Ut:I
innovations in chemical detection
NPO:
selective for NITROGEN or PHOSPHORUS compounds
TID-2-H
2/AIR
&
TID-4-H
2/AIR
Equipment:
This detection mode uses a
TID-2 or TID-4 type
thermionic source mounted in a concentric cylinder
geometry relative to a collector electrode. Thedetector
gases are a low H
2
flow (2 - 6mL/min) and a flow of
air (50mL/minl . The source is heated by a constant
current supply and is polarized at -5 Volts with respect
to the collector. During operation, the surface
temperature of the source is in the range of 600 BOODC which produces a visible orange glow.
Principle:
Thismode uses a thermionic surface of moderatework
funct ion operated in a dilute H
2/air
environment . High
select ivity is obtained for organic compounds
containing N or P atoms. The NP selectivity turns on
abrupt ly when the temperature of the source exceeds
an "ignition" point (about 600 DC) for the H
2/air
mix
ture. This "ignition" of H
2
and O
2
in the detector gases
creates a hot , chemically react ive gas boundary layer
containing radical species such as H,
0,
and OH in the
immediate vicinity of the source. Sample compounds
impact this boundary layer and are decomposed.
Samples containing N or Patoms form decomposition
products which are elect ronegative and which are
converted to gas phasenegative ions by the ext raction
of electrons f rom the thermionic surface.
Response:
The existence of a chemically reactive boundary layer
is an essential requirement for responses to a wide
range of N or Pcompounds. Unlike an FlO, there is not
sufficient H
2
supplied to sustain the "flame like "
boundary layer when the heat ing current .to t he
thermionic source is turned off. The two most impor
tant controls are the source heat ing current and the H
2
f low . The thermionic source is normally operated at a
heat ing current just sufficient to give "ignition" of
HEATING
CURRENT
TID-2
THERMIONIC
SOURCE
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POLARIZATION
VOLTAGE
ELECTROMETER
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COLLECTOR
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t
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t~t
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SAMPLE INLET
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AIR
J
:
LAIR
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L SAMPLE
&
Hz (2-6rn1.1mln)
theboundary layer. For organic compounds containing
N or P atoms, the detection limit is 0.07pg N(PI/sec,
the selectivity versus hydrocarbons is 10
5gC/gN(P).
NPdetectors from different manufacturers varywidely
in performance. The ceramic coatings on OET
thermionic sources are formulated to provide long term
responses. TheTID-2 type source is recommended for
applications requiring both Nand/or P detection, and
features minimal tailing of P compounds. The TID-4
type source is recommended for applications requiring
only N detection, and represents our best possible N
response.
All DET
sources are tested
chromatographically before shipment. Also, a cost
saving recycling service is available for depleted
sources.
Website :
E-mail :
TelNo : 03 9762 29034 . . . inAUSTRALIA
