Chromatography Forum

I am currently experiencing problems with a Related Substances method I have used successfully many times before. The method is HPLC(gradient) using an Electrochemical detector and the baseline is very noisy and therefore making it very difficult to integrate peaks of a small response. I have already changed the filters, and a new guard cell and analytical cell have been used. I have changed the column twice and all mobile phase components are of the correct grade and quantities. The mobile phase is: MP A: 92% 25mM K2HPO4 20mM octane sulphonic acid/8%acetonitrile ph=8.5. Mobile phase B is exactly the same but pH is 6.0. Nothing has changed, as far as I am aware, between my last analysis and this one. We have recently had a software upgrade from: Chemstore C/S Revision B.03.02Build(183) SR2 and Chemstation B.01.03(204) to Chemstore CS Rev.B.01.03 and Chemstation A.08.03(847). Does anyone know of any change in the software, affecting my method, that would lead to a noisy baseline?

Can you replace the detector unit with another to see if therein lies the problem? Or has the gain control been inadvertantly adjusted (are your peak sizes the same as before?)

Dear ajohnston:

I think you should check the simple fixes first.

If the baseline noise is regular, check the pump pulsation, and possible air trapped in flow cell (adding an online degasser should help).

If the baseline noise is irregular, check for leaks. Also, check lamp's age (if it's 2000 hrs, you may consider to change).

You could inject the mobile phase (as sample), and pump at 0mL/min (no flow), to see if baseline still exists. (If there isn't, then the pump is pulsating a lot). Also, check for air flow directly to the detector (I have seen this, and we had to use a shield to cover our detectors from the air from AC unit).

Alfred.

Hi,

in my experience, octane sulphonic acid is not well suited for an electrochemical detector...it "blocks" the graphite surface.