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

1
Introduction
NOTE: The contents of this manual apply to either the HID/ECD dual mode
detection system (D-2) or the HID single mode detection system (D-2-I). In
general, the manual describes the dual mode detector; information which
applies specifically to the single mode detector is so indicated.
Description and Operating Principle
The pulsed discharge detector is a non-radioactive universal detector which
can easily be configured to function as a helium ionization detector (HID),
photoionization detector (PID), or electron capture detector (ECD). A
schematic representation of the two configurations 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.
HELIUM INLET
DISCHARGE ZONE
SPRING WASHERS
QUARTZ
INSULATOR
DOPANT INLET
SAPPHIRE
INSULATORS
GROUND
BIAS ELECTRODE
COLLECTOR ELECTRODE
CAPILLARY COLUMN
COLUMN INLET
DOPANT INLET
VENT
BIAS ELECTRODE
COLLECTOR ELECTRODE
BIAS ELECTRODE
CAPILLARY COLUMN
GROUND
QUARTZ
INSULATOR
SAPPHIRE
INSULATORS
VENT
COLUMN INLET
SPRING WASHERS
DISCHARGE ZONE
HELIUM INLET
DISCHARGE ELECTRODES
Figure 1: Schematic of the D-2 detector in the PDECD mode (left)
and the PDHID mode (right)