VICI D-2-IM - Manual (Page 5)

1
Introduction
Description and Operating Principle
The pulsed discharge detector Model D-2-IM is a universal helium photoionization
detector optimized for trace level work with micropacked and capillary columns.
A schematic representation is shown in Figure 1.
As the name implies, a stable, low power, pulsed DC discharge in helium is utilized
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 PDHID 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 or sub
picogram range. The response to fixed gases is positive (the standing current
increases), with MDQs in the low ppb range.
Figure 1: Schematic of the D-2-IM detector
BIAS ELECTRODE
COLLECTOR ELECTRODE
BIAS ELECTRODE
CAPILLARY COLUMN
GROUND
QUARTZ
INSULATOR
SAPPHIRE
INSULATORS
VENT
COLUMN INLET
SPRING WASHERS
DISCHARGE ZONE
HELIUM INLET
DISCHARGE ELECTRODE