standard problem

[jto]

I have been running aflatoxin samples by HPLC post column derivitization for several years now with the usual problems that are diagnossed fairly easy; high back pressure, leaks, degassing issues, etc... For some reason, my standard mixture containing aflatoxins B1, B2, G1, and G2 are coeluting not on just one system but both HPLC systems. This has never happened before. I have not changed anything in the method. I have tried the usual- fresh mobile phase, fresh standard, changing the column. Do you think the check valves could possibly need cleaning/replaced? I do not have a service contract on these instruments only on my mass spec; just try to troubleshoot the HPLC'S myself due to budget issues.

Appreciate any advice.

Thanks.

[tom jupille]

It looks like you have checked the most likely culprits.

Some other details would be helpful:
- isocratic or gradient separation?
- if isocratic, are you using "premixed" mobile phase or are you "on-line mixing"?
- Does the retention time of the coeluting peaks match what you expect for one of the standards?
- are you using individual standards or a mixture?
- if a mixture, is it possible that it has degraded (by exposure to light, for example)?