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).
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.
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.
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.
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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.
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.
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.
The maximum rate is up to 200 data points per second, giving you a high-resolution real-time measurement suitable for fast reactions.
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.
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.
The working temperature (given normal room temperature) is 4-70 °C with a stability of ± 0.02 K.
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.
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.
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.
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.
Sample degassing will have to be done offline. There is no inline degassing.
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.
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.
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:
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.
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.
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.
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.
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.
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:
The angle of incidence for the ellipsometry module is 65°, and this equals the angle of reflection.
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.
The electrochemical module can withstand a maximum of 40 °C, which is limited by the reference electrode.
The crystal may react to other things beside surface interactions:
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.
Two basic requirements have to be fulfilled:
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.
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.
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.
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.
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.
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.
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.
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.
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.