Benzene is one of those organic compounds that is considered to be very hazardous to health, even in sub part per million concentrations.
The good news is that PID’s can easily measure benzene – the bad news is that a PID can’t tell it apart from similar compounds that tend to co-exist with benzene, but are not considered nearly as hazardous.
Many PID’s can now measure into the low ppb range, so you may find a high sensitivity to the presence of benzene.
The Ionization Potential (IP) for all the aromatics is quite low, and there are many chemicals that will cross interfere when using the standard 10.6 eV lamp.
So how does one read less than 1 ppm benzene in the presence of a few hundred ppm of BTEX (benzene, toluene, ethyl benzene, xylene) and their chemical relatives?
There are a few strategies and SOP’s that manufacturers and companies can use, knowing the properties of both the chemicals being measured and the PID’s being used:
- Use a lower power lamp
- Use a benzene scrubber tube
Using a lower power lamp, like a 9.8eV or 10.0 eV, reduces the energy of the photons striking and ionizing the gas molecules.
If we look at this IP table courtesy of Ion Science (pictured right), you can see that all of the aliphatic chemicals require higher IP to ionize then aromatic compounds. So if we use a 10.0 eV lamp, response from aliphatic compounds is effectively ignored, and we only see a response when aromatic compounds are present. Thus, the reading is sometimes called “TAC” or “Total Aromatic Compounds” concentration.
In other words, using a 9.8/10.0 eV lamp (MgF2 window and Xenon gas) one can measure many aromatics and unsaturated VOC’s containing 6 or more carbons. However, you cannot detect VOC’s that contain 2, 3 or 4 carbons. These Xenon lamps generally have a very good service life.
If there are VOC’s present but the display is less than 1 ppm of TAC’s, then there is also no benzene present, so no further testing for benzene is required. This saves a lot of benzene tests, and since TAC monitors can be worn as personal portables, you can have continuous protection against benzene exposure.
If there is a positive TAC result, then we need to test with a benzene scrubber tube. This scrubber tube will remove all aliphatic and aromatic compounds with the exception of benzene, so the only thing reaching the 10.0 eV lamp is benzene. The reading on the display will be only the benzene component of the gas matrix being sampled.
It is best to use a new scrubber tube for each test, because any breakthrough that occurs may result in a false positive benzene reading.
Because these tubes tend to be expensive, some people will tend to use them more than once since there is a colour change in the tube once they become ‘used up’. Just be careful with that, since false positives are sometimes hard to explain after the fact.
The best strategy is to use a TAC monitor and see if there are any aromatics present. If there are, then use a benzene specific analyser that incorporates a benzene scrubber tube and do a spot check.
It is not a perfect solution, but since we are not measuring benzene directly and continuously, it is a solution that moves the bar up a bit, and is better than what we used to do when measuring benzene.
There are affordable benzene specific fixed gas monitors on the horizon as of November 2015, but not quite yet. We’ll be sure to let you know when they’re on the market.
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