DETector i::nglneenng
&
I
ecnnoioqy,
me,
486 NorthWiget Lane
Shadelands BusinessPark
DET
Walnut Creek, CA 94598 USA
innovations inchemical detection
Phone: (925) 937-4203, FAX: (925) 937-7581
e-mail:
CUSTOMAPPLICATIONSOFDETTHERMIONICSOURCES
General Construction of the Ion Sources
All DETThermionicSources have a finewire core covered by layers of ceramicmaterial. The outermost
ceramic surface coating determines the ionizing characteristics (i.e., TID-1, TID-2, etc. type of ion source).
The 1/16 inch diameter ceramic part of the source is fragile and can be brokenwith excess tranverse
force. Handle the fragile ion source
with
care. carefully remove the protective Tygon tubing from the
metal source mounting flange. When placing the ion source down onto a surface, make sure the source is
resting on itsmetal flange rather than on the ceramic. In operation, the source and itsmetal mounting
flange get hot. Handle a hot source by touching only the fiberglass sleeving, and do not place a hot source
down onto a surface that will not withstand the heat. Standard sourcemounting flanges are a hexagonal
mount (010- designation at beginning of source part number) and a 1/4 inch tubemount (110- designation
in part number). Sources are availablewith a Twinexelectrical connector (-00 designation at end of part
number), orwith bare Ni ribbon terminations (-01 at end of part number).
Usual IonSource - IonCollectorConfiguration
The cylindrically shaped ion source isnormally located on the center axis of a cylindrical metal collector
electrode. Electrical polarization between the ion source and collectorprovides an efficient radial electrical
field that causes ions formed on the source surface to move to the collector. All current DET ion sources
are intended to provide negative ions upon impactwith incoming samplemolecules. Normal gas flows are
directed from the ceramic coating toward themetal mounting flange.
Electrical Considerations
The ion sources)legin heating rap"dlywhen supplied
with
heating currents in excess of 1.5Amps. Usually,
the ion sources aremounted in a structurewhich is alsoheated externally, and the amount of electrical
current supplied to the source depends on the surrounding structure temperature, as well as on the type
of ion source. Forexample, a TID-1 type source is intended to operate at surface temperatures of 400
600°C (no visible color to the hot source). For a TID-1 source, typical source heating currents are 2.6 - 2.7
Amps for a surroundingwall at 100°C, 2.5 - 2.6A for wall at 200°C, and 2.4 - 2.5 A forwall at 300°C. TIt:>2, TID-3, TID-4, and TID-5 type ion sources typically operate at 600 -
aoooe
(glowing orange color), and
require higherheating currents up to about 3.5 Amps. Themagnitude of gas flow past the ion source, as
well as the thermal conductivityof the flowing gas are heat losses from the source that also affect the
magnitude of heating current supplied. In typical operation, thewire core in the ion sources has
approximately 1Ohm
t
"'It resistance. Surges
of
e
'cal cunent inexcess
of
4 Amps can bum out
the heaterwire
co . '
addition to the electrical heating current, power to the sources usually also
indudes polarizing ..;.....ater wire coreat a negative voltage relative to the collector electrode. Current
DETelectronics provide polarizations of
-45
Volts for TID-1. TlD-3, and TlD-5 type sources, and -5 V for
TlD-2 and TlD-4 sources. Hig er
po
rizations up
to
100or200Volts uld probably further improve the
detection limits for TID-1, TID-3, andTlD-5 sources.
Typical GasEnvironments for
the
IonSources
TIt:>-1and TID-3 ion sourcesare intended for operation in gas environments of
N:z,
Air, or
°
2 ,
while TID-2,
TID-4, andTID-5 sources usuallyoperate in an environment consisting of a dilutemixture of
~
in Air,
where themixture ratio is not sufficient to support ignition of a self sustaining flame. TID-1 sources have
been used in total gas flows ranging from 20 to 1000mUmin in magnitude.
. ,,:a...
::r-nr-~eIlIlSUY
COUTcro~/eTgnlteC
again - colected positive ions as opposed to the rest of the NPD
wortd
Whi~h
collectednegative ions.
V
~IAN
- first
cera~ic
ion source developed in
1978
by same personnel who later founded DET - ceramic materials
~lrmlnated
the
softe~
lng
and
~elting
limitations of Perkin Bmer andHewlettPackardsources - side insertionof largesize
Ion source
res~lted
In excessive detector dead volume - incoming samples impacted only one small spot on the bottom
surface of.the
10~
source - exposedmetal support posts for the ion sourceweresubject to corrosion from alkalimetals in
theceramic coating - excessive tailingof Phosphorus peaks.
DET - introduced in
1982 -
a cylindrically shaped ion source fabricated of multiple layers of ceramic materials that
completely
prot~ct~d
th~
heatin~
wire core from exposure to corrosive chemicals - also introduceda concentriccylinder
NPD configuration Inwhich.the lo.n sourcewas installed in the top of the detector hardwareso
it
self-aligned in the center
of the.collectorelectrode - Incomingsamples could impact the entire ion sourcecoating rather than just a small portionon
oneSide
~
detector geometry providedoptimum stream-lined flowwith no sidepocket dead volumes - radial electric field
~etwee~
Ionsource and collector providedefficient ion collection - topmounting of ion source providedeasy access for
Installatlon/re~oval
of
repl~cement
ion sources - well defined detector components allowed better understandingof the
general
operatln~
mechams~
of
th~rmionic
surface ionization and easy extension of the same basic equipment to other
modes of
d~tectlon
-
ce
coating technology allowed development of many different coating formulations without
concern for.Ion source melting - a Black Ceramic ion source eliminated Phosphorus tailing, while a White Ceramic ion
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