Journal or CI'lromall.'9'aon.e $coence. Vol. 2•• Feo,uat'f 1986
the samplemolecule. Consequently, thismode of response pro–
vides exceptionally high
specificity
and sensitivity tomany com–
pounds containing the nitro (NO,) functional group
(5,13),
as
well as
to
certain other electronegative compounds
(e.g.,
pen–
tachlorophenol, diazepam, and methaqualone) . Thismode of
detection is very sensitive
10
the detailed
electrcnegauvny
of
the sample's molecular structure, as has
been
illustrated by the
observation that a larger signal is obtained for
thez.a-ecmer
of dinitrotoluene in comparison
10
the
z.s-Iscmer (5).
The
TID-I-N, mode is superior
to
a conventional NP
detector
or
an electroncapture detector
(EeO)
for detectionof trace level
nitro-compounds such as nitro-PAH, nitro-explosives, nitro–
pesticides
te.g.,
parathion,
merhyl
parathion), nitro-drugs, nitro–
derivatives.
For many nitro-compounds, the specificity
vs.
hydrocarbons isan astonishing 10', and detecnvityis in the0.1–
to I.o-pg range
(5).
TlD-1..
lr:
Hslogenlnltro speci fic re1900"
When theTID-Ilhermionic source isoperated in anoxygen–
containing gas environment rather than one of N" specific
responses to halogenated compounds arc enhanced while
responses to
nirro-ccmpounds
are
decreased.
somewhat
(14).
The
TID- I-air mode of detection is generally not as sensitive as an
ECD or Hall detector for
chlorinated
compounds, bUI it pro–
vides halogen specificity at higher concentrations where
ECO
and Hall are saturated . Typical specificity is
10"
and
detectivity
is0.1
to
1.0 ng. This is an especially simplemode of detection
for ethylene dibromide (EDB) in the
headspace
vapors of food
products.
TID-2·H
2
/slr:
Nitrogen/phosphorus specIfic
response
The schematic illustrationof Figure 2
represents
the
situation
rhat prevails in an NP detection mode. For Ihismode, H, and
air gases are supplied to the detector, and a thermionic source
ofmoderatework
function
(i.e. ,
TID·2) isoperated hOIenough
(600°
to gOO·C) to cause thermal/chemical decomposition of
the HI and
0 ,
gases . A critical parameter in this NP mode is
the
resrricnon
of the H, to low flows
(e.g.,
3106
ml/min)which
are nOI
sufficient
to
maimain
a
self-sustaining
flame at the sam–
ple conduit
(i.e. ,
jet structure) depicted in Figure 2. Instead,
a flame-likegaseous boundary layer is created in the immediate
vicinityof the hot thermionic source. Since this boundary layer
isvery
reactive
chemically, samplecompoundsaredecomposed
by the
activegas
phase chemistry, and
electronegative
products
of decomposition are
selectively
ionized by surface ionization
on
the thermionic
source.
N
or P compounds are ionized with
especially high specificity by this process.
An
essential condition
for the onset of NP detection is that the
thermionic
sourcemust
be hot enough to " ignite" the boundary layerchemistry. Under
these
conditions, a thermionic source of
moderate
work
Iunc–
tion provides the optimum compromise of sample response
signal
vs.
detector
background
signal.
A
lowwork function
them-
CfID
~FlAM£
..
(!'i,O, OM]
,
,
HEATING
CUUENT
IC
I ' AS
VOlTAGE
f''''1'-
.-
+
~f'
f'
f'
~~f'
I-
ElECTlOMETER
I'
,
~'"
,
+
I
\
,
r-,
",f'
,
\
1\"""
J
I--COllECTOI:
,
,
\
/
+
•
I
,
I
I
,
I
,
,
AIR
SAMPLE
+
",
CATAlYT
SOUICE
~~u
RE,lI,CTIVE
-
~80UND,lI,R Y
I
lAYER
I
I
I
SAM~ lE
CONOUI
T
"–
-,
,
v
,
I
,
I
I
SAM~l E
+
GAS 1
",
HERMIONIC
HEATING
r-
SOURCE
CURRENT
IOD~E~
SURFACE
l IAS
VOLTAGE
r-:
I ::
K
,
r
+
,
.
~
'~l
,
,
,
,
1
~
f-
ElECTROMETE lt
1:-
,
,
r-,
,
+
,
.
, I.
,
r-,
I .
}-I .
I
r--COLlECTOR
.~'
, ~ : ~. ~
';A
,
,
1
,
I
I
GAS
2
AIR
T
Figure
2. sceeeanc iUustratlOfl
01
the
detection
COnfiQlJrallOn
ror
the
TIO·2-H2"air
or NPmcoeOr tl'lermomc IOnlzallon.
FiQure3. Schematic iUlJStration
01
tile oetecnoe configulaltOn tormecatalytic
flame
lonLZallon oerectcr
(CFIO)
mode
01 response.
43
