cage blower (Buffalo Forge Co.) powered by a 0.10-hp. motor rated a t 1500 to 3000 linear feet per minute of air flow. The filtered air is discharged into the fume hood system of the laboratory. The rate of air flow a t the port of the burner housing was determined to be 1200 linear feet per minute. The auxiliary blower and the filter should be mounted on a shelf above the flame spectrophotometer a t a minimum distance of 3 feet from it. This arrangement facilitates proper handling of radioactive solutions and permits the use of lead shields if necessary. The entire assembly fits conveniently inside the average hood space of 4 by 8 feet. The most important feature of the filter system is the filter medium, which is a specially prepared asbestos-impregnated paper developed by Arthur D. Little, Inc. This paper is arranged in deep multiple pleats over fluted aluminum foil separators. The filter medium is mounted in a fram: t o form a compact unit 8 inches square and 4 inches deep. rests (1) made on atmospheric dusts and on aerosols with average particle-size diameters of 0.3 to 0.5 microns indicate that the efficiency of the filter is as high as 99.999%. The tests were made at air flow rates cquivalent to a static pressure differential of 1 inch of mercury and a t temperatures up to 250" F. Periodic examination of the duct system and filter unit has indicated that the filter efficiently removes the radioactivity from the air stream being discharged into the fume hood system of the laboratory. A typical radiation survey showed 1 milliroentgen per hour of radiation a t contact along the exterior of the duct that leads from the burner housing t o the filter unit; radiation of 8 t o 13 milliroentgens per hour was dctected a t the filter unit. No radioactivity was detected inside the burner housing or along the duct that leads from the filter case t o the hood exhaust duct. The filter is replaced when the rate of air flow decreases to less than 800 linear feet per minute or when the radioactivity accumulated on the filter causes background radiation greater than 7.5 milliroentgens per hour a t the position where the flame spectrophotometer is operated. This filter system has been in use for approximately two years in conjunction with n Beckman Model B flame spectrophotometer, equipped with a photomultiplier tube and a sensitive external photoamplifier. Use of the system, which results in an increase in the rate of air flow through the burner housing, does not appreciably change the luminosity readings obtained with the flame spectrophotometer. However, new calibration curves should be prepared when the exhaust fan and filter unit are put into operation with the flame spectrophotometer. LITERATURE CITED
(I) Little, Arthur D., Inc., Cambridge 42, Mass. "Interim hIemorandum Report on Filter Development and Discussion on Availability of Materials," May 19, 1950. (2) Smith, W. J., "Noncombustible and Chemical Resistant Air Filters for High and Low Temperature Use," Air Cleaning Seminar, Ames Laboratory, Sept. 15-17, 1952, U. S. Atomic Energy Commission, W A S H 149 (March 1954). (3) Stafford, E., and Smith, W. J., I n d . Eng. Chem. 43, 134G (1951). (4) Stockdale, W.G., ORKL Chemical Technology Division, private communication to J. H. Edgerton, hIarcl1 12, 1953.
