ELS detector gain

Hello.
When we use ELS detector in our UPLC system and raise the gain over 150 or so,
there's no signal obtained. It seems like it went out of the proper range somehow.
(No natural noises.)
Measurements can be done normally with lower gain.
What can be the reason and remedy?

Answers

  • You can consider three types of setting low, normal & high, A low-gain setting is used for milligram samples and a normal-gain setting for nanogram to microgram samples. Low is for low gain

    detector operation, and is useful when dealing with high concentrations of analyte (I mean in mg). Like from 4-25 gain

    High Gain setting for very low concentrations (you can say traces amount) like 1-10 nanogram. Dont try 150, i think even 50 is enough for traces

    You can also use Dynamic Range setting, when you are looking many analytes from very low to high in amounts.

    ~ Salman

  • Thank you for your comment, Salman.

    But, sorry, it seems my question was not clear enough.

    What I mean is that it happens even without injection of samples.

    When you are monitoring baseline of ELSD signal without injection, you would see

    natural fluctuation of the signal. When we raise the gain during such monitoring,

    baseline makes step at each occasion of gain-change, and when the gain comes

    to ca. 150 or more, the natural fluctuation disappears and the baseline becomes

    perfectly straight, which is unusual (and it seemed it went out of the proper range).

    Even if we try auto-zero at this situation, baseline keeps perfectly straight, and even

    if we inject samples at this gain, the situation is the same: perfectly straight and

    no signal obtained at any concentration of the sample.

    I would appreciate further comments.

  • Dear it’s the matter of sensitivity, when you increase the gain your sensitivity level increase & high gain in ELSD delivers maximum sensitivity with minimum baseline noise…. with or without injection, so lamp starts work according to that… your ELSD gain high so that baseline looks straight.

    the gain is like you can say attenuation settings in UV..

    i think 150 gain is very high...no signal will appear.... did you check Nitrogen? as ELSD requires high purity N2.. as you said, you getting noises at lower Gain... & temperature settings also important...?

    ~ Salman

  • Thank you again for your comment, Salman.

    I think raising the gain would normally cause increasing in apparent natural noises when we are looking

    at the same LSU range... isn't it right?

    Nitrogen might be a possibility, but we could run measurements with the gain of 500 before some time, for

    the same sample.

    My colleague says once she saw a peak got saturated (I mean signal became perfectly-straight line and

    looks like the peak-top has cut off) before it reaches the upper limit of LSU 2000. So, we rather suspect

    some kind of software or electronic issue. Hmm...

  • Kimiya:

    We believe that what you are looking at is a large background that is being disguides by the autozero on injection. Whay we suspect is that by increasing the gain will only makes the PMT current (the ELS signal) larger. All PMT's will saturate if the gain is too large, which we suspect is the case here. The fact that the signals look "normal" at lower gains tends to suggest that the background is too big.

    To be sure we will nee to diagnose:

    (1) Ddisable autozero in the instrument method

    (2) Observe background scattering level.

    (3) Adjust drift tube temp and nebulization conditions to minimize the background level.

    Note the ELSD does not discriminate between scattering of solvent droplets and scattering of analyte "slush balls." the drift tube temp should be high enough to desolvate but low enough to ensure that semi volatile peaks are preserved.

    Let us know.

    Liz

  • Thank you for your comment, Liz. We have tried as you have suggested.
    After disabling autozero, the ELS signal readings were high, close to
    2000 LSU.
    However, any change in ELS detector setting including drift tube and
    nebulizer temperature, as well as in solvent, did not lower the signal
    readings. What should we do then?

  • Hello

    There are two possibilities, there is just too much background. It may be that we will need to change the flow rate. We may have a nebulizer contamination issue too.

    Can you send a chromatogram and the instrument settings and the detector settings, the composition flow rate etc and we will go from there.

    Best regards,

    Liz

  • Here's some chromatograms and conditions used, with our system,
    with the auto-zero turned off. Please see the file attached.
    Gain was set as 100, so the line barely goes within the range.
    But when we make the gain 150, it goes out of the range.

    Some contamination in the drift tube or the nebulizer might be
    suspected, as some ghost (?) peaks appear even without injection.
    However, cleaning procedures that we are taught from Japanese Waters
    staff did not make the situation better.

  • Hello again:

    Thank you for the chromatograms and information and so now we have (of course) a few more questions to ask - apologies.

    (1) Since the signal offset and the noise are so high, we are suspicious of the Normalization factor. Is it possible that you ran "Normalization" with liquid flow to the nebulizer? I do want to be confident that the instrument PMT was normalized properly, just to be sure can you run the normalization diagnostic once again. Ensure that the liquid flow off (no flow), with the gas flow set to something like 35 psi. The diagnostic itself has some instructions along the way.

    (2) After this the next concern is that the nebulizer "off" cooling the nebulizer may reduce the background dramatically.

    Let me know.

    Liz

  • Hello, Liz. It's been a long time.

    It took us a long time, as Waters (Japan) did not recommend us to
    do normalization of PMT by ourselves. A Waters staff visited us
    and normalized PMT, but the situation did not change. He recommended
    us to send the detector back to Waters for checkup. So we did.
    Eventually, they told us that what have worked was the normalization of
    the lamp, not the PMT. Now our ELS detector works well.

    Thank you anyway, for your comments.

    Kimiya

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