The present invention relates to the filtration field, and more particularly, to an improved centrifugal filtration apparatus for filtering and concentrating a solution. The improvement comprises eliminating any retentate pockets below the filter and instead employing a port on the filtrate side of the filter that is above the bottom level of the filter, thereby not permitting filtering to dryness.
The filtration of fluids may be accomplished through the use of filtration devices which utilize microporous filters to filter and concentrate a macro-molecular solution is well known. This technique has been used in centrifugal filtration apparatuses that rely on centrifugal forces to create pressure in the apparatus to force solutions through a filter which separates liquid solutions into filtrate and concentrate.
There are certain drawbacks, however, associated with conventional centrifugal filtration apparatus. While many apparatus designs can prevent filtering to dryness in certain applications, they do not in other applications, such as a swinging bucket centrifuge. Those that work in both a fixed angle centrifuge and a swinging bucket centrifuge without filtering to dryness may present the problem of either retaining a different amount of concentrate when used in a fixed angle centrifuge than they will when used in a swinging bucket centrifuge or retaining a different amount of concentrate when used in fixed angle centrifuges with different fixed angles.
Some of the conventional centrifugal filtration apparatus require a second spin to remove the retained concentrate (i.e. dead stop volume). In these apparatus it is difficult or impossible to remove the retained concentrate from the concentrate tube with a pipette.
Another problem with some of the conventional centrifugal filtration apparatus is that as the concentrate volume approaches its final retained volume, the active filter area approaches zero. Therefore, the filtration rate will slow dramatically as the concentrate volume approaches its final retained volume.
Another problem with the conventional centrifugal filtration apparatus is that they are open systems (i.e. they contain a venting means that vents to the atmosphere in the centrifuge).
Yet another problem with some of the conventional centrifugal filtration apparatus is that they are not scaleable (i.e. they are designed to be used as small volume centrifugal filters, or large volume centrifugal filters, not both).
Certain types of filtration devices, such as that disclosed in U.S. Pat. No. 4,632,761 to Bowers et alia, are capable of preventing filtration to dryness and contain a dead stop feature which causes filtration to cease while there is concentrate remaining within the apparatus. This device, however, filters to dryness when spun at a 90xc2x0 angle and therefore the dead stop feature will not work if the device is spun in a swinging bucket centrifuge. Also, in this type of filtration device, the amount of concentrate remaining after dead stop is dependent upon the angle of the centrifuge rotor. The filter surface area in this type of device is limited by the diameter of the device, and the surface area is relatively small when compared to the volume of liquid solution within the housing. Another problem with this type centrifugal filtration apparatus is that is an open system (i.e. it contains a venting means that vents to the atmosphere in the centrifuge). This type of filtration device is conductive to clogging because the heaviest and denser molecules within the liquid solution are forced into the membrane filter. Accordingly, this device is limited because it will filter to dryness when spun in a swinging bucket centrifuge, it will filter to different dead stop volumes when used in fixed angle rotors with different angles, it is an open system and will vent potentially harmful gases to atmosphere during filtration, and its active filter surface area is limited by the diameter of the device.
The types of filtration devices disclosed in U.S. Pat. No. 4,722,792 to Miyagi et alia are capable of preventing filtration to dryness and contain a dead stop feature which causes filtration to cease while there is concentrate remaining within the apparatus. In this type of filtration device, the amount of concentrate remaining after dead stop is dependent upon the angle of the centrifuge rotor. Therefore, the dead stop volume will be different when the device is used in a swinging bucket rotor centrifuge than it will be when used in a fixed angle rotor, and will also be different when the device is used in fixed angle rotor centrifuges of different rotor angle. This means that the results obtained from this type of device when used in one type of centrifuge rotor cannot be compared to the results obtained from this type of device when used in another type of centrifuge rotor. Another problem with this type centrifugal filtration apparatus is that is an open system (i.e. it must contain a venting means that vents to the atmosphere in the centrifuge). Another problem with this type of centrifugal filtration apparatus is that the filtration rate starts out high because of its relatively large filter surface area. However, as the concentrate volume approaches its dead stop volume, the active filter area approaches zero. Therefore, the filtration rate will slow dramatically as the concentrate volume approaches its final retained volume. Accordingly, this device is limited because it will filter to different dead stop volumes when used in centrifuge rotors with different angles, it is an open system and will vent potentially harmful gases to atmosphere during filtration, and the filtration rate will slow dramatically as the concentrate volume approaches the dead stop volume.
The types of filtration devices disclosed in U.S. Pat. No. 5,112,484 to Zuk are capable of preventing filtration to dryness and contain a dead stop feature which causes filtration to cease while there is concentrate remaining within the apparatus. In this type of filtration device, the amount of concentrate remaining after dead stop is dependent upon the angle of the centrifuge rotor. Therefore, the dead stop volume will be different when the device is used in a swinging bucket rotor centrifuge than it will be when used in a fixed angle rotor, and will also be different when the device is used in fixed angle rotor centrifuges of different rotor angle. This means that the results obtained from this type of device when used in one type of centrifuge rotor cannot be compared to the results obtained from this type of device when used in another type of centrifuge rotor. Another problem with this type centrifugal filtration apparatus is that is an open system (i.e. it must contain a venting means that vents to the atmosphere in the centrifuge). Another problem with this type of centrifugal filtration apparatus is that as the concentrate volume approaches its final retained volume, the active filter area approaches zero. Therefore, the filtration rate will slow dramatically as the concentrate volume approaches its final retained volume. Although the retained concentrate can be removed from this device with a pipette, the retained concentrate is best removed from this type of device using the concentrate cup with a second spin. This type of device is not well suited to filter small volumes. Accordingly, this device is limited because it will filter to different dead stop volumes when used in centrifuge rotors with different angles, it is an open system and will vent potentially harmful gases to atmosphere during filtration, the filtration rate will slow dramatically as the concentrate volume approaches the dead stop volume, it is not designed to filter small volumes, and it is not easy to remove the dead stop volume with a pipette.
The types of filtration devices disclosed in U.S. Pat. No. 5,490,927 to Herczeg are capable of preventing filtration to dryness and contain a dead stop feature which causes filtration to cease while there is concentrate remaining within the apparatus. In this type of filtration device, the amount of concentrate remaining after dead stop is dependent upon the angle of the centrifuge rotor. Therefore, the dead stop volume will be different when the device is used in a swinging bucket rotor centrifuge than it will be when used in a fixed angle rotor, and will also be different when the device is used in fixed angle rotor centrifuges of different rotor angle. This means that the results obtained from this type of device when used in one type of centrifuge rotor cannot be compared to the results obtained from this type of device when used in another type of centrifuge rotor. Another problem with this type centrifugal filtration apparatus is that is an open system (i.e. it must contain a venting means that vents to the atmosphere in the centrifuge). This type of device is not well suited to filter small volumes. This type of device utilizes the entire filter surface area until the dead stop volume is reached when used in a swinging bucket centrifuge. However when this type of device is used in a fixed angle rotor centrifuge the active filter area will approach zero as the concentrate volume approaches its final retained volume. Therefore, the filtration rate will slow dramatically as the concentrate volume approaches its final retained volume when used in a fixed angle centrifuge. This type of device requires a second spin to remove the retained concentrate solution. Accordingly, this device is limited because it will filter to different dead stop volumes when used in centrifuge rotors with different angles, it is an open system and will vent potentially harmful gases to atmosphere during filtration, the filtration rate will slow dramatically as the concentrate volume approaches the dead stop volume when used in a fixed angle rotor centrifuge, it is not designed to filter small volumes, and it requires a second spin to remove the retained concentrate solution.
Finally, the art discloses the use of a lower retentate pocket to address the issue of filtering to dryness with centrifugal filtration devices. U.S. Pat. No. 5,647,990 to Vassarotti reveals a device having a concentrate retention pocket located at the bottom of a pre-filtration chamber, below the level of the filter membrane. Fluid flows through the filter into filtrate outlet channels, exiting these channels into the filtrate reservoir. These exit channels do not collect filtrate.
It is therefore an object of the present invention to provide a filtration device that may be used in a swinging bucket centrifuge, as well as a fixed angle centrifuge, without filtering to dryness.
It is also an object of the present invention to provide a filtration device that will filter to approximately the same dead stop volume when used in either a swinging bucket rotor centrifuge, or when used in a fixed angle rotor centrifuge.
It is also an object of the present invention to provide a filtration device having a relatively high filtration membrane surface area thereby enabling filtration to occur at a higher rate.
It is also an object of the present invention to provide a filtration device that continues to use a relatively high filtration membrane surface area until the dead stop volume is attained, thereby enabling filtration to occur at a higher rate throughout the entire filtration process.
It is also an object of the present invention to provide a filtration device, which minimizes the clogging of the semi-permeable membrane thereby maximizing filter efficiency.
It is also an object of the present invention to provide a filtration device from which the retained concentrate solution can be easily removed with a pipette.
It is also an object of the present invention to provide a filtration device which can be used as a closed system (i.e. does not vent to atmosphere), or that can be used as an open system (i.e. vents to atmosphere).
It is a further object of the present invention to provide a filtration device, which can be manufactured as a disposable small volume device, or as a disposable large volume device.
The foregoing problems of the prior art are solved, and the objects of the present invention are achieved, by use of a filtration apparatus constructed in accordance with the principles of the present invention.
In accordance with the present invention, the filtration apparatus for filtering and concentrating a solution includes a concentrate tube, which is divided into two sections. The upper cylindrical section acts primarily as a reservoir for unfiltered solution and directs unfiltered solution from the upper section into the lower section of the concentrate tube. The upper section is shaped so that all of the solution in the upper section will flow into the lower section regardless of the centrifuge rotor angle. The lower section of the concentrate tube is mainly rectangular, but may be triangular shaped at the bottom. The volume of the lower section is made as small as possible to insure that the entire filter surface area is used to filter the maximum amount of solution. The lower section may contain a pipette tip channel to facilitate the removal of the retained concentrate with a pipette. The lower section is open on one side, or two sides.
A micro-porous membrane or membranes may be sealed to the lower section in several ways. First, the membrane can cover the open side of the lower section of the concentrate tube, or the membranes can cover the open sides of the lower section of the concentrate tube. Alternately the micro-porous membrane may be sealed to the filter cover, or covers. A second alternative is to seal the micro-porous membrane or membranes between the filter cover and a filter sealing gasket or gaskets. A third alternative is to seal the micro-porous membrane between the filter cover and the concentrate tube using a compression rib that compresses a portion of the outer periphery of the micro-porous filter between the filter cover and the concentrate tube.
A filter cover is sealed to the open side of the lower portion of the concentrate tube, or two filter covers are sealed to the two open sides of the lower portion of the concentrate tube. The filter cover or covers contain a filter support means such as filter support ribs. The filter cover or covers also contains a filtrate outlet port. The position of the filtrate outlet port or ports determines the retained concentrate volume. The concentrate tube may contain a port or ports to allow displaced filtrate air to flow into the upper part of the concentrate tube to replace the filtered solution that flows from the concentrate tube into the filtrate tube.
The means for collecting the filtrate may include a filtrate tube. The concentrate tube may be sealed into the filtrate tube. The filtrate tube cap may contain a means to nest the concentrate storage tube.