Almost daily we get requests for the use of a PID (Photo Ionization Detector) for field screening purposes. We always ask which lamp is needed or what constituents are you screening for. More times than not the person on the other end is a little unsure of what to say next.
To explain, PIDs can be outfitted with a variety of lamps that vary in ionization potential (IP or eV). The most common choices are 10.6 eV lamps and 11.7 eV lamps. All PID manufacturers have reference materials to help you decide the correct lamp needed for your job such as RAE System’s Tech Note TN-106. The way you go about using the chart is simple: compile the list of compounds you need to screen for then locate them on the chart. You need to look for the IP of each compound. Once you have located that, you can then determine the correct lamp to use. For instance, if I need to sample for Benzene, by looking at TN-106 I see it has an eV of 9.25. Since 9.25 eV is less than 10.6 eV a PID outfitted with a 10.6 eV lamp will be sufficient when monitoring for Benzene. If you weren’t tell me that you have a compound such as Methylene Chloride which has an ionization potential of 11.32 eV (greater than 10.6 eV), I would recommend you use a PID with an 11.7 eV lamp in order to “see” this chemical compound.
Something to keep in mind, a PID will not be able to discern between different chemical compounds existing in a given sample. In other words, all compounds present in your sample that the lamp is capable of detecting for will be part of your readings. Also, if you are wondering why not just use an 11.7 eV lamp all the time, it’s because these lamps have a very short shelf life, and for better accuracy you should always use the lamp that is closest in ionization energy to the chemical you are screening for.
Enviro-Equipment, Inc. rents and sells photoionization detectors. Below are a few links to our rental and product pages: