For the first time, scientists have come up with an effective way to combine retention data from liquid chromatography with mass data from a mass spectrometer (MS) and concentration data from a refractive index (RI) detector. Using this approach, they have managed to develop an accurate technique for assessing the molecular weight distribution (MWD) of synthetic polymers.

A problem with polymers is that they are not all created equal, even when consisting of the same monomer units. For the vagaries of polymer production ensure that they are made up of polymer chains with a variable number of individual monomer units.

This means that a single polymer sample will actually contain polymer chains of varying lengths and therefore also varying molecular weights. Thus the polymer sample will possess a certain molecular weight distribution, reflecting the range and concentrations of the different length chains, with a wide range of chain lengths resulting in a large MWD.

Knowing the MWD of a polymer sample can help to reveal important information about the original polymer production process, including the specific reaction pathways and kinetics. Unfortunately, accurately determining a polymer's MWD is not easy.

The current standard method utilises size exclusion chromatography (SEC), which separates the polymers according to their size, but is limited by the fact that polymer size is only indirectly related to molecular weight. This means that the SEC method requires a calibration step to produce accurate MWD measurements.

The best type of calibration utilises polymer chains of known molecular weight, but these are not available for many polymers. Otherwise techniques such as light scattering and viscometry have to be used. These assess SEC properties that are only indirectly related to polymer molecular weight and therefore require a number of assumptions to be made, introducing a major source of potential error.

To make matters worse, chromatographic band broadening can act as a major source of interference. As a result, what appear to be polymers of different molecular weights eluting at different times may simply be polymers of the same weight forming a broad band due to the effects of diffusion and different paths through the SEC stationary phase.

A team of chemists led by Christopher Barner-Kowollik from the Universität Karlsruhe in Germany thought that many of these limitations could probably be overcome by linking SEC to both an electrospray ionisation (ESI) MS and a RI detector. The idea being that the ESI-MS would provide accurate data on the masses of the different length polymer chains, while the RI detector would provide accurate data on the concentrations of the different polymer chains. Combining this data should then produce an accurate measure of the polymer's MWD, without the need for any calibration.

This still left the problem of band broadening. But the chemists thought this could be dealt with by utilising a statistical technique known as the maximum entropy procedure to combine the ESI-MS and RI data.

'This procedure takes as inputs the concentration trace recorded by the RI detector as well as the peak positions and shapes of the single oligomer profiles [from the ESI-MS] to arrive at an accurate and de-broadened MWD,' team member Till Gruendling told separationsNOW. '[It] ensures that the impact of measurement noise, which would otherwise significantly distort the outcome of the procedure, is minimised.'

So Barner-Kowollik and his team designed an SEC system in which the effluent stream is split in two, with half going to an ESI-MS and half to an RI detector. Testing this combination of SEC, ESI-MS and RI on different polymer samples, they found that it could accurately determine their respective MWDs.

The researchers then assessed the MWDs of a range of commercially-available polymers and found that their technique measured the MWDs as being 5-14% lower than the values reported by the manufacturers. However, as the manufacturer's assessed the MWDs using the standard SEC method, the chemists are confident that their values are the more accurate.

Barner-Kowollik and his team are now trying to extend their MWD measuring technique to mixtures of polymers with varying endgroups. '[This] will provide valuable additional information on the reaction pathways during polymerisation, polymer degradation and endgroup modification,' says Gruendling.

Related links:

• Analytical Chemistry (Article in Press): "Quantitative LC−MS of polymers: determining accurate molecular weight distributions by combined size exclusion chromatography and electrospray mass spectrometry with maximum entropy data processing"

Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.