Journal orCh'omal09'ap'"CScf6nce. VOl. 24. F$O,uary 1966
Remote FlO: OrgancMead specific respon..
This mode of detection is a simple variation of the FTID in
which noelectrical heating is supplied to the thermionic source.
and the ion-suppress voltage depicted inFigure 4 isnot applied.
In thiscase. the thermionic source servesmerelyas a polarizer
to drivenegat ive ions in the flame effluent to the TIDcollector
electrode. The ionization sensed in this mode corresponds
to
long-lived negative ions originally produced in the flame.
Becauseof the large separation belweenthe name and theTID
collector electrode. the bulk of the hydrocarbon ionization pro–
duced by the flame is dissipated
(i.e.,
posit ive-negative ion
recombination or neutralization at a wall surface) before
reaching the TID collector. However. certain hereroatcmcom–
pounds appear to combust to negative ion products which are
especiallystable and long-lived. The outstandingdemonsrratiOll
FTIO: Nl1rogenlhalogen specifIc respon..
Figure4depicts a further detectionmode inwhich the therm–
ionicsource and collector electrodestructure are positioned well
downstreamof the active region of a self-sustaining flame. The
basic concept (6) of this flame thermionic ionization detection
(frIO)mode is to burn samplecompounds ina self-sustaining
H,/ air flame at a flame jet, and to selectively re-ionize elec–
tronegative combustion products bymeans of the thermionic
ionization components located downstream. In the frIO, a
large physical separation between the flame and the thermionic
source/collector electrode providesminimal collection of ioniza–
tion produced in the flame, but excellent collection of ioniza–
tion produced at the heated surface of the thermionic source.
Ah auxiliaryion-suppress voltage can
be
applied10further pre–
vent ionization produced in the flame from reaching the ioniza–
tioncollector. Thismode of detection provides specific responses
to compounds containingNor halogen atomswith a specificity
of 10' and detecrivity of 1.0 ng. The precombustion ofsamples
in the flame minimizes interferences from sample matrices
and provides more uniform responses independent of the
original molecular structureof the samplecompound. Both the
lowwork function (TID-I) and moderate work function (TID-2)
thermionicsources have been used in this frIOconfiguration.
FTIO-I provides good responses to both nitrogenand halogen
compounds. whereas FTIO-2 responds best for halogen com–
pounds
with
suppressed nitrogen response.
tion processes identical to thosewhichoccur in a conventional
FID, and surface ionization processes at the catalytic: source
structure which especially enhance the ionization efficiencyof
many
herercatom
compounds (especiallyhalogenated and phos–
phorus compounds).
Like
a
conventional FID. the magnitude of the gas phase
ionization is
determined
primarily bythemagnitudes of
H,
and
air flowsand the size of the jet orifice, so that additional elec–
trical healing of the CFIO source has
little
effect on the gas
phase ionizat ion. However, the magnitudes of ionization pro–
duced bysurface processes at theCFIOsource isstronglydepen–
dent on the electrical heat ing of the source. Hence. in many
cases. response factors for heteroarom compounds can be
enhanced to be comparable to hydrocarbons by a judicious
selection of source heating current. For this CFIO mode of
detection. the thertmcnic/catalytic source of highestwork func–
tion is most suitable because the flame heat would otherwise
causean excessive thermionic emissionbackground signal . The
CFIO provides
detecuvu ies
in the
!O-
to lOO-pg range formost
organic compounds.
HU MIONIC
~
Hf.ATlNG
sec ece
CUIIENT
.OO:E-:\
SU.. ' ....CE
.1....
S
VOUAGE
-
~
~
+
1.-',
r-,
I'-.
I-
flfCTlOMf.f£1
I'-.
4-
I'-.
I
~
~COllECTO"
L
....
,
.
t I-
I
t
I
I
I
I
I
I
I
I
I
COM' Ust ION
,Ioouets
b
/\
[
~
HAMf ION
SU.. IUS
~
VO lTAGf
I
~
HAMf
I
,
I
COM'Ust ION
I
I
Figure 4 Scnemac Illustration
01
thedetl!'Ctlon configuration tor
the
name thermo
cee toneanondetector
(FTID)
mode 01 response,
T
)
,
O X iDANT
FTI O
CFID:
Unlvel'Slll
response to
.11
org.nlcs
Figure 3 depicts a mode of response achieved when the H,
flow
[Q
thedetector is sufficient
to
produce a true self-sustaining
flame burning at the jet structure. This mode of operation has
been designated (5) catalytic flame ionization detection (CFlO)
becauseof its dose similarity 10a conventional FID. TheCFID
is essentially an FlOwhich has been modified by inserting an
electri -ally-heared catalytic source comprised of a Ni-impreg–
nated ceramic into the center of theactive flame region. In this
detectionmode. the catalytic source structure serves the three–
fold function of name ignitor. flame polarizer. and catalytic
combustion modifier/thermionic surface ionizer. In the CFID.
tWO types of ionization processes are active:
gas
phase Ionize-
ionic source te.g.•TID-I )operated under these conditionswould
produce an overwhelmingly large background signal. Conversely.
a high
work funcucn
thermionic source
(e.g.•
catalytic name
ionizationdetect ion)operated under these ccnditicnswould pro–
duce smaller
NP
signals and less specificity than the moderate
work function thermionic source. Typical performance specifi–
cations for NP detectors are detecuvuies in the range of I to
10
pg,
and specificity with respect to hydrocarbons in the range
of 10' to 10'.
44
