Peak Purity Problems-Allantoin

[usallevyn]

I need your help of Peak Purity testing for Allantoin on PDA detector. For some reason the Purity Angles are greater than the Purity Threshold for Allantoin in a lotion that I am testing using Zorbax NH2 column. My lotion matrix contains parabens. I have tried different options- solvents like 90% acetonitrile, 65/35/5 0.1H3PO4/ACN/MEOH. Even the allantoin standards are behaving the same way i.e. the purity angles are greater than purity threshold suggesting that the Allantoin peak is spectrally not pure. Please advise any resolution to the problem. I am using Water Instruments/EMPOWER SOftware. flow rate is 1ml/min and wavelength is 240nm. Allantoin has very good absorbance from 190 to 240nm.

[Ben Buglio]

As a check on peak purity you could try running the separation under normal phase conditions. The change in separation mechanisims make it unlikely that any impurity would coelute in both modes. Look for additional peaks and run your software peak check w the second method.

[vimal]

what is the concentration used for finding out the peak purity.

[Uwe Neue]

It may be due to column bleed. Please look at the "decrease (3x) ..." post.