WO1990013809A1

WO1990013809A1 - Methods and apparatus for the determination of chlorine

Info

Publication number: WO1990013809A1
Application number: PCT/GB1990/000728
Authority: WO
Inventors: Ramater Narayanaswamy; Saschi Ahmed Momin; Ashutosh Sharma
Original assignee: The University Of Manchester
Priority date: 1989-05-11
Filing date: 1990-05-11
Publication date: 1990-11-15
Also published as: AU5731490A; GB8910813D0

Abstract

Apparatus for determining amounts of chlorine in a gaseous or liquid sample comprises a sensor element including a reagent which is capable of luminescence which is quenched by chlorine, means for exciting the luminescence of the reagent, and means for detecting changes in the luminescence resulting from quenching by chlorine.

Description

METHODS AND APPARATUS FOR THE DETERMINATION OF CHLORINE

The present invention relates to a method and apparatus for the determination of chlorine in a gaseous or liquid sample.

Chlorine is a highly toxic gas that has found extensive use in the chemical industry. It is used in the manufacture of paper products, textiles, petroleum products, medicines, antiseptics, insecticides, foodstuffs, solvents, paints, plastics and many other consumer products. Another major use of chlorine is in the sterilisation of water. It is administered to reservoirs, storage tanks and swimming pools.

The human olfactory sense can recognise chlorine in a concentration of about four parts per million parts of air volume by volume. Also, it is irritant to the mucous membranes of the eye, nose and respiratory passages. This provides a warning which is probably adequate to prevent acutely dangerous exposure but inadequate to prevent delayed effects from prolonged exposure.

The recommended maximum concentration as published by the Ministry of Labour is one part of chlorine per million parts of air, approximately 3mg m^~3, averaged over the normal eight hour working day. Thus, the danger of chlorine pollution demands a reliable and rapid method for its determination.

Both continuous and discontinuous methods for the determination of chlorine are known in the art.

The conventional discontinuous method depends upon the formation of reagent complexes with chlorine. A typical method of chlorine determination based on the formation of reagent complexes is disclosed in Czechoslovakian Patent No. CS 240787. Another method employs the absorbency of radiation by chlorine itself as a function of concentration and is disclosed in United States Patent No. US 4647210. The latter method is not selective as other substances present in the sample may absorb in the same region and interfere with the signal measured due to chlorine. Both methods often employ relatively bulky, inconvenient and expensive collection devices which are tedious to use and also time consuming. Furthermore, such methods cannot be employed in the continuous monitoring of chlorine in various environments.

Amperometric detection of trace chlorine in air or other gases is also known in the art, and one method is disclosed in Polish Patent No. 245689. An electrochemical method of sensing chlorine by use of electrodes is disclosed in Japanese Patent No. 84/26596. Although many amperometric and electrochemical methods for sensing chlorine are described in literature and patents, such devices suffer memory effects with prolonged use. It is an object of the invention to obviate or mitigate the abovementioned disadvantages .

According to a first aspect of the present invention there is provided apparatus for determining amounts of chlorine in a gaseous or liquid sample comprising a sensor element including a reagent which is capable of luminescence which is quenched by chlorine, means for exciting the luminescence of the reagent, and means for detecting changes in the luminescence resulting from quenching by chlorine.

According to a second aspect of the present invention there is provided a method of determining the amount of chlorine in a gaseous or liquid sample comprising exposing to the sample a sensor element including a reagent which is capable of luminescence which is quenched by chlorine, exciting the luminescence of the reagent, and monitoring a change in the luminescence resulting from quenching by chlorine.

The luminescent reagent used in the invention may be a phosphorescent compound but is most preferably a fluorescent compound. Most preferably, the compound is one for which the fluorescence is dynamically quenched by chlorine.

The present invention Is based on the observation that chlorine quenches the luminescence of various compounds. The quenching luminescence of a fixed concentration of compound can be monitored as a function of chlorine present.

A sensing principle based on dynamic quenching of fluorescence is preferable since it offers the advantages of full and rapid reversibility (see Appendix which describes the theory of fluorescence quenching). The relation between the fluorescence intensity and the concentration of quencher (chlorine) can be given by the Stern- Volmer relationship,

F°/F = 1 + K [Q_C12] (1)

where F° and F are the fluorescence intensities in the absence and presence of chlorine respectively, and [Q_Cι ₂] i-^s the concentration of chlorine and K_sv is the Stern-Volmer quenching constant for the system. The Stern-Volmer constant for the system may be determined by calibrating the system using known concentrations of chlorine and determining the fluorescence intensities F at these concentration. Similar relationships to (1) may be used in the case where the luminescent reagent in phosphorescent or is a reagent for which the luminescence is statically quenched.

The luminescent reagents which may be employed in this invention are preferably those which have a high quantum yield of fluorescence, with possibility of visible or near visible excitation and availability in pure form.

Examples of reagents which may be used are fluorescent reagents, eg. polynuclear aromatic hydrocarbons and their derivatives. In particular such derivatives may be halo

(particularly chloro or cyano) derivatives. More specifically, preferred fluorescent reagents include anthracene, perylene, pyrene, fluoranthene, acridine and their derivatives. A specific reagent which may be used is 9,10-diphenylanthracene.

Alternatively the luminescent reagent may be metal complex, most particularly a metal-ligand complex in which the metal is ruthenium (II), osmium (II) or iridium (III) and the ligand is 2,2'- bipyridine, 1,10-phenanthroline, or 2,2',2"-tripyridine as well as derivatives of these compounds. Generally the luminescent reagent will be immobilised in or on a support matrix therefor. Support matrices which may be used include cross-linked syrene/divinyl benzene polymers (eg. XAD2 or XAD4), silicone rubber, nylon, cellulose, aluminium oxide, gels (eg. polyacrylamide gel and silica gel), polyacrylamide, polyvinyl alcohol, polyvinyl acetate, polyethylene glycol, polyvinyl chloride, Vycor glass and ion-exchange resins.

Depending on the particular luminescent reagent and support matrix used, the reagent may be entrapped in the matrix or absorbed thereon. The amount of the luminescent reagent will be minor compared to the amount of the matrix. Typically the sensor element may comprise (when provided on a 1 mm optical fibre-see infra) ca 0.5 mg of the luminescent reagent/support matrix, of which about 1-2 μg of the mixture will be the luminescent reagent.

In one embodiment of the invention, the sensor element may be provided in an aperture in the wall of a tube (or other conduit) so that the luminescent reagent is exposed to the interior of the tube. The sample to be monitored may be passed along the interior tube whilst the means for exciting the reagent and for detecting changes in the fluorescence are provided exterior of the tube. In this embodiment, the sensor element may comprise a film of the reagent on a solid support which is transparent to the radiation which excites fluorescence of the reagent. The intensity of the fluorescence may be monitored at those surfaces of the film which are exposed and unexposed to the chlorine so that the change resulting from exposure to chlorine may be determined.

In an alternative embodiment of the invention, the apparatus may comprise an optical fibre at the end of which is provided the sensor element. Such a device provides a probe which may be inserted into a sample whereof the chlorine concentration is to be monitored. Excitation radiation may be passed along the optical fibre and the fluorescence detected by any suitable means.

Excitation of the immobilised reagent and detection of fluorescence may be effected by known methods. The detector may for example be a photomultiplier tube (PMT) or a photodiode. Chlorine concentrations may be determined by measuring the changes in intensity of quenched fluorescence of the reagent or by measuring changes in the fluorescence decay time of the reagent due to the presence of chlorine.

At low concentrations of chlorine, there will generally be a reversible interaction between the chlorine and the luminescent reagent. For example we have ascertained that a sensor element comprised of an anthracene derivative (eg. 9,10-diphenylanthracene) immobilised on XAD2 (by adsorption) may be used for measuring dissolved chlorine concentrations of 0-300 pp and for such concentrations the reagent is deemed to have a reversible reaction. However above a certain maximum chlorine concentration (which will depend on the particular luminescent reagent being used) the reaction with chlorine will- be irreversible. However is such instances the sensor element that a particular concentration of chlorine has been exceeded and thus may be used to provide an alarm system. Obviously, the sensor element would subsequently require replacement due to the irreversible interaction with chlorine.

The invention will be further described by way of example only with reference to the accompanying drawings, in which,

Fig. la illustrates the quenching of the fluorescence of 9- vinyl anthracene by chlorine;

Fig. lb is a calibration curve derived from the data of Fig. la;

Figs. 2a and 2b illustrate embodiments of apparatus for the continuous monitoring of chlorine in a sample and;

Figs. 3a and 3b illustrate embodiments of optical fibre probe for monitoring of chlorine.

Fig. la illustrates the quenching of 9-vinyl anthracene in methanol at various (known) concentrations of chlorine, and along a range of excitation wavelengths. Fig. lb is a plot of F°/F (at wavelength of maximum quenching, ca 430nm) vs chlorine concentration. This graph may be used as a calibration curve. The value of the quenching constant obtained using this data is 12,770 M-¹. Fig. 2a and 2b illustrate apparatus for the continuous monitoring of chlorine Each such apparatus comprises a tube 1 having in a wall thereof an aperture 2 in which is mounted a sensor element which includes a film of immobilised reagent exposed to the interior of the tube.

In the embodiment of Fig. 2a, the reagent is illuminated by a light source 4 and the fluorescence is monitored by a detector 5 which may for example be a conventional fluorometer.

In the embodiment of Fig. 2b, an optical fibre 6 is used to transmit light from the exciting source and back to the detector (neither shown in Fig. 2b).

Figs. 3a and 3b illustrate embodiments of optical fibre probe which may be inserted into a sample for measuring the chlorine present therein. Each probe comprises an optical fibre 10 with a sensing tip including the fluorescent reagent.

In Fig. 3a, the reagent is physically or chemically bound directly onto the fibre and is bounded by a membrane 11 which is permeable to chlorine.

In Fig. 3b, the reagent is immobilised in a matrix 12. Methods of immobilising the reagent include adsorption, ion exchanges, covalent binding and entrapment within a support matrix. Support matrices which may be employed include silica gel, polystyrene (XAD), nylon, ion exchange resins, silicone rubber, and gas permeable membranes which are not destroyed by chlorine.

It will be appreciated that the invention provides a number of advantages, including,

1. Geometric flexibility - allows the analysis of samples that are difficult to reach and where there is no "line of sight".

2. Environmental versatility - measurements may be made when the sample is hot, cold radioactive or in hostile environments.

3. Real-time analysis - there is no sample collection and therefore analysis can be performed in almost real-time.

4. Multiple analysis - more than one location may be monitored by several sensors multiplexed to one central instrument. 5. Sample integrity - there is no sample alteration hence eliminating the problems encountered with sample collection.

6. Cost effectiveness - optical fibre sensors offer cost advantages over electrodes because of low cost fibres.

7. Physical size - the ease of miniaturisation allows the development of very small, light, low volume sensors that are easy to handle.

8. Low loss - optical fibres allow the transmission of optical signals over long distances with minimal loss of energy, enabling remote sensing.

9. Non-electrical - the sensor makes use of optical signals and so is not subject to electrical interferences.

10. No separate reference system will be required as compared to conventional potentiometric devices.

APPENDIX

Theory of Fluorescence Quenching

The fluorescence intensity or quantum yield of luminescent species may be decreased or even eliminated by interactions with other chemical species. This phenomenon is called fluorescence quenching and occurs by a variety of mechanisms:

1. Static Quenching

Interaction between the fluorescent molecule (F) and the quencher (Q) takes place in the ground state, forming a non- fluorescent complex (F~Q):

F + Q = F~Q

The efficiency of quenching is governed by the formation constant of the complex, K_s, and the concentration of the quencher (Q)

F/F° = 1

1 ⁺ K_s [Q]

K_s = [F^~Q]

[F] [Q]

where F° is the fluorescence intensity in the absence of the quencher and F is the fluorescence intensity in the presence of the quencher.

2. Dynamic Quenching

The fluorescent molecule (F) and the quenching species (Q) undergo a collisional process during the lifetime of the excited state of the fluorescent molecule. The processes may be represented as,

F + hv ^ F* excitation

F* ^- F + hv' fluorescence F* + Q ^ F + Q* quenching

This phenomenon is mathematically represented by the Stern-Volmer equation,

F/F° = 1 = 1

1 ⁺ k_QT0 [Q] 1 + K_SV[Q]

where K_ΞV is the Stern-Volmer constant, and K_sv = k_QT0 k_Q is the diffusion controlled rate constant, and TO is the fluorescence lifetime.

For fluorescence, the lifetime is,

10 -11 < T₀ < 10 -7

thus,

10^{" 1} M^{" 1} < k_QT0 (K_sv) < 10³ M- 1

The dependence of F/F° on the concentration of quencher (Q) is identical to that observed for static quenching, except that the quenching constant in the latter case is the association constant. Fluorescence quenching data, obtained by intensity measurements alone, cannot distinguish between dynamic or static processes unless additional information is provided. The lifetime, temperature, and viscosity dependence of quenching can be used to discriminate between static and dynamic quenching. Another method is by careful examination of the absorption spectrum of the fluorophore. Collisional quenching only affects the excited state of the fluorophores and does not change the absorption spectrum. In contrast, ground-state complex formation will frequently result in perturbation of the absorption spectrum of the fluorophore.

3. Resonance Energy Transfer

Fluorescence energy transfer is the transfer of excited state energy from a donor (D) to an acceptor (A). This transfer occui without the appearance of a photon and is primarily the result of dipole-dipole interactions between the donor and acceptor

hv + D - D* excitation of donor A + D* - D + A* transfer of energy A* -> A + hv' fluorescence of acceptor

Energy transfer processes depend strongly on the distance between the donor and acceptor, the amount of overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor, and the relative orientation factor of the donor and acceptor transition dipoles.

4. Inner-Filter Effects

An important, often unavoidable, dominant factor in fluorescence quenching measurements is the absorption of the excitation and/or emission light. Such processes are termed inner- filter effects. The primary inner filter effect involves absorption of the excitation radiation by various chromophores in the solution or matrix. The secondary inner filter effect involves the absorption of the emitted fluorescence radiation by the same chromophores. The self-absorption of the fluorescer is assumed to be negligible. This assumption can virtually always be satisfied experimentally by employing a small concentration of fluorescer and by the proper choice of emission wavelength.

SUBSTITUTE SHEET

Claims

1. Apparatus for determining amounts of chlorine in a gaseous or liquid sample comprising a sensor element including a reagent which is capable of luminescence which is quenched by chlorine, means for exciting the luminescence of the reagent, and means for detecting changes in the luminescence resulting from quenching by chlorine.

2. Apparatus as claimed in claim 1 wherein the reagent is a fluorescent reagent.

3. Apparatus as claimed in claim 2 wherein the fluorescence of the reagent is dynamically quenched by chlorine.

4. Apparatus as claimed in claim 2 wherein the fluorescent reagent is a polynuclear aromatic compound or derivative thereof.

5. Apparatus as claimed in claim 4 wherein the polynuclear aromatic compound is selected from anthracene, perylene, pyrene, fluoranthene, acridine, and derivatives thereof.

6. Apparatus as claimed in any one of claims 1 to 5 wherein the luminescent reagent is a metal-ligand complex.

7. Apparatus as claimed in claim 6 wherein the metal is ruthenium (II), osmium (II), or iridium (III) and the ligand is 2,2'- bipyridine, 1,10-phenanthroline, or 2,2*,2"-tripyridine, and derivatives of these compounds.

8. Apparatus as claimed in any one of claims 1 to 7 wherein the luminescent reagent is immobilised on a support matrix.

9. Apparatus as claimed in claim 8 wherein the support matrix is selected from cross-linked syrene/divinyl benzene copolymers, silicone rubber, nylon, cellulose, aluminium oxide, gels, polyacrylamide, polyvinyl alcohol, polyvinyl acetate, polyethylene glycol, polyvinyl chloride, Vycor glass, and ion-exchange resins.

10. Apparatus as claimed in any one of claims 1 to 9 wherein the sensor element is provided on an optical fibre.

11. Apparatus as claimed in any one of claims 1 to 9 wherein the sensor element is provided in an aperture in the wall of a tube or other conduit.

12. Apparatus as claimed in any one of claims 1 to 11 wherein the detector comprises a photomultiplier tube or a photodiode.

13. A fibre optic probe comprising an optical fibre provided with a sensor element which includes a luminescent reagent whereof the luminescence is quenched by chlorine.

14. A method of determining the amount of chlorine in a gaseous or liquid sample comprising exposing to the sample a sensor element including a reagent which is capable of luminescence which is quenched by chlorine, exciting the luminescence of the reagent, and monitoring a change in the luminescence resulting from quenching by chlorine.