psi readbacks UPLC vs 2695

I've got a method that is run on both the 2695/Qtof and UPLC/TQD and had a question about system psi anomolies. The column guide when speaking to Waters apps chemists is supposed to be kept below 1500 psi (IC Pak Anion HR, 4.6x75, WAT026765).It's been running on the UPLC/TQ at ~900 psi for the past several days and I put it on the 2695 today and it's psi is running at ~1450 with the same physical column and mobile phase (solvent bottles are used on UPLC and then moved over to the 2695), so it's not an issue of different solvent systems. Today, I put it back on the UPLC and it still reads back at ~900 psi, again the solvent bottles are the same, meaning I don't have two bottles of A, there is only one A bottle, I'm moving it between systems. Same with B solvent. Flow rate on both systems is 0.75 ml/min, column specs say not exceed 1.0 ml/min.Any reason as to why the 50% discrepenancy between the two systems? I've ran the same column type (WAT026765) before and it's been within specs (~1000 psi on 2695). I'm wondering is there an issue with the readbacks on the 2695 (or the UPLC reading back too low)?Thanks,


  • Hello:

    This is a really interesting post.

    As I first read your thread I was ready to type, well the UPLC system always comes up a little higher as the tubing ID's are much smaller. However, the discrepancy is the reverse!

    I think therefore it must be where and how the system pressure is being read. The pressures may be different, but must be working reproducibly as the systems are working fine.

    Anyway this long thread is simply to say "I have no idea" and need to consult some smart people and get back to you!


  • Jason:

    I need to wait for further replies,but here are some engineering guesses are below. If the pressure transducer in the Alliance is behind the restrictor this is likely the reason.

    (1)There is a possibility that the in-line filter in the 2695 (between the solvent and sample manager) could be partially clogged which could contribute a fair amount of backpressure.

    (2) Another consideration might be how well the needle-hole is aligned in the injector seal (seal calibration), but typically in IC one would see peak-splitting (doubling).

    (3) Finally, some of the pressure difference may be due to the different sample management systems. Recall the 2695 has a restrictor in the flow path that allows a 90:10 split (during injection thru analysis) that becomes 0:100 split during sampling. I believe this is quite different than the ACQUITY Sample Manager.

    Thank you again,


  • Hello Grand,

    if you are using peek capillaries please check if the capillaries on the 2695 were overtightened. That could lead to increased backpressure, and it's getting worse with smaller IDs.

    Also check which part of the system is responsible for increased pressure. Start at the QTof and disconnect the capillary coming from the 2695 and continue till you reached the end of the column. Somewhere in between there has to be a pressure difference of nearly 500 psi.


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