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Quartz Crystal MicrobalancesSome new innovations stand alongside

the standard, reliable workhorse.Judith Handley

What do NASAs Mars Rover, hu-

man serum albumin antibodies,and oily wastewater have in common?The answer isthe quartz crystal mi-crobalance (QCM).

Some of these instruments have notchanged in the five years sinceAnalyti-cal Chemistrylast reviewed them (Anal.Chem. 1996, 68, 625 A628 A), butthey are still the instrument of choicefor many applications. If one thinks ofthe QCM as a mass or thickness deviceonly, then there are many competingtechnologies, says K. Keiji Kanazawaof Stanford University. But, he says,the versatility of the QCM, with itsability to be used in liquid environ-ments as well as [gas or vacuum and]the current ability to assess the qualityfactor of the resonance, provides infor-mation not available using these othermethods.

Analytical Chemistrys earlier reviewcovered the main principles behind theQCM and the characteristics of quartzcrystals. In general, the standard QCM

measures the mass of a material deposit-ed on a quartz crystal surface as a linearfunction of a change in the oscillatingcrystals resonant frequency. The fre-quency is affected by the environmentat the crystals surface, the mass andcharacteristics of the coating, and theproperties of the solution near the elec-trode surface. These factors includeviscosity, density, concentration, andcharge density. QCMs can measuremasses ranging from micrograms tofractions of a nanogram, the mass of a

layer or even a partial layer of atoms.

A basic QCM includes a source ofalternating current (the oscillator), aquartz crystal, two metal electrodeson opposite sides of the thin crystal

wafer, and a frequency counter. Otherelectronic components control processconditions and data manipulation.

Table 1 lists representative QCM

components and ancillary equipment.

The reader is encouraged to contact themanufacturers for further information.

QCM considerationsThere are many choices of QCM com-ponents. Some systems are limited tomanual control, while others have dif-ferent levels of electronic modules, soft-

ware, or interfaces with PCs. Regardless

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of the size of the company or the extentof its product line, choosing a QCM isa matter of finding the right match forthe analytical objective and sample con-ditions, say the experts.

Depending on the company, QCM isa general term loosely applied to differ-ent components: the sensor; the powersource for generating the oscillation fre-quency; or a set of components, suchas the crystal, electrodes, and electronicsthat convert frequency change into mass.

Some companies use variations of the

term for different applications: cryo-genic (CQCM) for an instrument thatfunctions below the boiling point ofliquid nitrogen or EQCM for a deviceused in electrochemical studies. An elec-trochemical nanobalance is an EQCN.

With thermal control, often by a heat-ing/cooling Peltier heat exchanger, theterm is TQCM.

QCM monitors usually display fre-quency or rate and thickness. Controllerfunctions vary with the instrument. They

handle one or more sensors, each with

single or multiple crystals, and adjustone or more power sources to maintaina constant rate and uniformity of surfacecoating. Some controllers close shutterson sensors to stop processes at predeter-mined levels and also extend the crystalslife. If a crystal fails, the controller clos-es the shutter of the failed crystal andautomatically switches to a new crystal.Some controllers keep track of a singlefilm layer, whereas others track hun-dreds of layers. Other controller func-

tions are graphing data and controlling

Table 1. Representative QCM systems and ancillary equipment.

Manufacturer

Example QCMsystem(s)

ResolutionMass

Frequency (Hz)

Applications

Accessories

CrystalsResonancefrequency (MHz)Surface shapeCrystal diameter(mm)Electrodematerial(s)Surface finish(es)

Special features

ElchemaP.O. Box 5067Potsdam, NY 13676

315-268-1605www.elchema.net

EQCN-700

EQCN-900 quartz crystal immit-tance measurement system

0.01 ng

0.01

Adsorption, electroplating, cor-rosion, biosensors, anti-genantibody interactions, DNAstudies, ion ingress, polymerstudies, viscoelastic materials,intercalation

Rotacell RTC-100 cell system, 3.5-to 100-mL EQCN cells, flow anddemountable cells, DAQ-616SCreal-time data acquisition, PS-205B potentiostat/galvanostat

10

14

Ag, Al, Au, Co, Cr, Cu, blankPolished and unpolished

1 ms response; series resonance;low noise; grounded/floating highprecision f/V: 0.0012%; outputs: f,Df, V; QCI: admittance and phase

InficonTwo Technology PlaceEast Syracuse, NY 13057

315-434-1100www.inficon.com

IC/5 thin-film deposition con-troller for closed-loop control ofeither sequential or codeposi-

tion processes

XTM/2 thin-film deposition moni-tor for either deposition or etchmonitoring

0.005 (IC/5); 0.1 (XTM/2)

Vacuum deposition

Quartz crystals, quartz crystal sen-sor heads, vacuum feedthroughs

5 or 6Plano-convex

14

Au, AgUnpolished

Full line of thin-film products forvacuum deposition applications;all thin-film monitors and con-

trollers use patented measure-ment system for highest meas-urement resolution possible

Institute of Physical ChemistryPolish Academy of SciencesKasprzaka 44

01-224 Warsaw, Poland48 22 632 32 21, X3217malina.ichf.edu.pl/zd-2/quartz.htm

Quartz crystal holder, electro-chemical cell with reference andauxillary electrodes as well as acontroller; user's potentiostatneeded

0.35 ng and 0.03 ng

0.1

Batteries, corrosion, depositionand dissolution, plating, etching,adsorption, ion dynamics, elec-

tropolymerization, ion exchange,sensors and biosensors, HPLCand FIA detection

Dip-type crystal holder and com-plete electrochemical cell, self-contained flow-through quartzholder, radial thin-layer flow, de-

tection volume 1 L.

5 or 10Plano-convex

14 and 8

Au, Pt, Ti, Ag, Ni, otherPolished and unpolished

No potentiostat of special de-sign required (nongroundedworking electrode); RS-232 con-

trol/data acquisition

Intellemetrics, Ltd.35 Cable Depot Rd.Riverside Industrial Estate

Clydebank, ScotlandG81 1UY, United Kingdom44 141 952 0087www.intellemetrics.com

IL150 film thickness monitor

89.3 ng/cm2

General vacuum deposition; EMsample preparation; thermal, elec-

tron beam, and sputter deposition

Range of crystal holders for highvacuum and ultrahigh vacuum

6Plano-convex

14

Au, Ag, alloy

Deposition rate display,eight-material memory,RS-232 interface, automatic

thickness termination

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water cooling tubes and electrical circuitsto maintain the temperature of the crys-tal environment.

Some other factors to consider when

purchasing a QCM are the cut, size,shape, oscillation frequency, and surfacetexture of crystals; the size and volumecapacity of liquid cells; and the possibleuse of a reference crystal along with thesensor crystal. Many experts agree thatthinner quartz crystals have greater sen-sitivity because they resonate at higherfrequencies, but they are also quite frag-ile. Crystals can be ground so that theyare thin in the center and thick on theedges to make them stronger for han-dling and placing in a crystal holder. Ac-

cording to Michael Ward of the Univer-

sity of Minnesota, most crystals have a0.25- to 1.0-in. diameter and resonatebetween 5 and 30 MHz.

Most QCMs use AT cut crystals,

which Richard Cernosek of AuburnUniversity says are a temperature-compensated cut for the thickness shearmode at room temperature. He saysthat each temperature-compensated cutis for only one temperature. Small varia-tions in the temperature or the angle ofthe cut can cause small variations in themeasured frequency. Cernosek says thatmost people choose a crystal cut for tem-peratures within 25 C of the workingtemperature to make these effects negli-gible. But he warns that one must be

wary of determining what is negligible,

because a frequency shift of even afew hertz can be important in high-precision measurements. He says thathe either controls the temperature or

measures the temperature and compen-sates for it.Cernosek says that controlling the

temperature of the crystal in liquid cellsis not easy because of heat conductionfrom the liquid to the crystal. One solu-tion to this problem, he says, is to builda large structure around the QCM andallow the temperature to equilibrateafter heating or cooling the structure;alternatively, a thermocouple or otherheat-measuring device near the QCM

will monitor the temperature so that a

correction factor can be implemented.

Table 1. Representative QCM systems and ancillary equipment (continued).

Manufacturer


ResolutionMassFrequency (Hz)

Applications

Accessories


Special features

Maxtek11980 Telegraph Rd., Suite 104Sante Fe Springs, CA 90670

562-906-1515www.maxtekinc.com

PM-710 and TPS-550

PLO-10 and CHT-100

0.375 ng/cm2

0.03 (6-MHz crystal)

Electrochemistry, polymers, bio-logicals, gas sensing, corrosion,electrolytic/electroless plating

Flow cell, data acquisitionsoftware

5, 6, and 9Plano-convex, plano-plano

12.5, 14, 25.4

Au, Ag, Al, PtPolished and unpolished

Capacitance compensation,crystal resistance output; crystalholders: well, dip, or flow cellsof Teflon or Kynar

PerkinElmer Instruments, Inc.Attn: Princeton Applied Research801 S. Illinois Ave.

Oak Ridge, TN 37831865-481-2442www.par-online.com

QCA-917 quartz crystal analyzer

1 ng/cm2

0.1

Electrochemical deposition, elec-troplating, adsorption, biosensordevelopment, batteries

Sherbrooke cell, PowerSuiteelectrochemistry software

9

8, square

Au, PtUnpolished

Nongrounded crystal functionswith any potentiostat; resonantadmittance also measured

QCM Research2825 Laguna Canyon Rd.P.O. Box 277

Laguna Beach, CA 92652949-497-5748www.qcmresearch.com

Mark 20 TQCM sensorLab controller model 2000

Mark 18 CQCMFEU flight controller

0.008842.45 ng~0.05

Contamination control on space-craft and in laboratories; outgas

testing; TML, CVCM, QTGA,ASTM E595 and E1559, HD, andsemiconductors

Cables, hosts, software

3, 5, 10, 15, 25, 50 (168 soon)Plano-plano, plano-convex

6.3, 12.7

Au, AgPolished and unpolished

Working on submersible models;radiation-insensitive models alsoavailable

Q-Sense1000 Quail St., Suite 230Newport Beach, CA 92660

949-250-0273www.q-sense.com

Q-Sense D300 including QE 301drive electronics for simultane-ous multifrequency (142 MHz)measurements, QAFC 301 cham-ber for batch or flow measure-ments, and QSoft 301 acquisitionsoftware

1 ng/cm2

0.01

Adsorption, hydration, cross-linking and phase transitions ofmacromolecules such as pro-

teins, lipids, and polyelectrolytes

Spin coater holder, crystalcleaning holder

5

14

SiO2, polystyrene, Au, Ti

Optically polished

QCM-D (simultaneous multifre-quency and multidissipationmeasurements); data analysissoftware enables extraction offilm density, thickness, viscosity,and elasticity

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A. Robert Hillman of the Universityof Leicester (U.K.) points out that tem-perature affects the density and viscosityof liquids and the chemistry of processessuch as kinetics, and these variations arereflected in the frequencies measured bya QCM. I expect that chemically orient-ed factors show a larger temperature de-

pendence [than the temperature changeof the crystal], he says.Experts say that another factor to be

aware of with liquid cells is longitudinalor compression waves from out-of-plane

vibrations of the quartz crystal. Cernoseksays that these waves do not lose energyas easily as shear waves, so they can re-flect back from distant surfaces and in-terfere with the frequency measurement.To keep this from happening, he says toorient the cell at an angle to the surfaceof the crystal or roughen the surface to

diffuse any reflection.

Build or buyJohn Hildebrand of Maxtek notes thattheres a lot of varied opinion out thereas to what gives you the best measure-ment. That is why theres quite a fewpeople making QCMs commercially,and theres also quite a few people mak-ing them on their own.

This statement resonates with Ward,who says that he recommends that peo-ple build their own. Although Wardsays that commercial instruments can bereliable, he stresses that whether usersbuild or buy, they should know the prin-ciples and be aware of conditions thataffect results. For example, he suspectsthat viscoelastic properties of liquids ordeposited films may have contributedpart of the frequency signal reportedin some early QCM papers.

Rob Roberts of PerkinElmer expresses

a different opinion. Purchasing a com-

mercial QCM is not only a convenience,but it offers the added advantages ofaccess to service and technical supportfrom the supplier, he says.

Whats new?One type of frequency distortion in apositive-feedback oscillator, says Kana-zawa, who has worked with Maxtek,arises from the static arm of the res-onator because it is independent of themotion of the quartz and deposited film.He has helped develop a compensatedphase-lock oscillator that measures theequivalent resistance of the resonatorand also offsets distortions in frequencycaused by static capacitance. The result,

according to Hildebrand, is that frequen-cy and resistance measurements give in-formation about viscosity changes inpolymer coatings or any other substan -ces at the crystal surface. Hildebrandalso says that the measurement resolu-tion of instruments has improved withhigher-speed circuitry that detectssmaller mass changes.

Kanazawa says that another innova-tion is an instrument that simultaneous-ly measures frequency and dissipation,

which is useful for examining viscoelas-tic properties. This method takes advan-tage of the fact that a viscoelastic-coatedcrystal exhibits high dissipation, unlikea crystal with a rigid coating. After the

viscoelastic-coated crystal is broughtto a constant oscillating frequency andthe power is cut off, the oscillation am-plitude decreases quickly (high dissipa-tion). Kanazawa says that the devicerecords the ensuing decay of the cur-rent. From the decay curve, both theresonant frequency and the decay time

can be determined. The decay time isrelated to the resonator losses and canbe expressed as a dissipation, D. [Thismethod] is also insensitive to any [fre-quency distortion] effects from the di-electric capacitance.

Other companies have developedsmall systems for use in space or forhand-held field instruments. Scott Wal-lace of QCM Research reports the de-

velopment of QCMs that can cancelfrom their frequency measurementsthe effects of absorbing radiation from

the sun.

Table 1. Representative QCM systems and ancillaryequipment (continued).

Manufacturer


ResolutionMassFrequency (Hz)

Applications

Accessories


Special features

Sigma Instruments1318 Duff Dr.Fort Collins, CO 80524970-416-9660www.sig-inst.com

SQM-160 rate/thickness monitor

SID-142 codeposition controller

0.32 ng/cm2

(2-s measurement)0.025

Deposition rate/thickness measure-ment and control in a vacuum system

Full line of crystal sensors and vacuumfeedthroughs for use in high-vacuumsystems

6Plano-convex

14

Au, AgUnpolished

Up to eight sensors can be measuredat once; up to four sources can be con-trolled at once with the SID-142

Universal Sensors, Inc.5258 Veterans Blvd., Suite DMetairie, LA 70006504-885-8443intel.ucc.ie/sensors/universal/

PZ-1001 immunobiosensor

PZ-105 gas-phase detector

2 ng1

PZ-1001 for direct, real-time monitoringof biomolecular reactions in liquid orgas phase; PZ-105 for measuring gassamples or dry crystals

Cell in acrylic or PEEK for flowing (70-L chamber) or static (up to 1 mL) liquidsamples; software for real-time graphi-cal display of crystal response

10Flat disc

14

AuUnpolished

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A little palm-readingQCMs have been increasingly used tofollow biological processes, includingmechanisms and kinetic studies, and to

investigate the mechanical properties offilm coatings. Cernosek sees a push touse QCMs as biosensors. He also notesthat QCMs are now used for monitoringfluids and deposition at high tempera-tures, either with or without tempera-ture compensation. But he says that anup-and-coming area of research is newmaterials being developed for use attemperatures high enough to destroythe piezoelectric character of quartz.

Maria Hepel of the State Universityof New YorkPotsdam thinks that the

QCMs biggest impact will be on stud-ies of biologically significant systems, suchas transport through lipid bilayer mem-branes, drug interactions and drug de-

livery systems, and biotechnology withDNA and antigenantibody interactions.

Kanazawa sees growing interest in in-terfacing the QCM to electrolytic solu-tions; exploring coatings for chemicalspecificity; and making QCMs part of hy-brid systems, possibly together with scan-ning tunneling microscopy or surface plas-mon resonance. He also says that there isan exciting amount of activity in devel-

oping mathematical models . . . to reflectproperties of the film and/or liquid inter-face that will aid the interpretation ofdata. Kanazawa adds that the means for

acquiring undistorted data is now availablein several forms. But the ability to go di-rectly from measurements to film proper-ties would be a great step forward for theQCM.

Judith Handley is an assistant editor of

Analytical Chemistry.

Upcoming product reviewsJuly 1: Proteomic systemsAugust 1: Automated sample preparationfor MALDISeptember 1: Capillary electrophoresis

QCMs can measure masses ranging from

micrograms to fractions of a nanogram, the

mass of a layer or a partial layer of atoms.
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