# Dispersion in Chromatography Columns - The Alternative Axes of a Chromatogram > Page 3

Thus, assuming
there are (n) non-interacting, random dispersive processes occurring in the
chromatographic system, then any process (p) acting alone will produce a
Gaussian curve having a variance _{ },

Hence, _{}

where,
_{()}
is the variance of the solute band as sensed by the detector.

The above equation is the algebraic enunciation of the principle of the summation of variances and is fundamentally important. If the individual dispersion processes that are taking place in a column can be identified, and an expression for the variance arising from each dispersion process evaluated, then the variance of the final band can be calculated from the sum of all the individual variances. This is how the Rate Theory provides an equation for the final variance of the peak leaving the column.

# The Alternative Axes of a Chromatogram

An elution
curve of a chromatogram can be expressed using parameters other than the *volume
flow* of mobile phase as the independent variable. The Plate theory provides
an equation that expresses the retention and standard deviation of a peak in
terms volume flow of mobile phase. However, instead of using milliliters of
mobile phase, as the independent variable, solute concentration in the mobile
phase can be related, *time*, or *distance* traveled by the
solute band along the column and proportionally the same chromatogram will be
obtained. This is illustrated in figure (1)