Variance contributed by pressure induced injection in capillary electrophoresis

The contribution of variance from a pressure injection to the final analyte peak in capillary electrophoresis (CE) is studied quantitatively. The band broadening process of CE can be described by the product of the Laplace transformed contributing factors; the inverse transform of the product gives the final peak shape which is the convolution of the original functions. The CE process is, therefore, similar to the chromatographic processes described by Sternberg. Based on the additivity of the variance, the injection induced variance is transferred directly to the final peak. The differences between the concepts of band broadening and peak broadening are discussed. Because the injection length from a pressure induced injection is the same for all analytes, and the migration rates during the CE separation are different for these analytes, the variance contributed by injection is larger for slower migrating analytes. The pressure forced flow generates a parabolic component in an injected sample plug, and this should also be considered in calculating the total injection length (tau). The variance is (1/16)tau(2) for a sample plug with a Gaussian concentration profile, and (1/12)tau(2) for one with a rectangular profile. It is demonstrated that the total variance of an analyte peak increases at the same rate as the injection variance. The difference between the total variance and the injection variance is contributed by longitudinal diffusion and other factors.