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

pre-concentration methods are often necessary. A number
of detection methods have been used in CE to meet this
challenge, many of which are similar to those employed in
liquid column chromatography. As in HPLC, UV-Visible
detection is by-far the most common. Table 19 contains a
list of many of the detection methods investigated, along
with detection limits, and advantages/disadvantages.
4.3.1 UV-Visible absorption
UV-Visible absorption is the most widely used detection
method primarily due to its nearly universal detection
nature. With fused-silica capillaries, detection below 200 nm
up through the visible spectrum can be used. The high
efficiency observed in CE is due in part to on-capillary
detection. Since the optical window is directly in the capillary
there is no zone broadening as a result of dead-volume
or component mixing. In fact the separation is still occurring
while in the detection window. As with all optical detectors,
the width of the detection region should be small relative to
the solute zone width to maintain high resolution. This is
best accomplished with a slit designed for specific capillary
dimensions. Since peaks in CE are typically 2 to 5 mm wide,
slit lengths should be maximally one third this amount.
Detector design is critical due to the short optical path
length. The optical beam should be tightly focused directly
into the capillary to obtain maximum throughput at the slit
and to minimize stray light reaching the detector. These
aspects are important to both sensitivity and linear
detection range.
Instrumentation/Operation
4.3.2 Sensitivity
Sensitivity is defined as the slope of the calibration curve
(detector signal versus sample concentration). A steeper
slope indicates better sensitivity. For absorptive
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