Stop flow test

Applies to

Electrochemical detector

Summary

In case there is a problem with noise or high background current, the stop-flow test can be helpful to check if the problem originates from a contamination somewhere in the HPLC system (flow cell only being the messenger of the problem). For the test, the flow cell is temporarily disconnected from the LC flow path by short-circuiting the flow cell in and outlet. This stops the supply of any electrochemically active substances to the flow cell. The resulting drop in background current and noise are indicators for the source of the problem.

Fig.1. Method [1]. Modifications of the LC flow path to the stop-flow set-up: the flow path at the flow cell is short-circuited.

In a simplified version, only the pump is stopped;

Fig 2. Method [2]. Monitor baseline and stop the pump (flow rate = 0 mL/min). A significant drop in baseline indicates a problem of EC active contaminants in the mobile phase. The supply is cut off by stopping the flow, resulting in this drop in baseline.

Procedure

The stop flow test can be done in 2 ways. The Easiest method [1] is to put the flow rate to 0 mL/min, while monitoring the baseline take enough time to let the pressure drop to really 0 bar. No changes in setup are needed, and the baseline trace will show what is going on (Fig. 2).

In case of severe contamination or pump pulsations this will be enough to diagnose the problem. In case of doubt, a more accurate method [2] is disconnecting the tubing as indicated in Fig.1, to make sure there is no residual flow due to pressure or height differences.

1. Note the noise and cell current of the system
2. Disconnect the outlet tubing from the column
3. Disconnect the outlet tubing from the flow cell
4. Connect both tubing to the column and flow cell according to Fig. 1. Do not remove the tubing that is connected at the inlet of the flow cell! This may cause an air bubble over the working electrode.
5. Measure the noise and cell current again under these stop-flow conditions.

Compare the data of noise and current from both set-ups:

– Background current (I-cell) and noise did not change significantly

• example: I-cell 85  =>  75 [nA] (with/without flow)
• example: noise 18  =>  15 [pA] (with/without flow)
• Conclusion: the high background current is not caused by an EC active contamination in the flow path. Most likely the flow cell itself needs service or repair.

– Significant drop in I-cell

• Example: 85 => 15 [nA], or 500 => 80 [nA] (with/without flow).
• Conclusion: electrochemical contamination in flow path (origin for example: mobile phase, tubing, pump, autosampler, column leaching)

– Significant drop in noise

•  example: example: 20 pA => 4 pA (with/without flow)
• Conclusion: the problem is not located in the flow cell (it is only the messenger). Suspect areas are a malfunctioning pump, pulse damper, degasser (bubbles), or contamination.

Depending on the outcome, take remedial steps to get the system back to work. Apply service to the cell or send in for service to Antec, or methodically check and replace suspected parts to find out what is causing the high current and/or noise.