Journal of ChromatographiCSCience. VOl. 24. Feoruary 1986
response10halogenatedcompounds can
be
selectivelyenhanced
by increasing the source heating current, while the CFID
responses to hydrocarbons remain unchanged. Consequently,
by judiciously adjusting the source heating current, theCFID
response to halogenated and some other hetercatcmcompounds
can
be
tuned to yield about the same response factor asobtained
for hydrocarbons.
The developrnernsof thermionic techniques inGC have also
spawned applications of the rechnolngv in liquid chromatog–
raphydetecucn (17), thin layerchromatography (18). and mass
spectromet ryionsources(191. Since there remain to be studied
many different combinations of thermionic source compost–
lions. thermionic source temperatures. and gas environment
compositions. it is probable that the technology will continue
to evolve in coming years.
Summary
Figure 16, CompansonorCFIOandFlO responsestoawater sampleccmao–
ing acetoneand carton telractllonde, Column: OJromosortl 102. isothermal
12ll"C.
The precedingdatahavedemonstrated tbat thermionic ioniza–
tion techniques and equipment have applications in gas
chromatography that go well beyond the usual NP detection,
such as that ofTID-2·H,/air. To achieve the best possiblesignal–
to-noise ratio and specificity for each mode of detection, the
specificchemical composition of the thermionicemissionsource
needs 10 be matched with the temperature and gas phase en–
vironment in which the source is operated.
Fromtheir extensive use in NP detection. thermionic ioniza–
tion detectors are known to often exhibit decreasing sensitivity
with increasing operating time as a result of depletion of the
thermionic scurceactivny. Consequently, the thermionicsource
usually needs 10
be
replaced al periodic intervals. Of Ihe dif–
ferent modes ofdetection described in this report, theNPmode
is the most demandingwith regard to the operating lifeof the
thermionic source. Generally, in modes of detection (e.g.,
TID· I·N,) where theoperating temperature is lower and thegas
environment is less reactive. the thermionic sources maintain
their responses over longer periods of lime. For all modes of
detection. a practicalguideline for achieving the longest possi–
ble source lifetime is to operate me thermionicsource just hot
enough to achieve the response required.
.
•
v
o
v.
•
v
CFIO
-'
FlO
--
CF I C 3><10- 8 A
It%
v / v
EACH COMPOUND
.
"' ,
..
HEAT
..
•
•
-
z
v
v
•
•
,
•
•
v
>
,
•
e
,
~
z
-
z
.:
.-
v '
,
,
v e
Z
-
•
•
,
v
v
0
,
FiQure17. Companson ofCFIDresponses tosample ormemaeneclllOndeand
venous hydrocarbons at lWO differenl maQniludes or Ileating current to
tt\e
catalytic source. Column: SP·2100. 40° 10 1800(;at IOo/min.
".
!'I-I...-----
t
' ''H C1
C.. ( [ ..
n
wiTH " " . Ol.
100.._ [ 0.
" OOf 0
Figure 15 ExampleofTID-1'a1f applicationtodetermine EOBIII
take
mil .
$ample:
reacscace vaporsrrom0.7QmDuncan HinesDeluleYelowCakeMil Inasealed
2·ml Vial,SamphnQ procedure: neat sealedsamplevial 10 12llOC: extract 200
" llleadSjlace vapor wltll
gaS'~Qllt
syrinQe: injectIntoGC andstart column oven
temperatureprogram. Column: SP·2100. 40° to 1100C ar mvrmn.
51
