Why are only odd harmonics being measured?

  1. This has to do with wave physics in that a QCM cannot resonate at the 2nd, 4th and 6th (etc.) multiples of a half wavelength that defines the fundamental tone and that depend on the thickness of the crystal. In theory, there are also resonances at these even multiples of the eigen frequency (the fundamental tone) but they can only be excited mechanically, and would only have an amplitude inside of the quartz, and therefore meaningless as we want to study what is happening on the surface of the quartz. This is why it would be wrong to say that we use the 1st, 2nd, and 3rd resonances referring to 5, 15, and 25 MHz, and that is why we refer to the 3rd, 5th, and 7th overtones (or harmonics) of the crystal. 
  2. It is a question of physics—the even overtone resonances have a node on both surfaces, and for the QCM to be able to excite oscillation/standing wave at resonance it requires a node on one surface and an anti-node on the other. A QCM can only oscillate at half-wavelengths: the fundamental tone has a wavelength twice that of the thickness of the crystal, thus a node on one surface and an anti-node on the other. The third overtone has a wavelength a third of the fundamental tone, thus having a node on one surface, a node two-thirds of the way through the crystal and an anti-node on the other surface. The fifth overtone has a wavelength one fifth of the fundamental tone, thus a node on one surface and two nodes inside the crystal (at 2/5 and 4/5 of the thickness of the crystal) and an antinode on the other surface.

What is ‘buffer effect’? 

 

Since the technique is sensitive to any change in the medium above the sensor surface, a shift between two buffers will affect the measurement. The easiest way to identify them is that they occur instantaneously, in contrast to most adsorption processes that take place more slowly. A ‘buffer step’ is recognized as instantaneous, reversible and surface independent.

The best way to avoid buffer steps is to keep the concentration of the analytes down and, of course, to use the same buffer as much as possible throughout the experiment. Also, if you can do a final rinse with the ‘baseline buffer’ after a completed measurement you will be certain that no buffer effect will disturb the interpretation. Then any jump in f and D due to buffer changes will have returned to their original levels.

Factors such as salt concentration and pH will immediately change the baselines. The effect also occurs if very high concentrations of analytes (e.g. proteins) are used alternately with low ones. 

Buffer effects may occur together with surface-related (wanted) effects. It is therefore recommended to always return to the original buffer after finished analyte/sample experiment steps to read off the true signals. Alternatively, a reference measurement with buffers alone can be made first, and the values subtracted afterwards from the analyte results (changes in f and D due to surface related effects and bulk effects are usually additive).


My baselines are drifting—why?

A clean system normally shows stability, with as little as 1 Hz variation per hour on an E4 system and less than 0.5 Hz per hour on a D300 system. If drifts occur, there can be several possible causes. See below for details.

There can be many different causes for drifts in f and D. Following here is a list of possible causes, and suggestions for solutions.

Problem

Description

Solution

Leaks

If a tube is leaking, or the sensor is not properly mounted (or even cracked) this can cause liquid to enter spaces in the measurement chamber where it should not be. For example if the back side of the sensor becomes exposed to only small amounts of liquids or vapors large changes in f and D may occur. Also, if liquids enter the electrical parts of the measurement chamber large variations in the measured signal may result. Eliminate leaks. Check for leaks by mounting a sensor in a dry measurement module. Close the outlet tube by clamping it. Then fill a syringe with air and connect it to an inlet tube. Press gently for about 30 seconds and then release. You have a leak if the syringe does not return to the initial state. 

Bubbles

Gas bubbles may form on the surface of the sensor if you use a liquid that is not properly degassed. Such bubbles will of course influence f and D. For example, the gas solubility of water decreases when the temperature is increased. If water with a lower temperature than the measurement chamber is injected then there is a large risk for formation of bubbles. 
Note that the risk for bubble formation generally increases with decreasing salt concentration of a water solution.
Use only degassed liquids or make sure that the gas solubility of the liquid is not lowered during the measurement. 
Bubbles can often be removed by using high flow speed of the pump.

Temperature changes

The measurement chamber is temperature stabilized but large variations in the environment may not be fully compensated. Temperature changes will change the viscosity and density of a liquid and thereby change f and D
Large temperature changes of the electronics unit will also change the frequency of the reference clock. This will directly change the measured frequency (but not dissipation). 

 

At certain temperatures the resonant modes may coincide with unwanted modes that are very temperature dependent. If measuring at such a point, the signal may be affected and drifts occur.

Make sure the temperature controller is turned on! 
Keep a constant environment around the measurement chamber and the electronics unit. Make sure air circulation is adequate and constant around the measurement chamber and electronics unit. 
Avoid direct sunshine and air-streams (e.g., from an air-conditioner) to be pointed directly at the instrument. 

 

Change the temperature slightly to ‘come loose’ from the mode intersection.

Surface reactions

The QCM-D is designed to measure surface reactions. However, sometimes there are reactions going on that the user does not anticipate. For example, on a bare gold sensor in contact with water there may be a slow change of the ion content in the Helmholtz double layer. There might also be a slow transfer of contaminants from the module walls to the sensor surface. Or there might be a slow desorption, degradation, or restructuring of the surface layer on the sensor. It is also not uncommon that, e.g., a sensor with a polymer coating absorbs or desorbs solvents or even water that will change the measured mass. 
These kinds of ‘drifts’ often induce relatively larger shifts in f than in D (just like most measurements of very thin films do). The frequency ‘drifts’ then follow the mass sensitivities of the overtones, i.e., the shifts in frequency goes as 1:3:5:7 for the fundamental and the 3rd, 5th, and 7th overtones.
This is not really a drift since it is a result of surface processes, which the QCM-D is designed to measure. You can check if some kind of surface process is the cause of the ‘drift’ by passivating the sensor surface. We have found that passivation of the sensor by lipid layers, proteins, or thiols have significantly reduced these kinds of ‘drifts’. 
It is also important to thoroughly clean all parts of the module regularly. It may be necessary to exchange all tubing and the O-ring for new ones.

Pressure changes

(D300) If you have evaporation from the end of the outlet tube and the tube is hanging down, then the pressure on the crystal will change as the liquid level changes. You can easily test how much this pressure changes f and D by starting a measurement in liquid and then slowly move the end of the outlet tube up and down. 
The same thing can happen if the valve into the sensor is slowly leaking. Then the pressure (and possibly the temperature) can slowly change causing a drift. 
Keep the outlet tube firmly in place and not allowed to move around. Ideally, the last stretch of the outlet tube is horizontal. Then a small evaporation from the end of the tube will not change the water level and hence not the pressure felt by the sensor. It is also possible to put a valve at the end of the tube which stops evaporation altogether. 

Mounting stresses

Most physical stresses on the sensor influence all resonant frequencies and dissipation factors even if the effects are usually more pronounced in the fundamental mode than the overtones. Mounting the sensor in a module will inevitably induce sensor stresses since the O-ring, Teflon® ring (D300) and the spring-loaded gold contacts all exert a force on the sensor. If these forces are held absolutely constant during the run of the measurement, they will usually not cause a problem. 
Any changes in these stresses will, however, more or less, induce changes in f and D. For example, if the sensor is unevenly placed on the O-ring there might be a slow creep in the O-ring that will change the mounting stresses. 
Also, changes in temperature will change the diameter and elastic properties of the O-ring as well as the dimensions of the measurement module by tiny amounts. This may be enough to significantly chance the mounting stresses.
Be careful when mounting the sensor to make sure it is placed evenly and centered on the O-ring. Make sure the O-ring sensor, and Teflon ring (D300) are clean and free from dust particles. Stresses can sometimes be released by knocking hard with one finger on the measurement chamber (D300).

Backside reactions

The backside of the sensor is as sensitive to surface reactions as the front side. If, for example, there is a large change in humidity on the backside of the sensor due to leaks or a large change in temperature, the amount of adsorbed water will change and thereby influence f and D. Note that the air at the backside of the sensor is in contact with the ambient through a small whole. The reason for this small whole is to prevent pressure changes (see Pressure changes above) when the temperature of the measurement chamber is changed. Make sure the dew point of the air around the measurement chamber (which is in contact with the backside of the sensor) is significantly higher than the measurement temperature. See also Leaks above.

O-ring swelling

Upon changing from working in one solvent to another the O-ring might swell/de-swell due to the changed properties of the liquid (see also Mounting stresses above). Use different O-rings for different solvents, or pre-soak for long time when changing from one liquid to another.

Bad electrical contact

The measured dissipation factor will increase if there is a bad electrical contact (high electrical resistance) between the sensor and the  gold contacts. This will increase noise and possibly also drifts. Indicative of this problem is that a clean sensor has a high dissipation factor (>40·10-6) when measured in air. Make sure the sensor is properly mounted so that the  gold contacts will make good contact with the sensor electrodes. Look in the manual if you are unsure how the sensor should be mounted. 
It is important that the backside electrodes are clean. This can be a problem if the front side of the sensor has been spin coated with a non-conducting layer, which easily can spill over the edge of the sensor. 
Make sure the spring-loaded gold contacts are undamaged.