Journal ot Chromal09raDhoC
$c,.roe•.
Vol. 2". Fetlruary 1986
Dlnvited
Paper'[l
================
Recent Advances in Thermionic Ionization Detection
for Gas Chromatography
P.L. Patterson
Detector Englneenng
&
TechnolOgy. Inc.• 2212 Brampton Road. Walnut Creek. California 94598
Abstract
1'tMrmk)niC Ionlz.atlondatec1:Of1I an
m~
widety
used
for
1M
spKtflc
detection
01
nIil~
compounda
lnga~ography.
1'heopetatlng
n..cNnisrn
ofthne
del.etOf8 .. . surface lontzaUon
proc. ..
lnwhk:h the
k.y
patamet.,...re the wor1t funcUon
01
tIM tlMrmkmk:
.mi..
aion surfaca,
ttt.
tamparJIture of tM thlIrmlonic surface.
and tMeompo"'Uon
01
tMgaa anvironment In tM imm.–
dllile viclntty
01
1M tMrmloniC IWrface.
By
s)'*tamlltic
variations
01
aKtl
01
tha..
ttl,... kay pa,..malan. tM
111Chnlque of
~rmloniC
ionization
delectlon
hu
been
graatly axpanded to encompaaa a nl.lmbar of dlffarent
moonof
'"90"..,
all 01
whiCh use similar datllClOl'
hard–
wara and electronic eomponents.
Introduction
Thermionic ioniza tion detectors (TID) are best known in gas
chromatography(GC) for their application to thespecific detec–
lion of nitrogen (N) or phosphorus (P) compounds. All modern
TIDs are tuenlially derivationsof a basic design first described
by Kolb and Bischoff (I) in 1974. The main component in this
type of detector is an electrically-healed thennionic emission
source
in
the fann of a
bead
or cylinder which
is
usuallycomposed
of an alkali-metal compound impregnating a glass or ceramic
mat rix. In
th~
TID. the thermionic source is positioned so that
sample compounds may impinge upon its surface. and any
lcnuancn produced is measured by an adjacent collector elec–
trode.
Kolband Bischoffwere
the
firsl lOreport
that
a thennionic
source comprised of a Rb-silicale glass bead produced very
specific NP responses when the bead was operated at high temp–
erarares in a gas environment of dilute H. in air.
Since the original work ofKolb and Bischoff. there have been
conlinuingdevelopments in NP detectors. withmuchemphasis
on improved melhods of construction and composition of the
thermionic emission sources. Themost imponant development.
however,
has
been
the recognmcn in recent years lhat me opera–
non
mecnanemof aTIDisa surface ionization process(2) rather
than the gas phase lcmeaucn process originally proposed by
Kolb
et
&I.
11 ,3). Once it was clear Ihal a surface ionizalion
process was operanve, il was possible to identify three key
operating parameters which control the ionization produced.
These parameters Ire:
the
electronic work functionof the
therrn–
ionic
emission surface which isdetennined by the chemical com–
position of Ihe
surfa~;
the temperature of the thermionic sur–
face; and the chemical composition of the gas environment im–
mediately surrounding the thermionic surface.
The identification of these parameters
has
led to a dearer
understanding of the complexchemistry active in NPdetection,
andhas
provided an important guide for expanding the applica–
lions of thermionic ionization teehniques , Through syuernane
variations in each of the key parameters. many different modes
of detector response have been achieved
(3-6).
Hence, the tech–
nique of thermionic ionization detection now correctly refers
to a number ofGC detector responseswhich are related through
lhe useofmanycommon hardwareand electronic components.
This article reviews the present state of development of the
members of this unique group of detectors.
Types of Thermionic Emission Sources
All commercially available TIOs use thermionic emission
sources formed according to one of the following four general
methods:
(A) homogeneous alkali-glass bead formed on a loop of bare
platinum wire
(I);
(8) alkali salt aclivator coated on a ceramic cylinder core con–
taining an embedded heater coil (7);
(0 homogeneous alkali-ceramic bead formed on a coil of
nichrome heater wire (2,8.9);
(0)
multiplelayersofcylindrically-shaped ceramic coatings. with
a nca-ecrresbe. elearically<onducting sub-layer of Ni–
ceramic completely covering a loop of nichrome wire. and
a surface layer comprised of alkali and/or other additives
in a ceramic matrix
(S,10).
Thermionic sources representing all four categories cited
above have been used in NP detectors available from differenl
manufacturers. Generally. those sources formed fromceramic
materials provide grealer flexibility for varying the chemical
compcsincn of the source. This is because Ihe ceramic eom–
positions are formulated and coated from a slurry at room
temperature (91.whereas the glass eempcsmons are formed in
"--LP" )
'11) 01
01....._.
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41
