VICI D-2-I - Manual (Page 27)

23
Mode Selection and Setup
Helium Ionization Mode
Since the PDHID mode provides a better indication of the cleanliness and
the integrity (leak tightness) of the system, that mode is utilized for initial
testing and startup. If the system is operating according to the parameters
described thus far, it is ready for operation in the PDHID mode.
In the PD-D2-IS, the column connects directly to the inlet, and cannot be
adjusted. However, the PD-D2 may require some additional work on the
column positioning. If the instructions of Step 1 at the top of page 17 were
properly executed, the column should already be properly positioned at
116 mm (±1 mm). The suggested range for the column position is between
112 mm and 117 mm, wherein the detector response (not the background
current) is within 90% of the maximum. Since there may be some variation
in the flow rate for the different types of capillary columns, the user may
want to optimize the column position within this suggested range. DO NOT
insert the column more than 118 mm.
With this flow configuration, only pure helium passes through the discharge
region, minimizing the chance of discharge electrode contamination through
contact with eluting samples. However, if very high concentrations of
organic compounds are introduced for extended periods of time, they could
diffuse into the discharge region and contaminate the electrodes. Under
normal chromatographic use with capillary columns, such contamination is
negligible even over extended periods.
Selective Photoionization Mode
Since the pulsed discharge detector is essentially a windowless helium
photoionization detector, changing the discharge gas from pure helium
to helium doped with argon, krypton, or xenon changes the discharge
emission profile. This results in a change in the photon energy due to
additional resonance atomic emissions and diatomic emissions from the
rare gas added. Thus a single detector can be operated in any of the three
photoionization detector (PID) modes: Ar-, Kr-, or Xe-PID.
Doped helium is used rather than other pure gases in order to retain the
benefits of the helium: namely, its transparency for Ar, Kr, an Xe resonance
radiation and its efficient cooling of the electrodes. Any problems associ-
ated with the presence of a window between the photon source and the
ionization chamber are eliminated. In most applications involving current
commercial PIDs, analyte condensation and decomposition on the window
attenuate the lamp energy, necessitating frequent cleaning and recalibration.
Custom gas blends for the pulsed discharge detector are available from
leading gas suppliers at special prices. Alternatively, they may be formu-
lated on the spot by using appropriate fixed restrictors to mix appropriate