VICI D-3-I-7890 - Manual (Page 5)

1
Introduction
Description and Operating Principle
The D3 is a non-radioactive pulsed discharge ionization detector (PDID), with
models available for “plug and play” installation on the Agilent 6890 or 7890
GC. A schematic representation of the detector is shown in Figure 1.
The D3 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 D3 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 D3 also functions as a
selective photoionization detector. (Suitable dopants include Ar for organic
compounds, Kr for unsaturated compounds, or Xe for polynuclear aromatics.)
Figure 1: Schematic of the D-3 detector
COLLECTOR ELECTRODE
GROUND PIN
HIGH VOLTAGE
VENT
COLUMN INLET
CAPILLARY COLUMN
DISCHARGE REGION
SAPPHIRE INSULATORS
HELIUM INLET