34
A.4 VISCOMETER DETECTOR
A.4.a Theory
The viscometer detector utilizes the patented technology of the four-capillary bridge to
minimize the influence of flow rate fluctuations and maximize the sensitivity of detection.
GPC/SEC IN
OUT
R
1
R
2
R
3
R
4
+ ∆
P -
Delay
Column
+
P
i
-
Figure 12 Four-Capillary Bridge
Consider that the four capillaries R
1
- R
4
have equal flow resistances,
Figure 12
. Then when
pure solvent of viscosity
η
0
is flowing in all four capillaries, the differential pressure
DP
=
P
2
(+) -
P
2
(-)
will be zero, or have some near zero baseline value. Then as the polymer solution elutes
through the bridge, the viscosity in capillaries R
1
, R
2
, and R
3
will increase to
η
, while the pure
solvent remains flowing through capillary R
4
due to the delay volume column. Now it is found
that
DP
is no longer zero, but increases in approximate proportion to the specific viscosity of the
solution. The exact equation is shown below.
17
[A.4-1]
η
sp
DP
IP
DP
=
−
4
2
where
DP
= Pressure across middle of bridge =
( )
( )
P
P
2
2
+ −
−
IP
= Inlet pressure through bridge top to bottom
=
( )
( )
P
P
1
1
+ −
−
17
See Haney, M. A. , American Laboratory, 17 (4), 116-126 1985, for the complete derivation.
