I have an impurity peak that is not very well resolved from the main peak (I am working on optimizing the method but have to live with it for now). Some days it merges into the main peak and some day it is separated. I have ruled out the column and mobile phase. Could it be that the column fitting is not quite right? We have stainless steel ferrules now because the peek fittings were leaking. I'm not sure if we should switch back. Also, can anyway recommend the best way to cut stainless steel tubing?

The best, and most expensive way, to cut tubing is with one of those fancy electric saws sold for that purpose. I've always had good luck cutting the tubing with a Terry tool, filing the cut end flat, then reaming out the bore with a small needle. The most important thing is to be sure the ferrule is set at the proper point on the tubing. Fittings from different manufacturer's are not interchangable. The length the tubing extends into the fitting past the ferrule varies. When setting a stainless steel ferrule, push the tubing all the way into the fitting and hold it there, then slide the ferrule and nut up to the fitting and tighten. If you change the fitting, cut off the old ferrule and set a new one.

The nice thing about PEEK is that the ferrules are not permanently swaged to the tubing so you don't have to replace them each time you change the fitting. The bad thing is that the tubing tends to slide out of the ferrule as you tighten the nut. However, holding the tubing in place as you tighten the nut generally eliminates this problem.

Best Regards,

Michael Hinsberg
http://www.hinsbarlabs.com

If your loss of resolution is not consistent and you're using the same column and fittings, it is probably not a fitting problem. It is more likely due to slight inconsistencies in the mobile phase (pH especially), inconsistent equilibration time, strongly retained materials, etc. Do you ever see any extraneous peaks? How much do the retention times of your critical pair vary?

Perhaps I am wrong but it seems that if you have permanently swaged stainless fittings, if there was a fitting problem it would be consistent because the connecting tubing would be installed the same each time. Are you experiencing any temperature fluctuations? What is the mobile phase? I have heard that buffers used outside "normal" pH ranges can cause variations in resolution.