US3378725A

US3378725A - Electron capture detector having separate ionization and sensing regions

Info

Publication number: US3378725A
Application number: US359827A
Authority: US
Inventors: Julius H Bochinski; James C Sternberg
Original assignee: Beckman Instruments Inc
Current assignee: Beckman Coulter Inc
Priority date: 1964-04-15
Filing date: 1964-04-15
Publication date: 1968-04-16
Anticipated expiration: 1985-04-16

Description

P 1968 J. H. BOCHINSKI ETAL 3,378,725

ELECTRON CAPT URE DETECTOR HAVING SEPARATE IONIZATION AND SENSING REGIONS Filed April 15, 1964 A n 1 ll] WA \YN II/l),

5:, I fiifi jgfikngm s2 II /34 40 32 22 I ZO 3O RECORDER -l INVENTORS JULIUS aeocumsm JAMES c. STERNBERG T: MUM

ATTORNEY United States Patent 3,378,725 ELECTRON CAPTURE DETECTOR HAVING SEP- ARATE IONIZATION AND SENSING REGICNS Julius H. Bochinski, Fasadena, and James C. Sternberg, Fullerton, Calif., assignors to Beckman Instruments, Inc., a corporation of California Filed Apr. 15, 1964, Ser. No. 359,827 7 Claims. (Cl. 315-111) ABSTRACT OF THE DISCLOSURE An electron capture detector for use such as in gas chromatography having separate ionization and sensing portions with discharge electrodes and a gas chemically inert with respect to the detector in the discharge portion. An anode and cathode are positioned in the sensing portion having a gradient established thereacross not exceeding approximately .7 of a volt per centimeter per millimeter of mercury pressure therebetween to provide for electron capture operation. A sample gas contained in a carrier, inactive with respect to the sample and having no electron capture capability, flows past the anode into the sensing portion and is exhausted from a point between the anode and cathode together with the inert gas from the discharge portion.

This invention relates to a detector for use in gas chromatography and more particularly to an improved form of such a detector adapted to operate in the electron capture mode.

Many different forms of detectors have been used in the prior art in gas chromatography such as thermal conductivity, hydrogen flame, solution conductivity, breakdown voltage and photoelectric effect detectors, detectors employing radioactive materials for ionization, etc. Electron capture detectors, heretofore employing radioactive isotopes as a source of ionization, have been widely used in the analysis of electroncgative materials such as pesticides, steroids and amino acids. These detectors are particulariy advantageous because of their great sensitivity and high degree of selectivity. However, the radioactive isotopes present both health hazards and require government licenses, requiring specialized handling. Also, the upper operating temperatures of such detectors have been a serious limitation, for example where more sensitive detectors have employed tritium sources. Tritium employed in detector structure vaporizes appreciably above 220 C, Accordingly, it is an object of this invention to provide an electron capture detector which does not employ radioactive isotopes.

Another object of the invention is to provide such a detector having an upper operating temperature limited only by the materials of construction, for example 400 to 500 C.

Still another object of the invention is to provide such a detector in which the source of ionization is a combination of electric discharge and ultraviolet light.

A further object is to provide such a detector employing ceramic and metal body portions to permit operation at higher temperatures.

A still further object of the invention is to provide such a detector in which the electron current may be varied over several orders of magnitude and which may operate at current levels greatly exceeding those presently available in detectors employing radioactive sources.

In carrying out the invention in one form thereof a chamber is provided having a discharge portion with a pair of discharge electrodes and a sensing portion. An apertured anode and cathode are positioned in the sensing portion and a restriction is provided connecting the two portions of the chamber. A source of potential, to establish a gradient not exceeding .7 of a volt per centimeter per millimeter of mercury pressure, is connected across the anode and cathode and a second source is connected to create a discharge across the discharge electrodes. A gas which is chemically inert with respect to the materials of the detector and its other contents is injected into the discharge portion to flow past the dis-charge electrodes through the restriction and then through the cathode. A carrier gas which may contain a sample gas capable of capturing electrons is injected through the anode into the sensing chamber. The two gases are exhausted from a point between the cathode and anode. The relative potential of the electrodes is set such that the discharge electrode field is negative and the anode positive with respect to the cathode to enhance the flow of electrons from the discharge electrodes into the sensing portion of the detector. Typical gases which may be used in the discharge portion are hydrogen, nitrogen, carbon dioxide and the noble gases. For carrier gases examples are carbon dioxide, nitrogen and a mixture of the noble gases and others such as argon and methane.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof, can best be understood by reference to the following description taken in connection with the accompanying drawing illustrating one embodiment of the invention in cross section.

Turning now to the drawing, a detector is illustrated having a cylindrical sensing chamber 10 contained in a cylindrical housing 12 which may be made from alumina, for example. Opposite ends of the sensing portion 10 are defined by a cathode 14 and an anode 16 which may be made of stainless steel mesh (approximately 200 mesh). The chamber 10 may be approximately A2 inch in diameter and ,3 inch from the anode to the cathode and has an exhaust port 18 which may be approximately /s inch OD. and positioned about /3 of the way from the cathode to the anode. An inlet port 20 is provided for passing carreir gas, which may contain a sample, through the anode 16 into the sensing chamber 10. A lead 22 is taken from the cathode 14 through the housing 12 to provide an electrical connection to a source of potential 24 and thence to ground. A second lead 26 is taken from anode 16 through housing 12 to provide an electrical connection to an amplifier 23 and then to a recorder 30. Potential source 24 thus establishes the required gradient between the anode 16 and the cathode 14.

The housing 12 is attached to a housing 32, which may be of stainless steel, by means of a cylindrical collar 34, which may be made of nickel. Housing 32 has inserted within it an inner housing 36, which may be of quartz or ceramic, which defines a discharge chamber 38. Discharge chamber 38 contains two discharge electrodes 40 which may have platinum tips of approximately .03 inch in diameter spaced approximately .03 inch apart. The discharge electrodes 48 are sealed in bushings 42 which may be of alumina or cezamic and electrodes: 40 may be approximately inch from cathode 14. Bushings 42 each have a collar 44, which may be of nickel, on top of which a gasket 46, which may be of silver, is placed. A seal is provided by screwing the retaining nuts 48, which may be of steel, down on the gaskets 46.

The inner housing 36 is held in place in the housing 32 by means of a gasket 50 which may be of silver and a back-up plate 52 which may be of stainless steel. A second back-up plate 5'4, which may also be of stainless steel, forces back-up plate 52 against gasket 5 and is held in place by means of screws 56 which pass through housing 32. A tube 58 is inserted in back-up plate 52 and provides an inlet port which extends through the back-up plate 52 and washer 50 and through housing 36 into the discharge chamber 38, through which a gas for the discharge chamber, such as hydrogen, nitrogen, carbon dioxide or a noble gas may be admitted. The discharge chamber 38 may have dimensions of approximately .18 inch in drameter by .36 inch long.

A flow path is provided through a restriction 60 which may be approximately .7 inch in diameter, from the discharge chamber 38 and through an intervening space 62 between the discharge chamber 38 and the sensing chamber 10 of the detector, such that the gas sweeping through tube 58 may pass the discharge electrodes 40 in the chamber 38 and sweep electrons through the restriction 60 into the sensing chamber 10 past cathode 14.

Discharge electrodes 40 are connected together by

leads

64 and 66, which pass through the bushings 42, through a source of potential 68 and a resistor 70, which may be for example approximately -40,000 ohms, in order to cause a discharge between the electrodes. The terminal 72 between source of potential 68 and resistor 70 is grounded as shown.

The source of potential 68 may be approximately 200 to 400 volts, when using helium as a discharge chamber gas; and source 24 may range from 0 to 500 volts for example with polarities as indicated. With the sources of potential of the magnitudes described, and the other typical dimensions given, a gradient is established across the anode 16 and cathode 14 of less than approximately .7 of a volt per centimeter per millimeter of mercury pressure in the chamber which may operate around atmospheric pressure. This is done in order to operate in the electron capture mode avoiding electron multiplication. The flow of gas past the discharge electrodes 40 and through the restriction 60 serves to prevent the carrier gas, containing any sample, flowing through port 20 from coming into contact with the platinum-tipped discharge electrodes 40. This then prevents the discharge electrodes from becoming contaminated. The relative potentials of the field between the discharge electrodes 40, cathode 14 and anode 16 serve to accelerate electrons generated in the are at discharge electrodes 40 through cathode 14 to anode 16 where they are collected. Resistor 70 is employed specifically to accomplish this by establishing the discharge electrode nearest ground potential also negative with respect to the cathode 14. An additional effect taking place within the detector occurs in that photons generated at the discharge electrodes 40 radiate through the restriction 60 and strike the cathode 14 releasing additional electrons. The flow of gas from the discharge chamber 38 through the cathode 14 also serves to prevent contamination and modification of the cathode 14 with respect to its photoelectric emission properties.

Helium is one preferred gas used in the discharge chamber 38 because of the relatively low potential required to strike an arc. A preferred carrier gas for the sensing chamber is nitrogen which is made to flow counter to the stream of electrons passing between cathode 14 and anode 16. When electronegative materials such as pesticides, steroids and amino acids that exhibit the property of capturing some of the electrons out of the flow are contained in the nitrogen, the electron flow, the density of which may be conveniently modified by controlling the applied potentials, is diminished.

The decrease in the electron current flowing between cathode 14 and anode 16 yields an indication of the type, presence and amount of electronegative sample gas contained in the carrier when calibrated with time after injection of the sample into a chromatograph column which is thus indicated by a reduction in the current passing through the amplifier 28 and resulting variation recorded on the recorder 30. The suggested materials of construction also permit operation of the detector at temperatures up to 400 to 500 C., which is ab ve t e column peratures normally employed. The detector then can operate in an oven to avoid condensation of sample.

While the principles of the invention have now been made clear in the illustrative embodiment, there will be immediately obvious to those skilled in the art many modifications in structure, arrangement, proportions, elements and components used in the practice of the invention and otherwise, which are particularly adapted for specific environments and operating requirements, without departing from these principles. The appended claims are therefore intended to cover and embrace any such modifications that fall within the limits only of the true spirit and scope of the invention.

What is claimed is:

1. An electron capture detector comprising, a chamber having a discharge portion and a sensing portion, a pair of discharge electrodes positioned in said discharge por tion, means for connecting a source of potential across said discharge electrodes, an anode and a cathode positioned in said sensing portion, means for connecting a source of potential across said anode and cathode to establish a gradient not exceeding approximately seven tenths volt per centimeter per millimeter of mercury pressure therebetween, means for admitting a gas chemically inert with respect to the detector and its other gas contents into said discharge portion to flow past said discharge electrodes and said cathode, means for admitting a carrier gas for containing a sample into said sensing portion to flow past said anode, said carrier gas being inactive with respect to said sample and having substantially no electron capture capability, and means for exhausting said gases from a point between said anode and cathode.

2. An electron capture detector for a gas chromatog raph comprising, a chamber having a discharge portion and a sensing portion, a pair of discharge electrodes positioned in said discharge portion, means for connecting a source of potential across said discharge electrodes, an anode and a cathode positioned in said sensing portion, means for connecting a source of potential across said anode and cathode to establish a gradient not exceeding approximately seven tenths volt per centimeter per millimeter of mercury pressure therebetween, means for admitting a noble gas into said discharge portion to flow past said discharge electrodes and said cathode, means for admitting a carrier gas into said sensing portion to flow past said anode, and means for exhausting said gases from a point between said anode and cathode.

3. An electron capture detector for a gas chromatograph comprising, a chamber having a discharge portion and a sensing portion, a pair of discharge electrodes positioned in said discharge portion, means for connecting a source of potential sufiicient to cause an arc across said discharge electrodes, an apertured anode and an apertured cathode positioned in said sensing portion, means for connecting a source of potential across said anode and cathode to establish a gradient not exceeding approximately seven tenths volt per centimeter per millimeter of mercury pressure therebetween, means for admitting a gas chemically inert with respect to the detector and its other gas contents into said discharge portion to flow past said discharge electrodes and said cathode, means for admitting a carrier gas for containing a sample gas having an afiinity for electrons into said sensing portion to flow past said anode, said carrier gas being inactive with respect to said sample and having substantially no electron capture capability, means for exhausting said gases from a point between said anode and cathode, and said sources of potential establishing the field between said discharge electrodes negative and said anode positive with respect to said cathode.

4. An electron capture detector for a gas chromatograph comprising, a chamber having a discharge portion and a sensing portion, a pair of discharge electrodes positioned in said discharge portion, means for connecting a source of potential sutficient to cause an arc across said discharge electrodes, an apertured anode and an apertnred cathode positioned in said sensing portion, means for connecting a source of potential across said anode and cathode to establish a gradient not exceeding approximately seven tenths volt per centimeter per millimeter of mercury pressure therebetween, means for admitting a noble gas into said discharge portion to flow past said discharge electrode and through said cathode, means for admitting a carrier gas for containing a sample gas having an affinity for electrons into said sensing portion to flow past said anode, means for exhausting said gases from a point between said anode and cathode, and said sources of potential establishing the field between said discharge electrodes negative and said anode positive with respect to said cathode.

5. An electron capture detector for a gas chromatograph comprising, a chamber having a discharge portion and a sensing portion, a pair of discharge electrodes positioned in said discharge portion, means for connecting a source of potential sutficient to cause an arc across said discharge electrodes, an apertured anode and an apertured cathode positioned in said sensing portion, means for connecting a source of potential across said anode and cathode to establish a gradient not exceeding approximately seven tenths volt per centimeter per millimeter of mercury pressure therebetween, means for admitting helium gas into said discharge portion to flow past said discharge electrodes and through said cathode, means for admitting nitrogen gas for containing a sample gas having an afiinity for electrons into said sensing portion to flow past said anode, means for exhausting said gases from a point between said anode and cathode, and said sources of potential establishing the field between said discharge electrodes negative and said anode positive with respect to said cathode.

6. An electron capture detector for a gas chromatograph comprising, a chamber having a discharge portion and a sensing portion, a pair of discharge electrodes positioned in said discharge portion, means for connecting a source of potential sufiicient to cause an arc across said discharge electrodes, an apertured anode and an apertured cathode positioned in said sensing portion, means for connecting a source of potential across said anode and cathode to establish a gradient not exceeding approxmiately seven tenths volt per centimeter per millimeter of mercury pressure therebetween, means for admitting helium gas into said discharge portion to flow past said discharge electrodes and through said cathode, means for admitting nitrogen gas for containing a sample gas having an aflinity for electrons into said sensing portion to flow past said anode, means for exhausting said gases from a point between said anode and cathode, said sources of potential establishing the field between said discharge electrodes negative and said anode positive with respect to said cathode, and the centers of said anode and cathode and the center point: between said discharge electrodes being substantially in a straight line.

7. An electron capture detector for a gas chromatograph comprising, a discharge chamber and a sensing chamber, a pair of platinum tipped discharge electrodes positioned in said discharge chamber, means for connecting a source of potential sufficient to cause an arc across said discharge electrodes, an anode of stainless steel mesh positioned in said sensing chamber, a cathode of stainless steel mesh positioned across one end of said sensing chamber, a necked down chamber connecting said discharge and sensing chambers, means for connecting a source of potential across said anode and cathode to establish a gradient not exceeding approximately seven tenths volt per centimeter per millimeter of mercury pressure therebetween, means for admitting a helium gas into said discharge portion to flow past said discharge electrodes and said cathode, means for admitting a mixture of nitrogen and a sample gas having an afiinity for electrons into said sensing portion to first flow past said anode, means for exhausting said gases from a point between said anode and cathode such that said mixture remains substantially out of contact with said cathode to avoid contamination of said cathode, said sources of potential establishing the field between said discharge electrodes negative and said anode positive with respect to said cathode, and the centers of said anode and cathode and the center point between said discharge electrodes being sub stantially in a straight line.

References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, Primary Examiner. STANLEY D. SCI-ILOSSER, Examiner. R. JUDD, Assistant Examiner.