The photoionization detector (PID) utilizes ultraviolet
light to ionize gas molecules, and is commonly
employed in the detection of volatile organic
compounds (VOCs). This technique originally
found use in bench top laboratory instruments, but
its complexity limited its use elsewhere. The heart of
the photoionization detector is an ultraviolet source,
which is essentially a lamp. Early versions of this lamp
used electrodes inside the lamp similar to those used
in the early days of the vacuum tube and were quite
costly to manufacture.