Ion Chromatography: Spikes Appear on Changing Flow Rate

[M. Farooq]

Hello,

I am using a Dionex ion-chromatography ICS 3000 with conductivity detection and self regenerating suppressor instrument for academic purposes. Actually, I was trying to clean the system with water without a column (we use different self prepared columns).

However, during cleaning something interesting was observed. It may too common, but I am trying to understand why chaging a flow rate would cause an "apparent" change in conductance.

The question is that when the column is removed and the rest is connected as it should be i.e.
pump-->injector-->suppressor-->conductivity detector,

sharp peaks are seen whenever the flow rate is changed without stopping the pump. For example if flow rate is 2.0 mL/min is changed to 1.5 mL/min a spike appears in the signal. The eluent is nothing but pure water. I initially thought that the injector valve or sample loop is dirty, but if the injector valve is by-passed same behaviour is observed.

Even if the configuration is pump -->detector i.e. the suppressor and injector valve are bypassed spikes appear when flow rate is either increased or decreased.

Similarly, on manually changing the injector valve to load to inject or vice versa, similiar peaks appear.

This might be a common phenomenon, but I am not well experienced with IC systems. Does changing the flow rate causes spikes in the conductivity detectors?

Thank you very much for some clues.

M. Farooq

[tom jupille]

Most detectors (of *any* type) are at least somewhat flow sensitive, so seeing a baseline shift or even a brief spike when you change flow is not something I would worry about. As to what *causes* it on a conductivity detector, I can only speculate:
you measuring *very* small currents, so anything that changes the diffusivity of ions (for pure water, hydronium and hydroxide) will generate a response: a shange in temperature (frictional heating)? a change in density?

[M. Farooq]

You are right that most flow cells are sensitive to flow but especially for conductivity detector the origin of a peak does not make sense to me. I agree baseline shift makes sense but why a peak?

Pressure changes (without column) from 1.00 to 1.50 mL/min are minimal so density changes in water and temperature changes can be ruled out. Infact, the conductivity cell shows are peak whether it is thermostated at above room temperature or not thermostated at all.

Thanks.