DET REPORT
No.53 JUNE
2011
figure, NP selectivity turns on when the ion source is
heated sufficiently to ignite theHydrogen-Air detector
gases. An important NPD parameter is that the
Hydrogen ismaintained at a low enough flow that the
ignitedchemistry does not flash back and form a self
sustained flame at the Hydrogen/sample inlet orifice.
Instead, the ignitedchemistry remains asachemically
reactive boundary layer in the vicinity of the hot ion
source. Incoming samples are decomposed in this
reactive boundary layer, and electronegative Nand P
decompositionproducts extract electrons from the ion
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From the recognition that the NPD mechanism is a
surface process, ithas been possible to identifyother
surfaces and other detector gas environments that
produce other modes of selective detection. This
expansion to other modes has been facilitated by
ceramic fabrication technology which allows
development of amany different ion source coatings.
FlO
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>1.0 'I
.UPONII
source surface to form gas phase ions that
+
Gas Phase Process I
subsequently move to the collector electrode for
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I-
equal
+
and. Ions /
-
detection.
J
I
-
Data shown in Figure 5 were a milestone in
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+
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demonstrating that NPO ionization was a Surface
Ionization process rather than aGas Phase process.
- 0.5
I
f
These data show a comparison of FlO and NPO ion
,
'*
I
currents measured as a function of positive and
negative polarization voltages.
As iswell know, FlO ionization isaGasPhaseprocess
that producesequal numbers of positive and negative
ions.That isclearlyshown by the symmetry in theFlO
data.
In contrast, ion current data measured with an NPO
equippedwithaceramic ionsource demonstrated that
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-_
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>100
.200Votta
POLARIZATION VOLTADI
NPD
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Surface Process
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unequal
.ACXGaOUNO...........
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and· Ions
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the ion source was a prolific emitter of large
magnitudes of positive ion background with positive
polarizations, but a negligible emitter of background
current at negative polarizations. At the same time,
therewasasignificant ioncurrent response to the test
N compound at negative polarizations, while any
responseatpositivepolarizationswasswampedbythe
highbackground.Theunsymmetrical NPOdataclearly
indicated that the responsemechanism in thatdetector
was a Surface Ionization process rather than a Gas
Phase process.
- 0..
;-N aUPONIl
SXlO- 11
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-1.0
---+---'"
-10
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s
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10
Votta
Ceramic Ion SourceCharacteristics
• Ions: large N response compared to small background
+
Ions: very large background swamps any N response
Figure5.PositiveandNegative IonCurrent versusPositive
andNegativePolarization Voltages. FIDandNPD.
4.) LIFETIMECONSIDERATIONS FORNPD IONSOURCES.
A unique characteristic of NP detectors is that the
absolutemagnitudesof signal andnoisecanbevaried
over a wide range by changes in the ion source
heating current and the Hydrogen flow. Therefore,
signal-to-noise as well as selectivity should be the
main considerations in judging NPO performance
rather thanabsolute signal magnitudes.
The necessary and sufficient condition for turn on of
NP selectivity is that the ion source must be hot
enough to ignite the Hydrogen-Air chemistry. While
absolute signal magnitudes can be increased by
heating the ion source beyond its ignition value, that
practice leads to shorter operating life for the ion
source. Some GC-NPO manufacturers have
incorporateda flawed operating concept (e.g.,Agilent
Adjust Offset feature) that automatically increases the
ion source heat in order to maintain a constant NPO
4
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