Benzene is toxic, carcinogenic and flammable, yet essential to industries including the chemical industry, where it is used as a feedstock ingredient for preparation of widely used derivatives including cyclohexane, cumene and ethylbenzene, the fossil fuels industry, with uses in petroleum refining and coal manufacturing, and in the rubber tire industry. As such, these industries and others require rapid, accurate monitoring of benzene with detection limits sufficiently low to allow safe industrial operations.
The industrial user has the following choices of real-time benzene monitor:
Photoionization detectors (PIDs) with pre-filter tubes: Photoionization involves exposure of a gas sample to ultraviolet light. The excited molecules lose electrons to become positive ions, and the current resulting from movement of these ions is detected. The pre-filter removes molecules that would also ionize and result in a false positive signal. These can allow continuous, real-time rapid monitoring of very low concentrations of benzene with a high degree of precision.
Photoionization detectors (PIDs) incorporated into gas monitors: Rather than a pre-filter, competing technology uses infrared absorption to simultaneously monitor for benzene and other gases of interest. Sensitivity is excellent.
Multi-gas monitors: These machines are designed to detect volatiles present at levels posing a hazard for explosion. They will not detect concentrations of benzene at lower levels and as such cannot be used to monitor compliance with applicable safety regulations.
Colorimetric gas detection tubes: These are sampling tubes impregnated with a reagent that reacts with benzene to give a color change. Sensitivity is acceptable. However, industrial use requires a monitoring system with an alarm. Although this method could be used as a double-check on a monitor, the tubes expire, and waiting to ship a fresh batch of tubes before double checking an important safety monitor is not acceptable.
Portable gas chromatography (GC) systems: GC provides sensitive and specific measurements of benzene. However, they are large and heavy, and they require a highly trained technician to operate. Perhaps useful to double check a spot flagged by real-time monitors (if the technician can get the machine into that spot), but not practical as a stand-alone approach.
Metal oxide-based sensors: Old technology, they are sometimes used because they are cheap. However, they are not sufficiently sensitive to monitor compliance with safety regulations. They also aren’t specific for benzene and environmental conditions will affect readings.
Compact mass spectrometer (CMS): These units do measure benzene to 0.5 ppm, but that is not nearly as sensitive as other methods used for safety regulation compliance. Hand-held monitors are not yet commercially available as they are with some other options; the requirement for cartridges can add to cost.
Sampling followed by off-line chemical analysis methods can be required for regulatory compliance. One method is standard:
Gas chromatography with mass spectrometric detection (GC-MS): Gas is either sampled directly or pumped through an adsorbent and subjected to standardized, laboratory-based analytical procedures. This technique is the gold standard for quantifying benzene, but readings will take hours (if laboratory is onsite) or days (if offsite).