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limited to that a t the pump throat, since no effect of this change is reflected in the measured pressures. The advantages of this system for our application are obvious, and further interesting consequences might be predicted. For example, if additional baffling or trapping is desired in a vacuum system and if these are installed in a manner which maintains a curtain continuously down to the pump throat, it would still be possible to achieve the ultimate pressure of the diffusion pump throat or very close to it throughout the critical volume. In other words, the pressure achievable in a selected region of a vacuum system is limited only by the gas flow rate entering the entire envelope and the size of the pump evacuating it, and can be made independent of the baffles, traps, and isolation valves also used. Another potentially useful prediction is that a lower pressure in the critical volume might be obtained by using a larger diffusion pump even if the diameter of the pump exceeds that of the port and vacuum system to which it is attached. In a singly pumped system, one would gain little by this alteration because the net conductance of the pumping stack is limited by its smallest dimension. The major limitation of the vacuum design described here compared to a conventional differentially pumped system is, as has been shown, the ultimate vacuum achievable with it is inherently limited for a given gas flow rate and vacuum pump. As is possible with the truly differentially pumped system, the pressure of the critical volume cannot be con-
tinuously lowered by decreasing the size of the aperture connecting the two envelope volumes. Furthermore, for a given system, much lower pressures in the critical volumes can be obtained with a conventional differentially pumped system. In our application, for example, the pressure in region B could be reduced to about 1 X Torr by the proper addition of a second diffusion pump stack of modest size. Nevertheless, for many applications, the envelope design described here can offer satisfactory results a t a minimum of expense and instrumental complexity, and certainly provides a significant improvement in performance over the conventional singly-pumped vacuum envelope.
LITERATURE CITED (1) B. Munson, Anal. Chem., 43 (13), 28A (1971). (2) E. C. Horning, M. G. Horning, D. I. Carroll, I.Dzidic, and R. N. Stillwell, Anal. Chem., 45, 936 (1973). (3) M. McKeown and M. W.Siegel, Am. Lab., Nov. 1975. (4) E. P. Grimsrud and R. A. Rasmussen, Atmos. Environ., 9, 1010 (1975). (5) Extranuclear Laboratories, Inc., Instruction Manuals for Preamp counting head, model 032-3, and for particle multipliers, model 051, Oct. 1975. (6) S. Dushman, "Scientific Foundations of Vacuum Technique", 2nd ed., J. M. Lafferty, Ed., John Wiley and Sons, New York, N.Y., 1962, Chapter 2.
RECEIVED for review October 4, 1977. Accepted November 14, 1977. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society; to Research Corporation; and to the National Science Foundation for support of this research.
CORRECTION Chemically-Bonded Aminosilane Stationary Phase for the High-Performance Liquid Chromatographic Separation of Polynuclear Aromatic Compounds In Table I of this article by S. A. Wise, S.N. Cheder, H. S. Hertz, L. R. Hilpert, and W. E. May, Anal. Chem., 49, 2306
(1977), the logarithm of the retention index for pyrene on pBondapak NH2 should be 3.26 rather than 3.68.
