Introduction To Photoionization
A Photo-Ionization Detector (PID) measures real-time
concentrations of organic and inorganic vapors in low levels
from parts-per-billion (ppb) up to 10,000 parts-per-million
(ppm). A PID is a very sensitive instrument used in a wide
variety of applications such as:
*Hazardous waste sites
*Clean-up of spills
*Protecting workers from toxic vapors
*Refineries
*Leak detection for compliance
*Pulp and paper plants
*Hazardous materials response
*Terrorist chemical attacks
How Does a PID Work?
A PID uses an ultraviolet (UV) lamp to ionize molecules, causing
them to release an electron and form a positive ion. This will
occur when the ionization potential (IP) of the molecule is less
than the electron volt (eV) output of the UV lamp (i.e., 10.6eV,
11.7eV, or 11.8eV). Ions formed in the air are driven in one
direction by a bias electrode and accumulated at a collecting
electrode. The ion current is then amplified and converted to a
digital meter readout in parts-per-million (ppm). The most
common UV lamps used in field work are 10.6eV, 11.7eV, and
11.8eV. The useful lifespan of the 10.6EV is about 3,000 hours,
the 11.7eV is about 200 hours, and the 11.8eV is about 500
hours. Of course, these lifespans are all dependent on how
carefully the lamps are used and maintained.
View a
diagram.
When to use a PID
PID's are primarily screening devices. They will detect any
volatile organic compound (VOC) with an ionization potential
below the electron volt rating of the lamp. Simply put, a PID
tells you that something is there, never what it is (besides
that it is a voc), and only sometimes how much. For example, the
standard calibration for a PID is 100ppm Isobutylene, if
isobutylene is the only VOC in the air the ppm reading on the
instrument is an accurate representation of how much Isobutylene
is present. However, if there is or can be other VOC's in the
air, you can only be certain that something is there, not what
or how much. Sometimes PID's can be calibrated to other gases,
like Hexane. Many PID's contain built in correction factors that
allow the instrument to be calibrated to Isobutylene but have
the reading adjusted for other gases.