I'm using XTerra RP18 column for 5 months, running mobile phase with pH about 3 (10 mM KH2PO4 + H3PO4). After purchasing the column I performed identical test as the vendor (acenaphthene). The difference in RT (about 0.5 min.) was probably caused only by different dwell volume. My analytes: thiomethyl uracil, benzylnicotinium carboxylate, anisaldehyde. Before using Xterra I used Nova-Pak C18. Methylthiouracil eluted in the void and benzylnicotinium carboxylate (BNC) had bad RT reproducibity. After switching to XTerra both problems were solved easily. But recently, RT of BNC shifted to much higher values while other peaks stayed in original locations. So I again performed manufacturer's test with acenaphthene and RT was 1.8 min. shorter than original value. I thought that XTerra technology should give a high resistance against hydrolysis even in such non-agressive mobile phase. While comparing to my 3 years old Nova-Pak C18 which is used under similar conditions I see no such loss in carbon loading. Can anyone explain this fast changes on expensive column?

I compared my Nova-Pak columns and the difference between a new and the old one is about 1.1 min. under similar conditions for the same analyte. I also tried these columns for routine run and the difference in RT was negligible. So propably it is the same for the mentioned XTerra column. But anyway I would like to hear some opinion about it.
Thanks.

At acidic pH, there is little difference between an XTerra column and a classical C18 column with the same bonding procedure. The advantage of XTerra comes into play, when alakline pH values are used.

At acidic pH, the important part is whether the silane is monofunctionally bonded or trifunctionally bonded. You would have gotten better results using the XTerra MS C18 column, which is based on a trifuntional silane. The XTerra RP18 is based on a monofunctional silane.

Hi Vojtech!
Uwe gave absolutely correct explanation. Xterra technology based on complex three-dimensional network-like incorporation of polymeric material into silica support was developed to solve the problem with alkaline anakytes. Unfortunately these type of columns is not very stable to low mobile phase pH. This information I gained from my contacts with Waters specialist. The newer doesn't mean better. However, did you tried change phosphate salts to organic ones.
Best regards
Kirill

Hello Kirill,
I followed Waters' instructions and they recommended phosphate buffer for low pH work. The column is said to be stable in range 2-11. But anyway I dot not complain, the column works well only the RT shifted a bit over time, but peak shape and selectivity are propably unaffected.
Vojtech

One of the things we are working on at LC Resources is a systematic way to adjust mobile phase conditions to compensate (as far as possible) for things like column aging. We have a preliminary (ugly spreadsheet) version of a program for doing this available as a free download. Click on the LC Resources link in the upper left to get to the web site and then click on "Resources" and then "Exchange". The spreadsheet is called "LC-FIXIT".

Or, send me an e-mail and I'll attach a copy via return e-mail.

-- Tom Jupille