Sensors & transducers technology
FlaMe IonIsatIon Detectors (FIDs)
how It works
FIDs measure the concentration of ions produced when hydrocarbons are burned in a flame fuelled by hy- drogen or a hydrogen/ helium mix- ture. The ions create a polarisation voltage between two electrodes, which is proportional to the VOC concentration
gc/Ms
Gas Chromatography separates the components of a VOC mixture and Mass Spectroscopy quantitatively and qualitatively analyses the individ- ual VOCs.
aDvantages
• Measures all VOCs including methane • Standard reference method for regulatory emissions monitoring • Sensitive
• Linear response 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
• Highly sensitive selective speciation • Traditional lab method
• 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
FtIr
Spectroscopic analysis delivering simultaneous analysis of multiple components
• Highly sensitive selective speciation • Traditional lab method • Standard reference method for regulatory emissions monitoring
• 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
therMal DesorPtIon or teDlar bag saMPlIng bags
Samples are collected by pump or adsorbed by a passive diffusion tube for subsequent lab analysis – usually GC/MS
• Low capital cost
• Time delay before result • No real-time alarm capability • Results are averages over longer time periods
colorIMetrIc (‘staIn’) tubes
A target compound in the sample induces a colorimetric change in a tubed solid reagent
• 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 capil- lary to the working electrode where it is oxidised or reduced. This electro- chemical reaction results in a current that is limited by diffusion, so the out- put from the sensor is linearly propor- tional to the gas concentration.
Metal oxIDe seMI- conDuctor sensors (Mos)
The sensing principle relies on the interaction between the porous gas sensitive layer and the target gas: ad- sorption 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, gener- ating a photionisation current.
• Targets a specific group of VOCs (e.g. ethyl- ene oxide) • Low cost • Low power • Compact • Continuous monitoring
• Low cost • Compact • Continuous monitoring • Alphasense p-type metal oxide sensors have more stable baselines and very low humidity sensitivity • Measures CFCs
• Fast response (1-2 secs) • Responds to most VOCs • Low cost • Choice of PID lamps for different applica- tions • Known response factors enable quantitative analysis of specific VOCs
• Responds only to VOCs that are electroactive • Sensor requires electronic optimisation for tar- get VOC
• Responds to families of VOCs, not specific VOCs.
• Traditional n-type metal oxide sensors suffer from baseline drift and 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
Instrumentation Monthly February 2019
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