# Plate Theory and Extensions - The Position of the Points of Inflection > Page 56

# The Position of the Points of Inflection

In order to
measure the column efficiency, the position of the points of inflection, where
the peak width is to be measured, must be identified. The inflection points are
not easily located on a peak, so it is necessary to know the fraction of the
peak height at which they occur. The peak represents the concentration profile
of the eluting solute, so the fraction of the peak height at which the points
of inflexion are located will be the ratio of the solute concentration after (_{})
plate volumes of mobile phase has passed through the column to the solute
concentration after (n) plate volumes of mobile phase have passed through the
column.

Therefore, if (f) is the fraction of the height (h) at which the points of inflection occur, then

Replacing and with elution function that describes them,

Thus,
_{ } (50)

Now, if^{ }x<<1,

Consequently, since (n) will always be greater than 100,