Chromatography Forum

We have a RP method using a C18 column and a ACN/water gradient. Our analyte peak normally elutes at about 14 min. and usually has no problems passing the USP system suitability criteria of < 2% RSD and < 2 tailing. Recently we had problems with both of these failing. We tried a new column and had the same failures after 1 day. We moved both old and new columns to a different instrument and had great results so now we know that the problem is related to the instrument and not the column. Our pressure is steady and reproducible so we don't think it is a pump problem. Any other suggestions as to what instrumental problems will cause our peaks to become broad with variable peak areas?

Thanks for any suggestions.

"USP system suitability criteria of < 2% RSD and < 2 tailing"

I'm assuming no leaks as you're not seeing a pressure drop or fluctuation.

If they're both failing, my first port of call would be the injector, it's one place where both issues can originate.

Has the rotor seal become worn?The 6 port valve rotors can develop "grooves" where solvent from the sample can seep in during the injection cycle, only to bleed out when the injection switches to "run" this could be one cause.

Have you changed the injector loop recently ? Could be a void space in the pipework

I'd also check for:

Worn syringe plunger
Air bubbles in the injection system
And if its a really old system wear on the screw threading of the injection system.

The suggestion that the source of the problem is mechanical rather than "chemcal" is well-observed.
We had an instance of similarly poor chromatography and traced the problem to the Syringe Injector Assembly. A teflon screw that stabilized the needle plunger on siphoning was severely eroded and caused irregular losses in volume loads; which manifested as such in poor chrom. I must admit that I was surprised that the problem was fixed with only a $14 part.