GB2400174A

GB2400174A - Portable filtration apparatus and method of detecting arsenic using the same

Info

Publication number: GB2400174A
Application number: GB0403136A
Authority: GB
Inventors: Neil Francis Durham; Nicholas Jon Price
Original assignee: WAGTECH INTERNAT Ltd
Current assignee: WAGTECH INTERNAT Ltd
Priority date: 2003-02-18
Filing date: 2004-02-12
Publication date: 2004-10-06
Anticipated expiration: 2024-02-12
Also published as: GB2423582A; GB0403136D0; GB2400174B; GB0609522D0; GB0303719D0; GB2423582B

Abstract

A portable filtration apparatus comprising a container 1 for containing a sample and a container, a lid 2 having first 5 and second 6 filter receiving means. The filter receiving means 5, 6 are shaped with grooves or projections such that a first filter 30 can be received by the first filter receiving means 5, but not the second filter receiving means 6 and a second filter 40 can be received by the second filter receiving means 6, but not the first filter receiving means 5. This way it is ensured that the filters are inserted in the correct order. The apparatus is particularly useful for field testing for the presence of arsenic in a sample of water. The first filter 30 may be a detector for detecting arsine gas and the second filter 40 may be a filter for removing excess arsine gas which has passed through the first filter 30. Colour change of the first filter 30 may be detected by an electronic sensing device 50.

Description

À 1 24001 74 Detection Method and Apparatus The present invention relates

to an apparatus and method for filtration and/or testing or detection procedures, in which it is necessary or desirable to pass a fluid sequentially through two different filters. It also relates to an apparatus and method for detecting arsenic.

Arsenic is a common contaminant of ground water. It is a naturally occurring element, indigenous to rock formations in many countries and has been found to be highly toxic. In developing countries, such as Bangladesh, arsenic contamination of the water supply is a very significant problem. In many developing countries water is primarily obtained from wells, many of which may be contaminated with arsenic. Arsenic can be dangerous to human life even at comparatively low levels, perhaps even as low as lO parts per billion (ppb). In Bangladesh wells are considered to be unsafe if the level of arsenic is above 50 ppb. Therefore there is a need for testing the level of arsenic present in water from these wells.

J

Conventionally water samples are taken locally at the wells and transported back to a test centre, where they are tested by atomic absorption spectroscopy. Atomic absorption spectroscopy is an accurate method, but also expensive and requires a team of highly trained technicians. Thus, especially in developing countries, there can only be a limited number of test centres, but there are a very large number of wells. Furthermore, is thought that the level of arsenic can vary over time (arsenic creep) so that a once 'safe' well may become unsafe a few months later. Therefore, each well needs to be tested regularly.

The logistics of taking samples from tens of thousands of wells, transporting them to one or two national test centres, keeping track of the results and the origin of the samples and returning the results to the local authorities is extremely difficult. Thus, there is a need for a method of testing for arsenic locally (i.e. at the well sites), to the required degree of accuracy, so that samples do not have to be transported to a test centre.

A number of field test kits exist. These field test kits use a form of the Gutzeit method for detecting arsenic. According to this method certain substances are added to the water sample being tested to react with the arsenic (which may be in one of its dissolved forms, e.g. arsenite or arsenate), so as to generate an arsine gas.

The arsine gas is passed through a detector paper in the form of filter paper that has been impregnated with mercuric bromide (HgBr). The arsine gas reacts with the mercuric bromide and this reaction stains the detector paper causing it to change from a white colour to a yellow or brownish colour. The degree of colour change is directly related to the amount of arsine gas released and thus indicates the amount of arsenic present in the water sample (low levels produce a light yellow stain, higher levels tend towards a dark yellow or brown stain). A colour chart is provided so that the tester can visually compare the colour of detector paper with the colours on the chart to determine the level of arsenic.

Sulphides are often present in the water being analysed and they can react to give off sulphide gas (e.g. Hydrogen Sulphide) at the arsine generation stage.

Hydrogen Sulphide (HS) tends to colour the detector paper black, it is also poisonous. Therefore it is necessary prevent, HS from contacting the detector paper and desirable to prevent it from leaving the testing apparatus. Some prior art test kits provide a scrubber filter for removing gas) for this purpose, in the form of cotton wool soaked in lead acetate, which is placed below the detector paper. Other prior art kits provide an extra oxidising agent for adding to the water sample to prevent generation of HS gas.

These prior art test kits provide a container,

having a bung or lid device on or in which the detector paper can be mounted and a colour chart as described

above. The prior art test kits have several

disadvantages, detailed below.

They are inaccurate because visual comparison of the detector paper colour with a chart is not a reliable method of determining arsenic levels.

Arsine gas is toxic and any excess arsine gas which passes through the detector paper (and does not react with the mercuric bromide) can be dangerous to the tester and the external environment. Excess arsine gas can escape the apparatus, even if the arsine gas is directed to only a small part of the detector filter paper.

Lead acetate is toxic and therefore needs to be handled with care. The prior art kits simply provide cotton wool, which is to be soaked in lead acetate and has to be manually fitted to the lid or bung device.

While this approach is suitable in the laboratory it is not very practical in the field. The tester will not necessarily have access to protective gloves (to protect himself from contact with the lead acetate) or the time to meticulously assemble the testing apparatus each time before it is used.

Furthermore, the detector paper can easily become contaminated when handled by the tester. This can lead to inaccuracy in the test results.

Recently a new testing apparatus has been developed, which uses electronic sensing equipment to measure the colour change of the detector paper and calculates the level of arsenic present in the water sample from the opto-electronically detected colour change. This apparatus goes some way to solving the inaccuracy of (human) visual colour comparison associated with the prior art field test kits, but the other problems still remain. For example the user is still at risk from contact with lead acetate and from arsine gas which can S escape into the surrounding environment. Furthermore, this apparatus has a very cumbersome bung device, which is fitted on the mouth of the flask carrying the water to be tested. It is necessary to handle the detector paper, using tweezers or the like, when it is first positioned in the bung device and later to remove the detector paper from the bung device and place it in a separate electronic colour change measuring device. At each handling step there is a risk that the detector paper will become contaminated and it is difficult to achieve consistency in the location of the detector paper, which is measured for colour change. If different portions of the paper are measured at the calibration and colour change detecting steps then inaccuracies can result.

There can even be inaccuracies if the orientation of the paper differs between the calibration and detecting steps.

In summary there is a need for a simple and

practical field-testing apparatus, which is capable of accurately determining the level of arsenic in a water sample.

The present invention, in its various aspects, aims to go some way towards overcoming the above-mentioned

disadvantages of previous field-testing kits.

A first aspect of the present invention provides a method of testing for the presence of arsenic in a sample, comprising adding one or more substances to the sample to react with any arsenic or arsenic compounds present in the sample to generate arsine gas, allowing the arsine gas to contact a detector element which changes colour upon contact with arsine gas, and using a filter (which may be a scrubber) to remove excess arsine gas remaining after the arsine gas has passed by or through the detector element.

The filter removes excess arsine gas and thus protects the environment and the tester from the harmful effects of exposure to arsine.

After the detector element has been exposed to the arsine gas, it can be tested for colour change, visually or with an electronic sensor, so as to determine the level of arsenic present in the sample A second aspect of the present invention provides an apparatus for field testing for the presence of arsenic in a sample (e.g. water), comprising: a container for containing a sample to be tested; a first filter which changes colour upon contact with arsine gas; and a second filter for removing excess arsine gas. The second filter may be a scrubber.

The apparatus has to be suitable for field-testing

and therefore must be portable; usually the container will be of handheld size. There may also be provided an electronic sensor device for optically detecting the colour change of the first filter. It may be necessary to provide a means (e.g. a scrubber, such as a lead acetate filter) for preventing hydrogen sulphide and/or other sulphide or other contaminant gases from contacting the first filter. Preferably the apparatus has a third filter for removing Hydrogen Sulphate, positioned between the second and third filters.

Preferably the apparatus includes a lid for the container and the filters are adapted to be mounted on or in the lid. Generally the apparatus will be designed so that, in use, arsine gas passes through the first filter and then comes into contact with the second filter.

Preferably the lid has a bore through which arsine gas from the container can pass and the filters can be fitted to extend across the bore and are axially spaced from each other (along the axis of the bore).

The filter may be in the form of a filter cartridge comprising a plastic casing and a filtering element (e.g. a strip of filter paper) mounted in the casing.

Preferably the plastic casing surrounds the filter paper and two openings on either side of the casing expose the filter paper. When the cartridge is mounted in the apparatus the openings can be aligned with a bore in the apparatus (e.g. a bore in a lid of the container) to allow passage of gas through, or at least into contact with, the filtering element inside the casing.

The apparatus of the second aspect of the present invention can be used to carry out the method of the first aspect, in which case the first and second filters correspond respectively to the detector element and the arsine gas filter of the first aspect of the present invention.

As the above aspects of the present invention use two filters, (an arsine filter and a detector), it is necessary to mount these filters correctly in the apparatus. If the second filter (the arsine filter) is accidentally placed upstream of the first filter (the detector), then it will remove arsine gas before it reaches the first filter and the first filter will erroneously indicate that no arsine is present in the sample. If an electronic sensor device is used then it is necessary to ensure that the first filter (the arsine detector filter), and not the second filter (the arsine scrubber) is inserted into the detector device. This may be achieved by colour coding of the filters, however while this helps, it may not be in itself sufficient, as it does not actually prevent the filter from being placed in the wrong filter receiving means.

Accordingly, a third aspect of the present invention provides a portable filtration apparatus comprising a filter receiving member having first and second filter receiving means, the first filter receiving means being shaped or otherwise configured such that it can receive a filter designed for (e.g. designed to fit) the first filter receiving means, but cannot receive a filter designed for the second filter receiving means, the second filter receiving means being shaped or otherwise configured such that it can receive a filter designed for the second filter receiving means, but cannot receive a filter designed for the first filter receiving means.

Preferably the filtration apparatus includes first and second filters designed to fit and removeably mountable to the first and second filter receiving means respectively.

Thus the first filter receiving means is shaped or otherwise configured so that it can receive the first filter but not the second filter. The second filter receiving means is shaped or otherwise configured so that it can receive the second filter but not the first filter. Therefore, it is not possible to accidentally mount the first and second filters in the wrong locations.

It is to be understood that the filters may be provided as filter cartridges, e.g. filter elements in a surrounding plastic casing. The important thing, in this case, is that the cartridge casings fit their respective filter receiving means. The apparatus may be provided as the filter receiving member and first and second filter cartridge casings, but without the filter elements (which the user might purchase separately) . Therefore, in this aspect of the present invention, the term 'filter' is to be interpreted as including a filter cartridge casing.

Preferably, but not necessarily, the filter cartridge casing contains a filter element. Preferably the filter cartridge casing is formed of plastic.

Another aspect of the invention is a separate device (e.g. an electronic sensing device for detecting colour change) that one of the two filters (e.g. the first filter) is to be used with after it has performed the filtering operation. This separate device has a filter receiving means shaped or configured such it can receive that filter but not the other filter (e.g. so that it can receive the first filter, but not the second filter). In this way it can be ensured that only the correct filter is mounted in the separate device.

It is thought that this third aspect of the present invention will find particular application to arsenic testing methods and apparatus for detecting impurities (such as arsenic) in liquids (such as water). For example a method and apparatus as described in the first and second aspects, having a first filter in the form of arsine detector and a second filter in the form of an arsine filter or scrubber. However it is not limited to such methods and apparatus. It is thought that it will find general application to other filtration or testing apparatus and methods, in which it is necessary or desirable to pass a fluid sequentially through two different filters.

As mentioned above, the first and second filters may conveniently be provided as filter cartridges comprising a plastics material casing in which a filter element (e.g. filter paper) is mounted. Such a cartridge allows for easy handling and also helps to prevent contamination of the filter elements.

Preferably the casing generally surrounds the filter element and has a bore through which fluid can pass to contact the filter element. In one embodiment the casing comprises two opposed members, which can be placed on either side of the filter element. Preferably each of the opposed members has an opening and said openings form a bore when the opposed members are aligned, most preferably the opposed members are hinged together. The casing may have a filter element locating means for keeping the filter element in the correct position. This locating means could e.g. be a projecting spike.

There are various ways of achieving the selective fit of the filters whereby the first filter fits in the first filter receiving means, but not the second filter receiving means and the second filter fits in the second filter receiving means, but not the first filter receiving means. For example, there can be a system of grooves and projections (which may be in the form of ridges) on the first and second filters and the first and second filter receiving means.

Preferably the first filter receiving means has a first groove for receiving a projection on the first filter, and the second filter receiving means has a second groove for receiving a projection on the second filter, wherein said first and second grooves have different shapes and/or are differently positioned relative to their respective receiving means. Thus in use, this different shaping and/or positioning means that S the first filter receiving means cannot receive the second filter and vice versa. A similar principle can be applied to a filter receiving means on a separate device for analysing the first filter after it has performed the filtering operation.

Alternatively the grooves can be provided on the filters and the projections (or ridges) on the filter receiving means.

Thus in general, said first and second filters and said first and second filter receiving means have a system of grooves and projections whereby the first filter receiving means can receive the first filter, but not the second filter and the second filter receiving means can receive the second filter, but not the first filter.

The grooves or projections may be shaped or positioned so that a filter can only be received when in a given predetermined orientation (e.g. the right way up). The arrangement may be such that there is only one possible orientation of the filter in which it can be received by the filter receiving means. This is particularly useful for applications where the filter is analysed as its orientation may affect the analysis.

Preferably the filtration apparatus is designed so that when the first and second filters are mounted on the first and second filter receiving means, they are in overlapping relation to one another so that, in use, fluid passes through the first filter before contacting the second filter. Preferably the filtration apparatus has a bore and the first and second filters, when mounted, extend across the bore and are spaced from each other along the axis of the bore.

Preferably the filtration apparatus is part of a testing kit (e.g. a field kit for testing for arsenic) which includes a container for containing a sample to be tested and the filtration apparatus is shaped to fit over an opening of the container through which fluid can pass (e.g. a lid to fit over the mouth of the container if the container is a test tube or flask).

The apparatus may be provided as a kit of parts without the filter elements. The user could then obtain the filter elements (e.g. detector filter paper and arsine scrubber filter paper) separately. Accordingly the first and second filters in the third aspect of the present invention may in fact be filter holders or the plastic casing part of filter cartridges, without the filtering elements. The same applies to the fourth aspect of the invention below.

In fact, it is quite possible that the filter receiving member could be sold separately, from the other parts of the apparatus. Accordingly, a fourth aspect of the present invention provides a lid member shaped or configured to fit over the mouth of a container and having a bore for communicating with the mouth of the container, a pair of filter cartridge receiving slots passing though said bore, said slots being substantially perpendicular to said bore and spaced apart from each other along the axis of said bore, a first groove on said first receiving slot for receiving a projection on a filter cartridge body, a second groove on said second receiving slot for receiving a projection on a filter cartridge body, wherein the first and second grooves are shaped and/or positioned differently from each other such that a filter cartridge having a projection and designed for the first slot cannot fit into the second slot, and such that a filter cartridge having a projection and designed for the second slot cannot fit the first slot.

Alternatively the filter receiving member may be designed for filter cartridges having grooves and may have filter receiving slots with one or more projections shaped and positioned to fit said filter cartridge grooves.

Preferably the lid member is provided together with a container. Preferably the lid member is provided together with said first and second filter cartridges that it is designed to fit.

Mention above has been made of filter cartridges. It is thought that the concept of a filter cartridge in itself represents an improvement over the prior art.

Accordingly a fifth aspect of the present invention

provides a field test kit for testing for arsenic

comprising a container for containing a sample, a container lid having a filter cartridge receiving means and a filter cartridge removably mountable in said filter cartridge receiving means, the filter cartridge comprising a plastic casing and a filtering element (e.g. filter paper impregnated with mercury bromide) that changes colour when it contacts arsine gas.

Preferably the plastic casing generally surrounds the filtering element and exposes just a portion thereof (e.g. through an aperture or bore) so that said exposed portion can contact any gases released by the sample.

Preferably the filter cartridge is shaped such that it can only be mounted on the filter cartridge receiving means in the correct orientation (e.g. the right way up).

This may, for example, be achieved by providing the filter cartridge with a ridge or protrusion which mates with a groove in the filter receiving means.

Alternatively the filter cartridge may have a cross section such that it can only fit one way into a filter receiving means (e.g. the receiving means may be a slot and the slot and the cartridge may have complementary tapered cross sections, wider at one end than the other).

This principle of the filter cartridge having a correct orientation and being shaped so that it can only be mounted in the correct orientation may also be applied to one or both of the filters of any of the above aspects of the present invention.

Lead acetate is toxic and therefore a scrubber consisting of an absorbent material (e.g. cotton wool) soaked in lead acetate is dangerous to handle.

Furthermore the absorbent material can be difficult to fit to other parts of the apparatus in which the scrubber is to be used.

Accordingly a sixth aspect of the present invention provides a scrubber comprising an absorbent material in a filtering substance e.g. lead acetate, and a non-porous tube (e.g. a rubber or plastic tube) surrounding the absorbent material.

The provision of the (e.g. rubber or plastic) tube allows much more easy assembly of the scrubber into a filtering apparatus because the tube can easily fit and attach to e.g. a bore in the apparatus, such as the lid member, filter receiving member or filtration apparatus bore mentioned in the above aspects of the invention.

Furthermore, because the tube is non-porous the filtering substance does not leak or become contaminated.

A further advantage is that, if the filtering substance is hazardous to humans, then a person assembling apparatus including the scrubber is protected, by the non-porous tube, from contact with the hazardous substance and need not wear protective gloves.

Preferably the filtering substance is a substance capable of acting as a scrubber for removing hydrogen sulphide (e.g. lead acetate).

Preferably the filtering elements (e.g. filter paper) used in the above aspects of the invention have a plastic edging (e.g. plastic around the circumferential the edges). This enables the filter paper to be handled, without contamination, before mounting in the apparatus or a filter cartridge.

Preferably the filtering elements are colour coded, so that it is clear to the user which part of the apparatus, or which filter cartridge they are to be mounted in. This colour coding may be achieved by using a particular colour of plastic edging.

Preferably the first filter cartridge and the plastic edging of filtering elements for the first filter are provided in a first distinctive colour (e.g. black) and the second filter cartridge and the plastic edging of the filtering elements for the second filter are provided in a second distinctive colour (e.g. red). Thus it will be clear to a user which filtering elements are to be put into which filter cartridges.

Preferably the filter or filter cartridge for detecting the arsine gas are colour coded in a first distinctive colour (e.g. black) and the filters or filter cartridges for removing gas (e.g. the HS and arsine scrubbers) are colour coded in a second distinctive colour (e.g. red). In this way the filters can easily be identified.

can be ensured that the filtering cartridges are inserted into the correct locations in the apparatus by use of further colour coding (e.g. of the filter receiving means or slots) and/or by selective fits as described above in the third and fourth aspects of the present invention. Similarly where there is a separate apparatus for analysing just one of the first and second filters (e.g. an electronic sensing device for measuring colour change), colour coding or a selective fit can be used to ensure that the correct filter is used.

As will be clear to a person skilled in the art, the above aspects of the present invention may be combined with each other and any feature described in relation to one of the aspects may also be used in the other aspects.

A seventh aspect of the invention provides a kit of parts for making an apparatus according to any one of the above aspects.

An eighth aspect of the present invention is an electronic sensing device for detecting colour change in a detecting element (e.g. an arsenic detecting filter).

The electronic sensing device comprises means for detecting colour change in the detecting element and a slot for receiving a cartridge carrying a detecting element.

Preferably the slot is shaped so that the cartridge can only be inserted in a single predetermined orientation, i.e. it cannot be inserted the wrong way up.

Preferably this is achieved by means of a mating groove and protrusion on the cartridge and receiving slot.

Where the detecting means is also used in an apparatus together with a scrubber held in a scrubber cartridge, the detector receiving slot is preferably shaped (e.g. with a slot or protrusion) so that it cannot receive the scrubber cartridge, but so that it can receive the detector cartridge.

As will be evident to the skilled man, colour coding and other features of the above aspects may be used with this eighth aspect of the invention.

Further aspects and preferred features of the present invention may be found in the claims.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a cross-sectional view of an arsenic testing kit; Fig. 2 is a cross-sectional view of a hydrogen sulphide scrubber for use with the arsenic testing kit of Fig. 1; Fig. 3 is a top-down view of a plastic filter holder for use with the arsenic testing kit of Fig. l; Fig. 4 is a perspective view of the arsenic testing kit; Fig. 5 is a perspective view of a detector filter and an electronic sensing device having a slot for receiving the detector filter; Fig. 6 is a plan view of a first side of the lid of an arsenic testing kit; Fig. 7 is a plan view of the opposite side of the lid means to Fig 6; Fig. 8 is a perspective view of a digital arsenic testing instrument having a slot for receiving a detector fi lter cartridge; and Fig. 8a is an enlarged perspective view of the slot of Fig. 8 and the detector filter to be received by the slot.

As shown in Fig. l, the arsenic testing kit comprises a filtration apparatus having a container l for containing the sample to be tested and a lid portion 2 designed to fit the open mouth of the container. In use, the sample to be tested (e.g. water 3) is placed in the container l and appropriate substances are added to the sample 3 to react with any arsenic or arsenic compounds in the sample and cause the release of arsine gas.

The lid device 2 is placed over the mouth of the container 1. The lid device 2 is a filter holding member and has first and second filter receiving means in the form of first and second filter receiving slots 5 and 6.

Filter receiving slots 5 and 6 are in an upper portion of the lid 2; a lower depending portion 4 of the lid can be fitted snugly to make a sealing fit with the mouth of container 1.

Lid 2 has a central bore which extends through the depending portion 4 and through the first and second filter receiving slots 5 and 6. Thus the bore (not shown) facilitates communication between the interior of container 1 and any filters placed in the filter receiving slots 5, 6. A hydrogen sulphide (HS) scrubber is fitted in the portion of the bore in

the depending portion 4 of the lid 2. The HS scrubber is provided in the form of a bullet as shown in Fig. 2. The bullet comprises a substantially cylindrical plastic or rubber tube 10 and absorbent l material, such as cotton wool, which is placed inside the tube lO and soaked with lead acetate. The tubing lO conveniently protects the user assembling the filtration apparatus from contact with the lead acetate which is a hazardous substance. A first filter is placed in first filter receiving means 5 and the second filter is placed in second filter receiving means 6. The first filter includes a detector element in the form of filter paper impregnated with mercuric bromide (HgBr) which changes colour on contact with arsine gas. The second filter is an arsine scrubber for removing arsine gas.

Thus, in use the arsine gas released from the sample 3 travels upward towards the lid 2 and passes through the lid's bore. The gases released from the sample 3 first contact the HS scrubber lO and any HS present is removed.

The gas then contacts the first filter in the first filter receiving slot 5 and the detector element changes colour in proportion to the amount of arsine gas present.

Remaining arsine gas which travels through or by the filter in the first filter receiving means 5 is removed by the arsine scrubber in the second filter receiving means 6. In this way arsine gas (which is toxic) is not released into the external environment.

The first and second filters each comprise a plastic filter holder or cartridge and a filtering element (in this case filter paper) which is placed inside the filter holder. The filter holder generally surrounds at least a portion of the filtering element and slots into the appropriate filter receiving slot 5 or 6. An example of a filter holder 20 is shown in Fig. 3.

The filter holder 20 has first and second members 21, 22 which are hinged together end to end. In Fig. 3 the filter holder 20 is shown in an open configuration.

A filtering element such as a strip of filter paper can be placed on the first member 21 and the filter holder closed by hinging the second member 22 around to oppose the first member 21 and thus enclose at least a portion of the filtering element. The first member 21 has a projecting spike 23 for engaging the filtering element and keeping it in position. This projecting spike 23 opposes and can engage with a corresponding hole 24 on the second member 22 when the filter holder is closed.

Each member 21, 22 has an aperture 25, 26 for allowing gas or liquid outside of the filter holder to contact the filtering element when the filter holder is closed.

These apertures 25, 26 are aligned with each other to form a through bore when the filter holder is closed.

Handle portions 27, 28 on the end of each member 21, 22 facilitate opening and closing of the filter holder.

Fig. 4 is a perspective view showing the filtration apparatus including the container 1, the lid device 2, first and second filter holders 30 and 40 and their respective filtering elements 35 and 45. Fig. 4 also shows the central bore 2a of the lid device 2 as previously described.

The first and second filter holders are colour coded so that the first filter bears a first distinctive colour and the second filter holder bears a second distinctive colour. In this embodiment the first filter holder is black and the second filter holder is red. The HS scrubber 10 is colour coded in the same colour as the second filter holder (red), so both gas removing filters are colour coded in the same colour and differently to the detector filter (the first filter). The lid member 2 and the filter receiving slots 5 and 6 may be colour coded too in order to help the user place the right filter holder in the right slot.

The first filter holder 30 holds an arsine detecting element 35 in the form of mercuric bromide impregnated filter paper. The second filter holder 40 holds an arsine scrubber filtering element which is also provided as filter paper. Optionally, the detector element can have a periphery lined with a first distinctive colour (e.g. black) corresponding to the distinctive colour of the first filter holder and the arsine scrubber element can have its periphery lined with a second distinctive colour (e.g. red) corresponding to the colour code of the second filter holder 40. A coloured plastic trim would be a suitable means for achieving this. When the filter elements 35 and 45 are colour coded in this manner it can be assured that the user will place the detector element in the first filter holder and the arsine scrubber in the second filter holder.

Once the arsine gas has been allowed to contact the detector element 35 for a sufficient amount of time, e.g. fifteen to twenty minutes, the first filter holder 30 can be removed from the lid device 2 and placed in an electrical optical sensing device 50 for detecting colour change in the first filter, as shown in Fig. 5. The optical sensing device 50 has a slot 55 adapted to receive the first filter holder 30. This slot may also be colour coded with the distinctive colour of the first filter holder (e. g. black). The optical sensing device 50 has been previously calibrated with an un-stained detector element from the same batch as the detector element 35. When the detector element 35 and filter holder 30 are inserted into the slot 55 the optical sensing device detects the change in colour of the detector element 35 and produces a reading on its screen 60.

Optionally, the user may compare the colour of the (stained) detector paper 35 after it has been in contact with the arsine gas with a colour chart and only use the electronic sensing device 50 if the colour change is within certain predetermined critical levels (e.g. if precise determination of the colour change is needed).

In some cases the level of arsenic in the sample and corresponding colour change in the detector paper 35 will be so great that it will be clear from a visual comparison alone that the sample water 3 is not safe for human consumption. In other cases, where the level of arsenic present is close to the maximum prescribed by the local health authorities then a more accurate electronic determination of the colour change is needed.

It is important that the first and second filters 30, 35 and 40, 45 are inserted into the correct slots (5 and 6 respectively) of the lid device 2 so that the arsine gas contacts the detector paper 35 before it contacts the arsine scrubber 45. Similarly it is important that the user inserts the first filter 30, 35 into the detector 50 and not the second filter 40, 45.

Obviously, it is the colour change in the detector paper which needs to be measured.

Colour coding goes some way to ensuring the above.

However, as a further measure the filter receiving means (slots 5, 6 and 55) are designed so that they can only receive the correct filter holder. Thus the first filter receiving slot 5 is shaped so that it can receive the first filter holder 30, but not the second filter holder 40. The second filter receiving slot 6 is shaped so that it can receive the second filter holder 40, but not the first filter holder 30. The electronic sensing device slot 55 is shaped so that it can receive the first filter holder 30, but not the second filter holder 40.

This can be achieved by providing each filter holder 30, 40 with a respective projection or ridge on its outer surface and corresponding recesses in the filter receiving slots, as shown in Figs 6- 8.

Thus, Fig 6 is a schematic plan view of a first side of the lid 2 of Figs 1 and 4. The lid 2 has an upper slot 6 which has a groove 65 in its bottom right corner (this is ringed in the plan view of Fig 6). The second filter holder 40 for the arsine scrubber 45 has a ridge 46 which fits this groove 65. Fig 6 shows the filter holder 40 both in plan and perspective views. The groove 65 is ringed in Fig 6.

Fig 7 is a corresponding plan view of the opposite side of the lid 2. It shows the lower slot 5 for receiving the first filter holder 30. The filter holder has a ridge on its left side and this is received by the groove 75 on the left of the slot 5 when the filter holder is inserted.

As the respective grooves of the upper and lower slots are in the bottom right and bottom left corners respectively, the filter holders cannot be inserted into the wrong slots. The positioning of the recesses 65, 75 in slots 5, 6 and corresponding ridges 36 and 46 also makes sure that each filter can only be inserted in the correct orientation.

As can be seen in Fig. 8 and Fig 8a, the electronic sensing device for determining the amount of arsenic present has a slot 55. The slot 55 has a recess 56 positioned so that it is possible to insert the first filter holder 30 (containing the detector filter element), but not the second filter holder into the slot 55. The recess 56 also ensures that the filter holder 30 is inserted in the correct orientation so that readings are consistent.

Thus, in general, the slots 5, 6 and 55 are provided with recesses capable of accommodating the projections or ridges on the respective filter holders which they are designed to fit. The positioning or shape of the projections or ridges is such that the first filter holder can be received by the first filter receiving slot and the electronic sending receiving device slot 55, but not by the second filter receiving slot 6. The second filter receiving slot 6 can receive the second filter holder 40, but not the first filter holder 30. In this way it is ensured that the first and second filters are placed in the correct position in the lid 2 and that only the detector filter is inserted into the electronic sensing device.

Furthermore, as shown in Fig 8a, each filter holder has a first end 30a which is to be inserted into the S slots and an opposite end 27, 28 having an enlarged flange which is too broad to be inserted into the slots.

Thus the filter holder has to be inserted with the first end 30a first. The same applies to the second filter holder 40.

Claims

Claims A portable filtration apparatus comprising a filter receiving

member having first and second filter receiving means, and first and second filters removeably mountable to said first and second filter receiving means, the first filter receiving means being shaped or configured such that it can receive the first filter, but cannot receive the second filter, the second filter receiving means being shaped or configured such that it can receive the second filter, but cannot receive the first filter.
2. A filtration apparatus according to claim 1 wherein said first and second filters are filter cartridge casings.
3. A filtration apparatus according to claim 2 wherein each of said filter cartridge casings contains a filter element.
4. A filtration apparatus according to any of the above claims wherein said first and second filters and said first and second filter receiving means have a system of grooves and projections whereby the first filter receiving means can receive the first filter, but not the second filter and the second filter receiving means can receive the second filter, but not the first filter.
5. A filtration apparatus according to claim 4 wherein the first filter receiving means has a first groove for receiving a projection on the first filter, and the second filter receiving means has a second groove for receiving a projection on the second filter, wherein said first and second grooves have different shapes and/or are differently positioned relative to their respective receiving means.
6. A filtration apparatus according to claim 4 wherein the first filter receiving means has a first projection for engagement with a groove on the first filter, and the second filter receiving means has a second projection for engagement with a groove on the second filter, wherein said first and second grooves have different shapes and/or are differently positioned relative to their respective filters.
7. A filtration apparatus according to any one of the above claims, further comprising a container for containing a sample and having an opening, and wherein said filter receiving member is shaped to fit over said container opening.
8. A kit of equipment comprising the filtration apparatus according to any one of claims 1 to 7 and an electronic apparatus having a filter receiving means shaped or configured to be capable of receiving the first filter, but not the second filter.
9. A filtration apparatus according to any one of claims 1 to 7 wherein the first filter comprises an arsine detecting filter element, which changes colour on contact with arsine gas.
10. A filtration apparatus according to claim 9 wherein the second filter comprises a filter element for removing arsine gas.
11. A filtration apparatus according to claim 9 or 10, the apparatus further comprising a third filter for removing Hydrogen Sulphide.
12. An apparatus for field testing for the presence of arsenic in a sample, comprising: a container for containing a sample to be tested) a first filter which changes colour upon contact with arsine gas, and a second filter for removing excess arsine gas.
13. An apparatus according to claim 12 and further comprising a third filter for removing Hydrogen Sulphidei said first filter being positioned between said second and third filters.
14. An apparatus according to claim 12 or 13 wherein one or more of the filters is a filter cartridge comprising a plastic casing and a filtering element inside the casing.
15. An apparatus according to any one of claims 12 to 14 wherein said apparatus has first and second filter receiving means to which said first and second filters are removeably mountable, said first filter receiving means being shaped or configured such that it can receive said first filter, but not said second filter.
16. An apparatus according to claim 15 wherein said second filter receiving means is shaped or configured such that it can receive said second filter, but not said first filter.
17. An apparatus according to any one of claims 12 to 14 wherein said first filter, second filter and container are part of a filtration apparatus according to claim 7.
18. A kit for field testing for the presence of arsenic in a sample, the kit comprising an apparatus according to any one of claims 12 to 17 and an electronic sensor device for optically detecting colour change of the first filter.
19. A kit according to claim 18, wherein the electronic sensor device has a filter receiving means for receiving the first filter, said filter receiving means being configured or shaped so that the first filter can be inserted into the electronic sensor device only in a predetermined orientation.
20. A kit according to claim 18 or 19, wherein the electronic sensor device has a means for receiving the first filter, said means being shaped or configured such that it can receive the first filter, but not the second filter.
21. A method of testing for the presence of arsenic in a sample, comprising adding one or more substances to the sample to react with any arsenic or arsenic compounds present in the sample to generate arsine gas, allowing the arsine gas to contact a detector element which changes colour upon contact with arsine gas, and using a filter to remove excess arsine gas remaining after the arsine gas has passed by or through the detector element.
22. A method according to claim 21, further comprising the step of testing the detector element for colour change, after it has been in contact with the arsine gas, so as to determine the level of arsenic present in the sample.
23. A method according to claim 22 wherein the testing step is carried out with an electronic sensing device.
24. A field test kit for testing for arsenic comprising a container for containing a sample, a container lid comprising a filter cartridge receiving means and a filter cartridge removeably mountable in said filter cartridge receiving means, the filter cartridge comprising a plastic casing and inside said plastic casing an arsine detecting filter element that changes colour when it contacts arsine gas.
25. A field test kit according to claim 24 wherein the filter cartridge receiving means is shaped or configured such that it can receive the filter cartridge only in one predetermined orientation.
26. The field test kit of claim 25 wherein the container lid is a filtration apparatus according to any one of claims 1 to 6, 9 or 10.
27. An electronic sensor device for receiving a filter cartridge and a filter cartridge containing an arsine detecting filter element which changes colour when it contacts arsine gas, the electronic sensor device being configured to detect the difference in colour between said filter element and a pre-set calibration colour.
28. An apparatus according to claim 27 wherein device has a filter receiving means configured or shaped so that it can only receive said filter cartridge in a predetermined orientation. s
29. A scrubber comprising a filtering substance held by or in an absorbent material, and a non-porous tube surrounding the absorbent material.
30. A scrubber according to claim 27 wherein the filtering substance is a filtering substance for removing Hydrogen Sulphide.
31. A scrubber according to claim 30 wherein the filtering substance is lead acetate.
32. A scrubber according to any one of claims 28 to 31 wherein the nonporous tube is made of rubber or plastic.
33. An apparatus according to claim 11 or 13 wherein the filter for removing Hydrogen Sulphide is a scrubber according to any one of claims 29 to 32.
34. A kit of parts for making a portable filtration apparatus according to any one of claims 1 to 7, 9 or 10, said kit comprising: a filter receiving member having first and second S filter receiving means, and first and second filters removeably mountable to said first and second filter receiving means, the first filter receiving means being shaped or configured such that it can receive the first filter, but cannot receive the second filter, the second filter receiving means being shaped or configured such that it can receive the second filter, but cannot receive the first filter.
35. A kit of parts according to claim 34 further IS comprising a container having an opening over or in which the filter receiving member is mountable.
36. A kit of parts for making a field testing apparatus for detecting the presence of arsenic in a sample according to any one of claims 12 to 17, the kit comprising: a container for containing a sample to be tested; a first filter which changes colour upon contact with arsine gas, and a second filter for removing excess arsine gas.
37. A portable filtration apparatus substantially as described herein, with reference to Figs. 1 to 4, 6 and 7.
38. An apparatus for field testing for the presence of arsenic in a sample, substantially as described herein with reference to Figs. 1 to 4, 6 and 7.
39. A set of equipment for field testing for the

presence of arsenic in a sample, substantially as described herein with reference to Figs. 1 to 8 and 8a.
40. An electronic sensing device for detecting colour change in a detecting element, the device comprising means for detecting colour change in the detector element and a slot for receiving a cartridge carrying a detecting element.