VICI D-4-I-TQ-R - Manual (Page 17)

13
Mode Selection and Setup
Helium Ionization Mode
If the instructions at the top of page 11 were properly executed, the column
should already be properly positioned. 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 ± 2.0 mm. DO NOT insert the column
more than 15 cm.
With this flow configuration, only pure helium passes through the discharge
region, minimizing the chance of discharge electrode contamination through
contact with the eluting sample. 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 associated
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 form-
ulated on the spot by using appropriate fixed restrictors to mix appropriate
amounts of pure helium and pure dopant through a tee. Since all gas
streams must pass through a Valco purifier, the second option requires
an additional purifier for each dopant. This may still be more cost effective
than requesting a custom blend of the more expensive Kr or Xe; since the
typical flow rate required for the pure dopant rare gas is about 0.3 -
1 mL/min,
a small lecture bottle can last for a long time. In either case, the total dis-
charge gas flow rate should be the same as specified in “Gas Connections”
on page 7.