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QCM-Z500 FAQ 1 / 1

October 2004

KSV Instruments Ltd Tel +358 9 54973300 VAT: FI07865293Hylmtie 7 Fax +358 95497 3333 Trade reg no: 459.442

FIN-00380 Helsinki e-mail: info@ksvltd.fi VAT RegFINLAND www.ksvltd.com HELSINKI

KSV QCM-Z500 FREQUENTLY ASKED QUESTIONS

Q. Can QCM-Z500 measure dissipation or monitor dissipation changes?

A.YES ! The QCM-Z500 instrument can measure dissipation and also monitor dissipation changes.

The measurement principle of QCM-Z500 is based on impedance (Z) analysis of the quartz crystalproperties, which enables the determination of the Quality factor of resonance of the crystal, i.e. theQ-factor. The Q-factor is a characteristic property of quartz crystals, and actually dissipation isdefined as D = 1/Q.

The Q-factor contains all the information about how well the quartz crystal is resonating in the

surrounding media. This means that the influence of all kind of adsorbed layers on the quartz crystalor liquids in contact with the quartz crystal surface are reflected in the Q-factor, especially if thesurrounding media damps the oscillation of the quartz crystal i.e. energy is dissipated into thesurrounding media or adsorbed layer. An example of how the Q-factor of a quartz crystal is affectedby the surrounding media is shown in the figure below.

Admittance (A) spectra of a quartz crystal in contact with different surrounding media.

Admittance = 1/Impedance.

The sharper and higher the admittance peak the higher the Q-factor and less energy dissipates intothe surrounding media. Dissipation is reflected in the admittance peak as a broadening of the peakand lowering of the peak height.

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Q. What is the difference between KSV QCM-Z500 and other QCM instruments on the market?

A. The QCM-Z500 (Quartz Crystal Microbalance with Impedance (Z) Analysis) extracts the frequency

and Q-factor (dissipation) of a quartz crystal by making use of impedance analysis. To our knowledgethis is one of the few if not even the only commercial QCM instrument on the market making use ofimpedance analysis. In a simplified description one could say that by using impedance analysis it ispossible to extract a series of electrical components of the quartz crystal that reflects the acousticproperties of the quartz crystal and its surrounding media. These electrical components can then beused for calculating the frequency and Q-factor (dissipation), even at several overtone frequencies.

Most of the other QCM instruments on the market are based on connecting the quartz crystal to asimple electronic oscillator circuit to extract the frequency of the quartz crystal. However, thedisadvantages of using an electronic oscillator are a) you only measure one frequency and are notcapable of measuring overtone frequencies or Q-factors (dissipation) as can be done by the QCM-Z500 instrument, b) the frequency shift of an oscillator circuit always contains some admixture fromthe energy supplying elements which can interfere with the true frequency shift due to a massincrease*, and c) the measured change in frequency depends on the phase shift introduced by theelectronic oscillator, which is in most cases unknown.*

The use of impedance analysis enables unique features of the QCM-Z500 instrument:

- Avoids the problems induced by using an electronic oscillator mentioned above asimpedance analysis enables the measurements of impedance magnitude and phase of theQCM.*

- It enables the measurement of frequency and Q-factors not only for the fundamentalresonance frequency of the crystal but also for a series of overtone frequencies. For

example, for a 5 MHz crystal the QCM-Z500 can measure the frequencies and Q-factors upto the 11

thovertone frequency i.e. 15, 25, 35, 45, 55 MHz. By measuring overtones it is

possible to determine the validity of the Sauerbrey equation, as well as it gives an indicationof the softness of the adsorbed layer.

- It enables fast jumps between different overtones and therefore also quick measurementsi.e. 1 data point/second. The data collection rate can be increased up to 5 datapoints/second if you allow for a lower signal/noise ratio. However, to our experience so far afaster acquisition time is only needed in 1 case of 100, if even that often.

- The QCM-Z500 can be used as a conventional network impedance analyzer i.e. it is notrestricted to only Quartz Crystal Microbalance application. For example, it can be utilized forimpedance spectroscopy used a lot in corrosion science, and to determine the characteristic

impedance of other kind of solid substrates used in coating industries.

Furthermore, impedance analysis is a very well established technique and has been used for severaldecades for characterizing fundamental quartz crystal properties. Hence, it is very well known andthe theories and treatment of the obtained data are well accepted.

* Rodahl, M.; Kasemo, B. Rev. Sci. Instrum.1996, 67, 3238-3241

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Q. What exactly is Q-factor?

A. The Q-factor reflects how purely elastic the oscillation of the crystal is. If there is something thatresists this purely elastic oscillation for example a liquid in contact with the crystal it will damp thecrystal and the crystal oscillation will loose some energy i.e. dissipation occurs. Hence, damping isthen the sum of all energy losses in the system per oscillation cycle. A soft film adsorbed to thequartz crystal is deformed during the oscillation, which gives a low Q-factor (high dissipation) while arigid film gives a high Q-factor (low dissipation).

Q. Why measure Q-factors (dissipation) and overtones?

A. There are several reasons for this, and they all contribute to the fact that by combining themeasurement of frequency and Q-factors obtained at different overtones one can obtain valuableinformation about the properties of the adsorbed layer that cannot be extracted by only measuringthe frequency.

The measurement of the Q-factor (dissipation) is important as the frequency signal reflects the totalmass of the adsorbed layer on the quartz crystal, while the Q-factor reflects the softness of the filmformed on the surface. For example, the amount of the surrounding liquid in the film may varybetween 10-90% depending on the type of molecule and the way the molecules adsorbs on thequartz crystal surface. In this kind of situations a soft film (liquid rich) and a rigid film (less liquid) maygive close to the same frequency change, while they induces completely differences changes in theQ-factor. The measurement of Q-factor (dissipation) also can provide information about structuraldifferences between different adsorbed systems, or structural changes in the same film during the

actual adsorption process because it is reflects the shear viscous losses induced by the adsorbedfilms.

The measurement at several overtones is very useful in such cases where the adsorbed film is sosoft that the upper region far away from the quartz crystal surface, do not couple to the oscillation ofthe quartz crystal sensor. If one would try to use the normal Sauerbrey relation to calculate the massof the adsorbed film directly from the frequency change one would underestimate the mass.However, by measuring both dissipation and frequency at several harmonics it becomes possible toestimate visco-elastic properties and even film thickness and film density in the case of soft films.Furthermore, the different overtones give information about the homogeneity of the adsorbed films,because the detection range out from the quartz crystal surface decreases with increasing overtonenumber. Hence, often an abnormal frequency behavior is the reason for variations in the film

properties in the thickness direction. The fact that the detection range decreases with increasingovertone number can also be used to estimate the thickness of films that do not fully couple to theoscillation of the crystal.

Another advantage of using higher overtones is the increased sensitivity and signal-to-noise ratio athigher overtones, which is good when better sensitivity is required.

There is a vast amount of scientific publications available dealing with the modeling of data obtainedfrom impedance analysis of quartz crystals coated with soft films. These articles are readily availablefor everybody that is interested in using the data from impedance analysis for modeling of the visco-elastic properties of soft films adsorbed to quartz crystals.

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Q. How can I make a stable and good QCM measurement in a liquid environment?

A. The most important things we have found out for making a stable measurement is the type of

crystal used and a stress-free mounting of the quartz crystal to the measuring chamber. If these arenot optimized there may be strange jumps in the frequency. In QCM-Z500 this is obtained by usingoptically polished quartz crystals with a little bit thicker edges combined with a good design of gentlemounting of the crystal into the measuring chamber.

Secondly, it is important to control the temperature of the measuring chamber in order to avoidinfluences from changes in the bulk liquid viscosity and the quartz crystal frequency as thetemperature is fluctuating. The QCM-Z500 can be equipped with a temperature control unit forcontrolling the temperature between 15 and 60

oC with a resolution of 0.1

oC.

Furthermore, sophisticated electronics is needed to make a good impedance analysis of the quartz

crystal, and especially building an impedance analyzer for the QCM application. This has beenenabled by the well over 20-years of experience that KSV has as a company of designing electronicsfor sophisticated measurement instruments, complete developments and manufacturing of laboratoryinstruments, and of course a careful choice of the components for the impedance analyzer.

Q. How is the temperature of the measurement controlled?

A. The temperature of the bypass and the actual compartment holding the quartz crystal is controlled

by a temperature controller connected to a peltier element that on the other hand is attached to alarge aluminum block that holds both the bypass and the compartment with the quartz crystal. Thebypass is used for exchange and pre-temperaturize the sample liquid before it is flushed into thecompartment holding the quartz crystal. The temperature can be controlled between 15 and 60

oC

with a resolution of 0.1oC.

Q. What kind of crystals in terms of manufacturer, size and frequency can be used?

A. There are a range of manufacturers of quartz crystals out on the market and the standard

measuring chamber for the QCM-Z500 hold quartz crystals that are max 14 mm in diameter (min. 13mm), has the connection electrodes on one side of the crystal, and the frequency is in the range of 3-55 MHz. KSV of course provides standard 5 MHz crystals for the instrument, but if you find any othersupplier that can supply you similar crystals and you would like to try them out, then be free to do so!

Furthermore, we have a wide experience of customizing instruments, so if there is a need for using aspecial size quartz crystal, then please contact us for more discussions.

Note also that the large frequency range of the QCM-Z500 instrument is an important feature, whichenables the determination of several overtones for crystals in the frequency range 3-10 MHz.

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Q. What kind of coating or surfaces can be used on the quartz crystals?

A. A quartz crystal can be coated with almost any material as long as it can be deposited firmly as athin homogenous film to the quartz crystal surface. The layer thickness normally used is in the range50-100 nm, but can vary depending on the properties of the applied material. KSV has asubcontractor that can coat the quartz crystals with the following materials:

Metals: Ag, Al, Ca, Cr, Cu, Li, Mg, Ni, Sn, PtInorganics: ITO, LiF, MgF2, SiO, SiO2, TiO2, ZnS, Ti3NOPolymers:PANi, Teflon, PMMA, PS, PC, PE, PP, AKD

Q. How many times can the quartz crystal be re-used?

A. The number of times the quartz crystal can be re-used is totally dependent on the system you

study. We have experience of re-using the same quartz crystal up to 10 times in a vesicle adsorptionstudy, but in that case a very good and gentle cleaning procedure was available. For re-use purposescareful handling and cleaning procedures that do not harm the crystal or its coating is required.However, for other situations like covalently bound self-assembled monolayers or polyelectrolyteadsorption studies it can be difficult to find proper cleaning procedures and the quartz crystals usedtend to be disposable.

Q. What amount of sample liquid volumes is needed for the measurements?

A. The standard flow through measuring chamber contains a bypass that takes 2.5 ml of liquid, while

the actual compartment holding the quartz crystal takes 1 ml of liquid measured from the 3-valvepoint that separates the bypass and the inlet to the compartment. So, in principle if working verycarefully 2-2.5 ml is enough for each exchange, but 4-5 ml is recommended for optimal results.

Additionally, the QCM-Z500 instrument can be equipped with an open air/open beaker measurementprobe that can be immersed in any open beaker for measurements. This measurement probe mightrequire large amounts of sample liquid, but it has the advantage that it is not closed into any

compartment and the quartz crystal surface can easily be viewed or illuminated with light for examplethrough the bottom of a glass/quartz beaker.

Q. What kind of fluids can I use for my measurements ?

A. The tubes and O-rings used in the standard measuring chamber for the QCM-Z500 instrument is

made ofViton. Many different kind of fluids can be used with these tubes and O-rings ranging fromwater, water based inorganic salt solutions and buffers, most alcohols and organic solvents.

For example, the manufacturer for Viton tubes and O-rings claims that at least the following differentfluids and organic solvents can be used with Viton; water, inorganic salt solutions, Butanol, Carbon

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tetrachloride, Chloroform, Cyclohexane, Ethanol, Hydrochloric acid (dilute), Hydrogen peroxide(dilute), Methyl chloride, Nitric acid (dilute and medium), Toluene, Trichloroethylene, Xylene, just to

mention a few of the most fluids used in the laboratory.

Q. How thick layers can I use with the QCM-Z500 instrument?

A. The maximum thickness of the layer that can be deposited on the quartz crystal depends on the

visco-elastic and density properties of the material deposited on the quartz surface. As a thumb ofrule one can say that the more rigid the layer that is deposited on the quartz is, the larger thickness ispossible. One can estimate the maximum thickness of the layer from the approximate penetrationlength of the shear waves induced by the oscillatory movement of the quartz crystal i.e.

= (/ f) , where = shear viscosity of the layer, = density of the layer,

and f = resonance frequency

For example for water ( = 1 Pa s, and = 1 g/ml) and a 5 MHz crystal one obtains as thepenetration depth 250 nm, and for a 10 MHz crystal the penetration depth reduces to about 178 nm.

However, by applying a very rigid film on the quartz surface such as a metal will still give the samedetection range in a liquid i.e. a pre-coated metal or other high density solid do not affect the

detection range as long as the rigid film have been coated on the quartz surface prior to themeasurements.

Q. What is the smallest mass change I can detect with the QCM-Z500?

A. First of all this depends on the frequency of the quartz crystal used. The higher the frequency the

smaller changes can be detected. The mass sensitivity of a standard 5 MHz (optional 10 MHz)crystal used in the QCM-Z500 instrument is 17.5 ng/cm

2(4.24 ng/cm

2) per 1 Hz change in the

frequency. This means that as the QCM-Z500 instrument can detect frequency changes down to

0.03 Hz in air the mass sensitivity become around 0.5 ng/cm2 (0.15 ng/cm2) for a 5 MHz (10 MHz)crystal. In the case of liquid measurements the QCM-Z500 can detect changes down to 0.2 Hzmeaning that the mass sensitivity become around 4 ng/cm2 (1 ng/cm2). Furthermore, the sensitivityincreases with the overtone number, so if higher sensitivity is needed one can always use theovertone measurements in the calculations.

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Q. What is the noise level of the measurement data and how much can it drift on a long termscale?

A. The noise level of the measured data is dependent on the mounting of the quartz crystal into the

measuring chamber, the electronics and the software algorithms used for running the instrument.Due to KSV:s proprietary electronic design, innovative measuring chamber design and softwarealgorithms the noise level of the QCM-Z500 instrument is superior compared to other QCMinstruments on the market. This is illustrated by the measurement below. The frequency of the crystalmonitored in air is shown in figure A and the crystal monitored in a water solution is shown in figureB. Note, the small scales in the images.

Figure A. Figure B.

The RMS values of the noise level for air and the water solution are 0.005 Hz and 0.08 Hz,

respectively.

The drift of the measurement data is largely dependent on if the measurement cell is temperaturecontrolled or not. The data in figures A and B above have been measured with the QCM-Z500 in airand water, respectively, by using the temperature control unit controlling the temperature with aresolution of 0.1

oC. The drift of the signal in air is less than 1 Hz/hour and hardly anything in liquid as

can be seen from the figures above. Generally the signal measured with the QCM-Z500 drifts lessthan 1 Hz/hour in both air and liquid if the temperature control unit is used to control the temperatureof the measuring chamber.