a.) Air in mobile phase, detector cell or pump.
Solution: Degas mobile phase. Flush system to remove air from detector cell or pump.
b.) Incomplete mobile phase mixing.
Solution: Mix mobile phase by hand or use less viscous solvent.
c.) Temperature effect (column at high temperature, detector unheated.)
Solution: Reduce differential or add heat exchanger.
d.) Pump pulsations.
Solution: Clean or exchange check valves of the pump head. If problem still persists, incorporate pulse damper into the system.
e.) Pressure close to maximum.
Solution: Minimize back pressure by reducing flow rate or heating the column.