Run failure - concentration increase over time

I've been thrown back into the world of HPLC after I thought I'd left it behind 8 years ago!

The company I work for has 1 HPLC system and the two methods used on it were transferred from an external company. It is used for determine levels of glutaraldehyde and formaldehyde in separate solutions.

We use DNPH to derivatise the glutaraldehyde and formaldehyde in the sample solutions and are sometimes finding that the concentration increases over the course of the run ~ 6-8 hours.

Does anyone have any experience of this? What can we look at to stop this happening?


Mobile phase: acetonitrile and water

Gradient elution with on-line mixing

Column: C8, 4.6 x 150mm

Diluent: 50:50 acetonitrile and water


Thank you!

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