Lamp energy

<p><span>Hi all,</span></p><p></p><p><span>just one further question:</span></p><p><span>We are using a PDA-detector. </span></p><p><span>As a routinely performed test, we check the use "verify calibration" and "read energy". </span></p><p><span>And here is my problem with "read energy". There is no "pass"- or "fail"-criterion given. So, what is a good energy, what is bad?</span></p><p></p><p><span>To give you an example:</span></p><p><span>We had to change the lamp, because of the s/n ratio. So after the change we performed the tests and found 50.000 counts. Four weeks later I did the test again and we had already lost 25% of the energy.</span></p><p></p><p><span>So, what is your opinion on this?</span></p>


  • Hello again:

    This is another situation where the answer will not be exactly "Pass/Fail". Clearly you have found that there can be a relationship between energy and signal to noise performance, but that may not always be the case.

    The lamp energy read back takes into account several factors. The age and use of the lamp itself. The condition of the flow cell, which can change rapidly based on the type of usage. The condition of the optical components such as the mirrors, optical windows and grating. All of these can factor into the appearance that a lamp has degraded.

    If using an ACQUITY PDA detector then the best test of reading the energy is the use of a medium shunt flow cell. This eliminates any potential flow cell issues from making the lamp appear to be deficient. It also allows you to be confident that if the lamp is new and the energy reads low that the problem is likely due to an optical component.In either case the detectors firmware will adjust sampling time to the proper level based on the energy and the background absorbance as long as autoexposure is turned on. What all of this means is that there is really no pass/ fail criteria that we can provide strictly related to lamp energy. Signal to noise levels that are acceptable to the customer are probably the best indicator of how long a lamp can be used with all of the other factors taken into consideration.

    As an aside, the reason that lamp energy cannot be used as a strict guide for the PDA is the use of the Lamp Optimization Software feature, which automatically maintains a minimal noise level throughout the useful life of the lamp. It automatically selects an appropriate exposure time for the photodiodes based on lamp energy, mobile phase and wavelength range for the analysis. This optimization is performed before each injection and requires no operator adjustments to the detector. It is dependent on what is in the flow cell at the time. So a mobile phase containing an absorber, such as TFA, would have longer exposure time than pure ACN. For every injection a value for exposure time is documented, in msec and for a given analysis/set of samples it should not vary. Personally, I check the exposure time value periodically and see what it is as the lamp ages.

    What was the value for your last anlysis?

    Hope this helps,


  • Liz,

    I'm sorry, I'm not sure where to find the value for exposure time. Shame on me...

    The only thing I found was "auto" in the instrument method.

    Can you - or someone - tell me how to find the value when using Empower?


  • Hi

    You need to create a view filter for you channel tab in the project window, and add the PDA exposure time column to the view filter. See the two screen dumps from Empower.

    Hope this helps.

    best regards


  • Hi,

    thanks. I have tried and found out, that my new data won't give any exposure time readback, as there is no spectra but only one wavelength-channel.

    So I opened some data which is now about 4 weeks old - and my exposure time was then 25.0 msec. This is already with my "new" lamp.


    I've performed another sample with spectra today: It is still 25.0 msec.

  • Hello:

    And what is the mobile phase today and yesterday. As I mentioned the exposure time will depend on the mobile phase in the flow cell.

    And can you confirm which PDA you have, is it one that can go up to 500 nm or to 800 nm

    Thank you,


  • Hello Liz,

    the mobile phase is exactly the same mixture of water and ACN.

    No, we have the "small" PDA, not the extended.


  • Hello

    Apologies for the delay, this seems a little long, but it is always difficlut to be sure. I would suggest that you consider cleaning the flow cell to eliminate that issue. The directions are available with the flow cell or in the operator's guide, version D.


  • Hello Liz,

    thanks for the reply.

    I have already cleaned the flow cell following the procedure -> no difference...

    I already thought about trying the "larger" cleaning procedure for the whole system...?

  • Hello:

    Then I think this should be a call to the FSE, for guidance. I would like to see what else may be the problem.


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