If the retention times are unstable across a plate or shifting over time:
- Make sure the mobile phases are purged and there are no bubbles in the system.
- Make sure no air is drawn or injected by the autosampler.
- Make sure the entire LC-MS system is well conditioned and the column is sufficiently equilibrated.
Otherwise, check the following parts with your instrument's vendor:
- Degasser
- Mixing cahmber (needs to be replaced)
- Intake/outlet valve at pump head
- Seals at pump head
- Replace the analytical column (it may have reached the end of its lifespan)