It is common knowledge that a 10 MHz QCM crystal has higher mass sensitivity than a 5 MHz one. And a 27 MHz crystal beats them both. But is the mass sensitivity really the important parameter when determining how small masses a QCM can detect? The short answer: No. The longer answer: Please read on.
Mass sensitivity – the theoretical value vs the value in a measurement situation
Some parameters mentioned in the context of QCM can be misleading. These parameters are true, but may still be irrelevant in an actual measurement situation. One example of such a parameter is the mass sensitivity, often referred to as the ‘sensitivity’.
The theoretical mass sensitivity is a value that depends purely on the fundamental resonant frequency of the QCM crystal. The higher the fundamental mode, the higher the theoretical mass sensitivity. A 5 MHz crystal will have a mass sensitivity of 17.7 ng/(cm2∙Hz), and a 10 MHz crystal will have a theoretical mass sensitivity of 4.4 ng/(cm2∙Hz). The mass sensitivity is how many ng of material per cm2 that is needed to shift the resonance frequency 1 Hz. A smaller mass sensitivity value means that a smaller amount of material is needed to shift the frequency, and hence the mass sensitivity is higher.
Is a 10 MHz crystal better than a 5 MHz crystal?
Reading these numbers, it is close at hand to assume that a 10 MHz crystal will be better, i.e. will be able to sense smaller changes, than a 5 MHz one, and the higher the fundamental frequency the better. However, it must be considered that the noise level also increases with a higher fundamental resonant frequency. This means that a higher theoretical sensitivity does not necessarily correlate with a better mass detection limit (the useful mass sensitivity) in the actual measurement situation. A much better value is, therefore, the signal-to-noise ratio which gives an indication of how small masses can be theoretically measured. In this context, it is important to remember that not even the signal-to-noise ratio parameter tells the full story. Other important parameters, such as temperature stability, ease of handling, simultaneous multiharmonic measurements, etc. also influence the end result and the conclusions that can be drawn from an experiment.
So, when choosing which QCM instrument to use, forget about the mass sensitivity parameter and instead look at the factors that matter for your measurements.
Download our guide below to learn more about how to assess the QCM sensitivity and other parameters that are related to the QCM data quality.