GB2328789A - Photoionisation detectors - Google Patents

Abstract

In a photoionisation detector (PID) in which a gas or vapour is passed through a sampling chamber 11 between inlet and outlet ports 15,116 and radiation, e.g. uv, is shone through a window 19 in the chamber to ionise material which is then attracted to positive and negative electrodes 123,124, the negative electrode 123 is located centrally opposite the window and is encircled by a cavity 117 leading to the outlet port. The positive electrode 124 may be a ring positioned near the window but shielded from it by an annular wall 25. Such a configuration reduces fouling of the window, reduces clear-down time of the detector and provides greater output stability.

Description

Detectors This invention is concerned with detectors, and relates in particular to novel forms of photolonisation detector.

Photoionisation detectors - PIDs - are well-known devices for the detection of readily-ionisable gaseous or vaporised materials in a gas stream by means of the photoionisation of those materials. In essence, such a device is a chamber - the ionisation chamber - through which can be passed the gas-borne material to be detected, the chamber containing an electric-fieldproducing pair of electrodes and having a window through which can be shone light (from a chamber-external lamp) of an appropriate frequency and energy such that it ionises some of the materials molecules, forming charged particles that can be picked up by the pair of electrodes to give rise to an electric current that indicates the presence of the material.

The ionisation process involves photons - the elementary energy packets of which the light consists interacting with molecules in the gas stream; photons having energy in excess of the ionisation potential of a particular gas stream constituent can cause molecules of that constituent to be fragment into ions - that is, moieties that bear an electric charge. Usually, two such ions are formed from one interacting photon, one positive (lacking a negatively-charged electron), the other negative (having gained that electron). If the pair of electrodes in the chamber is suitably connected across a voltage source, one electrode being positively charged while the other is negatively charged, an electric field is generated across the chamber which results in the ions experiencing a force which drives them towards the relevant electrode - the negativelycharged ions going to to the positive electrode, the positively-charged ions to the negative electrode. When the ions arrive at the electrodes they are neutralised by an electric current flowing to or from the electrodes. Although this current is very small, perhaps less than a thousand millionth of an amp, it can be amplified and measured, and thus can be used to provide a very sensitive and rapid method for measuring the amount of ionisable gas in the ionisation chamber.

PIDs have traditionally been employed in the detection of organic materials emanating from a gas chromatographic column. Increasingly, however, PIDs are finding utilisation in the detection of organic gases and vapours of organic volatiles both in industry and the environment, where such volatiles present a health risk, an environmental threat, a fire risk, or an economic loss. Such materials are typically the large molecules commonly found in scents, solvents and liquid petroleum-based fuels (mostly the alkanes, from butane on). The detector may be used for continuous monitoring, or for the location of a leak emanating from some means of containment of an organic material.

The physical processes which enable photoionisation detection are very fast, and the time taken for a detector to respond to the presence of an ionisable material - or, more usually, to an increase in the concentration of an ionisable material in some other, non-ionisable gaseous carrier medium - is usually determined by the rate at which the material is displaced into the detector's inlet means and then through the ionisation chamber itself. However, conventional detectors frequently suffer from several inadequacies which significantly reduce their performance; careful analysis of the situation reveals that these are, for the most part, due to the configuration of the both inlet and outlet means and the electrodes, and to the manner in which these components form part of the ionisation chamber itself. This is now explained further.

Firstly, the utility of photoionisation detection is in many instances not only determined by the rate at which the detector responds to increased ionisable material concentration, but also the rate which it responds to a decreased concentration. The two rates are commonly described by the terms "response time" and "clear-down time" respectively. In conventional PIDs the clear-down time is found to be significantly longer than the response time, such that if the ionisation chamber is briefly exposed to high levels of ionisable material it is for an inconvenient length of time thereafter unable to detect that material at low concentration levels. Secondly, the lower sensitivity threshold of the detector to a given organic material is, in conventional PID detectors, occasionally or even constantly limited by an unstable current generated within the photoionisation chamber itself, rather than by instabilities traceable to the material composition, unstable gas flow, variable photon flux emanating from the lamp constituting the light source, or the electronics conferring current amplification and signal processing. A third problem is that the lamp window through which the light enters the chamber becomes fouled with an organic film subsequent to exposure of the conventional detector to even modest levels of ionisable organic material. This fouling effect attenuates the flux emanating from the lamp, thus reducing the amount of light available to cause photoionisation and so lowering the sensitivity of the detector to any particular ionisable material. The fouling must be rectified by removing and carefully cleaning the lamp window, which is disruptive both for the user and in terms of detector stability. However, the fouling occurs only in the event of the detector being used, and not in the event of a zero field being applied between the electrodes, or a zero light output from the lamp, and therefore the fouling effect must relate at least in part to the field applied between the electrodes.

The present invention proposes changes to the design and construction of PIDs which should remove, or at least significantly mitigate, the identified inadequacies. More specifically, the invention suggests, firstly, that the negative electrode, to which are attracted the positive ions (these are usually an organic moiety of the material, and give rise to the fouling effect), should be located centrally opposite the light input window; this ensures that the potentially window-fouling ions are kept physically as far away from the window as possible. Secondly, it proposes that this negative electrode be almost totally (and most preferably completely) encircled by a cavity which leads to the chamber's gas outlet means; this ensures that the general gas throughput is most rapidly enabled to remove ions derived from the subsequent re-ionisation (by the W light) of any organic deposit formed on the electrode, and thus avoids what is thought to be one of the major causes of the extended clear-down time of conventional PIDs. And thirdly, and preferably, it is proposed that the positive electrode, to which are attracted the negative ions, should be positioned out of sight of the light input window, and thus out of sight of the light source; this ensures a greater stability in the device's output.

In one aspect, therefore, the invention provides a photoionisation detector for the detection of a gaseous or vaporised material by the ionisation of a sample thereof, the detector including a sampling chamber having: inlet and outlet ports through which the material to be detected can be passed into and then out of the chamber; a light input window through which can be shone into the chamber ionising electromagnetic radiation so as to ionise some of the material therein to form charged particles; and a positive and negative electrode pair positioned internally of and across the chamber so as to be able to pick up the charged particles; and wherein: the negative electrode is located centrally opposite the light input window, and is almost totally encircled by a cavity which leads to the chamber's gas outlet port.

The photoionisation detector of the invention has a sampling chamber with inlet and outlet ports, a light input window and a positive and negative electrode pair.

Other than the position and status of the negative electrode (centrally opposite the light input window, and almost totally encircled by a cavity which leads to the chamber's gas outlet port), the detector as so far defined is more or less conventional, and needs no detailed discussion here. Nevertheless, it may be useful to point out the following: Photoionisation detectors are regularly used to detect for organic materials that have a significant vapour pressure. Such materials are commonly found in solvents, perfumes and liquid fuels, and specific examples are 2,2,4-trimethylpentane (a constituent of petrol), dichloromethane (a common solvent used in dry-cleaning), styrene (a precursor in the manufacture of the well-known plastic polystyrene), isobutane (a common refrigerant), ammonia (an important precursor in the synthesis of many nitrogenous organics), and dichlorodifluoromethane (CFC12, a common aerosol propellant).

The chamber itself may be of any material as may be appropriate to its use (and to the material it is to detect) - thus, which provides sufficient mechanical support for the chamber members, is not significantly degraded by the ionising radiation employed (high-energy W light), is not permeable to any of the gases to be tested for, and is electrically insulating. A typically such material is PTFE.

The chamber may be of any size and shape as may be appropriate to its use (and to the gas it is to detect).

Generally, though, it will conveniently be cylindrical in shape in conformity with the circular lamp window of the normally-available UV lamps, and since very little W light shines beyond the edges of the window the chamber conveniently matches the window in size, as well (and so is about 10 mm in diameter). The depth of the cell is conveniently anywhere from 1 to 10 mm, depending on the specific use to which the PID is to be put.

Shallow chambers are preferred where the carrier gas is air (air absorbs W light strongly, and within a millimetre or two, so deeper chambers are of no value).

Deeper chambers are preferred where, as in the case of a chromatographic column, the carrier gas is a weakly W-absorbing one such as helium).

The inlet and outlet ports are in essence no more than sealable openings into the chamber to which may appropriately be attached feed lines from the source of the gas to be detected and to a disposal point for that gas. Thus, the combination of lines and ports provides inlet and outlet means for the conveyance of gas through the ionisation chamber. These may comprise cavities or tunnels through or between the chamber's walls. They may be connected by piping, so as to permit conveyance of material-carrying gas into and out of the ionisation chamber from some remote environment. Such cavities and piping desirably have internal walls which physically isolate the conveyed gas from the external environment and through which neither the constituents of the conveyed gas nor the constituents of the external environment can leak appreciably. It is preferable for gas flow to be laminar, and therefore the internal walls exposed to the conveyed gas are smooth and clean, having no sharp edges or recesses by which a substantial portion of the gas might be delayed in its journey to or from the ionisation chamber. It is preferable for gas piping to have an internal cross-section of no more than a few millimetres, and to have internal walls made of a relatively impermeable plastic such as PTFE, or a metal such as stainless steel.

It is through the chamber's light input window that there is shone into the chamber electromagnetic radiation (that is, light radiation of a frequency capable of ionising the material to be detected, and normally in the ultraviolet (W) region of the spectrum). The window is commonly an integral part of the lamp light source, sealingly closing a corresponding aperture in the chamber walls, and normally comprises an optically-flat disc attached (in a W-lamp) to a cylindrical glass lamp body to confine a volume of gas at low pressure. The lamp includes either or both of internal and external electrical components to enable the confined gas to form a light-emitting plasma. The lamp window is naturally made of a material which transmits the ultraviolet fraction of light generated in the lamp.

Disposed internally of and across the chamber is a positive and negative electrode pair. It is the manner in which these are so disposed that provides the PID of the invention with its major benefits; the negative electrode is located centrally opposite the light input window, and is almost totally - and, indeed, preferably wholly - encircled by a cavity which leads to the chambers gas outlet port.

As explained hereinabove, having the negative electrode, to which are attracted the large, positive material ions, located centrally opposite the light input window ensures that these possibly window-fouling ions are kept physically as far away from the window as possible, while having this negative electrode almost or actually totally encircled by a cavity which leads to the chamber's gas outlet means ensures that any ions derived from the subsequent re-ionisation (by the W light) of any organic deposit formed on the electrode are rapidly swept away from the chamber by the general gas throughput.

The negative electrode is located centrally opposite the light input window - that is to say, it is positioned at the side of the chamber opposite the window, and is central thereof (ie, is at the centre of the area defined by a "projection" of the window onto that side). "Central" does not necessarily mean at the exact centre, merely roughly central, as within a distance - typically a millimetre or so - that is small relative to the dimensions of the chamber.

The negative electrode is almost or actually totally encircled by a cavity which leads to the chamber's gas outlet port. Indeed, the electrode is most conveniently a pin-like member (referred to hereinafter as a "pin electrode") located axially of the cavity and approximately normal to face of the lamp window and protruding no more than very slightly - just its tip, probably less than a millimetre - from the chamber wall into the chamber space. In the simplest case, the cavity is a generally tubular passageway leading through the chamber wall directly to the outside (the port), and the electrode is located axially of that passage. In another embodiment, the cavity is a shorter tubular passageway which then connects to a smaller pipe-like passage itself leading off, at an angle, to the port.

The wall of the chamber in which the negative electrode is located (in its cavity) may - rather like the window - be a separate wall member, sealably mountable in an aperture in the chamber, rather than an integral part of the chamber itself.

The PID of the invention employs a positive and negative electrode pair positioned internally of and across the chamber so as to be able to pick up the charged particles. The negative electrode is, as just described, located in a cavity located centrally opposite the light input window. While in principle the positive electrode could be almost anywhere across the chamber (so that the pair produces an electric field across the chamber that drives the negatively-charged ions to one electrode and the positively-charged ions to the other), most preferably the positive electrode is positioned at the side of the chamber opposed to that side containing the negative electrode, and thus at that side containing the light input window. Moreover, most preferably the positive electrode is positioned out of sight of the light input window, and thus out of sight of the light source, so as to ensure a greater stability in the device's output. Advantageously, therefore, the positive electrode is placed in close proximity to the perimeter of the lamp window - indeed, it is most conveniently shaped as a ring, and mounted around and just outside the edges of the window (such an electrode is referred to hereinafter as the "ring electrode").

It is desirable for the ring electrode not actually to abut the lamp window, due both to the brittleness of the window, and also to the adverse affect of high energy light on the electrode material (this can result in an increase in the ionisation current elicited thereby even in the absence of an ionisable gas).

Instead, it is preferable for a gasket-like wall of an inert and deformable material to be positioned between the ring electrode and the lamp window.

The electrodes can be made of any suitable material - the pin electrode is conveniently made of brass or stainless steel, while the ring electrode is preferably made of an inert and readily machinable metal such as brass.

The photoionisation detector of the invention requires in operation to be suitably connected both to gas provision means for supplying to (and removing from) its chamber the gas-borne material to be detected and also to electronic means for providing an appropriate voltage across its electrodes, and then for amplifying and measuring the current generated by the neutralisation of the electrons and ions formed in the chamber. In each case, these may be whatever is conventionally utilised for these purposes, and need no further comment here. Nevertheless, briefly, the gas supply (and removal) means involves a constant flow of gas into and out of the ionisation chamber (the rate of flow of gas through the detector may affect the measured detector response), and so, to make a quantitative determination of the concentration of ionisable gas within the conveyed gas, there is required a constant pressure differential between the enclosure input means and output means. The detection system may include or be operatively connectable to components such as pumps, fans, or regulated gas cylinders, which provide this constant pressure differential.

As to the electronic equipment required, this involves basically little more than means to provide a field-forming voltage across the two electrodes - a field of around a few tens of volts per millimetre is required, necessitating a voltage of several hundred volts - coupled with a means to amplify whatever small current flows at one or other electrode when any generated ions are being collected and neutralised. It should, though, perhaps be noted that because the current is extremely small - of the order of a few nanoamps - the amplification circuitry (and the leads to and from the electrodes) should be very well shielded, else the noise they pick up will swamp the signal. In this connection it is normal to enclose the majority of the system components - the chamber and its lamp, for instance - in a Faraday cage.

As will be appreciated, in its more preferred embodiment the invention provides a system for the detection of a readily-ionised gas passed by inlet and outlet means through an ionisation chamber, which chamber includes: a window through which W light can be caused to emanate; and a first electrode which is located centrally opposite the window and within and thus encircled by a cavity (in a chamber wall) which provides the gas outlet means, which electrode can be caused to be at a negative potential relative to a second electrode spaced therefrom across the chamber; and wherein the electrodes are electrically connectable to a means of measuring an electric current.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying diagrammatic Drawings in which: Figure 1 shows a schematic "sectional" representation of a conventional photoionisation detector of the Art; Figures 2A,B show a similar schematic "sectional" representation, but of a photoionisation detector of the invention, together with a detail showing the position of the pin electrode relative to some other ionisation chamber members; and Figure 3 shows a schematic diagram of the PID of Figure 2 in use in a gas leak testing situation.

Figure 1 shows a conventional photoionisation detector of the Art. The detector has a tubular chamber defining a space (generally 11) having side walls (12), an end wall (13; this is shown integral with the side walls 12, but is preferably a separate member sealingly mounted to close the end aperture formed by the side walls), and a window aperture (14).

The end wall 13 is penetrated by two passageways though which may flow the gas-borne material to be detected; one is an inlet passageway (15), the other an outlet passageway (16). The outlet passageway 16 is in two parts; an internal part (17), immediately adjacent the chamber space 11, and an external part (18), leading from the internal part 17 to the outside.

The window aperture 14 has sealingly mounted therein the W-transparent window (19) of a W-light lamp (generally 20). The lamp has a body (21) enclosing a volume (generally 22) of gas that is excitable (by means not shown; normally this will be a radio-frequency inducer) into a plasma form that will emit W light, which light can exit the space 22 through the window 19 and thus into the chamber space 11.

The detector's chamber space 11 contains two electrodes (23,24) connected to a voltage source via a circuit including a current-measuring system (represented by an ammeter A). One (23) of these electrodes is a negatively-charged electrode; it is formed like a pin, and located roughly centrally between the input and output passageways 15 and 16,17. It extends deeply into the chamber space 11 very close to the window 19. The other (24) of these electrodes is a positively-charged electrode; it is formed like a sleeve, or collar, and is located within the chamber space 11 and around the side walls thereof. Much of it is in direct sight of the window (and thus the light therefrom).

In operation, the gas-borne material to be detected - usually a very small amount of detectable material within a much larger volume of some other, "inert", carrier gas - is passed through the ionisation chamber space 11 via the gas inlet and outlet means 15,16. W light generated by the lamp 20 is shone through the window 19 into the chamber space 11, and causes some of the ionisable material present to be ionised. Any negative ions generated are attracted to the positively-charged sleeve electrode 24, while the positive ions are attracted to the negatively-charged pin electrode 23. Because this latter is so close to the window 19, it is inevitable that many of these positive ions will collide with, and probably adhere to, the inside surface of the window, so fogging it (and thus reducing the intensity of the light available in the chamber space 11).

The detector of Figure 1 is for the most part an integral device, being a one-piece tube closed at one end (13) by and integral end wall with apertures therein and closed at the other end by the window 19. The tube (and other) material is of course of an inert material that does not react with either the high energy (W) light emanating from the lamp window 19 (any such reaction would lead to degradation and possibly a detector response even in the absence of an ionisable material within the ionisation chamber). Also, the tube (and other) material must not be prone to adsorb any significant quantities of ionisable material, relative to the lowest quantity thereof which may be detected as a consequence of its conveyance into the ionisation chamber by the gas inlet and outlet means. Also again, the inert material must electrically isolate the pin electrode 23 the other electrode, so as not to cause a pathway for the flow of electric current other than by means of gas ionisation in the chamber. A suitable material for the tube is polytetrafluoroethylene (PTFE).

Figures 2A,B show a photoionisation detector of the invention. The detector is very like the Art detector shown in Figure 1; it too has a tubular chamber defining a space (generally 11) having side walls 12, an end wall 13, and a window aperture 14, and the end wall 13 is penetrated by two passageways though which may flow the gas-borne material to be detected; one is an inlet passageway 15, the other an outlet passageway (116) in two parts; an internal part (117), immediately adjacent the chamber space 11, and an external part (118), leading from the internal part 117 to the outside. The detector's chamber space 11 contains two electrodes (123,124) connected to a voltage source.

One (123) of these electrodes is a negatively-charged electrode; it is formed like a pin, but unlike the corresponding electrode 23 in the Art detector it is located axially of the internal part 117 of the outlet passageway 116, and protruding only very slightly (if at all) into the chamber space 11 (see Figure 2B) towards the window 19. The other (124) of these electrodes is a positively-charged electrode; unlike the sleeve electrode 24 of the Art detector, in the invention the electrode 124 is formed like a ring or annulus, and though located within the chamber space 11, is adjacent the periphery of the window 19 but shielded from direct sight of the window (and thus the light therefrom) by a low annular wall (25).

Operation of the detector of the invention is much like operation of the Art detector. The material to be detected - usually a very small amount of material within a much larger volume of some other, "inert", carrier gas - is passed through the ionisation chamber space 11 via the gas inlet and outlet means 15,116. W light generated by the lamp 20 is shone through the window 19 into the chamber space 11, and causes some of the ionisable material present to be ionised. Any negative ions generated are attracted to the positively-charged ring electrode 124, while the positive ions are attracted to the negatively-charged pin electrode 123 (and thus away from the window 19).

As gas continues to be fed into the chamber space 11 so it exits therefrom via the cavity 117 around the pin electrode 123 and leading to the outlet 18. In so doing it directly passes out across part or all of the surface of the pin electrode 123, so removing from therearound any remaining positively-charged gas ions.

The schematic diagram of Figure 3 shows a simply instance of the use of a PID (31) to detect a leak from a crack (32) in a pipe (33) carrying a detectable material. The inlet (116 in Figure 2A) of the PID 31 is connected by a narrow-bore conduit (34) to a probe (35), and the outlet (15 in Figure 2A) of the PID is connected by another narrow-bore conduit (36) to a pump (37) which exhausts back to the ambient atmosphere.

In operation, the probe 35 is moved around in the area of the pipe 33 where a leak is suspected, and a sample of the ambient atmosphere around the pipe 33 is drawn by the pump 37 into the probe 35 and through the conduit 34 into the PID, and thence out of the PID to and through the conduit 36 and the pump, and vented to atmosphere.

The PID is also electrically and electronically connected to a lamp driver (38) and to a source (39) of electrode voltage and current measurement. The latter is shown with an indicator light (41) and a meter (42) for showing when, and how much, material is detected.

Claims (10)

Claims

1. A photoionisation detector for the detection of a gaseous or vaporised material by the ionisation of a sample thereof, the detector including a sampling chamber having: inlet and outlet ports through which the material to be detected can be passed into and then out of the chamber; a light input window through which can be shone into the chamber ionising electromagnetic radiation so as to ionise some of the material therein to form charged particles; and a positive and negative electrode pair positioned internally of and across the chamber so as to be able to pick up the charged particles; and wherein: the negative electrode is located centrally opposite the light input window, and is almost totally encircled by a cavity which leads to the chamber's gas outlet port.

2. A detector as claimed in Claim 1, wherein the material from which the chamber is made is PTFE.

3. A detector as claimed in either of the preceding Claims, wherein the chamber is cylindrical, 10 mm in diameter and from 1 to 10 mm in depth.

4. A detector as claimed in any of the preceding Claims, wherein the illuminating, ion-producing electromagnetic radiation is light radiation in the ultraviolet (low) region of the spectrum.

5. A detector as claimed in any of the preceding Claims, wherein the negative electrode is within and wholly encircled by the cavity which leads to the chamber's gas outlet port.

6. A detector as claimed in any of the preceding Claims, wherein the negative electrode is a pin-like member located axially of the cavity and normal to face of the lamp window.

7. A detector as claimed in any of the preceding Claims, wherein the positive electrode is positioned at the side of the chamber opposed to that side containing the negative electrode, and out of sight of the light input window, and thus out of sight of the light source.

8. A detector as claimed in Claim 7, wherein the positive electrode is shaped as a ring and placed around and outside the edges of the window.

9. A detector as claimed in Claim 8, wherein the ring-shaped positive electrode does not abut the window, there being a gasket-like wall of an inert and deformable material positioned between the electrode and the window.

10. A photoionisation detector as claimed in any of the preceding Claims and substantially as described hereinbefore.