Loss of retension in ion pair chromatography

[ether]

Dear members,
I have a new coloum that lost its retention after I using it only for 3 month with almost 300 injections. This coloum is dedicated to separation for one product. Since the method is new, i rarely using it for sample analysis. Most of the time, it is used to run standard.
I am using C18 coloum for separation polar substances using ion pair chromatography. The coloum dimension is 10um 4.6x300 mm endcapped, with sodium hexansulfonate as an ion pairing reagent. Ion pairing solution mobile phase is adjust to pH 2.7 with the addition of acetic acid glasial.
I notice that the retension is lost gradually. After 3 month, retention time of the last peak, that in the first injection is in 20 min, decreased become 14 min.
I can't figure it out why it happen. Do you have any suggestion or theory to explained it?

[tom jupille]

If retention is dropping fast enough that you can measure the loss in a reasonable time (maybe a couple of days?) there is a diagnostic test you can run to determine whether the problem is coming from the mobile phase (contamination? pH?) or from the samples (contamination). It's described on our web site:
http://www.lcresources.com/resources/TSWiz/hs390.htm

That said, columns are cheap and your time is valuable, so the most cost-effective solution may simply be to replace the columns frequently. If you're getting 300 injections from a column, that works out to $1.xx per injection; it probably costs you much more than that to prep the samples and standards and document the results.