DET
innovations in chemical detection
REMOTE FlO:
selective for
LEAD, TIN, PHOSPHORUS,
or
SILICON
compounds
Equipment:
is greater than 5 x 10
5
gC/g(Pb,Sn,Pl. Applications
include detection of lead in gasolines and gasoline
contaminated soil and water, organo-tin contaminants
inwater, phosphorus pesticides, and silicon bleed from
gel implant prostheses. Since the ions detected in this
mode are produced in the flame combustion process
as in an FlO, this detection mode is characterized
by
stable signal magnitudes over long operat ing times as
long as the gas flows are maintained.
COLLECTOR
POLARIZATION
VOLTAGE
ELECTROMETER
FLAME
+
I
I
I
L_
OXIDANT
•
I
I
I
SAMPLE
&
FUEL
..
..
,
\
\
\
I
LONG-UVED
NEGATIVE IONS
HEATING
CURRENT
+
I
I
I
OXIDANT --'
CFID
SOURCE
POlARIZER
This detection mode uses a
CFIO
type thermionic
source mounted in either a
REMOTE FlO
or
FTIO
TOWER.
The CFIO source is used because it has the
highest work function of the available source types.
and in this mode the source is used simply as a flame
ignitor and a polarizer electrode instead of as a source
of surface ionization. The thermionic source polarizer
and ion collector are located several centimeters
downstream of a flame jet rather than immediately
adjacent the jet as is the case in an FlO. The detector
gases are air and a mixture of H
2
and an organic fuel
such as methane. Heating current to the CFIO source
is increased to ignite a self-sustaining flame which
flashes back and burns at the jet structure. After
ignition, the heating current is reduced to zero or some
low value . The source in this mode is normally
polarized at -15 Volts relative to the collector for best
sensitivity and dynamic range , or -5 Volts for best
selectivity.
Negative ion currents arriving at the
collector electrode constitute the detection signal.
Principle:
Response:
This mode is based on the discovery that some
heteroatom compounds form very long-lived negative
ion species in flame combustion. These stable ions
survive to be measured by the downstream collector,
whereas ions from hydrocarbon combustion are
dissipated by recombination processes before reaching
the collector. The thermionic source serves only as a
polarizer, and provides no additional ionization of
samples. The detector uses an air-rich flame, and an
organic fuel is added to further improve selectivity
versus complex hydrocarbon matrices.
This mode provides selectivity for lead (Pb), t in (Sn),
phosphorus (Pl. and silicon (Si), but it is unresponsive
to N, 0, S, or CI. Oetectivity for Pb, Sn, or P is 1pg
(Pb, Sn, Pl/sec,
and-the
selectivitywith an organic fuel
DETector Engineering
&
Technology, inc.
