Backpressure drop HPLC system


I have a agilent 1200 series with agilent 1260 infinity II.

After ramping my flow 0.2ml/min - 20 min. 0.4ml/min - 20 min and 0.6ml/min 20 min.

And ramping my temperature for the same period of time 30-40-50 °C

The pressure on system is around 66 bar.

The pressure used to be around 59bar. Which already shows. Increase in pressure.

However, the question is :

If I click on the instrument HiP sampler and perform a needle wash 

The pressure drops to 56bar after the wash.

And during injection the pressure goes back to 66bar.

I never noticed this. Is this normal?

This drop in backpressure during a needle wash prior to starting any analysis. ( The needle wash performed was not the one performed during the injection or the injector program).

All the best,


  • Hello Carlos,

    What is the model number of the sampler?  Some samplers have an active needle wash with a peristaltic pump and others "dip" the needle into a vial.  In either case, the injection valve will switch from main pass to bypass causing around 5-8 bar pressure difference.  

  • This is correct. The main pass flow path includes the sample loop, whereas the bypass skips the sample loop so that the needle can be washed without losing the sample. Removing the sample loop from the flow path results in a drop in system back pressure that returns upon injection of the sample.

    As for the increase in pressure, is it possible that you may have some contaminants that have reached your column. Look into performing a column backflush per the manufacturer's instructions for your specific column. Column temperature can also have an impact on the system back pressure. Are you comparing those pressure values from when the temperatures were at equal values?

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