WO1987006011A1

WO1987006011A1 - Monitoring the presence of materials

Info

Publication number: WO1987006011A1
Application number: PCT/AU1986/000076
Authority: WO
Inventors: David William James
Original assignee: University Of Queensland
Priority date: 1986-03-24
Filing date: 1986-03-24
Publication date: 1987-10-08

Abstract

Raman scattered radiation is monitored at a remote location with monochromatic radiation fed (19) to the remote location and the scattered radiation returned (20) for processing along an optical fibre bundle (27).

Description

Title: "MONITORING THE PRESENCE OF MATERIALS"

FIELD OF THE INVENTION This invention relates to the monitoring of the presence or concentration of a chemical component in an environment of interest by stimulation of the environ¬ ment with electromagnetic radiation to produce scattered radiation which is collected and from which a portion or portions characteristic of the component of interest is isolated. BACKGROUND OF THE INVENTION

Raman scattering is a technique widely used for identifying molecules through their vibrational spectrum. The principle of the technique is illustrated in FIG. 1, where both the experimental requirements (FIG. 1a) and the energy level changes (FIG. 1b) are shown, and this is described in greater detail below. In the scattered light the major intensity is due to Rayleigh scattering, with a small contribution from the Raman scattering. The difference in energy between the two is measured in the spectrum analyser which is usually a double monochromator. This difference in energy corresponds to one or more vibrational energies of the molecules in the sample. The intensity at any particular energy gap corresponds to the concentration of the component vibrating with that energy. The scattered light is emitted in all directions and so may be collected at any angle relative to the incident light.

OBJECT OF THE INVENTION It is an object of the invention to utilise scattered radiation so as to monitor the presence or concentration of a particular chemical, or combination of chemical components in an environment to be monitored by a means which is remote from the environment of inter¬ est. The environment of interest may be a gas or liquid stream within a chemical plant, a fluid under pressure, a fluid at a temperature other than atmospheric temperature, a gas dissolved in a liquid or the surface of a solid. The technique of the present invention enables remote monitoring of materials not normally accessible for examination. Other objects and advantages will hereinafter become apparent.

FEATURES OF THE INVENTION The invention provides a monitor for determining the presence or concentration of a particular chemical or combination of chemical components in an environment to be monitored comprising a source of electromagnetic radiation, a means for directing the electromagnetic radiation into an environment to be monitored, a means within the environment for receiving scattered electro- magnetic radiation, a means for delivering the scattered electromagnetic radiation to a location remote from the environment to be monitored and a means sensitive to a predetermined component or combination of components within the scattered electromagnetic radiation which is characteristic of the chemical or combination of chemical components to be monitored, characterised in that the means for delivering electromagnetic radiation to the environment and the means for delivering the scattered electromagnetic radiation to the remote location comprise optical fibres, the fibres terminating in the environment to be monitored with the end of the delivery fibre or fibres being disposed relative to the end or ends of the fibres which collect scattered radiation in a relation¬ ship which minimises any return of the electromagnetic radiation other than scattered radiation.

Further the invention provides an optical monitor ij for determining the presence or concentration of a particular chemical or combination of chemical components in an environment to be monitored comprising a laser providing monochromatic radiation, means for isolating Raman scattered radiation characteristic of the chemical or combination of chemical components to be monitored and a detection means for receiving the isolated Raman scattered radiation to provide the determination of the presence or concentration of the component characterised in that monochromatic radiation generated by the laser is delivered to the environment via one or more optical fibres and scattered radiation is collected therefrom by one or more optical fibres and transmitted thereby to the means for isolating Raman scattered radiation.

Finally the invention provides a method for monitoring the presence or concentration of a particular chemical or combination of chemical components in an environment to be monitored from a remote location com¬ prising the supply of electromagnetic radiation to the environment, collecting and transmitting scattered radiation from the environment to a remote location, isolating Raman scattered radiation from the scattered radiation, and determining the presence or concentration of a component to be monitored by detecting a character¬ istic portion, or portions of the Raman spectra of that component in the collected Raman scattered radiation characterised in that the collection of Raman scattered radiation and its transmission to the remote location is achieved in isolation from the supply of monochromatic radiation by means of optic fibres.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1a illustrates the basic component of a Raman scattering device.

FIG. 1b illustrates the energy level changes of interest in scattering. I

FIG. a is a view of an optical fibre bundle. FIG. 2b is a cross section through the bundle of FIG. 2a. FIG. 3 is an axial section through a fibre bundle at the collection end.

FIG. is an axial section through another fibre bundle at the collection end. FIG. 5 is a schematic layout illustrating the optical circuit of a monitoring device in accord¬ ance with the present invention.

FIG. 6 is a schematic layout illustrating the optical circuit of an alternate monitoring device in accordance with the present invention.

FIG. 7 is a schematic layout illustrating the electrical circuit of a monitoring device of the type of FIG. 5.

DESCRIPTION OF PARTICULAR EMBODIMENTS FIG. 1a shows the basic requirements for deter¬ mining the constitution of a sample wherein a source of monochromatic radiation such as a laser 10 is used to create a beam of radiation 11 made to pass through a sample 12 to create a transmitted beam 13 from which some energy has been lost through scattering in the sample 12, the scattered radiation 14 being collected and analysed in a spectrum analyser 15 of suitable form to identify spectral components which are characteristic of whatever material is being monitored. In FIG. 1b is shown an energy level diagram indicating the two contributions to scattering caused by Rayleigh and Raman scattering. The vibrational energy gap can have a number of different values for any part¬ icular molecule, each being associated with one of .the vibrational modes characteristic of the particular mole¬ cule to produce scattered Raman photons each shifted in frequency from that of the incident photon to generate a characteristic spectrum which may be viewed to identify the material generating it. The technique can in principle use any monchrom- atic light source. The shorter the wavelength the greater the scattered intensity. The device to be described is designed to operate most usefully with the 488nm emission from an argon ion laser. It will work with slightly decreased sensitivity from the 5l4nm emission from an argon ion laser and with increased sensitivity from the lower wavelength emissions from He/Cd and He/Sr metal vapor lasers. When the device is being used at some distance from the laser and optical loss in the fibre is significant lasers having a longer wavelength (Cu vapour 578nm, He/Ne 628nm or Krypton ion 64lnm) will be preferable. The signal intensity is dependent on the power of the laser emission used. Laser powers from 10mw and greater are suitable for the technique.

To enable monitoring from a remote location, and to enable coupling into a closed environment, such as the internal spaces of a chemical process plant, the laser incident radiation is delivered to the sample via an optical fibre or an optical fibre bundle. The scatt¬ ered light is collected from the sample by an optical fibre bundle. Fibres having fibre diameters from 100^,to 500,0,with numerical apertures from 0.2 to 0.5 may be used depending on the application. Both silica and plastic fibres have been used. The choice of fibre will depend on the environment being measured and the loss character- istics of the fibre.

The optic fibre bundle may be of a number of different forms as set out below.

Concentric Assembly. The simplest assembly uses a single fibre for the delivery of the incident light. Concentric with this fibre is a fibre bundle of 6, 15 or 30 fibres for collection of the scattered light. The complete probe is encased in stainless steel or other protective envelope and sealed with a flourinated polyester resin. The deliv¬ ery end of the fibre is polished so that the ends of all fibres are in the same optical plane. An assembly of this design using six collection fibres is shown in FIG.2 wherein an input fibre 16 is surrounded by six coll¬ ection fibres, such as 17, all encased in a sheath 18. Multiple Delivery Concentric Assembly. In applications where the intensity of the incident light must be dis¬ persed for safety reasons several fibres are used for delivery of the incident light. Each of these carries only a portion of the total light intensity and each has collection fibres as nearest neighbours. The sheathing and polishing is similar to 2a and b.

Acentric assembly. In some applications it is necessary to separate the delivery and collection fibres. In this case the delivery fibre 19 is placed acentrically with delivery and collection bundles separately sheathed in stainless steel or other protective sheathing. This is shown in FIG. 3. The delivery fibre 19 is mounted at an angle 22 to the collection bundle 20 so that the volume illuminated by the incident beam coincides with the collection cone from the collection bundle. The complete acentric assembly is sheathed in stainless steel 21 or other protective envelope.

Protected Acentric Assembly. When monitoring is being carried out in a particularly hostile environment where environmental damage to the optical fibres is possible a variant of 3 is used. In this, the ends of the delivery fibre 19 and the collection fibres 20 are protected by thin sapphire windows 23 and 24. fused to the ends of the two fibre systems as shown in FIG. 4. It is important that the protective windows for delivery and collection fibres are separate so that lateral flare is not trans¬ mitted to the collection fibres. Alternative protection of the delivery fibre using a sapphire sphere and with subsequent focussing of the beam may be used. Connection of Scattering Assembly to Measurement System. The scattering assembly may be integrated with the meas- urement system in which case the delivery fibre is continuous between the laser and the scattering assembly and the collection fibres are similarly con¬ tinuous between the scattering assembly and the detect- ion analysis system.

Alternatively the scattering assembly may be a separate removable device. In this case the laser light is transmitted through a fibre which is then conn¬ ected through a standard, low loss, connector to the delivery fibre in the scattering assembly. The collection fibres in the scattering assembly are fused to yield a single transmission fibre which is connected, again through a low loss connector to a single fibre which transmits the integrated collected light to the detection/ analysis system.

Coupling to Laser. The optical coupling of the delivery fibre to the laser is preferably made through a system which is mechanically integrated to the laser in order to eliminate variations due to differential vibration. The laser can be focussed by a lens which matches the numerical aperture of the particular fibre utilised. Any of the commercially available devices can be utilised for this purpose. Spectrum Analysis. It is necessary to examine the inten- sity of the scattered light at selected characteristic wavelengths. This may be accomplished by using a mono¬ chromator and in a few cases this type of system may be necessary. In a system of a few components however, better sensitivity may be achieved by examining the total scatter- ed intensity transmitted through a narrow bandpass filter. Monochromator System. When a monochromator is required for the measurement of the signal it may be either a single or double monochromator. For a single monochromator a pre- filter is required to reduce the level of the Rayleigh scattering entering the monochromator. The output from the end of the fibre would be treated as a point light source and would be coupled with appropriate optics into the monochromator. A block diagram for this arrangement is shown in FIG. 5. In FIG. 5, laser 25 is joined by coupling optics 26 into a fibre 27 for directing monochromatic radiation to a sample or environment to be tested or monitored 28. Scattered radiation is directed by collection fibre 29, via coupling optics and a pre- filter 31 to screen Rayleigh scattered radiation, to a monochromator and detector 32. Suitable signal pro¬ cessing and readout is catered for at 33. Bandpass Filter System. There are three components necessary for spectrum analysis using the bandpass filter system and these are (i) prefilter, (ii) band¬ pass filter system, (iii) detector.

(i) The prefilter - The prefilter is necessary because the intensity of the Rayleigh scattered light is many times more intense than the scattered light of interest. This intensity gives rise to a high background signal over a wide spectral range. An edge prefilter having an optical density of ~-4 at the position of the Rayleigh scattering falling to low values ^~-15nm to the red is effective in reducing this background. The effectiveness of the prefiltration is improved if the laser is chopped or pulsed and phase sensitive detection at the chopping frequency is employed. Chopping or pulse frequencies above 200hz are found to be satisfactory. Normal sectored disc choppers are used. (ii) The bandpass filter system - This is made up of a series of narrow bandpass interference filters each having a bandpass between 100 cm -1 and 400cm-1. Each filter is chosen to give maximum transmission of the Raman scatter¬ ing of a particular component to be analysed. The choice of particular filters is dependent on the wavelength of the laser light used to produce the scattered radiation.

In addition to the bandpass filters for the components being analysed there must be a filter to provide information on the background level at a wave- length where minimal scattering is present. The filters are mounted in a turret to allow automatic changing from one to another to allow sequential recording of the scattered intensity transmitted by each filter. This filter changing is under microprocessor control. (iii) The detector system - The detector used to detect the scattered light may be either a photomultiplier or a diode detector depending on the sensitivity required. The scattered light transmitted by the filter system is foc- ussed on the detector. The signal from the detector is amplified by a phase sensitive amplifier and the result¬ ing voltage for each of the filter positions provides the raw data which is processed by a preprogrammed microproc¬ essor into concentrations of each of the components of interest. Complete Analysis System. The collected scattered light delivered by the collection fibre(s) is in the form of a diverging cone of light the geometry of which is dependent on the particular fibre or fibre bundle used. This diverg¬ ent cone of light is collimated by an appropriate lens system which is integrated with the fibre termination to optimise coupling efficiency. This collimated beam is passed through the prefilter and the filter assembly before being focussed by a further lens system onto the photomultiplier or diode. An optical block diagram show- ing the arrangement of components in the system utilising the filter system is given in FIG. 6. In addition an electrical block diagram showing the relationship of the electrical components is given in FIG. 7.

In FIG. 6, laser 3 provides an output which may be chopped by a chopper means 35 prior to being coupled to a delivery fibre, or bundle of fibres by coupling optics 36. Light scattered from the sample, or environ¬ ment being tested, or monitored, 37 may be processed by foreoptics 38 prior to passing prefilter 39 to remove Rayleigh scattered radiation. The Raman scattered rad¬ iation is processed by any of one or more band pass filters 41 which may be carried into the optic path by a suitable carrier 40 such as a revolving turret. A background filter 42 is provided to establish a back- ground level. Focussing optics 43 may be used to focus the Raman scattered radiation on a suitable sensor 44. In FIG. 7, the electrical output of a sensor 46 is fed to a microprocessor 48 with amplification 47. The microprocessor 48 is fed signals which indicate the position of a filter carriage 49 so as to determine which filter is in use and from the chopper 45 to indicate the chopping rate and phase. Where phase sensitive amplific¬ ation is employed, signals from the chopper are also fed to the amplifier 47. The above described invention may find applic¬ ation in environments wherein components of gases, liquids or solids are monitored. A. APPLICATION TO GASES 1. Gas Mixtures at Pressures up to Two Atmospheres The method is applicable equally to static or flowing gas streams. For most gas components absolute measurements of component concentration are possible from 1% while a relative measurements (ratio of component concentration) can be made to lower component concentrat- ions. When the measurement is made in the presence of condensable components the fibreoptic probe is provided with an inbuilt heater to maintain the optical surface clear of condensation. This heater is a combination resistance heater and temperature sensor. No interfer- ence is found in the optical signal from the resistance heating. The probe is maintained at a temperature ^~-5 above the temperature of the medium being measured. 2. Gas Mixtures at Elevated Pressures

The fibreoptic system is particularly useful for the analysis of gases under pressure. Signal levels are directly proportional to the number density of gas molecules which is related to the pressure and so the determination of minor components is improved at higher pressure. The fibreoptic probe may be installed dir¬ ectly in the gas flow path and the components may be continuously monitored with negligible delay between sampling and measurement display.

These characteristics make this type of measur¬ ing system ideal for use in a feedback system for process control. B. APPLICATIONS TO LIQUIDS

The Raman spectral bands observed for liquids are, in general, much broader than those of gases and the spectra are frequently much more congested than gas spectra. The monitoring of one component using the bandpass filter system is dependent on having one region of scattered light unique to that component and this condition is sometimes difficult to fulfil for liquids. When this separation of one component is not possible it may be necessary to use a monochrometer for the analysis. The optimisation of a synthetic process may be possible if the relative concentrations of two components can be continuously monitored. Such reacting systems are frequently corrosive which would require the use of a protected probe. Dissolved gases are frequently of importance in a continuous flow liquid system. Provided the nature of the possible equilibria involving the gases in solution are understood the dissolved gases may also be monitored. C. APPLICATIONS TO SOLIDS The composition of the surface for solids may be analysed using this technique. The scattering volume is relatively close to the surface of the probe and the scattering occurs from a relatively small thickness of the surface.

Claims

1. A monitor for determining the presence or concentration of a particular chemical or combination of chemical components in an environment to be monitor¬ ed comprising a source of electromagnetic radiation (25, 34), a means for directing the electromagnetic radiation into an environment to be monitored (27) a means within the environment for receiving scattered electromagnetic radiation, a means for delivering the scattered electro¬ magnetic radiation to a location remote from the environ¬ ment to be monitored (27) and a means sensitive to a predetermined component or combination of components within the scattered electromagnetic radiation which is characteristic of the chemical or combination of chemical components to be monitored (32, 44) characterised in that the means for delivering electromagnetic radiation to the environment (19) and the means for delivering the scatter¬ ed electromagnetic radiation to the remote location (20) comprise optical fibres, the fibres terminating in the environment to be monitored with the end of the delivery fibre or fibres (22) being disposed relative to the end or ends of the fibres which collect scattered radiation in a relationship which minimises any return of the electro¬ magnetic radiation other than scattered radiation.

2. A monitor as claimed in claim 1 wherein a single delivery fibre is combined in a bundle with from 5 to 30 collection fibres with the delivery and collection ends in the same optical plane.

3- A monitor as claimed in claim 2 wherein the fibre bundle is provided with a protective envelope scaled with a material which is inert relative to the environment.

4. A monitor as claimed in claim 4 wherein the protective envelope is stainless steel and the sealing material is a flourinated polyester resin.

5. A monitor as claimed in claim 4 wherein the delivery and collection end of the bundle is polished so that the fibre ends are in the same optical plane.

6. A monitor as claimed in claim 1 wherein a plurality of delivery fibres is provided in a bundle with a plurality of collection fibres.

7- A monitor as claimed in claim 6 wherein the fibre bundle is provided with a protective envelope seal¬ ed with a material which is inert relative to the environ¬ ment.

8. A monitor as claimed in claim 7 wherein the protective envelope is stainless steel and the sealing material is a flourinated polyester resin.

9. A monitor as claimed in claim 8 wherein the delivery and collection end of the bundle is polished so that the fibre ends are in the same optical plane.

10. A monitor as claimed in claim 1 wherein the delivery fibre or fibres is or are provided acentrically with respect to the collection fibre or fibres.

11. A monitor as claimed in claim 1C wherein the delivery and collection fibres are provided within prot¬ ective envelopes sealed with a material which is inert relative to the environment.

12. A monitor as claimed in claim 11 wherein the delivery and collection fibres are separately sheathed with the delivery fibre or fibres oriented to illuminate a volume within the collection cone of the collection fibres.

13- A monitor as claimed in claim 12 wherein the complete acentric assembly of fibres is sheathed in stain¬ less steel.

14. A monitor as claimed in claim 3 wherein a sapphire window is provided adjacent to the end of the fibre bundle.

15. A monitor as claimed in claim 7 wherein a sapphire window is provided adjacent to the end of the fibre bundle.

16. A monitor as claimed in claim 11 wherein the delivery and collection fibres are provided with sapphire windows at their ends.

17. A monitor as claimed in claim 16 wherein the delivery end is provided with a sapphire sphere serving as the window.

18. A monitor as claimed in claim 5 wherein the fibres have diameters from 100^ to 500 AA. and numerical apertures from 0.2 to 0.5.

19. A monitor as claimed in claim 9 wherein the fibres have diameters from 100 v. to 500 and numerical apertures from 0.2 to 0.5.

20. A monitor as claimed in claim 13 wherein the fibres have diameters from λ QQ ^, to 00 ^ and numerical apertures from 0.2 to 0.5.

21. An optical monitor for determining the pres¬ ence or concentration of a particular chemical or combin¬ ation of chemical components in an environment to be monitored comprising a laser (25,34) providing monochrom¬ atic radiation, means for isolating Raman scattered rad¬ iation characteristic of the chemical or combination of chemical components to be monitored (31,39,41) and a detection means for receiving the isolated Raman scattered radiation to provide the determination of the presence or concentration of the component (32, 44) characterised in that monochromatic radiation generated by the laser (25, 34) is delivered to the environment via one or more optical fibres (19) and scattered radiation is collected therefrom by one or more optical fibres (20) and transmitted thereby to the means for isolating Raman scattered radiation (32, 41).

22. An optical monitor as claimed in claim 21 further characterised by the means for isolating Raman scattered radiation comprising one or more band pass filters (41), selectively insertable in the optical path between the collection fibre and the detection means (44), together with a filter effective to isolate back¬ ground radiation (42).

23- An optical monitor as claimed in claim 22 further characterised by a prefilter means (31,39) in the optical path between the collection fibres and the band pass filters, the prefilter being an edge filter to screen Rayleigh scattered radiation.

24. An optical monitor as claimed in claim 23 further characterised by a chopper means (35) in the optical path between the laser and the delivery fibre or fibres to convert the monochromatic radiation into a pulse train and the detection means being phase sensitive.

25. An optical monitor as claimed in claim 22 wherein the detection means is a photomultiplier or diode detector and the Raman scattered radiation is focussed thereon.

26. An optical monitor as claimed in claim 21 wherein the collection fibres deliver scattered radiation to a monochromator.

27. A method for monitoring the presence or concen¬ tration of a particular chemical or combination of chemical components in an environment to be monitored from a remote location comprising the supply of electromagnetic radiation to the environment, collecting and transmitting scattered radiation from the environment to a remote location isolating Raman scattered radiation from the scattered radiation, and determining the presence or concentration of a component to be monitored by detecting a characteristic portion, or portions of the Raman spectra of that component in the collected Raman scattered radiation characterised in that the monochromatic collection of Raman scattered radiation and its transmission to the remote location is in isolation from the supply of monochromatic radiation by means of optic fibres.

28. The method as claimed in claim 27 further characterised in that the monochromatic radiation is provided as a train of pulses and in that the detection of a characteristic portion of the Raman spectra is achieved in a phase sensitive manner.

29. The method as claimed in claim 27 wherein the level of background radiation is determined so as to enable the level of Raman scattered radiation to be adjusted to indicate the concentration of the component.

30. A monitor as claimed in claim 1 wherein the fibres are provided in a bundle which is provided with a heating means to heat the bundle end to a temper¬ ature above that in the environment to be monitored.