Some parameters mentioned in the context of QCM can be misleading. These parameters are true, but they are most often irrelevant in the 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 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 are 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.
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 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.
Read about how and why the QCM fundamental frequency matters in measurements
The fundamental resonance frequency of QCM-sensors is often 5MHz. Learn about the theory behind the number.
Read about why it is possible to gain valuable information from a viscoelastic sample by monitoring multiple overtones in QCM measurements.
The D-factor provides information that is complementary to the frequency response. Read about how it can be understood and what information it reveals.