Anal. Chem. 1984, 56, 319-320
ACKNOWLEDGMENT We thank Michele Perella (Institute of Enzimology, University of Milan, Italy) for his interest in this work.
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(3) Papeschi, G.; Bordi, S.; Beni, C.; Ventura, L.; Biochim. Biopbys. Acta 1978. 453. 192-199. (4) Katsube, T.: Lanks, I.; and Zemel, J. N. Sens. Actuators 1982, 2, 399-4 10. (5) Macur, R. A. German Patent 2 121 047, 1971; Apr. 30, 1970 U S . APPl. (6) Papeschi, G.; Bordi, S . ; Carll, M.; Criscione, L.; Ledda, F. J . Med. Eng. Technoi. 1981, 5 , 84-86. (7) Douzou, P. ”Cryobiochemistry”; Academic Press: Lonon, New York. and San Francisco, 1977.
Registry No. Ir, 7439-88-5;IrOz, 12030-49-8.
LITERATURE CITED (1) Maurei, P.; Travers, F.; Douzou, P. Anal. Biocbem. 1974, 57,
555-563. (2) Larroque, C.; Maurei, P.; Balny, C.; Douzou, P. Anal. Biocbem. 1978, 73, 9-19.
RECEIVED for review July 1, 1983. Accepted September 21, 1983.
Modified Valve Seat for the Static Mercury Drop Electrode R. E. Morton
Instrument Shop, University
of
Georgia, Athens, Georgia 30602
0. M. Evans*
Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, Georgia 30613 The introduction of the static mercury drop electrode (SMDE) (1) was welcomed as a solution to our problem of producing constant-area mercury drops. Our mission is to develop and evaluate new techniques for trace metal speciation. Therefore we use the instrument in both the HMDE and DME modes. Over the past several years, however, we have encountered some difficulty with air getting into the system. The problems were recognizable as drops with varying areas, breaks in the mercury causing discontinuity, and failure of drops to remain on the capillary-all of which led to distorted, undersirable wave forms. We have attempted many solutions, but only temporary corrections were achieved. Modified Valve Seat. Sturrock and William’s (2) approach appeared to offer a viable alternative that could be applied to our situation. However, with the introduction of the modified capillary we decided to redesign the valve seat in a manner to eliminate or a t least minimize air leakage into the system. Figure 1A shows the design of the original valve seat with the modified capillary inserted. This new capillary performed much better than the original and clogging was virtually eliminated. Air entrapment and leakage remained as difficulties, however. A helium leak detector attached to the system revealed that air was diffusing into the system. Probable points of entry were around the capillary nut (upper portion) and the point where the capillary projects from the capillary nut (Teflon tape wrapped around the threads of the valve seat can prevent air from entering at this point). Our conclusion was that the “0” ring was not providing an effective seal. When the capillary nut is tightened the “0” ring seal tends to flatten, perhaps pucker, and become distorted depending on the degree of tightness. In our design we retained the exact height and dimensions of the original valve seat but included a retaining shoulder (-0.056 in. in height) to prevent flattening of the “0”-ring seal. The top of the ferrule is buttressed against this shoulder resulting in an effective “four-point’’seal. We further modified the valve seat (Figure 1B) to obviate electrical conductance problems with the tin oxide film. We used the measurements of ref 2 to provide a basis for comparison. The “0”-ringseal retaining wall is shown (Figure 1B). We have evaluated the performance of the new designs and
I l l
1h I
Figure 1. (A) Original configurations of valve seat and “modified” electrode: (a) valve seat, (b) solenoM valve stem, (c) valve stem rubber tip, (d) capillary, (e) “0”-ring seal, (f) metal ferrule. Figure at right shows compressed condition of “0”-ring seal. (B)Modified valve seat: (a)-(f) same as A, (9) mercury-filled cavity, (h) valve seat retaining shoulder or “0”-ring seal retaining wall. (C) Original valve seat configuration with “modified” capillary: (a)-(f) same as 6,(h) groove in ferrule, (i) ferrule retaining wall.
0003-2700/84/0356-0319501.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
have not observed breaks in the mercury, nor detachment of drops even after extended periods of idleness. Filling the Reservoir. Air trapped in the cavity above the capillary can pose particular problems. Therefore, how one fills the reservoir merits attention. We initially plug the small opening in the center of the reservoir, attach a vacuum line torr) to the end of the capillary for approxto imately 10-15 min, and slowly tighten the capillary nut (the valve is operated while the evacuation procedure is in progress). We subsequently fill the reservoir (continue vacuum) and then remove the plug slowly. All of the air should have been removed prior to removing the plug; however, the flow of mercury should force any residual air out through the capillary. This method also allows very rapid filling of the reservoir if circumstances necessitate draining the system several times. Modified Capillary. During the course of modification of the valve seat we also modified the capillary for use with the old valve seat. The change entailed extending the height
of the ferrule to encompass the “O-ring seal (Figure 1C). The “0’’-ring seal in effect sat in a shallow groove (the outer edge of the ferrule acted as a retaining “wall”). When the capillary nut was tightened, the “0”-ring seal was pressed against the upper surfaces of the valve seat. Essentially the effect was a contained “four-point’’ seal. This modified capillary was installed and the system checked with the leak detector. No leaks were detected over several 48-h test periods.
LITERATURE CITED (1) Peterson, W. M. Am. Lab. (Fairfield, Conn.) 1979,Dec, 69. (2) Sturrock, P. E.: Williams, W. K. Anal. Chem. 1982, 5 4 , 2629.
RECEIVED for review July 14,1983. Accepted October 12,1983. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the U.S. Environmental Protection Agency.
