VICI D-4-I - Manual (Page 5)

1
Introduction
Description and Operating Principle
The PD-D-4-I is a non-radioactive pulsed discharge ionization detector (PDID),
designed as a universal retrofit to existing GCs. A schematic representation
of the basic D4 detector is shown in
Figure 1.
The D4 utilizes a stable, low power, pulsed DC discharge in helium as the
ionization source. Elutants from the column, flowing counter to the flow of
helium from the discharge zone, are ionized by photons from the helium
discharge above. Resulting electrons are focused toward the collector
electrode by the two bias electrodes.
The principal mode of ionization is photoionization by radiation arising from
the transition of diatomic helium
He
2
(A 1 ∑ + )
u to the dissociative 2He(1S 1 )
ground state. This is the well-known Hopfield emission. The photon energy
from the He
2
continuum is in the range of 13.5 eV to 17.7 eV.
The D4 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 (MDQs) in the low
picogram range. The response to fixed gases is positive (the standing
current increases), with MDQs in the low ppb range.
Detector response is universal except for neon, which has an ionization
potential of 21.56 eV. Since this potential is close to the energy of the
He* metastable (19.8 eV) but greater than the photon energy from the
He
2
continuum, neon exhibits a low ionization efficiency and low detector
response.
When a dopant is added to the discharge gas, the D4 also functions
as a selective photoionization detector. (Suitable dopants include Ar for
organic compounds, Kr for unsaturated compounds, or Xe for polynuclear
aromatics.)
BIAS ELECTRODE
COLLECTOR ELECTRODE
CAPILLARY COLUMN
GROUND
INSULATOR
VENT
COLUMN INLET
SPRING WASHERS
DISCHARGE ZONE
HELIUM INLET
DISCHARGE ELECTRODES
Figure 1: Schematic of the D4 detector