Clarity discussion forum

Dear fellow scientiscs.
It is in our interest to calculate polydispersity accurately and yet we have experienced a problem that might be of significan importance to us. In the first step we obtain a peak (in our case a PS standard of narrow molar mass distribution) for which we calculate Mw and Mn. Afterwards we alter the column properties which lead to change (or decrease if you will) in selectivity. As a result the same polymer in the same column elutes faster (retention decreases) and with lower selectivity, providing us with a broader peak in our chromatogram. Obviously we expect an increase in polydispersity and that's what we're looking for in our calculations. Strangely enough, we have found out that the Mw/Mn ratio which defines polydispersity was kept close to the previous results despite significant peak broadening. Why? As peak width increases, Mw/Mn should most definitely increase as well. We have not observed this from the calculations that were carried out in Clarity. We're looking forward to
receive some valuable feedbacks on this issue.
Best regards,
Albert Russ & Maria Simekova

Dear Albert,

polydispersity (Mw/Mn) is a polymer property. Narrow polystyrene standards usually have a low polydispersity (close to 1), which means all the molecules have practically the same size. In case you observe wider peak on column with lower selectivity (i.e. steeper calibration curve), there is something wrong with your separation and unless you expect polymer degradation, the increase in polydisperzity is the last likely explanation for this phenomenon.

You can send me your chromatograms (as *.prm files) and the used calibrations (as *.gal files) for inspection. Please, use the vins@dataapex.com address, the conference does not allow for sending attachments (for security reasons).

Best regards

Ivan Vins

DataApex