0.02% was obtained using the precision pipet. The same precision was obtained with the weight buret. This good comparison for the precision pipet was obtained despite the fact that samples were less than half the size of those used with the weight buret. As will be noted, the mean assay figures by the two methods agreed well with each other. The excellent performance of the precision pipet as indicated by these tests has been confirmed in other experience with it. The assembly as shown in Figure 1 is especially convenient where it may be desired to measure volumes of different solutions sequentially. When making a change, simple precautions will prevent contamination arising from traces of an old solution that cling to the outside of the sampling tube. After swinging the table aside and removing the supply of previous solution, the outside of the sampling tube is rinsed with several changes of the new solution, each time letting a little of the
new solution syphon into the sampling tube. Where only a minimum amount of the new solution may be available for use, the outside of the sampling tube is rinsed by a stream of distilled water from a wash bottle and then dried with a clean piece of filter paper or paper toweling. The solution meniscus inside the sampling tube is allowed t o move to a position 1 or 2 cm. from the end of the tube. The capillary can then be safely immersed in a fresh solution. Hydrostatic pressure causes solution to enter a little way into the capillary, but trapped air prevents mingling of the two solutions. The interior of the precision pipet is promptly filled with new solution, first letting 10 mi. or so syphon to waste. The equipment is now ready to measure out a sample by completing the operations previously described. This latter technique of changing sample solution is probably sufficiently safe from possibilities of contamination to permit sampling directly from an original container
without first removing a portion for the sampling procedure. To summarize, the semiautomatic precision pipet is the equal of a weightburet in sampling precision and accuracy but is more easily and rapidly operated than a weight buret. The precision pipet is uniquely suited to situations where identical sample volumes are desired. Although the precision pipet is believed to have particular utility in coulometric titration work, it should find general application wherever small volumes of solution must be repetitively measured with high accuracy. LITERATURE CITED
(1) Eckfeldt, E. L., Shaffer, E. W., Jr., ANAL.&EM. 37, 1634 (1965). ( 2 ) Leeds and Northrup Co. 7960 coulometric analyzer and 7961 coulometric titrat'ion cell kit, Data Sheet E-94(4)262, Philadelphia 44, Pa. (3) Seligson, D., A m . J . Clin. Pathol. 28, 200 (1957). (4) Tutundaic, P. S., Mladenovic, S., Anal. Chim. Acta 8, 184 (1953).
All-Teflon Sampling Valve Made for Gas Chromatography W. M. Graven and H. R. Harmon, Aeronutronic Division, Philco Corp., Newport Beach, Calif. with the analysis of gaseous effluent from the catalyst bed was encountered during the course of a study of the catalystic oxidation of dimethyl methylphosphonate (DMMP) vapor on platinum-alumina. The gas sampling valve of the Perkin-Elmer vapor fractometer, which was used for periodic monitoring of the reactor effluent stream, became contaminated with condensable reaction products and interference with the DMMP analysis resulted. Although the sampling valve and all connecting tubing exterior to the chromatograph column oven were thermostated a t an elevated temperature and the interior walls of the system were frequently cleaned, the difficulty was not eliminated. However, when the stainless steel sampling loop was replaced with one constructed of glass the performance of the analytical system was greatly improved. Further improvement was obtained by replacement of the entire sampling valve with an arrangement of two four-way glass stopcocks, the springloaded, glass plugs of which were lubricated with powdered Teflon. Because this all-glass system was somewhat fragile and was not designed for ease of cleansing, a sampling valve constructed entirely of block Teflon which could readily be cleaned xas substituted. The entire system from the IFFICULTY
1626
ANALYTICAL CHEMISTRY
Side
TOP
Crow-sectional views
Figure 1 . Sampling valve made of Teflon exit of the reactor to the chromatograph column was made of Teflon and was maintained a t an elevated temperature. An aliquot gas volume could be removed from the reactor effluent stream and transferred to the chromatograph column without being exposed to any metal surface. The design of the Teflon valve is shown in Figure 1. A cylindrical block of white Teflon, 3 inches in diameter by 2 inches in height, formed the body of the six-port valve. Either of two positions could be selected by rotation of the Teflon plug through a 60' angle, thereby alternately directing either the reactor effluent stream or the chromatograph carrier gas stream through the sample loop. A section of l/r-inch i.d. Teflon tubing, which was cemented into two opposite ports of the valve to make the sample loop, formed a sample volume
of approximately 10 cc. The Teflon valve was housed in an air thermostat which was maintained at 63' A l o C., and the connecting Teflon tubing was kept slightly above this temperature. Except for those periods when the temperature of the thermostat was being raised above ambient there was no evidence of leakage, even with a carrier gas pressure of 10 p.s.i.g. With this sampling system, peak areas which were reproducible to within 5% were obtained after several samples had been introduced to condition the capillary column, R-hich was coated with Carbowax-4000. To test the sensitivity of the analytical system an air stream containing known concentrations of DMMP vapor (previously determined gravimetrically by adsorption on activated charcoal) was diluted by known factors to obtain DMMP concentrations ranging from 2 to 3500 mg. per cubic meter. The corresponding chromatographic peak areas were measured with a Disc chart integrator. A linear relationship between concentration and peak area which extended over a range of three orders of magnitude was observed. Although not demonstrated by direct test, it is estimated that a concentration of 0.2 mg. of DMMP per cubic meter of air would have been clearly perceptible with the analytical system.
