Anal Bioanal Chem (2007) 387:249–257 DOI 10.1007/s00216-006-0966-4

REVIEW

Ultrasound in analytical chemistry F. Priego Capote & M. D. Luque de Castro

Received: 5 September 2006 / Revised: 27 October 2006 / Accepted: 27 October 2006 / Published online: 14 November 2006 # Springer-Verlag 2006

Abstract Ultrasound is a type of energy which can help analytical chemists in almost all their laboratory tasks, from cleaning to detection. A generic view of the different steps which can be assisted by ultrasound is given here. These steps include preliminary operations usually not considered in most analytical methods (e.g. cleaning, degassing, and atomization), sample preparation being the main area of application. In sample preparation ultrasound is used to assist solid-sample treatment (e.g. digestion, leaching, slurry formation) and liquid-sample preparation (e.g. liquid–liquid extraction, emulsification, homogenization) or to promote heterogeneous sample treatment (e.g. filtration, aggregation, dissolution of solids, crystallization, precipitation, defoaming, degassing). Detection techniques based on use of ultrasonic radiation, the principles on which they are based, responses, and the quantities measured are also discussed. Keywords Ultrasound . Analytical process . Sample preparation . Ultrasound-based detection techniques Abbreviations CE capillary electrophoresis HPLC high-performance liquid chromatography ICP–MS inductively coupled plasma–mass spectrometry MAD microwave-assisted digestion MAL microwave-assisted leaching SFL supercritical-fluid leaching USAL ultrasound-assisted leaching F. Priego Capote (*) : M. D. Luque de Castro Department of Analytical Chemistry, University of Córdoba, Marie Curie Building, Annex C-3, Campus of Rabanales, 14071 Córdoba, Spain e-mail: [email protected]

USASD USASTD USN USNs

ultrasound-assisted soft digestion ultrasound-assisted strong digestion ultrasonic nebulization ultrasonic nebulizers

Introduction Ultrasound is simply sound pitched above human hearing that is currently used for a growing variety of purposes in industry (large-scale cleaning, emulsification of cosmetics and food), medicine (nonsurgical removal of kidney stones, treatment of cartilage injuries, imaging fetal development during pregnancy), engineering (welding plastics, cutting alloys), and chemistry (synthesis of fine chemicals). Traditionally, the scientific community engaged in work on ultrasound has been divided into those who use it to induce physical or chemical effects in a medium (by using high-power, low-frequency ultrasound from 20 kHz to 2 MHz in sonochemistry) and those who use it for measurement without altering the medium (e.g. use of high-frequency (5 MHz to several GHz), low-power ultrasound for nondestructive testing). Analytical chemists fall largely in the former group; thus although an increasing number of analytical processes is being facilitated or improved by use of ultrasound, analytical chemists have contributed to the development of new, interesting modes of ultrasound-based and ultrasound-assisted detection. The different steps of the analytical process which can be expedited and (or) improved by use of ultrasound energy, including those less known by analytical chemists, are revised here. Figure 1 shows the different steps which can be assisted by ultrasound energy; a distinction is made between application of ultrasound before the analytical

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process, during it (for sample preparation), for assisting detection, and as a tool proper. The paper focuses on the ways analytical chemists can use ultrasound energy properly if provided with adequate knowledge of the potential of ultrasound in each step of the analytical process.

outside the scope of sample preparation, degassing can indeed be a part of the analytical process, although it usually constitutes a preliminary step. A physical process such as atomization (understood as the formation of finely divided droplets) is rarely used in analytical chemistry, but can replace freeze-drying for sample conservation. Ultrasound-assisted cleaning

Uses of ultrasound before the analytical process The steps discussed in this section differ in their nearness to sample preparation. Thus, whereas cleaning clearly falls Fig. 1 Steps of the analytical process which ultrasound energy can assist

Ultrasonic cleaning has become a major application of power ultrasound now few laboratories have no access to an ultrasonic cleaning bath. Ultrasound is widely used to Cleaning

PRIOR TO THE ANALYTICAL PROCESS

Degassing Atomization Digestion Solid samples

Leaching Slurry formation

SAMPLE PREPARATION

Acceleration of reactions Liquid–liquid extraction Liquid samples Emulsification Homogenization Filtration Aggregation Dissolution of solids Heterogeneous samples Sonocrystallization/ sonoprecipitation Defoaming Degassing Nebulization

ASSISTANCE TO DETECTION

Sample levitation Assistance to electroanalytical techniques Velocity Primary

Attenuation Reflectance

ULTRASOUND-BASED DETECTION TECHNIQUES

Acoustic response/ measurement conditions

Resonance Impedance Secondary

Interference Relaxation Diffraction

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facilitate the cleaning of analytical equipment ranging from glassware to columns to a variety of devices liable to clogging or malfunctioning as a result of the effect of dirt. Applications such as cleaning and disinfection of surgical and dental instruments [1], decontamination of minimally processed fruits and vegetables [2], and cleaning of membranes of various materials [3], metal surfaces [4], and fine mineral suspensions [5] are quite commonplace.

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continuous systems than in discrete systems, in the former it has greater advantages in terms of process control, efficiency, solvent consumption, and process time, as is widely documented in the analytical literature [8]. One immediate distinction in sample preparation can be made in terms of the physical state of liquid, solid, and heterogeneous samples, which require different treatment. Solid samples

Ultrasound-assisted degassing Ultrasound-based degassing involves removing gases from solutions without heating or evacuation. Cavitational effects, which are the basis of sonochemical action, are also the origin of the extreme efficiency of ultrasound for degassing liquids. Although degassing is a very commonplace in analytical chemistry (e.g. in connection with liquid chromatography), it is rarely mentioned in descriptions of analytical procedures. Ultrasound-assisted atomization Atomization is the popular name given to the first stage of spray-drying, that is, the fine division of a liquid, suspension, or emulsion into fine, narrow-size-range droplets or particles which can rapidly lose their liquid phase under appropriate physical conditions. Typically, a liquid, suspension or emulsion is atomized by forcing it at a high rate through a small opening; ultrasound is, however, an effective way of achieving this objective, and furnishes very fine droplets of high sphericity and uniform size distribution. Although spray-drying is not commonplace in analytical chemistry, it is occasionally used instead of freeze-drying for sample conservation. The former is faster and cheaper than the latter, and is also the only way of effectively removing the liquid phase when the sample is rich in fat [6].

Uses of ultrasound for sample preparation Almost all analytical processes involve some samplepreparation step, because the target analytes are usually excessively diluted or concentrated in the sample or, simply, because the sample must be rendered compatible with normal instrument operation. The steps involved in sample preparation are usually those benefiting from ultrasonic assistance to the greatest extent. Such assistance is effectively provided by ultrasonic baths and, especially, ultrasonic probes [7], and encompasses both discrete and continuous systems—in which ultrasound-assisted steps can be connected on-line to other steps of the analytical process. Although ultrasound is used less frequently in

Solid samples are the most difficult to analyze directly, because they are incompatible with most analytical instruments. The first step in solid-sample preparation, therefore, inevitably involves obtaining the target analytes in a liquid phase. This can be achieved in a variety of ways ranging from complete dissolution of the test sample to partial dissolution or separation of part of the sample. Ultrasound-assisted digestion Sample dissolution can be accomplished with the assistance of heat, chemical reagents, high pressures, and/or moisture. The underlying process is called “digestion” and involves decomposition of the sample matrix and loss of its initial structure. Thermal energy can be replaced by or supplemented with auxiliary energy, for example ultrasound, to accelerate sample digestion. Ultrasound can assist digestion by mechanical and chemical effects, which, despite they being simultaneous, have different effects. Thus, ultrasound-assisted soft digestion (USASD) is usually the result of using ultrasound as the sole auxiliary energy at an ambient or warm temperature and atmospheric pressure, whereas ultrasound-assisted strong digestion (USASTD) involves drastic conditions equivalent to those of exhaustive sample treatment, including high temperatures and highly reactive chemicals (e.g. pure or highly concentrated oxidants or reductants) in addition to ultrasonication. In USASD mechanical effects are the main agents effecting sample digestion, but chemical effects are also present; in USASTD both types of effect are combined. The digestion approach of choice depends on the composition of the particular sample. If the sample contains no insoluble or refractory compounds, USASD may suffice to accelerate its dissolution; one such example is the analysis of pharmaceutical formulations in diverse dosage forms (tablets, capsules, suppositories, and powder) with different solvents, from chloroform [9] or ethyl acetate [10] to sodium hydrogen sulfate solution [11] or water [12], depending on sample composition. In contrast, a complex matrix must usually be decomposed by drastic treatment; one example is the determination of metals in biological samples (particularly tissues), for which a strongly basic medium (tetramethylammonium hydroxide/

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water) may be required in addition to ultrasound [13]. Seafood products have also been digested with ultrasonic assistance and high concentrations of a mixture of HNO3, HCl, and H2O2 [14]. Ultrasound-assisted strong digestion competes with classical digestion techniques such as dry ashing, wet digestion, and fusion, and also with more recent alternatives, for example microwave-assisted digestion (MAD), which has so far been more widely used than USASTD.

Ultrasound-assisted leaching (USAL) Several alternatives to classical leaching methods have been reported since ultrasound was first used as a form of auxiliary energy to assist leaching. The advantages of ultrasound-assisted leaching over classical leaching are obvious. Thus, ultrasound dramatically reduces leaching times [8], use of hazardous reagents, and intervention of the analyst. USAL can be also coupled with other steps of the analytical process in continuous systems to automate a process with dramatically reduced leachant consumption (usually a few milliliters compared with, for example, 100– 150 mL in most Soxhlet methods). Similarly to ultrasound, other auxiliary forms of energy, for example microwaves or use of high pressures and temperatures have proved effective means of accelerating and automating leaching. Compared with microwaveassisted leaching (MAL), USAL is normally simpler and safer, because it does not require high pressures or temperatures. This is really important in some applications, for example determination of methylmercury in biologically certified reference materials, in which degradation to inorganic mercury and losses by evaporation were observed when MAL, rather than USAL, was used [15]. Ultrasoundassisted leaching is strongly affected by particle size, which should, therefore, be strictly controlled. Quantitative leaching of metals from plants has been achieved by use of sample particles of small size (50 μm); below this a further decrease in size was not accompanied by an increase in the amount extracted [16]. USAL requires much simpler equipment than supercritical-fluid leaching (SFL), more commonly known as “supercritical fluid extraction”; overall leaching costs are, as a result, much lower, which is of special interest to laboratories performing routine analysis; USAL also enables leaching of a wider variety of compounds without the need to use a co-leachant, enables operation at ambient temperature and pressure, which ensures the stability in thermolabile analytes, and is usually more expeditious than SFL [8]. In contrast, SFL, with removal of the CO2 leachant during despressurization, enables the leached species to be dissolved in a fairly small volume of solvent [17].

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Ultrasound-assisted slurry preparation Slurry formation in analytical chemistry is practically limited to sample preparation before introduction into atomizers (including ionization, if required) for subsequent atomic (or mass) detection. Slurries are prepared by adding a liquid to a previously ground, sieved, and weighed solid sample; this ensures the stability of the slurry during the time required to withdraw an sample for transfer to the measuring instrument, whether by hand or automatically. Low-frequency, high-energy, ultrasound power in the kHz range is used to assist slurry formation, so mechanical and chemical effects of cavitation are significant. Ultrasoundassisted slurry formation is a simple, efficient means of circumventing problems associated with digestion of samples with complex matrices, because of the hazardous conditions required; it is also an alternative to leaching when efficiencies are not quantitative. Applications involving slurries prepared with ultrasonic assistance are continuously proposed with different detection systems; these demonstrate the versatility of slurries for determination of metallic elements. Thus, slurries prepared by ultrasonication have been analyzed by use of atomic detectors such as ICP–MS [18] or ICP–OES [19] after cold vapor or hydride generation. Another possibility is direct insertion of the slurry into an atomizer or vaporizer, such as in the determination of As, Pb, Se, and Sn in sediments by electrothermal vaporization ICP–MS [20]. In such analysis the duration of the ultrasonication step is dictated by the nature of the sample matrix. Ultrasound-assisted slurry formation has been compared mainly with MAD for a variety of matrices and analytes. A distinction can be made here depending on whether a cleaning ultrasonic bath or a probe was used; the better choice for ultrasound-assisted slurry preparation [21] and MAD [22] is determined by the characteristics of the sample and the liquid phase. Liquid samples Although liquids seem less likely to require energy assistance for proper development, acceleration, or automation of a given analytical step, several of the sample preparation steps used can be improved by use of ultrasound. Figure 1 shows selected examples with and without chemical reaction. The enormous effect of ultrasound on chemical reactions (mainly in organic synthesis) has been widely exploited for more than two decades. In contrast, analytical reactions have been only sparingly assisted by use of ultrasound, although the few examples reported testify to the high accelerating potential of this form of energy. Thus, derivatization has been assisted by use of ultrasound in

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both continuous [23] and batch approaches [24]; this has always resulted in substantial improvements in product formation. The accelerating effect of ultrasound on the determination of phosphate by the molybdenum blue method has been ascribed to depolymerization of molybdate, which accelerated its reaction with phosphate and increased the sensitivity as a result [25]. Biocatalyzed reactions assisted by use of ultrasound are mostly of the enzymatic digestion type [26]. This process, which can involve solid, liquid, or heterogeneous samples, is not digestion proper, because only the target analytes are removed; the initial appearance of the remaining sample is usually preserved. This is a relatively novel use of the ultrasound–enzyme combination, which is more commonplace in hydrolysis reactions [27]. Oxidation reactions are also dramatically promoted by use of ultrasound energy. Ultrasound-assisted oxidation reactions ranging from purely analytical processes, for example determination of the oxidative stability of edible oils [28], which is reduced from hundreds of hours to a few minutes, to the degradation of highly polluting organic compounds, which is typically 104 times faster than natural aerobic degradation [29], are of enormous interest. Ultrasound is also an excellent means of acceleration and activation in organic synthesis, which is usually triggered by heat, light, or another form of energy. One example is the mediated electrosynthesis of carbon– carbon bonds in totally “green” surfactant-free emulsion media generated by application of ultrasonic power to a biphasic water–organic medium [30]. Ultrasound-assisted hydrolysis, which usually involves organic systems, most often requires use of high-intensity ultrasound. One example is processes benefiting from the improved photocatalytic activity of titania-only materials produced by ultrasoundassisted hydrolysis [31]. In contrast, ultrasound has rarely been used for generation of reagents, although it has proved very effective for this purpose (e.g. in generating oxidant species) [32, 33]. The effect of ultrasound on these reactions has not yet been efficiently exploited or explained. The effect of ultrasound in accelerating the physical steps of liquid-sample preparation is poorly known and difficult to generalize owing to the disparate nature of the phenomena to be enhanced. The absence of chemical reaction is sometimes unclear, even though the final result reveals no chemical changes. Liquid– liquid extraction from an aqueous to an organic phase or vice versa is highly affected by the presence of ultrasound, albeit not always favorably. Some systems are improved [34], others undergo undesirable emulsification. Systems in which a chemical reaction is coupled with liquid–liquid extraction are more markedly improved, because the ultrasound affects both. Examples include continuous liquid–liquid extraction without phase separation and hydrolysis of paracetamol in suppositories [35].

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Emulsification is greatly promoted by ultrasound. The effect of ultrasound here is based on droplet disruption in sonicated liquid–liquid systems as a result of cavitation. These systems are barely used in analytical chemistry, but have a wide scope in the pharmaceutical industry. Homogenization is also effectively assisted by use of ultrasound, without altering the chemical characteristics of the system. This step can be required one or several times during sample preparation to facilitate contact between solutions; alternatively, it can be introduced before sampling to ensure the representative nature of the target system. Ultrasoundassisted homogenization is widely used in analytical laboratories to bring solutions into contact, but is rarely studied or optimized during the development of analytical methods.

Heterogeneous samples The main purpose of ultrasound as applied to heterogeneous media is in separating a solid from a liquid phase in which it is dissolved; this favors or accelerates formation of the solid phase. The solid phase can exist in the sample as such or be formed as a result of, mainly, a chemical reaction (e.g. precipitation); a physical phenomenon such as crystallization or aggregation can, however, also be the origin of the solid phase. Filtration is one conventional means of separation of phases that is dramatically aided by use of ultrasound. Analytical applications of filtration include: 1. separation of a solid phase formed in a chemical step involved in sample preparation (e.g. in an automated, on-line system for bioprocess control based on flow injection, ultrasound filtration and coupled charge detection [36]); and 2. preliminary use in sample preparation to remove undesirable particles. The latter use is rarely discussed in the analytical literature but has been widely studied during assessment of the performance of filtration membranes in the presence and absence of ultrasound under different conditions and frequencies [37]. Aggregation (also known as agglomeration) is the formation of large particles from small ones which agglomerate rapidly and efficiently when subjected to ultrasound; this enables easier separation of the solid phase, if isolation of the phases is the pursued objective. This technique can be used before or after sampling, and also at an intermediate stage if finely divided solid is formed during sample preparation. Analytically, dissolution of solids present in a heterogeneous sample is performed to determine an analyte present

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in the solid phase or randomly distributed between both phases. The specific ultrasound-assisted process involved is rarely stated, and usually misnamed. Thus, in the preparation of milk samples for determination of mercury by AFS, sonication of the samples in the presence of 8% (v/v) aqua regia, 2% antifoaming agent, and 1% hydroxylamine hydrochloride for 10 min then treatment with 8 mmol L−1 KBr and 1.6 mmol L−1 KBrO3 in hydrochloric acid medium to obtain a liquid phase was given the name “slurry sampling” [38]. The treatment was also used for subsequent determination of 45 elements in milk by ICP– MS [39]. Sonocrystallization is the name currently given to use of ultrasound power to control and accelerate the course of a crystallization process. This effect had been used in the salting-out process for years and has proved to have a favorable effect on the initial nucleation stage of crystallization. The wide use of this form of energy in food technology [40] clearly testifies to the advantages of ultrasound in this discipline. Ultrasound has also been used to assist the formation of extremely finely divided, uniform particles; this is known as sonoprecipitation. This effect, which has been exploited in analytical chemistry, can facilitate sample preparation in nephelometric and turbidimetric methods. Defoaming is the operation performed to remove foam (viz, a dispersion of a gas in a liquid in which the distances between the individual bubbles are very small). Foam can cause problems in analytical operations, for example chromatographic separations and molecular spectrometric detection, among others. Acoustic transducers operating at 10 and (or) 20 kHz are capable of defoaming liquids if the acoustic source is placed above the liquid surface on which the foam is being generated. No dedicated acoustic defoamers have been developed, even though foaming is a frequent problem in analytical laboratories, particularly when working with surfactants [41]. Although the analytical uses of ultrasound-assisted degassing are almost restricted to the step preceding sample preparation, several analytical methods produce gas during one of the stages. One example is the ion chromatographic determination of bromate in drinking water, in which the decrease in the concentration of chloride and carbonate on an on-guard Ag and H cationic column produces an eluate containing CO2 and H2 bubbles that are effectively removed by 10 min sonication in an ultrasonic bath [42].

Uses of ultrasound to improve detection Ultrasound has been used in other ways to improve the performance of detection techniques. It has, for example,

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been used to assist nebulization, for sample insertion into detection systems, since the earliest ultrasonic nebulizers (USNs) were reported in the 1980s [17]. The energy required to form an aerosol with a narrow droplet-size distribution is emitted by a vibrating transducer. After the aerosol is formed it is carried to the detector by the nebulizer gas stream [43]. The analytical sensitivity, selectivity, and resolution of atomic detectors has been markedly improved by use of USNs. Use of these nebulization systems should not be regarded as exclusive to atomic detectors, however, because USNs have also been successfully implemented in the electrospray-formation devices used as interfaces between CE or HPLC equipment and mass detectors [44]. It is well-known that USN results in higher analyte transport efficiency than does pneumatic nebulization (usually 8–15 times better); this results in improved sensitivity and lower detection limits, which is particularly important for analysis of species at trace or ultratrace levels [45, 46]. Sample levitation can be accomplished in different ways, one of which is the use of ultrasonic energy. This use was first reported by Bucks and Müller in 1933 [47]. The flexibility and potential of acoustic levitation in a variety of disciplines, especially analytical and bioanalytical studies, is widely documented [48–50], which testifies to its maturity. Acoustic levitation is based on the production of a standing wave with equally spaced nodes and antinodes by multiple reflections between an ultrasonic radiator and a solid reflector; this enables levitation of small samples in the pressure nodes of the acoustic standing wave. The main advantage of acoustic levitation over other levitation techniques is that materials in different states of aggregation (solids, liquids, suspensions, and gases) can be levitated by use of acoustic forces irrespective of their physical properties (viz whether they are insulators or conductors, magnetic or nonmagnetic) [49], with the sole exception of the restrictions imposed by the density of the material. Although ultrasound can, in theory, be used to levitate any type of material, applications of acoustic levitation to solids have been restricted to a few studies in analytical chemistry and related areas, largely because of the unavailability of appropriate devices for sample positioning and detection systems for extracting information from solids. Gas levitation has, essentially, been limited to basic studies. Ultrasound energy has also been used to improve the performance of electroanalytical techniques by exploiting the chemical and physical effects of cavitation (including solvent sonolysis, acoustic streaming, microstreaming, and microjetting) when this form of energy is applied to the electrolyte–electrode interface (as for any solid–liquid interface) [51]. Ultrasound assistance can be provided before and (or) during analysis. In techniques involving

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no electron exchange at the solution–electrode interface, ultrasound is usually applied before analysis—and, hence, in the absence of the sample—to activate the electrode surface [52]; in electron–exchange processes ultrasound can be used for pretreatment but also during analysis. In the latter example ultrasound can be applied during the measurement step and, in stripping techniques, during deposition of the target analyte on to an electrode, either electrochemically or by physisorption. Use of ultrasound during the electroanalytical process is known as sonoelectroanalysis, mainly focused on voltammetry for determination of inorganic species in different types of sample in which the sensitivity of silent classical voltammetric techniques is usually reduced by fouling of the electrode [53–56].

Ultrasound-based detection techniques Ultrasound-based detection encompasses a broad group of techniques that use ultrasound energy either to interrogate and measure the response of a given specimen or merely as an interrogating force. No conceptual distinction has yet been made between techniques and methods, which has raised confusion in this field. Analytical chemists have been reluctant to use ultrasound-based detection techniques, even though they have been widely used in other research fields, particularly in physical chemistry, in which they have enabled complex theoretical developments with which the analytical chemist is usually unfamiliar. Ultrasound-based detection could be a useful tool for analytical chemists if they acquire the basic knowledge required. Ultrasonic detection techniques rely on measurements of low-intensity waves or mechanical deformations at frequencies higher than 50 kHz (mostly in the MHz region, but also in the GHz region). The ultrasound intensity required is so low (typically <1 W cm−2) that it causes no physical or chemical alteration of the properties of the material through which the wave propagates; the amplitude of deformations is, therefore, extremely small, so ultrasonic spectroscopic techniques can be deemed nondestructive. In contrast with light waves, ultrasonic waves can propagate through most types of material and their wavelength is fairly easy to change as they are synthesized electronically. Ultrasonic detection modes differ in: 1. the manner in which ultrasound is applied (e.g. as a single frequency, as broad-band pulses, or as a scanning frequency), after which modes are named; 2. the manner in which ultrasound impinges on the sample (normal, parallel, oblique), after which the waves produced in the material (longitudinal, shear, oblique) are named; and

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3. the manner in which the experimental data are used (viz. as amplitude or phase spectra) or processed (viz. in the frequency or time domain).

Ultrasonic spectrometry usually relies on measurements of some characteristic of ultrasonic waves propagating through the sample that provide information on the interaction of the ultrasonic waves with the inside of the sample, thus enabling analysis of its physical and chemical properties. Propagation of ultrasonic waves is determined by ultrasonic velocity and attenuation; these, with reflectance, are the primary acoustic responses. Attenuation is the best property for characterizing dispersed phase composition and particle size [57]; in contrast, the speed of the sound is better for characterizing chemical composition at the molecular level [58]. Several additional secondary measurements, for example resonance, interference or impedance can, under specific conditions, be obtained from primary responses (attenuation or velocity) or are a fraction thereof (e.g. absorption, scattering, diffraction, relaxation). One particular example is ultrasound resonance spectroscopy, in which measurements of ultrasonic resonance involve use of a resonator cell comprising a resonator chamber and two transducers (usually piezoelectric). The sample is positioned in the resonator chamber and, after acoustic resonance, the two transducers excite and detect the corresponding ultrasonic vibrations. The ultrasonic velocity and attenuation are then determined from measurements of ultrasound frequency and energy losses under resonance conditions. This technique has poor measurement resolution, which is limited to 0.001% for ultrasonic velocity and approximately 1% for attenuation, even with specialized sophisticated devices; ultrasonic characteristics are also difficult to interpret, which hinders the use of this technique for routine analyses. The main field of application is characterization of materials [59]. Ultrasound-based detection techniques can benefit from use of lasers as energy sources. Laser-based ultrasound techniques have many attractive features, including the possibility of making noncontact, non-destructive, remote measurements. These modes of optical detection are, typically, one or two orders of magnitude less sensitive than their contact transducer counterparts, where available. Thus, acoustic microscopy uses transmitted ultra highfrequency ultrasound in the range 10 MHz to 2 GHz to produce images, mainly of biological structures, with a resolution approaching that of light microscopy. This technique, which is usually sensitive to the mechanical properties of the materials under inspection and contrast, provides useful information about the physical structure of the sample. The acoustic microscope has transmitting and

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receiving ultrasound transducers between which the target specimen is placed. Specimen areas of approximately 250 μm2 can be scanned within ca 2 s by use of highly focused ultrasound [60]. Brillouin scattering spectroscopy is a hyphenated technique in terms of energy source (viz. thermal and optical for phonon and photon production, respectively). Interactions between photons (generated, for example, by a laser) and phonons can be used to derive information about the interaction of ultrasound with the test specimen by examining changes in the light waves. Thermal surface phonons restrict practical application of this technique, owing to their low scattering efficiency; this results in overly long data-collection times (typically several hours for a single spectrum, even with advanced multipass interferometers) [61].

Conclusions Few types of energy can contribute to such a vast field of analytical applications. The different steps which can be aided by ultrasonic energy have been reviewed. These steps range from those widely used by the analytical community in solid-sample preparation (leaching, digestion, or formation of slurries) and assisting chemical reactions to those less known by analysts for preparation of liquid samples (liquid–liquid extraction and emulsification) and heterogeneous samples (filtration, agglomeration, crystallization, etc.). Other applications include improving the performance of detection techniques, and include sonoelectroanalysis, ultrasound-assisted levitation, and nebulization. Finally, to give a general view of the potential of these techniques, the state-of-the-art of detection techniques based on use of ultrasonic irradiation, scarcely known by analytical chemists, is also discussed. Acknowledgement The authors gratefully acknowledge the Spanish Comisión Interministerial de Ciencia y Tecnología (CICyT) for financial support (Project CTQ2006-01614). F. Priego-Capote is also grateful to Spain’s Ministerio de Educación y Ciencia for award an FPU scholarship.

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