GB1564499A - Inosation detectors - Google Patents

Description

(54) IONISATION DETECTORS (71) I, JAMES EPHRAIM LOVELOCK, a British citizen, of Coombe Mill, St. Giles on the Heath, Launceston, Cornwall and formerly of Bowerchalke, Salisbury, Wiltshire, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention concerns ionisation detectors, examples of which are electron capture detectors. In particular the invention concerns ionisation detectors which are capable of operating at elevated temperatures.

In the case of an electron capture detector, it is known that the sensitivity, that is the ability of the detector to detect the presence of an electron absorber in a gas flow through the detector, is a function of the properties of the absorber of interest and the operating temperature of the detector. Certain electron absorbers which dissociate after electron attachment, such as halocarbons, are more readily detected at elevated temperatures whereas other electron absorbers which attach electrons without dissociation, such as oxygen and certain oxygen containing organic compounds, are less readily detected at elevated temperatures.

At elevated operating temperatures, thermal decomposition of the absorbers can occur. Therefore in detectors designed to operate at elevated temperatures a balance is required between the rate of thermal decomposition of the electron absorber to be detected and the rate of reaction of the absorber with electrons in the ionisation chamber of the detector. An upper temperature limit is generally determined by the temperature at which the rate of thermal decomposition of the absorber of interest is comparable with its rate of reaction with electrons. The electron absorber is conveyedinto the detector in a carrier gas stream and in conventional detectors the incoming sample generally travels through a conduit, which can be formed from a metal, and which in practice will reach a temperature substantially equal to the temperature pertaining in the ionisation chamber of the detector.Consequently, a substantial decomposition of the absorber can occur within the flow conduit before entry into the ionisation chamber of the detector thus counteracting the advantages to be gained in operating at elevated temperatures.

According to the present invention there is provided an ionisation detector comprising an ionisation chamber having an inlet, an outlet, a source of ionising radiation within the chamber and means for heating the chamber to elevated operating temperatures and in which the inlet and a wall of the chamber about the inlet are each formed from a material of lower thermal conductivity than the remainder of the chamber whereby to minimise heat conduction along the inlet to thereby maintain the inlet at a temperature lower than the operating temperature of the chamber. This results in a significant improvement in performance as the absorber of interest, that is the sample in the carrier flow, can reach the ionisation chamber without encountering a destructive temperature along its path of travel to the chamber.

The invention will be described further, by way of example, with reference to the diagrammatic drawing accompanying the Provisional Specification which illustrates one embodiment of an electron capture detector.

The illustrated detector comprises a hollow cylindrical first electrode 1 having an elongate hollow stem 2 and an elongate second electrode 3 mounted substantially coaxially within the first electrode 1. The first electrode can constitute the cathode and the second electrode 3 the anode. The electrode 3 is supported by a block 4 of electrically insulating material, for example a PTFE bush, at the end of the hollow stem 2.

The cylindrical electrode 1 is located within a cup-shaped housing 5 of thermally insulating material and an electrical heating coil 6 is arranged about the exterior of the electrode 1 within the housing 5. The interior of the electrode I defines an ionisation chamber 7 and a radio-active source 71 is disposed around or adjacent the interior of the electrode 1.

The front open end of the chamber 7 is closed by a thin plate 8 carrying an inlet conduit 9 for conveying gas into the detector. The conduit 9 can be formed integral with the plate 8 or can be detachably connected to the plate 8. The conduit 9 can be provided with external cooling fins 10, or even a water jacket. The plate 8 and conduit 9 are formed from the same or different materials, the material or materials being poor heat conductors having a lower thermal conductivity than the material of the chamber.

Gas flowing through the conduit 9 enters the chamber 7 and exits through an outlet 11 arranged at or adjacent the end of the stem 2.

The detector differs from a conventional detector having -cylindrical geometry in that the base or one end of the- ionisation chamber is closed by the thin plate 8 which is exposed on its side remote from the ionisation chamber 7 to normal atmospheric conditions of temperature. The plate 8 can be formed from stainless steel and the inlet conduit 9 can be formed from the same material. The cooling fins 10 can be formed from copper discs spaced at intervals along the conduit 9.

By this arrangement the incoming flow can enter the ionisation chamber in a cool and undecomposed condition. In addition while the walls of the chamber can be heated by the coil 6 to elevated temperatures which can be in excess of 200"C, the exposure of the base or end of the chamber to ambient temperature can result in the formation of convection currents within the chamber 7. Such convection currents can assist in hastening the encounters between the sample and electrons within the chamber. The orientation of the detector can influence its performance and in order to nullify the effects of gravity it is preferable to arrange the detector in a horizontal axial position.

As examples only of dimensions, the chamber 7 can have a diameter of 1.25 cm.

and a depth of 1.25 cm. The plate 8 can have a thickness of 0.01 inch and the conduit 9 an internal diameter of 0.01 inch.

It will be realised that a detector according to the invention is not restricted to such dimensions and materials as mentioned above and likewise the detector is not limited to cylindrical geometry.

A detector of the type illustrated can be used to detect the level of methyl chloride in the atmosphere with significant improvefent over a conventional detector. With a conventional detector operating at the most favourable conditions the limit of detection of methyl chloride was 2 to 3 parts per 109 by volume.

With a detector of the invention the detection was enhanced to 8 parts per 1011 by volume. Similar results and improvements have been obtained with other compounds such as vinyl chloride, methyl chloroform and dichloro-difluoro-methane.

As an illustration of the invention it was observed that with the detector chamber at a temperature of 250"C, the temperature at the junction of the inlet conduit 9 and the plate 8 was in the region of 100 to 1200C.

Stainless steel is a suitable material as it has a low thermal conductivity for a metal and is chemically inactive or unreactive compared with other metals. However it is not the only material and other alloys, such as Nichrome (Registered Trade Mark), can be employed.

Examples of radio-active sources are Nickel 63, Iron 55, Promethium 147, Krypton 85, Samarium 141, and Scandium Tritide, the latter being a form of Tritium which is stable at high temperatures.

Attention is directed under Section 9 of the Patents Act 1949 to Specification Nos.

1,469,570 and 1,148,440.

WHAT I CLAIM IS: I. An ionisation detector comprising an ionisation chamber having an inlet, an outlet, a source of ionising radiation within the chamber and means for heating the chamber to elevated operating temperatures and in which the inlet and a wall of the chamber about the inlet are each formed from a material of lower thermal conductivity than the remainder of the chamber whereby to minimise heat conduction along the inlet to thereby maintain the inlet at a temperature lower than the operating temperature of the chamber.

2. An ionisation detector as claimed in claim 1 in which the inlet and wall are formed from stainless steel.

3. An ionisation detector substantially as herein described with reference to, and as illustrated in, the drawing accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. of the electrode I defines an ionisation chamber 7 and a radio-active source 71 is disposed around or adjacent the interior of the electrode 1. The front open end of the chamber 7 is closed by a thin plate 8 carrying an inlet conduit 9 for conveying gas into the detector. The conduit 9 can be formed integral with the plate 8 or can be detachably connected to the plate 8. The conduit 9 can be provided with external cooling fins 10, or even a water jacket. The plate 8 and conduit 9 are formed from the same or different materials, the material or materials being poor heat conductors having a lower thermal conductivity than the material of the chamber. Gas flowing through the conduit 9 enters the chamber 7 and exits through an outlet 11 arranged at or adjacent the end of the stem 2. The detector differs from a conventional detector having -cylindrical geometry in that the base or one end of the- ionisation chamber is closed by the thin plate 8 which is exposed on its side remote from the ionisation chamber 7 to normal atmospheric conditions of temperature. The plate 8 can be formed from stainless steel and the inlet conduit 9 can be formed from the same material. The cooling fins 10 can be formed from copper discs spaced at intervals along the conduit 9. By this arrangement the incoming flow can enter the ionisation chamber in a cool and undecomposed condition. In addition while the walls of the chamber can be heated by the coil 6 to elevated temperatures which can be in excess of 200"C, the exposure of the base or end of the chamber to ambient temperature can result in the formation of convection currents within the chamber 7. Such convection currents can assist in hastening the encounters between the sample and electrons within the chamber. The orientation of the detector can influence its performance and in order to nullify the effects of gravity it is preferable to arrange the detector in a horizontal axial position. As examples only of dimensions, the chamber 7 can have a diameter of 1.25 cm. and a depth of 1.25 cm. The plate 8 can have a thickness of 0.01 inch and the conduit 9 an internal diameter of 0.01 inch. It will be realised that a detector according to the invention is not restricted to such dimensions and materials as mentioned above and likewise the detector is not limited to cylindrical geometry. A detector of the type illustrated can be used to detect the level of methyl chloride in the atmosphere with significant improvefent over a conventional detector. With a conventional detector operating at the most favourable conditions the limit of detection of methyl chloride was 2 to 3 parts per 109 by volume. With a detector of the invention the detection was enhanced to 8 parts per 1011 by volume. Similar results and improvements have been obtained with other compounds such as vinyl chloride, methyl chloroform and dichloro-difluoro-methane. As an illustration of the invention it was observed that with the detector chamber at a temperature of 250"C, the temperature at the junction of the inlet conduit 9 and the plate 8 was in the region of 100 to 1200C. Stainless steel is a suitable material as it has a low thermal conductivity for a metal and is chemically inactive or unreactive compared with other metals. However it is not the only material and other alloys, such as Nichrome (Registered Trade Mark), can be employed. Examples of radio-active sources are Nickel 63, Iron 55, Promethium 147, Krypton 85, Samarium 141, and Scandium Tritide, the latter being a form of Tritium which is stable at high temperatures. Attention is directed under Section 9 of the Patents Act 1949 to Specification Nos.

1,469,570 and 1,148,440.

WHAT I CLAIM IS: I. An ionisation detector comprising an ionisation chamber having an inlet, an outlet, a source of ionising radiation within the chamber and means for heating the chamber to elevated operating temperatures and in which the inlet and a wall of the chamber about the inlet are each formed from a material of lower thermal conductivity than the remainder of the chamber whereby to minimise heat conduction along the inlet to thereby maintain the inlet at a temperature lower than the operating temperature of the chamber.

2. An ionisation detector as claimed in claim 1 in which the inlet and wall are formed from stainless steel.

3. An ionisation detector substantially as herein described with reference to, and as illustrated in, the drawing accompanying the Provisional Specification.