Chromatography Forum

Recently when qualifying one of our HPLC's we noticed that one of our peaks in the standard we use for PQ has a higher peak height than it does on any other system we have. Everyting still passes, its just this particular system seems to give us a higher and narrower peak with roughly the same area counts as it would on every other system. It is only for this peak (Aspirin), The other peaks are normal and on par with what we see across all of our systems. Anyone have any ideas as to what may cause this system to give higher, and narrower Aspirin peaks?

2487 detector
has occured with several differnt columns and mobile phases
We believe we have it narrowed down to the system itself, just do not know what component specifically.

Flow cell volume the same as in all other Systems, and the tubing ID also the same? Wavelength the same? New UV lamp?

Hi Metrology,

My vote is for the length and/or ID of the capillary tubing on the "special" system with the larger peak height for the aspirin peaks but yet the same peak area as the other Alliance systems...say if the "special" system had 0.007" ID tubing installed instead of the typical Waters 0.009", or if the "special" system had a shorter run of capillary tubing. [Probably tubing length would show this even if the IDs on all sets of tubing were the same.}

Particularly if...is the aspirin peak also the first one to elute?

I will take a look at the tubing. The Aspriin peak is actually the last peak to elute. All obvious variables are the same. Flow cell, wavelength etc.

Well, Metrology,

When you say that the aspirin peak is the last to elute...that doesn't give me a warm, fuzzy feeling about my educated guess...especially if all of the other earlier peaks eluting are affording similar areas and heights to the other systems under PM. But I can't think of anything else...doesn't make sense that the rise time would be less for one peak eluted on one instrument...and that, as ridiculous and far-fetched as it may seem, would be be next educated guess.

I am running with your tube diameter hypothesis for now. It does appear as if the tube from the soldered nut after the injector that runs to the bottom of the column is a bigger bore. I'm going to change that today and run some test injections to see what happens. Thanks for the help.

Knowing that aspirin is the last peak prompts me to ask if you are running a gradient, and if the gradient gets steeper towards the end. If you do and it does then small changes in mixing ratio, volume flow, or pre-column volume could put the aspirin in a region of increasing mobile phase strength that "chases" the back of the peak towards the front, and makes the peak narrower.

Peter

Hey Metrology,

My apology for being a nudge, and mostly since I don't like the educated guess I made anymore and kind of like Peter's, any new word on the specially high and narrow aspirin peak(s)? A difference in the gradient slope due to proportioning differences could well fit the bill, though it's weird to me that only one Alliance exhibits the phenomenon in your set.

Hi, if you are using a gradient, are you sure you have programmed the same gradient cure on the Alliance, like type 5 or type 6 etc? Very interesting issue.