with longer beam lines. Mass spectrometrists may be interested in exploring the technique's potential for high resolution.By pushing the technique with faster rise times of the BVM wave form, higher beam voltages, smaller BVM tubes, and energy selection of the continuous ion beam, vastly higher resolutions could be achieved. The technique shows great promise as a method for high-frequency population modulation and appears to be amenable to Fourier transform mass spectrometric techniques. ACKNOWLEDGMENTWe are also indebted to A. G. Marshall for discussions about this instrument and our neighbors who have lent us equipment, including R.

separations dramatically (22)(23)(24).While we have restricted attention to linear inlet gradients, similar effects could obviously arise for concave and convex gradients. Shocks could occur even sooner with convex gradients and later for concave gradients (which would tend to become more linear). The resulting effect on adsorbate separation would be along the lines of the foregoing discussion. CONCLUSIONSThe effect of accounting for the adsorption of the mobilephase modulator in gradient elution is examined quantitatively through numerical simulations for reversed-phase chromatography with water-acetonitrile as the mobile phase. The adsorption of the modulator gives rise to gradient deformation and, under extreme conditions, to the formation of a modulator shock layer. The possible consequences of such a shock layer on gradient separations include extreme concentration of adsorbates that lie just behind, or straddle, the shock layer and shoulders ("false peaks") on either the leading or the trailing edge of the band. Experimental verification is being attempted in our laboratory. ACKNOWLEDGMENTWe thank Richard Hendrickson, Paul Westgate, and Yiqi Yang for their helpful comments on the manuscript.

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