High performance capillary electrophoresis - T.E.A.M.

Principles
The contribution of injection to the total variance is given by
w i
2
s 2 = (18)
inj
12
where
w i
= injection plug length.
Ideally, the sample plug length should be less than the
standard deviation due to diffusion, (2Dt) 1/2 . The exact length
depends both on the diffusion coefficient of the solutes and
on the analysis time. Macromolecules can have diffusion
coefficients 100 times lower than small solutes and will
necessitate smaller sample plugs. The relationship between
injection plug length and diffusion coefficient and their
effect on efficiency is illustrated in table 6.
Injection length (mm) N (D=10 -5 cm 2 /s) N (D=10 -6 cm 2 /s)
Table 6
Effect of injection length and diffusion
coefficient on efficiency
1 238,000 1,400,000
2 164,000 385,000
10 81,000 112,000
A practical limit of injection length is less than 1 to 2 % of
the total capillary length. For a 70-cm capillary, a 1 %
length corresponds to 7 mm (or 14 nl for a 50- mm id). While
current instrumentation can load these small volumes reproducibly,
under typical conditions detection limit difficulties
often necessitate longer injection lengths. Methods to
improve detection limits without degrading efficiency are
discussed in chapter 2, in section 4.1.3 and in section 4.3.
2.3.4.4 Solute-wall interactions
Interaction between the solute and the capillary wall is
detrimental to CE. Depending on the extent of interaction,
peak tailing and even total adsorption of the solute can
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