WO1995018661A1

WO1995018661A1 - Casting of polymer bead seals in filter housings

Info

Publication number: WO1995018661A1
Application number: PCT/GB1995/000020
Authority: WO
Inventors: Richard Banfield Hicks
Original assignee: Intersep Limited
Priority date: 1994-01-07
Filing date: 1995-01-06
Publication date: 1995-07-13
Also published as: AU1324695A; GB2290243B; GB2290243A; GB9517467D0

Abstract

A filter housing in the form of a centrifugal concentrator filter unit (3) is dimensioned to be fully containable in a conventional centrifuge tube (2) when sealed with a cap (4) to provide a closed system centrifugal concentrator adapted to concentrate solution without creating external aerosol or fluid contamination. In accordance with a principle aspect of the invention the filter housing (3) comprises two mating parts (5, 6) which retain a filter element (7) between confronting formations of the two parts (5, 6). End wall formations of the two housing parts (5, 6) cooperate to define a capillary channel (16) into which a watery free-flowing cyanoacrylate polymer adhesive or sealant can be introduced from ledge (12A) through aperture (12). The adhesive will flow around the channel (16) by capillary action to form a bead seal (19) between the filter element (7) and the two parts (5, 6). The principle aspect of the invention is applicable to two-part filter housings generally. Conveniently and in accordance with other aspects of the invention, a first one of the two parts is provided by a generally cylindrical filter tube (5) which is a loose or sliding fit in the centrifuge tube (2) and which provides a filter reservoir between an open end of the filter tube and the filter element in the form of permeable filter membrane (7) disposed across an opening at or near the other end of the tube, and the second housing part is provided by elongate spacer member (6) extending from the filter tube oppositely to the reservoir which in use abuts a closed end portion of the centrifuge tube (2) to space the filter membrane (7) therefrom and to define a filtrate collection vessel of a predetermined volume at the closed end of the centrifuge tube (2).

Description


  
 



   OF POLYMER BEAD SEAIS IN FILTER HOUSINGS"
 This invention according to a principle one of its aspects relates to a method of casting a polymer seal between two mating parts of a filter housing and a filter element retained therebetween generally and more particularly but not exclusively to a centrifugal concentrator filter unit comprising a polymer seal made in accordance with said method of the invention.



   In conventional centrifugal separation a sample of solution comprising components of differing specific density is placed in a centrifuge tube which once sealed with a cap can be spun in a centrifuge to separate out the different components. More advanced applications use the centrifuge to filter a solution to remove fluid therefrom whilst retaining concentrate.



  For this a centrifugal concentrator is used in place of the conventional centrifuge tube. Usually a concentrator contains a filter membrane which retains microscopic or molecular sized particles and requires the high pressures generated in a centrifuge to force fluid molecules therethrough.



   Typically, centrifugal concentrators are in the form of the concentrator shown in Figure 1, and comprise a sample reservoir 100, a filter membrane 101 held on a support member 102 to which a filtrate reservoir 103 is attached, and a retentate cup 104. In use a full sample (not shown) of dilute solution is placed in the reservoir 100 and the whole concentrator is then placed in an angled head centrifuge, ie: it is placed in a centrifuge rotor in a retention bore so as to be inclined to the vertical as shown in Figure 1. Once the concentrator has been spun, for example, about axis A-A the sample has been separated into two components: the filtrate 105 and the concentrated solution 106. For this to occur air in the reservoir 103 must be displaced by the filtrate 105 and air must be allowed to enter reservoir 100 to replace the filtrate 105.

   Consequently, it will be appreciated that concentrators of this type require that both the cup 104 and the connection between the support member 102 and reservoir 103 must be vented. This can lead to possible aerosol or fluid contamination of the surrounding environment. In testing of benign or non-volatile substances this poses little health risk, although it can be a nuisance.



  However, often the concentrate may be of cancerous or infectious materials and in such circumstances it may be appropriate to provide the centrifuge  with a vented hood to remove any aerosol pollution or to operate the centrifuge in a controlled environment.



   It will be understood from Figure 1 that by using the described concentrator in an angled head centrifuge rotor the disposition of a drain hole 107 centrally of the filter membrane 106 means that not all solution will pass therethrough, ie: the volume 106 will not be able to pass through the membrane. This is particularly important because were all fluid able to pass through the membrane 101 the resultant concentrate would be dried out and caked on the membrane. This not only makes it difficult to remove, but also when the concentrate is a protein then dehydration effectively kills it and makes it useless for subsequent testing.



   If the aforedescribed concentrator is used in a conventional swivel head centrifuge then during spinning it is disposed radially of the axis of rotation. Clearly, all fluid will then be able to pass through the drain hole 107 unless the centrifuge is stopped before this occurs. Consequently, it is advisable that this sort of concentrator is always used in conjunction with an appropriately sized angled head centrifuge rotor. Obviously, if concentrators of different sizes are being used in a laboratory then a plurality of angled head centrifuges will need to be present.



   Clearly for effective filtration it is essential that fluid is not allowed to seep from sample reservoir 100 around the periphery of filter membrane 101 into the retentate reservoir 103. To prevent this an'0-ring' seal 108 is provided. It will be understood that an'O-ring'seal can fail l for many different reasons including poor seating on surfaces to be sealed and/or degeneration by materials in which it is in contact. Also, in microfiltration and nanofiltration the membrane filters 101 tend to be of fragile fibre composition which might rip or tear due to the contacting pressure of an elastcneric seal.



   It would be advantageous to replace a mechanical seal of the'0-ring' type with a sealant which is applied as a fluid or mastic permitting it to form to the surface contours of the fibre membrane 101 before setting.



  However, application of fluid or mastic type sealants or adhesives would be at best messy and at worst lead fo unwanted contamination of the operative surface of filter membrane 101.  



   It is an object of one principle-aspect of the invention to provide a filter unit in which a bead seal is cast from a free-flowing polymer sealant or adhesive and which may form a part of a closed system centrifugal concentrator adapted to concentrate solution without creating external aerosol contamination.



   According to a first aspect of the invention a method of casting a polymer seal between two mating parts of a filter housing having a throughbore extending therebetween and a filter element retained between confronting formations of the two housing parts to span the throughbore, in which during assembly the two housing parts are mated together to retain the filter element between said confronting formations is characterised in that;

   confronting peripheral wall formations of the two mating parts cooperate to define a capillary channel extending around the periphery of the filter element into which a peripherally outer portion of the filter element extends, and in that an aperture extends through the side wall of one of the said two mating parts of the filter housing permitting the introduction of a free-flowing polymer adhesive or sealant into the capillary channel, the arrangement providing that the adhesive is caused by capillary action to flow throughout and to generally fill the channel thereby contacting the peripherally outer portion of the filter element so that on curing of the adhesive or sealant a bead seal is formed between filter housing parts and the filter element.



   Typically, a second aperture extends through one or other of the two parts of the filter housing to provide a breather port to the capillary channel permitting air to exhaust therefrom as the channel fills with the adhesive or sealant. Preferably, on curing the bead seal rigidly attaches said one of the two parts of the filter housing to the other part thereof.



   Desirably, the filter element is a generally flat membrane the peripherally outer portion of which on assembly of the two parts of the filter housing is caused to seat in the base of the capillary channel. The filter element may be a microporous filter membrane typically used in microfiltration or ultrafiltration of medical specimens.



   Preferably, one of the two-parts of the filter housing is at least translucent and the sealant or adhesive contains an optical brightener and  the method further comprises quality-control involving illuminating the assembled filter housing with ultra-violet light to permit the integrity of the bead seal to be adjudged by visual means. Said visual means may comprise an automate computer aided image recognition system.



   Generally, the adhesive or sealant is cyanoacrylate polymer adhesive moisture curable at roan temperature. The capillary channel may have a width of between 0.1 millimetres and 1.0 millimetres more preferably between 0.20 millimetres and 0.35 millimetres.



   According to a second aspect of the invention a filter unit comprises a housing having two parts thereof retaining a filter element and a bead seal which has been provided cast in accordance with said first aspect of the invention.



   A centrifugal concentrator filter unit in accordance with said second aspect of the invention is characterised in that; the housing is dimensioned to be fully containable in a conventional centrifuge tube when sealed with a cap to provide a closed system centrifugal concentrator adapted to concentrate solution without creating external aerosol or fluid contamination, the housing comprises a male first part in the form of a generally cylindrical filter tube,

   which is a loose or sliding fit in the centrifuge tube and which provides a filter reservoir between an open end of the filter tube and the filter element being a permeable filter membrane disposed across an opening proximate the other end of the tube and the housing further comprises a female second part in the form of an elongate spacer member extending from the filter tube oppositely to the reservoir which in use abuts a closed end portion of the centrifuge tube to space the filter membrane therefrom and to define a filtrate collection vessel of a predetermined volume at the closed end of the centrifuge tube.



   Preferably, an end region of the elongate spacer member adapted for attachment to the filter tube is of generally tubular form so as to receive a complimentarily shaped end region of the filter tube and is provided with an inwardly extending abutment formation on which the peripherally outer portion of the filter membrane can seat and against which the membrane is compresse by an end face formation-at said other end of the filter tube.  



   Also, said end region of the spacer member may be provided with a plurality of circumferentially spaced sidewall apertures adapted to receive, locate and lock respective correspondingly shaped barbs extending outwardly from the filter tube to effect a snap-fit connection therebetween.



   Desirably, the end wall of the filter tube which compresses the membrane is of reduced diameter as compared to a confronting wall region of the bore of the elongate member so as to define the said capillary channel therebetween which can be accessed through one of the snap-fit connection sidewall apertures.



   According to a third aspect of the invention a centrifugal concentrator filter unit dimensioned to be fully containable in a conventional centrifuge tube when sealed with a cap to provide a closed system centrifugal concentrator adapted to concentrate solution without creating external aerosol or fluid contamination, in which the filter unit comprises a generally cylindrical filter tube, which is a loose or sliding fit in the centrifuge tube and which provides a filter reservoir between an open end of the filter tube and a permeable filter membrane disposed across an opening at or near the other end of the tube,

   and an elongate spacer member extending from the filter tube oppositely to the reservoir which in n use abuts a closed end portion of the centrifuge tube to space the filter membrane therefrom and to define a filtrate collection vessel of a predetermined volume at the closed end of the centrifuge tube. Preferably, the length of the elongate member is such that in use the said predetermined volume is less than the volume receivable by the filter reservoir.



   Typically, an end region of the elongate spacer member adapted for attachment to the filter tube is of generally tubular form so as to receive a complimentarily shaped end region of the filter tube and is provided with an inwardly extending abutment on which a peripheral region of the filter membrane can seat and against which the membrane is compresse by an end face at said other end of the filter tube. The whole of the elongate spacer member may be generally tubular in form and the sidewall of the region extending oppositely of the filter tube may be provided with one or more lateral drainage apertures or channels.

   The said end region of the spacer member may be provided with a plurality of circumferentially spaced sidewall apertures adapted to receive, locate and lock respective correspondingly  shaped barbs extending outwardly frcan the filter tube to effect a snap-fit connection therebetween.



   Preferably, the end of the filter tube which compresses the circumferential edge of the membrane is of reduced diameter as compare to a confronting region of the bore of the elongate member so as to define a capillary channel therebetween which can be accessed through one of the snap-fit connection sidewall apertures. An alkoxy-ethyl cyanoacrylate adhesive fluid sealant in which an optical brightener which fluoresces on illumination is dissolved may be introduced into the capillary channel.



   According to a fourth aspect of the invention, a method of concentrating a solution in which a given measure of unconcentrated solution is introduced into the filter reservoir of a filter unit in accordance with the first aspect of the invention; the filled filter unit is then introduced into a centrifuge tube and sealed therein with a cap to form a closed system concentrator; the concentrator is then spun in a centrifuge; subsequently the filter unit is removed from the centrifuge tube leaving a predetermined volume of filtrate therein and a known concentration of solution in the filter reservoir; the filter unit is then inserted upside dvwn into a second centrifuge tube which is subsequently capped and spun in the centrifuge to remove concentrate from the filter reservoir; and the filter unit is then removed to leave the concentrated solution in the second centrifuge tube.



   According to a further aspect of the invention, an assembly of a centrifugal concentrator comprises a filter unit in accordance with the third aspect of the invention, a conventional centrifuge tube and a cap; in which the cap is provided with a formation adapted in use to engage the open end region of the filter tube to prevent radial movement thereof and/or to provide an extraction tool to facilitate removal of the filter unit from the centrifugal tube on removal of the cap. Preferably, the formation has a generally cylindrical outer profile which is a push fit in the bore of the filter tube and p smits venting thereof.



   Embodazients of various aspects of the invention and methods in accordance with the various other aspects of the invention will now be described, by way of examples only, with reference to the accompanying drawings, in which:  
 Figure 1 is an open system centrifugal concentrator of conventional form as aforedescribed;
 Figure 2 is cross-sectional view of a closed system centrifugal concentrator embodying a preferred embodiment;
 Figure 3 is a side view of the preferred enbodiment; and
 Figure 4 illustrates the centrifugal concentrator of Figure 2 both before and after concentration of a solution, in accordance with the method of the fourth aspect of the invention, has taken place.



   The closed system centrifugal concentrator 1, shown in Figure 1, comprises a conventional centrifuge tube 2 of a given size, a filter unit 3 (shown in profile in Figure 3) being the preferred embodiment of the invention which is a loose sliding fit in the tube 2, and a sealing cap 4.



  Clear plastics are normally used for moulding the aforedescribed components except for the cap 4 for which coloured plastics is usual. The concentrator is usable in any centrifuge adapted for use with centrifuge tube 2 and does not require a fixed angle head rotor.



   The filter unit 3 comprises top part 5, lower part 6 and a nano-filtration membrane 7. The lower part 6 is of elongate generally cylindrical form and is open bottomed. Part way along its length it is provided with a radially disposed shelf or abutment 8 through the centre of which extends a drain hole 9 providing a throughbore between parts 5,6.



  Below the shelf 8 the lower part 6 has a bottom portion of elongate form which in use spaces the membrane 7 fran the base of the centrifuge tube 2.



  This bottom portion of part 6 is provided with a plurality of drainage apertures 10 and a drainage groove or channel 11. Groove 11 also assists in locating/orienting the camponent during assembly. Above the shelf 8 part 6 has a cylindrical upper bore the sidewall of which is provided with four circumferentially spaced apertures 12.



   Top part 5 is of generally cylindrical form having a lower portion adapted to be received by the upper bore of the lower part 6. This lower portion has three distinct outer profiles: the first at 13 is a push fit in the upper bore; the second is generally a push fit although is provided with four circumferentially spaced barbs 14 each adapted to be received by a corresponding one of the apertures-12; and the third at 15 providing an end wall formation is of reduced diameter as campared to the confronting wall  formation of the upper bore so as to-cooperate therewith to define an annular capillary channel or groove 16 therebetween.



   The filter unit 2 is assembled by first placing the filter membrane 7 on the shelf 8 of the lower part 6, and then introducing the lower portion of the upper part 5 into the upper bore of the lower part until the barbs 14 snap-fit into the apertures 12. At this point the end 17 of the upper part 5 pins the membrane 7 in place against the shelf 8. The assembled unit defines a filter reservoir 18 above the membrane 7 in which a sample solution can be received. To ensure during operation that fluid can only exit the reservoir through drain hole 9 it is desirable to form a bead of sealant 19 in the annular groove 16.



   Typically, formation of bead of sealant can be achieved by introducing, for example, a fluid cyanoacrylate adhesive/sealant through one or more of the apertures 12 into the groove 16. The dimensions of groove 16 are such that a thin adhesive will be caused by capillary action to travel along and flood the groove producing the continuous bead 19. It has been found that slight agitation of the unit 3 on introduction of the adhesive facilitates the creation of the bead 19.



   Typically, the groove 16 is between 0.20 and 0.35 millimetres across.



  However, it is believed that dimensions ranging between 0.10 and 1.00 millimetres will suffice dependent on ambient conditions of temperature and humidity and on the viscosity of the sealant or adhesive used.



   Generally, when adhesive or sealant is applied by manual or robotic dispensers on to upwardly facing edge or ledge 12A, of one aperture 12 of the lower part 6, a blob of adhesive or sealant will form which once surface tension is broken will disperse into the capillary groove 16. The adhesive or sealant will travel under capillary action in opposite direction around the annular groove 16 until the two flows meet generally diametrically opposite the point of introduction. Desirably, adhesive is introduced simultaneously into two opposing apertures 12 thereby reducing the distance adhesive must travel prior to a complete bead 19 being formed.



   It is always possible that-a perfect bead 19 has not been made.



  Consequently, quality assurance requires that each seal is tested. This  could be done by pressure testing although this method has the disadvantage of requiring a complex test rig and the careful monitoring of pressure decay using transducers and electronic means. Not only would the tooling and equipment be expensive but also the test time would normally be more than 10 seconds per component. Clearly, it is desirable that the integrity of the seal along the bead 19 be tested by swift optical means.



   Use of a coloured adhesive to form the bead 19 has been suggested.



  This may well be effective although has the attendant disadvantage that the bead 19 may not be perfectly formed and any overflow of sealant into the apertures 12 will at best look untidy. The inventor has found that an optical brightener can be dissolved in the otherwise clear cyanoacrylate solution. By using this modifie cyanoacrylate it is possible to test the seal by illuminating the bead 19 with a W light source. The sealant of bead 19 fluoresces under such illumination and the integrity of the seal can readily be observe in about a second. This has proved to a be a rapid and efficient way of testing the bead 19 whilst leaving the glue line transparent in normal light conditions.



   Visual testing can be done typically by quality control personnel directly inspxting the bead 19 under W illumination. Alternatively, a test system utilising an automated computer aided image recognition system could be used. In such automate systems an associated video camera could view the assembled filter unit coaxially thereby utilising the plastics material of the upper and lower parts 5,6 as a lens and the image recognition would need to determine the presence or absence of a complete circular illumination. If the video camera were to view the assembly offset or obliquely then the image recognition would need to determine the presence or absence of a complete elipsoid illumination.



   Typically, for the adhesive an alkoxy-ethyl cyanoacrylate is selected for its high molecular weight resulting in a low volatile characteristic.



  Molecules are not easily released from a bead of adhesive. The commonly available ethyl cyanoacrylate has a low   lecular weight and a high volatile characteristic. The free molecules of the latter form a thin film of adhesive on surfaces close by and will consequently block an ultrafiltration membrane and form a white bloom on transparent/opaque materials. For this  reason the commonly available ethyl cyanoacrylate is not suitable for use in the construction of devices containing ultrafiltration membrane.



   In use it is necessary after concentration to remove the filter unit 3 from the tube 2. To facilitate this the cap 4 is provided with a central boss 20 of generally cylindrical outer profile which is a push-fit in the bore of the upper part 5 of the filter unit 3. Thus, the cap 4 can be used to lift the unit 3 from the tube 2. To prevent the boss 20 sealing the upper part 5 it is provided with a peripheral groove so that in use (as shown in Figure 1) the reservoir 18 is provided with a vent 21 to the bore of the centrifuge tube 2.



   Advantageously, the reduced diameter at 15 on the top part 5 is dimensioned to be an interference fit in its own upper bore, so that it can be used as an extractor tool in place of the cap 4.



   Referring to Figure 4 the preferred method in accordance with the fourth aspect of the invention will naa be described. The closed system concentrator is shows immediately prior to concentration at (A) having an accurately measured volume of dilute solution 22 filling the reservoir 18 so as to be isolated from the bore of tube 2 by. the filter membrane.



  Subsequent, to centrifugal concentration in a swivel head centrifuge the concentrator is as shawn at (B) has a small volume of concentrated solution 23 retained in the reservoir and a greater volume of filtrate 24 filling the bottin of the tube 2 (the apparent volumes shows at (B) are exaggerated for ease of illustration). It will be understood that in the closed system illustrated filtration will occur until the level of filtrate 24 equals the level of solution 23, ie: both converge at level X. The precise position of
X will clearly vary depending on the volume of dilute solution 22 initially placed in the reservoir 18, although for any set volume the position will be the same.

   Consequently, so long as the known volume of solution 23 is greater than the available volume for filtrate below the level of the membrane 7, it is possible to determine the exact concentration of solution that will occur.



   Additionally, it will be appreciated that by having available lower parts 6 of differing dimensions it is possible to provide a range of filter units 3 adapted to provide different concentrations for any given known  volume of dilute solution. Similarly, it is possible to supply different sizes of upper and lower parts 5,6 to fit any standard size of centrifuge tube 2.



   To remove the concentrated solution 23 frcm the reservoir 18, the filter unit 3 can be extracted frcm the tube 2 using the cap 4 and placed inverted into a second centrifuge tube and subsequently spun so as to collect the solution 23 in this second tube.



   It will be further appreciated that the use of a closed system concentrator enables the original sample 22 to be reconstituted by adding the collected concentrated solution 23 once more to the filtrate 23.



   The aforedescribed method would also generally be applicable to conventional open system concentrators so long as in operation the criteria is fulfilled that the filter membrane and adjacent portion of the filter reservoir were contained within, ie: disposed below the upper level of, the filtrate reservoir. Also, it will be appreciated that the use of a capillary channel to facilitate formation of a seal could be applied to broader applications in the field of medical equipment technology or elsewhere.



   Although, the casting of a bead of sealant aspects of the invention have been described in relation to a centrifugal filter unit of aforedescribed form, it will be appreciated that these aspects of the invention are equally applicable to filter units having different formations and uses. In particular the invention is applicable to filters commonly referred to as syringe filters and to filters having non-medical applications.



   It should be understood that the invention as herein claimed is but one invention embodied in the design of centrifugal concentrator unit aforedescribed and that the Applicant reserves where applicable the right to file divisional Applications based on any subject matter contained herein and in particular based on the subject matter encompassed by statements of inventions filed which are not at present coterminous with the claims on file.

Claims

CLAIMS: 1. Method of casting a polymer seal between two mating parts of a filter housing having a throughbore extending therebetween and a filter element retained between confronting formations of the two housing parts to span the throughbore, in which during assembly the two housing parts are mated together to retain the filter element between said confronting formations and characterised in that;

confronting peripheral wall formations of the two mating parts cooperate to define a capillary channel extending around the periphery of the filter element into which a peripherally outer portion of the filter element extends, and in that an aperture extends through the side wall of one of the said two mating parts of the filter housing permitting the introduction of a free-flowing polymer adhesive or sealant into the capillary channel, the arrangement providing that the adhesive is caused by capillary action to flow throughout and to generally fill the channel thereby contacting the peripherally outer portion of the filter element so that on curing of the adhesive or sealant a bead seal is formed between the said filter housing parts and the filter element.

2. Method in accordance with claim 1, characterised in that; a second aperture extends through one or other of the two parts of the filter housing to provide a breather port to the capillary channel permitting air to exhaust therefrom as the channel fills with the adhesive or sealant.

3. Method in accordance with claim 1 or claim 2, characterised in that; on curing the bead seal rigidly attaches said one of the two parts of the filter housing to the other part thereof.

4. Method in accordance with any one of the preceding claims, characterised in that; the filter element is a generally flat membrane the peripherally outer portion of which on assembly of the two parts of the filter housing is caused to seat in the base of the capillary channel.

5. Method in accordance with any one of the preceding claims, characterised in that; the filter element is a microporous filter membrane typically used in microfiltration or ultrafiltration of biological fluids.

6. Method in accordance with any one of the preceding claims, characterised in that; one of the two parts of the filter housing is at least translucent and the sealant or adhesive contains an optical brightener and the method further ccamprises quality control involving illuminating the assembled filter housing with ultra-violet light to permit the integrity of the bead seal to be adjudged by visual means.

7. Method in accordance with claim 6, characterised in that; said visual means comprises an automate computer aided image recognition system.

8. Method in accordance with any one of the preceding claims, characterised in that; the adhesive or sealant is cyanoacrylate polymer adhesive moisture curable at rouan temperature.

9. Method in accordance with claim 8, characterised in that; the capillary channel has a width of between 0.1 millimetres and 1.0 millimetres.

10. Method in accordance with claim 9, characterised in that; the capillary channel has a width of between 0.20 millimetres and 0.35 millimetres.

11. A filter unit comprising a housing having two parts thereof retaining a filter element therebetween, characterised in that; a bead seal is provided cast in accordance with the method of any one of the preceding claims.

12. A centrifugal concentrator filter unit in accordance with claim 11, characterised in that; the housing is dimensioned to be fully containable in a conventional centrifuge tube when sealed with a cap to provide a closed system centrifugal concentrator adapted to concentrate solution without creating external aerosol or fluid contamination, the housing comprises a male first part in the form of a generally cylindrical filter tube,

which is a loose or sliding fit in the centrifuge tube and which provides a filter reservoir between an open end of the filter tube and the filter element being a permeable filter membrane disposed across an opening proximate the other end of the tube and the housing further comprises a female second part in the form of an elongate spacer member extending from the filter tube oppositely to the reservoir which in use abuts a closed end portion of the centrifuge tube to space the filter membrane therefrom and to define a filtrate collection vessel of a predetermined volume at the closed end of the centrifuge tube.

13. A concentrator filter unit in accordance with claim 12, characterised in that; an end region of the elongate spacer member adapted for attachment to the filter tube is of generally tubular form so as to receive a complimentarily shaped end region of the filter tube and is provided with an inwardly extending abutment formation on which the peripherally outer portion of the filter membrane can seat and against which the membrane is compressed by an end face formation at said other end of the filter tube.

14. A concentrator filter unit in accordance with claim 13, characterised in that; said end region of the spacer mamber is provided with a plurality of circumferentially spaced sidewall apertures adapted to receive, locate and lock respective correspondingly shaped barbs extending outwardly from the filter tube to effect a snap-fit connection therebetween.

15. A concentrator filter unit in accordance with claim 13 or claim 14, characterised in that; the end wall of the filter tube which compresses the membrane is of reduced diameter as compare to a confronting wall region of the bore of the elongate member so as to define the said capillary channel therebetween which can be accessed through one of the snap-fit connection sidewall apertures.