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Repeatedly blocked QQQ MS capillary

Recently our triple quad MS capillary has repeatedly been getting blocked.  Initially we thought it was dirty samples, however the problem appears to also occur with cleaner samples and standards and we filter all our samples so this really shouldn’t be a problem.  Then it was thought that using a direct infusion method (i.e. by-passing the column) to tune our compounds may be introducing particulates which would otherwise be removed by the column (or filtering), however it still occurs if this is not done.

The current theory is that the buffer in our mobile phase which we use to reduce the retention times, 4mM Ammonium Acetate in 100% Methanol mobile phase, is precipitating out within the capillary .  In order to reduce this we have lowered the desolvation temperature to 200oC, although early signs are not looking as if this has made a great deal of difference.  The next step may be to reduce the amount of buffer and increase the flow rate (from 0.1ml/min) to compensate, and/or add water to the mobile phase, however we’re not even 100% sure this is the problem (although the engineer today did seem to agree this was a likely candidate) and chromatographically speaking our current method works so I’m reluctant to play around with it.

The only way we are able to keep on top of our samples at the moment is to manually flush out the capillary with Methanol and or Water every few samples!

Has anyone else encountered repeatedly blocked capillaries?  If so, how did you work around it?  Are there any other suggestions from other QQQ MS users?

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