PHOTOIONIZATION DETECTOR SYSTEM FOR ORGANICS IN WATER

Abstract

A gas-equilibrated, volatile-in-water detector comprises a gas-sensing chamber having an orifice closed by a hydrophobic, vapour-porous membrane, typically PTFE, sealed to the periphery of the orifice. Membrane is also sealed to an external wall of a surrounding enclosure and forms an entry point to a second gaseous enclosure external of the gas-sensing chamber. A PID or similar sensor generates a measurable current or voltage in response to the partial pressure of the analyte within the gas-sensing chamber without the sensor significantly altering such equilibrium partial pressure.

Claims

1. A system for detecting or measuring gaseous or volatile analytes in an aqueous medium comprising: (a) a gas-equilibrated, volatile-in-liquid detector for immersing in an aqueous fluid or placing adjacent the surface of a body of aqueous liquid; (b) a vapour-porous or vapour-permeable membrane for enabling analyte flow through into an otherwise closed gas-sensing chamber, in order to achieve near chemical equilibrium between the analyte in the gas-sensing chamber and the concentration of the analyte in the adjoining liquid phase; and (c) current or voltage measuring means for measuring a current or voltage generated by a sensor in response to the partial pressure of the analyte within the gas-sensing chamber without the sensor significantly altering such equilibrium partial pressure, wherein the sensor is a photoionisation detector and the internal volume of the gas-sensing chamber is no greater than 100 mm.sup.3.

2. A system as claimed in claim 1, wherein the membrane forms the entry point into the gas-sensing chamber and is sealed to the periphery of a chamber orifice by means of a first seal.

3. A system as claimed in claim 2, wherein the membrane also forms an entry point to a second gaseous enclosure external to the gas-sensing chamber.

4. A system as claimed in claim 1, wherein the gas-sensing chamber orifice is in a planar supporting surface coplanar with the termination of a wall of a surrounding enclosure.

5. A system as claimed in claim 2, wherein a second seal seals the membrane to an external wall of the gas-sensing chamber or of a surrounding enclosure by means of a peelable adhesive.

6. (canceled)

7. A system as claimed in claim 2, wherein the seal to the periphery of the chamber orifice is a vapour-impermeable barrier throughout the thickness of the membrane.

8. (canceled)

9. (canceled)

10. (canceled)

11. A system as claimed in claim 1, in which the sensor is detachable from an assembly supporting the sensor.

12. A system as claimed in claim 2, wherein the seal to the periphery of the chamber orifice is a weld or an impermeable adhesive penetrating the membrane.

13. A system as claimed in claim 1, wherein elements of the sensor within the gas-sensing chamber are within 2 mm of the membrane.

14. (canceled)

15. A photoionisation detector (PID) sensor head for use in a detector of volatiles in aqueous media, which head includes a vapour-porous or vapour-permeable membrane forming an entry point into an otherwise closed gas-sensing chamber, wherein the internal volume of the gas-sensing chamber is no greater than 100 mm.sup.3.

16. A sensor head as claimed in claim 15, wherein the membrane is sealed to the periphery of a chamber orifice by means of a chamber orifice seal and forms an entry point to a second gaseous enclosure external to the gas-sensing chamber.

17. A sensor head as claimed in claim 16, wherein a surrounding enclosure seal seals the membrane to an external wall of the gas-sensing chamber or of a surrounding enclosure.

18. A sensor head as claimed in claim 17, wherein the external wall seal seals by means of a peelable adhesive.

19. (canceled)

20. A sensor head as claimed in claim 16, wherein the gas-sensing chamber orifice is in a planar supporting surface coplanar with the termination of a wall of a surrounding enclosure.

21. A sensor head as claimed in claim 16, wherein the seal to the periphery of the chamber orifice is a vapour-impermeable barrier throughout the thickness of the membrane.

22. (canceled)

23. A sensor head as claimed in claim 15, wherein the membrane is hydrophobic.

24. (canceled)

25. (canceled)

26. A sensor head as claimed in claim 16, wherein the seal to the periphery of the chamber orifice is a weld or an impermeable adhesive penetrating the membrane.

27. A sensor head as claimed in claim 15, wherein a sensor associated with the sensor head includes a plurality of electrode contacts which, in an assembled position, lie in a common plane in the gas-sensing chamber.

28. (canceled)

29. A sensor head as claimed in claim 15, wherein elements of the sensor are within 2 mm of the membrane so as to reduce the response time of the sensor.

30. A gas-equilibrated, volatile-in-liquid detector incorporating a sensor head, the sensor head comprising a vapour-porous or vapour-permeable membrane forming an entry point into an otherwise closed gas-sensing chamber, wherein the internal volume of the gas-sensing chamber is no greater than 100 mm.sup.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 shows a schematic representation of a gas-equilibrated, volatile-in-water detector according to the invention.

[0038] FIG. 2 is an elevation of the membrane assembly of FIG. 1 prior to its deployment.

[0039] FIG. 3 shows the membrane assembly of FIG. 2 from a different perspective.

[0040] FIG. 4 shows a schematic representation of a gas-equilibrated, volatile-in-water probe according to the invention wherein a gas sensor body contains a removable and replaceable gas receiving and sensing member.

[0041] FIG. 5 shows a gas sensing replaceable sub-assembly prior to assembly in the probe shown schematically in FIG. 4.

DESCRIPTION OF EMBODIMENTS

[0042] The invention will now be described in more detail by reference to the drawings provided.

[0043] FIG. 1 shows a schematic representation of a gas-equilibrated, volatile-in-water detector provided by the invention, in which a gas sensor 1 is contained within a gas sensor cradle 2. The gas sensor is preferably of cylindrical shape, and typically of 20 mm diameter and 16.6 mm height, excluding contact pins 3, as provided as a received industrial standard by gas sensor manufacturers such as Alphasense Ltd, Dynament Ltd and City Technology Ltd. The gas sensor preferably is disposed to sit upon, and make electrical connection via 3 to, a small printed circuit board (PCB), 4, itself electrically connected to a cable 5, through which power is delivered to the gas sensor and signals are communicated to and from the sensor to some external means of control such as a computer, via, by way of example, a universal serial port. The cradle 2 and PCB 4 are positioned so that the face, 1a, of the gas sensor containing a gas sensing orifice 1b, is approximately co-planar with the cradle wall flat surfaces 2a. Gas porous PTFE membrane 6, some 280-300 m.sup.−6 thick, is attached by means of a thin adhesive layer 7 to the sensor face 1a. A further portion 6b of the membrane is attached by means of a thin layer of adhesive 8 to the cradle wall flats 2a, thereby forming an entry point to a second gaseous enclosure external of the gas sensor 1. The adhesive on the membrane does not extend beyond its point of contact to the gas sensor face 1a at the periphery of the gas sensing orifice 1c. Over at least its area of coverage of the gas orifice, the membrane portion 6c is on the contrary, substantially free of adhesives which could prevent gas flow into or out of the gas orifice. The invention also provides in one aspect an additional membrane component 6d, which extends beyond cradle wall segments 2a, enabling the membrane to be peeled off after field use, rather than being stored, possibly wet and contaminated, attached to other detector members.

[0044] FIG. 2 shows the membrane assembly of FIG. 1 prior to its deployment. Additional to the components already described, the membrane includes a waxed paper or other removable member 9 to protect adhesive portions of the membrane before use. FIG. 3 shows the membrane assembly of FIG. 2 from a different perspective, with the adhesive-protective element removed.

[0045] FIG. 4 shows a schematic representation of a gas-equilibrated, volatile-in-water probe provided by the invention wherein a gas sensor body 10 contains a removable and replaceable gas receiving and sensing member 11. An example of such a sensor is the 16.6 mm×20 mm diameter miniature photoionisation detector (PID) manufactured by Ion Science Limited and containing an electrode pellet as described and claimed in our GB 2449664 B.

[0046] Sensor body 10 is seated on PCB 4, with which it makes electrical contact by means of pins 3. PCB 4 is also in electrical contact with cable 5, as described in reference to corresponding components 3, 4 and 5 in FIG. 1.

[0047] Gas sensing member 11 is approximately pellet shaped, and includes an orifice for receiving gas, 11a, on its outwardly facing major surface, 11b, opposing its other major surface proximal to the sensor body cavity (gas sensing chamber) 10b containing the gas sensing member 11. Surface 11b of the gas sensing member 11 is approximately co-planar with sensor body surface 10a, and also approximately co-planar with cradle wall flats 2a.

[0048] Gas sensing member 11 is attached at annulus 11c to gas-porous or gas-permeable membrane 6 by means of an adhesive or ultrasonic welding of membrane portion 6a to the gas sensing member flat surface 11b close to the periphery of gas orifice 11a. It is preferable for the membrane portion 6a joined to gas sensing member portion 11c not to be porous or permeable to gas, either by virtue of an impermeable adhesive being applied to and impregnating the membrane, or by the membrane 6 and gas sensing member face 11b being welded at the point of fusion so as to form a seal that is neither porous nor appreciably permeable.

[0049] The membrane 6 is also attached to cradle wall flats 2a by means of adhesive 13 at annulus 6b, thereby forming an entry point to a second gaseous enclosure 12 external of the gas-sensing chamber. The membrane is substantially porous or vapour-permeable over that portion overlaying gas orifice 11a and bounded by the impervious seal between annulus 6a and portion 11c. The membrane portion between annular seals 6a and 6b may or may not be porous or permeable according to the benefit conferred by enabling gaseous analyte gas irrigation at potential leak paths between probe members 10 and 11. The membrane 6 further may include a tab 6d for convenient removal of the membrane 6 after use.

[0050] FIG. 5 shows a replaceable gas sensing sub-assembly comprising components 11 and 6 prior to their assembly in the probe shown schematically in FIG. 4. This replaceable component includes a removable protective cover 14 to protect the adhesive portion of the membrane 6b.

[0051] The invention will now be described by reference to how it is operated in order to measure volatiles present in a watery liquid.

[0052] In the case of a gas sensor containing an integral means of gas admittance, such as is described in FIG. 1, it is preferable for the sensor 1 to be removed from the cradle 2 over times of significant storage. The sensor is manually fitted in the cradle ensuring pins from the sensor 3 fit snugly into PCB platform 4. The covering 9 is removed from a disposable membrane assembly such as shown in FIG. 2, and placed over the cradle wall flats 2a and sensor face 1a to make a seal. To ensure correct alignment of the membrane to the sensor face, such that a porous and permeable part of the membrane 6c overlays the gas sensing orifice 1b it is preferable for the membrane and cradle to include means of co-alignment such as notches (not shown).

[0053] The probe is now connected to a means of power supply and data communication via cable 5. The probe may be calibrated by placing a gas hood over the assembled probe, and presenting a suitable concentration of the analyte to the gas hood. Alternatively, the probe may be calibrated using an aqueous liquid, typically pure water, into which the gaseous analyte is dissolved. The former is generally preferable, and provides an advantage of using a gas-equilibrated, volatile-in-liquid detector over other volatile-in-water detection technologies. These previous detectors require aqueous reference samples, which are prone to loose the volatile as soon as their means of containment, typically a glass ampoule, is breached.

[0054] Typically the cable 5 connected to the probe is flexible and armoured or sheathed so as to provide protection when submerged in a watery fluid, perhaps under very adverse conditions. The probe may include additional members to ensure to protect the assembly from damage, particularly the probe itself. It is however deleterious for the membrane 6 to be appreciably obscured from the aqueous environment presented to it, insofar as free flow of fluids across it is needed for dynamic sensing.

[0055] The removal of the sensor may benefit from additional cradle members, not shown in FIG. 1, which enable the cradle walls surrounding the circular or cylindrical section of the gas sensor to be partly dismantled.

[0056] The sensor shown in FIG. 4 is best stored with its removable components removed from the sensor cradle 2. The sensor may be manually assembled as described above, although in this case the membrane assembly shown in FIG. 5 is affixed to the sensor cradle 2 only after careful orientation and assembly of gas sensor sub-components. For example, in the case of a PID sensor, a lamp is first inserted in the gas sensing member 11 opposite the membrane orifice 11a. Then the protective backing 14 to the adhesive section is removed. Then the membrane assembly is placed over the cradle wall flats 2a as shown in FIG. 4 and pressed down to ensure a water tight seal.

[0057] Following tests with the probe shown in FIG. 4, it is again advisable for the probe to be washed off in clean water and dried. The membrane 6 is peeled off cradle wall flats 2a. After removal of the sensor, the entire sub-assembly shown in FIG. 5 is removable and disposable, being a small, environmentally benign and modest cost item. The sensor body can then be removed and stored in a separate storage capsule or refitted with a new membrane assembly as depicted in FIG. 5, ready for subsequent use.

[0058] The instrument is calibrated as described above. In the case of a PID, the use of a gas for calibration is a particular advantage where the PID is being used to trace a volatile to which PID responds, with reasonable assurance as to the presence of that volatile. For example, it might be calibrated with 1 ppm xylene in air, using Henry's Law to determine the equivalent xylene-in-water concentration.

[0059] Possible applications of the invention include, for example, remediation of polluted water, monitoring of potable water, food and drink processing, and regulatory enforcement.