Gas Detection 15 Technology
Flame Ionisation Detectors (FIDs)
How it works
FIDs measure the concentration of ions produced when hydrocarbons are burned in a flame fuelled by hydrogen or a hydrogen/ helium mixture. The ions create a polarisation voltage between two electrodes, which is proportional to the VOC concentration.
GC/MS
Gas Chromatography separates the compo- nents of a VOC mixture and Mass Spectroscopy quantitatively and qualitatively analyses the individual VOCs.
FTIR
Spectroscopic analysis delivering simultaneous analysis of multiple components
Advantages
• Measures all VOCs including methane • Standard reference method for regulatory emissions monitoring • Sensitive • Linear response
• Highly sensitive selective speciation • Traditional lab method
Disadvantages
• Requires a fuel source • Different response factors for different VOCs • Bulky/heavy, so better suited to fixed and lab applications • No speciation unless coupled with a GC • Expensive
• Bulky/heavy, so better suited to fixed and lab applications • Not suitable for total organic carbon (TOC) measurements • Different columns necessary for different VOCs • Power hungry • Very expensive
• Highly sensitive selective speciation • Traditional lab method • Standard reference method for regulatory emissions monitoring
Thermal Desorption or Tedlar Bag sampling bags
Colorimetric (‘stain’) tubes
Samples are collected by pump or adsorbed by a passive diffusion tube for subsequent lab analysis – usually GC/MS
A target compound in the sample induces a colorimetric change in a tubed solid reagent
• Low capital cost
• Bulky/heavy so better suited to fixed and lab applications • Not suitable for total organic carbon (TOC) measurements • Power hungry • Unable to measure ppb in small volumes • Very expensive
• Time delay before result • No real-time alarm capability • Results are averages over longer time periods
• Low capital cost • Targets a specific VOC
• Poor accuracy
• Disposal of toxic tubes • Non-continuous • Only provides an average reading over several days/ weeks • analysed by laboratory, so results reported days or weeks later
Electrochemical sensors
Gas diffuses into the sensor via a capillary to the working electrode where it is oxidised or reduced. This electrochemical reaction results in a current that is limited by diffusion, so the output from the sensor is linearly proportional to the gas concentration.
Metal Oxide Semiconductor sensors (MOS)
The sensing principle relies on the interaction between the porous gas sensitive layer and the target gas: adsorption of the gas causes a change in the electrical resistance of the porous layer.
PhotoIonisation Detectors (PIDs)
Sample gas diffuses into and out of the PID cell via a capillary or slot. The gas is ionised by UV light, generating a photionisation current.
• Targets a specific group of VOCs (e.g. ethylene oxide) • Low cost • Low power • Compact
• Continuous monitoring
• Low cost • Compact
• Continuous monitoring
• Alphasense p-type metal oxide sensors have more stable baselines & very low humidity sensitivity • Measures CFCs
• Fast response (1-2 secs) • Responds to most VOCs • Low cost
• Choice of PID lamps for different applications • Known response factors enable quantitative analysis of specific VOCs
Summary
This article highlights that different sensor technologies are better suited to some applications and careful consideration should be given before making a choice, through discussions with manufacturers such as Alphasense.
Metal Oxides
most C3 and C4+ VOCs. Among Alphasense customers, the Krypton lamp is most popular because of its high sensitivity and longest lifetime: these lamps can operate for up to 10,000 hours. A filtered Krypton lamp, operating at 10.0eV is the best choice for BTEX due to its higher intensity than the 9.6eV lamp.
The Argon lamp (11.7eV) can measure halogenated VOCs, but has a much shorter lifetime.
Users of PID instruments should be aware of the variety of response between different VOCs. Manufacturers of PID sensors provide a comprehensive list of response factors. These figures represent the response of a lamp to a specific VOC relative to its response to a calibration gas – generally isobutylene. So, if the response of a PID to a particular VOC is eight times smaller than it is for the same concentration of isobutylene, then the response factor would be 8. Similarly, if the response factor for a particular VOC is 0.5, the PID response is twice that for isobutylene at the same concentration. Many instrument manufacturers build in response factors to enable the quantification of a specific gas when measured in isolation.
In addition to the technical considerations outlined above, it is also vitally important to choose the right supplier. For end-users, the effectiveness of their work relies on the accuracy and reliability of their monitoring equipment, and for instrument manufacturers, their brand reputation is built on the quality and reliability of their equipment. It is therefore important to seek suppliers with proven levels of quality and reliability.
For sensor manufacturers, quality management procedures should extend beyond the requirements of ISO 9001. All sensors should undergo a test and validation procedure ensuring complete stabilisation prior to characterisation. Test data should be stored for each and every sensor, including sensitivity, time of response and recovery, and zero off-set. This is important because some manufacturers simply record the average test data for a batch, or record test data for a sample from a batch. This increases levels of uncertainty and prevents traceability.
The choice of VOC sensor therefore starts with a discussion about the potential application and suitable technology, and ends with the delivery of an appropriate sensor with traceable test and validation data.
ETO-A1
Author Contact Details Alphasense Ltd • Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley, Essex CM77 7AA, United Kingdom • Tel: +44 (0) 1376 556 700 • Email:
sensors@alphasense.com •
www.alphasense.com
• Traditional n-type metal oxide sensors suffer from baseline drift & humidity sensitivity • Non-linear response
• Responds to some interfering inorganic gases
• No speciation of mixtures without GC or chemical filter • Wide variety of response factors requires knowledge of the suspected VOC. • Poor response to methane and halogenated VOCs
• Responds only to VOCs that are electroactive • Sensor requires electronic optimisation for target VOC • Responds to families of VOCs, not specific VOCs.
ETO-ElectrochemicalSensor
www.envirotech-online.com IET November / December 2018
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