Photoacoustic spectroscopy
Encyclopedia
Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy (particularly of light
) on matter
by means of acoustic
detection. The discovery of the photoacoustic effect dates to 1880 when Alexander Graham Bell
showed that thin discs emitted sound
when exposed to a beam
of sunlight
that was rapidly interrupted with a rotating slotted disk. The absorbed
energy
from the light is transformed into kinetic energy
of the sample by energy exchange processes. This results in local heat
ing and thus a pressure
wave or sound. Later Bell showed that materials exposed to the non-visible portions of the solar spectrum
(i.e., the infrared
and the ultraviolet
) can also produce sounds.
A photoacoustic spectrum of a sample can be recorded by measuring the sound at different wavelength
s. This spectrum can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to study solid
s, liquid
s and gas
es.
has become a powerful technique to study concentration
s of gases at the part per billion or even part per trillion levels. Modern photoacoustic detector
s still rely on the same principles as Bell’s apparatus, however to increase the sensitivity
the following modifications have been made:
variation of the concentration of nitric oxide
in the stratosphere
at an altitude of 28 km with a balloon-borne photoacoustic detector. These measurements provided crucial data bearing on the problem of ozone depletion
by man-made nitric oxide emission.
concentration have been developed and commercialized. Typically, low cost thermal sources are used which are modulated electronically. Diffusion
through semi-permeable disks instead of valves for gas exchange, low cost microphones and proprietary signal processing with digital signal processor
s has brought down the costs of these systems. The future of low cost applications of photoacoustic spectroscopy may be the realization of fully integrated micromachined photoacoustic instruments.
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
) on matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
by means of acoustic
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
detection. The discovery of the photoacoustic effect dates to 1880 when Alexander Graham Bell
Alexander Graham Bell
Alexander Graham Bell was an eminent scientist, inventor, engineer and innovator who is credited with inventing the first practical telephone....
showed that thin discs emitted sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
when exposed to a beam
Light beam
A light beam or beam of light is a narrow projection of light energy radiating from a source into a beam. Sunlight is a natural example of a light beam when filtered through various mediums...
of sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
that was rapidly interrupted with a rotating slotted disk. The absorbed
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
from the light is transformed into kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the sample by energy exchange processes. This results in local heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
ing and thus a pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
wave or sound. Later Bell showed that materials exposed to the non-visible portions of the solar spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
(i.e., the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
and the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
) can also produce sounds.
A photoacoustic spectrum of a sample can be recorded by measuring the sound at different wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s. This spectrum can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to study solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
s, liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s and gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es.
Uses and techniques
Photoacoustic spectroscopySpectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
has become a powerful technique to study concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
s of gases at the part per billion or even part per trillion levels. Modern photoacoustic detector
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s still rely on the same principles as Bell’s apparatus, however to increase the sensitivity
Sensitivity (electronics)
The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.Sensitivity is...
the following modifications have been made:
- Use of intense laserLaserA laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s instead of the sun to illuminate the sample since the intensitySound intensitySound intensity or acoustic intensity is defined as the sound power Pac per unit area A. The usual context is the noise measurement of sound intensity in the air at a listener's location.-Acoustic intensity:...
of the generated sound is proportionalProportionality (mathematics)In mathematics, two variable quantities are proportional if one of them is always the product of the other and a constant quantity, called the coefficient of proportionality or proportionality constant. In other words, are proportional if the ratio \tfrac yx is constant. We also say that one...
to the light intensity. - The ear has been replaced by sensitive microphoneMicrophoneA microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
s. The microphone signals are further amplified and detected using lock-in amplifierLock-in amplifierA lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment . It is essentially a homodyne with an extremely low pass filter...
s. - By enclosing the gaseous sample in a cylindrical chamber, the sound signal is amplified by tuning the modulationModulationIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
frequency to an acoustic resonanceAcoustic resonanceAcoustic resonance is the tendency of an acoustic system to absorb more energy when it is forced or driven at a frequency that matches one of its own natural frequencies of vibration than it does at other frequencies....
of the sample cell.
Example
The following example illustrates the potential of the photoacoustic technique: In the early 1970s, Patel and co-workers measured the temporalTime
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
variation of the concentration of nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
in the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
at an altitude of 28 km with a balloon-borne photoacoustic detector. These measurements provided crucial data bearing on the problem of ozone depletion
Ozone depletion
Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere , and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon...
by man-made nitric oxide emission.
Academic research
While most academic research has concentrated on high resolution instruments, some work has gone in the opposite direction. In the last twenty years, very low cost instruments for applications such as leakage detection and for the control of carbon dioxideCarbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
concentration have been developed and commercialized. Typically, low cost thermal sources are used which are modulated electronically. Diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
through semi-permeable disks instead of valves for gas exchange, low cost microphones and proprietary signal processing with digital signal processor
Digital signal processor
A digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...
s has brought down the costs of these systems. The future of low cost applications of photoacoustic spectroscopy may be the realization of fully integrated micromachined photoacoustic instruments.
External links
- General introduction to photoacoustic spectroscopy: http://www.spectroscopyeurope.com/images/stories/ArticlePDfs/PAS14_5.pdf
- Photoacoustic spectroscopy in trace gas monitoring http://www.mlf.science.ru.nl/publ/2000/0718a.pdf
- MIniaturized-Cell-Enhanced PhotoAcoustic Spectroscopy MICEPAS http://www.micepas.basnet.by/
- MTEC Photoacoustics, Inc.: http://www.mtecpas.com