Note: Descriptions are shown in the official language in which they were submitted.
10918~S
This application is a division of Canadian application
272,70~ filed February 25, 1977.
mis invention is a further development in the art
of electron capture detectors, and relates in particular
to an asymmetric cylinder electron capture detector suitable
for use in both the dc mode and the pulsed mode.
Description of the Prior Art
An electron capture detector is particularly useful,
for example, in measuring the électron absorptive properties
of the effluent of a gas chromatograph, and for indicating
the presence of an electronegative gas in leak detection
applications.
An electron capture detector usually i~cludes an
electrode configured to defi~e an ionization volume, with
a source of ionizing radiation being disposed within the
ionization volume. The source of ionizing radiation may be,
for example, a tritiated foil of titanium or scandium, or a
foil of nickel-63. A means is provided for passing a gas
through the ionization volume. The charged particle
emanations from the foil ionize the gas in the ionization
volume, thereby producing sécondary electrons having
relatively low energies. The gas passing through the
ionization volume may be, for example, the column effluent
of a gas chromatograph or the sampled gas of a leak detector
apparatus. A collector electrode is disposed in the vicinity
of the ionization volume defining electrode.
A difference of electrical potential is provided
between the collector electrode and the ionization ~olume
defining electrode, thereby creating an electr~c field
that causes the free electrons in the ionization volume to
migrate toward the coilector electrode. A means is provided
for measurins the current of the migratins electrons. lf
10113ZS
the gas contains an electron-absorbing constituent, fewer
electrons migrate to the collector than if no electron-
absorbing constituent is present in the gas. Thus,
measurement of the flow of electrons to the collector
electrode can provide qualitative and quantitative infor-
mation concerning electron-absorbin~ constituents in the gas.
Electron capture detectors have been made in a variety
of configurations. ~wo particular configurations are
those which have historically been designated as the
"concentric cylinder" detector and the "asymmetric cylinder"
detector. One reference discussing the prior art is an
article by Dr. J.E. Lovelock, entitled "Analysis by Gas
Phase Eleatron Absorption", which appeared in Gas
Chromatography 1968, The Institute of Petroleum, London,
1969, pages 95-108.
A concentric cylinder detector for use in conjunction
with a gas chromatographic apparatus typically comprises
a cylindrical electrode structure housing a radioactive
foil, and a cylindrical collector electrode disposed con-
centrically inside the electrode that houses the radio-
active foil. Carrier and sample gases are caused to flow
through the annular volume between the two electrodes.
Charged particles emi~ted from the radioactive foil ionize
the carrier gas inside the electrode structure housing the
radioactive foil, thereby producing free electrons.
Appropriate electronic circuitry causes a difference of
electrical potential between the two electrodes, thereby
causing the free electrons to migrate toward the collector
electrode. A means is pro~ided for measuring the flow, or
current, of the free electrons.
An asymmetrical cylinder electron capture detector
for use in conjunction with a gas chromatographic appara~us
109182S
also typically comprises a cylindrical electrode structure
housing a radioactive foil for ionizing the column effluent.
A cylindrical collector électrode is likewise disposed
coaxially with respect to the electrode structure housing
the foil, but is displaced longitudinally from the interior
of the foil-housing electrode structure. An electrically
insulating cylinder mechanically connects the two electrodes
so as to provide a flow path for the gaseous effluent,
without permitting electrical conduction between the two
electrodes. As in the case of the concentric cylinder
detector, charged particles emitted by the radiation source
ionize the carrier gas, thereby producing free electrons.
Electronic circuitry is p~ovided for causing these free
electrons to migrate to the collector electrode, and for
measuring the resulting electron current.
In general, the migration of free electrons can be
accomplished in either a dc mode or a pulsed mode.
In the dc mode, a dc voltage is applied between the
electrode housing the radiation source and the collector
electrode. Variations in the continuously flowing current
to the collector electrode are measured to obtain a
quantitative indication of the amount of free electrons not
absorbed by the sample gas constituents.
In the pulsed mode, voltage pulses of uniform width
and amplitude are impressed across the electrode housing the
radiation source and the collector electrode, while a
separate generator produces a reference current. A frequency
modulator is used to vary the rate of the voltage pulses,
until the current to the collector electrode balances the
re~erence current. The frequency required to balance the
free electron current with the reference current provides a
quantitative indication of the amount of electron-absorbing
1091825
material present in the sample gas.
Both the pulsed mode of operation and the asymmetric
cylinder configuration have considerable advantages. The
pulsed mode of operation, however, has not hexetofore been
used in commercial applications with electron capture
detectors of asymmetric cylinder configuration, because
pulse widths short-enough:ito provide a satisfactory dynamic
range could not be obtained.
The pulsed mode of operation provides a more nearly
linear response than does the dc mode over a wider range of
concentrations for electron-absorbing constituents in the
sample gas. The asymmetric cylinder configuration provides
~ a superior response to électron-absorbing constituents of
the sample gas at highsr electrode voltages than the
concentric cylinder configuration.
If higher electrode voltages can be used to cause free
electrons to migrate, the electron transit time between the
' electrode defining the ionization volume and the collector
electrode is thereby reduced. The duration of the voltage
pulses can thus be correspondingly reduced, thereby providing
a wider dynamic range for the instrument. It is generally
; desirable that the pulse width be as small as possible,
because the maximum variation in the pulse rate between zero
and the rate at which the pulses begin to overlap is an
inverse function of the pulse width. For a constant pulsé
amplitudé, as the pulse width is reduced, energy can be
imparted to the free electrons for shorter time durations.
When the pulse widths are very narrow, each pulse endures
for only a very short time. However, if the maximum transit
time for the electrons from the ionization volume to the
collector electrode is greater than the pulse width, all
of the electrons cannot reach the collector electrode during
1091825
the life of a single pulse. Thus, for short pulse widths,
the measured current reaching the collector electrode may
provide an erroneous indication of the actual concentration
of electron-absorbing constitubnts in the sample gas.
Electron capture detectors have in the past been
significantly affected by "field-free background current",
which is a term used to designate an electron current that is
independent of the current caused by the electronic
circuitry. Field-free background current can result from a
number of causes: e.g., high-energy beta particles from the
radioactive foil that reach the collector electrodé directly;
charged particles that diffuse through the effluent to the
collector electrode independently of the electric field;
,,~
and/or charged particles that ~re carried to the collector
electrode by convection of the movi~g yas~s. Field-free
background current can vary with concentration of the sample
gas in the effluent, thereby making a quantitative determi-
nation of the amount of electronegative material in the
sample gas difficult to ~btain.
The field-free background current is generally a greater
problem in the pulsed mode than in the dc mode, because
the pulses are generally off more than they are on. Since
the field-free background current is not affected by the
pulses, it tends to mask the current caused by migration of
free electrons under the influence of the pulses. In the
dc mode, the field-free background current, while inevitably
present to some extent, is nevertheless a much smaller
component of the total current detected than in the pulsed
mode. The greater linearity of response provided by the
pulsed mode, however, would make operation in the pulsed
mode preferable, if the adverse features of pulsed mode
operation experienced by the prior art, viz., the effects
-- 5 --
1091825
of field-free background curren~ and the long electron
~ transit times, could be overcome.
With the concentric cylinder configuration, the
collector electrode is often directly exposed to the radio-
active foil, thereby rendering the collector electrode
susceptible to impact by the beta particles emitted by
the radioactive foil. Also, in the concentric cylinder
configuration, the collector electrode is generally
surrounded by an ionized gas volume, thereby exposing the
collector electrode to impact by diffusing or convecting
charged particles.
Field-free background current can be reduced in the
concentric cylinder electron capture detector by increasing
the separation between the electrodes. This, however,
reducés the dynamic range of the detector due to the larqer
electron transit distances and the correspondingly longer
pulse widths required to provide sufficient energy to the
electrons to enable the electrons to traverse such distances
during the life of a single pulse. It has been found that
electron transit times for the concentric cylinder detector
can be reduced by using an argon-methane mixture as the
carrier gas. A 90% argon - 10% methane mixture is effective
in cooling free electrons to thermal energies, while still
permitting them to have a high drift velocity in the electric
field. ~owever, the argon-methane mixture is more ~xpensive
and is more difficult to obtain than commonly used nitrogen
as a carrier gas.
'! ' In the asymmetric cylinder elec-tron capture detector,
the collector electrode is generally positioned upstream of
the radioactive foil so that the effluent flow is directed
away from the collector electrode. By locating the collector
electrode outside the ionization volume, direct impingement
.
~ - 6 -
. _,. . . ~ . , .
10918ZS
of beta particles on the collector electrode is minimized.
~he flow of the effluent gas away from the collector
electrode minimizes the likelihood of charged particles,
including negatively charged ions formed by the ionization
process, reaching the collector electrodes by mass transport
effects such as diffusion or convection. m us, with respect
to field-free background current, the asymmetric cy inder
detector is superior to the concentric cylinder detector.
However, asymmetric cylinder detectors known to the prior
art required a long insulative path to maintain electrical
isolation between the electrode defining the ionization
volume and the collector electrode. -~
The long insulative path between the electrodes in
;i
prior art ~lectron capture detectors resulted in long
tran~it times for the free electrons, thereby reducing the
dynamic response. Furthermore, in asymmetric electron
capture detectors known in the prior art, the long insula-
tive path required to prevent leakage between the electrodes
was typically provided by an insulating ceramic cylinder of
relatively large size. The size of the insulating cylinder
provided a considerable surface area on which surface charge
would accumulate as free electrons passed therethrough.
Such surface charge would adversely affect the migration of
electrons to the collector electrode, thereby introducing
inaccuracy in the indication of the concentration of
electron-absorbing constituents in the sa~ple gas.
Heretofore, because of the disadvantages haracteristic
of the existing asymmetric cylinder electron capture
detectors, as discussed above, their performance was not
substantially improved by operation in 4~he pulsed mode,
and pulsed mode operation was confined to use with the
concentric cylinder conriguration.
1091825
According to the present invention there is provided
an asymmetric cylinder electron capture detector comprising
a generally cylindrical collector electrode, a generally
cylindrical structure housing a source of ionizing radiation,
and a generally cylindrical electrical insulator disposed
intermediate said collector electrode and said radiation
source housing structu~e; said collector electrode,
insulator, and radiation source housing structure being
generally coaxially aligned and being configured to provide
a flow path for gas thereth~ough in a direction through
said collector electrode toward said radiation source housing
structure via said insulator; said collector electrode having
an elongate portion extending into the interior of said
,.~
insulator to substantially preclude the formation of sùrface
charge on the ~urface of said insulator; and means for pro-
I viding an electric field to cause free electrons produced
by ionization of gas in said radiation source housing
structure to migrate toward said collector electrode.
The electrode defining the ionization volume is
preferably, but not necessarily, of cylindrical configuration.
e salient feature of the configuration of the electrodes
of this invention is that the electrodes support an electrïc
field, whose field pattern is substantially the same as the
field pattern of an electric field that would be forme~
between a hypothetical first electrode of right-circ~lar
cylindrical configuration and a hypothetical second electrode
of plate-like configuration disposed perpendicular to the
axis of the first electrode at a position adjacent one end
of the first electrode. The precise location of the face
3~ of ~he second electrode may be anywhere along the axis of
the first electrode in the region extending from the precise
end of ~he first electrode outward to a position away from
.
1 - - 8 -
1091l325
the first electrode at which acceptable operation of the
- detector in the pulsed mode is still feasible. The concept
of acceptable operation, with respect to the pulsed
operating mode, is discussed hereinafter. The preferred
configuration for the ionization-volume defining electrode
of the invention is a right-circular cylindrical configura-
tion. Nevertheless, it is anticipated that other electrode
configurations may be suitable for certain particular
applications.
In gas chromatographic applications, the effluent
f~rom a chromatographic column would be directed past the
collector electrode structure into the electrode defining
the ionization volume, and thence out from the electrode
,
defining the ionization volume to effluent gas receiving
means or, depending upon the kinds of gases involved, to
atmo5phere. A radioactive foil disposed within the ioniza-
tion volume emits charged particles to ionize the effluent
passing therethrough.
A cylindrically configured ceramic insulating-structure
mechanically connects the electrode defining the ionization
volume with the collector electrode. The collector electrode
in the preferred embodiment has an elongate portion extending
substantially through the interior of the insulator to a
point pxoximate the adjacent facing end of electrode
defining the ionization volu~e. This elongate portion of
the collector electrode has a smaller diameter than the
interior of the insulator in the vicinity of the adjacent
facing end of the ionization-volume defining electrode,
so as to maintain a relatively small clearance therebet~een.
This configuration provides a relatively long insulative
path to minimize electrical leakage between the collector
electrode and the electrode derining tne ionization volume,
:. _ 9 _
lO9i82S
and provides a relatively short migration path for electr-
ons f~om the interior of the ionization volume to the face
of the collector electrode. The gap between the two
electrodes is large enough to provide high electrical
resistance, yet is short enough to provide relatively
short transit times for electrons migrat~ng to the col-
lector electrode. The limited exposure of the interior
surface of the insulator to charged particles minimizes
the accumulation of surface charge on the insulator.
In the preferred embodiment, a transversely extend-
ing gas exit port is provided in the collector near the
end of its elongate portion adjacent the radiation source.
The transverse exit port causes effluent gas from the col-
lé~torto be directed into the insulator at right angles to
the overall direction of gas flow through the detector,
thereby creating turbulence which inhibits the build-up of
stagnant effluent gas in the insulator.
A feature of the detector of this invention is the
minimal field-free background current. In particular,
the impact on the collector electrode of beta particles
is minimal hecause the collector electrode is not physic-
ally located within the ionization volume. Also, since
gas flow is directea away from the collector electrode,
the impact on the collector electrode of diffusing and
convecting charged particles is likewise minimal.
For operation in the pulsed mode, the energy that
drives the free electrons to the collector electrode is
dependent upon the pulse ~idth for a given constant pulse
, amplitude. In general, it is desirable to make the pulse
width as short as possible in order to provide as wide a
dynamic range as possible. As the average flight path,
or transit time, of the free electrons from the ioniza-
--10--
1091825
tion volume to the face of the collector electrode increa-
- ses, the pulse width must necessarily also increase, for
a siven constant pulse amplitude, in order to provide
sufficient energy to the electrons to permit their coll-
ection by the collector electrode during a single pulse.
Thus, it is desirable, in terms of minimizing pulse width,
to locate the collector electrode as close as possible to
the adjacent facing end of the electrode defining the ion-
ization volume. However, in terms of minimizing the field-
free background current, the collector electrode should
not enter into the ionization volume.
It has been found that for the preferred configura-
tion of the electrodes and of the ceramic insulating
structure, as described hereafter in greater detail in the
speciication, the ~ield-free background current is reduc-
i ed to such an extent that, for commercial purposes, the
collector electrode can provide a satisfactory dynamic
range, if the face of the collector electrode is located
precisely at the adjacent facing end of the electrode de-
fining ionization volume. It has also been found that,
for nitrogen and for argon-methane carrier gases, a satis-
factory dynamic range can likewise be obtained if ~he
collector electrode is located coaxially spaced apart
from the adjacent facing end of the electrode defining the
ionization volume, provided that the separation between
the electrodes is short enough so that the free electrons
can tra~el to the collector electrode during a pulse width
of one microsecond or less. Thus, for specialized appli-
cations in which a narrower dynamic range can be tolerated
in order to reduce field-free bac~ground current to its
lowest possible extent, ~he collector electrode of this
invention can be located coaxially spaced apart from the
- 1 1-
1091825
adjacent facing end of the ionization-volume defining
electrode. However, the separation of the face of the
collector electrode from the adjacent facing end of the
electrode defining the ionization volume can be no more
than that which provides an "acceptable" trade-off be~
tween reduced field-free background current and reduced
dynamic range. Preliminary experiments by the inventor
indicate that such coaxial separation would in most cases
not exceed 0.125 inch (approximately 0.32 cm).
Particularly suitablé electronic circuitry for
operating the electron capture detector of this invention
in the pulsed mode is disclosed in U.S. Patent No.
4,117,332 (issued September 26, 1978~ by John R. Felton
and Russell S. Gutow, and assigned to the assignee of the
present invention.
~he achievement of lower electron transit times
improves the dynamic range of the asymmetric cylinder
electron capture detector in the pulsed mode. The dynamic
range of the detector of the present invention has been
found to be about 106, using sulphur hexafluoride as the
sample gas and using nitrogen as the carrier gas.
' With the detector of this invention, the linearity
of response with respect to pulsed frequency of the con-
S centration of electron-absorbing constituents in the
sample gas continues practically up to the dc limit, which
is the point at which the pulses begin to overlap and
become an essentially uninterrupted dc signal. Thus, it
is a general object of this invention to provide an
asymmetric cylinder electron capture detector that is
capable of implementing the advantages of linear opera-
tion in the pulsed mode, while exhibiting low field-free
bac~ground current and a wide dynamic range.
-12-
1091825
Other objects and advantages of the present
invention may be discerned from thé following detailed
specification in conjunction with the accompanying drawing
and appended claims.
Description of the Drawing
FIGURE 1 is a diagrammatic view of a gas chromato-
graphic system incorporating the asymmetric cylinder
electron capture detector.
FIGURE 2 is an elevational view, partially in block
form, showing the electron capture detector portion of the
system of FIGURE l.
Description of the Preferred Embodiment
FIGURE 1 shows a gas chromatographic system 10,
which incorporates the asymmetric electron capture detec-
tor of this embodiment.
The system l~ includes a pressurized container ll
for storing a supply of carrier gas, such as nitrogen.
The container 11 delivers a stream of carrier gas to a
chromatographic column 14. A quantity of sample gas is
added to the carrier gas stream via an injection port 15
located in a conduit between the container 11 and the
column 14. Stationary phase material within the column
14 adsorbs some or all of the constituents of the sample
gas in varying degrees, such that the effluent from the
column 14 exhibits a particular measurable property that
is a ~ime-varying function of the nature and amount of
the constituents of the sample gas. A detector 16 senses
variations in this measurable property of the effluent,
and actua'es a recorder 20 for providing a permanent
record 22 of the time variations of this measurable
property.
The carrier gas supply container 11 is highly
1091825
pressurized, and is preferably made of steel. It is a
featu~e of this e~bodiment that the detector 16 performs
well using relatively inexpensive and widely available
nitrogen as the carrier gas. A more costly argon-methane
gas mixture can also be used, but is not necessary for
achieving a dynamic range as wide as 10 for an electro-
negative gas such as sulphur hexafluoride. ffme carrier
gas supply container 11 may also include a flow meter 24
for adjusting the rate of flow of the carrier gas toward
the column 14.
The injection port 15 may comprise any suitable type
of device known to those skilled in the art for injecting
I the sample gas into the high-pressure carrier gas stream
flowing between the carrier supply contalner 11 and the
column 14.
The column 14 likewise may be of a type known to
those skilled in the art, and comprises an elongate
tubular portion 30 containing a stationary phase material
32. The mixture of carrier gas and sample gas percolates
through the stationary phase 32 within the tubular por-
tion 30. The stationary phase 32 is a liquid or solid
material chosen for its property of differentially adsorb-
ing certain substances~ preferably the anticipated consti-
tuents of the sample gas. By reason of such differential
adsorption, at least one property of the effluent from
i the column 14 is caused to vary as a function of time,
the time function being related to the capability of the
stationary phase 32 to adsorb the constituents of the
sample ~as.
One property of the effluent which varies by the
action of the stationary pnase on the effluent is the
capability of the e~fluent, when ionized, to capture free
-14-
109~8ZS
electrons.
The detector 16, which is of the electron capture
type, receives and analyzes the column effluent. The
effluent passing through the detector 16 is ionized so as
to generate free electrons, which are thereupon formed
into a measurable electron current by an impressed elec-
tric field. Fluctuations in this measurable electron
current are indicative of variations in the capability of
the sample gas to capture free electrons. Thus, fluctua-
tions in the electron current can provide a quantitative
measurement of the presence of electronegative constitu-
ents in the sample gas.
The recorder 20 is connected by suitable electronic
circuitry to the detector 16 so as to indicate the time-
~arying capability of the ionized effluent to capture free
! electrons. The recorder 20, which is preferably a strip
chart recorder, produces a permanent strip chart record-
ing 22 indicating the time variations in the capture of
free electrons.
; 20 FIGURE ~ illustrates in detail the structure of the
-`~ detector 16, and provides a functional representation of
the associated electronic circuitry. Effluent gas from
the column 14 is supplied to the detector 16 by way of a
feed tube 50. The effluent gas is directed through a
first tubular insulator 52 connecting the feed tube 50
with a generally cylindrical collector electrode struct-
ure 54. The collector electrode 54 extends from the
first insulator 52 to a second tubular insulator 56. The
effluent flowing through the collector electrode 54, and
thence through the second insulator 56, is directed to-
ward a tubular radiation source cell 60. The radiation
source cell 60 and the collector electrode 54 are aligned
-15-
.~
:1091825
coaxially with and longitudinally spaced apart from one
another. The second insulator 56 provides gas communica-
tion between the collector electrode 54 and the radiation
source cell 60. The insulators 52 and 56 maintain the
so~rce cell 60 and the collector electrode 54 isolated
electrically from ground and from each other.
The insulator 52 is preferably made of an electri-
cally insulating ceramic material having sufficient
rigidity to support the facing ends of the feed tube 50
and the collector electrode 54 in fixed relationship with
respect to one another. The collector electrode 54 is
I preferably a metallic cylindrical member having a bore 61,
; which provides gas communication between the interiors of
the first insulator 52 and the second insulator 56. The
collector electrode 54 is made of electrically conductive
material, such as stainless steel or Kovar metal. The
insulator 56 is similar to the insulator 52 in configura-
tion and material. The insulator 56 holds the adjacent
ends of the collector electrode 54 and the tubular radia-
tion source cell 60 in fixed relationship with respect to
one another. The collector electrode 54 thus provides
gas communication from the feed tube 50 and the insulator
52 to the insulator 56 and the interior of the radiation
source cell 60.
The radiation source cell 60 is of generally hollow
cylindrical configuration. A source 65 of ionizing radia-
tion, such as a foil of tritiated titanium or scandium,
or a foil of nickel-63, is disposed adjacent the interior
surface of the radiation source cell 60. The radioactive
foil 65 irradiates the effluent gas flowing through the
cell 60 with charged particles, thereby ionizing the
effluent gas so as to generate free electrons.
.
-16-
~091825
Electronic circuitry is connected to the radiation
source cell 60 and to the collector electrode 54 for
establishing an electric field so as to cause the free
electrons generated by the ionization process to migrate
toward the collector electrode S4, (i.e., in the direction
contrary to the direction of gas flow), and to measure
the rate of such electron migration. Suitable circuitry
for producing an electric field includes a negative pulse
generator 70, which is connected to the conductive materi-
al comprising the radiation source cell 60. The negative
pulse generator 70 produces pulses of negative voltage,
and impresses these pulses on the radiation source cell
60. The pulses are uniform in width, being approximately
O.6 microseconds in duration. The negative pulse genera-
tor 70 is of a known type, and includes means for adjust-
ing the frequency of the negative pulses impressed upon
the cell 60.
The impression of a negative pulse on the cell 6
; establishes an electric field, which causes the free
electronc produced by the ionization process to migrate
toward the collector electrode 54. The collector elec-
trode 54 thus receives a negative charge flow, which is
a function of the rate at which the free electrons migra-
te from the cell 60 to the collector electrode 54, and
of the fraction of free electrons absorbed by the
effluent gas.
A direct current electrometer 72 is connected to
the collector electrode 54 in order to measure the flow
1~ of the migrating free electrons. The electrometer 72 is
a known type of instrument for accurately measureing
minute current flow. The fre~ electron current (-IE)
from the collector electrode 54 is combined at the input
-17-
109~825
of electrometer 72 with a reference current (IR) that is
generated by a dc reference current generator 76. The
electrometer 72 amplifies the IR-IE signal, and produces
a signal on a lead 74 which is a function of the current
difference IR -IE-
A voltage-to-fre~uency converter means 80 causes
the negative pulse generator 70 to produce pulses with a
frequency dependent upon the voltage signal on the electro-
meter output lead 74. The pulse frequency of the negative
pulse generator 70 is adjusted until the current difference,
IR-IE, becomes zero. A frequency-to-voltage converter 82
~ produces an output signal proportional to the pulse fre-
; quency output of the negative pulse generator 70. The
frequency of the pulses impressed on the radiation source
cell 60 is thus utilized as an indication of the concen-
tration of electron-absorbing constituents in the sample
gas.
The insulator 56 is configured so that one end
thereof overlaps an adjacent end of the collector electrode
54, and the other end thereof overlaps an adjacent end of
the radiation source cell 60. Thus, the adjacent ends of
the collector electrode 54 and radiation source cell 60
are received within the insulator 56. A caping member 62
fits over and coaxially surrounds the overlapping ends of
the insulator 56 and the collector electrode 54. Similar-
ly, A caping mel~er 63 fits ov~r and coaxially surrounds
the overlapping ends of the insulator 56 and the radia-
tion source cell 60. The caping members are bonded to the
members they join, as by brazing, in order to provide a
gas-tight seal. In a similar manner, the insulator 52 is
sealed to the feed tube 50 and to the other end of the
collector electrode 54.
-18-
10918ZS
The collector electrode 54 has an elongate portion
55 extending longitudinally into the interior of the
insulator 56. The elongate portion 55 does not contact
the interior surface of the radiation source cell 60, but
rather has an outside diameter that is smaller than the
inside diameter of the insulator 56, thereby precluding
physical contact therebetween. This cohfiguration minimi-
zes electrical leakage between the collector electrode
54 and the radiation source cell 60 by providing a rela-
tively long insulative path from the radiation source
cell 60, received in one end of the insulator 56, to
that portion of the collector electrode 54 which is in
contact with the insulator 56 at the other end thereof.
The outstanding feature of this configuration is that the
~pacing between the radiation ~ource cell 60 and the face
of the collector electrode 54 can be minimized, while
still providing a relativeIy long insulative path between
the electrodes to minimize electrical leakage therebetween.
It is generally desirable, from the standpoint of
achieving wide dynamic range, to minimize the transit
time required for free electrons generated in the ioniza-
tion volume to migrate to the face of the collector
electrode 54. Thus, it is generally desirable to locate
the face of the collector electrode 54 as close as possi-
- ble to the facing end of the radiation source cell 60.
In the preferred embodiment shown in FIGURE 2, the
elongate portion 55 of the collector electrode 54 extends
into the interior of the insulator 56 to a terminus co-
planar with the facing end of the radiation source cell
60. The resulting electric field pattern is substantial-
ly the same, for purposes of mathematical analysis, as
the field formed between a right-circular cylindrical
--19--
' ,
. .
109182S
electrode of one polarity and a plate-lika electrode of
opposite polarity disposed perpendicular to the axis of
the cylindrical electrode at a position adjacent one end
of the cylindrical electrode.
It is recognized that the close proximity of the
face of the collector electrode 54 to the ionization
volume, as shown in FIGURE 2, theoretically renders the
collector electrode 54 more susceptible to field-free
background current, due to direct bombardment by beta
particles from the ionization source 65, and due to the
impingement of negatively charged particles carried
thereto by mass transport phenomena such as diffusion
and convection, than would occur if the face of the
collector electxode 54 were disposed further away from
; the facing end of the radiation source cell 60. It has
been found, however, that for commercial applications,the
disposition of the collector electrode 54 with respect to
the radiation source cell 60, as shown in FIGURE 2, is
for the most part untroubled by the field-free background
current problem that plagued concentric cylinder electron
capture detectors in the prior art.
: For particular specialized applications where the
field-free background current must be reduced to the
greatest possible extent, even at the expense of dynamic
; range, the terminus of the elongate portion 55 of the
collector electrode 54 need not extend into the interior
of the insulator 56 quite so far as shown in FIGURE 2.
The face (i.e., the terminus) of the elongate portion 55
.i could be spaced apart from the plane defining the facing
end of the radiation source cell 60 by whatever amount is
necessary to accomplish the desired ultra-minimization
of field-free background current while still providing
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109~8ZS
feasible pulsed mode operation.
Investigations by the inventor indicate that feasi-
ble operation of the detector of this invention in ~he
pulsed mode would require that the separation between the
end of the elongate portion 55 of the collector electrode
54 and the plane defining the facing end of the radiation
~ource cell 60 be not greater than about 0.125 inch
(0.32 cm). Greater separation than that would require
pulse widths of longer than one microsecond in order to
permit the electrons to travel from the ionization volume
to the collector electrode 54 during a single pulse.
Such long pulsewidths would severely limit the dynamic
range of the instrument, and would therefore severely
limit the utility of the instrument for large sample
concentrations. The maximum pulse frequency that could
be impressed on the radiation source cell 70 is that
fxequency at which the pulses overlap. The wider the
pulse width is, the lower is the frequency at which the
- , pulses overlap. Thus, any lowering of the dynamic range
lowers the sample concentration for which the instrument
can be effective.
T,he bore 61 extends axially throughout the entire
length of the collector 54, th~reby providing gas commun-
ication from the chromatographic column, via the feed
tube 50 and the insulator 52, to the interior-of the
cell 60. In the preferred embodimen,t, the elangate por-
' tion 55 has a transverse gas exit port 58 for directing
the effluent gas into the interior of the insulator 56
at right angles to the bore 61. This configuration causes
gas turbulence within the insulator 56, which prevents
the accumulation of stagnant effluent gas and minimizes
the build-up of surface charge on the interior surface
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~)91825
of the insulator 56. Thus, the likelihood of spurious
output signals being generated by delayed passage to the
radiation source cell 60 of sample gas that has been de-
tained in the insulator 56 is minimized.
The detector described above is an asymmetric
cylinder electron capture detector suitable for use in the
pulsed mode, and is capable of achieving the advantages
normally associated with pulsed mode operation. This
detector possesses favorable linearity of response and
low field-free background current, which are characteris-
tics of asymmetric cylinder detectors generally, and in
addition provides the shorter electron transit times
required for good dynamic range.
Although the primary advantages of this detector
are associated with operation in the pulsed mode, it is
to be empha~ized that this détector also performs well in
the dc mode.
This detector can also be used in leak detection
and related applications. It can be employed in any
application requiring detection of electronegative sample
gases contained in a non~electronegative carrier gas.
For example, electron capture detectors are frequently
used in leak detection apparatus where electron-absorbing
gases are employed to pinpoint leaks in pnéumatic systems.
In a particular application, the gas that is caused to
flow through the collector electrode into the ionization
` volume is gathered from one side of an object to be leak
tested. An electronegative gas such as sulphur hexafluor-
ide is then introduced to the other side of the object to
be leak tested. When a leak occurs, thé electronegative
gas passes through the leak, and can be detected as a
constituent of the gas passing through the ionization
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., ,
~091825
volume.
The description of the embodiment set forth aboveis intended to be illustrative rather than exhaustive of
the pr~sent invention. It should be appreciated that
those of ordinary skill in the art may make certain modi-
fications, additions or changes to the described embodi-
ment without departing from the spirit and scope of this
invention as claimed hereinafter.
'~ ,
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