Patent Publication Number: US-6666978-B2

Title: Apparatus producing a vacuum in several cavities of a microtitration filter plate, and corresponding method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for producing a vacuum in several cavities that which are configured in a microtitration filter plate. 
     2. Description of Related Art 
     Apparatus and methods of this above-mentioned kind are used in parallel processing of several liquid samples by means of vacuum filtration in microtitration filter plates. Like conventional microtitration plates, microtitration filter plates have, for instance, 96 or 384 cavities, but contrary to the case of microtitration plates, the microtitration filter plates have a lower cavity aperture fitted with a filter covering the aperture&#39;s cross-section. 
     Herein the concept of “microtitration filter plates” covers not only the conventional formats (96, 384), but also any planar test unit comprising several cavities to be used in a similar way. 
     When the samples are processed conventionally, they are first pipetted into the cavities of the microtitration filter plates. Then the lower cavities&#39; apertures are subjected to vacuum whereby the sample liquid is aspirated through the filters. As a rule a microtitration plate is mounted underneath the microtitration filter plate and the filtrates from the microtitration filter plate latter are collected in the microtitration plate. 
     Typically, the above-described vacuum filtration is carried out using apparatus comprising a chamber allowing a vacuum to be produced therein. First, the microtitration plate that serves to collect the sample liquid is inserted into the chamber. Then the microtitration filter plate is mounted in or on the vacuum chamber, with appropriate seals between the rim of the microtitration filter plate and the vacuum chamber assuring proper sealing. The typically applied vacuum ranges from 100 to 900 hPa. 
     Accordingly, when testing, the upper apertures of the microtitration filter plate are freely accessible outside the apparatus, whereas the lower apertures are subjected to the vacuum produced in the vacuum chamber. 
     However, a problem may arise when not all of the microtitration filter plate cavities are filled with sample liquid. These unfilled cavities induce an effect of air leakage that may degrade the applied vacuum. 
     In order to avoid this air leakage effect, it is commonplace to tape shut or otherwise cover the upper apertures of unfilled cavities before vacuum filtration takes place. Alternatively, however, unfilled cavities might be filled with, for instance, water. While both measures reduce or even avert the problem of air leakage, they are comparatively time-consuming and practically preclude automation. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an objective of the present invention to create an apparatus and a method allowing carrying out the vacuum filtration of microtitration filter plates or of similar test units comprising several cavities in a simpler manner. 
     Accordingly, the apparatus of the present invention comprises a flexible plate that, at the side of the upper cavity apertures, may be assembled in a way to make planar contact with and on the microtitration filter plate. The properties of the flexible plate are selected such that, following the plate&#39;s mounting on the microtitration filter plate and in the presence of applied vacuum, the plate shall be able to hermetically seal the upper apertures of the unfilled cavities. 
     In general, appropriate flexible plates are designed so that, upon application of vacuum, the plates will be aspirated slightly into the upper apertures of the unfilled cavities and then the plates will rest in a sealing manner on the upper apertures&#39; rims. 
     The concept of “plate” as used herein denotes all suitable forms, therefore including also mats, foils, etc. The term “flexible plate” also includes a basically rigid, planar structure fitted on its side engaging the microtitration filter plate with an appropriate, flexible coating such as, for instance, a silicone mat. 
     Preferably, the flexible plate will only close the upper apertures of the unfilled cavities, not the apertures of the filled ones, because the suction at the upper apertures of the filled cavities is considerably lower than that at the unfilled ones. 
     However, the scope of the present invention also covers the case of the flexible plate hermetically covering the filled cavities. In this mode, which applies in particular to microtitration filter plates having large numbers of cavities, the plate need only be lifted off the microtitration filter plate once or several times during vacuum filtration and be lowered again after the cavities have been vented. 
     In both modes of implementation, air leakage during vacuum filtration due to the unfilled cavities can be averted in an especially simple manner. 
     The flexible plate designed in the manner of the invention is required, as already mentioned above, to seal the apertures of unfilled cavities when it makes planar contact with a microtitration filter plate to which a vacuum is applied. 
     Flexible plates preferably made of resilient plastic were found especially suitable in this respect. A silicone mat about 1.5 to 3 mm thick and with a shore hardness of 30-40 for instance is particularly appropriate. Other materials, however, also are applicable. For example, rubber and the like may also be used besides plastics. 
     One substantial advantage offered by the flexible plate of the present invention is the new freedom from having to check, before vacuum filtration begins, whether any unfilled cavities are present in a microtitration filter plate, which, if found, then would have to be taped shut or the like or be separately filled. 
     The flexible plate of the present invention is mounted over all cavities of the microtitration filter plate and thereupon, depending on the nature of the plate and the density of the cavities, will hermetically and selectively seal only the unfilled cavities or all upper apertures of the cavities. The latter case does require venting one or more times during the vacuum filtration stage by briefly raising and then lowering again the plate. Even so, this operation is a substantial operational simplification over the state of the art. 
     Accordingly, the invention makes automation possible. More specifically, the flexible plate of the invention can be lowered onto and subsequently raised again from the microtitration filter plate in a suitably controlled manner by means of a displaceable adjustment device or a corresponding gripper tool cooperating with the apparatus. Furthermore, to improve handling, the plate may be received in a support frame or the like. 
     In this regard, the control device may be designed in that the flexible plate will automatically make planar contact with the microtitration filter plate when a vacuum is applied. 
     The control device may further be programmed such that during the applied vacuum stage, the plate may be lifted off and then repositioned on the microtitration several times during the vacuum stage. 
     Alternatively, and just as well, the plate of the invention may be designed to be a manual accessory of the apparatus of the invention. 
     The invention not only relates to the apparatus, but also to a corresponding method whereby the above-described plate can be moved into planar contact with the upper side of a microtitration filter plate for the purpose of avoiding air leakage during vacuum filtration, and to using a plate for such purposes. 
     As already mentioned above, to avoid air leakage the prior art only taught to tape shut or the like the upper apertures of unfilled cavities. 
     Relative to that state of the art, the method of the invention offers a simplified implementation. In the invention, merely one flexible plate exhibiting the above-described properties need be mounted on the microtitration filter plate such that all cavities will be covered. 
     When thereupon the vacuum conventionally used is applied for vacuum filtration, namely in a range from 100 hPa to 900 hPa, then the flexible plate will rest in a sealing manner on the upper rims of the unfilled cavities due to the suction arising there. The suction in the filled cavities, on the other hand, will be substantially lower and, as a rule, no sealing takes place in the filled cavities and enough air may flow into the cavities as the sample liquid is aspirated through the filters. 
     If some of the filled cavities are expected to be hermetically sealed, then the plate only needs to be raised off the microtitration filter plate one or more times during vacuum filtration and venting said cavities shall be carried out in this manner. 
     The flexible plate may also be held, for instance, in a support frame to facilitate handling. However, the plate may also be used directly in its formatted form in the method of the invention. 
     The invention also includes using appropriately flexible plates in order to preclude air leakage during vacuum filtration of microtitration filter plates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and further features of the invention will be apparent with reference to the following description and drawing, wherein: 
     The drawing FIGURE schematically shows a vacuum producing apparatus according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawing, the apparatus  10  has a housing  11  defining a chamber  12 , which is connected through a borehole  13  and a corresponding tube  14  to a vacuum-generating device  15 . 
     A removable frame  16  rests on the housing  11 . A peripheral sealing ring  17  is disposed between the frame  16  and the housing  11 . 
     A microtitration filter plate  19  is mounted in the frame  16  above a peripheral sealing strip  18  and defines several cavities  20 ,  21 ,  22  and  23 , of which some cavities  21 ,  23  are unfilled and some cavities  20 ,  22  are filled with the sample liquid  25 . 
     Each cavity has an upper aperture  25  and a lower aperture  26 . A filter  27  is mounted in the zone of the lower aperture and covers the aperture&#39;s cross-section, the sample liquid  24  being aspirated through the filter during vacuum filtration. 
     In order to collect the sample liquid  24 , i.e. its particular filtrates, a microtitration plate  29  is inserted into the vacuum chamber  12  and is held in place by an internal peripheral lip  28 . The microtitration plate  29  defines cavities  30  that are associated with the cavities  20 - 23  of the microtitration plate  19 , as illustrated. 
     The shown configuration will be the typical testing configuration. In this case, the device  15  applies a vacuum to the chamber  12  and the liquid  24  is aspirated through the filters  27  into the associated cavities  30  of the microtitration plate  29 . 
     Following filtration, the frame  16  together with the microtitration filter plate  19  is removed from the housing  11 , whereupon the microtitration plate  29  can be removed from the apparatus  10  for further processing. 
     As mentioned above, there is an air leakage problem in conventional apparatus in the region of the unfilled cavities  21 ,  23 . This air leakage problem may interfere with the appropriate vacuum. 
     To remedy this situation, the invention provides a flexible plate  32  received in a support frame  31  and displaceable manually or by means of appropriate adjustment devices into planar contact with the upper surface of the microtitration filter plate  19 . 
     The plate  32  is designed such that, when a vacuum acts on the unfilled cavities  21 ,  23 , the plate will be slightly aspirated into the cavities and, in this manner, will seal the upper aperture  25  of these cavities. However, the suction effect applied to the zone of the upper aperture  25  of the filled cavities  20 ,  22  will be insufficient in the preferred case to entail a hermetic seal. Therefore, in the preferred case, enough air for filtration may flow through these filled cavities. 
     In case of doubt, the flexible plate  32  may be raised one or more times off the microtitration filter plate  19 , whereby all the cavities will be vented and then filtration may continue even if the plate  32  were to seal the filled cavities when making contact with the filled cavities. This case may be encountered, in particular, with formats of a large numbers of cavities. Illustratively, in microtitration filter plates having 384 cavities, one filled cavity may be surrounded by unfilled cavities. Therefore, the plate will then be aspirated so tightly against the surface of the microtitration filter plate that air is precluded from flowing even into the said filled cavity. Accordingly, venting of the cavities by lifting the flexible plate  32  will avoid this problem.