CompactGC manual
14
PDD
Operation principle
The Pulsed Discharge Detector (see figure 3.7) is a non-radioactive ionization detector
with a universal concentration dependant response. A stable, low power, pulsed DC
discharge in helium acts the ionization source. Eluants from the column, flowing counter
to the flow of helium from the discharge region, are ionized by photons from the helium
discharge above. Resulting electrons are focused towards the collector electrode by a
bias electrode.
Figure 3.7: Diagram of the PDD
The principle mode of ionization is photoionisation by radiation arising from the transition
of diatomic helium to the dissociative ground state. This is the well known Hopfield
emission. The photon energy from the diatomic helium continuum is in the range of 13.5
to 17.7 eV.
The PDD is essentially non-destructive (0.01-0.1 % ionization) and highly sensitive. The
response to organic compounds is linear over five orders of magnitude with minimum
detectable quantities (MDQ) in the low picogram range. The response to fixed gasses is
positive (the standing current increases), with MDQ’s in the low ppb range. The
detector response is universal except for neon and helium.
Operation
Since the PDD is highly sensitive, also to air, extreme care should be taken to leakages,
quality of carrier gas, and all possible sources of detector pollution. Although a helium
purifier is used, gas quality should be 5.0 (or N50) or better.