Basic HTML Version
257
6.5. SMV: SEC-‐MALLS WITH AN ADDITIONAL VISCOSITY
DETECTOR
6.5.1. General
Adding an on-line viscosity detector to a SEC-MALLS system allows
additional determination of the solution viscosity, from which the specific
viscosity is directly calculated:
η
sp
=
η
−
η
0
η
0
In practise, the viscosity detector is based on a thin capillary. The pressure
difference across the capillary is continuously monitored, and the viscosity is
calculated from Poiseuille’s equation:
U
=
dV
dt
=
π
(
P
1
−
P
2
)
r
4
8
η
l
U
= Flow rate (determined by the pump)
P
1
−
P
2
: Pressure difference (measured)
r
: radius
l
: length of tube
Since the concentration is also recorded simultaneously (the RI detector),
combining data from both detectors provides
η
sp
/c for each elution slice. The
intrinsic viscosity is then calculated from Huggins’ equation:
η
sp
c
=
η
[ ]
+
k
'
η
[ ]
2
c
The calculation requires in principle knowledge of Huggins’ constant (k’).
However, the experiments are usually carried out at such a low concentration
that k’[
η
]
2
c << [
η
], hence [
η
]
≈ η
sp
/c.
The figure (next page) shows data obtained from an alginate with M
w
= ca.
200.000 Da
48
and a chitosan with M
w
= 38.000 Da. Note the limitation in R
G
at
about 20 nm (
λ
/20), whereas no such limitation applies to the intrinsic
viscosity. In this case the signal can be improved by increasing the
concentration.
48
PT180-3.xls (2003)