Module
dRI detector module
Concentration. Mass at every slice, via dn/dc. The universal reference channel.
Concentration. Mass at every slice, via dn/dc. The universal reference channel.
About the differential refractive index detector
What a differential refractive index detector measures
The refractive index detector is one of a few “universal” detector types. Its signal is determined by the difference in refractive index between the reference, which is the solvent, and the sample, which is the polymer in solvent. The response is linear with concentration.
The detector response factor, K, is determined using a narrow polymer standard. Of the terms in that relationship, dn/dc is the most important.
dn/dc
Here n is refractive index and c is concentration, so dn/dc is the change in refractive index of the solvent per unit change of concentration. It is specific to a polymer and solvent pair. You can look it up or measure it, and it can be measured online or in batch.
When dn/dc is very low, between 0.00 and 0.009, the pair is isorefractive and the response is zero or very low. Neither the RI detector nor a light scattering detector will work for these combinations. Polyethylene and polypropylene in decahydronaphthalene, and PDMS in THF, are examples.
Where the RI detector fits among the detectors
The RI detector is a concentration detector. UV-Vis absorbance is the other, and it needs a sample that absorbs light in the 200 to 900 nm range.
In conventional calibration the concentration comes from the RI response and the molecular weight comes from the calibration curve. In absolute calibration the concentration still comes from the RI signal, and the molecular weight comes from the light scattering signal. The detectors used for each type of calibration are:
- Conventional: RI or UV
- Absolute: RI and light scattering
- Universal: RI or UV, and a viscometer
- Triple: RI, light scattering and a viscometer
- Quad: RI, UV, light scattering and a viscometer