A reusable microconcentrator device may be disassembled, cleaned and reassembled before use. The microconcentrator device of the invention is typically useful in filtering and separating small samples of solutions and suspensions. The microconcentrator device of the invention includes an elongate sample sleeve, a base element which can be removable and replaceably coupled to a centrifugal portion of the sleeve, a compliant membrane compressing feature, and a disposable membrane. Preferably, the base element includes an O-ring which compresses the membrane to seal it to the centrifugal surface of an annular ledge portion of the sleeve, which also forms a deadstop. Among the advantages of the microconcentrator device of the invention is that a reliable retentate volume remains following filtration. The elements of the microcentrator device can also be permanently assembled to one another.

BACKGROUND OF THE INVENTION 
The invention relates to a centrifugal filtration device for separating and 
concentrating macromolecules or particles, such as viruses, from a 
solution or suspension. More particularly, the invention relates to 
centrifugal ultrafiltration devices that protect against filtration to 
dryness. 
Many chemical and biochemical techniques require separation of 
macromolecules or particles from a solution or suspension. Many such 
techniques involve the removal of buffer and salts from a protein solution 
to yield a highly concentrated protein sample that is to be further 
analyzed or utilized. 
A technique commonly used for concentrating such solutions is centrifugal 
ultrafiltration. One problem that is inherent to the process of subjecting 
samples of a solution or suspension to centrifugal ultrafiltration 
techniques is the potential that the sample will be filtered to dryness. 
Filtration to dryness can reduce the biological activity of any retained 
macrosolute (e.g., protein) and can also reduce the total mass recovery of 
macromolecules. Attempts to re-dissolve the retained macrosolute after 
filtration to dryness, by adding a buffer, are not always effective to 
overcome the reduced biological activity or the reduced recovery. It is 
most effective to cease filtration once a desired final retentate volume 
is achieved. 
This end is well served by the "deadstop" means of the microconcentrator 
taught by Bowers and Rigopulos in U.S. Pat. No. 4,632,761. This deadstop 
means is a hydrostatic barrier created by the membrane support base which 
has filtrate duct or ducts offset inward from the edge of the wall of the 
sample sleeve. When used in a fixed angle centrifuge rotor and the 
retentate meniscus reaches the centrifugal radial level of the outermost 
edge of the outermost filtrate duct, filtration stops due to the 
counterbalancing hydrostatic pressure of filtrate contained between the 
remaining wetted membrane area and the membrane support base. This design 
has the significant advantage of offering maximal available membrane 
surface area and resulting filtration rate for a given size flat membrane 
disk fitting into a given device tube diameter. 
An alternative known deadstop means is to provide an impermeable cover or 
coating to a portion of the membrane area. U.S. Pat. No. 3,817,379 
describes an impermeable coating on a portion of the membrane in the 
bottom of an adsorbent-driven concentrator. This coating only partially 
impedes filtration, due to the wicking by surface tension seen at ambient 
gravity, which allows continued slower filtration after the meniscus 
reaches the coated portion of the membrane. Several known centrifugal 
concentrators now provide a plastic molded deadstop ledge as part of the 
retentate chamber. Centrifugal devices have the advantage that wicking is 
eliminated due to the higher gravitational field of the centrifuge. 
However, the membrane area covered by the deadstop ledge is unavailable 
for filtration, which results in a lower filtration rate, compared to the 
same size membrane contained in a device with a hydrostatic deadstop. 
The hydrostatic deadstop design of U.S. Pat. No. 4,632,761 offers the 
additional benefit of membrane seal reliability. Because the deadstop 
means is beneath the membrane, in larger volume devices a conventional 
O-ring may be used above the membrane to form a seal between the delicate 
solute-rejecting top skin of the ultrafiltration membrane and the outer 
edge of the retentate sleeve, to prevent some volume of the concentrated 
macrosolute from being lost by seeping through the perimeter seal into the 
filtrate tube. The compliant O-ring elastomer provides a uniform crushing 
of the porous membrane, and easily corrects for small differences in 
membrane thickness or dimensions of the retentate sleeve and membrane 
support. 
With impermeable deadstop centrifugal microconcentrators, and even for 
hydrostatic deadstop devices of smaller device sizes, it is not practical 
to use an O-ring to form the seal between the membrane top skin and the 
retentate sleeve. The axial thickness of the O-ring occupies the volume 
just above the membrane needed for retentate storage with an impermeable 
deadstop. Furthermore, in smaller devices, the radial thickness of the 
ring becomes too large a fraction of the total disk area, resulting in 
unacceptably slow filtration rates. Another objection to O-rings used as 
seals above the membrane has been the potential for contamination of 
macrosolute retentate by oils added to many elastomer formulations as 
processing aids. 
These factors have led to designs for smaller volume microconcentrators 
which seal the ultrafiltration membrane by simply squeezing it between the 
retentate sleeve and the membrane support. Variations in membrane 
thickness, in molded sleeve and membrane support dimensions, and in 
assembly processing make consistent reliable crush sealing quite 
challenging, and seal leakage has been a significant ongoing problem with 
many noncompliant crush-sealing designs. 
Currently available centrifugal microconcentrator devices, which are 
designed to concentrate volumes of a few milliliters or less, are all 
manufactured as preassembled disposable integral units. These devices can, 
at best, be reused only a few times by washing out remaining retentate 
product to minimize carryover between samples. The need to use disposable 
microconcentrator devices for centrifugal filtration applications can add 
significantly to the cost of basic research and other procedures that 
require separation and concentration of macromolecules and other suspended 
solids from small volumes of liquids. 
Accordingly, an object of the present invention is to provide a 
microconcentrator device for use in centrifugal ultrafiltration 
applications that avoids the problems associated with filtering to dryness 
by yielding a reliable fixed volume of retentate. A further object of the 
invention is to provide a compliant microconcentrator device that has good 
sealing properties so as to prevent the seepage of unfiltered sample into 
the filtrate cup. It is also an object of the invention to prevent 
contamination of macrosolute retentate by oils from an elastomeric seal. 
Another object of the invention is to provide a reusable microconcentrator 
device that can be simply and reliably assembled, disassembled and 
reassembled by the user with fresh membranes of a variety of different 
pore sizes. Yet another object of the invention is to maximize the 
available membrane surface area, consistent with other objects. These and 
other objects will be apparent to one of ordinary skill in the art upon 
reading the disclosure that follows. 
SUMMARY OF THE INVENTION 
The invention provides a microconcentrator device for use in centrifugal 
ultrafiltration applications. Among the advantages of the 
microconcentrator device of the invention is that the microconcentrator 
device provides a reliable retentate volume, thus eliminating problems 
associated with filtering to dryness. Further, in one embodiment the 
microconcentrator device is reusable and may be easily disassembled, 
cleaned and reassembled by the user with a fresh membrane. 
The microconcentrator device of the invention is useful for separating and 
concentrating macromolecules or particles from a solution or a suspension. 
The microconcentrator device of the invention fits within a filtrate 
collection tube that has a closed centrifugal end and an open centripetal 
end. The microconcentrator device comprises an elongate sleeve that is 
matable into the filtrate collection tube. The sleeve is of a 
substantially cylindrical shape and has openings at centripetal and 
centrifugal ends thereof. Further, the elongate sleeve includes an annular 
ledge disposed at a centrifugal portion of the sleeve. A base element can 
be removably mountable upon the centrifugal end of the sleeve, adjacent to 
the centrifugal side of the ledge. The base element has a centripetal 
surface and a centrifugal surface with one or more apertures or flow ports 
extending therethrough. The device also includes a coupling arrangement 
that succeeds in securely mounting the base element to a centrifugal end 
of the sleeve such that the base element is joined to the sleeve while 
abutting the centrifugal side of the annular ledge. The base element must 
be coupled to the sleeve in a manner that prevents seepage of unfiltered 
sample. 
In one embodiment, a removable membrane is adapted to be placed between the 
centripetal surface of the removable base and the centrifugal surface of 
the annular ledge. This embodiment envisions the mounting of a gasket on 
the centripetal surface of the base element in contact with the 
centrifugal (inactive) surface of the membrane to enhance sealing 
characteristics. 
In one embodiment the centripetal surface of the base element has a 
plurality of concentric, raised circular ridges with the apertures or flow 
channels disposed between the ridges. Alternatively, the centripetal 
surface of the base element has a convex cross section and flow channels 
in the surface of the base element are formed within one or more stepped 
shoulders. 
The microconcentrator device of the invention may also exist in the form of 
a disposable embodiment in which the elements noted above are 
substantially permanently assembled. In this embodiment the device cannot 
normally be disassembled and reassembled.

DETAILED DESCRIPTION OF THE INVENTION 
As illustrated in FIG. 1, microconcentrator device 10 includes a sample 
sleeve 14, a removable base element 16, an O-ring or gasket 40, a 
semipermeable membrane 18, and a closure cap 20. The microconcentrator 
device is adapted to fit, at least partially, within filtrate collection 
tube 12. In one embodiment, the various elements that make up the 
microconcentrator device may be conveniently disassembled and reassembled. 
This feature permits the microconcentrator device to be reused simply by 
disassembling the device, cleaning the various components, and 
reassembling the device while using a new membrane. Alternatively, the 
components of the microconcentrator device can be substantially 
permanently assembled such that it is not normally possible to disassemble 
the device. 
The microconcentrator device of the invention may be used in well known 
ultrafiltration techniques to concentrate, separate and purify small 
volume samples of solutions and suspensions, such as proteins, viruses, 
and nucleic acids. The device can be used with ultrafiltration membranes 
having desired retention characteristics. 
Among the advantages of the microconcentrator device of the invention is 
its reusability. This feature renders the device economical to use and 
thus significantly reduces costs in comparison to conventional disposable 
devices. Despite the ability of the microconcentrator device to be 
disassembled and reassembled, the device possesses suitable sealing 
properties such that separation and concentration techniques can be 
reliably performed with little or no risk that macro solute will seep from 
the sample sleeve to the filtrate collection tube. In addition., the 
microconcentrator device features an impermeable deadstop region which 
prevents filtration to dryness and ensures that a retentate volume in the 
range of 3-30 .mu.L, and most preferably about 3-5 .mu.L, will remain in 
the sample sleeve. The deadstop region is constructed such that it ensures 
that retentate will not be lost through leakage, for example. 
As shown in FIG. 1, the microconcentrator device 10 includes an elongate 
sample sleeve 14 having centripetal and centrifugal ends 22, 24. When 
assembled for use, the sleeve is placed inside a filtrate collection tube 
12, which is substantially conically shaped. A centripetal portion of 
sleeve 14, including centripetal end 23, protrudes from the open, 
centripetal end 26 of collection tube 12. A vented cap 20 seals the 
opening in the centripetal end 22 of the sample sleeve to prevent or 
minimize spillage and evaporation during or after filtration procedures. 
FIGS. 2-5 further illustrate the microconcentrator device of the invention. 
The centrifugal end 24 of sleeve 14 includes an interior, annular ledge 28 
which extends from the interior wall 30 of sleeve 14 and which has 
centripetal 39 and centrifugal 38 surfaces. Two or more legs or tabs 32 
form a part of the centrifugal end 24 of sleeve 14 and extend below the 
ledge 28. The tabs 32 include surface features 34 that are able to engage 
the removable base element 16 to couple the base element 16 to the sleeve 
14. In an embodiment illustrated in FIGS. 3A and 4, surface features 34 
are in the form of a centripetal-facing shoulder 35. The base element 16 
is coupled to the sleeve such that the semipermeable filtration membrane 
18 is sandwiched between the centripetal surface 36 of base 16 and the 
centrifugal surface 38 of annular ledge 28. Preferably, a gasket or O-ring 
40 may be mounted on the centripetal surface 36 of the base to abut the 
centrifugal surface 42 of the membrane 18. 
In one embodiment, as illustrated in FIG. 3A, the portion of the sleeve 
centripetal to the ledge 28 is of a substantially constant diameter along 
virtually all of its length. However, as illustrated, the diameter 
increases in one or more steps at the centripetal end 23 of the sleeve. 
As illustrated in FIGS. 2-5, tabs 32 extend below annular ledge 28 to form 
the bottom-most portion of the sleeve. Preferably, the tabs 32 are 
somewhat compliant, permitting them to be flexed outwardly when subjected 
to a sufficient force. This force imparted to tabs 32 increases the inner 
diameter of that portion of the sleeve defined by the tabs to an extent 
sufficient to allow the base element 16 to be inserted within the space 
defined by the tabs 32. The tabs include one or more surface features, 
such as centripetal-facing shoulder 35, which engages the base 16 to 
couple it to the sleeve 14 once the tabs return to their natural position. 
FIGS. 1, 3B, 5, 9 and 10 illustrate an alternative embodiment of the sample 
sleeve 14. As shown, the portion of the sleeve centripetal to the annular 
ledge 28 has a narrow diameter region 50 and a more centripetally disposed 
wider diameter region 52. At least two compliant extensions 54 (similar to 
tabs 32) extend from the region of the sleeve centrifugal to ledge 28. 
Extensions 54 include an aperture 56, which preferably is elongated in the 
axial direction. 
The annular ledge 28 should be oriented such that in its normal position, 
when not subjected to any forces, the centripetal 39 and centrifugal 38 
surfaces of ledge 28 are angled at between about 0.degree. and 89.degree. 
centripetal to a line drawn perpendicular to an interior wall 30 of the 
sleeve. More preferably these surfaces are angled at about 0.degree. to 
60.degree.. These surfaces may also be angled up to about 15.degree. below 
(centrifugal) to a line drawn perpendicular to an interior wall 30 of the 
sleeve. It is understood that the centripetal 39 and centrifugal 38 
surfaces of the ledge 28 need not be disposed at the same angle. For 
example, FIG. 1 illustrates a design in which centrifugal surface 38 is 
oriented at about 45.degree. while centripetal surface 39 is oriented at 
about 8.degree.. The angles referred to herein, unless otherwise 
indicated, are understood to be centripetal to a line dram perpendicular 
to an inner wall 30 of the sleeve. 
The inclined (e.g., approximately 45.degree.) angle of the centrifugal 
surface 38 of ledge 28 is believed to provide an advantageous sealing 
compressive force to the centripetal surface of membrane 18. 
In one embodiment, illustrated in FIG. 3B, ledge 28 includes a 
substantially vertically oriented annular lip 31. The lip 31 extends by 
approximately 0.005 to 0.02 inch above the base surface 33 of ledge 28. 
Lip 31 is useful in that it further ensures that retentate of a desired 
volume will remain within the sample sleeve, retained on ledge 28. As the 
angle of surface 39 becomes rather steeply inclined, lip 31 may be 
unnecessary. 
Preferably, the sample sleeve is able to accommodate a volume of fluid 61 
in the range of approximately 0.5 to 1.0 mL, and more preferably about 1.0 
mL. These sleeves for the present device are adapted to fit within 
standard 1.9 mL microcentrifuge tubes, however a range of larger and 
smaller devices are intended to be covered by this invention to handle 
larger and smaller volumes in centrifuge rotors made for larger or smaller 
tubes. 
By way of example, the diameter of the sleeve, in a region adapted to fit 
within the collection (microcentrifuge) tube 12 is in the range of 
approximately 0.28 to 0.35 inch. The diameter of a wider portion of the 
sleeve that does not fit within the collection tube is non-critical and 
can be in the range of about 0.35 to 0.50 inch. Typically the sleeve has a 
length, measured from the centripetal-most part of the sleeve to annular 
ledge 28, in the range of about 0.80 to 1.12 inch. The length of the tabs 
32 and extensions 54 is typically about 0.15 to 0.30 inch. The annular 
ledge preferably extends toward the center of the sleeve by a distance of 
about 0.03 to 0.10 inch for devices designed to fit 1.9 mL microcentrifuge 
filtrate tubes. 
As noted above, the collection tube 12 can be virtually any standard 
microcentrifuge tube. Preferably, the collection tube is a 1.9 mL 
microcentrifuge tube. Collection tube 12 is substantially conically shaped 
having a closed centrifugal end 58 and an open centripetal end 60. As 
illustrated in FIG. 1 sleeve 14 is positioned within the open end 60 of 
collection tube 12 in a secure, frictional fit. One of ordinary skill in 
the art will appreciate that surface features are normally present on the 
outer walls of the sleeve, or on the inner walls of the collection tube, 
to allow venting of the collection tube and also to facilitate easy 
removal of the sleeve from the collection tube. The surface features can 
be in the form of ridges, channels, protrusions, and the like. 
The tabs 32 or extensions 54 disposed on the centrifugal end 24 of sleeve 
14 can also serve as flexible, self-venting features of the sleeve. As 
noted above, tabs 32 and extensions 54 are somewhat compliant. That is, 
the tabs 32 or extensions 54 can be flexed outwardly to increase the inner 
diameter of the area defined by tabs 32 or extensions 54 in order to fit 
the base element 16 within this area. Once the base is properly 
positioned, the tabs 32 or extensions 54 return to their normal position 
to firmly engage base 16. The positioning of the centrifugal end 24 of 
sleeve within a filtrate collection tube also forces tabs or protrusions 
32, 54 to compress the side walls of the base 16. This ensures that the 
sleeve 14 will securely mate within a variety of filtrate collection tubes 
having mouths of varying inner diameters, and that the base will be 
securely mounted within the sleeve. 
A cap 20 is also provided to mate with the centripetal, open portion of the 
sleeve 14 to prevent spillage and/or evaporation of any fluid contents 
before, during or after centrifugation. As illustrated in FIGS. 8 and 9, 
the cap 20 may be attached by way of hinge 64 to a centripetal portion of 
the sleeve 14. This design is useful with respect to sleeves, such as 
those illustrated in FIGS. 3B, 9 and 10, which have a centripetal portion 
extending well above the open, top (centripetal) portion of the collection 
tube 12. 
Base element 16 is adapted to be removably and replaceably coupled to a 
portion of the sleeve adjacent to and centrifugal to the annular ledge 28. 
Base 16 is a substantially disk-shaped object having a centripetal surface 
36 and a centrifugal surface 37. One or more apertures 70 extend between 
the centrifugal and centripetal surfaces of the base element to provide 
flow conduits for filtrate. The base also includes one or more surface 
features 72 that interact with surface features 34 of tabs 32 and/or with 
apertures 56 of extensions 54 to permit coupling of the base to the 
sleeve. 
In one embodiment, illustrated in FIG. 4, the surface feature 72 of base 16 
is in the form of an annular, centrifugally-facing shoulder 74. As 
illustrated, a centrifugal-most portion of the base is of a slightly 
smaller outside diameter than is the remainder of the base, thus creating 
centrifugal-facing shoulder 74. Shoulder 74 is adapted to engage the 
complementary surface feature 34 of tabs 32. Shoulder 74 is approximately 
0.015 to 0.03 inch in width. 
Alternative surface features that can be formed on base 16 are illustrated 
in FIGS. 5 and 11-14. As illustrated, base 16 may have at least two 
outwardly flared protrusions 78 that fit within apertures 56 of the 
compliant extensions 54 formed in the sleeve of the type illustrated in 
FIGS. 1, 5, 9 and 10. 
The centripetal surface 36 of base 16 is designed to promote flow of 
filtrate to and through the apertures 70. Preferably, the centripetal 
surface 36 of the base is of a substantially convex cross section. As 
illustrated in FIGS. 6 and 7, the centripetal surface 36 may have a 
plurality of concentric grooves 80 that are separated by concentric raised 
ridges 82. One or more apertures 70 preferably is disposed within each of 
the grooves 80. Apertures 70 typically are of a small diameter in the 
range of about 0.01 to 0.02 inch. 
Alternatively, as illustrated in FIGS. 11-13, the centripetal surface 36 of 
base 16 may include a central raised ridge 84 and have one or more 
apertures 86 disposed radially outward from the ridge 84. Apertures 86 
generally are of a diameter somewhat larger than apertures 70, thus making 
it possible to use fewer apertures. Apertures 86 have a diameter in the 
range of about 0.025 to 0.06 inch. 
FIG. 14 illustrates another embodiment of base 16 that is specifically 
adapted to house an O-ring 40. In this embodiment; base 16 includes a 
tapered, annular spool 88 formed adjacent (and centripetal to) 
intermediate surface 43. An O-ring (not shown) is mounted around spool 88 
such that a bottom (centrifugal) surface of the O-ring abuts intermediate 
surface 43 of base 16. One or more filtrate flow ducts 90 are formed in 
the centripetal surface 36 adjacent the tapered annular spool 88, 
extending up the tapered annular walls of spool 88 as grooves to conduct 
filtrate past the inner surface of the O-ring. 
The membrane useful with the present invention can be virtually any 
semipermeable anisotropic (skinned) ultrafiltration membrane. Suitable 
membrane materials include polysulfone, polyether sulfone, cellulose 
esters, and regenerated cellulose polymers. Exemplary commercially 
available membranes include the MILIPORE BIOMAX, the AMICON YM and the 
FILTRON OMEGA. Obviously, the filtration device of the invention is 
intended to accept membranes having a variety of desired molecular weight 
cutoffs and other properties useful in filtration and separation. 
One of ordinary skill in the art can readily determine a membrane diameter 
suitable for use with a microconcentrator device of the present invention. 
Generally, the membrane should have a diameter that, on the large end, is 
at least about equal to or slightly greater than the inside diameter of 
the sample sleeve. On the small end, the membrane diameter can be slightly 
greater than the diameter of the flow path defined by the annular ledge 28 
plus the width of one side of the annular ledge 28, thereby ensuring that 
even with maximal misalignment of the membrane and device axes during 
assembly there will not be a void space in which ledge 28 fails to crush 
and seal the membrane. In a preferred embodiment the membrane diameter is 
approximately 9/32 inch. The membrane should be of sufficient thickness to 
be crushable between the base and the annular flange in order to create a 
sufficient seal and to prevent leakage of unfiltered sample into the 
filtrate collection tube. Generally, the membrane thickness is in the 
range of 0.008 to 0.012 inch. 
As noted above, the microconcentrator device of the invention is reusable. 
As a reusable device, it must be able to be easily disassembled to 
facilitate cleaning, and then easily reassembled. The device may be 
assembled for use as follows, with reference to FIG. 1, unless otherwise 
noted. 
The sleeve 14 is most conveniently inverted upon a work surface, and a 
suitable ultrafiltration membrane 18 is placed on the centrifugal surface 
38 of the ledge 28, with its skin side facing down, in the centripetal 
direction. The base 16 is then pushed down into the sleeve by hand, or 
with a suitable insertion tool. 
In one embodiment, illustrated in FIG. 4, the shoulders 35 of tabs 32 
engage the shoulders 74 of the base 16. This engagement imparts a force on 
the base 16 which causes membrane 18 to be securely held between the 
centripetal surface 36 of the base 16 and the centrifugal surface 38 of 
the ledge 28. This engagement provides a suitable seal between the 
membrane, base and the ledge to prevent any leakage of any unfiltered 
sample. In one embodiment, the coupling force imparted upon the base also 
causes the ledge 28 to deflect and to flex upwardly (in the centripetal 
direction) up to an additional 30.degree., creating a stress in the sleeve 
ledge 28 and wall which serves to maintain the crushing seal between ledge 
28 and the membrane skin. 
As illustrated in FIG. 5, O-ring 40, when desired for use, may be placed 
beneath (centrifugal to) the membrane 18 to energize the sealing crush of 
the membrane against ledge 28. The O-ring 40 may be positioned on the 
centripetal surface 36 of the base such that it engages the centrifugal 
side 42 of the membrane. Although not illustrated, the O-ring may also be 
used to augment the seal shown in the embodiment illustrated in FIG. 4. 
FIG. 5 illustrates the use of a base 16 which includes a tapered annular 
spool 88 that houses O-ring 40. As shown, the membrane 18 mounts atop the 
centripetal surface 36 of base 16 and the centripetal surface 41 of O-ring 
40. 
The O-ring preferably is a ring-shaped object that has an outside diameter 
sufficient to compress the membrane 18 against the centrifugal surface 38 
of ledge 28. The inside diameter of the O-ring should be such that it fits 
around spool 88, without falling off, when the device is disassembled. 
Preferably the relaxed outside diameter of the O-ring is about 0.25 inch 
and the inside diameter of the O-ring is approximately 0.125 inch, as in a 
standard commercial ring size 2-006. The O-ring has a thickness of about 
0.03 to 0.08 inch, and preferably a thickness of about 0.07 inch. The 
O-ring can be made of virtually any elastomeric material. A preferred 
material is an ethylene propylene (EPR or EPDM). 
The device design shown in FIGS. 1 and 14 has the advantage of a dramatic 
increase in available filtration surface area and resulting filtration 
rate due to the use of a steeply angled (45 degree) centrifugal surface 38 
of ledge 28. This design fits a standard 1.9 mL microcentrifuge tube. When 
tested at 12,000 rcf using a commercial polysulfone 10 kD membrane it has 
been found to concentrate 1 mL (maximal capacity) of 0.1% Blue Dextran 
down to a deadstop volume 67 of 4 .mu.L in less than 30 minutes without 
visible leakage. The initial 2.times. volume reduction was accomplished in 
5 minutes, due to the 214 psi initial transmembrane pressure generated by 
the 0.5 inch meniscus height of 1 mL in this design mounted in a 45 degree 
microcentrifuge at 12,000 ref. The 45 degree centrifugal ledge surface 
angle (centripetal to a line drawn perpendicular to a wall of the sleeve) 
design achieves an active filtration surface area of 0.0506 in.sup.2. 
By contrast, a design with a 16 degree ledge surface angle (centripetal to 
a line drawn perpendicular to a wall of the sleeve) having equivalent 
initial and deadstop ledge volume has an active surface area of only 
0.0278 in.sup.2. This is due in part to the larger inner diameter of a 
ledge having a centrifugal surface disposed at a 45 degree angle, made 
possible, for the given 4 .mu.L deadstop volume, by the downward angle of 
the centripetal surface of ledge 28, which results in a 1.28 fold increase 
in active area over the ledge design in which the centrifugal surface of 
the ledge is disposed at 16 degree angle. The fact that the membrane is 
able to be formed into a smoothly curved dome shape by spool 88, O-ring 
40, and ledge surface 38, when the centrifugal ledge surface is disposed 
at 45 degrees, results in a further 1.55 fold increase in active area over 
the equivalent diameter flat membrane. The overall result is a 1.98 fold 
increase in active filtration area. 
Once assembled the microconcentrator device 10 of the invention can be used 
in a typical manner. Following filtration, the ledge enables a reliable 
volume of retentate 67, approximately 3 to 30 .mu.L, and more preferably 
about 3-5 .mu.L, to remain in the sample sleeve 14, thus preventing 
filtering to dryness. 
After use disposable versions of the device can be disassembled and the 
membrane can typically be removed and discarded. The various components of 
the microconcentrator device may be cleaned, such as with a detergent 
solution, and reused. Those of ordinary skill in the art will appreciate 
that the microconcentrator device of the invention can be manufactured in 
disposable versions as well in which the various elements of the device 
are substantially assembled. 
FIGS. 15 and 16 illustrate convenient designs for filtration membranes and 
a filtration membrane dispensing system useful with the microconcentrator 
device of the invention. 
As illustrated (FIG. 15), membrane 18 includes a non-filtering handling tab 
92 which is appended to a filtering portion 93 of membrane 18 by bridge 
94. Preferably, the filtering portion, tab and bridge are integrally 
formed. The tab serves a useful function in that it promotes easy handling 
of the filtering portion of the membrane for positioning the membrane 
within a microconcentrator device and for removing the membrane from a 
microconcentrator device. This feature allows the membrane to be easily 
and conveniently handled and manipulated without contacting or 
contaminating the sensitive skin surface of the membrane. The tab can be 
of a variety of useful shapes, such as substantially circular or 
rectangular. 
The tab may also be used as a surface upon which to affix indicia 96 
concerning membrane properties such as molecular weight cutoff. Indicia 
may be affixed to the tab with, for example, a heated die to discolor the 
tab with an image representing the desired information concerning membrane 
properties. Indicia 96 may also be formed on the tabs 92 by printing 
techniques. Preferably, it has been found that indicia 96 may be formed 
simply by die stamping an impression which compresses the tab and 
substructure to cause the affected portions of the tab to become 
transparent. 
Generally, the size of tab 92 is non-critical and suitable dimensions can 
be determined by one of ordinary skill in the art. In preferred 
embodiments the tab and bridge are combined into a rectangular shape which 
is about 0.125 inch wide by 0.4 inch long. 
In one embodiment, illustrated in FIG. 16, a plurality of membrane-tab 
units can be affixed to a dispenser strip 98. Preferably the filtering 
portion 93 of the membrane lies on a non-adhesive portion 100 of the 
dispenser strip 98. The tab and bridge elements 92, 94 may be disposed on 
an adhesive portion 102 of the dispenser strip. In addition, a protective 
release strip 104 may be disposed over the dispenser strip so as to cover 
the membranes, bridges and tabs. In many cases the adhesive present on the 
dispenser strip is sufficient to maintain the release strip 104 in place. 
Alternatively, adhesive may be applied to cover all or part of the 
dispenser strip- and membrane-contacting surface of the release strip 104. 
Adhesives suitable for this purpose include repositionable and transfer 
silicones and acrylics, which are well known in the art. Suitable examples 
include the Drybonder.TM. instant tack repositionable transfer adhesive 
system manufactured by Chartpak, and Post-it.TM. 658 correction and 
cover-up tape and Scotch.RTM. brand 2070 SAFE RELEASE masking 
repositionable tapes both manufactured by 3M. 
Once assembled in this manner the membranes, dispenser strip and release 
strip may be perforated to facilitate removal of waste tape after use, and 
collected on a reel or spool for easy handling and dispensing. 
The following non-limiting examples serve to further illustrate the 
invention. 
EXAMPLES 
Membrane and device flux comparisons were performed using a 0.1 g/dL 
solution of Blue Dextran in distilled water. This solute, obtained from 
Sigma Chemical Company, has an average molecular weight of 2,000,000 
daltons, and is made visible by incorporation of 0.1 mmol Reactive Blue 
dye/g dextran. A 0.001 g/dL solution was clearly visible by eye, 
permitting convenient confirmation of &gt;99% membrane retention and seal 
integrity by visual inspection of filtrates. 
EXAMPLE 1 
FIG. 17 is a comparison of microconcentrator devices according to the 
design of FIG. 1, having a 45 degree membrane sealing ledge (centrifugal 
surface of annular ledge). Modified polysulfone 10,000 dalton cutoff 
ultrafiltration membranes (here designated as P10) and regenerated 
cellulose 10,000 dalton cutoff ultrafiltration membranes (here designated 
as RC10) were tested with 1 mL starting volumes in an Eppendorf 5412 
microcentrifuge. The 1.9 mL filtrate tubes were initially weighed, and 
weighed following each successive 5 min spin. Filtration rate is faster 
for 0.1% Blue Dextran with the P10 membrane, with maximal concentration 
seen in about 25 min. The RC10 regenerated cellulose membrane, known to be 
less porous, took an additional 10 min to reach maximal concentration. 
EXAMPLE 2 
FIG. 18 is a similar study of the microconcentrator device of FIG. 1 using 
the P10 membranes compared to two commercially available disposable 
microconcentrator devices. Comparative Device 1 has an impermeable 
deadstop and a large surface area vertically-mounted P10 membrane. 
Comparative Device 2 is a deadstop-free, high filtration rate P10 
microconcentrator. All devices were tested with 0.5 mL starting volume, 
and filtrate weights were again measured successively. As seen in Example 
1, the flux curves for the FIG. 1 devices again required 15 min to go from 
0.5 mL down to about 30 .mu.L. In this study, all devices tested reached 
equivalent retentate volumes below 40 .mu.L in 15 min, with no visual 
leakage. 
The invention being thus disclosed, other variations and modifications, as 
well as adaptations to known microconcentrator devices will occur to those 
having ordinary skill in the art. All such variations, modifications and 
adaptations are within the spirit and scope of the invention as defined in 
the claims appended hereto. 
The entirety of all references noted herein are expressly incorporated by 
reference.