Flow Rates - Acquity UPLC Binary Solvent Manager

This question for is Waters.What are the uncertainties for flow rates @ 2.0, 1.0 , 0.5 and 0.2 ml/min when measured and compared to the actual settings under normal room temperature conditions (i.e., 23°C and using water)? Is there data listed anywhere?Thanks, Lotus


  • They should not be too different, but I do not have the exact values, the data that we have tends to compare RT. The data is collected that way to assess chromatography performance because the ACQUITY BSM runs at very high pressures and compensates for the compressibility of different mobile phases at different backpressures and temperature etc. This makes the operation very different to establish from a classic beaker and drip volumetric test, which I assume is your question. However, if you use 100% solvent and measure at ambient and below 1,000 psi backpressure the differences from expected should not be too high.

    For high pressure gradient systems, compositional accuracy is driven by flow rate performance of the individual pumps. HPLC pumps meter solvent at system pressure with the solvent pre-compressed to system pressure before delivery to the column; but most laboratory measurements of flow rate are performed at atmospheric pressure. Consequently, the pre-compression of the mobile phase is not accounted for in the measurement and calculation of the apparent flow rate. The positive bias of the apparent flow rate increases with system backpressure and is dependant upon the specific mobile phase, the partial pressure of dissolved air in the mobile phase, and the operating temperature. Waters' flow rate specification defines a solvent and system pressure which minimizes the positive bias of the measurement imposed by the solvent pre-compression.

    The Waters' SQ test used for qualification measures flow rate based on the measurement of retention time and not volume. The method uses an unretained peak (thiourea) which is not retained (on reverse phase columns) and elutes when the void volume has been pumped through the column. A plot of the inverse of the retention time of the peak at different flow rates is linear and any non-zero X-intercept implies a systematic flow rate bias. This test verifies both the linearity and accuracy of flow rate at system pressure. This type of test is more suitable for measurements of UPLC systems, at multiple flow rates and where the flow rates measured exceed 1 ml/min and the system backpressure can exceed 10,000 psi.

    I am not sure if this answers you question exactly, but an experiment that mirrors the qualification test rather than a volumetric test will correspond more closely to the actual function of the pump and will give more representative results. The deviations in a manual volumetric test will give results that may not differ too highly from expected, but truly do not represent the actual performance.

  • Thanks.

    That was a very detailed and satisfactory response.


  • At our site, we need to check the flow rate daily. We use the timer & volumetric method since setting up to run the unretained peak test isn't practical. The UPLC's tend to run on the high side; 101-104% of the set flow rate (our spec is +/- 5%).

  • Many thanks for a practical solution, I did pass on to our community at Waters!!

  • As a further note from one of the engineers on the flw rate difference...

    It is expected for the volumetric flow rate to be higher because the BSM delivers flow at pressure and your test measures flow at atmosphere. The higher the pressure the apparent discrepancy will become. It may be good to record the pressure that you run for your test.


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