Gas Detection 7
FAIMS microchip - courtesy Owlstone Nanotech Ltd
remains high and tuning to a specific molecule frequently requires knowledge of the local environment and competing VOCs.
Gas Chromatography and/ or Mass Spectrometry (GC/MS) is the laboratory method to identify and quantify specific VOCs. Advances occur continually for both of these analytical techniques and their combination. But shrinking them to be low cost and portable represents their own challenges including the view by some that the market size does not support a major R&D effort. Micro MS measuring systems exist, but the requirement for a vacuum pump and sampling system have been the stumbling blocks. The smallest battery powered vacuum system suitable for MS still costs about $5,000. Miniaturised GC on its own, using a PID as the detector is available, with rapid advances expected: lower cost, smaller size and better separation.
FTIR and Raman Spectroscopy are already available as handheld tools for identifying solid and liquids but cannot yet achieve adequate sensitivity/ LoD for the gas phase. Costs start at typically $8,000 and their performance is impressive, considering the complexity and required robustness of the optics. Multipass gas cells should be possible to improve resolution, but the current direction of developments means that these spectrometers will struggle to identify uniquely a single VOC if more than a few other known VOCs are present. This will be especially difficult if the chosen target analyte is present in a relatively low concentration, compared to other VOCs. As understanding of Surface Enchanced Resonant Raman Spectroscopy (SERRS) improves, there may be a more concerted effort to use SERRS for gas detection; current SERRS efforts are focusing on liquids and biological samples.
Sensor Transducers are simply platforms that measure changes in the property of a sensing layer. By themselves, they are not gas sensors- they require a sensing layer to complete the gas sensor system. We consider the future for three popular transducer structures.
Integrated Optic Sensors is a general term for a variety of technologies, usually centered around waveguides or optical fibers. Various combinations of light sources, Fibre Bragg Gratings to filter the light sources then a detection stage define an optical sensor. Evanescent waves are used frequently to detect small changes in the optical properties of a gas-sensitive sensing layer. Detection methods include Surface Plasmon Resonance (SPR), loss of light intensity, polarisation shift, single peak measurements such as Cavity Ring Down Spectroscopy (CRDS) and SERRS. These platforms allow silicon electronics to be integrated with the optics to provide potentially a low cost integrated package, so why don’t we see these yet? Performance depends on sensitive, selective, reversible, fast-responding sensing layers- see below. Also, to
MEMS array - courtesy of the Deptartment of Engineering, Warwick University
Conventional configuration for TDLS employing a laser diode. Ramping the laser current has the effect of scanning the emitted wavelength through the gas line, with the recovered signal shown to the right. Signal generation and recovered signal processing take place in a PC or microcontroller via DAC / ADC.
reduce cost the volume must be significant, so until the ideal sensing layer and integrated platform join up, volumes will remain low and prices high.
MEMS Sensor Arrays are a technology improvement over a sensing method that has been with us since the ‘90s. Sensor arrays were used in the early electronic noses with limited success. Moving to depositing organometallic and/ or semiconductor metal oxides in arrays theoretically offers sensitivity and selectivity; coupled with good correcting algorithms should provide the required stability. But if the sensing layer variants are not reversible, fast responding and sensitive, then many poor sensors formed into an array does not make a good sensor system, even with advanced analytic techniques. The capability of reliably depositing arrays of sensing layers onto MEMS platforms is becoming available; MEMS also helps with reproducibility, lower cost and reduced power demand. Like the optics platform, sensor array platforms need better sensing layers.
GasFETs are discussed regularly, so why are they not more commonly used? GasFETS offer theoretically the best sensing platform: Stephenson at MIT taught chemical FET sensing in the early ‘70s and Art Janata has been educating us since the early ‘80. But the difficulty and cost of manufacture and sensor irreproducibility have made this sensing platform still awaiting a breakthrough. Again, better sensing layers is the key.
Advanced Organometallic Sensing Layers and catalysts are the key to a breakthrough in the next generation of gas sensors; chemistry, materials and physics departments world wide are working on new gas sensing materials. This quick review cannot start to describe all the work undertaken to find better gas sensing materials. Research directions include unusual materials families such as chalcogenides and lanthanides, carbon
polymorphs (carbon nanotubes, graphene), 2-dimensional crystals, nanometal oxides and scaffolded organometallics. Such a variety of research is confusing but hopefully some successes will emerge, mainly targeted at VOCs and difficult gases such as ammonia and hydrogen. Only then will the transduction platforms be used to their full capabilities to provide the next generation of gas sensors.
Ignore the Internet at Your Peril
Advances in the use of cloud-based data storage/ access and more apps are going to change how we use gas detectors in the future. Two immediate developments are: (1) multilayer mapping where GPS information allows overlay of data layers- positions of persons, gas concentration maps, safety areas that can be redefined dynamically, weather conditions to predict plumes and subcontractor activity are just some layers that can be combined. (2) Each gas detection manufacturer has or will develop apps specific to their gas detectors. This can allow operators to change STEL and TWA levels in real time, for example when plant construction places new temporary safety constraints.
Emerging markets are pushing for gas detection breakthroughs. New materials and their understanding will provide us with better sensing layers. MEMS and optical integration production and assembly processes will reduce sensor systems’ cost and improve performance. Advanced algorithms will improve sensor selectivity and stability. The internet will integrate gas detectors with other monitoring platforms and information systems.
The future looks bright. But when? We wait, watch, encourage and support.
Author Contact Details John Saffell, Alphasense Ltd • Tel: +44 (0)1376 556700 • Email:
jrs@alphasense.com • Web:
www.alphasense.com
About the Author
John Saffell has been Technical Director of Alphasense Ltd since it was founded in 1997. John is Chairman of the Council of Gas Detection and Environmental Monitoring (CoGDEM), a fellow of the Institute of Measurement and Control, a member of both ASTM and BSI standards committees and was previous Chairman of Sensors for Water Interest Group (SWIG).
www.envirotech-online.com IET January / February 2016
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64