Shifting retention times and poor peak shape on M-Class - WKB319025
SYMPTOMS
- Shifting retention times and poor peak shape
ENVIRONMENT
- M-Class
CAUSE
The gradient initial condition on the B pump is 1% of 260nL/min which is 2.6nL/min and too close to 0 which can cause back flow.
FIX or WORKAROUND
- Increase pump B initial condition to 4% (10.4nL/min)
ADDITIONAL INFORMATION
Never go below 10nL/min at any time in the gradient and trapping. The flow controllers are never exactly zero at 0nL/min. Going below 10nL/min can cause back flow due to the tolerance of the flow controllers. When stopping flow, never enter 0ul/min. Always select Stop Flow to stop the plungers movement.
The issue is where the auto zero calibration lands. We create a 0 flow condition and tell the flow controllers to calibrate to that 0 flow. After the calibration, it measures the flow. The criteria is, that measurement of the 0 flow needs to be within +/- 5nL/min.
If the result is +4nL it passes. But 1% of 300nL/min is 3 nL/min. The system thinks it's already pumping 4nL/min, so the plunger will pull in the solvent at 1nL/min (back flow) to satisfy the offset. This disturbs the introduction of the organic
The best scenario is to have a -4nL result after the auto zero. If you ask for zero flow, it will flow 4nL/min to satisfy the offset. If you want 3nL/min, the pump will pump positive 1nL to satisfy the offset.
One system can have a negative offset and work where another system will have a positive offset and not work.
To make sure you never have negative flow, we say never go below 10nL/min at any time.