1. Mobile phase contaminated/deteriorated (causing retention times and/or selectivity to change).

2. Obstructed guard or analytical column.

1. Prepare fresh mobile phase.

2. Remove guard column (if present) and attempt analysis. Replace guard column if necessary. If analytical column is obstructed, reverse and flush. If problem persists, column may be clogged with strongly retained contaminants. Use appropriate restoration procedure. If problem still persists, change inlet frit or replace column.