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

time and other experimental parameters can be used to
calculate the apparent solute mobility using
Principles
m a
=
l lL
=
tE tV
(9)
where: m a
= m e
+ m EOF
.
V = applied voltage
l = effective capillary length (to the detector)
L = total capillary length
t = migration time
E = electric field
In the presence of EOF, the measured mobility is called the
apparent mobility, µ a
. The effective mobility, µ e
, can be extracted
from apparent mobility by independently measuring
the EOF using a neutral marker that moves at a velocity
equal to the EOF. Examples of neutral markers include
DMSO, mesityl oxide, and acetone. A sample calculation
using a vitamin separation is illustrated in figure 10.
mAU
(93.1 s)
Solute Migration m a
(cm 2 /Vs) m e
(cm 2 /Vs)
time(s)
Cation 38.4 3.05 × 10 -3 7.40 × 10 -4
Neutral 50.7 2.31 × 10 -3 2.31 × 10 -3
Anion 93.1 1.26 × 10 -3 -1.05 × 10 -3
(38.4 s)
(50.7 s)
lL (50) (58.5)
Cation: m a
= = = 3.05 × 10 -3
Vt (25,000) (38.4)
Neutral: m EOF
= (50) (58.5) = 2.31 × 10 -3
(25,000) (50.7)
Figure 10
Calculation of electro-osmotic flow
mobility and effective solute mobility
m e
= m a
- m EOF
= (3.05 × 10 -3 ) - (2.31 × 10 -3 ) = 7.40 × 10 -4
0.5 1.0 1.5 2.0
Time [min]
(note: m e
will be negative for the anion)
The two different capillary lengths, effective and total, are
described schematically in figure 11. The effective length is
that from the point of injection to the point of detection. For
on-capillary spectroscopic detection, this length is typically
5 to 10 cm shorter than the total length. For off-column
27