By Anonymous on Wednesday, June 19, 2002 - 11:30 am:
Please help- I have a method in which the mobile phase is an acetate buffer, pH 4.0 (70%) / acetonitrile (30%) on a spherisorb ODS2 column. For certain reasons I'm trying to modify the method to use methanol instead of acetonitrile, but it seems I can no longer see my peak of interest, even by varying the MeOH amount. With ACN it elutes at ~5 min. Why might this be? Thank you...
By Anonymous on Wednesday, June 19, 2002 - 11:49 am:
Usually methanol, instead of Acetonitrile will increase the retention time. What is the concentration of the analyte? How long did you run your chromatogram for? Try injecting a higher cocentration, say 5-10 times more than what is used in the method with Methanol in the mobile phase and run for 30-40 minutes. You got to get your peak:-) Let me know what happens
By tom jupille on Wednesday, June 19, 2002 - 12:48 pm:
To amplify a little on Anononymous #2's good suggestion:
Generally, it takes a higher % MeOH than of ACN to get a given retention (MeOH is "weaker" as a reversed-phase solvent). If you started with 30% ACN and switched to 30% MeOH, your retention time will probably go from about 5 minutes to about 15 minutes.
The nomogram relating reversed-phase solvent strengths is widely available. You can find a copy on our web site here:
http://www.lcresources.com/software/drylabmd.html
(scroll down to about the middle of the page; just before "Step IV").
-- Tom Jupille / LC Resources Inc.
By Anonymous on Wednesday, July 10, 2002 - 11:09 pm:
What is your peak of interest n what is its conc.R u working at meoh cutoff wavelength.