Random High Peak Areas - Agilent 1100 autosampler 100

[Haslamm]

Hi ,

I have a recurring but intermittent problem. We are currently analysing dissolution samples by HPLC and every now and then we get a higher than expected result for one of the timepoints, which at the next timepoint returns to 'normal' (i.e 10 mins=85% dissolved, 15 mins=109%, 30 mins=100%). Upon re-injection of these vials, the 109% would be 99% for example - fitting back in with the expected profile. On closer analysis of the particular chromatograms we usually notice that the solvent front peak is also bigger. So far we have just documented a 'suspected HPLC injection error' and reported the repeat injection result, but it's happened a few too many times now to ignore. The HPLC system is in a calibrated state and system suitability is within acceptance limits both pre- and throughout the run. The injection volume is 100 µl (which is the maximum calibrated volume on the system). The phenomenon has happened on more than one HPLC system in our lab.

Any suggestions?

[AdrianF]

This may be due to the substance being absorbed somewhere in the system,once it builds up the next injection will cause it to be eluted. You could check this by having a blank sample using an organic solvent as blank between each injection, if the problem goes away that is probably the answer.

[DR]

Or, you have a late eluting compound that likes to coelute with your analyte of interest. If this is the case, there would occasionally be another smallish random peak showing up periodically throughout the run.

[wrighty]

we have recently had similar problems with our 1100 series too.

One of our thoughts was that some of the sample being tested hadnt dissolved correctly and had remained on/in the syringe when injected.

i dont know how plausble this is but it is quite simple to remove the syringe and clean in a suitable solvent.

worth giving it a try...

steve

[JM]

If solvent peak is also increases with your analyte peak , it indicate more volumn is injected intermittently, reason could be ,

1. Drain pipe of injector blocked.
2. Rotar seal has developed some dead cavities.

check and replace faulty parts and use needle wash ( i hope you know what it is)during injection.

It is not prefered to run your injector at maximum calibrated volume. Rather change to bigger injection loop ( 200 µl) and inject 100 µl.

let us know if this solve your problem.

[Haslamm]

Thanks for all your replies so far. I hope to look into it shortly. We are not currently using a needle wash on this method - which is something I had thought about - so could incorporate that. Also, the systems have been in quite a lot of use recently so maybe rotor seal damaged? I find these intermittent problems always difficult to investigate as can never seem to make it happen when I want to! Thanks again, will keep you posted.....

[Consumer Products Guy]

1100 Rotor seal is straightforward to replace, just be aware of the order the parts come out. We don't remove the six capillaries feeding in to it (like Agilent instructions say), we just use a long-handled allen/hex wrench. Most of the parts only fit on one-way due to offset holes,tabs, etc. Should take 10 minutes or less. If there's dark particles in there, wash them out, use cotton applicator, etc.