techniques for which concentrations as low as 10" 1 2 M have been detected in a 50-μηι-diameter capillary. To take advantage of exciton detec tion, an appropriate molecular crys tal with the diameter of the capillary being used for electrophoresis is placed at the point where the sepa rated molecules exit. This placement of a crystal full of donor excitons at the edge of the capillary allows one to t a k e a d v a n t a g e of t h e vastly i n creased excitation efficiency inherent in energy transfer (as opposed to di rect-laser excitation). The molecules to be separated are appropriately tagged with acceptor fluorophores. Those that pass within the excitation envelope of the crystal have a 50% chance of being excited. How would the sensitivity of such a detection system compare with the current state of the art? Let us con sider a concentration of 10" 1 2 M for the separated molecules. If a 50-μπι capillary is used together with a flow rate of 1 mm/s, there would be 1500 molecules/s to sample. At this rate, each molecule would remain in the excitation envelope of the crystal for 5 μβ. Given an average fluorescence of 5 ns, there would be 500 excita tions per molecule, which would pro vide an overall signal of 7.5 χ 10 5 photons/s. Because the signal is so large, we can consider scaling down the capil lary size from 50 μπι to 2 μηι, the smallest size used to date. Smaller capillaries produce better s e p a r a tions, and for this dimension a count r a t e of 1 0 3 p h o t o n s / s w o u l d be achieved without having to increase the concentration from the 10" 1 2 M used in the 50^m-capillary experi ment. Such count rates are easily achieved with sensitive photomultipliers and, given the sensitivity lim its of photomultipliers, the concen tration could probably be dropped to 10" 13 M.
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The authors thank Michael Rudman for provid ing us with Figure 7, which was obtained as part of his Ph.D. research at the Hebrew Uni versity of Jerusalem. We also thank Raoul Kopelman for many stimulating discussions, espe cially on molecular exciton microscopy and exciton detection. We also thank Edward Yeung for helpful comments on the CZE section. This research was supported by the Innovative Sci ence and Technology Office of the Strategic De fense Initiative, the Letterman Army Institute of Research of the U.S. Army, and the Israeli Ministry of Science. References (1) a. Synge, E. H. Phil. Mag. 1928, 6, 356. b. McMullan, D. Proc. Roy. Microsc. Soc. 1990 25 127. (2) O'keefè, J. À. / Opt. Soc. Am. 1956, 15, 359.
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 11, JUNE 1, 1991 · 637 A
