By definition, organic compounds contain the element carbon, and exhibit similar chemical properties, which is advantageous from a monitoring perspective. However, these properties unfortunately vary widely between the many thousands of different VOCs. It’s important to understand the factors affecting sensor choice – for both end-users and manufacturers of monitoring instruments – and key questions must be addressed, but firstly it’s important to be aware of the tech available (see table).
Main application
This is the most important consideration, because it impacts the choice of technology. For example, if the requirement is to single out and measure a specific VOC in the presence of others, this could rule out many of the technologies. Similarly, whilst the cost of metal oxide sensors might be attractive, the potential presence of certain inorganic gases may render them unsuitable. (In applications where the identity of other gases may be known, the response of a specific type of sensor may be solely attributable to the VOC of interest.)
Regulatory monitoring of VOCs in applications such as industrial stack emissions necessitate certain technologies, such as GC/MS and FTIR. However, they’re less well suited to applications like workplace safety, leak detection, and HAZMAT, due to cost, portability, and power requirements. Popular sensors for these applications are electrochemical, metal oxide and PIDs.
Electrochemical sensors
Offering a resolution from 10 to 50 ppb, electrochemical cells are relatively low cost, low power and compact. These sensors need to be optimised for the target VOC, because each one requires a different ideal bias voltage for best sensitivity. Also, electrochemical cells require about 25 seconds to respond, in comparison with one to two seconds for PIDs. Nevertheless, electrochemical sensors are suitable for applications where cost is important and performance characteristics are known.
Metal oxide sensors (MOS)
These are relatively compact and low cost, but require more power than electrochemical sensors. Humidity sensitivity and baseline drift are all characteristics of traditional n-type MOS sensors, although Alphasense p-type units may perform better. Also, compared with PIDs, MOS are not as sensitive at low concentrations. MOS sensors also respond to high concentrations of some inorganic gases such as NO, NO2, and also CO. MOS may be a more suitable than PIDs in applications requiring the measurement of halogenated VOCs such as chlorofluorocarbons.
Photoionisation sensors (PID)
PIDs respond to most VOCs except for small hydrocarbons such as methane, and some halogenated compounds. Each VOC has a characteristic ionisation potential, and the peak photon energy generated in a detector depends on the PID lamp used. Alphasense offers three: xenon (9.6eV), krypton (10.6eV) and argon (11.7eV). Of these, the argon lamp provides the largest detection range of VOCs, whereas a xenon lamp can increase selectivity.
The choice of lamp is dictated by the likely VOCs to be measured, lamp lifetime considerations, and the sensitivity and level of selectivity required. The xenon lamp is suitable for selective detection of many aromatics and unsaturated VOCs containing at least six carbon atoms (C6+), for example, BTEX compounds (benzene, toluene ethyl benzene and xylenes). The krypton lamp detects most non-halogenated C2, most C3 and C4+ VOCs. A filtered krypton lamp operating at 10.0eV is better for BTEX than the xenon lamp due to its higher intensity. The argon lamp, meanwhile, 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 that 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 eight. 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 conclusion, different sensor technologies are better suited to certain applications. Careful consideration, following discussions with manufacturers, should be given before making a choice.
Summary: Take care
In addition to technical considerations, it is also vitally important to choose the right supplier. If the effectiveness of the work of end-users relies on the accuracy and reliability of their monitoring equipment, then the brand reputation of instrument makers is built on product quality and reliability. It is important to seek suppliers with proven capabilities there.
For sensor manufacturers, quality management procedures should extend beyond the requirements of ISO 9001. All sensors should undergo a test and validation procedure that ensures complete stabilisation prior to characterisation. Test data should be stored for each sensor, including sensitivity, time of response and recovery, and zero off-set. This is important because some makers simply record the average test data for a batch. This prevents traceability and increases uncertainty.
In summary, the choice of VOC sensor should start with a discussion about the application and suitable technology, and end with the delivery of an appropriate unit with traceable test and validation data.
| Technology & how it works | Advantages | Disadvantages |
| Flame ionisation detectors (FID) 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 | • Measures all VOCs including methane • Standard reference method for regulatory emissions monitoring • Sensitive • Linear response | • Expensive; 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 gas chromatograph |
| Gas chromatography (GC) separates the parts of a VOC mixture and mass spectroscopy (MS) quantitatively and qualitatively analyses the individual component VOCs, or species | • Highly sensitive selective speciation • Traditional lab method | • Very expensive; power-hungry; bulky/heavy • Not suitable for total organic carbon measurements • Different columns necessary for different VOCs |
| FTIR (Fourier-transform infrared spectroscopy) involves 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 applications • Not suitable for total organic carbon (TOC)
measurements • Unable to measure ppb in small volumes • Power hungry; very expensive |
| Thermal desorption/Tedlar Bag processes collect samples using a pump, or by adsorption using a passive diffusion tube for subsequent lab analysis, usually GC/MS | • Low capital cost | • Time delay; no real-time alarm capability • Results are averages over longer time periods • Analysed by laboratory, so results are not
returned for days or weeks |
| Colorimetric (‘stain’) tubes employ a tubed solid reagent to induce a colour change in response to a target compound in the sample | • Low capital cost • Targets a specific VOC | • Poor accuracy; non-continuous • Requires disposal of toxic tubes • Provides average reading over days/weeks |
| Electrochemical sensors rely on an oxidation or reduction reaction with gas that diffuses into the sensor via a capillary to the working electrode. This reaction results in a current that is limited by diffusion, so the output from the sensor is linearly proportional to the gas concentration | • Targets a specific group of VOCs • Low cost and low power • Compact • Continuous monitoring | • Responds only to VOCs that are electroactive • Sensor requires electronic optimisation for target
VOC • Responds to VOC families, not specific VOCs |
| Metal oxide semiconductor sensors (MOS) feature a porous layer whose electrical resistance changes when the target gas is adsorbed | • Low cost and compact • Continuous monitoring • Measures CFCs | • Traditional n-type metal oxide sensors suffer from
baseline drift and humidity sensitivity • Non-linear response and inorganic gas interference |
| Photoionisation detectors (PIDs) ionise sample gas by UV light, generating a photoionisation current. The gas diffuses into and out of the PID cell via a capillary or slot | • Fast response; low cost; responds to most VOCs • Choice of PID lamps for different applications • Known response factors enable quantitative
analysis of specific VOCs | • No speciation of mixtures without GC or chemical filter; poor response to methane/halogenated VOCs • Wide variety of response factors requires knowledge of the suspected VOC |