Photoacoustic spectroscopy is the measurement of the effect of absorbed
electromagnetic energy
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
(particularly of
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
) on
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...
by means of
acoustic detection. The discovery of the
photoacoustic effect dates to 1880 when
Alexander Graham Bell
Alexander Graham Bell (, born Alexander Bell; March 3, 1847 – August 2, 1922) was a Scottish-born inventor, scientist and engineer who is credited with patenting the first practical telephone. He also co-founded the American Telephone and T ...
showed that thin discs emitted
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 ...
when exposed to a
beam of
sunlight
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when ...
that was rapidly interrupted with a rotating slotted disk. The
absorbed energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
from the light causes local
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
ing, generating a
thermal expansion
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kin ...
which creates a
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
wave or sound. Later Bell showed that materials exposed to the non-visible portions of the solar
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.e., the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
and the
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
) can also produce sounds.
A photoacoustic spectrum of a sample can be recorded by measuring the sound at different
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s of the light. This spectrum can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to study
solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
s,
liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, ...
s and
gases.
Uses and techniques
Photoacoustic
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 ...
has become a powerful technique to study
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', ''number concentration'', ...
s of gases at the part per billion or even part per trillion levels.
[ Modern photoacoustic ]detector
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s still rely on the same principles as Bell's apparatus; however, to increase the sensitivity, several modifications have been made.
Instead of sunlight, intense laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...
s are used to illuminate the sample since the intensity of the generated sound is proportional to the light intensity; this technique is referred to as laser photoacoustic spectroscopy (LPAS).microphone
A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publ ...
s. The microphone signals are further amplified and detected using lock-in amplifiers. By enclosing the gaseous sample in a cylindrical chamber, the sound signal is amplified by tuning the modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informat ...
frequency to an acoustic resonance
Acoustic resonance is a phenomenon in which an acoustic system amplifies sound waves whose frequency matches one of its own natural frequencies of vibration (its ''resonance frequencies'').
The term "acoustic resonance" is sometimes used to nar ...
of the sample cell.
By using cantilever enhanced photoacoustic spectroscopy sensitivity can still be further improved enabling reliable monitoring of gases on ppb-level.
Example
The following example illustrates the potential of the photoacoustic technique: In the early 1970s, Patel and co-workers measured the temporal variation of the concentration of nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its ...
in the stratosphere
The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
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 consists of two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth's atmosphere, and a much larger springtime decrease in stratospheric ozone (the ozone lay ...
by man-made nitric oxide emission. Some of the early work relied on development of the RG theory by Rosencwaig and Gersho.
Applications
One of the important capabilities of using FTIR photoacoustic spectroscopy has been the ability to evaluate samples in their ''in situ'' state by infrared spectroscopy
Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functi ...
, which can be used to detect and quantify chemical functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the r ...
s and thus chemical substance
A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wit ...
s. This is particularly useful for biological samples that can be evaluated without crushing to powder or subjecting to chemical treatments. Seashells, bone and such samples have been investigated. Using photoacoustic spectroscopy has helped evaluate molecular interactions in bone with osteogenesis imperfecta .
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 dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
concentration have been developed and commercialized. Typically, low cost thermal sources are used which are modulated electronically. Diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
through semi-permeable disks instead of valves for gas exchange, low-cost microphones, and proprietary signal processing with digital signal processor
A digital signal processor (DSP) is a specialized microprocessor chip, with its architecture optimized for the operational needs of digital signal processing. DSPs are fabricated on MOS integrated circuit chips. They are widely used in audio s ...
s have 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.
The photoacoustic approach has been utilized to quantitatively measure macromolecules, such as proteins. The photoacoustic immunoassay labels and detects target proteins using nanoparticles that can generate strong acoustic signals. The photoacoustics-based protein analysis has also been applied for point-of-care testings.
Photoacoustic spectroscopy also has many military applications. One such application is the detection toxic chemical agents. The sensitivity of photoacoustic spectroscopy makes it an ideal analysis technique for detecting trace chemicals associated with chemical attacks.
LPAS sensors may be applied in industry, security (nerve agent
Nerve agents, sometimes also called nerve gases, are a class of organic chemicals that disrupt the mechanisms by which nerves transfer messages to organs. The disruption is caused by the blocking of acetylcholinesterase (AChE), an enzyme that ...
and explosives detection), and medicine (breath analysis).
References
Further reading
* Sigrist, M. W. (1994), "Air Monitoring by Laser Photoacoustic Spectroscopy," in: Sigrist, M. W. (editor), "Air Monitoring by Spectroscopic Techniques," Wiley, New York, pp. 163–238.
External links
* General introduction to photoacoustic spectroscopy
* Photoacoustic spectroscopy in trace gas monitorin
* Photoacoustic spectrometer for trace gas detection based on a Helmholtz Resonant Cell (www.aerovia.fr)