Biomedical Engineering Reference
In-Depth Information
Detector
EOF
+
-
+
+
S
Outlet
(A)
Inlet
T
L
BGS
+
Detector
EOF
-
-
+
+
BGS
T
L
BGS
+
(B)
Detector
EOF
+
+
+
-
BGS
L
T
+
(C)
CZE
FIGURE 12.8
Schematic diagrams of a transient-ITP model for cations. (A) The capillary
is conditioned with a BGS, the leading electrolyte, sample solution, and terminating electro-
lyte are then injected in turn, and a high positive voltage is applied; (B) concentration of the
analytes occurs between the leading and the terminating ions during transient-ITP migration;
(C) the concentrated analyte zones are separated by the CZE mode. (From Lin, C.-H. and
Kaneta, T.,
Electrophoresis
, 25, 4058, 2004. With permission.)
pseudostationary phase in MEKC and to produce a focusing effect. Samples are
injected onto the capillary in a buffer solution with a conductivity similar to the
one used in the BGE, but in the absence of a pseudostationary phase (micelles).
The capillary inlet is then placed into an anionic micellar BGE solution and the
separation is performed in the reverse polarity mode. The BGE is kept at low
pH to suppress the electroosmotic l ow (EOF), allowing the anionic micelles to
migrate toward the detector. As the micelles migrate they “sweep” the neutral
analytes along. The effect is dependent on a uniform electric i eld and the absence
of micelles in the sample solution. The effectiveness of this sample preconcentra-
tion has been shown to be dependent on the afi nity of the analytes for the pseu-
dostationary phase. This technique allows to obtain up to a 1000-fold lower limits
of detection (LODs) [27,28].
12.3.2 I
MPROVEMENTS
IN
D
ETECTION
S
YSTEMS
The sensitivity of CE when on-column UV-Vis detection is used, is limited by the
short optical path, which is the capillary inner diameter itself. One way to increase
UV-Vis detection sensitivity is to use capillaries with extended light path, which
should increase the signal-to-noise ratio simply as a result of Beer's law. The cap-
illary can be angled into a “z” shape (z-cell) or extended (bubble-cell), and both
designs are commercially available. Sensitivity improvement is in the range of —two
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