DETector Engineering &Technology, inc.
486N.Wiget Lane,Walnut Creek, CA94598 USA
DET
telephone: (925) 937-4203
fax: (925) 937-7581
innovations in chemical detection
e-mail:
Ionization Detector (FlO). A main difference between an NPO and an FlO is that the NPO
requires much lower flows of Hydrogen and Air, so detector pneumatics controls must be
capable of controlling these low flows. Also unlike an FlO, an NPO does not involve a self
sustained flame burning at a jet structure, so anyNPO jet structure serves only as a means
of routing theGC column, sample, and detector gases into the detection volume.
5. NPDfTlO electronics include a power supply that electrically heats and polarizes the ion
source, and an electrometer to measure signals. All OET ion sources produce negatively
charged ions so the ion source is polarized at a negative voltage relative to the collector,
and the electrometermeasures negative rather than positive ion current.
A power supply that provides controlled levels ofConstant Current to heat the ion source is
preferred to a power supply that provides controlled levels of Constant Voltage. With
Constant Voltage power, the magnitude of heating current through the ion source's wire
core depends on the resistance of thewire core plus all the electrical leads and connectors
to the ion source. Conversely, a Constant Current supplyprovides the same heating current
irrespective of these resistances.
As
the electrical resistance of the ion source increases
slightly with thermal equilibration and age, Constant Voltage power produces an
undesirable decrease in heating current and a corresponding decrease in detector
response that is purely caused by t he electronic powermeans r ather than by a loss in
ionizing activityof the ion source's surface.
The polarizing voltage applied to the ion source produces an electric field that causes ions
formed at the ion source surface to migrate to the collector electrode. In an optimum
concentric cylinder detector configuration, the internal diameter of the collector cylinder is
normally0.25 to 0.30 inches, and an optimum polarization for NP detection is in the range of
3 to 5 Volts. Higher polarizations for the NPmode generally result in decreases in signal to
noise and in selectivity. For other modes of detection, higher polarizations in the range of
50 to 100Volts can produce improved signal to noise byasmuch as a factor of 10.
Currently, the NPO electronics on a Thermo Scientific Trace GC provide themost versatility
for all modes of NPOfTlD detection. Thermo's power supply provides Constant Current
heating in the range of 1.000 to 3.500 Amps settable in increments of 0.001 Amps; negative
polarization voltages settable in the range of 1 to 99 Volts; capability of turning ion source
heating current On/Off as a timed event; and readout indication of an open circuit in the
source wiring. A limitation of NPO electronics such as those built into Thermo and Agilent
GCs is that the NPD power electronics are subject to thermal gradients inside the
instrument package corresponding to when the GC oven cools, and that can cause
excessive upsets and wander noise in the detector baseline. Specifications for a stand
alone DET Current Supply include constant heating currents in the range of 0.000 to 4.000
Amps settable in 0.001 Amp increments; switch selection of - 5, - 15, or - 45 Volt
polarizations, and a Status Lightwarning indication ofan open circuit in the sourcewiring.
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