DET
innovations in chemical detection
PTID:
(Phosphorus Thermionic Ionization) selective detection and very large signals
for P with suppressed N response.
Equipment:
This detectionmode uses a T10-6 type thermionic
sourcemounted inaPTIOTower. The PTIOtower
structure is similar to a Remote FID Tower in
positioning the thermionic source several
centimeters downstream of the jet. However, the
PTIOTower contains a reduced internal diameter
for high gas velocity to prevent flame front
flashback fromthehot source to the jet. This allows
higher HydrogenandAir gas flows to be used than
are possiblewith anNPD. In the PTID, typical gas
flows areH
2
=
20 - 30
mUmin, and Air
=
250 - 500
mUmin. The thermionic source is heated by a
constant current supply and is polarized at -
15
V
with respect to the collector. During operation, the
surface temperature of the source ismaintained in
the range of
600 -
BOO°C
which produces a visible
orange glow.
Principle:
This mode uses a thermionic source of highwork
function operated inanFID-IikeH
2/air
environment.
However, unlike an FlO, an internal flow restrictor
prevents flame front flashback to form a self
sustaining flame at the jet orifice where H
2
and Air
are initially mixed together. Instead, an ignited,
chemically activegas boundary layer ismaintained
about the hot source surface similar to an NPD.
Becauseof thehigher Hydrogen andAir, this PTID
boundary layer hasamuchhigher concentration of
chemically active radical species. NPD thermionic
sources do not hold up well in this harsher
environment, so this mode of detection requires a
moredurableceramicsourcesurface. LikeanNPD,
sample compounds are decomposed in the
gaseous boundary layer, and P compounds form
PTID
TID-6
THERMIONIC
SOURCE
COLLECTOR
REACTIVE GAS
BOUNDARY LAYER
HIGH GAS VELOCITY
PREVENTS FLASHBACK
OF FLAMETO ..lET
'" '" ,fI.
In1n l
I
I
I
I
I
I
Air - -
j
I
L - -
Air
I
Hydrogen
&
Sample
decomposition products which are converted with
high efficiency to gas phase negative ions by
extracting electrons from the thermionic surface.
Response:
PTID response to P compounds is generallymore
than
10
times larger than the corresponding
responseofanNPD.However, thebackgroundand
noisearealso larger, sodetectivity iscomparableto
anNPD
(0.07
Pg P/sec). The dynamic range
0
f
responseof aPTIDexceeds5ordersof rnaqnitude,
and it hasexcellent selectivity vs. hydrocarbons, as
well as vs. N, 0, CI, Br, S, and Si
DETector
Engineeri'ng
&Technology, inc.
II
