Superior performance when applied to photoacoustic gas detection
Cantilever sensor
High sensitivity is achieved by using a patented cantilever pressure sensor that is over hundred times more sensitive compared to a membrane, which is used in conventional techniques.
Gasera’s patented cantilever-type pressure sensor is designed to significantly improve the sensitivity of photoacoustic spectroscopy (PAS). An extremely thin cantilever portion moves like a flexible door due to the pressure variations in the surrounding gas. The movement of the free end of the cantilever can be about two orders of magnitude greater than the movement of the middle point of the tightened membrane under the same pressure variation. This is because the cantilever only bends and does not stretch.
The sensor is made out of single crystal SOI-silicon with a specially developed dry-etching process that leads to a highly stable component; this is why the sensor is practically totally immune to temperature and humidity variations. In addition, the sensor is not suffering from wearing. The manufacturing process of the cantilever has been developed in co-operation with the Department of Micro- and Nanosciences of Aalto University in the research group of Micro and Quantum systems which is lead by professor Ilkka Tittonen.
Readout Interferometer
The displacement of the cantilever is measured optically to avoid the so called “breathing effect”, which occurs in capasitive measurement principle where the other electrode damps the movement of the sensor. For precise measurement of the cantilever movement Gasera has developed a laser based readout inteferometer which is able to accurately measure displacement from well under a pico-meter up to millimeters.
Together the cantilever sensor and the readout interferometer form an optical microphone that has demonstrated over hundred times better sensitivity compared to conventional microphone technology when applied to photoacoustic gas detection.
The need for rapid and reliable monitoring of air pollutants and hazardous gases is growing and the photoacoustic spectroscopy provides an efficient technology for this demand.
The photoacoustic infrared spectroscopy is based on the fact that infrared light energy is absorbed by gas molecules. In the photoacoustic spectroscopy the light energy is converted into pressure variations i.e. sound energy. The sound energy in the gas sample cell is then converted into electric signal using a microphone.
Photoacoustic principle
The sample gas is sealed into a photoacoustic measurement chamber and irradiated with infrared light of a frequency that corresponds to a resonant frequency of a known sample gas molecule. If this sample gas is present in the measurement chamber a portion of the infrared energy is absorbed by that gas. This results in local increase of the heat energy in the gas molecules and the pressure and temperature of the sample gas will increase. When the infrared radiation is modulated with a certain frequency the pressure variation in the photoacoustic sample chamber creates acoustic wave of the same frequency. This acoustic wave is then converted into electric signal by the use of a microphone.
Advantages of photoacoustics
Very high sensitivity in gas detection can be achieved with the photoacoustic spectroscopy. Especially utilizing Gasera’s novel optical cantilever microphone, below ppb limits of detection can be reached.
In the photoacoustic spectroscopy the absorption is measured directly, not relative to the background as in other infrared absorption techniques. This means that it is a zero-background technology and the zero-point stability of the system is extremely good. Furthermore, the response of the optical cantilever microphone is extremely stabile. This means that very low amount of drift occurs and calibration interval is very long.
Dynamic measurement range of over 100 000 times the detection limit is possible with photoacoustic spectroscopy. This allows simultaneous analysis of very high and low concentrations without any range adjustments.
Low sample volume, only a few milliliters, is required to achieve similar sensitivity compared to multi-pass gas cells of several meters and several liters in volume in other infrared techniques. This is particularly useful when only a small amount of sample gas is available for analysis.
Due to the short optical path length required the response is highly linear over a wide dynamic range. This is advantageous in compensating for the effect of other gases in the sample gas mixture. This is particularly useful when analyzing wet gases.
Photoacoustic spectroscopy is well suited and advantageous method for measurement and analysis of solid and semi-solid samples. It is also suitable for powders, fibers, and samples of very small size. The shape of the photoacoustic spectrum is independent of the morphology of the sample.
Signal generation process involves absorption of light in the sample and production of heat followed by propagation of heat-generated thermal waves to the sample surface. Heat is then transferred into the adjacent gas, varying its pressure, which is then measured by a microphone as the photoacoustic signal.
Photoacoustic principle in solid and liquid phase measurements
The sample is sealed in the photoacoustic measurement chamber and irradiated with modulated infrared light through a window. The principle of signal generation is similar for solid- and liquid-phase samples. Periodic heating in the sample is generated when the sample is irradiated with the modulated infrared light. The periodic heat flow to the surrounding gas in the chamber from the sample surface generates expansion and contraction in a thin layer of gas close to the surface. This mechanism is called thermal coupling. Periodic heating of the sample causes also pressure variations to propagate in all directions and a superposition of these acoustic waves at the sample surface generates a surface motion that is coupled to the surrounding gas. This mechanism is called acoustic coupling. The pressure signal that is detected in the gas by a pressure sensor (i.e. microphone) is a combination of these two mechanisms. In a typical solid-phase photoacoustic experiment, thermal coupling is dominant and acoustic coupling can be neglected. The acoustic coupling can be the dominating mechanism for some liquids.
Part of the infrared radiation is reflected from the sample. The amount of reflection depends on the absorption into the sample. Rest of the radiation is absorbed into the sample according to Beer’s law and absorption coefficient to different wavelengths. If the sample is thin, part of the radiation might transmit the sample and hit another surface. In thicker samples the radiation just penetrates deeper into the sample. The temperature rise in a gas layer on the sample surface gets the energy from a certain depth from the sample depending on its thermal diffusivity.
Solid -phase photoacoustic FTIR
Typical photoacoustic Fourier transform infrared (FTIR-PAS) setup for analysis of solid and liquid samples contains an interferometer, a focusing mirror, and a photoacoustic cell. FTIR interferometer consists of a beamsplitter and two mirrors. The infrared beam is split into two beams: one is reflected from a fixed mirror and one from a moving mirror. By combining the two beams each wavelength of the light is modulated with a different modulation frequency. The combined beam is then focused into the solid or liquid sample in the photoacoustic cell. The generated photoacoustic signal can be directly transformed into absorption spectrum.
Depth-varying information of the sample can be obtained by varying the mirror velocity or phase angle of detection. Typically for polymer materials the depth from where the spectra is obtained can be varied from few micrometers to about 100 micrometers.
Advantages
The FTIR analysis of solid- and liquid-phase samples has a great variety of applications and advantages compared to other techniques. The most important and best-known advantages are
minimal sample preparation required,
suitability for opaque materials,
possibility for depth profiling,
and nondestructive measurement, which means that the sample is not consumed.
Applications
The typical applications of solid- and liquid-phase photoacoustic FTIR are the study of
carbons,
coals,
hydrocarbons,
hydrocarbon fuels,
corrosion,
clays and minerals,
wood and paper,
polymer layers,
food products,
biology and biochemistry e.g. proteins, bacteria and fungi,
medical applications such as human tissue,
drug characterization and penetration,
teeth, hair and bacteria,
and nondestructive measurement of carbonyl compounds, textiles, and catalysts.
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Typical photoacoustic Fourier transform infrared (FTIR-PAS) setup for analysis of solid and liquid samples contains an interferometer, a focusing mirror, and a photoacoustic cell. FTIR interferometer consists of a beamsplitter and two mirrors. The infrared beam is split into two beams: one is reflected from a fixed mirror and one from a moving mirror. By combining the two beams each wavelength of the light is modulated with a different modulation frequency. The combined beam is then focused into the solid or liquid sample in the photoacoustic cell. The generated photoacoustic signal can be directly transformed into absorption spectrum.