QSENSE - PRINT - QCM-D FAQ

• What is the difference between QCM-D and traditional QCM?
The QCM-D technology (Quartz Crystal Microbalance with Dissipation monitoring) extracts the frequency and dissipation of the freely oscillating quartz crystal after excitation. Major benefits with this procedure compared to other QCMS are:

- Extremely quick measurements that gives a kinetic resolution of up to 200 data points per second with the latest instrument. Higher measurement rate means higher sensitivity since several data points can be used to average data.

- Record the response of the crystal when it is not a part of a feedback loop (active oscillation circuit). This means that the instrument only record changes occurring at the surface. In conventional QCMs part of the recorded changes in resonant frequency is due to changes in the feedback loop (such changes occur when the mechanical load on the crystal changes) and not due to changes taking place at the surface.

• What is dissipation?
The dissipation (damping) is the sum of all energy losses in the system per oscillation cycle. It is defined as 1/Q, i.e. the energy dissipated per oscillation, divided by the total energy stored in system. With QCM-D, the dissipation factor is measured every time the drive generator output is stopped and the sensor oscillation starts to decay exponentially. A soft film attached to the quartz crystal is deformed during the oscillation, which gives a high dissipation while as a rigid material gives a low dissipation.

• Why is it so important to measure the dissipation?
The frequency response of a quartz crystal represents the total oscillating mass. This oscillating mass always includes a certain amount of water. However, the amount of water may vary between 10 and 90% depending on the type of molecule and the way it adsorbs on the surface (an elongated protein that adsorbs flat to the surface gives low dissipation while the very same molecule standing up on the surface gives high dissipation). By measuring the dissipation it becomes possible to determine if a soft film (water rich) has formed on the surface or if the film is rigid (less water). Only when the film is fairly rigid, the Sauerbrey relation gives a good estimation of adsorbed mass. Measuring of the dissipation means that it is possible to determine if the Sauerbrey relation is valid or not.

• When is it correct to use the Sauerbrey relation to calculate the mass adsorbed on the quartz crystal?
The Sauerbrey relation describes the linear relation between frequency changes and changes in mass for thin films adsorbing to the sensor surface. It gives a good estimation of mass/ thickness, as long as the dissipation is relatively low. When the dissipation value typically reaches above 1×10-6 per 10 Hz, the film is too soft to function as a fully coupled oscillator- the upper parts, far away from the surface, do not couple to the oscillation of the sensor. This means that the normal relation to calculate the mass directly from the change in frequency, will underestimate the mass. However, by measuring both dissipation and frequency at several harmonics it becomes possible to extract correct thickness estimations even in these cases and also calculate viscoelastic properties using a viscoelastic model incorporated in Q-Sense software.

• What are the benefits of measuring at several frequencies simultaneously?
Simultaneous measurement of multiple overtones is required to model viscoelastic properties and extract correct thickness for films outside the Sauerbrey region. With the Q-Sense E4 system, 14 incoming parameters (7 frequencies and 7 dissipation values) per sensor provide a well-determined model of the particular film properties. Moreover, the different overtones give information about the homogeneity of applied layers: as the detection range out from the crystal surface decreases with increasing overtone number, abnormal frequency behaviour suggests vertical variations in film properties. The fact that the detection range out from the crystal surface decreases with increasing frequency is also used by the modeling software to calculate accurate thickness of films that do not fully couple to the oscillation of the crystal. For rather soft films, containing much water (films made of large proteins for example), you will not get accurate thickness information without measuring at several frequencies.

Another advantage of using higher overtones is the decreasing noise to signal ratio, which is good when extra high sensitivity is desired.

• What is the difference between QCM-D and SPR measurements?
The ability to evaluate kinetics is quite similar with both systems from a technical point of view, but when it comes to sensitivity they differ quite a lot. QCM-D systems are more sensitive for water rich and extended layers, while the SPR system is favoured for compact and dense layers. The reason to this difference is due to the different physical principles by which the coupled mass is measured. Since the mass-uptake estimate from SPR data is based on the difference in refractive index between the adsorbed biomolecules and water displaced by the biomolecules upon adsorption, water associated with the protein film, e.g. the hydration shell, is essentially not included in the mass determination. In contrast, changes in frequency acquired with QCM-D measures water coupled as an inherent mass via direct hydration, viscous drag and/or entrapment in cavities in the adsorbed film. This means that the SPR response is proportional to the coupled "molar mass", while in QCM-D measurements the layer is essentially sensed as "hydrogel" composed of the macromolecules and coupled water.

However, while SPR measures one parameter only, the additional information contained in energy dissipation measurements when using QCM-D increases the base for a detailed interpretation. Changes in the dissipation are related to the shear viscous losses induced by the adsorbed layers, and thus provide information that has the potential to identify structural differences between different adsorbed systems, or structural changes in the same type of molecule during the adsorption process.

• What is the mass sensitivity of Q-Sense instruments?
The maximum mass sensitivity in liquid is about 0.5 ng/cm2 for Q-Sense E4 and 1 ng/cm2 for Q-Sense D300 if measuring at a rate of 1 data point / 5 seconds. Q-Sense E4 operating at all four sensors and at 3 harmonics has a sensitivity of about 2 ng/cm2 if all data are collected in 1 second. For example, consider a monolayer (<100% surface coverage) of Myoglobin (17800 Da): the monolayer corresponds to 177 ng/cm2 (change in frequency, 10 Hz).

• What is the time resolution?
The maximum rate is up to 200 data points per second with Q-Sense E4.

• What is the frequency range of Q-Sense instruments?
The frequency range is 1 to 70 MHz with Q-Sense E4 and 1 to 42 MHz with Q-Sense D300. A large range is important to be able to use the unique features of multiple frequency and dissipation sampling.

• What coatings/surfaces can be used?
The sensor crystal can be coated with almost any material as long as it can be applied as a thin (nm range) homogenous layer firmly attached to the underlying surface. The layer thickness can vary between nanometers and micrometers, depending on the viscoelastic properties of the applied material.

Q-Sense offers precoated crystals with for example Gold, Ti, SiO2, AlO3, stainless steel, hydroxyapatite and polystyrene. Several other materials are available, for example most metals, metal oxides, spin-coated polymers etc.

• What is the maximum thickness of an applied/adsorbed film?
The maximum allowed thickness of a coating depends on the viscoelasticity of the coating and may vary from a couple of hundred nanometers to a few micrometers. The more rigid layer, the larger thickness is possible.

• What is the detection range from the surface?
The detection range varies from nanometers to micrometers, depending on the viscoelasticity of the applied film. In pure water it is approximately 250 nm. Applying a very rigid film such as a metal still gives the same detection range in water, which means that you do not affect the measurement principle while thin films have been coated on the surface prior to measurements. Compared to optical methods the detection range is a main benefit of QCM-D. Consider for example polyelectrolyte multilayers of several hundred nanometers. These are easily sensed by QCM-D.

• What is the temperature range of Q-Sense instruments?
Proper temperature stabilization and function of the chamber can be obtained at temperatures between 18ºC (64ºF) and 45ºC (113ºF) in normal room temperature around 20ºC. The stability of the actual temperature is +/- 0.02 K at 25º Celsius.

• How large is the sensor area?
The standard sensor crystal has a diameter of 14mm. Since the sensor crystal is most sensitive in the middle you need to know that the same reaction takes place all over the surface in order to compare measurements.

• How many times can the sensor crystal be reused?
Depending on application and possibility to clean. If only mild cleaning conditions are needed and the application is insensitive to increased surface roughness, a crystal can be used for numerous experiments. However, for certain applications a standard surface can be used only once.

• What sample volumes are needed?
The volume above the sensor crystal is 40 µl with Q-Sense E4 and 80 µl with Q-Sense D300. With E4, the total volume needed from inlet to outlet is about 300 µl. With D300, the volume needed to exchange the content in the temperature loop is approximately 0.8 ml, but for optimal result 2 ml is recommended for each exchange.

• What fluids can I use?
Many different fluids including water, inorganic salt solutions, alcohols, and organic media (even e.g. hexane and toluene) can be used. Except from the titanium wall of the chamber, the fluid is exposed to tubing and o-rings that can easily be changed for different types of measurements. Check chemical compatibility charts for details.