Peak area and height decreasing over time

[haagiboy]

Hi,

I am trying to make a calibration curve for ethylene glycol in water. Column has been cleaned, guard column is changed etc. The retention time is consistent. Mobile phase is 100% distilled water.

Why is my peak area and height changing? It is more prominent in my higher concentrated EG standards. If I take 2 samples after eachother, the 2nd one will be lower. If I wait a day, it will be even lower again. I have taken 8 samples after eachother, and they decrease each run.

My calibration curve is therefore not linear or reliable at all, and I believe I should get consistent peak areas/heights right?

[James_Ball]

I have seen similar problems with EG before. Years ago I had trouble making standards that would stay consistent and it turned out that it was bacterial activity in the water. Take some of the water in a beaker and heat it in a microwave or on a hotplate until boiling, let it cool to room temperature and see if the standards work then. It may not be the problem, but it should be a quick experiment just to test it out.