DETector Engineering
&
Technology, inc.
486 N.Wiget Lane,Walnut Creek, CA 94598 USA
DET
telephone: (925) 937-4203
fax: (925) 937-7581
innovations in chemical detection
e-mail:
AGILENT 6890 & 7890 NPD - adopted sarne concentric cylinder geometry as OET - 3 types of NP ion sources are
available - Black Ceramic supplied by OET, White Ceramic made by Agilent (old formula originally licensed from OET),
and "Bios" glass bead similar to the Perkin Elmer bead except it is inserted in the concentric cylinder geometry like that of
OET.
THERMOSCIENTIFIC TRACE GC - uses ceramic ion sourcesmanufactured by OET, but side insertion of the ion source
is similar to theVarian configuration and has more dead volume than the concentric cylinder geometry.
IDENTIFICATIONOF THENPDOPERATINGMECHANISM
AS A SURFACE IONIZATION PROCESS
The accompanying graph was an important milestone
in the evolution of understanding the NPO operating
- FlO
I
I
, ....._-::.
.....
aUPON51
.:
Gas PhaseProcess
+0.5
-
- equal
+
and - ions
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-
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-+-.-+- +
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. . 100
POLAIlIZATION VOLTAGE
-200
- 100
o
. 200 Volts
u
z
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0 '-
NPO'
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Ceramic Ion Source Characteristics
- ions: largeN response compared to small background
+ ions: very large background swamps anyN response
mechanism. The data in the graph compared ion
currents measured for representative samples using
an FlO versus anNPOwith aCeramic Ion Source.
As
is well known, the ionization in an FlO is a Gas
Phase process that produces equal numbers of
positive and negative ions. That is clearly shown by
the symmetry in the FlO data as ion currents were
measured versus negative and positive polarization
voltages.
In contrast to the FlO, ion current data measured with
the NPO demonstrated that the ceramic ion source
was a prolific emitter of large magnitudes of positive
ion background with increasing positive polarizations,
but a negligible emitter of background current at
negative polarizations. At the same time, there was a
significant ion current response to the test N
compound at negative polarizations while any
response at positive polarizations was swamped by
the high background. The unsymmetrical NPO data
clearly indicated that the response mechanism in that
detector was a surface ionization process rather than
a gas phase process. From the recognition
of
the
NPO mechanism as a surface process, it was then
possible to proceed on to identify other surfaces and
other detector gas environments that produced other
modes of selective detection. The ceramic fabrication
technology facilitated this expansion to other modes
by not being limited by ion sourcemelting constraints.
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E-mail :
TelNo : 03 9762 29034 . . . inAUSTRALIA
