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

Detector response
[arbitrary units]
0.4
0.2
0
0 40 80
0.4
0.2
0
0 40 80
Length [arbitrary units]
a)
b)
There is an interdependent relationship between sensitivity
and linear detection range. For a given design, increasing
the light throughput, by increasing the slit size for example,
can improve detection limits by increasing the light level to
the photodiode and thereby decreasing the baseline noise.
However, if the slit allows stray light through the capillary
walls, the linear detection range will be compromised. If the
slit is increased along the length of the capillary, resolution
will be reduced. Depending on the analysis, slit size can be
chosen to optimize either sensitivity, or resolution and
linear detection range.
4.3.4 Aspects of quantitative analysis
Peak area reproducibility is critical for quantitative analysis.
Typically, better than 2 % RSD can be obtained under
wellcontrolled conditions. The major factors affecting peak
area are temperature variations, sample adsorption, precise
injection of small sample plugs, and integration of signals
with low signal-to-noise ratios. A more complete list is
given in table 20. Many of these factors can be directly
affected by the user, while a few are completely instrument
dependent. As with migration time reproducibility, use of
an internal standard can often be useful.
Instrumentation/Operation
Peak width
10
6
2
1000 2000
Migration time [s]
c)
Figure 58
detector response as a) a function of
time; b) as corrected for differences in
zone velocity; c) peak width corrected (•)
and uncorrected (o) 6
Conditions in c): dansylated lysine run at
different voltages to
produce different
migration times
An interesting aspect of quantitative analysis of peak area
results from different migration velocities of the solutes.
This is in contrast to chromatographic techniques in which
all solutes travel at the same rate when in the mobile phase.
One must correct for velocities since different residence
times in the detection region artificially affect peak area.
Solutes of low mobility remain in the detection window for
a longer time than those of higher mobility, and thus have
increased peak area. This phenomenon can be corrected
simply by dividing integrated peak area by migration
time.This is illustrated in figure 58.
101