Photoacoustic Spectroscopy

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Photoacoustic Spectroscopy
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What is Photoacoustics?
What is Photoacoustics ?
Photoacoustic Detection
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Gas Detection in The Early Days
Old fashioned gas monitor
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Photoacoustic Spectroscopy

• Typical examples of applications
• Monitoring and identification of toxic and
  polluting gases in the atmosphere
• Monitoring of organic compounds in production
  plants, laboratories and hospitals
• Process control such as Fermentation Monitoring
  and Pure Gas Manufacturing

Global Warming
CH4?CO?CO2
N2O
NH3?NH4-
Acidification
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Photoacoustic Detection
Alexander Bells experiment in 1880
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Gas Detection of Today
A more modern set-up
The essential components of a Photoacoustic
set-up

• A chamber to contain the gas sample
• A light source
• Some means of modulating the light
• (usually a chopper)
• A detector to measure the sound
• (usually a microphone)
• Some method of processing the signals
• (the level of sophistication of the
• signal analysis depends entirely on
• the requirements)

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The Photoacoustic Effect
Sequence of events

• Gas sample is sealed in measurement chamber
• Chamber is irradiated with pulsed, narrow-band
  light
• Gas absorbs light proportional to its
  concentration and converts it to heat
• Gas heats and cools as the light is chopped
• Temperature fluctuations generate pressure waves
• Pressure waves are detected by microphones

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Infrared Absorption
Photoacoustic Detection
Infrared Absorption
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Electromagnetic Radiation
The infrared region is most useful for the
quantitative and qualitative analysis of gases.
The absorption in the region from 900 cm-1 to
1400 cm-1 is highly selective - the fingerprint
region.
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Molecular Vibrations
The number of vibration modes depend on the
number of atoms. The resonance frequency of these
modes are determined by the molecular structure
and are always the same for a given molecule - in
the order of 1013 Hz.
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Absorption of Infrared Light
Infrared light is absorbed when the radiation
and vibration are of the same magnitude.
Infrared light is transmitted when the radiation
and vibration are of different frequency.
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Release of Absorbed Energy
The increased molecular speed means increased
temperature and pressure in the measurement
chamber.
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The Optical System
A condenser microphone consists of a thin
metallic membrane in close proximity to a rigid
backplate.
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Sound Pressure in The Measuring Cell
K Gas and cell dependent constant Cp, Cv Heat
capacity at const. pressure and volume Cp/ Cv
varies from Argon 1.7 to Butane 1.1 c Gas
concentration Io Incident light fc Chopper
frequency
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Typical Dimensions
Microphones are extremely sensitive. At the
detection limit, typical values are
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PAS Detection
What is Photoacoustics ?
Photoacoustic Detection
PAS Detection
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An Infrared Spectrum
Typical spectrum of a compound in solution.
High resolution transmittance spectrum of water
vapour.
High resolution absorbance spectrum of water
vapour.
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Qualitative And Quantitative Analysis
Qualitative Analysis
The infrared spectrum of the gas sample is
compared with the standard spectra of various
gases with which the gases present in the
substance can be identified.
Quantitative Analysis
For quantitative analysis of a small number of
known gases the sample is irradiated at the
wavelengths. The responsive gases absorb strongly.
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Detection of Specific Gases
If interference between two gases is unavoidable,
the monitor, which is able to detect several
gases in one sample, can reduce, sometimes
overcome the problem by cross-compensation for
the presence of interfering species.
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Interfering Species
The various gases which are found in normal
atmospheric air are potential interferents when
detecting toxic or polluting gases. However,
the main constituents of air do not absorb
infrared radiation at all. Only two minor
constituents, water and Carbon Dioxide, absorb
infrared light and these are present in low
concentration.
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PAS And Transmission Spectroscopy
In conventional transmission spectroscopy light
passes through the measurement chamber and a
light detector measures the amount of light
transmitted through the cell.
At very low concentration the difference between
two almost equal signals is being measured.The
signal to noise ratio is poorer with transmission
spectroscopy than with PAS and therefore is a
less sensitive technique.
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PAS vs. Transmission Spectroscopy
The PAS way
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The Transmission Spectroscopy way
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Response Time
In contrast to many other monitoring techniques,
the measuring chamber of a photoacoustic monitor
is sealed and the air is analysed as discrete
samples. This does not affect the response time
as the cell volume is very small, typical 3
ml. Compare this with a typical spectrometer,
which requires large cell volumes of 3-4 litres
to reach necessary sensitivity.
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Photoacoustic Detector
Photoacoustic Detection
Photoacoustic Detector
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The Photoacoustic Detector
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Optical Filters
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Photoacoustic Measurement Sequence

• An air sample is continuously drawn into the
  measurement chamber by a pump.
• Radiation from the IR-source passes through a
  chopper and optical filter into the chamber.
• The IR radiation is absorbed and generates heat
  and pressure variations.

4. The pressure variations correspond to
the chopper frequency, creating a pressure wave
which can be detected by the microphones. 5.
The microphone signal, proportional to the gas
concentration, is post processed and the
measurement result is calculated.
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Optical Filters

• 26 optical filters covering the infra-red region
  of interest.
• Each filter consist of three elements, to
  achieve
• Well-defined transmission.
• Low leakage, thereby high
• suppression of interference.
• Not damaged by humidity.
• Complies with MIL-SC-48497A requirements.

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Spectral Distribution of Optical Filters
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Spectrum of Atmospheric Air
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Microphone Stability
Change in sensitivity at 150C lt 10 /2
hours Change in sensitivity at 25C lt 10 /600
years
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The IR-Source
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The Photoacoustic Cell
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Why PAS?
What is Photoacoustics ?
Photoacoustic Detection
Why PAS ?
Photoacoustic Detector
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Advantages of PAS?

• Advantages
• Extremely stable means calibration maybe once
  per year
• Good sensitivity detection limits typically in
  ppb-range
• Dynamic range factor 104 105
• Very low sample volume required min. 10 ml
• Two microphone system minimises interference
  from vibration

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