Chromatography Forum

I have a rather complicated method, which we want to improve. Until now with only limited success.

We have a product PO (propylene oxide). Under certain circumstances this can polymerize to PPO, which should not happen. Therefore we have to determine if the quantity of PPO (everything above 1000 da) is below 1 ppm.

At the moment we use a single pore column with very low porosity. All polymer should elute as one peak just after the exclusion limit. We use RID as detection method. To meet this detection limit, we have to do an evaporation step which is laborious and not very efficient.

Ideal we want to do this analysis without any sample pre-treatment. Does someone have experience with this analysis? Or suggestions?

Carl,

I guess you are using a method similar to this one : http://www.freepatentsonline.com/5248794.html

If you need more sensitivity you will need another detector. CAD would be my first choice, ELSD my second.

Best whishes,

bhuvfe

The detection limit of a good ELSD is around 20 ng for a single peak per injection. CAD is three to five times better, say 5 ng. Thus it will take at least 5 uL of sample to detect and 15 uL of sample to quantify the 1 ppm sample. However, if PPO elutes as multiple peaks or a broad peak, the detection limit will be much worse, probably requiring injecting more than 100 uL of sample. I think CAD may not provide sufficient sensitivity, in which case MS is the choice of detection.

The analysis we’re doing is indeed something as described in the patent. This is how everyone does it, but it’s a very poor method.

Regarding detection. What would you advise? Single quad with APCI as ionization method? And of course looking than to the TIC plots. I agree that peaks will look much better with MS compared to ELSD/CAD. I experienced that al evaporation based detectors gave very “noisyâ€

If you want to quantify a certain MW range, then it would usually be best to select a column that can provide some separation in this range. Choosing a column designed to separate larger molecules (say, up to 20,000) would give you that ability. As an added benefit, you could also see the distribution in this range. That is, rather than saying, "I have 15% over 1000," you could differentiate the amounts in the 15 - 20k, 10-15k, 5 - 10k, and 1 - 5K ranges. This may not seem like useful information right now, but you may find in the future that the presence of a certain MW distribution points to a particular problem or cause. More information means better decisions and easier troubleshooting.

I don't think there is a big problem with just meauring the excluded mass. Perhaps Uwe or some of the others with more classical GPC experience would have a reason to avoid this situation.

APCI will help for identification till 2000 Da but it will be tricky for quantification without standards. Besides that you will miss the rest of your sample (> 2KDa).

An alternative to CAD/ELSD might be fluorescent labeling ( naphtyl isocyanate or equivalent...).