35
η
sp
= Specific viscosity of the solution =
η η
η
−
0
0
DP
is typically measured in Pa;
IP
is typically three orders of magnitude larger than
DP
and is
measured in kPa. Thus, Equation [A.4-1] can be approximated as
[A.4-2]
η
sp
DP
IP
≈
4
The inlet pressure is more or less constant, being primarily a function of the flow rate of
the solvent and its viscosity. The pressure
DP
provides the “viscometer chromatogram,” as it is
nearly proportional to the change in viscosity,
i.e.
, specific viscosity. Note that the
IP
will
sometimes show a slight change as the sample elutes. This is essentially a “single capillary
viscometer” response to the sample. The full power of the bridge to cancel flow rate pulsation
and noise is seen in the
DP
signal.
DELAY COLUMNS
It is clear that the delay column is an important component of the bridge, as it provides
the reference solvent to the reference capillary R
4
during the period of elution of the polymer. If
the delay volume provided by the delay column is well-matched to the elution profile
(chromatogram), the polymer solution will not break through into capillary R
4
until well after the
sample has eluted and
DP
has returned to baseline,
Figure 13
.
Viscometer Chromatogram
2,0
6,6
11,2
15,8
20,4
25,0
-25
-3
20
43
65
88
110
Retention Volume (ml)
Figure 13
DP
Chromatogram Showing Ideal Delay Volume
However, if the delay volume is too small, the breakthrough will occur too early, interfering with
the
DP
chromatogram, as shown in
Figure 14
below.