It is known that elution of Polystyrene, PS using DMF eluent is later than
that with THF eluent when organic SEC (GPC) columns packed with styrene-divinylbenzene
polymer. Its separation was analyzed for THF and DMF eluents using Multi Angle
Light Scattering (MALS) detector. NBS706, a linear type poly disperse Polystyrene
standard, was used as a sample, which is supplied by the US National Bureau
of Standard (NBS).
Figure 1 shows the relationship between the RMS radius and the elution volume
for each eluent. The elution with DMF eluent is rather late, but the correlation
between the molecular size and the elution volume is good except for the small
RMS area. In this area, separation of both eluents is done by GPC. The absolute
molecular weights of NBS706 by GPC/MALS for both eluents are similar (286,000
by THF and 283,000 by DMF), the recovery rate by a RI-detector is 100%, which
means nothing was retained in the column.
Figure 2 is similar with Fig. 1, but its y-axis is molar mass. There is a
big difference between the two eluents. This means that the GPC separation
is performed by the molecular sizes, but not molar mass.
Based on Fig. 1 and 2, the relationship between molar mass and RMS radius
is shown in Fig. 3. Comparing molecules which have the same molar mass between
DMF and THF, the RMS radius of molecules in DMF is smaller than that of THF
due to shrinkages in DMF. Therefore, MALS enables the measurement of the absolute
molecular weight and size, which can track the separation behavior and facilitate
the evaluation.
Sample:
(1) 0.198% NBS706 in THF, 50µL
(2) 0.0935% NBS706 in 10mM LiBr in DMF, 100µL
Columns : (1) Shodex GPC KF-806 (8.0mmID*300mm)
(2) Shodex GPC KD-806 (8.0mmID*300mm)
Eluent : (1) THF , (2) 10mM LiBr in DMF
Flow rate : 1.0mL/min
Detector : MALS + RI
Column temp. : 26°C
