Molar Masssensitive Detectors

The response of such a detector depends on the concentration and also on the molar mass of the fraction, hence it has to be combined with a concentration-sensitive detector. The following types of molar masssensitive detectors are on the market: low angle light scattering detectors (LALLS); multi-angle light scattering detectors (MALLS); and differential viscosi-meters.

From light-scattering detection, the absolute molar mass distribution (MMD) can be determined directly but no information is obtained on polymer conformation. However, SEC with viscosity detection yields the intrinsic viscosity distribution (IVD). Hence it makes sense to combine a light-scattering detector with a viscosimeter detector. In light-scattering detectors (LALLS, MALLS) the light of a laser beam is scattered by the dissolved polymer coils in the measuring cell and the intensity of the scattered light is measured at angles different from zero.

The (excess) intensity R(6) of the scattered light at the angle 6 is correlated to the weight average of molar mass Mw:

+ 2A2c optical constant containing Avogadro's number NA, the wavelength refractive index n, and the refractive index increment dn/dc:

In a plot of K*c/R(6) versus sin2(6/2), Mw can be obtained from the intercept and the radius of gyration from the slope. It must be mentioned, that dn/dc of copolymers may vary strongly with composition.

Viscosity detectors yield the intrinsic viscosity [y], the so-called limiting viscosity number, which is defined as the limiting value of the ratio of specific viscosity (ysp = (y — y0)/y0) and infinitely low concentration c:

yo=lim yp where c is the concentration of the polymer, A2 is the second virial coefficient, and P(6) describes the scattered light's angular dependence. K* is an

Since the concentrations in SEC are typically very low, [y] can be approximated by ysp/c. In viscosity detection, one has to determine the viscosity, y, of the sample solution as well as the viscosity, y0, of the pure mobile phase.

This is typically done by measuring the pressure drop across a capillary; the pressure drop is proportional to the viscosity of the streaming liquid (using a differential pressure transducer). The problems of single capillary viscometers (SCV) are obvious: the viscosity changes, Ay = y — y0, will typically be very small compared to y0. Moreover, pressure differences due to pulsations of a reciprocating pump will be much larger than those resulting from the viscosity change caused by the eluted polymer.

A better, but still not perfect approach is the use of two capillaries in series, each of which is connected to a differential pressure transducer and a sufficiently large hold-up reservoir in between. Pulsations will be eliminated in this setup, because they appear in both transducers simultaneously.

A sophisticated approach is used in a differential viscometer, which is commercially available from Viscotek. In this instrument, four capillaries are arranged similar to a Wheatstone bridge. The detector measures the pressure difference AP at the differential pressure transducer between the inlets of the sample capillary and the reference capillary, which have a common outlet, and the overall pressure P at the inlet of the bridge. The specific viscosity, ysp = Ay/y is thus obtained from AP/P.

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