The Molecular Weight Cut-Off (MWCO) value often is used by membrane manufacturers to characterize their porous ultrafiltration membranes. It is a typical permeation-related membrane characteristic. Molecular weight cut-off refers to the lowest molecular weight solute in which 90% of the solute is retained by the membrane, or the molecular weight of the molecule that is 90% retained by the membrane. The rejection is defined as:
R = (cF – cP)/cF = 1 – (cP/cF)
where R is the rejection [-], cF the feed concentration of a solute and cP the permeate concentration of this solute. The rejection depends on many solute and process parameters like the type of solute, concentration, hydrodynamics, pressure, temperature and pH. MWCO measurements are usually carried out in separate experiments using different solutes, each with a certain MW. This results in a time consuming procedure for MWCO determination, in addition also neglecting differences in shape and flexibility of the solute and phenomena like concentration polarization and adsorption of the solute on the membrane.
At the EMI Twente we therefore do not use single solutes but a mixture of dextranes or polyethylene glycols (PEG) covering a broad molecular weight range around the expected MWCO. Tests are carried out in a standardized cell, where pressure and cross flow velocity is carefully controlled in order to create standard conditions for the measurements. After the permeation experiment is carried out, the MW distributions of feed and permeate are determined using Gel Permeation Chromatography (GPC). By calculating the rejection for each MW using the equation mentioned before, the rejection as a function of MW can be determined in a single permeation experiment.
In house: Agilent 1200 series and Agilent 1260 Infinity
- Accurate molecular weights – light scattering detection does not require column calibration and when combined with PLgel columns delivers perfect results about the true polymer molecular weight, size, shape, and long chain branching
- Accurate size and structure – viscometry can be used to measure Rg (sample radius) and sample structure over a wide molecular weight range extending from millions to tens of Daltons, and connects with light scattering to give the ultimate detector combination
- Minimal dispersion – a low-volume light scattering detector flow cell of just 10 μL ensures correct values for polydispersity
- Efficient, reproducible answers -– each detector is independently temperature controlled from 30 to 60 °C to ensure high baseline stability and minimal baseline drift
- Fully upgradable – to expand the system’s capabilities, you can upgrade at any time from a single, advanced detector to full triple detection
- Simple instrument operation – in a single software solution you get complete instrument control, data collection, and data analysis from conventional GPC through to triple calculations
- Low molecular weight sample analysis – the viscometer can measure molecular weights down to as low as 100’s thanks to the latest advances in silica and pressure transducer technology
- Versatile – any combination of detectors is possible and a touch-sensitive keypad enables easy operation or fast visual inspection of status summary; full control is through the Agilent GPC/SEC software
- Triple detection delivers the ultimate in polymer analysis – absolute molecular weights are determined by light scattering detection and molecular size and solution behavior comes from the light scattering and viscometry detectors.