Laser Photoacoustic Spectroscopy

Laser-based sensing techniques are attracting great interest in various fields of applications. Among various detection schemes, photoacoustic detection offers some unique features over other laser-based techniques such as low sample volume, high sensitivity and wide linear dynamic range. Signal is generated only from the absorption thus being a so called zero-background technique. The performance of a laser photoacoustic spectroscopy system depends on the availability of appropriate laser sources, modulation schemes and the pressure sensor (i.e. microphone).

Gasera’s ultra-sensitive cantilever microphone enables high sensitivity even with low cost telecom diode lasers that operate at the weak overtones of fundamental vibration bands. On the other-hand combination with high power Mid-IR quantum cascade lasers provides detection limits that no other method can achieve.

In the tunable diode laser photoacoustic spectroscopy the laser wavelength is selected so that only a single absorption line, characteristic to a target molecule, is used. This allows highly selective measurements. Power density of laser light is very high allowing high sensitivity and short response time. With tunable diode lasers a wavelength modulation across the absorption line is performed in order to minimize background signal.

Laser-PAS applications include

• Environmental monitoring
• Greenhouse gas flux measurements
• Engine exhaust measurement
• Breath analysis
• Safety and security
• Leakage monitoring
• Air quality monitoring
• Automotive
• Process control

Optical Filter Photoacoustic Spectroscopy

Photoacoustic spectroscopy (PAS) is based on the absorption of modulated infrared light leading to the local warming of the absorbing gas volume element. The generated pressure waves are detected by a pressure detector (i.e. microphone) producing signal proportional to the absorption.

Conventional photoacoustic infrared spectroscopy with mechanical chopper and optical filters

 

In conventional photoacoustic spectroscopy (PAS) setup the gas to be analyzed is sealed into a measurement chamber and irradiated by a modulated light of a pre-selected wavelength. The infrared radiation is focused with a mirror onto the window of the photoacoustic measurement chamber after it has passed a light chopper and an optical filter. The optical filter is typically a narrow-band infrared interference filter, which is selected by the absorption band of the target gas. More light will be absorbed if the concentration of the target gas is higher in the measurement chamber.

Photoacoustic infrared spectroscopy for multi-gas analysis

For simultaneous analysis of multiple gas compounds in a complex gas mixture, a set of optical filters is installed in a filter wheel. High selectivity is achieved by choosing several different filters with narrow spectral bands for target gases as well as for interfering gases. The analysis is based on least squares fit of the optical filter response data to the calibration data of pure gas compounds.

A mathematical model based non-linearity compensation is applied to all filter responses which expands the linearity range to over 100 000 times the detection limit using only one point span calibration!

Differential Photoacoustic Spectroscopy

The differential photoacoustic method combines the sensitivity of photoacoustic spectroscopy with the long absorption path length used by conventional absorption spectroscopy and allows open-path and flow-through detection of gases.

The differential system consists of three separate gas cells: sample, reference, and differential photoacoustic cell. The sample cell is filled with the gas to be analysed. The reference cell is filled with a non absorbing zero-gas, e.g. nitrogen. The differential photoacoustic cell contains the gas to be analyzed, typically at high concentrations. The differential cell is divided into two equal halves, separated by the ultra-sensitive cantilever sensor.

The light emitted by the light source is also split into two equal parts, the top beam going through the sample cell into the top half of the differential cell and the bottom beam going through the reference cell into the bottom half of the differential cell. When there is no absorption in the sample cell, the two beams arrive at the differential cell with equal intensity and identical pressure waves are generated in the top and bottom halves which cancel each other out. If there is absorption in the sample cell, then there is a net pressure effect in the differential cell which is measured with the silicon cantilever sensor. The cantilever displacement is directly proportional to the concentration of the gas in the sample cell. Since the sample cell is not part of the photoacoustic detection cell, open-path measurements can be performed, even without relevant interference from the ambient noise.

The IR source can be a blackbody, LED or a laser. The modulation is performed by a mechanical chopper or electrically modulating the source. The most interesting advantage of this technique is that no optical filter or spectrograph is necessary, since the gas inside the differential cell acts like an optical filter (so-called gas correlation method). The selectivity is given by the real absorption spectrum and is not affected by any instrument function.

The differential photoacoustic spectroscopy is an ideal platform for single gas sensors. This measurement concept is the motivation for the MINIGAS EU-project that aims to a miniaturized high performance gas sensor based on combining cantilever sensor based differential PAS and infrared LED sources.

A fast and sensitive gas analyzer can be achieved by combining laser sources with the differential photoacoustic spectroscopy concept. This is ideal technology e.g. for gas flux measurements where low detection limits and short response time are required.

Photoacoustic Fourier Transform Infrared Spectroscopy (FTIR-PAS)

FTIR is a widely used method for obtaining IR spectra of a sample. The sample is exposed to IR radiation modulated by an interferometer and the intensity of transmitted radiation is monitored via an IR detector. At certain resonant frequencies characteristic of the specific sample, the radiation will be absorbed resulting in a series of peaks in the spectrum, which can then be used to identify the sample.

Gasera has improved the sensitivity of the gas phase FTIR significantly by combining FTIR and photoacoustic spectroscopy (FTIR-PAS). This technology can also be applied to solid, semi-solid and liquid samples.

 

Gas phase photoacoustic FTIR is based on the absorption of modulated light leading to the local warming of the absorbing volume element. The generated pressure waves are detected by a sensitive pressure detector producing a signal proportional to the absoption.

In comparison to an absorption spectrometer (conventional FTIR) a significantly smaller sample cell can be used. Therefore, small sample gas volumes are needed and the whole device is smaller. This makes it ideal for compact portable and hand-held instruments.

Short optical path length brings about other advantages, in particular the high linearity of response of the measured signal. Linearity is important for measuring the analyte response accurately and for accurately subtracting the signals from any interfering gases, in particular, from water and CO2. This feature is very important when measuring industrial exhaust gases, which usually contain high levels of water vapour.

The Gasera’s patented optical cantilever sensor provides huge dynamic range. This combined with the linear response opens up new interesting applications such as before and after analysis of scrubbers etc.

Furthermore, when using the novel silicon cantilever sensor, the sample cell can be heated up to 200°C, which is required in many industrial emission applications. As a result of these advantages, the photoacoustic technique has a high potential for the measurement of industrial emission gases, in all fields.