High performance capillary electrophoresis - T.E.A.M.
High performance capillary electrophoresis - T.E.A.M.
High performance capillary electrophoresis - T.E.A.M.
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Instrumentation/Operation<br />
4.3.6 Extended light path capillaries<br />
Sensitivity and linear detection range can usually be<br />
improved by increasing the inner diameter of the <strong>capillary</strong>.<br />
This approach is limited, however, by the increase in current<br />
and subsequent heating within the <strong>capillary</strong>. For example, a<br />
two-fold increase in diameter will yield a twofold increase<br />
in absorbance but a four-fold increase in current. Special<br />
<strong>capillary</strong> designs can be used to extend the optical pathway<br />
without increasing the overall <strong>capillary</strong> area. One such<br />
design is the “bubble cell” (figure 61).<br />
Light<br />
"Bubble"-cell<br />
Figure 61<br />
Schematic of extended light path<br />
<strong>capillary</strong><br />
The bubble cell offers a unique method to extend the pathway<br />
with nearly no degradation of separation efficiency<br />
and resolution. It is made by forming an expanded region,<br />
a bubble, directly within the <strong>capillary</strong>. Since the bubble is<br />
located only in the detection region no increase in current<br />
occurs. In the region of the bubble the electrical resistance<br />
is reduced and thus the field is decreased. Concomitant to<br />
this is a proportional decrease in flow velocity due to the<br />
expanded volume of the bubble. When the zone front enters<br />
the bubble its velocity decreases and the zone concentrates<br />
or “stacks” in a manner similar to electrophoretic stacking<br />
during injection. As the sample zone expands radially<br />
104
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