Operation range?

I'm a postgraduate student, recently we purchased one of this OpenQCM. We are using it to measure the absorption capability of some ionic liquids.

Our concern is about the operation range of the apparatus. For example, we tried to use a small drop of the ionic liquid (approximately 2mg) on top of the gold plate, but the plot for the frequency goes to 0 when using 6MHz, or 16MHz when using 10MHz as quartz frequency. When using an even lighter amount of IL, we can achieve some data, but it's still far of we expected. Is there a limit weight we can use on top of the gold plate?

Until now, we're using a gas flowrate of 10 cc/min. Is it too much? According to your test, you used 0.1 cc/min and I wonder if this could be the reason why it's not properly working.

Thank you so much for your help!


  • Hello jmhm,
    thank you for your interesting question.
    Unfortunately you will not be able to retrieve a frequency-mass relation for a such "big" droplet by measn QCM devices.
    First (but secondary as importance) the nominal saturation limit for a 10MHz sensor ideally (for an uniform and rigid deposited layer) is about f/100 of the resonance frequency. As a consequence by considering the sensitivity of a 10MHz quartz you will have a nominal saturation value of about 0.44 mg/cm2.

    Really the most important effect that you should consider is that a droplet is not a rigid layer, as a consequence the sample will introduce viscoelastic effects that further complicate the QCM frequency behaviour.

    Furthermore you should also consider that for a liquid sample the depth of penetration in water at MHz frequencies is of the order of 100 nm.
    In few words the frequency-mass starts to begin "accurate" only if your sample layer will be sufficiently low these 100nm (but also in this case you should ever consider the vicoelastic properties of your sample).

    Relatively your last question, we prefer to use a flowrate of 0.1 cc/min only in order to reduce "peristaltic fluctuations", that are visible on the frequency baseline, during the pumping. You can also use 10 cc/min, if you do not need for a hight S/N ratio. Alternatively, if you can do it for your experiment, you can stop pumping before the frequency measurement (by waiting for f stabilization), and restart again after measurement.

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