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Photo Ionization Detector
- Disadvantages
- Nonselective among organic vapors below ionization potential of lamp.
- Affected by high humidity
- Higher potential lamps (11.7 eV) needed to measure methylene chloride have short life (one month)
- UV lamps are expensive
- Examples of Instruments: Photovac 2020, MSA Passport PID II, RAE Systems MiniRAE PLUS
Lower energy lamps use magnesium fluoride windows. Doesn't absorb water vapor. 11.7 eV lamps use a lithium fluoride window which discolors with time. They can absorb water vapor and turn cloudy. Lamps greater than 12 eV won't work because they will ionize components of air and will interfere with the measurement of interest
- oxygen = 12.07 eV
- water vapor = 12.62 eV
- nitrogen = 13.77 eV
- carbon dioxide = 13.77 eV
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