Modes of Response
Schematic illustrations of four different versions of therm–
ionic ionization detection equipment are shown in Figures
I
through
4.
Common components in eachversion are
as
follows:
(AI an electrically-heated, thermionic/catalyt ic source con-
structed of mu ltiple layers of ceramic coatings;
(B) a cylindrical collector electrode surrounding thecylindrically
shaped thermionic source;
(0
a sourcepower supply that provides heatingcurrent to heat
the source to typical temperatures of
400°
to
8OO"C,
and
a bias voltage to polarize the source structure at a negative
voltage with respect to the collector;
SAM'lE
CONDUIT
~
I
,
I
I
I
...
-,
/
V
,
1
,
,
,
SAMPLE
+
GAS I
",
{AIRJ
HelMIONIC
HEATING
souICE:
CUIUNT
IODYe~
SURFACe
liAS
VO~UGE
~
K
rr:
f
t-,
r',
-,
-
ELECTROMETEI
<,
+
-,
I,
"-'
.
~COLLECTOI
. ,~
'-
-..-,.... .#
'-
1
I
,
,
I
,
GAS 2
" 2
(AI R)
T
(0 1an electrometer that measures negative ionizat ion
currents
ar riving at the collector electrode.
The TID hardware usually mounts onto an FID- IYpedetector
base thai is resident on a GC. so that two different detector
gasesmay
be
supplied in addition to the GCeffluent. Therefore.
changes in [he modes of detector response [hat correspond to
the schematics of Figures
I
through
4
are accomplished by
changes in the type of thermionic source. changes in the
com–
position of gases supplied to the detector. or by changes in Ihe
operating temperature of the thermionic surface.
Most of the TiCs availablecommercially function by the
col–
lection or negative ionization rather than posi tive ionization.
In the discussion that follows. it wiUbe shown that theconcepts
of negative ion chemistry provide a logical
panern
for correlating
the responses of the different modes of thermionic detection.
T1D-I-N 2 [AIR]
T1D-l ·N1; Nltrof.leetronevatlve specific response
The simplest mode of thermionic detection is represented by
[he schematic in Figure 1. In thismode. the lowwork function
thermionic source designated by the TlD-J nomenclature is
operat ed in
a
detector gas
environment
of
N,.
Because the
derec–
tor gases are inert , samplecompounds interact directly with the
TID-I surface. which is typically heated
(0
temperatures in the
range of
400°
to
6Q0
0
c.
The ionization process in (his
case
is
direct transfer of negative charge from the TID-I surface to
Low
'Co'
Tl0-2
Table
I.
Thermionic
Source Surface
Layers
a
precess
that proceeds through a molten glass state (II).
The detailed chemical eomposit ions of thermionic emission
sources are usually regarded
as
eonfidential proprietary infor–
mat ion by the manufacturer. Since the first alkali-glass bead
reponed byKolb and Bischof! usedRb
as
the alkali compound.
there
existed
for many years a
belief
that Rb was an essential
component for optimum
NP
respon~.
However, in
recent
years. l'iP detectors with state-of-the-an performance specifica–
tions have been reponed in which Cs rather than
Rb
is used
as the alkali component
(S) .
Also. another recent report
(12)
has described an
NP
detector which
uses
a LaB./SiO, bead and
no alkali additive. In accordance with a mechanismof surface
ionization prevailing in theTID. themost important
character–
istic of the
thermionic
emission surface is its electronic work
function (i.e., the amount of energy required to emit a unit of
electrical charge from the surface). Alkali-metal compounds
have beenespecially successful additi vesbecause they lower the
work function of the glass or ceramic matrix. therebyfacilitating
the emission of charged particles from the heated thermionic
surface. The mathematical relationships between work func–
tion. surface temperature. and thermionicemission current have
been discussed
(2.12).
The development ofmultiple-layered, ceramic-coated tberm–
ionic emission sources has allowed examination of coatings of
many different chemical compositions without the risk of
materials in the surface layer corroding the heater wire. In the
search for expanded applications for thermionic ionization
techniques.
lhe
basic
task
is to defme a speafic
match
of a
toerm–
ionic source type with
an
operating
ps
environment
and
a range
of operating source temperatures. To date, three different
chemical
compositions of thermionic sources have been shown
{5,61
to
have
useful applic:ations
in
differingmodes of thermionic
detection.
These
source compositions are shown inTable
I.
Data
obtained using these three types of thermionic emission sources
are presented in the following sections.
CFIO
Fl\lu,,~
1. SChemanc
~luS1ratlOll
Olltledeteetl()l'l conflqurauon forme!llem'»Ol'llC
IO/UZiItlonmodes
TlO·I ·N.
i ll(! T1D·1·i lr, Detector gas
1.FIO·Ht
ll"Ilet
lme:
detector
!liS
2..
nn-a«
Inlet hne.
42
