- Email: [email protected]
Analytical uses of ultrasound
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
Trends
Analytical uses of ultrasound II. Detectors and detection techniques F. Priego-Capote, M.D. Luque de Castro
Most analytical chemists know that ultrasound (US) can help steps involved in sample preparation (e.g., particularly in dealing with solid samples) and transportation to atomic detectors (e.g., with US nebulization or slurry formation). But not many are aware of other interesting and less common uses. We therefore place emphasis on US-based spectroscopic techniques – mainly, US-attenuation and US-velocity spectroscopy – and other techniques, such as voltammetry, which US assistance has dramatically improved to give so-called sonoelectrochemistry. Additionally, we comment on the use of US for improving sample conditioning and transport – namely, acoustic levitation, nebulization and slurry formation. This article constitutes an alert on the potential of US energy, which analytical chemists almost neglected. ª 2004 Elsevier Ltd. All rights reserved.
1. Introduction F. Priego-Capote, M.D. Luque de Castro* Department of Analytical Chemistry, Annex C-3, Faculty of Sciences, University of Cordoba, Campus of Rabanales, E-14071 C ordoba, Spain
*Corresponding author. Tel./Fax: +34-957-21-8615; E-mail: [email protected]
Although the use of ultrasound (US) for assisting different steps in sample preparation has not enjoyed the development that the effects of US deserve, there are not too many analytical chemists who do not know to some extent the applications of US to improve, accelerate or automate the preliminary steps of the analytical process. However, this is not so when it comes to the variety of US uses in the detection step, which can vary from improving the way that the sample reaches the detector or the detection itself to being the basis of unknown, or little known, detection techniques. With the aim of diffusing knowledge of the use of US in the detection step, we pay special attention in this article to: • US spectroscopy (USS) in its different modes; • the improvement, with US, in established techniques, such as voltammetry, which has led to sonoelectrochemistry; and, finally,
0165-9936/$ - see front matter ª 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2004.07.013
• the different ways in which US can facilitate the sample to reach the detector (namely, levitation, nebulization and slurry formation).
2. US-based detection techniques 2.1. USS A common error in many textbooks is to define spectroscopy as a subject dealing with the interaction of electromagnetic radiation and matter. This is explained by the dominance of the electromagnetic wave. However, USS uses an alternative wave type – the high-frequency acoustic wave. When this energy acts over a given system or sample, inter-molecular forces are generated with the subsequent oscillating compression and decompression of the US wave [1–3]. These forces cause molecular arrangements in the sample, resulting in inter-molecular attraction or repulsion. No alteration of the sample is provided by the small amplitudes of deformation employed in analytical chemistry. The novel technique of USS enables, as explained below, the monitoring of certain parameters caused by US waves as they propagate through a sample. The main characteristic of high-resolution USS is the possibility of carrying out direct, non-destructive measurements [2–4]. This advantage makes this technique very competitive with other well-known detection techniques. Thus, minimal or no pre-treatment is required and measurements of the intrinsic properties of the system are achieved without sampling, the addition of anything, or a change of state. 829
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
In addition to the characteristics outlined above, USS can be classified in the group of universal detection systems, because US waves can propagate through most materials, including those that are opaque to light. Moreover, because US waves are synthesized electronically, it is relatively easy to change the wavelength of an US wave in comparison with optical waves as the latter originate from a light source so special care must be taken to ensure spectral purity. Although these features would qualify USS to be a powerful technique, its application has been limited to certain areas because of several technical factors, namely limited resolution, the need for large sample volumes and complicated measuring procedures. Recent advances in the modern principles of US measurements, electronics and digital processing have solved problems arising from these limitations [2]. Concerning objectives, there are two general ways of using USS as regards sample state, namely: measuring the current state of the system under study; or, monitoring its chemical and structural transformations. The two main and most frequently measured parameters in high-resolution USS are US attenuation and US velocity. While US-velocity spectroscopy (USVS) is mainly used for the study of inter- and intra-molecular processes [5], US-attenuation spectroscopy (USAS, also known as US-extinction spectroscopy) has found its major application in particle sizing [6], including studies of processes in relation to particle size. Attenuation is determined by the energy losses in compressions and decompressions of US waves, which includes absorption and scattering contributions. 2.1.1. USAS. Many publications related to USAS have appeared since 1991 when McClements’s review of this technique was published [7]. The greatest advantage of USAS in comparison to other techniques with similar applications, such as light scattering, is its capability to characterize intact, concentrated, dispersed systems. As measurements of attenuation do not require precise stability of temperature, they can be performed on large samples. In addition to information concerning particle size, other information about the compressibility and the state of aggregation of the dispersed phase can be obtained. The measurements may be made rapidly and in-line, in a pipeline or in a tank [8]. Significant applications of USAS [9–18] can be seen in Table 1. USAS has become one of the most promising measurement techniques in the field of on-line particlesystem characterization [14], including protein-stabilized emulsions. In comparison to optical techniques, USAS has no limits in applicability to opaque and/or highly concentrated liquid dispersions so it eliminates the need for sample dilution and preparation. Furthermore, data can be obtained from samples with a wide range of sizes (10 nm–1 mm). This, in turn, offers an 830
http://www.elsevier.com/locate/trac
Table 1. Selected applications of ultrasound-attenuation spectroscopy-based detectors Application
Reference
Molecular characterization of proteins Whey-protein-aggregation studies Particle-size measurements Monitoring particle-size distribution of highly concentrated slurries Particle-size-distribution analysis of casein in water Particle-sizing studies Monitoring of oil-in-water emulsions Monitoring the flocculation of droplets in protein-stabilized emulsions Characterization of quantum states at ultralow temperature
[9] [10] [11–13] [14] [8] [15] [16] [17] [18]
opportunity to study particle systems in their original state of dispersion, thus facilitating the determination of agglomeration or flocculation parameters [16] (e.g., monitoring whey protein aggregation [10] or flocculation of droplets in protein-stabilized emulsions [17]). Various physical mechanisms are responsible for the dependence of sound attenuation on particle size, as follows: • There is an analogy to optical acoustic scattering, which includes all mechanisms responsible for redirecting sound waves incident on particles. • A second phenomenon is thermal coupling, referring to the non-steady heat exchange between particles and the continuous phase that occurs when temperature fluctuations in the respective phases, caused by compression fluctuations, differ in phase and magnitude. • Another phenomenon is viscous coupling that can be defined as the retardation of the oscillatory motion of particles experienced within the continuous phase as a result of inertia differences. • Both thermal and viscous coupling are relaxation processes that reduce the original mechanical disturbances (i.e., sound intensity), so they are also involved in the so-called dissipative attenuation mechanisms. Their magnitudes are significant for particles much smaller than the US wavelength, in which acoustic scattering can be neglected. • In addition to the size-dependent attenuation mechanisms, US absorption occurs in both the continuous and the disperse phase. Total absorption therefore depends on the composition of the disperse system [19]. Nevertheless, because of strong attenuation caused by scattering, methods based on attenuation measurements cannot very often be used on small-sized specimens of such materials. Examples of this behavior are many ceramic materials with certain porosity or metallic
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
materials, which are generally weakly attenuating materials [20]. In these cases, the most widely used experimental techniques are based on measuring velocities of propagating pulses or wave packets. Concerning other independent applications of particle sizing, particularly important investigations are those developed at low temperature (e.g., the joint use of a Fourier transform device and USAS in the determination of the energy gap of superfluid 3 He–B at very low temperatures, where no other methods provide successful results [18]). 2.1.2. USVS. US velocity is determined by the density and the elasticity of the medium. This parameter is extremely sensitive to the molecular organization and the intermolecular interactions in the medium and can be used to analyze a broad range of molecular processes. However, its application requires high-resolution measurements that cannot be carried out in large samples because of the difficulties associated with temperature control [2]. USVS has been used both with USAS [21–29] and as a stand-alone detection technique [6,20,30–39], as can be seen in Table 2. In spite of the fact that in the past the most widely used approach for measurement of US parameters, whether attenuation or velocity, was based on the pulse technique, an important shortcoming is the limit in resolution by the path length of the pulse or sample size. In this sense, new commercialized high-resolution spectrometers have established novel principles for the measurement of US parameters. The novel principles
Trends
utilize modern advances in US design, electronics and digital processing, thus enabling US measurements with record resolution (down to 10)5% for US velocity) in a broad range of the sample volumes, down to just one droplet [2].
2.1.3. Other US-based techniques. Resonant USS (RUSS) is a relatively new technique for measuring the resonant frequencies of a sample of nearly ideal geometry and with known symmetry, dimensions and mass. In RUSS, a parallelepiped, cylinder or sphere of the material of interest is suspended between two transducers across opposite corners (in the cases of a parallelepiped or a cylinder). One transducer excites the specimen through a specified range of increasing vibration frequencies. At the opposite transducer, the response of the solid to the signal is converted to an electrical signal and fed to a computer for analysis. The resonance peaks are amplifications of waves through constructive interference within a solid at specific frequencies. By analyzing the spectrum of waves that are scattered by the object, one can determine its resonant frequencies. These can then reveal information about the material properties or geometry of the item [40]. RUSS has been satisfactorily applied (see Table 3) in the characterization of explosively welded clad rods [41], determination of elastic stiffness of materials [42], the high-precision measurements in small samples of elastic constant (e.g., GaN, quasi-crystals and perovskites) [43], and quality control (QC) in both manufacture of small
Table 2. Selected applications of ultrasound-velocity spectroscopy (USVS) alone or in conjunction with ultrasound-attenuation spectroscopy (USAS) Application
Technique
Reference
Studies of mechanical properties of glasses Material analysis Diagnosis of bone diseases Pharmaceutical research Characterization of pyrolysis stability of oil Studies of the acid gelation of milk Evaluation of heat stability of calcium-fortified milk Real-time analysis of chemical reactions Determination of critical micelle concentration Evaluation of the ring-opening metathesis polymerization of dicyclopentadiene Elastic modulus measurements of extremely porous ceramic materials Miscibility determination of isobutylene-co-isoprene rubber Miscibility determination of polychloroprene Characterization of binary blends and compatability Compatibility and miscibility studies Intermolecular association studies Elastic coefficient measurements of paper Study of membrane filters On-line process control of directional solidification Investigation of DNA condensation Determination of compressibility of alkyltrimethylammonium bromide micelles
USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USVS USVS USVS USVS USVS USVS USVS USVS USVS USVS USVS USVS
[21] [22] [23] [24] [25] [26] [27] [28] [29] [6] [20] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]
http://www.elsevier.com/locate/trac
831
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
Table 3. Selected applications of other ultrasound-based detection techniques Application
Technique
Reference
Characterization of explosively welded clad rods Determination of the elastic stiffness of materials High-precision measurements of elastic constant in small samples Quality control in manufacturing process Determination of structural integrity in construction Crystallographic texture determination Inspection of composite parts in manufacturing process Microstructural characterization of polycrystalline materials Characterization of elastic properties of materials Characterization of evolution of microstructure of steel at various temperatures Mechanical characterization of polymer films Determination of diffusion bond strength Characterization of aerated foods Detection and characterization of delaminations in thin composite plates diagnostics of micro-damage of materials Determination of Mycobacterium tuberculosis Monitoring the oxidative damage induced by the vitamin C–Fe(III) system Inhibition studies on the growth of Escherichia coli Determination of a-amylase Determination of rheumatoid factor End-point determination in titration Monitoring of mutagenic process Study of DNA–platinum-based drug interaction mechanism Determination of nitrate and nitrite Determination of total NH3 and total CO2 in blood Determination of sulfite in wines and fruit juices Determination of volatile acidity of fermentation products
RUSSa RUSS RUSS RUSS RUSS LGUSSb LGUSS LGUSS LGUSS LGUSS URSc URS URS URS NEWSd AWISe AWIS AWIS AWIS AWIS AWIS AWIS AWIS AWIS AWIS AWIS AWIS
[41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [65] [66] [67] [68]
a
RUSS, resonant ultrasound spectroscopy. LGUSS, laser-generated ultrasound spectroscopy. c URS, ultrasonic reflection spectroscopy. d NEWS, non-linear elastic wave spectroscopy. e AWIS, acoustic wave impedance spectroscopy. b
parts (e.g., aircraft landing gears and wheels and precision ball-bearings) [44] and construction of bridges in order to test the structural integrity [45]). It is also worth commenting on the technique known as laser-generated USS (LGUSS), which uses one shortpulsed laser to generate US waves and another laser coupled to an interferometer to detect the corresponding mechanical displacements [46]. The technique yields huge savings in inspection time in comparison to conventional US techniques. Unfortunately, for some material and process combinations, LGUSS suffers from a lack of sensitivity. Averaging signals compensate for this lack at the cost of increased inspection time and decreased savings. At present, the CO2 laser is used to generate the US waves. However, although efficient, the CO2 wavelength and its pulse shape are not optimal [47]. Applications that take advantage of this technique [46–50] can be seen in Table 3. Another US-based technique is US reflection spectroscopy (URS) that has been used relatively rarely compared with USAS or USVS, with applications including determination of the viscoelastic properties of polymer films [51], measurement of diffusion bond strength be832
http://www.elsevier.com/locate/trac
tween two identical materials [52], detection of changes in bubble characteristics in aerated food (where USAS and USVS are unsuitable because of the high attenuation of the US signal that cannot be propagated far enough through the sample [53]) and detection of delaminations in thin composite plates [54] (see Table 3). A specific group of techniques have been developed in order to quantify the level of non-linearity in the elastic response of materials containing structural inhomogeneity and damage. These techniques have been called non-linear elastic wave spectroscopy (NEWS) and they are much more sensitive to damage-related structural alterations than any other technique used for investigation of linear material parameters, such as US velocity or attenuation [55]. Finally, acoustic wave impedance spectroscopy (AWIS) has found several applications in many fields. The detector is constructed by connecting a piezoelectric quartz crystal and one or two conductivity electrodes in series to make the feedback circuit of the oscillator, and it can be represented by an electrical equivalent circuit. The fundamentals of AWIS applications are the change of any physical or chemical property of a system (e.g.,
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
solution or culture) because of any phenomenon (e.g., bacterial growth in culture medium), which causes variations of equivalent circuit parameters of piezoelectric quartz crystal. The changes in equivalent circuit parameters reflect the phenomenon. It is known that the AWIS detector is sensitive, not only to the mass change at the electrode surface, but also to the density–viscosity change of the test solution. The main advantages are high sensitivity, simplicity of use, and capability of providing multi-dimensional and real-time information to reflect some physical and chemical properties of the investigated system [56,57]. Table 3 shows that AWIS has been used in a number of applications [56–63]. Moreover, the coupling between an AWIS detector and flow injection (FI) systems can have the advantages of automation and reduced consumption of reagents and samples [64]. Table 3 also shows examples of this coupling [65–68].
2.2. US for monitoring evolving systems The characteristics of US-detection techniques (e.g., nonintrusiveness, fast response and potential for application at high pressures and temperatures) make them particularly useful in monitoring evolving systems in general and for on-line monitoring of industrial processes in particular. There are many publications about the construction and use of US sensors for these tasks, for which practically all types of US-based techniques have been used. The US signal can penetrate through the walls of a given line or vessel, offering the advantage of a truly non-invasive, non-destructive technique. A US pulse of a certain energy emitted from a vibrator (transducer) into a medium propagates through this medium and reaches the receiver at a lower energy after a given time. US velocity monitoring has been applied in crystallization processes (e.g., to satisfy the customer’s requirements in terms of product purity and crystal-size distribution and the manufacturer’s economical requirements). One of the main variables to be controlled in crystallization is the supersaturation in the solution, which is defined as the difference between actual and equilibrium concentrations of a solution. The USmeasuring technique enables the range of supersaturation and metastability to be determined. According to this technique, proposed by Omar and Ulrich, the metastable limit, which defines the operating zone of the industrial crystallizer, is a function of the US velocity and temperature [69]. Another industrial application is the control of grinding by on-line particle size analyzers. Grinding to a finer size than necessary results in reduced throughput, higher energy costs and increased consumption of mill liners, grinding media and reagents. Too coarse a grind results in reduced recovery of valuable minerals. The benefits of US-based control of grinding are better milling
Trends
efficiency, more stable operation, higher throughput and improved downstream processing. Industry requirements for an on-line slurry particle-size analyzer include direct on-line measurement without dilution, rugged and reliable equipment, and easy calibration and standardization, for which US techniques provide a number of advantages [70]. US-velocity monitoring is also used for QC in production of admixtures and new binders and for QC in concrete production [71], for which a testing device has been developed to monitor the setting and the hardening of cement materials in order to obtain reproducible results. The method is adjusted to concrete and can also be used for materials such as gypsum, lime, starch and other stiffeners. Another case is the monitoring of polymer melt extrusion, where the operator can use on-line US-velocity measurements to detect in a fast, sensitive way a change in this process, but not the cause of the change [72]. Combining measurements of US velocity and attenuation with gamma-ray transmission to provide a full size distribution of the particles in range 0.1–1000 lm, CSIRO Minerals has developed an in-stream slurry particle-size analyzer for use in grinding circuits [70]. These types of measurements are also the basis for the development of a highly sensitive, self-calibrating, on-line sensor to measure the density, the velocity and the attenuation of US. This approach can be applied to a liquid or slurry flowing through a pipeline, although it is also valid for measurements made in vessels. Interesting applications in this context are the determination of the density and the concentration of solids in radioactive waste slurries. This probe configuration does not depend upon passage of US through the fluid; the density measurement depends upon only US reflection at the walls of the container, so the probe can be used to measure the densities of very attenuative slurries [73]. Moreover, by basing the measurement on multiple reflections, the sensitivity of the instrument is increased by the power of the reflection coefficient. The measurement method is self-calibrating because the measurement of acoustic impedance is independent of changes in the voltage of the pulsed source. Field observations of debris flow have also benefited of the evolution of US sensors; specifically, direct measurements of the hydrological conditions for the occurrence of debris flows in the sense of determining flow behavior are of the outmost importance in developing effective flow-prevention techniques [74]. Automation in the food industry requires fast, reliable measurements of the physical properties of materials during processing. The possibility of carrying out these measurements on-line provides benefits to the industry in terms of better control of product quality, improved processing efficiencies and reduction in wastage. Moreover, operators can adjust processing parameters during, rather than after, production runs [75]. http://www.elsevier.com/locate/trac
833
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
3. US-improved technique: sonoelectroanalysis The coupling of US power with well-established but under-exploited electrochemical techniques, such as voltammetry, particularly stripping voltammetry, has led to the emergence of the powerful new analytical technique of sonoelectroanalysis. Where classical electroanalytical techniques were plagued with electrodefouling and/or sensitivity limitations, the introduction of US into the system has provided a dramatic increase in analytical efficiency and substrate applicability, predominantly through enhanced mass transport and electrode–surface activation. Sonoelectroanalysis has been applied to a range of modern analytical problems, allowing sensitive determination of a wide number of analytes from a variety of otherwise hostile matrices. This section presents a view of recent advances in this field. This combination of technologies has allowed the improvement of such techniques, which have hitherto been relatively limited, largely because of passivation, reliability and sensitivity problems. US equipment working in the range of 20–100 kHz is relatively inexpensive and readily available, and has proved to be an excellent technology for electroanalysis (some high frequency (500 kHz) equipment has also been used for more fundamental investigations of mechanisms). Application of US leads to a major enhancement of sensitivity of stripping analysis because of the significant increase in mass transport resulting from acoustic streaming. This gives the ability to make quantitative measurements in complex media with a minimum of sample pretreatment because cavitation cleans the electrode surface in situ [76,77]. Moreover, it is possible to extract electroactive species from binding sites in organic matrices and to emulsify otherwise immiscible organic and aqueous layers without the need for additional emulsifying agents. The benefits of coupling US power with well-established electrochemical techniques have rapidly made sonoelectroanalysis a powerful new tool for analytical chemists. Moreover, sonoelectroanalysis offers a real alternative to the current industry standards of AAS and ICP–MS, which carry much greater cost and time restrictions. An additional aspect is that the sonovoltammetric techniques lend themselves well to miniaturization, and portable versions of sonovoltammetric equipment are currently undergoing field trials. The influence of US on chemical processes can produce (e.g., in homogeneous sonochemistry, via acoustic hot spots) novel chemical species [78], providing enhanced yields (energy savings) and/or selectivity (reduced waste) of existing reactions, with the possibility of replacing hazardous reagents and making certain reactions ‘‘green’’. 834
http://www.elsevier.com/locate/trac
Sonochemical activation in heterogeneous (solid– liquid, liquid–liquid) systems has been traditionally thought to be a consequence of the mechanical effects of cavitation. Electrochemistry in biphasic systems is an area that had been exploited only minimally because of the difficulties of creating and maintaining emulsions in the absence of surfactants, which, if present, can affect the analysis detrimentally. The use of US to form emulsions ensures that, regardless of the relative densities of the two liquids, droplets of both are in contact with the electrode surface during voltammetric analysis [79]. Concerning applications of sonoelectrochemistry, examples of analytical, determinative applications can be found in the literature with both inorganic [76,77] and organic species [79–81]. The analysis of lead in petrol provides an example of the use of sonoanodic stripping voltammetry [82]. The process consists of sono-extraction with an organic phase sono-emulsified with the target medium. In this sense, the separation of a multitude of target species from one phase with their introduction into a second phase allows the application of analytical voltammetry.
4. US-assisted sample transport to (or position at) the detection point There are a number of ways in which US provides a unique or a better way for positioning the sample at, or transporting it to, the detector. The following are the most common. 4.1. US-assisted levitation Sometimes, a crucial aspect of sampling and sample storage relates to the contact between the sample and container walls. This situation is particularly problematic in microanalysis, where such effects can be decisive. One of the definitive aspects in this case is maintenance of sample composition without possible alterations. Changes can be caused by adsorption of the analyte at the walls or desorption of either the analyte or interfering substances from the walls. The best way to minimize these alterations is to perform at least some of the steps of an analytical method, especially sample pretreatment, without any contact between the sample and container walls. One possibility of handling small samples without this contact is to apply acoustical levitation. This sampling technique simplifies the analytical process and allows direct determination when applied with suitable detectors. Since it maintains the levitated object in a fixed position, the process under investigation is not disturbed by the influence of any surface contact other that the surrounding medium, commonly air. This is of importance when striving for single-molecule detection [83].
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
Acoustical levitation requires no specific physical properties of the sample, in contrast to most levitation techniques, such as those based on electrostatic or magnetic fields. Consequently, almost all samples – solid or liquid, conductor or insulator, magnetic or nonmagnetic – are acoustically levitable. The maximum diameter of a levitated sample is a function of the US wavelength and turns out to be about half the wavelength under ambient atmospheric conditions. Usually, the levitators operate with US frequencies of 15–100 kHz, resulting in wavelengths of 2.2– 0.34 cm. However, the maximum volume of a drop is related to liquid properties; specifically, surface tension and specific density. Fig. 1 shows typical levitation system. The use of a levitation technique in analytical chemistry should comply with the following requirements: (a) stable sample position with effortless adjustment and measuring; that means a vertical and horizontal gradient of the levitating force is desirable; (b) easy access to the sample; (c) no special sample properties; (d) a wide range of sample volumes, and; (e) low costs for supply and operation [84]. Only acoustical levitation meets all the above requirements, at least to a certain degree, but the use of this model of levitation in analytical chemistry is still at an early stage, so other techniques are frequently chosen at present. An important application field for acoustical levitation is the combination of micro and trace analysis, for which liquid–liquid extraction, solvent exchange, and analyte enrichment have been demonstrated successfully under
Trends
levitation conditions. Other analytical procedures that can be developed in this way include wet decomposition and solid-phase extraction [85]. Free levitation of microsamples is a good alternative for trace and ultra-trace analysis because it offers the possibility of ending up, after the necessary preconcentration and clean-up steps, with a final sample volume of 1–2 ll (preceding, e.g., injection into a GC-MS system). This property offers an important advantage over many official trace-analysis methods, with end volumes in the range 10–100 ll. A promising area for acoustic levitation is off-line enrichment by solvent evaporation in a levitated drop for microanalysis techniques, such as capillary electrophoresis (CE). Despite the great resolution power, the use of CE is hampered by its low detection sensitivity, which is inherent in the short path length when on-line absorption detectors are used. Sample volumes in the ll range can be enriched by solvent evaporation with the aid of acoustical levitation and then directly injected into a CE capillary. With this containerless approach, the sample is easily accessed and effects are reduced (e.g., contamination by compounds desorbing from, and analyte adsorption on container walls). The drawback is that not only the analytes of interest will be enriched but also any other non-volatile components present in the sample. This may lead to high ion concentrations that will impair CE performance [86,87]. The applicability of acoustical levitation to the investigation of atmospheric processes has also been highlighted [88] so a wealth of methods has been developed to study the microphysical properties of aerosols, their
Figure 1. Droplet levitated in a sound–pressure node of a standing ultrasonic wave: (a) reflector; (b) ultrasound transducer; (c) flow-through liquid microdispenser; (d) capillary. (From [88] with permission.)
http://www.elsevier.com/locate/trac
835
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
chemical compositions and their phase behavior. For this, particles are levitated to make them accessible to analytical monitoring. 4.2. US-assisted nebulization Direct injection of an aqueous or organic phase containing the analytes of interest by nebulization has been widely used with practically all types of absorption, emission or mass detectors based on atomization (e.g., by flame or plasma) [1]. Pneumatic and US nebulization (USN) are the usual ways to produce the aerosol, the latter being used mainly in plasma techniques. 4.2.1. For plasma-based spectrometry. The pneumatic nebulizer has for many years been the most universal sample-insertion device for plasma-based spectrometry. The inherent lack of transport efficiency, coupled with the continuing need for increased sensitivity, has promoted research into the use of US nebulizers to boost detection capabilities. In conclusion, USN in combination with a desolvation system has been found to improve the limits of detection provided by conventional pneumatic systems [89]. USN has made it possible to improve the introduction of organic solvents, otherwise made difficult by overloading and pyrolysis effects, because of the combination of two effects: more efficient nebulization; and, desolvation of the organic aerosol by a heater and cooler system. This methodology has been proposed, e.g., for the determination of trace elements in crude oil and its fractions by USN-ICP-MS after sample dissolution in toluene [90]. This approach is a promising alternative to time-consuming sample preparation by conventional or microwave-assisted digestion prior to analysis. USN has also improved the possibilities of some detection techniques, such as ICP-OES, where low concentrations of the target analytes are incompatible with the determination limit required. The combination of a FI system – for carrying out an on-line preconcentration step – and ICP-OES [91] through USN can solve this problem. Another example is the use of microflow US nebulizers that may solve the problems of coupling a liquid chromatograph (LC) to an atomic spectrometer that arise from the low transfer of the analyte across the LC/ ICP interface [92]. 4.2.2. Electrospray formation. Electrospray ionization emerged as a promising technique for interfacing LC and CE to mass spectrometry (MS). All types of MSs have been used with electrospray ionization. However, in the case of LC/MS, its use has been limited by restrictions on mobile-phase composition and volumetric flow-rate that are amenable to electrospraying. These restrictions are that the flow-rate does not exceed 5 ll/min, and does not possess high conductivity or high surface tension [1]. 836
http://www.elsevier.com/locate/trac
In order to improve the efficiency of the electrospray technique, researchers suggested that the spray should be produced by mechanical means, such as US vibration, instead of applying an electric field to make spray formation independent of the physical properties of the LC mobile phase and flow-rate [93]. In this way, LC mobile phases can be used, extending the range to include high flow-rates, high electrical conductivities and high-surface tension solutions. 4.3. Slurry preparation US slurry sampling is a fast, convenient preparation step that has been used successfully, as demonstrated by the numerous examples in the literature [1]. An advantage of this system in comparison with magnetic agitation and vortex mixing is that the analyte of interest is partly extracted into the liquid phase, due to the US action. The dislodging efficiency is nearly to 100% after relatively short time of US agitation [94]. Significant improvement in the precision of the slurry technique is achieved using a small US probe to mix the slurry in the autosampler cup just before introducing it into the detection system [95]. Nevertheless, this preparation step also has some shortcomings that limit its implementation, so only more experience and understanding of the critical parameters for the success of the method can lead to progress. Detection systems, such as flame atomic absorption spectroscopy, require very small particle size for slurries in order to avoid nebulizer clogging. This fine particle size is difficult to obtain in fibrous samples, unless US agitation is used for homogenization. This agitation helps in the formation of fine slurry through the rupture of particles because of both the cavitation phenomenon, which generates a high local temperature, and the mechanical action at the solid–liquid interface [96].
5. Conclusion This overview shows the versatility of US energy, which can participate in every step of the analytical process. As well as the many aspects of its involvement in sample preparation [97], US can also transport sample to the detector, and facilitate or perform detection. Better knowledge of the analytical uses of US will provide analytical chemists with a very versatile tool that will often be able to increase the application of laboratory equipment with low costs.
Acknowledgements The Spanish Comisi on Interministerial de Ciencia y Tecnologıa (CICyT) is gratefully acknowledged for
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
financial support (Project No. BQU-2002-1333). F. Priego-Capote is also grateful to the Ministerio de Educaci on y Ciencia for an FPU scholarship. References [1] M.D. Luque de Castro, J.L. Luque-Garcıa, Acceleration and Automation of Solid Sample Treatment, Elsevier, Amsterdam, The Netherlands, 2002. [2] V. Buckin, B. O’Driscoll, LabPlus Int. June (2002) 17. [3] V. Buckin, E. Kudryashov, B. O’Driscoll, Am. Lab. (Shelton, Conn.) 34 (5) (2002) 28. [4] L. Cartz, Nondestructive Testing Radiography, Ultrasonics, Liquid Penetrant, Magnetic Particle, Eddy Current, ASM International, Materials Park, Ohio, USA, 1995. [5] G.S. Constable, A.J. Lesser, E.B. Coughlin, J. Polym. Sci., Part B: Polym. Phys. 41 (2003) 1323. [6] D.J. McClements, Langmuir 12 (1996) 3454. [7] D.J. McClements, Adv. Colloid Interf. Sci. 37 (1991) 33. [8] M.J.W. Povey, M. Golding, D. Higgs, Y. Wang, Int. Dairy J. 9 (1999) 299. [9] C.M. Bryant, D.J. McClements, J. Sci. Food Agric. 79 (1999) 1754. [10] C.M. Bryant, D.J. McClements, Food Hydrocolloid 13 (1999) 439. [11] D.J. McClements, Langmuir 12 (1996) 3454. [12] F. Hinze, Beitrag zur Charakterisierung hochkonzentrierter Suspensionen mit submikronen Partikeln mittels Ultraschallspektroskopie, Dissertation, Technische Universit€ at Dresden, Germany, 2001. [13] L.D. Kachanovskaya, E.V. Datskevich, V.S. Sperkach, Yu.D. Usenko, Colloid Surface A: Physicochem. Eng. Aspect 106 (1996) 103. [14] S. Takeda, P.J. Goetz, Colloid Surface A: Physicochem. Eng. Aspect 143 (1998) 35. [15] P. Mougin, D. Wilkinson, K.J. Roberts, R. Jack, P. Kippax, Powder Technol. 134 (2003) 243. [16] D. Hibberd, A. Holmes, M. Garrood, A. Fillery-Travis, M. Robins, R. Challis, J. Colloid Interf. Sci. 193 (1997) 77. [17] K. Demetriades, D.J. McClements, Colloid Surface A: Physicochem. Eng. Aspect 150 (1999) 45. [18] N. Masuhara, B.C. Watson, M.W. Meisel, J. Low Temp. Phys. 121 (2000) 815. [19] F. Babick, S. Ripperger, Part. Syst. Charact. 19 (2002) 176. [20] A. Wanner, Mater. Sci. Eng. A248 (1998) 35. [21] A.H. Khafagy, Phys. Status Solidi A 186 (2001) 105. [22] V. Buckin, B. O’Driscoll, C. Smyth, A.C. Alting, R.W. Visschers, Spectrosc. Eur. 15 (1) (2003) 20. [23] J. Wu, F. Cubberley, Ultrasound Med. Biol. 23 (1) (1997) 129. [24] V. Buckin, E. Kudryashov, B. O’Driscoll, Pharm. Tech. Europe 14 (12) (2002) 33. [25] J.F. Schabron, J.F. Rovani, Petroleum Chem. 49 (2) (2004) 138. [26] D. Dalgleish, M. Alexander, M. Corredig, Food Hydrocolloid 18(5) (2004) 747. [27] B. O’Driscoll, C. Smyth, A.C. Alting, R.W. Visschers, V. Buckin, Am. Lab. (Shelton, Conn.) 35 (3) (2003) 54. [28] V. Buckin, C. Smyth, C. Dwyer, E. Kudryashov, Abstract, in: 227th ACS National Meeting, Anaheim, CA, USA, 28 March–1 April, 2004. [29] V. Buckin, E. Kudryashov, Polym. Paint Colour J. 192 (2002) 10. [30] K. Singh, K.N. Pandey, K.K. Debnath, R.S. Pal, D.K. Setua, Bull. Mater. Sci. 19 (1996) 587. [31] K.N. Pandey, K.K. Debnath, P.T. Rajagopalan, D.K. Setua, G.N. Mathur, J. Therm. Anal. 49 (1997) 281. [32] D.K. Setua, K.N. Pandey, A.K. Saxena, G.N. Mathur, J. Appl. Polym. Sci. 74 (1999) 480.
Trends [33] P.K. Sahoo, R. Mohapatra, A. Sahoo, S.K. Swain, J. Appl. Polym. Sci. 88 (2003) 3196. [34] A. Awasthi, J.P. Shukla, Ultrasonics 41 (2003) 477. [35] P.L. Ridgway, A.J. Hunt, M. Quinby-Hunt, R.E. Russo, Ultrasonics 37 (1999) 395. [36] T.E. G omez Alvarez-Arenas, J. Membrane Sci. 213 (2003) 195. [37] A. Drevermann, C. Pickmann, R. Tiefers, G. Zimmermann, Ultrasonics 42 (1–9) (2004) 105. [38] B.I. Kankia, V. Buckin, V.A. Bloomfield, Nucl. Acid Res. 29 (2001) 2795. [39] E. Kudryashov, T. Kapustina, S. Morrisey, V. Buckin, K. Dawson, J. Colloid Interf. Sci. 203 (1998) 59. [40] R.B. Schwarz, J.F. Vuorinen, J. Alloy Compd. 310 (2000) 243. [41] Y.M. Cheong, S.S. Kim, Y.S. Kim, J. Nucl. Mater. 303 (2002) 83. [42] Y. Fan, B. Tysoe, J. Sim, K. Mirkhani, A.N. Sinclair, F. Hornavar, H. Sildva, A. Szecket, R. Hardwick, Ultrasonics 41 (2003) 369. [43] M. Lei, A. Migliori, H. Ledbetter, Physica C (Amsterdam) 282–287 (1997) 1077. [44] Anon., Using Resonant Inspection in Production Environment, Quasar International, Inc., Albuquerque, NM, USA. http://www.qusarintl.com/ndi. [45] A. Migliori, T.W. Darling, Los Alamos National Laboratory, Report LALP-95-9, 1995. [46] A. Moreau, D. Levesque, M. Lord, M. Dubois, J.P. Monchalin, C. Padioleau, J.F. Bussiere, Ultrasonics 40 (2002) 1047. [47] M. Dubois, P.W. Lorraine, R.J. Filkins, T.E. Drake, K.R. Yawn, S.Y. Chiang, Ultrasonics 40 (2002) 809. [48] W. Gao, C. Glorieux, S.E. Kruger, K. Van de Rostyne, V. Gusev, W. Lauriks, J. Thoen, Mater. Sci. Eng. A 313 (2001) 170. [49] Y. Hong, S.D. Sharples, M. Clark, M.G. Somekh, Ultrasonics 42 (109) (2004) 515. [50] G. Lamouche, S. Bolognini, S.E. Kruger, Mater. Sci. Eng. A 370 (102) (2004) 401. [51] D. Lellinger, S. Tadjbach, I. Alig, Macromol. Symp. 184 (2002) 203. [52] A.I. Lavrentyev, J.T. Beals, Ultrasonics 38 (2000) 513. [53] A. Kulmyrzaev, C. Cancelliere, D.J. McClements, J. Food Eng. 46 (2000) 235. [54] R. Kazys, L. Svilainis, Ultrasonics 35 (1997) 367. [55] K.A. Van Den Abeele, A. Sutin, J. Carmeliet, P.A. Jonson, NDT & EInt. 34 (2001) 239. [56] F. He, J. Zhao, L. Zhang, X. Su, Talanta 59 (2003) 935. [57] B. Guo, Y. Yuan, Y. Wu, Q. Xie, S. Yao, Anal. Biochem. 305 (2002) 305. [58] H. Tao, W. Wei, S. Zhang, J. Photochem. Photobiol. A 161 (2004) 193. [59] L.L. Bao, Q.J. Xie, Y. Xu, W.Z. Wei, Anal. Lett. 32 (1999) 885. [60] H.O. Ghourchian, N. Kamo, T. Hosokawa, T. Akitaya, Talanta 41 (1994) 401. [61] D.Z. Shen, Z.Y. Li, L.H. Nie, S.Z. Yao, Anal. Chim. Acta 280 (1993) 209. [62] J.Z. Zhang, W.Z. Wei, T.A. Zhou, D.L. He, S.Z. Yao, Q.J. Xie, Talanta 53 (2000) 525. [63] H. Su, P. Williams, M. Thompson, Anal. Chem. 67 (1995) 885. [64] M. Valc arcel, M.D. Luque de Castro, Flow Injection Analysis: Principles and Applications, Ellis Horwood, Chichester, UK, 1987. [65] X. Su, X. Qu, W. Wei, S. Yao, Microchem. J. 59 (1998) 341. [66] X. Su, B. Yu, H. Tan, X. Yang, L. Nie, S. Yao, J. Pharm. Biomed. Anal. 16 (1998) 759. [67] X. Su, W. Wei, L. Nie, S. Yao, Analyst (Cambridge) 123 (1998) 221. [68] X. Su, L. Nie, S. Yao, Fresenius’ J. Anal. Chem. 360 (1998) 272. [69] S. Titiz-Sargut, J. Ulrich, Chem. Eng. Process. 42 (2003) 841. [70] P.J. Coghill, M.J. Millen, B.D. Sowerby, Miner. Eng. 15 (2002) 83. [71] H.W. Reinhardt, C.U. Grosse, Construct. Build. Mater. 18 (2004) 145.
http://www.elsevier.com/locate/trac
837
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
[72] P.D. Coates, S.E. Barnes, M.G. Sibley, E.C. Brown, H.G.M. Edwards, I.J. Scowen, Polymer 44 (2003) 5937. [73] M.S. Greenwood, J.A. Bamberger, Ultrasonics 39 (2002) 623. [74] P.R. Tecca, A. Galgaro, R. Genevois, A.M. Deganutti, Hydrological Process. 17 (2003) 1771. [75] J. Salazar, A. Tur o, J.A. Ch avez, M.J. Garcıa, Ultrasonics 42 (109) (2004) 155. [76] R.G. Compton, J.C. Eklund, F. Marken, Electroanalysis (NY) 9 (1997) 509. [77] A.J. Saterlay, R.G. Compton, Fresenius’ J. Anal. Chem. 367 (2000) 308. [78] L.S. Berstein, M.R. Zakin, E.B. Flint, K.S. Suslick, J. Phys. Chem. 100 (1996) 6612. [79] J.L. Hardcastle, C.J. Paterson, R.G. Compton, Electroanalysis (NY) 13 (2001) 899. [80] A.M. Oliveira Brett, F.M. Matysik, Bioelectrochem. Bioenerg. 42 (1997) 111. [81] A.M. Oliveira Brett, F.M. Matysik, Ultrason. Sonochem. 4 (1997) 125. [82] A.N. Blythe, R.P. Akkermans, R.G. Compton, Electroanalysis (NY) 12 (2000) 16. [83] W.B. Whitten, J.M. Ramsey, S. Arnold, B.V. Bronk, Anal. Chem. 63 (1991) 1027. [84] E. Welter, B. Neidhart, Fresenius’ J. Anal. Chem. 357 (1997) 345.
838
http://www.elsevier.com/locate/trac
[85] D. Skouteris, Stofftransport an Phasengrenzfl€ achen Ultraschalllevitierter Mikroproben, Ein neuer Anstz zur L€ osung atmosph€ arenchemischer Fragestellungen, Dissertation, PhilippsUniversit€ at, Germany, 1996. [86] M. Petersson, J. Nilsson, L. Wallman, T. Laurell, J. Johansson, S. Nilsson, J. Chromatogr. B 714 (1998) 39. [87] L. Schweitz, M. Petersson, T. Johansson, S. Nilsson, J. Chromatogr. A 892 (2000) 203. [88] C. Mund, R. Zellner, J. Mol. Struct. 661–662 (2003) 491. [89] Y. Madrid, M. Wu, Q. Jin, G. Hieftje, Anal. Chim. Acta 277 (1993) 1. [90] C. Duyck, N. Miekeley, C.L. Porto da Silveira, P. Szatmari, Spectrochim. Acta B 57 (2002) 1979. [91] J. Posta, A. Alimonti, F. Petrucci, S. Carola, Anal. Chim. Acta 325 (1996) 185. [92] M.A. Tarr, G. Zhu, R.F. Browner, Anal. Chem. 65 (1993) 1689. [93] J.F. Banks, J.P. Quinn, C.M. Whitehouse, Anal. Chem. 66 (1994) 3688. [94] J.C. Yu, K.F. Ho, S.C. Lee, Fresenius’ J. Anal. Chem. 369 (2001) 170. [95] R.A. Conte, M.T.C. Loos-Vollebregt, Fresenius’ J. Anal. Chem. 367 (2000) 722. [96] K. Ashley, R.N. Andreus, L. Cavajos, M. Demange, J. Anal. At. Spectrom. 16 (2001) 1147. [97] F. Priego-Capote, M.D. Luque de Castro, Trend Anal. Chem. 23 (2004).
Trends
Analytical uses of ultrasound II. Detectors and detection techniques F. Priego-Capote, M.D. Luque de Castro
Most analytical chemists know that ultrasound (US) can help steps involved in sample preparation (e.g., particularly in dealing with solid samples) and transportation to atomic detectors (e.g., with US nebulization or slurry formation). But not many are aware of other interesting and less common uses. We therefore place emphasis on US-based spectroscopic techniques – mainly, US-attenuation and US-velocity spectroscopy – and other techniques, such as voltammetry, which US assistance has dramatically improved to give so-called sonoelectrochemistry. Additionally, we comment on the use of US for improving sample conditioning and transport – namely, acoustic levitation, nebulization and slurry formation. This article constitutes an alert on the potential of US energy, which analytical chemists almost neglected. ª 2004 Elsevier Ltd. All rights reserved.
1. Introduction F. Priego-Capote, M.D. Luque de Castro* Department of Analytical Chemistry, Annex C-3, Faculty of Sciences, University of Cordoba, Campus of Rabanales, E-14071 C ordoba, Spain
*Corresponding author. Tel./Fax: +34-957-21-8615; E-mail: [email protected]
Although the use of ultrasound (US) for assisting different steps in sample preparation has not enjoyed the development that the effects of US deserve, there are not too many analytical chemists who do not know to some extent the applications of US to improve, accelerate or automate the preliminary steps of the analytical process. However, this is not so when it comes to the variety of US uses in the detection step, which can vary from improving the way that the sample reaches the detector or the detection itself to being the basis of unknown, or little known, detection techniques. With the aim of diffusing knowledge of the use of US in the detection step, we pay special attention in this article to: • US spectroscopy (USS) in its different modes; • the improvement, with US, in established techniques, such as voltammetry, which has led to sonoelectrochemistry; and, finally,
0165-9936/$ - see front matter ª 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2004.07.013
• the different ways in which US can facilitate the sample to reach the detector (namely, levitation, nebulization and slurry formation).
2. US-based detection techniques 2.1. USS A common error in many textbooks is to define spectroscopy as a subject dealing with the interaction of electromagnetic radiation and matter. This is explained by the dominance of the electromagnetic wave. However, USS uses an alternative wave type – the high-frequency acoustic wave. When this energy acts over a given system or sample, inter-molecular forces are generated with the subsequent oscillating compression and decompression of the US wave [1–3]. These forces cause molecular arrangements in the sample, resulting in inter-molecular attraction or repulsion. No alteration of the sample is provided by the small amplitudes of deformation employed in analytical chemistry. The novel technique of USS enables, as explained below, the monitoring of certain parameters caused by US waves as they propagate through a sample. The main characteristic of high-resolution USS is the possibility of carrying out direct, non-destructive measurements [2–4]. This advantage makes this technique very competitive with other well-known detection techniques. Thus, minimal or no pre-treatment is required and measurements of the intrinsic properties of the system are achieved without sampling, the addition of anything, or a change of state. 829
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
In addition to the characteristics outlined above, USS can be classified in the group of universal detection systems, because US waves can propagate through most materials, including those that are opaque to light. Moreover, because US waves are synthesized electronically, it is relatively easy to change the wavelength of an US wave in comparison with optical waves as the latter originate from a light source so special care must be taken to ensure spectral purity. Although these features would qualify USS to be a powerful technique, its application has been limited to certain areas because of several technical factors, namely limited resolution, the need for large sample volumes and complicated measuring procedures. Recent advances in the modern principles of US measurements, electronics and digital processing have solved problems arising from these limitations [2]. Concerning objectives, there are two general ways of using USS as regards sample state, namely: measuring the current state of the system under study; or, monitoring its chemical and structural transformations. The two main and most frequently measured parameters in high-resolution USS are US attenuation and US velocity. While US-velocity spectroscopy (USVS) is mainly used for the study of inter- and intra-molecular processes [5], US-attenuation spectroscopy (USAS, also known as US-extinction spectroscopy) has found its major application in particle sizing [6], including studies of processes in relation to particle size. Attenuation is determined by the energy losses in compressions and decompressions of US waves, which includes absorption and scattering contributions. 2.1.1. USAS. Many publications related to USAS have appeared since 1991 when McClements’s review of this technique was published [7]. The greatest advantage of USAS in comparison to other techniques with similar applications, such as light scattering, is its capability to characterize intact, concentrated, dispersed systems. As measurements of attenuation do not require precise stability of temperature, they can be performed on large samples. In addition to information concerning particle size, other information about the compressibility and the state of aggregation of the dispersed phase can be obtained. The measurements may be made rapidly and in-line, in a pipeline or in a tank [8]. Significant applications of USAS [9–18] can be seen in Table 1. USAS has become one of the most promising measurement techniques in the field of on-line particlesystem characterization [14], including protein-stabilized emulsions. In comparison to optical techniques, USAS has no limits in applicability to opaque and/or highly concentrated liquid dispersions so it eliminates the need for sample dilution and preparation. Furthermore, data can be obtained from samples with a wide range of sizes (10 nm–1 mm). This, in turn, offers an 830
http://www.elsevier.com/locate/trac
Table 1. Selected applications of ultrasound-attenuation spectroscopy-based detectors Application
Reference
Molecular characterization of proteins Whey-protein-aggregation studies Particle-size measurements Monitoring particle-size distribution of highly concentrated slurries Particle-size-distribution analysis of casein in water Particle-sizing studies Monitoring of oil-in-water emulsions Monitoring the flocculation of droplets in protein-stabilized emulsions Characterization of quantum states at ultralow temperature
[9] [10] [11–13] [14] [8] [15] [16] [17] [18]
opportunity to study particle systems in their original state of dispersion, thus facilitating the determination of agglomeration or flocculation parameters [16] (e.g., monitoring whey protein aggregation [10] or flocculation of droplets in protein-stabilized emulsions [17]). Various physical mechanisms are responsible for the dependence of sound attenuation on particle size, as follows: • There is an analogy to optical acoustic scattering, which includes all mechanisms responsible for redirecting sound waves incident on particles. • A second phenomenon is thermal coupling, referring to the non-steady heat exchange between particles and the continuous phase that occurs when temperature fluctuations in the respective phases, caused by compression fluctuations, differ in phase and magnitude. • Another phenomenon is viscous coupling that can be defined as the retardation of the oscillatory motion of particles experienced within the continuous phase as a result of inertia differences. • Both thermal and viscous coupling are relaxation processes that reduce the original mechanical disturbances (i.e., sound intensity), so they are also involved in the so-called dissipative attenuation mechanisms. Their magnitudes are significant for particles much smaller than the US wavelength, in which acoustic scattering can be neglected. • In addition to the size-dependent attenuation mechanisms, US absorption occurs in both the continuous and the disperse phase. Total absorption therefore depends on the composition of the disperse system [19]. Nevertheless, because of strong attenuation caused by scattering, methods based on attenuation measurements cannot very often be used on small-sized specimens of such materials. Examples of this behavior are many ceramic materials with certain porosity or metallic
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
materials, which are generally weakly attenuating materials [20]. In these cases, the most widely used experimental techniques are based on measuring velocities of propagating pulses or wave packets. Concerning other independent applications of particle sizing, particularly important investigations are those developed at low temperature (e.g., the joint use of a Fourier transform device and USAS in the determination of the energy gap of superfluid 3 He–B at very low temperatures, where no other methods provide successful results [18]). 2.1.2. USVS. US velocity is determined by the density and the elasticity of the medium. This parameter is extremely sensitive to the molecular organization and the intermolecular interactions in the medium and can be used to analyze a broad range of molecular processes. However, its application requires high-resolution measurements that cannot be carried out in large samples because of the difficulties associated with temperature control [2]. USVS has been used both with USAS [21–29] and as a stand-alone detection technique [6,20,30–39], as can be seen in Table 2. In spite of the fact that in the past the most widely used approach for measurement of US parameters, whether attenuation or velocity, was based on the pulse technique, an important shortcoming is the limit in resolution by the path length of the pulse or sample size. In this sense, new commercialized high-resolution spectrometers have established novel principles for the measurement of US parameters. The novel principles
Trends
utilize modern advances in US design, electronics and digital processing, thus enabling US measurements with record resolution (down to 10)5% for US velocity) in a broad range of the sample volumes, down to just one droplet [2].
2.1.3. Other US-based techniques. Resonant USS (RUSS) is a relatively new technique for measuring the resonant frequencies of a sample of nearly ideal geometry and with known symmetry, dimensions and mass. In RUSS, a parallelepiped, cylinder or sphere of the material of interest is suspended between two transducers across opposite corners (in the cases of a parallelepiped or a cylinder). One transducer excites the specimen through a specified range of increasing vibration frequencies. At the opposite transducer, the response of the solid to the signal is converted to an electrical signal and fed to a computer for analysis. The resonance peaks are amplifications of waves through constructive interference within a solid at specific frequencies. By analyzing the spectrum of waves that are scattered by the object, one can determine its resonant frequencies. These can then reveal information about the material properties or geometry of the item [40]. RUSS has been satisfactorily applied (see Table 3) in the characterization of explosively welded clad rods [41], determination of elastic stiffness of materials [42], the high-precision measurements in small samples of elastic constant (e.g., GaN, quasi-crystals and perovskites) [43], and quality control (QC) in both manufacture of small
Table 2. Selected applications of ultrasound-velocity spectroscopy (USVS) alone or in conjunction with ultrasound-attenuation spectroscopy (USAS) Application
Technique
Reference
Studies of mechanical properties of glasses Material analysis Diagnosis of bone diseases Pharmaceutical research Characterization of pyrolysis stability of oil Studies of the acid gelation of milk Evaluation of heat stability of calcium-fortified milk Real-time analysis of chemical reactions Determination of critical micelle concentration Evaluation of the ring-opening metathesis polymerization of dicyclopentadiene Elastic modulus measurements of extremely porous ceramic materials Miscibility determination of isobutylene-co-isoprene rubber Miscibility determination of polychloroprene Characterization of binary blends and compatability Compatibility and miscibility studies Intermolecular association studies Elastic coefficient measurements of paper Study of membrane filters On-line process control of directional solidification Investigation of DNA condensation Determination of compressibility of alkyltrimethylammonium bromide micelles
USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USAS/USVS USVS USVS USVS USVS USVS USVS USVS USVS USVS USVS USVS USVS
[21] [22] [23] [24] [25] [26] [27] [28] [29] [6] [20] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]
http://www.elsevier.com/locate/trac
831
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
Table 3. Selected applications of other ultrasound-based detection techniques Application
Technique
Reference
Characterization of explosively welded clad rods Determination of the elastic stiffness of materials High-precision measurements of elastic constant in small samples Quality control in manufacturing process Determination of structural integrity in construction Crystallographic texture determination Inspection of composite parts in manufacturing process Microstructural characterization of polycrystalline materials Characterization of elastic properties of materials Characterization of evolution of microstructure of steel at various temperatures Mechanical characterization of polymer films Determination of diffusion bond strength Characterization of aerated foods Detection and characterization of delaminations in thin composite plates diagnostics of micro-damage of materials Determination of Mycobacterium tuberculosis Monitoring the oxidative damage induced by the vitamin C–Fe(III) system Inhibition studies on the growth of Escherichia coli Determination of a-amylase Determination of rheumatoid factor End-point determination in titration Monitoring of mutagenic process Study of DNA–platinum-based drug interaction mechanism Determination of nitrate and nitrite Determination of total NH3 and total CO2 in blood Determination of sulfite in wines and fruit juices Determination of volatile acidity of fermentation products
RUSSa RUSS RUSS RUSS RUSS LGUSSb LGUSS LGUSS LGUSS LGUSS URSc URS URS URS NEWSd AWISe AWIS AWIS AWIS AWIS AWIS AWIS AWIS AWIS AWIS AWIS AWIS
[41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [65] [66] [67] [68]
a
RUSS, resonant ultrasound spectroscopy. LGUSS, laser-generated ultrasound spectroscopy. c URS, ultrasonic reflection spectroscopy. d NEWS, non-linear elastic wave spectroscopy. e AWIS, acoustic wave impedance spectroscopy. b
parts (e.g., aircraft landing gears and wheels and precision ball-bearings) [44] and construction of bridges in order to test the structural integrity [45]). It is also worth commenting on the technique known as laser-generated USS (LGUSS), which uses one shortpulsed laser to generate US waves and another laser coupled to an interferometer to detect the corresponding mechanical displacements [46]. The technique yields huge savings in inspection time in comparison to conventional US techniques. Unfortunately, for some material and process combinations, LGUSS suffers from a lack of sensitivity. Averaging signals compensate for this lack at the cost of increased inspection time and decreased savings. At present, the CO2 laser is used to generate the US waves. However, although efficient, the CO2 wavelength and its pulse shape are not optimal [47]. Applications that take advantage of this technique [46–50] can be seen in Table 3. Another US-based technique is US reflection spectroscopy (URS) that has been used relatively rarely compared with USAS or USVS, with applications including determination of the viscoelastic properties of polymer films [51], measurement of diffusion bond strength be832
http://www.elsevier.com/locate/trac
tween two identical materials [52], detection of changes in bubble characteristics in aerated food (where USAS and USVS are unsuitable because of the high attenuation of the US signal that cannot be propagated far enough through the sample [53]) and detection of delaminations in thin composite plates [54] (see Table 3). A specific group of techniques have been developed in order to quantify the level of non-linearity in the elastic response of materials containing structural inhomogeneity and damage. These techniques have been called non-linear elastic wave spectroscopy (NEWS) and they are much more sensitive to damage-related structural alterations than any other technique used for investigation of linear material parameters, such as US velocity or attenuation [55]. Finally, acoustic wave impedance spectroscopy (AWIS) has found several applications in many fields. The detector is constructed by connecting a piezoelectric quartz crystal and one or two conductivity electrodes in series to make the feedback circuit of the oscillator, and it can be represented by an electrical equivalent circuit. The fundamentals of AWIS applications are the change of any physical or chemical property of a system (e.g.,
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
solution or culture) because of any phenomenon (e.g., bacterial growth in culture medium), which causes variations of equivalent circuit parameters of piezoelectric quartz crystal. The changes in equivalent circuit parameters reflect the phenomenon. It is known that the AWIS detector is sensitive, not only to the mass change at the electrode surface, but also to the density–viscosity change of the test solution. The main advantages are high sensitivity, simplicity of use, and capability of providing multi-dimensional and real-time information to reflect some physical and chemical properties of the investigated system [56,57]. Table 3 shows that AWIS has been used in a number of applications [56–63]. Moreover, the coupling between an AWIS detector and flow injection (FI) systems can have the advantages of automation and reduced consumption of reagents and samples [64]. Table 3 also shows examples of this coupling [65–68].
2.2. US for monitoring evolving systems The characteristics of US-detection techniques (e.g., nonintrusiveness, fast response and potential for application at high pressures and temperatures) make them particularly useful in monitoring evolving systems in general and for on-line monitoring of industrial processes in particular. There are many publications about the construction and use of US sensors for these tasks, for which practically all types of US-based techniques have been used. The US signal can penetrate through the walls of a given line or vessel, offering the advantage of a truly non-invasive, non-destructive technique. A US pulse of a certain energy emitted from a vibrator (transducer) into a medium propagates through this medium and reaches the receiver at a lower energy after a given time. US velocity monitoring has been applied in crystallization processes (e.g., to satisfy the customer’s requirements in terms of product purity and crystal-size distribution and the manufacturer’s economical requirements). One of the main variables to be controlled in crystallization is the supersaturation in the solution, which is defined as the difference between actual and equilibrium concentrations of a solution. The USmeasuring technique enables the range of supersaturation and metastability to be determined. According to this technique, proposed by Omar and Ulrich, the metastable limit, which defines the operating zone of the industrial crystallizer, is a function of the US velocity and temperature [69]. Another industrial application is the control of grinding by on-line particle size analyzers. Grinding to a finer size than necessary results in reduced throughput, higher energy costs and increased consumption of mill liners, grinding media and reagents. Too coarse a grind results in reduced recovery of valuable minerals. The benefits of US-based control of grinding are better milling
Trends
efficiency, more stable operation, higher throughput and improved downstream processing. Industry requirements for an on-line slurry particle-size analyzer include direct on-line measurement without dilution, rugged and reliable equipment, and easy calibration and standardization, for which US techniques provide a number of advantages [70]. US-velocity monitoring is also used for QC in production of admixtures and new binders and for QC in concrete production [71], for which a testing device has been developed to monitor the setting and the hardening of cement materials in order to obtain reproducible results. The method is adjusted to concrete and can also be used for materials such as gypsum, lime, starch and other stiffeners. Another case is the monitoring of polymer melt extrusion, where the operator can use on-line US-velocity measurements to detect in a fast, sensitive way a change in this process, but not the cause of the change [72]. Combining measurements of US velocity and attenuation with gamma-ray transmission to provide a full size distribution of the particles in range 0.1–1000 lm, CSIRO Minerals has developed an in-stream slurry particle-size analyzer for use in grinding circuits [70]. These types of measurements are also the basis for the development of a highly sensitive, self-calibrating, on-line sensor to measure the density, the velocity and the attenuation of US. This approach can be applied to a liquid or slurry flowing through a pipeline, although it is also valid for measurements made in vessels. Interesting applications in this context are the determination of the density and the concentration of solids in radioactive waste slurries. This probe configuration does not depend upon passage of US through the fluid; the density measurement depends upon only US reflection at the walls of the container, so the probe can be used to measure the densities of very attenuative slurries [73]. Moreover, by basing the measurement on multiple reflections, the sensitivity of the instrument is increased by the power of the reflection coefficient. The measurement method is self-calibrating because the measurement of acoustic impedance is independent of changes in the voltage of the pulsed source. Field observations of debris flow have also benefited of the evolution of US sensors; specifically, direct measurements of the hydrological conditions for the occurrence of debris flows in the sense of determining flow behavior are of the outmost importance in developing effective flow-prevention techniques [74]. Automation in the food industry requires fast, reliable measurements of the physical properties of materials during processing. The possibility of carrying out these measurements on-line provides benefits to the industry in terms of better control of product quality, improved processing efficiencies and reduction in wastage. Moreover, operators can adjust processing parameters during, rather than after, production runs [75]. http://www.elsevier.com/locate/trac
833
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
3. US-improved technique: sonoelectroanalysis The coupling of US power with well-established but under-exploited electrochemical techniques, such as voltammetry, particularly stripping voltammetry, has led to the emergence of the powerful new analytical technique of sonoelectroanalysis. Where classical electroanalytical techniques were plagued with electrodefouling and/or sensitivity limitations, the introduction of US into the system has provided a dramatic increase in analytical efficiency and substrate applicability, predominantly through enhanced mass transport and electrode–surface activation. Sonoelectroanalysis has been applied to a range of modern analytical problems, allowing sensitive determination of a wide number of analytes from a variety of otherwise hostile matrices. This section presents a view of recent advances in this field. This combination of technologies has allowed the improvement of such techniques, which have hitherto been relatively limited, largely because of passivation, reliability and sensitivity problems. US equipment working in the range of 20–100 kHz is relatively inexpensive and readily available, and has proved to be an excellent technology for electroanalysis (some high frequency (500 kHz) equipment has also been used for more fundamental investigations of mechanisms). Application of US leads to a major enhancement of sensitivity of stripping analysis because of the significant increase in mass transport resulting from acoustic streaming. This gives the ability to make quantitative measurements in complex media with a minimum of sample pretreatment because cavitation cleans the electrode surface in situ [76,77]. Moreover, it is possible to extract electroactive species from binding sites in organic matrices and to emulsify otherwise immiscible organic and aqueous layers without the need for additional emulsifying agents. The benefits of coupling US power with well-established electrochemical techniques have rapidly made sonoelectroanalysis a powerful new tool for analytical chemists. Moreover, sonoelectroanalysis offers a real alternative to the current industry standards of AAS and ICP–MS, which carry much greater cost and time restrictions. An additional aspect is that the sonovoltammetric techniques lend themselves well to miniaturization, and portable versions of sonovoltammetric equipment are currently undergoing field trials. The influence of US on chemical processes can produce (e.g., in homogeneous sonochemistry, via acoustic hot spots) novel chemical species [78], providing enhanced yields (energy savings) and/or selectivity (reduced waste) of existing reactions, with the possibility of replacing hazardous reagents and making certain reactions ‘‘green’’. 834
http://www.elsevier.com/locate/trac
Sonochemical activation in heterogeneous (solid– liquid, liquid–liquid) systems has been traditionally thought to be a consequence of the mechanical effects of cavitation. Electrochemistry in biphasic systems is an area that had been exploited only minimally because of the difficulties of creating and maintaining emulsions in the absence of surfactants, which, if present, can affect the analysis detrimentally. The use of US to form emulsions ensures that, regardless of the relative densities of the two liquids, droplets of both are in contact with the electrode surface during voltammetric analysis [79]. Concerning applications of sonoelectrochemistry, examples of analytical, determinative applications can be found in the literature with both inorganic [76,77] and organic species [79–81]. The analysis of lead in petrol provides an example of the use of sonoanodic stripping voltammetry [82]. The process consists of sono-extraction with an organic phase sono-emulsified with the target medium. In this sense, the separation of a multitude of target species from one phase with their introduction into a second phase allows the application of analytical voltammetry.
4. US-assisted sample transport to (or position at) the detection point There are a number of ways in which US provides a unique or a better way for positioning the sample at, or transporting it to, the detector. The following are the most common. 4.1. US-assisted levitation Sometimes, a crucial aspect of sampling and sample storage relates to the contact between the sample and container walls. This situation is particularly problematic in microanalysis, where such effects can be decisive. One of the definitive aspects in this case is maintenance of sample composition without possible alterations. Changes can be caused by adsorption of the analyte at the walls or desorption of either the analyte or interfering substances from the walls. The best way to minimize these alterations is to perform at least some of the steps of an analytical method, especially sample pretreatment, without any contact between the sample and container walls. One possibility of handling small samples without this contact is to apply acoustical levitation. This sampling technique simplifies the analytical process and allows direct determination when applied with suitable detectors. Since it maintains the levitated object in a fixed position, the process under investigation is not disturbed by the influence of any surface contact other that the surrounding medium, commonly air. This is of importance when striving for single-molecule detection [83].
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
Acoustical levitation requires no specific physical properties of the sample, in contrast to most levitation techniques, such as those based on electrostatic or magnetic fields. Consequently, almost all samples – solid or liquid, conductor or insulator, magnetic or nonmagnetic – are acoustically levitable. The maximum diameter of a levitated sample is a function of the US wavelength and turns out to be about half the wavelength under ambient atmospheric conditions. Usually, the levitators operate with US frequencies of 15–100 kHz, resulting in wavelengths of 2.2– 0.34 cm. However, the maximum volume of a drop is related to liquid properties; specifically, surface tension and specific density. Fig. 1 shows typical levitation system. The use of a levitation technique in analytical chemistry should comply with the following requirements: (a) stable sample position with effortless adjustment and measuring; that means a vertical and horizontal gradient of the levitating force is desirable; (b) easy access to the sample; (c) no special sample properties; (d) a wide range of sample volumes, and; (e) low costs for supply and operation [84]. Only acoustical levitation meets all the above requirements, at least to a certain degree, but the use of this model of levitation in analytical chemistry is still at an early stage, so other techniques are frequently chosen at present. An important application field for acoustical levitation is the combination of micro and trace analysis, for which liquid–liquid extraction, solvent exchange, and analyte enrichment have been demonstrated successfully under
Trends
levitation conditions. Other analytical procedures that can be developed in this way include wet decomposition and solid-phase extraction [85]. Free levitation of microsamples is a good alternative for trace and ultra-trace analysis because it offers the possibility of ending up, after the necessary preconcentration and clean-up steps, with a final sample volume of 1–2 ll (preceding, e.g., injection into a GC-MS system). This property offers an important advantage over many official trace-analysis methods, with end volumes in the range 10–100 ll. A promising area for acoustic levitation is off-line enrichment by solvent evaporation in a levitated drop for microanalysis techniques, such as capillary electrophoresis (CE). Despite the great resolution power, the use of CE is hampered by its low detection sensitivity, which is inherent in the short path length when on-line absorption detectors are used. Sample volumes in the ll range can be enriched by solvent evaporation with the aid of acoustical levitation and then directly injected into a CE capillary. With this containerless approach, the sample is easily accessed and effects are reduced (e.g., contamination by compounds desorbing from, and analyte adsorption on container walls). The drawback is that not only the analytes of interest will be enriched but also any other non-volatile components present in the sample. This may lead to high ion concentrations that will impair CE performance [86,87]. The applicability of acoustical levitation to the investigation of atmospheric processes has also been highlighted [88] so a wealth of methods has been developed to study the microphysical properties of aerosols, their
Figure 1. Droplet levitated in a sound–pressure node of a standing ultrasonic wave: (a) reflector; (b) ultrasound transducer; (c) flow-through liquid microdispenser; (d) capillary. (From [88] with permission.)
http://www.elsevier.com/locate/trac
835
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
chemical compositions and their phase behavior. For this, particles are levitated to make them accessible to analytical monitoring. 4.2. US-assisted nebulization Direct injection of an aqueous or organic phase containing the analytes of interest by nebulization has been widely used with practically all types of absorption, emission or mass detectors based on atomization (e.g., by flame or plasma) [1]. Pneumatic and US nebulization (USN) are the usual ways to produce the aerosol, the latter being used mainly in plasma techniques. 4.2.1. For plasma-based spectrometry. The pneumatic nebulizer has for many years been the most universal sample-insertion device for plasma-based spectrometry. The inherent lack of transport efficiency, coupled with the continuing need for increased sensitivity, has promoted research into the use of US nebulizers to boost detection capabilities. In conclusion, USN in combination with a desolvation system has been found to improve the limits of detection provided by conventional pneumatic systems [89]. USN has made it possible to improve the introduction of organic solvents, otherwise made difficult by overloading and pyrolysis effects, because of the combination of two effects: more efficient nebulization; and, desolvation of the organic aerosol by a heater and cooler system. This methodology has been proposed, e.g., for the determination of trace elements in crude oil and its fractions by USN-ICP-MS after sample dissolution in toluene [90]. This approach is a promising alternative to time-consuming sample preparation by conventional or microwave-assisted digestion prior to analysis. USN has also improved the possibilities of some detection techniques, such as ICP-OES, where low concentrations of the target analytes are incompatible with the determination limit required. The combination of a FI system – for carrying out an on-line preconcentration step – and ICP-OES [91] through USN can solve this problem. Another example is the use of microflow US nebulizers that may solve the problems of coupling a liquid chromatograph (LC) to an atomic spectrometer that arise from the low transfer of the analyte across the LC/ ICP interface [92]. 4.2.2. Electrospray formation. Electrospray ionization emerged as a promising technique for interfacing LC and CE to mass spectrometry (MS). All types of MSs have been used with electrospray ionization. However, in the case of LC/MS, its use has been limited by restrictions on mobile-phase composition and volumetric flow-rate that are amenable to electrospraying. These restrictions are that the flow-rate does not exceed 5 ll/min, and does not possess high conductivity or high surface tension [1]. 836
http://www.elsevier.com/locate/trac
In order to improve the efficiency of the electrospray technique, researchers suggested that the spray should be produced by mechanical means, such as US vibration, instead of applying an electric field to make spray formation independent of the physical properties of the LC mobile phase and flow-rate [93]. In this way, LC mobile phases can be used, extending the range to include high flow-rates, high electrical conductivities and high-surface tension solutions. 4.3. Slurry preparation US slurry sampling is a fast, convenient preparation step that has been used successfully, as demonstrated by the numerous examples in the literature [1]. An advantage of this system in comparison with magnetic agitation and vortex mixing is that the analyte of interest is partly extracted into the liquid phase, due to the US action. The dislodging efficiency is nearly to 100% after relatively short time of US agitation [94]. Significant improvement in the precision of the slurry technique is achieved using a small US probe to mix the slurry in the autosampler cup just before introducing it into the detection system [95]. Nevertheless, this preparation step also has some shortcomings that limit its implementation, so only more experience and understanding of the critical parameters for the success of the method can lead to progress. Detection systems, such as flame atomic absorption spectroscopy, require very small particle size for slurries in order to avoid nebulizer clogging. This fine particle size is difficult to obtain in fibrous samples, unless US agitation is used for homogenization. This agitation helps in the formation of fine slurry through the rupture of particles because of both the cavitation phenomenon, which generates a high local temperature, and the mechanical action at the solid–liquid interface [96].
5. Conclusion This overview shows the versatility of US energy, which can participate in every step of the analytical process. As well as the many aspects of its involvement in sample preparation [97], US can also transport sample to the detector, and facilitate or perform detection. Better knowledge of the analytical uses of US will provide analytical chemists with a very versatile tool that will often be able to increase the application of laboratory equipment with low costs.
Acknowledgements The Spanish Comisi on Interministerial de Ciencia y Tecnologıa (CICyT) is gratefully acknowledged for
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
financial support (Project No. BQU-2002-1333). F. Priego-Capote is also grateful to the Ministerio de Educaci on y Ciencia for an FPU scholarship. References [1] M.D. Luque de Castro, J.L. Luque-Garcıa, Acceleration and Automation of Solid Sample Treatment, Elsevier, Amsterdam, The Netherlands, 2002. [2] V. Buckin, B. O’Driscoll, LabPlus Int. June (2002) 17. [3] V. Buckin, E. Kudryashov, B. O’Driscoll, Am. Lab. (Shelton, Conn.) 34 (5) (2002) 28. [4] L. Cartz, Nondestructive Testing Radiography, Ultrasonics, Liquid Penetrant, Magnetic Particle, Eddy Current, ASM International, Materials Park, Ohio, USA, 1995. [5] G.S. Constable, A.J. Lesser, E.B. Coughlin, J. Polym. Sci., Part B: Polym. Phys. 41 (2003) 1323. [6] D.J. McClements, Langmuir 12 (1996) 3454. [7] D.J. McClements, Adv. Colloid Interf. Sci. 37 (1991) 33. [8] M.J.W. Povey, M. Golding, D. Higgs, Y. Wang, Int. Dairy J. 9 (1999) 299. [9] C.M. Bryant, D.J. McClements, J. Sci. Food Agric. 79 (1999) 1754. [10] C.M. Bryant, D.J. McClements, Food Hydrocolloid 13 (1999) 439. [11] D.J. McClements, Langmuir 12 (1996) 3454. [12] F. Hinze, Beitrag zur Charakterisierung hochkonzentrierter Suspensionen mit submikronen Partikeln mittels Ultraschallspektroskopie, Dissertation, Technische Universit€ at Dresden, Germany, 2001. [13] L.D. Kachanovskaya, E.V. Datskevich, V.S. Sperkach, Yu.D. Usenko, Colloid Surface A: Physicochem. Eng. Aspect 106 (1996) 103. [14] S. Takeda, P.J. Goetz, Colloid Surface A: Physicochem. Eng. Aspect 143 (1998) 35. [15] P. Mougin, D. Wilkinson, K.J. Roberts, R. Jack, P. Kippax, Powder Technol. 134 (2003) 243. [16] D. Hibberd, A. Holmes, M. Garrood, A. Fillery-Travis, M. Robins, R. Challis, J. Colloid Interf. Sci. 193 (1997) 77. [17] K. Demetriades, D.J. McClements, Colloid Surface A: Physicochem. Eng. Aspect 150 (1999) 45. [18] N. Masuhara, B.C. Watson, M.W. Meisel, J. Low Temp. Phys. 121 (2000) 815. [19] F. Babick, S. Ripperger, Part. Syst. Charact. 19 (2002) 176. [20] A. Wanner, Mater. Sci. Eng. A248 (1998) 35. [21] A.H. Khafagy, Phys. Status Solidi A 186 (2001) 105. [22] V. Buckin, B. O’Driscoll, C. Smyth, A.C. Alting, R.W. Visschers, Spectrosc. Eur. 15 (1) (2003) 20. [23] J. Wu, F. Cubberley, Ultrasound Med. Biol. 23 (1) (1997) 129. [24] V. Buckin, E. Kudryashov, B. O’Driscoll, Pharm. Tech. Europe 14 (12) (2002) 33. [25] J.F. Schabron, J.F. Rovani, Petroleum Chem. 49 (2) (2004) 138. [26] D. Dalgleish, M. Alexander, M. Corredig, Food Hydrocolloid 18(5) (2004) 747. [27] B. O’Driscoll, C. Smyth, A.C. Alting, R.W. Visschers, V. Buckin, Am. Lab. (Shelton, Conn.) 35 (3) (2003) 54. [28] V. Buckin, C. Smyth, C. Dwyer, E. Kudryashov, Abstract, in: 227th ACS National Meeting, Anaheim, CA, USA, 28 March–1 April, 2004. [29] V. Buckin, E. Kudryashov, Polym. Paint Colour J. 192 (2002) 10. [30] K. Singh, K.N. Pandey, K.K. Debnath, R.S. Pal, D.K. Setua, Bull. Mater. Sci. 19 (1996) 587. [31] K.N. Pandey, K.K. Debnath, P.T. Rajagopalan, D.K. Setua, G.N. Mathur, J. Therm. Anal. 49 (1997) 281. [32] D.K. Setua, K.N. Pandey, A.K. Saxena, G.N. Mathur, J. Appl. Polym. Sci. 74 (1999) 480.
Trends [33] P.K. Sahoo, R. Mohapatra, A. Sahoo, S.K. Swain, J. Appl. Polym. Sci. 88 (2003) 3196. [34] A. Awasthi, J.P. Shukla, Ultrasonics 41 (2003) 477. [35] P.L. Ridgway, A.J. Hunt, M. Quinby-Hunt, R.E. Russo, Ultrasonics 37 (1999) 395. [36] T.E. G omez Alvarez-Arenas, J. Membrane Sci. 213 (2003) 195. [37] A. Drevermann, C. Pickmann, R. Tiefers, G. Zimmermann, Ultrasonics 42 (1–9) (2004) 105. [38] B.I. Kankia, V. Buckin, V.A. Bloomfield, Nucl. Acid Res. 29 (2001) 2795. [39] E. Kudryashov, T. Kapustina, S. Morrisey, V. Buckin, K. Dawson, J. Colloid Interf. Sci. 203 (1998) 59. [40] R.B. Schwarz, J.F. Vuorinen, J. Alloy Compd. 310 (2000) 243. [41] Y.M. Cheong, S.S. Kim, Y.S. Kim, J. Nucl. Mater. 303 (2002) 83. [42] Y. Fan, B. Tysoe, J. Sim, K. Mirkhani, A.N. Sinclair, F. Hornavar, H. Sildva, A. Szecket, R. Hardwick, Ultrasonics 41 (2003) 369. [43] M. Lei, A. Migliori, H. Ledbetter, Physica C (Amsterdam) 282–287 (1997) 1077. [44] Anon., Using Resonant Inspection in Production Environment, Quasar International, Inc., Albuquerque, NM, USA. http://www.qusarintl.com/ndi. [45] A. Migliori, T.W. Darling, Los Alamos National Laboratory, Report LALP-95-9, 1995. [46] A. Moreau, D. Levesque, M. Lord, M. Dubois, J.P. Monchalin, C. Padioleau, J.F. Bussiere, Ultrasonics 40 (2002) 1047. [47] M. Dubois, P.W. Lorraine, R.J. Filkins, T.E. Drake, K.R. Yawn, S.Y. Chiang, Ultrasonics 40 (2002) 809. [48] W. Gao, C. Glorieux, S.E. Kruger, K. Van de Rostyne, V. Gusev, W. Lauriks, J. Thoen, Mater. Sci. Eng. A 313 (2001) 170. [49] Y. Hong, S.D. Sharples, M. Clark, M.G. Somekh, Ultrasonics 42 (109) (2004) 515. [50] G. Lamouche, S. Bolognini, S.E. Kruger, Mater. Sci. Eng. A 370 (102) (2004) 401. [51] D. Lellinger, S. Tadjbach, I. Alig, Macromol. Symp. 184 (2002) 203. [52] A.I. Lavrentyev, J.T. Beals, Ultrasonics 38 (2000) 513. [53] A. Kulmyrzaev, C. Cancelliere, D.J. McClements, J. Food Eng. 46 (2000) 235. [54] R. Kazys, L. Svilainis, Ultrasonics 35 (1997) 367. [55] K.A. Van Den Abeele, A. Sutin, J. Carmeliet, P.A. Jonson, NDT & EInt. 34 (2001) 239. [56] F. He, J. Zhao, L. Zhang, X. Su, Talanta 59 (2003) 935. [57] B. Guo, Y. Yuan, Y. Wu, Q. Xie, S. Yao, Anal. Biochem. 305 (2002) 305. [58] H. Tao, W. Wei, S. Zhang, J. Photochem. Photobiol. A 161 (2004) 193. [59] L.L. Bao, Q.J. Xie, Y. Xu, W.Z. Wei, Anal. Lett. 32 (1999) 885. [60] H.O. Ghourchian, N. Kamo, T. Hosokawa, T. Akitaya, Talanta 41 (1994) 401. [61] D.Z. Shen, Z.Y. Li, L.H. Nie, S.Z. Yao, Anal. Chim. Acta 280 (1993) 209. [62] J.Z. Zhang, W.Z. Wei, T.A. Zhou, D.L. He, S.Z. Yao, Q.J. Xie, Talanta 53 (2000) 525. [63] H. Su, P. Williams, M. Thompson, Anal. Chem. 67 (1995) 885. [64] M. Valc arcel, M.D. Luque de Castro, Flow Injection Analysis: Principles and Applications, Ellis Horwood, Chichester, UK, 1987. [65] X. Su, X. Qu, W. Wei, S. Yao, Microchem. J. 59 (1998) 341. [66] X. Su, B. Yu, H. Tan, X. Yang, L. Nie, S. Yao, J. Pharm. Biomed. Anal. 16 (1998) 759. [67] X. Su, W. Wei, L. Nie, S. Yao, Analyst (Cambridge) 123 (1998) 221. [68] X. Su, L. Nie, S. Yao, Fresenius’ J. Anal. Chem. 360 (1998) 272. [69] S. Titiz-Sargut, J. Ulrich, Chem. Eng. Process. 42 (2003) 841. [70] P.J. Coghill, M.J. Millen, B.D. Sowerby, Miner. Eng. 15 (2002) 83. [71] H.W. Reinhardt, C.U. Grosse, Construct. Build. Mater. 18 (2004) 145.
http://www.elsevier.com/locate/trac
837
Trends
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
[72] P.D. Coates, S.E. Barnes, M.G. Sibley, E.C. Brown, H.G.M. Edwards, I.J. Scowen, Polymer 44 (2003) 5937. [73] M.S. Greenwood, J.A. Bamberger, Ultrasonics 39 (2002) 623. [74] P.R. Tecca, A. Galgaro, R. Genevois, A.M. Deganutti, Hydrological Process. 17 (2003) 1771. [75] J. Salazar, A. Tur o, J.A. Ch avez, M.J. Garcıa, Ultrasonics 42 (109) (2004) 155. [76] R.G. Compton, J.C. Eklund, F. Marken, Electroanalysis (NY) 9 (1997) 509. [77] A.J. Saterlay, R.G. Compton, Fresenius’ J. Anal. Chem. 367 (2000) 308. [78] L.S. Berstein, M.R. Zakin, E.B. Flint, K.S. Suslick, J. Phys. Chem. 100 (1996) 6612. [79] J.L. Hardcastle, C.J. Paterson, R.G. Compton, Electroanalysis (NY) 13 (2001) 899. [80] A.M. Oliveira Brett, F.M. Matysik, Bioelectrochem. Bioenerg. 42 (1997) 111. [81] A.M. Oliveira Brett, F.M. Matysik, Ultrason. Sonochem. 4 (1997) 125. [82] A.N. Blythe, R.P. Akkermans, R.G. Compton, Electroanalysis (NY) 12 (2000) 16. [83] W.B. Whitten, J.M. Ramsey, S. Arnold, B.V. Bronk, Anal. Chem. 63 (1991) 1027. [84] E. Welter, B. Neidhart, Fresenius’ J. Anal. Chem. 357 (1997) 345.
838
http://www.elsevier.com/locate/trac
[85] D. Skouteris, Stofftransport an Phasengrenzfl€ achen Ultraschalllevitierter Mikroproben, Ein neuer Anstz zur L€ osung atmosph€ arenchemischer Fragestellungen, Dissertation, PhilippsUniversit€ at, Germany, 1996. [86] M. Petersson, J. Nilsson, L. Wallman, T. Laurell, J. Johansson, S. Nilsson, J. Chromatogr. B 714 (1998) 39. [87] L. Schweitz, M. Petersson, T. Johansson, S. Nilsson, J. Chromatogr. A 892 (2000) 203. [88] C. Mund, R. Zellner, J. Mol. Struct. 661–662 (2003) 491. [89] Y. Madrid, M. Wu, Q. Jin, G. Hieftje, Anal. Chim. Acta 277 (1993) 1. [90] C. Duyck, N. Miekeley, C.L. Porto da Silveira, P. Szatmari, Spectrochim. Acta B 57 (2002) 1979. [91] J. Posta, A. Alimonti, F. Petrucci, S. Carola, Anal. Chim. Acta 325 (1996) 185. [92] M.A. Tarr, G. Zhu, R.F. Browner, Anal. Chem. 65 (1993) 1689. [93] J.F. Banks, J.P. Quinn, C.M. Whitehouse, Anal. Chem. 66 (1994) 3688. [94] J.C. Yu, K.F. Ho, S.C. Lee, Fresenius’ J. Anal. Chem. 369 (2001) 170. [95] R.A. Conte, M.T.C. Loos-Vollebregt, Fresenius’ J. Anal. Chem. 367 (2000) 722. [96] K. Ashley, R.N. Andreus, L. Cavajos, M. Demange, J. Anal. At. Spectrom. 16 (2001) 1147. [97] F. Priego-Capote, M.D. Luque de Castro, Trend Anal. Chem. 23 (2004).