Ultrasonic waves and material analysis:recent advances and future trends

Reprinted from LabPlus internationalJune 2002

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Reprinted from LabPlus international - June 2002

High-resolution ultrasonic spec-troscopy measures certain parametersof ultrasonic waves as they propagatethrough a sample. This novel techniquefor material analysis allows direct andnon-destructive measurements of theintrinsic properties of materials, with-out taking the sample apart, addingsomething to it, or changing its state.Samples may be analysed in their cur-rent state or alternatively, this tech-nique can be used to monitor chemicaland structural transformations in thesamples.Non-destructive characterisation of theinterior of the analysed sample normal-ly includes measuring the characteris-tics of signals which have travelledthrough the sample. These characteris-tics provide information on the inter-action of the signal with the sample.Any signal is a combination of wavesand up until now, only one type ofwave has dominated the field of materi-al analysis, electromagnetic waves. Thiswave probes the electromagnetic proper-ties of materials and is employed in awide variety of techniques, includingoptical spectroscopy and its variations(infrared, fluorescence, etc.), NMR,dielectric (microwave) measurementsand others. The domination of the elec-tromagnetic wave is reflected in the

common understanding of the term'spectroscopy'. In many textbooks spec-troscopy is defined as a subject dealingwith the interaction of electromagneticradiation and matter.Ultrasonic spectroscopy utilises analternative wave type: high-frequencyacoustic waves similar to those used by dolphins for communication and bats for navigation. The wave probes intermolecular forces in materials.Oscillating compression (and decom-pression) in the ultrasonic wave causesmolecular arrangements in the sampleto oscillate, resulting in intermolecularattraction or repulsion. The amplitudesof deformations in the ultrasonic wavesemployed in analytical ultrasound areextremely small, thus ultrasonic analy-sis is non-destructive.The second well-distinguished featureof ultrasonic waves is their ability topropagate through most materials,including opaque samples. It is also rel-atively easy to change the wavelength ofan ultrasonic wave. In contrast tooptics, where the wave originates froma light source and therefore special caremust be taken to ensure spectral purity,ultrasonic waves are synthesised elec-tronically. Therefore a typical ultra-sonic spectrometer can cover a broadrange of wavelengths (from 10 to 100 and

sometimes even more). The techniquecould be described as probing the inter-ior of the sample under analysis with aset of fingers which differ in theirlength by more than one order of mag-nitude.Although the analytical power of ultra-sound has been known for some time,the application of this technique tomaterials analysis has been limited tocertain areas due to a number of fac-tors. These include limited resolution,large sample volumes and complicatedmeasuring procedures. These limi-tations have now been overcome byadvances in the modern principles ofultrasonic measurements, electronicsand digital processing.

Measured parametersThe two major parameters measured inhigh-resolution ultrasonic spec-troscopy are ultrasonic attenuation andultrasonic velocity (speed of ultra-sound). Attenuation is determined bythe energy losses in the compressionsand decompressions in ultrasonicwaves, which includes absorption andscattering contributions. As measure-ments of attenuation do not requirehigh temperature stability of the med-ium, they can be performed in largesamples. Thus attenuation was theparameter responsible for the largestportion of the applications of ultra-sound in material analysis in the past.These include the field of kinetics offast chemical reactions, and particlesizing in emulsions and suspensions.Ultrasonic velocity is determined bythe density and the elasticity of themedium. This parameter is extremely

Ultrasonic waves and material analysis:recent advances and future trends

High-resolution ultrasonic spectroscopy is a novel tech-nique for the direct and non-destructive analysis of theintrinsic properties of materials. It is now possible tomeasure both ultrasonic attenuation and ultrasonicvelocity to a very high resolution in a broad range ofsample types and volumes, thus greatly expanding theapplications for this technique.

by Vitaly Buckin & Breda O’Driscoll

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Reprinted from LabPlus international - June 2002

sensitive to the molecular organisationand intermolecular interactions in themedium and can be used to analyse abroad range of molecular processes.However, its application requires highresolution of the measurements andthis cannot be achieved in large sam-ples because of the difficulties associat-ed with temperature control.As it was previously difficult to measureultrasonic parameters in a broad fre-quency range at the required resolution,some research groups focussed theirefforts on the measurements of onecharacteristic (attenuation or velocity),thus optimising their devices for thebest performance in the selected area.

Recent technologicaladvancesThe general principles of high-resolu-tion ultrasonic measurements areshown in Figure 1. The generated elec-tronic signal is transferred by the piezo-transducer into the ultrasonic wavetravelling through the sample. Another

piezotransducer then transfers thereceived ultrasonic wave into an elec-tronic signal, which is subsequentlyanalysed.In the past, the most widely usedapproach for the measurement of ultra-sonic characteristics was based on thepulse technique. In this technique anultrasonic pulse generated at a certainfrequency is sent through a sample andeither received at the opposite side, orafter reflection by the wall of the con-tainer and travelling back to the sourceof the ultrasound. Measurements of theamplitude of the wave in the pulseallow the determination of the ultra-sonic attenuation, while the propaga-tion time (or related parameters)characterises the ultrasonic velocity.The resolution of the measurements islimited by the pathlength of the pulse,or effectively by the size of the sample.A new high-resolution spectrometerlaunched recently by UltrasonicScientific Ltd., Dublin, Ireland (theHR-US 101) employs novel principlesfor the measurement of ultrasonic

parameters, where the length travelledby the ultrasonic wave in the sampleexceeds the size of the sample. Thenovel principles utilise modernadvances in ultrasonic design, electron-ics and digital processing, thus enablingultrasonic measurements with recordresolution (down to 10-5% for ultra-sonic velocity) in a broad range of thesample volumes, down to just onedroplet.

Analytical power ofultrasonic spectroscopyThese advances in ultrasonic analyticaltechnology have made commercialultrasonic instruments available for awide spectrum of analytical tasks inChemistry, Physics, Biotechnology,Pharmaceuticals, Food, Agriculture,Environmental Control, Medicine, Oil,Petroleum and Gas industries.Applications are found in many fieldsincluding research, product develop-ment, quality control and process con-trol analysis.

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Figure 1.

The general

principles of

high resolution

ultrasonic

measurements.

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Reprinted from LabPlus international - June 2002

As ultrasonic signals generated elec-tronically with the use of small piezo-transducers do not have large actuatorssuch as those in dynamic rheology orbulky light sources and other opticalparts, it is possible to build robust,multipurpose instruments which per-form a broad range of analytical func-tions for fast, non-destructive andnon-expensive analysis. In addition, thegeometry of modern ultrasonic cellsexcludes any cavities and sharp cornersallowing easy filling, refilling, cleaningand sterilisation. They can accommo-date even aggressive liquids such asstrong acids or organic solvents, with-out evaporation during a course ofmeasurements. Measurements are com-pletely computer controlled and resultsare presented in a graphical and digitalformat, which is compatible with Exceland most available data analysis soft-ware.Users of modern, high-resolutionultrasonic spectrometers can measureconcentrations of components, transi-tion temperatures and temperatureintervals, characterise the temperaturestability and shelf life of their materialsand analyse enzymatic activities. Thesizes of particles in suspensions andemulsions, the kinetics of sedimenta-tion and the kinetics of chemical andphysical processes in materials may alsobe determined, in addition to stoi-chiometries and affinities in ligandbinding and further parameters.

Technological advantagesHigh resolution ultrasonic spec-troscopy offers a number of advan-tages. Fast measurements allow highthroughput analysis of samples and theability to perform the measurement insmall volumes make it possible to usehigh-resolution ultrasonic spectrome-ters in HPLC and similar applications.In addition, the ultrasonic velocity andattenuation can be measured simult-aneously at different wavelengths as a

function of the time for analysis ofkinetics of chemical reactions andprocesses. Analysis of enzymatic activ-ity is an application of this technique.The ability of ultrasound to analyseopaque samples makes it possible tomeasure the speed of enzymatic react-ions in aqueous solutions as well as insamples where traditional spectroscopyfails, for example, in blood or tomatojuice.The construction of modern ultrasoniccells includes the possibility of con-trolled stirring of the sample, thusallowing measurements under shearand measurements in sedimentingsamples. This also allows utilisation ofthe titration regime, i.e. measurementsof ultrasonic velocity and ultrasonicattenuation in a course of titration.This technique is used for ligand bind-ing studies, to analyse the adsorption ofmolecules on the surface of particles incolloidal systems and to study complexformation phenomenon.The novel design of this system allowsresearchers to carry out ultrasonicmeasurements in the temperature rampregime. This technique is used for theanalysis of heat stability, phase transi-tions and conformational transitions inpolymers and other materials. Thesmall sample requirement reduces thecost of such analyses, which is a keyissue in pharmaceutical, biotechnologi-cal and biomedical research.Another important feature of novelultrasonic spectroscopy is the impres-sive dynamic range. The superbly highresolution allows the analysis of solu-tions of low concentration, down toµg/ml scale. Conversely, the samedevice can be used for concentratedmixtures and non-liquid samples suchas biological tissues, hard gels and foodproducts. The application of othertechniques, for example differentialscanning calorimetry, to these sampletypes would require two differentdevices: a high resolution calorimeterfor dilute solutions and a semisolidcalorimeter for concentrated samples.

Future trendsThe current position of high-resolutionultrasonic spectroscopy in the field ofmaterial analysis can be compared withthat of ultrasound techniques whenthey were first introduced for medicalapplications. When the technologyreached a required level, it generated an'explosion' in medical diagnosticsthrough its ability to visualise internalparts of a patient's body, analyse bloodstreams and other features. Now it isdifficult to imagine a modern hospitalwithout a set of ultrasonic instruments.Similarly, there will be a broad penetra-tion of ultrasonic spectroscopy intoanalytical laboratories in various fieldsof material analysis and productionlines of various industries. Novel high-resolution ultrasonic spectroscopy hasthe ability to perform a wide range ofanalyses, which other methods cannotdo, as well as to increase the effective-ness and reduce the cost of a number ofanalytical tasks currently performed bytraditional techniques.

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The authors

Vitaly Buckin,Dept. of Chemistry,Universiy College Dublin,Belfield, Dublin 4, Ireland.

Breda O’Driscoll,Managing Director,Ultrasonic-Scientific Ltd,Guinness Technology & Enterprise Centre,Taylors Lane, Dublin 8, Ireland.Email: bodriscoll@ultrasonic-scientific.com

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