What is the mass sensitivity of QSense instruments?

The maximum mass sensitivity in liquid is about 0.5 ng/cm2 for QSense Pro and Analyzer/Explorer  if measuring at a rate of one data point every five seconds. QSense Pro and Analyzer/Explorer operating with four sensors and at three harmonics has a sensitivity of about 2 ng/cm2 if all data are collected in one second. For example, consider a monolayer (<100% surface coverage) of myoglobin (17.8 kDa): the monolayer corresponds to 177 ng/cm2 (change in frequency, 10 Hz).

What is the frequency range of QSense instruments?

The frequency range is 1-70 MHz with QSense Pro and Analyzer/Explorer. A large range is important to be able to use the unique features of multiple frequency and dissipation sampling.

What are the benefits of taking measurements at several frequencies simultaneously?

Simultaneous measurement at multiple overtones is required to model viscoelastic properties and to calculate the correct thickness of films that do not obey the Sauerbrey relation. With the QSense Analyzer system, 14 incoming parameters (seven frequencies and seven 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 sensor surface decreases with increasing overtone number, abnormal frequency behavior suggests vertical variations in film properties. The fact that the detection range from the sensor surface decreases with increasing frequency is also used by the modeling software to calculate an accurate thickness of films that do not fully couple to the oscillation of the sensor. For rather soft films, with high water content (e.g., films made of large proteins), you will not obtain accurate thickness information without taking measurements at several frequencies. Another advantage of using higher overtones is the decrease in signal to noise ratio, which is beneficial when extra-high sensitivity is desired.

What is the exact detection range into the medium above the sensor?

The detection range depends on the penetration depth of the oscillatory motion of the liquid/film above the sensor and varies from nanometers to micrometers, depending on the viscoelasticity of the applied film and overtone number. In pure water, the detection range is approximately 250 nm for the fundamental mode. Applying a very rigid film, such as a metal, still allows the same detection range in water. This means that the measurement principle is not affected when a thin film is coated on the surface prior to taking measurements. Compared to optical methods, the detection range of QCM-D is an advantage. Consider, for example, polyelectrolyte multilayers several hundred nanometers thick. These are easily sensed by QCM-D.

See here for more details.

What is the maximum thickness of an applied/adsorbed film?

The maximum thickness of a sensor coating depends on the viscoelasticity of the coating and may vary from a couple of hundred nanometers to a few micrometers. The more rigid the layer, the thicker the layer can be. It is always possible to contact Q-Sense to request new surfaces.

What is the minimum sample volume required for the QSense Pro

The volume in the module above each sensor is 15 µL and the minimum volume required (including inlet and outlet channels) is 50 µL for each sample port. 

What is the volume of the measurement chamber and the module as a whole?

The volume on top of the sensor is 40 µL and the total volume inside the module (no tubing included) is 150 µL. This includes the temperature loop going from the inlet to the sensor volume that ensures the liquid that enters the sensor volume has the right temperature.

What is the time resolution?

The maximum rate is up to 200 data points per second, giving you a high-resolution real-time measurement suitable for fast reactions.

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 for the titanium wall of the chamber, the fluid in the Analyzer, Explorer and Initiator is exposed to tubing and O-rings that can easily be changed for different types of measurements (the so called high resistant kit). In QSense Pro, the fluid is also exposed to the materials in the pumps and the sample probe. For dull details on chemical compatibility, check the manual for your specific instrument or contact support. 

Why does the baseline shift when I change buffers?

The resonant frequency and dissipation factors of a QCM-D sensor measuring in a liquid are influenced by the liquid’s density and viscosity. So, if you change buffer from one to another, and the buffers have different viscosities and/or densities, there will be a shift in the f and D baselines. These baseline shifts are often referred to as bulk shifts since they depend on the bulk properties of the liquid.

When, for example, measuring protein adsorption, one can minimize bulk shifts by first obtaining the baseline in the buffer that the proteins are dissolved in. A dilute concentration of, for example, proteins will not significantly change the bulk liquid’s viscosity and density.

If you have a significant bulk shift, you will need to take this into account when analyzing your data. When using QTools, you can compensate for bulk shifts in a measurement if you know how the bulk fluid’s viscosity and/or density change. If you don’t know your test liquids densities and viscosities you can make a QCM-D measurement where you only have a bulk shift and then analyze this with QTools to calculate the change in viscosity or density when going from liquid to another.

What is the temperature range of the QSense Pro?

The working temperature (given normal room temperature) is 4-70 °C with a stability of ± 0.02 K. 

What is the temperature range of QSense Analyzer/Exporer?

Proper temperature stabilization and function of the chamber can be obtained at temperatures between 15 °C and 65 °C when the instrument is at normal room temperature around 20 °C. However, the Q-Sense High Temperature Chamber enables temperature control between 4 °C and 150 °C. The stability of the actual temperature is ± 0.02 K at 25 °C.

How accurate is the temperature reading of QSense instruments?

The temperature measured with Q-Sense instruments (shown in QSoft as Tactual) is guaranteed to be within 0.5 °C from the true temperature. In reality, this means that a temperature reading of 25.00 °C may in fact be any value between 24.50 °C and 25.50 °C. Also, there may be a temperature gradient because the temperature sensor is located in the chamber base. Please note that this information does not concern the temperature stability of our instruments, which is ± 0.02 K.

There are eight sensors and four pumps in the Pro. Can these pumps be controlled individually?

Yes, the four pumps are controlled individually which means you can run four completely different experiments with different flow rates, different times, different point of switching sample and so on simultaneously.

Are the cleaning programs for the Pro specific to different applications?

There are different cleaning programs for different samples such as proteins, acids, basic solutions, biological solutions. Note that this is for cleaning of the instrument and not the sensors. The cleaning recommendation for the sensors will be as previously since the sensors are the same.

In the Pro, can sample degassing be done inline or is it done offline?

Sample degassing will have to be done offline. There is no inline degassing.

How does the sample mixing mechanism work in the Pro?

Sample A and B are taken from different vials and dispensed, one by one, into the sample port (the cup). Once there, they are mixed by aspiration and dispensing by the needle.

In the Pro, is the data saved as the experiment script is running—or is the data saved at the end of the experiment?

QSoftOmega will either use a default data file name or ask you for one before a measurement starts (depending on how you set your preferences). Data from the experiment will be saved to this file every ten minutes. When the measurement is finished, all data will be saved to the file. So, in case of say power failure, you will lose no more than 10 minutes of measured data.

Positive f and negative D values—how can that be possible?

Typically, f curves decrease with adsorption and increase with desorption. In contrast, D curves typically increase with adsorption and decrease with desorption. Some reasons why curves would go in opposite directions are:

  1. The sensor was not clean when the measurement was taken, and some species have been desorbed during the measurement.
  2. The liquid above the sensor at the end of the measurement (or where the curves cross the baseline) has lower viscosity and/or density compared to at the beginning of the measurement.
  3. The sensor ‘settles’ during a measurement (more likely in the D300 than in the Explorer, Analyzer, Initiator systems  or Pro instruments). This might occur if mounting was not optimal, causing the sensor to shift due to an external force, such as someone tilting the instrument. A solution in this case is to gently tap the chamber or flow module prior to starting data acquisition.

When is it correct to use the Sauerbrey relation to calculate the mass adsorbed on the quartz crystal?

The Sauerbrey relation describes the linear relationship between changes in frequency and changes in mass for thin films adsorbing to the sensor surface. It gives a good estimate 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 regions distant from the surface do not couple to the oscillation of the sensor. This means that the Sauerbrey relation, which is normally used 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 is possible to extract the correct thickness estimations even in these cases. This also makes it possible to calculate the viscoelastic and structural properties using a viscoelastic model incorporated in the Q-Sense software, QTools.

What could be the problem if the instrument does not start?

  1. Check that the power cord is connected.
  2. Check that the voltage rating on the back panel of the instrument is properly set for local voltage conditions in the lab. If not, unplug the instrument, open the plastic cover and turn the drum so that the correct voltage appears through the window. Check that the fuse has not blown.

No resonance peaks can be found. What could be the reason?

  1. The settings to drive the sensor may need to be adjusted if the resonance frequency lies outside the search range specified in the settings. If running QSoft 401, uncheck ‘Automatically optimize all resonances’ in the ‘Find resonances’ window and widen the frequency range under ‘Individual resonance settings’, until a resonance peak is visible. If running QSoft 301, increase the value in the ‘Settings’ › ‘Sensor’  › ‘Search range (± kHz)’ cell. 
  2. The sensor resonance may be dampened too much. The dampening can become too high if the medium in which the sensor is immersed is too viscous or has too high density. A film attached to the sensor can also cause heavy dampening, as can certain defects in the sensor. In this case, the resonance peak in the sweep chart is small and can even be hard to distinguish from the background noise. 
    Solutions include:
    1. verify that the sensor is installed correctly and/or is not broken; 
    2. check whether a frequency can be found with another clean sensor or ‘Individual resonance settings’ (QSoft 401) or ‘Settings’ › ‘Acquisition’ (QSoft 301). Narrowing the ‘Frequency range’ (QSoft 401) may make the resonance peak more pronounced.

I see small peaks around the large resonance peak while searching for f and D. Could that disturb my measurement?

When searching for the resonances before starting a measurement, smaller resonance peaks may appear around the true overtone modes (mainly seen in air). These unwanted spikes are the result of small deviations in the sensor design (electrode and quartz dimensions, parallelism between interfaces, etc.) and do not normally affect the measurements because they are much weaker than the signals of interest. The only situation where you need to be concerned with the small peaks is when the unwanted modes show equally high peak values as the true one, as the instrument then will have difficulty determining which one to track. If this is the case, damage to the sensor crystal surface will probably be so severe as to be visible to the naked eye. Unwanted modes are much less stable than the true resonance modes, so it should be obvious from drifts and noise if the instrument is tracking the wrong peak.

The fundamental resonance is much less stable than the other resonances. Why?

It is not unusual for the fundamental frequency to be rather unstable. The reasons could be numerous, but the most important one may be that edge effects are much more pronounced for the fundamental tone than for the overtones, since this is the one that reaches the farthest out to the edge of the sensor. As it is sensing almost the entire sensor surface, the fundamental resonance may also be disturbed by the O-rings.

I obtain very regular fluctuations in f and D values. Why?

If symmetric, periodic noise is seen in both frequency and dissipation in any of the overtones when running QSoft 401 on your Analyzer/Explorer, it is most probably caused by an external source.

When both f and D show periodic behavior, the disturbance is probably not caused by temperature fluctuations, as temperature has a much smaller effect on D than on the frequency.

If f and D fluctuations occur while running a peristaltic pump with your instrument, try turning the pump off. It is of great importance to have a pump that does not send pressure waves through the tubing while it is running. A pump with many rotating rollers will give a more stable signal than one with few rollers.

The peristaltic pump that QSense supplies with our instruments does not induce any pulsation or disturbances when it is turned on or off. However, if the inner diameter (ID) of the Teflon® tubing used for connecting the Analyzer or Explorer flow module(s) is too small, the pressure in the flow system will be too low to allow a stable flow, and pulsation behavior may occur. It is recommended to use tubing with an ID of at least 0.75 mm.

The signal does not return to baseline or becomes unstable when rinsing with liquid. What could be the reason?

  1. There may be bubble formation on the sensor. If bubbles are trapped in the vicinity of the sensor, the measurement signal often does not return to the original baseline after rinsing. Bubbles make the signal unstable. Solution: Avoid the formation of bubbles. This is done by preferably using degassed liquid. Do not insert liquid with a temperature below the working temperature of the measurement chamber, as dissolved gas can be released during heating.
  2. Contaminants in the sample liquid can induce instability. Check the quality of the samples.
  3. The sensor may be experiencing a ‘pressure shock’. Irreproducible jumps in f and D can occur if the sensor is mechanically stressed by increasing the pressure on one side of the sensor when fluid is forced through the measurement chamber. Solution: Try to use lower pump flow rates.
  4. The sensor may be experiencing a ‘temperature shock’. Irreproducible jumps in f and D can occur if the sensor is thermally shocked, i.e., rapidly changing the temperature of the sensor. Solution: Make sure to keep sample solutions at roughly the same temperature as the Set temperature. Also, keep in mind that the larger the temperature difference between the sample and the set temperature, the slower flow speed needs to be used in order for the sample temperature to reach equilibration before it reaches the sensor. 

The measurement shows excessive noise. What could be the reason?

  1. Check that the sensor is mounted correctly with the arrow-shaped electrode towards the mark on the sensor holder (to the left). See sub Chapter ‘Mounting the Sensor Crystal’ in the Operator Manual. If the sensor is mounted in the wrong direction, the grounding of the electrodes will not be optimal, and current leakage may interfere with the measurement.
  2. If the sensor has scratches or damage on its surface, this may give rise to unwanted disturbing resonance peaks, seen adjacent to the true resonance peak in the frequency sweep window. If these unwanted modes are of the same amplitude as the true one, the instrument may jump over and follow them instead. If a false or non-pure resonance peak is followed, it could result in high noise.
  3. A sensor loaded with an inhomogeneous coating or a coating with high surface roughness could also give rise to non-pure resonance peaks and thus higher noise.
  4. Check for bubbles, or if measuring in air, residual liquid on the sensor or in the flow module/chamber.

Why does the frequency shift when I expose the surface to (UV) light?

It is very difficult to avoid temperature effects when working with (UV) light exposure. Even though the quartz itself is transparent in the UV-spectrum, the sensor electrodes are generally not. All the light an electrode will absorb is converted into heat. As a consequence, this will in many cases induce stresses in the electrode (generally, a material expands when heated) that will be transmitted to the underlying quartz. The resonance frequency will change as a consequence. The dissipation factor is usually not affected by these stresses. However, if you turn on the UV light in water you could see a shift in dissipation due to the heating of the water and hence a change in the water’s viscosity and density.

I find liquid outside the sensor area after opening the flow module/chamber. What could be the reason?

Liquid on the backside (the ‘anchor side’) of the sensor can cause a lot of problems for QCM-D measurements. If the backside of the sensor is facing any droplets of liquid, slow humidity stabilization processes will affect the f and D baselines, and destroy any attempt to make quantitative measurements. For prevention:

  1. Check that the sensor does not have any cracks or notches at the edges. Such damage may reduce the sealing capability of the O-ring.
  2. Check that the sensor is centered over the O-ring.
  3. For Analyzer/Explorer, check that the tubing connections are tight. Is there any liquid outside the black nut? It is very important that the Teflon® tubing is cut off at exactly 90° to ensure proper sealing. It is recommended to use the tubing cutter included in the Liquid Handling Set.

What is the angle of incidence for the ellipsometry module?

The angle of incidence for the ellipsometry module is 65°, and this equals the angle of reflection.

What setup is required for the ellipsometry module?

Note that the ellipsometry module can only be used with QSense Explorer chamber platform, as you need optical access from both ends of the module and the design of the Analyzer chamber platform does not allow for this. QSense does not recommend any particular ellipsometer but there are two suppliers that we know are compatible with our ellipsometry module and have a table especially designed to fit the Explorer chamber: Woollam and Accurion.

What is the maximum working temperature for the electrochemistry module?

The electrochemical module can withstand a maximum of 40 °C, which is limited by the reference electrode.

Which external factors might affect the resonance frequency and dissipation shifts?

The crystal may react to other things beside surface interactions:

  1. Temperature changes/stresses.
  2. Pressure changes/mounting stresses.
  3. Pressure waves.

What is the highest viscosity that can be measured with QCM-D?

We have measured an upper viscosity limit to 47 cP for a liquid of density 1204 kg/m3. The response will be also affected by density. In addition a maximum damping of ΔD ~2500E-6 has been measured. Higher damping hinders the oscillation of the sensor.

What microscope objectives can be used with the window module?

Two basic requirements have to be fulfilled:

  1. There must be enough room on the bench of the microscope to fit the chamber there.
  2. The focal distance of the objective lens has to be long enough to reach the sensor surface.

Our window module has a glass slide thickness of 1 mm and the distance between glass and sensor surface is another 1 mm, which gives a total distance of 2 mm down to the sensor surface.

What is the difference between measuring resistance and energy dissipation?

In order to completely describe the viscoelastic properties of a film adhered to a QCM sensor, it is necessary to measure all sources of energy losses or damping for the oscillation. Energy dissipation, D measurement as with QCM-D is the most direct way, but impedance measurements also obtain the full information on viscoelastic properties of the film. Resistance, R is only part of the impedance and thus cannot be directly correlated to viscoelastic properties of the film. To obtain complete information, capacitance measurement is also necessary, and it is possible to have a situation where resistance is not changed but where capacitance does change, or vice versa and thus changes in viscoelastic properties are missed or at least proportionally wrong. Another consequence of only measuring resistance is that it is not possible to quantify and also relative comparisons between measurements of R is of limited value.

Where is the temperature measured in the Analyzer instrument?

The temperature is measured immediately below the flow modules, in the center of the contact block on the chamber platform. Firm contact between the modules and the platform allows for good thermal conduction, so as soon as all curves (temp, f, and D) are stable, you know that the temperature has equilibrated to Tset over the sensor surface.

What is the maximum rate for temperature ramping on the E4/E1?

The typical maximum rate for the temperature programming in QSoft 401 is 0.5 K/minute, if the temperature control has enough time to adjust. It depends on how many flow modules are used and at which temperatures you are working. Quicker ramps will give larger over- or undershoots, so it is up to the user to determine if this is critical or not.

What is the Reynolds numbers for the flow module in the E4/E1?

The geometry of the flow module is complex to describe, with its beveled ceiling (the surface across the sensor surface); therefore, the Reynolds numbers are estimated. Assuming that the inner volume is a rectangular ‘pipe’ of 0.7×12 mm, the Reynolds number at 100 µL/min is 0.2, and at 800 µL/min, 2.0. Generally, a Reynolds number below 2300 is associated with a laminar flow, which means the flow through the E-Series Flow Module is well within the boundaries.

What is the power consumption of the E4 instrument?

When 240 V/50 Hz is used, between 105 VA and 123 VA have been measured. When the temperature reaches the set value, the consumption decreases somewhat.

What are the inner dimensions of the E4/E1 flow module?

The inner diameter of the O-ring is 11.1 mm; the distance between the sensor and the ceiling of the cell is 0.6 mm in the centre; the heat exchanger has a cross-section of 0.8×1.0 mm; the holes in the cell ceiling where the liquid enters to the sensor are 1.0 mm in diameter; the tubing is 0.75 mm in diameter.

There is no connection with the instrument (E4/E1). What could be the problem?

  1. Check that the USB cable is plugged into the PC and the instrument.
  2. Check if the hardware ‘QSense E-Series Device’ is found and installed on your computer. If not, open the installation file found in the C: directory under Program Files/QSense/QSoft 401. Follow the instructions (see also sub Chapter ‘Installing Software’). The software should be installed before the instrument is started for the first time.

What is the drive amplitude/voltage?

The drive amplitude, as used in QSoftPro and QSoft401, relates to the amplitude of the drive voltage over the sensor.

You can set it under ‘Individual resonance settings’, the settings tab that appears if the tickbox ‘Automatically optimize all resonances’ is unchecked. This value is proportional to the output voltage from the electronics unit. However, the actual voltage over the sensor depends on how loaded (dampened) is since there are capacitive losses in the transmission lines to the senor. So it is not possible to give an exact relationship between the value set in QSoft and the voltage over the sensor. Around 50 mV in air and 1 V in water are typical values over the sensor. 

Can you calibrate the temperature in the E4 instrument?

Yes. In QSoft, before you start a measurement, go to the ‘Tools’ menu, then select ‘Show Pre-Acquisition Form’. Select the desired ‘Set Temp’ and allow the ‘Tact’ to stabilize for at least 2 minutes.

Thereafter, under ‘Tools’, select ‘Calibrate Set vs Actual Temp’, and click ‘Apply’. The ‘Set temp’ line should adjust to the ‘Actual temp’. Normally this is done at 25 °C, but it can be done anywhere between 15 °C up to 50 °C.

The resonance frequency is lost, resulting in the error message ‘Warning: bad fitting of the decay curve’. Why is this?

  1. The environment of the sensor may be changing very fast, so that the changes in frequency and dissipation cannot be tracked effectively. Solution: Increase the data acquisition speed. Optimal speed is obtained by changing the settings under ‘Resonance optimization’ in the ‘Find resonances’ window. Move the pin towards ‘High speed’.
  2. The sensor has been loaded more and more during data acquisition and at some point the sensor cannot be loaded further as the dampening becomes too high. Solution: In some cases, the sensor can still be forced to resonate by optimizing the settings for acquisition. Check the effect of changing ‘Drive amplitude’ under ‘Individual resonance settings’.