Analysis of Chiral Drugs in Body Fluids - Chiral Separations by Capillary Electrophoresis - page 351

Detector

EOF

+

-
-

BGS -

(A) Inlet + Outlet

+

S

+ -

(High-conductivity)

(Low-conductivity)

Detector

EOF

-

+

BGS

+ BGS -

-

+
+ -

(B)

Stacking boundary

Detector

EOF

+ BGS

-

-

-- ++ +

(C)

CZE

FIGURE 12.7 Schematic diagrams of the FASS model. (A) The capillary is conditioned
with a BGS (a high-conductivity buffer), the sample, prepared in a low-conductivity matrix,
is then injected to a certain length, and a high positive voltage is applied; (B) focusing of the
analytes occurs near the boundaries between the sample zone and the BGS because of their
mobility changes; (C) stacked analytes migrate and are separated by the CZE mode. (From
Lin, C.-H. and Kaneta, T., Electrophoresis , 25, 4058, 2004. With permission.)

stacking (FASS). Very often a tenfold difference in ionic strength between the BGE
and sample provides satisfactory stacking efi ciency [30]. The need for a large
difference in ionic strength between sample and BGE is a limitation when samples
of high salt concentration (near 100 mM) are considered.
Enantioselective analysis of drugs and metabolites in biological samples carried
out by our group employing this stacking condition, combined with other techniques
of sample preconcentration, provided lim its of quantii cation (LOQs) similar to those
obtained by HPLC-UV. Therefore, it was possible to employ the methods for kinetic
disposition studies [23,31,32].
A variant of FASS is the stacking performed in micellar electrokinetic chroma-
tography (MEKC) for enhancing the detection of neutral analytes. In MEKC-FASS
the sample is injected hydrodynamically in a low-conductivity micellar solution into
the capillary containing a high-conductivity micellar BGE. The analytes may be
stacked in either normal or reverse polarity mode [28,30].
Another simple stacking method consists in the addition of two sample volumes of
organic modii ers, such as acetonitrile [29]. In this case, the addition of acetonitrile
could also promote protein precipitation, acting as an off-line sample preparation
procedure. However, this method of sample preparation does not allow the concen-
tration of the analyte so it is not suitable for the quantii cation of analytes at low
plasmatic concentration levels.