VICI D-3-I-7890 - Manual (Page 20)

16
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.
Selective Photoionization Mode
To maximize detector lifetime, turn off the discharge
power when the GC is not actually analyzing samples.
This is especially important in the Ar/Kr PID 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
discharge gas flow rate should be the same as specified in “Connecting
the Discharge Gas to the Detector” on page 12.