VICI D-2-I - Manual (Page 28)

24
Mode Selection and Setup
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 “Connecting the
Discharge Gas to the Detector” on page 16.
Ar-PDPID
Changing the discharge gas from helium to a mixture of 2% argon in
helium changes the photon energy level from the 17 - 13.5 eV range to the
11.8 - 9.8 eV range. The argon emission consists of resonance radiation
at 11.8 eV and 11.6 eV and the diatomic Ar
2
emission in the range of 9.2
- 10.3 eV. Except for fixed gases and a few organic compounds like CH
4
(IP = 12.5 eV), CH
3
CN (IP = 12.2 eV) and some fluro-chloro hydrocarbons,
the majority of organic compounds have ionization potentials lower than
11.8 eV. Thus the Ar-PDPID is nearly universal, like the flame ionization
detector, but without the risks associated with the presence of an open
flame and hydrogen.
Kr-PDPID
The recommended proportion is 1.4% Kr in He as the discharge gas.
The krypton emission consists principally of resonance lines at 10.6 eV
and 10.1 eV. The Kr-PDPID can detect compounds with IP < 10.6 eV,
which includes unsaturated and cyclic hydrocarbons, alcohols, aldehydes,
organic acids, esters, etc.
Xe-PDPID
The recommended proportion is 0.8% Xe in He as the discharge gas. The
xenon emission consists principally of resonance lines at 9.6 eV and 8.4 eV,
and can detect compounds with IP < 9.6, like aromatics, ethers, alcohols,
aldehydes, etc.
In addition to the specific compounds named in the three paragraphs
above, certain important inorganic compounds like ammonia, hydrogen
peroxide, arsenic trichloride, hydrogen sulfide, arsine, phosphine, nitric
oxide, carbon disulfide etc. can be selectively detected using the
appropriate photoionization mode.
Each dopant gas requires an additional helium purifier, which must be
purged and conditioned in the same manner as the purifier installed on
the discharge gas supply. If you are using more than one dopant, we
recommend use of a Valco multiposition stream selection valve so that no
fittings have to be disconnected. Not only is this convenient, it keeps the
system closed, minimizing chances of contamination. When changing from
one dopant to another, allow at least one hour for the old gas to be purged
from the system.