Chromatography Forum

We have been using shimadzu GPC for molecular weight analysis of our product since last two years. Recently, since last two months, we are getting major deviation in the standard molecular weights though the regression coefficient passes the test well within the limits. It is giving lot of deviation in our product results. Same column and the mobile phase if used on WATERS gpc, gives consistent results for molecular weight. Can anyone suggest what would be the culprit?

Flow rate variations?

To follow up on Uwe's suggestion. If you don't have a flow rate marker, sometimes you can use the system peaks to identify flow rate variations. First check that it is consistent in the "good" runs. Then, check it for the "bad" runs.

I'd also confirm that the concentration and injection volumes used on each of the two systems (Waters and Shimadzu) are the same.

Finally, you should see if the raw chromatograms (before baseline subtraction) match up reasonably well. I would expect some shift on the elution volume axis due to differing tubing volumes. And, there is going to be a difference in scaling (e.g., the concentration detector on one system may read 0.1 while the other reads 1 on the Y-axis). But, if one system is noisier (compared to peak height) or the baseline drift is really different, that may start to explain the observed differences in molecular weight.