# Extra Column Dispersion - Maximum Permissible Extra Column Dispersion > Page 9

Thus, _{}

where, dispersion due to the column alone (_{}) is_{}

Thus, _{} (5)

Now from (2) , _{}

Thus, substituting for (_{}) in (5),

(6)

and (7)

It is clear
that equations (6) and (7) also gives an expression for the permissible extra
column dispersion (variance and standard deviation) from all the remaining
dispersion processes not associated with sample volume dispersion. The maximum
sample volume and the maximum extra column variance are included in table 1 for
each column. It is seen that for solutes eluted close to the dead volume, the
maximum sample volume is very small, (1.57 ml
for the microbore column and about 5.0 ml
for the 4.6 mm I.D. and 3 mm I. D). In practice, such small sample volumes may
be difficult to handle, especially if the solutes are present at trace levels.
If the phase system is adjusted so that the solutes are eluted at a capacity
ratio of 5, then much larger sample volumes are permissible (*e.g.*, 9.4 ml for the microbore column and 30 ml for the large bore columns).