Page 130 - Tandem Techniques

Page 112

indicates that the early peaks are the most narrow, and any dispersion that takes place in the connecting
tube will have the maximum deleterious effect on peaks eluted at k'< 1. It is also seen that the increase
in peak width with (k') has become fairly small, even when k'=2. It follows, that if the chromatographic
conditions can be adjusted to ensure that the peaks of interest are all eluted at a (k') of two or more, then
the detrimental effect of extra column dispersion will be minimized. It is also seen that the optimum
linear velocity has an unique value for each (k') value of the solute. It follows that the choice of flow
rate will be optimum for only those solutes that are eluted at and around a specific (k') value. It is clear
that the optimum velocity should be chosen at the (k') value of the closest eluted solutes where
separation is the most difficult.

According to Klinkenberg [4], the variance due to extra column dispersion, should be restrained

to be no greater than 10% of the peak variance to ensure its effect on column resolution is not
serious. That is,

It follows that the dispersion from the connecting tube must not be more than one third of the dispersion
produced by the column. The dispersion from tubes 0.025 cm I.D. (ca .010 I.D.), and 0.0125 cm I.D.
(ca 0.005 I.D.), when used in conjunction with GC columns 30 m long and 0.025 cm I.D., and 0.053 cm
I.D., was calculated for the dead volume peak, i.e. the worst case scenario. The results are shown in
Figure 3.2.

It is seen that for the GC columns the contribution of the connecting tube to the column dispersion is
negligible, and far below the advised limit of one third the column dispersion. It is also seen that the
small diameter tubing need not be used and at least 50 cm of the 0.025 cm I.D. tubing can be tolerated,
without the inherent dispersion having any serious effect on the column resolution.