Varian 450-GC, Руководство пользователя

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Pulsed Discharge Helium Ionization Detector (PDHID) - Detectors
Varian, Inc. User Manual 450-GC Page: 295
PULSED DISCHARGE HELIUM IONIZATION DETECTOR (PDHID)
The PDHID is a non-radioactive pulsed discharge ionization detector (PDHID)
which is optimized for the Varian 450-GC. A schematic representation of the
detector is shown in Figure 48.
The detector utilizes a stable, low power, pulsed DC discharge in helium 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 2 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 to 17.7 eV.
The detector 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 picogram
range. The response to fixed gases is positive ( the standing current increases),
with MDQs in the low ppb range.
Detector response is universal except for neon, which has an ionization
potential of 21.56 eV. Since this potential is close to the energy of the He*
metastable (19.8 eV) but greater than the photon energy from the He
2
continuum, neon exhibits a low ionization efficiency and low detector response.
When a dopant is added to the discharge gas, the detector also functions as a
selective photoionization detector. (Suitable dopants include Ar for organic
compounds, Kr for unsaturated compounds, or Xe for polynuclear aromatics).