VICI D-2-I - Manual (Page 6)

2
Helium Ionization (PDHID)
The PDHID 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 or sub
picogram range. The response to fixed gases is positive (the standing
current increases), with MDQs in the low ppb range. For trace analysis
of fixed gases optimized for packed columns (low ppb range), we offer a
modified version of the detector dedicated to the ionization mode (Product
Number D-2-I).
The PDHID 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. (NOTE: Valco makes an enhanced mode detector for neon.
Consult the factory for information.)
Photoionization (PDPID)
Changing the discharge gas from pure helium to helium doped with argon,
krypton, or xenon changes the discharge emission profile, resulting in
resonance atomic and diatomic emissions of the rare gas added. Response
is limited to sample compounds with ionization potentials less than or equal
to the dopant gas emission energy. In this configuration, the detector is
essentially functioning as a specific photoionization detector for selective
determination of aliphatics, aromatics, and amines, as well as other species.
Since there is no lamp or window, sensitivity will not change with time.
Electron Capture (PDECD)
In the electron capture mode, the PDECD is a selective detector for
monitoring compounds with high electron affinity such as CFCs, chlorinated
pesticides, and other halogenated compounds. The MDQ for this type of
compound is at the femtogram (10
-15 ) level. Response characteristics and
sensitivity are similar to those of a radioactive 63 Ni ECD.
In addition to minor configuration changes, running in the ECD mode
requires the addition of a dopant gas (recommended is 3% xenon in
helium). The dopant gas is first ionized by the photons from the discharge.
Resulting electrons, in the absence of any electron-capturing compounds,
constitute the detector standing current. Electron capture processes occur
when electron capturing compounds enter the detector, resulting in a
decrease in the detector standing current which constitutes the PDECD
response.
That the constant-potential ECD response is not linear over a wide range is
well known. The signal from the PDECD is best described by:
( I
b
- I
e
)
/
I
e
= K A x [A]
where I
b
is the detector standing current, I
e
is the detector current measured
in the presence of analyte, A, K
A
is the electron capture coefficient, and [A]
is the concentration of A. The majority of commercially available ECDs use
a variable frequency, constant-current technique to increase the linear
Introduction