Historical real-time screening techniques include several portable VOC sampling instruments that can give instant results for TVOCs as well as individual VOCs. Although their precision and detection limits are generally not as good as equivalent analytical laboratory instruments, portable instruments provide the advantage of real time measurements for projects requiring on-site decisions, rapid capture of tempo- ral variations, or involving large survey areas.
Colorimetric indicator tubes (e.g. Drager, Gastec)(portable)
Screening or semi-quantitative analysis may be obtained for a variety of VOCs by drawing a known volume of air through a colorimetric indicator tube. Samples are usually collected by use of a simple hand pump supplied by the manufacturer of the indicator tubes. The presence of analyte is indicated by a color change to reagent within a glass indicator tube. Indicator tubes are available for a variety of VOCs, often covering detection ranges of several orders of magnitude (ppm to %).
Colorimetric tubes are generally suitable for use in environmental conditions associated with indoor air, with restrictions on use identified by the manufacturer. Tubes are often only semi-specific with cross-interference common, manufacturer data identifying potential types and degree of cross- interference may be available.
Flame ionization detection (FID) (portable / stationary)
FID involves the detection of VOCs resulting from the combustion of hydrocarbons. As “stand- alone” equipment, FID is unable to differentiate between VOCs. Operating as a “carbon counter”, FID can function under most indoor environmental conditions and is not subject to significant cross- interference provided oxygen content is stable. FID may be combined with separation techniques (e.g., gas chromatography) and allows qualification of specific hydrocarbons based on retention time.
A PID uses an ultraviolet light to ionize a chemical. It can accurately measure gases at low ppmv or even ppb levels, however it cannot differentiate between chemicals. The electrically charged gas produces a current that is amplified and displayed as a concentration.
PID can be operated under most indoor environmental conditions, cross-interference is limited however different analytes provide different responses precluding accurate interpretation of results obtained from gases containing mixed analytes. Portable GC systems with PID detectors provide some separation of VOC for improved identification and quantification.
Sensor technology (portable)
Different sensor systems have been used to provide continuous on-site monitoring of VOCs. Most field sensors rely either on electrochemical, mass sensitive, or optical transducers. Typical sensiti- vities are in the ppm range but ranges may be extended by coupling sensors with a analyte enrich- ment method. Acceptable environmental conditions and susceptibility to cross-interference is sensor specific.
Metal oxide sensors (portable / stationary)
Metal oxide sensors measure the change in conductivity in the presence of oxidizing and reducing gases and are capable of providing real-time measurements.
Proton transfer reaction – mass spectrometer (PTR-MS) (stationary)
The detection principle of the PTR-MS is based on reactions to most of the common VOCs but
not with the components of clean air. PTR-MS has potential for on-site detection of VOCs with the advantages of rapid response and high sensitivity without sample pretreatment. PTR-MS can be ope- rated in most indoor environmental conditions.
Ion mobility spectrometry (IMS) (portable / stationary)
IMS is an analytical technique used to separate and identify ionized molecules in the gas phase based on their mobility in a carrier buffer gas. Related ionization MS techniques include desorption electrospray ionization (DESI) electrospray laser desorption ionization (ELDI), direct analysis in real time (DART) and atmospheric pressure solids analysis probe.
SIFT-MS – (stationary)
SIFT-MS is an analytical technique that uses chemical ionisation to analyse for VOCs in a whole air sample and can be used for real-time quantification. SIFT-MS is suitable for moist / humid sam- ples and is routinely utilized for analysis of human breath without need for sample conditioning.
Photo-acoustic spectroscopy (PAS) – (stationary)
A PAS system includes: a chamber to contain the gas sample, a light source, some means of modu- lating the light, a detector to measure the sound, and a method of processing the signal. The inten- sity of sound emitted by a sample depends on the nature and concentration of the substance and the intensity of the incident light (the photo-acoustic effect). Combining with FTIR allows identification of unknowns and their concentrations.
Advantages include infrequent calibration of the microphone; and linear response to gas concen- tration over a wide dynamic range (1 ppm to 103 ppm). The major disadvantage is the potential for interference between two gases of similar structure due to overlap of absorption bands. PAS is operable under typical indoor environmental conditions.
The majority of hydrocarbons absorb energy when contacted by IR light, giving rise to a spectrum that can act as a ‘fingerprint’ that can be used to identify and quantify a contaminant. While IR analysis can provide instantaneous measurements, both contaminant identification and quantification may be subject to interference should air contain a mixture of contaminants.
An advantage of IR is the system stability allowing consistent results to be obtained over exten- ded periods with minimal quality assurance checks. Water vapour can impact IR analysers; sample conditioning is therefore required for analysis of high humidity samples.