The variance is generally used as a measure of dispersion of zones in
capillary zone electrophoresis (CZE). It is a quantitative measure of
the separation power of a system and different causes of dispersion ca
n be rated by their partial variances, which can be summed up to total
variance. However, the additivity is only valid for independent dispe
rsion sources, a fact that often seems to be ignored. The ubiquitous d
ispersion source diffusion is taken into consideration by the Einstein
term 2Dt. Other sources of dispersion are, e.g., injection, detection
, thermal gradients, adsorption, and hydrodynamic flow. For each of th
ese sources various variance expressions have been derived. The origin
of the term 2Dt and its relation to the variance is explained and the
calculation of variance in general is discussed. The equivalence of t
he diffusion variance and the term 2Dt is verified with some simple in
itial forms of sample zone and the additivity of variances in ideal zo
ne electrophoresis is demonstrated. The change of conductivity in zone
s results in asymmetrical zone forms which is an indication of nonidea
lity of a system. It is shown that in such cases the term 2Dt is no lo
nger valid and its use as an additive variance component leads to an e
rroneous total variance. Because in zone electrophoresis conductivity
in a zone always changes more or less, the additivity of variances is
never perfectly valid. However, in many cases the nonideality may be s
o small that the additivity in practice is still applicable. A general
ly valid way to calculate theroretically the total variance of a zone
is to derive a functional representation of the distribution and then
calculate the variance from it. This is possible only in the simplest
cases. Usually the distribution must be calculated by numerical method
s.