GB1595785A

GB1595785A - Optical analysis of liquids

Info

Publication number: GB1595785A
Application number: GB4516076A
Authority: GB
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1977-10-28
Filing date: 1977-10-28
Publication date: 1981-08-19

Description

(54) OPTICAL ANALYSIS OF LIQUIDS (71) We, WATER RESEARCH CENTRE, a British Company limited by guarantee, of 45 Station Road, Henley-On-Thames, Oxfordshire, RG9 lBW, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention is concerned with the measurement of optical properties of materials, particularly liquids.

It is well known to analyse various components of a liquid by directing a beam of light into the liquid and measuring the transmitted light. For example, by comparing the intensities of the input and transmitted light, it is possible to detect and qualify certain solutes and suspended materials in the liquid.

This technique has been used in the water industry to measure, for example, the amounts of dissolved organic carbon in water, the nitrate content, and the colour and turbidity of water. In one commercially available machine for this purpose, a beam of light is directed through the liquid sample, and the transmitted light split into two parts by a dichroic mirror.

Each part then passes through a filter to a photosensitive device which produces a signal correlated to the intensity of the light. One filter cuts out all but UV and the other all but a band of visible light, and from the outputs of the two photosensitive devices it is possible to calculate various properties of the liquid.

Whilst such an instrument is relatively simple and fairly reliable, it has a number of disadvantages. For example, the two photosensitive devices, though they be identical, may well not have precisely the same outputs for a given light intensity, and any such difference can cause error in the results. The likelihood of such a difference occurring increases with the age of the instrument. A second disadvantage is that there is no provision for simultaneously measuring the optical properties of the sample cell itself with, for example, pure water in it, i.e. as a standard.

In another more recent machine, provision is made for a standard. Two identical cells are mounted side-by-side, with sample liquid in one and standard liquid in the other. The beam from the light source is split into two parts, one for each cell.

The light transmitted by each cell is split into two parts as in the earlier machine.

Whlist this arrangement has some advantages over prior proposals, it has the drawback that it involves more photosensitive devices. In addition, it involves beam splitting upstream of the cells with the requirement that the intensity of light directed into each cell be the same. This is not always easy to achieve and maintain.

We have now devised certain improvements in these techniques, whereby greater accuracy can be achieved with less complicated (and generally cheaper) equipment.

According to one aspect of the present invention, there is provided a method of measuring optical properties of a sample which comprises passing light (as herein defined) into a cell, and sequentially detecting and comparing the intensities of the light from the cell at at least two different wavelengths or wavelength bands using a single photosensitive device, two or more filters being sequentially interposed in the light path so that the photosensitive device is sequentially exposed to light from the cell of the said two or more different wavelengths or wavelength bands, and wherein the cell is supplied with sample under test and then a standard.

In this specification, the term "light" includes not only visible light, but other radiation, for example, UV and infra-red radiation.

In this method, we avoid the use of two or more photo-sensitive devices. Instead, all the measurements are made using a single photosensitive device, and hence the various readings obtained at different wavelengths are precisely comparable providing that the characteristics of the photo sensitive device are known a priory: Furthermore, the necessity of a dichroic mirror (or other beam-splitting device) downstream of the sample cell is avoided.

In carrying out this method, preferably one or more filters are sequentially interposed in the light path between the sample and the photosensitive device, to provide the two or more selected wavelengths or wavelength bands to be fed to the photosensitive device. Conveniently, the filters are mounted on a moveable support, and the support is moved to bring different filters into the light path. The support may be rotatable about an axis perpendicular to the plane of the filters, the filters being arranged radially of the axis so that, upon rotation of the support about the axis, each filter moves into and out of the light path in sequence. Most preferably, the support is in the form of a wheel.

In this method of the invention, it is desirable to provide means for automatically selecting and changing the filter after a predetermined time, and to couple this to a control device correlating the output of the photosensitive device with the particular filter used, it then being possible to provide a final output reading directly related to a particular property of the liquid under test, e.g. its dissolved organic carbon content or its turbidity.

The particular wavelengths of light most suitable for measuring a particular property of an aqueous sample, e.g. dissolved organic carbon, nitrate content, colour and turbidity, are known in the art.

For example, dissolved organic carbon measurements are usually made at 250 to 260 nanometres.

The frequency of the filter change can vary widely, for example from 0.01 Hz to 1 kHz, although frequencies outside this range are not excluded. It will be appreciated, however, that the modern electronic equipment used for controlling the process and computing the final output readings operates extremely quickly and does not itself in practice impose any limitation on the method as a whole. The particular arrangements of electrical and electronic equipment which may be used do not form part of this invention and will, in any event, be clear to those skilled in the art.

Whilst it is preferred to interpose the filters in the light path downstream of the sample, they can be positioned upstream of the sample.

In the method of this invention, the cell is supplied with sample under test and then with a standard. In this way, we provide a way of conveniently making optical measurements on both a sample under test and an appropriate standard. Thus, direct comparison can be made between the intensities of the light from the cell, using the same light path and the same cell.

Errors due to light variations upstream of the sample cell, and due to any difference between two cells arranged side-by-side (as in a prior proposal) are eliminated. Differences between two cells can be substantial in practice where fouling of the sample cell walls occurs due, for example, to continuous deposition thereon of solid matter from the sample liquid.

A preferred type of cell for use in the method of the invention is described and claimed in our copending application No.

22754/80 (Serial No. 1595786) to which reference should be made in further details.

In a further preferred aspect of the method of the invention, there is disposed in the light path between the light source and the cell, a first moveable support carrying at least two filters, and there is disposed between the cell and the photosensitive device a second moveable support carrying at least two filters, and wherein the filters and supports are arranged so that the light passing into the sample is of a particular wavelength or wavelength band, and the light passing to the photosensitive device is of the same or a different wavelength or wavelength band.

As an example, the measurement of oil in water by UV fluorescence may be made by irradiating the sample at 254 nm and detecting at 350 nm, and there are other parts of the UV and visible spectrum at which measurements can be made to detect other dissolved and suspended substances.

Most conveniently both filters will be in the form of a filter wheel and will be automatically controlled. Both filters can, of course, contain a transparent element for measurements where it is not required to restrict the light, both upstream and downstream of the sample, to a particular wavelength.

Optical measurements which involve light scattering by d sample can be used to analyse the sample, and the present invention includes such methods in which the scattered light is measured. In such cases, the photosensitive device is arranged to receive only or mainly scattered light, for example at 900 to the direction of the incident light.

In order that the invention may be more fully understood, reference is made to the accompanying drawings, in which: FIGURE 1 illustrates schematically one arrangement suitable for carrying out the method; and FIGURE 2 illustrates schematically an other arrangement suitable for carrying out method.

Referring now to Figure 1, there is shown a sample cell (which is supplied with sample under test and then standard arranged in a light path 2 between a light source 3 and a photosensitive device 4. A collimating system 5 may be provided. The output from cell 4 is fed to electronic devices 6 to produce the desired output readings. Disposed between cell 1 and device 4 is a filter wheel 7 mounted for rotation about its axis 8 which is laterally offset from light path 2 so that, by rotation of wheel 7, the various radially disposed filters 9 can each be brought into and out of light path 2. It is emphasised that in this Figure, and in Figures 2 and 3, the representation is merely schematic and various well-known features are omitted for clarity.

For example, stopped apertures may be variously well-known features are omitted for clarity. For example, stopped apertures may be variously interposed between light source 3, lens 5, wheel 7, cell 1 and photosensitive device 4.

The filter wheel may be placed between cell 1 and photosensitive device 4, or a second filter wheel may be provided between cell 1 and collimating system 5.

Photosensitive device 4 may be arranged to receive scattered, rather than transmitted, light.

Means (not shown) are preferably provided to drive the wheel 7 and to lock it momentarily with a filter in the light path.

The means may be controlled by devices 6 or by separate devices (not illustrated).

In Figure 2, like numerals indicate like parts to Figure 1. Filter wheel 26 is interposed between light source 3 and sample cell 1 to selectively irradiate the sample at different wavelengths by rotation of the wheel 26 about its axis 27 which is laterally offset from light path 2 so that the various radially disposed filters 28 can each be brought into and out of light path 2. Rotation of wheel 26 is synchronised to rotation of wheel 7 so that individual measurements can be made at the same and different wavelengths to detect for example fluorescence. Control of the rotation of filter wheels 7 and 26 may be achieved by devices 6 or by separate devices not illustrated.

WHAT WE CLAIM IS: - 1. A method of measuring optical properties of a sample which comprises passing light (as herein defined) into a cell, and sequentially detecting and comparing the intensities of the light from the cell at at least two different wavelengths or wavelength bands using a single photosentitive device, two or more filters being sequentially interposed in the light path so that the photosensitive device is sequentially exposed to light from the cell of the said two or more different wavelengths or wave length bands, and wherein the cell is sup plied with sample under test and then a standard.

2. A method according to claim 1 where in the said filters are interposed in the light path between the sample and the photo sensitive device.

3. A method according to claim 1 or 2, wherein the said filters are mounted on a moveable support and are sequentially moved into and out of the light path by movement of the support.

4. A method according to claim 3 wherein the support is rotatable about an axis and the filters are located radially of the axis so that, upon rotation of the sup port about the axis, each filter moves into and out of the light path in sequence.

5. A method according to any of claims 1 to 4 wherein selection and change of the filter in the light path is automatically actuated and a measure of the output of the photosensitive device is automatically correlated to the filter in use to provide a reading relation to the particular pro perty under test in the sample.

6. A method according to any preceding claim which is operated to determine one or more of the dissolved organic carbon, the nitrate content, the colour and the tur bidity of an aqueous sample.

7. A method according to claim 1 wherein there is disposed in the light path be tween the light source and the cell, a first moveable support carrying at least two filters, and wherein there is disposed be tween the cell and the photosensitive device a second moveable support carrying at least two filters and wherein the filters and sup ports are arranged so that the light passing into the cell is of a particular wavelength or wavelength band, and the light passing to the photosensitive device is of the same or a different wavelength or wavelength band.

8. A method according to claim 7 where in each support is rotatable about an axis and the filters are located radially of the axis so that, upon rotation of each support about its axis, each filter moves into and out of the light path in sequence.

9. A method according to claim 7 or 8 wherein the photosensitive device is arranged to receive only or mainly light scattered from the sample.

10. A method according to claim 1 sub stantially as herein described with reference to Figure 1 or 2.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. shown a sample cell (which is supplied with sample under test and then standard arranged in a light path 2 between a light source 3 and a photosensitive device 4. A collimating system 5 may be provided. The output from cell 4 is fed to electronic devices 6 to produce the desired output readings. Disposed between cell 1 and device 4 is a filter wheel 7 mounted for rotation about its axis 8 which is laterally offset from light path 2 so that, by rotation of wheel 7, the various radially disposed filters 9 can each be brought into and out of light path 2. It is emphasised that in this Figure, and in Figures 2 and 3, the representation is merely schematic and various well-known features are omitted for clarity. For example, stopped apertures may be variously well-known features are omitted for clarity. For example, stopped apertures may be variously interposed between light source 3, lens 5, wheel 7, cell 1 and photosensitive device 4. The filter wheel may be placed between cell 1 and photosensitive device 4, or a second filter wheel may be provided between cell 1 and collimating system 5. Photosensitive device 4 may be arranged to receive scattered, rather than transmitted, light. Means (not shown) are preferably provided to drive the wheel 7 and to lock it momentarily with a filter in the light path. The means may be controlled by devices 6 or by separate devices (not illustrated). In Figure 2, like numerals indicate like parts to Figure 1. Filter wheel 26 is interposed between light source 3 and sample cell 1 to selectively irradiate the sample at different wavelengths by rotation of the wheel 26 about its axis 27 which is laterally offset from light path 2 so that the various radially disposed filters 28 can each be brought into and out of light path 2. Rotation of wheel 26 is synchronised to rotation of wheel 7 so that individual measurements can be made at the same and different wavelengths to detect for example fluorescence. Control of the rotation of filter wheels 7 and 26 may be achieved by devices 6 or by separate devices not illustrated. WHAT WE CLAIM IS: -

1. A method of measuring optical properties of a sample which comprises passing light (as herein defined) into a cell, and sequentially detecting and comparing the intensities of the light from the cell at at least two different wavelengths or wavelength bands using a single photosentitive device, two or more filters being sequentially interposed in the light path so that the photosensitive device is sequentially exposed to light from the cell of the said two or more different wavelengths or wave length bands, and wherein the cell is sup plied with sample under test and then a standard.