Acoustic resonance spectroscopy
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Acoustic resonance spectroscopy (ARS) is a method of
spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
in the acoustic region, primarily the sonic and
ultrasonic Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
regions. ARS is typically much more rapid than
HPLC High-performance liquid chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a technique in analytical chemistry used to separate, identify, and quantify each component in a mixture. It relies on pumps to pa ...
and NIR. It is non destructive and requires no sample preparation as the sampling waveguide can simply be pushed into a sample powder/liquid or in contact with a solid sample. To date, the AR
spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
has successfully differentiated and quantified sample analytes in various forms; (tablets, powders, and liquids). It has been used to measure and monitor the progression of chemical reactions, such as the setting and hardening of
concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...
from cement paste to solid. Acoustic spectrometry has also been used to measure the volume fraction of
colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
s in a
dispersion medium Interface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with colloids, heterogeneous systems consisting of a mechanical mixture of particles between 1 nm ...
, as well as for the investigation of physical properties of
colloidal dispersion A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exten ...
s, such as aggregation and
particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object which can be described by several physical property, physical or chemical property, chemical ...
size distribution. Typically, these experiments are carried out with
sinusoidal A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often in m ...
excitation signals and the experimental observation of signal
attenuation In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variable att ...
. From a comparison of theoretical attenuation to experimental observation, the particle size distribution and aggregation phenomena are inferred.


History

Dipen Sinha of the
Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development laboratories of the United States Department of Energy (DOE), located a short distance northwest of Santa Fe, New Mexico, ...
developed ARS in 1989. Most published work in acoustics has been in the ultrasonic region and their instrumentation has dealt with propagation through a medium and not a resonance effect. One of the first, if not the first publication related to acoustic resonance was in 1988 in the journal of Applied Spectroscopy. The researchers designed a V shaped
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
rod instrument that utilized ultrasonic waves to obtain signatures of
microliter The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: â„“) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3). ...
s of different liquids. The researchers did not have any type of classification statistics or identification protocols; the researchers simply observed ultrasonic resonance signatures with these different materials. Specifically, Sinha was working on developing an ARS instrument that can detect nuclear, chemical, and biological weapons. By 1996, he had successfully developed a portable ARS unit that can be used in a battlefield. The unit can detect and identify deadly chemicals that are stored in containers in matter of minutes. In addition, the instrument was further developed by a different research group (Dr. Robert Lodder, University of Kentucky) and their work was also published in Applied Spectroscopy. The researchers created a V-shaped instrument that could breach the sonic and ultrasonic regions creating more versatility. The term ''acoustic resonance spectrometer'' was coined for the V-shaped spectrometer as well. Since the study in 1994, the ARS has evolved and been used to differentiate wood species, differentiate pharmaceutical tablets, determine burn rates and determine dissolution rates of tablets. In 2007 Analytical Chemistry featured the past and current work of the lab of Dr. Lodder discussing the potential of acoustics in the analytical chemistry and engineering fields.


Theory


Vibrations

There are two main types of
vibrations Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, such ...
: free and forced. Free vibrations are the natural or
normal modes A normal mode of a dynamical system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The free motion described by the normal modes takes place at fixed frequencies. ...
of vibration for a substance. Forced vibrations are caused by some sort of excitation to make the analyte resonate beyond its normal modes. ARS employs forced vibrations upon the analyte unlike most commonly used techniques which use free vibrations to measure the analyte. ARS excites multiple normal modes by sweeping the excitation frequency of an analyte with no internal vibrations to obtain a resonance spectrum. These resonance frequencies greatly depend on the type of analyte being measured and also depend greatly on the physical properties of the analyte itself (mass, shape, size, etc.). The physical properties will greatly influence the range of frequencies produced by the resonating analyte. In general small analytes have megahertz frequencies while larger analytes can be only a few hundred hertz. The more complex the analyte the more complex the resonance spectrum.


Quartz Rod

The ARS is essentially set up to create a fingerprint for different samples by constructive and destructive interferences. Figure 1 is a schematic of the quartz rod ARS which illustrates the path of the sound through the quartz rod. A function generator is the source though any device that is capable of outputting
sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...
in
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
form could be used (i.e.
CD player A CD player is an electronic device that plays audio compact discs, which are a digital optical disc data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of audio material such as music or aud ...
,
MP3 player A portable media player (PMP) (also including the related digital audio player (DAP)) is a portable consumer electronics device capable of storing and playing digital media such as audio, images, and video files. The data is typically stored o ...
or
sound card A sound card (also known as an audio card) is an internal expansion card that provides input and output of audio signals to and from a computer under the control of computer programs. The term ''sound card'' is also applied to external audio i ...
).
White noise In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines, ...
is generated and the voltage is converted into a
sound wave In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...
by a piezoelectric disc coupled to the quartz rod. The sound resonates down the quartz rod which is shown as a blue sinusoidal wave and two key interactions occur. A portion of the energy (red) is introduced into the sample and interacts in a specific manner dependent of the sample and another portion of the energy (blue) continues unaltered through the quartz rod. The two energies will still have the same frequency though they will have changes in their phase and possibly
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
. The two waves recombine after the sample and constructive or destructive interference occurs depending on the
phase shift In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it v ...
and amplitude change due to the sample. The altered combined energy is converted to an electrical voltage by another piezoelectric disc at the end of the quartz rod. The voltage is then recorded onto a computer by a sound card. The sample is coupled to the quartz rod at constant pressure which is monitored by a
pressure transducer A pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually act ...
which also acts as the sample holder. Rubber
grommet Curtain grommets, used among others in shower curtains. A grommet is a ring or edge strip inserted into a hole through thin material, typically a sheet of textile fabric, sheet metal or composite of carbon fiber, wood or honeycomb. Grommets ar ...
s are used to secure the quartz rod to a stable stand minimizing coupling of the rod to the surroundings. Broadband white noise is used to obtain a full
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
; however, most sound cards only pick up between 20 and 22,050 Hz. The waveform that is sent to the computer is a time-based signal of the interactions of white noise with the sample.
Fast Fourier transform A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). Fourier analysis converts a signal from its original domain (often time or space) to a representation in th ...
(FFT) is performed on the waveform to transform the time-based signal into the more useful frequency spectrum.


Detection limits

A multidimensional population translation experiment was utilized to determine the
detection limit The limit of detection (LOD or LoD) is the lowest signal, or the lowest corresponding quantity to be determined (or extracted) from the signal, that can be observed with a sufficient degree of confidence or statistical significance. However, the ...
s of an ARS device, Populations with small multidimensional separation, in this case aspirin and ibuprofen, were used to determine that tablets with a 0.08 mm difference in thickness, 0.0046 g mass difference, and a difference in density of 0.01658 g/mL were not separable by ARS. Using vitamin C and acetaminophen for the largest multidimensional separation, tablets with a thickness difference of 0.27 mm, 0.0756 g mass difference, and 0.01157 g/mL density difference in density were inseparable. Experimentally the dynamic range of ARS is a factor of ten.


Applications

One potential application of ARS involves the rapid and nondestructive identification of drug tablet verification. Currently, there are no unfailing methods to eliminate contaminated or mislabeled products, a process which sometimes results in millions of pills having to be recalled. More studies need to be completed to determine if ARS could be used as a process analytical technique in industry to prevent problems with pills before they are shipped. ARS may also be useful for quantifying the active ingredient in pharmaceutical ointments and gels.


References


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