CA1091825A - Asymmetric cylinder electron capture detector - Google Patents
Asymmetric cylinder electron capture detectorInfo
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- CA1091825A CA1091825A CA345,183A CA345183A CA1091825A CA 1091825 A CA1091825 A CA 1091825A CA 345183 A CA345183 A CA 345183A CA 1091825 A CA1091825 A CA 1091825A
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- collector electrode
- insulator
- electron capture
- gas
- capture detector
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Abstract
Abstract An asymmetric cylinder electron capture detector comprises a cylindrical electrode configured to define an ionization volume, a source of ionizing radiation disposed within the ionization volume, a cylindrical collector electrode, the respective cylindrical electrodes being in coaxial alignment but having their ends spaced apart, means for directing a gas past the collector electrode into the ionization volume, and means for applying a difference of electrical potential between the electrodes.
The two electrodes are mechanically connected via an inter-mediately disposed cylindricaly insulator cylinder. The collector electrode is received in one end of the insulator cylinder, and the electrode defining the ionization volume is received within the other end of the insulator cylinder.
The collector electrode has an elongate portion extending into the interior of the insulator cylinder, but spaced apart from the inner surface of the insulator cylinder.
The elongate portion of the collector electrode may extend into the insulator cylinder up to a position coplanar with the end of the electrode defining the ionization volume.
The insulator cylinder provides a flow path from the collector electrode to the ionization volume and permits turbulence to be introduced into gas flow by means of a transverse gas exit port.
The two electrodes are mechanically connected via an inter-mediately disposed cylindricaly insulator cylinder. The collector electrode is received in one end of the insulator cylinder, and the electrode defining the ionization volume is received within the other end of the insulator cylinder.
The collector electrode has an elongate portion extending into the interior of the insulator cylinder, but spaced apart from the inner surface of the insulator cylinder.
The elongate portion of the collector electrode may extend into the insulator cylinder up to a position coplanar with the end of the electrode defining the ionization volume.
The insulator cylinder provides a flow path from the collector electrode to the ionization volume and permits turbulence to be introduced into gas flow by means of a transverse gas exit port.
Description
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 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 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 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 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;
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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 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 .
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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.
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 .
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-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 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.
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 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 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 .~
: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.
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~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 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.
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 ' , . .
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 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 ~)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 ., , ~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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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 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 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 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 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;
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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 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 .
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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.
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 .
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-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 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.
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 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 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 .~
: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.
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~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 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.
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 ' , . .
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 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 ~)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 ., , ~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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Claims (13)
1. 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 structure; said collector electrode, insulator, and radiation source housing structure being gen-erally coaxially aligned and being configured to provide a flow path for gas therethrough 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 surface charge on the surface of said insulator; and means for providing an electric field to cause free electrons produced by ionization of gas in said radiation source housing structure to migrate toward said collector electrode.
2. The electron capture detector of claim 1 further comprising means for measuring the rate of migration of free electrons toward said collector electrode.
3. The electron capture detector of claim 1 wherein said source of ionizing radiation is a foil structure mounted within said radiation source housing structure.
4. The electron capture detector of claim 3 wherein said foil structure comprises tritiated titanium.
5. The electron capture detector of claim 3 wherein said foil structure comprises tritiated scandium.
6. The electron capture detector of claim 3 wherein said foil structure comprises nickel-63.
7. The electron capture detector of claim 1 wherein said elongate portion of said collector electrode has a smaller diameter than the interior of said insulator so as to maintain a clearance therebetween.
8. The electron capture detector of claim 1 wherein said elongate portion of said collector electrode defines a gas exit port configured so as to direct gas from the collector electrode into the insulator in a direction trans-verse to the axis of said insulator, whereby turbulence is induced in said gas.
9. The electron capture detector of claim 1 further comprising means for mechanically coupling said collector electrode to a gas chromatograph column, whereby effluent from said column can flow through said collector electrode toward said radiation source housing structure.
10. The electron capture detector of claim 1 wherein said means for providing said electric field comprises means for producing a pulsed electric field.
11. The electron capture detector of claim 1 wherein said means for providing said electric field comprises means for producing a continuous electric field.
12. The electron capture detector of claim 1 wherein said collector electrode and said radiation source housing structure are both ungrounded.
13. The electron capture detector of claim 10 wherein said means for producing a pulsed electric field comprises a negative pulse generator connected to said radiation source housing structure, and an electrometer connected to said collector electrode, said negative pulse generator being electrically coupled to said electrometer via a voltage-to-frequency converter so as to vary the frequency of said pulse generator in response to the output voltage of said electrometer.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA345,183A CA1091825A (en) | 1976-02-27 | 1980-02-06 | Asymmetric cylinder electron capture detector |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/662,064 US4063156A (en) | 1976-02-27 | 1976-02-27 | Assymetric cylinder electron capture detector |
| US662,064 | 1976-02-27 | ||
| CA272,704A CA1087761A (en) | 1976-02-27 | 1977-02-25 | Asymmetric cylinder electron capture detector |
| CA345,183A CA1091825A (en) | 1976-02-27 | 1980-02-06 | Asymmetric cylinder electron capture detector |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1091825A true CA1091825A (en) | 1980-12-16 |
Family
ID=27164933
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA345,183A Expired CA1091825A (en) | 1976-02-27 | 1980-02-06 | Asymmetric cylinder electron capture detector |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1091825A (en) |
-
1980
- 1980-02-06 CA CA345,183A patent/CA1091825A/en not_active Expired
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