Two common types of QCM technologies are QCM-D and QCM-I. Both can provide information about mass changes and energy losses at the sensor surface, but they differ in how the energy loss is measured and how the data are acquired. In this post, we explain the technical principles behind QCM-D and QCM-I, how dissipation can be obtained, and what to consider when choosing a QCM setup.
Summary
- QCM-D stands for Quartz Crystal Microbalance with Dissipation monitoring.
- QCM-I stands for Quartz Crystal Microbalance with Impedance spectroscopy.
- Both approaches can provide information about resonance frequency and energy loss, which are needed for viscoelastic analysis.
- In impedance-based QCM, the resonance behavior is obtained from a frequency sweep across the resonance peak.
- In decay-based QCM-D, the crystal is excited and then allowed to freely decay; frequency and dissipation are extracted from the decay.
- Impedance-based approaches can characterize the electrical equivalent circuit, while decay-based QCM-D enables fast acquisition and high time resolution.
- The right choice depends on the type of measurement, required time resolution, need for multiple harmonics, and the complexity of the material being studied.
Different ways of measuring the energy losses
To get the most out of your QCM measurement and to be able to characterize viscoelastic materials, it is good to collect information about the energy losses, the so-called dissipation or damping. The energy loss can be measured in three different ways, via 1) the resistance, via 2) impedance spectroscopy, and via 3) the decay time of the oscillation. The latter two methods can collect enough information to enable viscoelastic modeling of the QCM data. Below we take a closer look at how the two methods work and how they relate to QCM-I and QCM-D.
What do QCM-D and QCM-I mean?
QCM-D is short for Quartz Crystal microbalance with Dissipation monitoring and QCM-I is short for Quartz Crystal Microbalance with Impedance spectroscopy. To complicate things a bit however, there are different types of QCM-D:s and there are multiple versions of QCM with impedance spectroscopy. I.e., not all QCM-D:s are based on the same technological principle, and QCM-I is not the only QCM based on impedance spectroscopy. As we mentioned above, the dissipation can be obtained in different ways, for example via the bandwidth, which is an impedance measurement, or via the decay time, also called pinging. Currently, there are QCM-Ds based on both the impedance approach and on pinging. Even if the technological principles of the QCM-D:s in the market differ, they all collect information about the energy losses of the system under study.
In addition to QCM-I, QCM:s with impedance spectroscopy can also be called for example QCM-Z and, as mentioned above, QCM-D. Since the abbreviation doesn’t reveal the technological principle on which the QCM is based, it can be helpful to learn more about the QCM that you are using or planning to purchase as this will give you better insight into the data and how to make the most of it.
Technical comparison of impedance spectroscopy and pinging
To compare the two approaches of measuring the energy loss, via impedance spectroscopy and via pinging, we start by looking at the electrical representation of a quartz crystal, Fig. 1. The equivalent circuit, a so-called Butterworth- Van-Dyke element, consists of a capacitance (C1), inductance (L1), and resistance, (R1), in parallel with a shunt capacitance (C0). In brief, L1 represents the oscillating mass and C1 represents the elasticity of the mass. The resistance, R1, represents the losses in the system, and C0 the electrical component due to the overlap of the electrodes on the crystal.
Figure 1. Representations of A) a quartz crystal and an B) equivalent circuit
With the electrical representation of a quartz crystal, the dissipation is defined according to Eq. 1. I.e., if R1 and L1 are known, the dissipation can be calculated.
Impedance-based QCM: measuring the resonance pea
As already mentioned, one way to assess the D value is to use impedance (or admittance) spectroscopy, which makes it possible to determine all the elements of the equivalent circuit, Fig. 1.1-3 In an impedance spectroscopy set-up, a sinusoidal voltage, V, is applied over the crystal and the input frequency is varied in a stepwise manner in the frequency range spanning the resonance peak, Fig. 2.
Figure 2. A schematic illustration of QCM with impedance spectroscopy.
At each frequency, the current and phase are recorded. The ratio of the voltage and current gives the frequency dependent impedance, Z, Eqs. 2-4 (or the admittance, Y, which is the reciprocal of the impedance). The impedance of the equivalent circuit can be expressed as
where |Ztot| is the amplitude, θ is the phase angle, and ω=2πf.
When a complete sweep over the frequency range has been recorded, the four parameters C1, L1, R1 and C0 of the electrical equivalent circuit can be fitted numerically. This result can then be used to quantify the parameters of the electrical equivalent circuit, of which L1 and R1 are needed to calculate D according to Eq. 1.
The dissipation, D, can also be obtained by measuring the bandwidth, BW, or half bandwidth at half maximum, Γ, of the resonance peak, and then using Eq. 5.
The equations used to describe the equivalent circuit are only valid in steady state when all transient processes have stopped. Hence, the frequency sweep must be made slow enough to let the crystal stabilize its oscillation at every new frequency step. This fact inflicts a speed limit on the acquisition rate, which makes impedance spectroscopy considerably slower than for example QCM and QCM-R.4 However the full characterization of the equivalent circuit enables the extraction of the dissipation in the system, not only a relative measure as with resistance measurements.
Decay-based QCM-D: measuring the free oscillation decay
The decay-based approach, also referred to as ‘pinging’ was developed by Rodahl et al. in the 90s.5 This technical solution was the origin of QSense QCM-D and is still the technical principle on which all QSense instruments are based. To our knowledge, QSense is the only QCM-D platform that uses a true decay-time, or ring-down, measurement without relying on a frequency sweep.
With the pinging method, the QCM crystal is rapidly excited to resonance, the driving voltage is turned off, and then the decay time of the oscillation is monitored. The decay is then numerically fitted to an exponentially damped sinusoidal and D can then be extracted along with the resonant frequency of the sensor. This measurement is very fast, and the high acquisition rate makes it is possible to record several harmonics with good data quality, which in turn enables high-resolution kinetic studies and viscoelastic modeling.5
Figure 3. Principle of decay-based Dissipation monitoring. The crystal is exited into oscillation during a short time, after which the driving frequency is switched off, and the freely oscillating decay is recorded. Since only one decay recording is necessary to determine both frequency and dissipation, the acquisition is very fast.
Pros and cons of impedance-based and decay-based measurements
As with all methods, both the impedance approach and the decay time measurement have their pros and cons. A strength of both approaches is that, compared to frequency-only QCM or resistance-based QCM (QCM-R) approaches, QCM measurements with impedance spectroscopy and QCM measurements with decay time monitoring contain enough information to extract the D-value. Both methods also allow for overtone measurements. As these are two prerequisites for viscoelastic modeling, both methods allow for such quantification.
A drawback with impedance measurements is that they can be slow. The frequency sweeps are lengthy, which limits the maximum time resolution. Also, the measurement is sensitive to small variations in the shunt capacitance, C0, which can influence the accuracy and/or noise of the measurement.
The decay-based QCM approach has important practical advantages for time-resolved measurements. Unlike the impedance approach, the pinging principle is very fast, which enables high time resolution. Also, the decay approach does not experience the same sensitivity to variations in the shunt capacitance since the short-circuit mode practically eliminates the interference from the shunt and stray capacitance. This makes the method very stable. A drawback relative to the impedance approach worth mentioning, however, is the cost of the instruments. Decay-based QCMs tend to be more expensive than impedance-based ones.
When to use which QCM
The pros and cons of the two types of QCM:s make them suitable for different kinds of applications. Impedance-based QCM can be useful when you want to characterize the electrical equivalent circuit and extract dissipation from the resonance peak or bandwidth. It can support viscoelastic analysis when enough harmonics and dissipation information are collected. Decay-based QCM-D is especially useful when you need high time resolution, stable dissipation data, and multi-harmonic analysis of soft, hydrated, or viscoelastic layers. This is often important for studying biomolecules, polymers, lipid layers, surfactants, hydrogels, coatings, electrochemical interfaces, and other dynamic surface-bound films. In short, the more complex, soft, or time-dependent the interfacial layer is, the more valuable it becomes to measure both frequency and dissipation at multiple harmonics with high data quality. Table 1 summarizes some of the key aspects and exemplifies when to use which method.
Table 1. Comparison of QCM, QCM with impedance and QCM with decay measurement.
Concluding remarks
QCM-D and QCM-I are two common QCM approaches that can be based on either impedance measurements or decay-time measurements, also known as pinging. The first QCM-D instrument on the market was based on the pinging approach, which was developed by Rodahl et al. in the 1990s. Since then, QCM-D instruments based on the impedance approach have also appeared. In general, both technical solutions can collect enough information, i.e. frequency, f, and dissipation, D, at multiple harmonics, to enable viscoelastic modeling. The practical differences are mainly related to how the data are acquired, including acquisition speed, time resolution, and sensitivity to factors such as shunt capacitance.
