Chromatography Forum

We are using a Waters LC-MS with FTN (flow through needle) autosampler. We are encountering carryover from "sticky" neutral aryl ureas. ESI+ mode MS (hence the formic acid).

We are using currently using:
Purge solvent:
90:10 (v/v) water:methanol + 0.1% formic acid

Needle wash (exterior of needle):
25:25:25:25 water:methanol:acetonitrile:2-propanol + 0.1% formic acid ("Magic Mix")

with 20 seconds of both pre- and post- needle wash and still experiencing carryover issues.

Any suggestions for better needle wash solvent mixtures to try or additional settings on the FTN injector?

(We can't really increase the "purge" solvent strength, because that remains in the needle, is part of the next injection, and will degrade the chromatographic separation.)

Run it twice if 20 seconds is as long as is allowed.
If that makes no difference, figure out which alcohol helps the most and replace the other one with more of the helpful one. you can also try EtOH instead of either IPA or MeOH.
I think FTNs also house a limited number of larger vials (4?) that can be used for pretreatment/rinse programs. If you need a "stronger" needle wash solvent, these vial positions can be used to flush it out of the needle etc. prior to your next injection.