METHOD AND DEVICE FOR DETECTING A DISTRIBUTION OF A CHEMICAL IN A PARTICULATE MEDIUM

The present invention relates to a method of detecting the distribution of a chemical in a section through a particulate medium, to a device and a kit for use in the method and to a visual image produced by the method.

The movement and distribution of chemicals through particulate media such as soil, sand or artificial substrates e.g. vermiculite and perlite is commonly the subject of scientific study. Such movement can be caused for example by simple diffusion, or can be the result of water percolation for example caused by rainfall. The movement itself is of interest, for example, to environmental scientists who study how fast and to what extent chemicals move through such media and where they might accumulate. It is also of interest to biologists studying chemicals that are biologically active in soil to see the depth and spread of such chemicals over time, for example to see if they remain in areas available for root uptake by plants, or over what area they might be effective against pests or diseases.

In the laboratory, the spread of chemicals through a particulate medium is generally monitored by first producing a radiolabeled sample of the chemical so that it can easily be detected using a radio detecting sensor. Radiolabelling is a well known technique which involves preparing a sample of the chemical which incorporates a radioactive isotope typically carbon- 14, ³ H (tritium) or Sulphur-35. This labelled chemical sample is then applied to the surface or at the base of a test plot as a liquid, granule or seed treatment to the particulate medium, for example a pot of soil, so as to model an anticipated 'real life' application. Such application can be for example an even distribution across the surface at a predetermined concentration, as a single or multiple spot applications or applied to the base to monitor upward movement. The test plot can be watered over a period of time to simulate irrigation or rainfall, or from the base to monitor effects of upward water movement.

At the end of a test period, samples are taken from the test plot and radioactivity levels are measured and compared with known standards to establish the final concentration and distribution of the chemical. In particular, many investigators are interested in producing a visual image of a cross-sectional view, usually a vertical or lateral cross- section, of the test plot on which is shown, either numerically or pictorially, the concentration of the chemical at various points.

There are two main conventional ways to take the samples. Core samples can be taken using a hollow tube and the radioactivity level can be measured along the core. From several such cores, the distribution of the chemical can be reconstructed, and a pictorial representation of the distribution of the chemical can be produced by feeding the relevant numbers into a suitable computer program. Alternatively, a cross-section can be taken directly by freezing the test plot, and then cutting it into slices. Such a slice can be used to produce a pictorial representation of the distribution of the chemical by laying it directly onto a piece of radioactivity sensitive photographic paper or phosphor image plate. There are disadvantages to these known sampling methods. Both with taking core samples and with slicing, there is a high risk of disturbing the soil or smearing the chemical with the cutting device and so redistributing the chemical so that the results of the tests are not wholly reliable. Considerable manual skill is required to take good core samples and even the best samplers will inevitably cause some soil disturbance. The alternative of freezing and slicing pots of soil and producing pictures directly from the slices, is a relatively time-consuming activity requiring dedicated equipment and skilled personnel.

There is a need therefore for a simple, reliable method of analysing the distribution of a chemical in a particulate medium such as soil, avoiding the disadvantages of the existing methods. There is also a need to be able to easily produce a high resolution pictorial representation of the distribution.

We have surprisingly found that we can effectively capture the concentration of such a chemical by means of a polyamide mesh within the particulate medium. Accordingly, the present invention provides a method for detecting the distribution of a chemical in a section through a particulate medium, the method comprising incorporating a polyamide mesh into the particulate medium at the desired position of the section prior to the introduction of the chemical.

During the test, the chemical in the medium immediately adjacent to the mesh adsorbs to the mesh. The amount of chemical which adsorbs to the mesh is proportional to the concentration in the immediately adjacent medium. The mesh can be removed after a predetermined test period and the concentration, or relative concentration, of the chemical adsorbed on the mesh can be measured and used to estimate or measure the corresponding concentration, or relative concentration, in the adjacent medium. This

can in turn be used to indicate the movement of chemical through the soil during the test period.

Preferably the method a method comprises

©incorporating a polyamide mesh into the particulate medium at the desired position of the section prior to the introduction of the chemical (ii) introducing the chemical, (iii) optionally watering the medium (iv) at the end of a predetermined test period, removing the mesh from the medium and analysing it to establish the concentration or relative concentration adsorbed across the surface of the mesh.

More particularly, the present invention also provides a method for recording the distribution of a radiolabeled chemical after a predetermined test period at a section through a particulate medium which comprises i) preparing a test plot of the particulate medium in which is incorporated a polyamide mesh at the desired section, ii) applying a sample of the chemical to the surface of the test-plot iii) optionally watering the test plot iv) at the end of the test period, removing the polyamide mesh from the medium and recording the radioactivity levels across its surface.

The present invention also provides a test kit for detecting the distribution of a chemical at a section through a particulate medium, the kit comprising a container of the particulate medium in which is incorporated a polyamide mesh at the desired position of the section.

Generally, the concentration of the chemical is determined by removing the polyamide mesh from the medium and measuring the distribution of chemical across its surface. This can be done in a number of ways. One way is by cutting the mesh into pieces and using chemical analysis on each piece, or when the chemical is radiolabeled, taking radioactivity measurements on each piece. A much more convenient way when the chemical is radiolabeled is by measuring the radioactivity level across the surface of the mesh and calibrating this against known standards.

Particularly conveniently when the chemical is radio-labelled, the concentration can be determined by laying the mesh onto a sheet of radioactivity sensitive photographic film or phosphor imaging, which will then give a direct visual image of the distribution,

coloured according to concentration. The image can be calibrated by using known concentrations of the labelled chemical.

Such coloured visual images are particularly useful in quickly comparing the movement of different chemicals or the movement of a particular chemical through different media and they also provide an easy way of demonstrating such movement to non-expert audiences who can easily relate the colours to the concentrations and can easily compare pictures of different samples.

The present invention also provides a visual image produced by the above method.

The particulate medium can be for example sand, soil or artificial substrates. Soil includes a wide variety of media in which plants can grow, such as, without limitation, clay soils, sandy soils, loam, compost and peat.

The test plot is generally a conveniently sized volume of the particulate material in a container. Examples of suitable containers for sand or soil are conventional flower pots or seed trays. The test plot can be watered by known means, for example from above by spraying or pouring in water, or from below, for example by standing the test plot in a shallow water reservoir, or on a wet absorbent material.

Polyamide mesh is convenient and easy to use and robust to handle. It allows the passage of water through it during the test. On removal from the medium, particles of the medium can be easily removed from the mesh by shaking, after drying if necessary. The mesh is easy to apply to suitable photographic paper or phosphor imaging plates so as to produce images of the distribution of the chemical. The mesh can easily be stored for later re-measurement.

Polyamides are a well-known class of polymers which includes Kevlar, nylon 6,6 and nylon 6. Of these, the nylons are preferred, particularly 6,6. Nylon mesh is conveniently and cheaply available as clothing material, such as stockings or tights

(pantyhose). Some pantyhose additionally comprise varying amounts of other polymers such as elastomers, cotton or silk. The polyamide mesh should comprise at least 25%, preferably more than 50% by weight of polyamide. Preferred mesh is between 10 and 30 denier, most preferably 20 denier.

Conveniently, the mesh can be supported on a support means, such as a frame, for example a wire frame which holds the mesh substantially planar to make insertion, removal and handling of the mesh easier. Accordingly, the present invention also provides a device for detecting the distribution of a chemical through a particulate medium, said device comprising a polyamide mesh suspended on a support means. The support means can be shaped so as to fit the shape of the container, for example the cross-sectional shape of a flower pot or seed-tray. As an example, material from tights or stockings can be sewn, stuck or clipped onto a wire frame.

The mesh can be of a size and shape so as to provide a complete or partial cross-section of the medium. The positioning of the mesh within the medium will depend on the particular investigation. For example, a vertical cross-section is useful in determining the penetration of a chemical down through the medium, and a horizontal cross-section for determining spread across the medium. Many other positions, including non-planar arrangements could also be used as desired. The chemical can be any chemical that can be absorbed by the polyamide mesh. Examples of chemicals of interest are chemicals that may be regarded as soil pollutants, or chemicals that are deliberately applied to soil such as agrochemicals. Examples of particular interest are agrochemicals such as insecticides, fungicides, herbicides, plant growth regulators, fertilizers, adjuvants, systemic acquired resistance compounds and safeners for any of these. A substantial list of specific agrochemicals can be found in The Pesticides Manual published by the British Crop Protection Council (14 ^th edition November 2006) and available online. Specific examples of more particular interest are insecticides, especially those of the anthranillamide class such as chlorantraniliprole, the neonicotinoid class such as thiamethoxam, and the pyrethroid class such as lambda- cyhalothrin or tefluthrin.

The invention will now be illustrated by means of the following examples;

Examples

Small sample pieces of polyamide mesh were cut from commercially available 20 denier nylon tights ("Simply..." brand) and clipped onto wire frames. The wire frames were shaped so as to fit vertically into 10cm plant pots. A cross-sectional view of the wire frame inserted into a plant pot is shown in Figure 1 and a photograph of a wire frame about to be inserted into a pot is shown in Figure 2.

Wire frames having polyamide mesh mounted on them were inserted into four pots as shown in Figures 1 and 2. Two different soils, one per pot, were poured around the mesh in the pots, giving two pots full of each soil.

A predetermined amount of a 14C radiolabeled insecticide, chloroanthraniliprole, was added as a spot application in the centre of each pot.

After 7 days the mesh was removed from two of the pots, one pot of each soil. Excess , soil was shaken from the mesh which was then brought into contact with phosphor imaging plates for 24 hours. This was repeated after 14 days with the other two pots. The resulting images are shown in Figure 3. This method has been repeated with radiolabeled thiamethoxam, lambda-cyhalothrin and tefluthrin with similar good results. This method represented a very simple and inexpensive way of obtaining striking pictures comparing the movement of this compound through different soils over time.