A number of laboratory operations require the handling of multiple small samples. For example, immuno-assays can be carried out in volumes of 100 .mu.l or less while involving a number of manipulations, such as adding and removing of reagents and carrying out serial dilutions of samples. In order to handle multiple small samples, including related samples such as are present in a serial dilution, a type of reaction vessel known as a microtiter plate has been developed. Such plates are typically made of polypropylene or a similar plastic and have a number of wells arranged in a geometric pattern that simplifies organizing and carrying out related operations. A commonly used plate is about 5.times.3.5 inches and contains 96 wells in an 8.times.12 rectangular pattern, each well having a total capacity of 1 ml or less. Other microtiter plates are available with different numbers of wells and capacities.
Recently, automatic or semi-automatic equipment has been developed to carry out a number of operations. For example, equipment is available to automatically add reagents, serially dilute samples, and optically read results of analyses carried out in microtiter plates.
An additional operation that has been automated is the washing of microtiter plates. A number of reactions require removal of excess reagent at various stages of the reaction. For example, immunological assays are carried out in which a reagent, such as an antibody that specifically binds to an analyte, is attached to the walls of the microtiter plate wells. In a typical assay, a sample is added, and analyte, if present, binds to the antibody. A color-forming reagent is next added, and, if the analyte was present in the sample, a color forms in the well. Reagents must be washed out of the wells in order to prevent color from forming in wells to which no analyte was added. Microtiter plate washers have been developed to automatically carry out the washing operation. Typical of these is a device produced by Bio-Rad Laboratories of Richmond, Calif. The microtiter plate resides in its upright position in a holder which moves it to the proper location. A series of downward directed needle-like nozzles add wash solution to each well in a plate (or a row of wells in a plate), followed by removal of the wash liquid by aspiration using a different set of needle-like nozzles inserted downward into the wells. Other washers of similar design are manufactured and/or marketed by Tri-Continent Scientific, Grass Valley, Calif.; Dynatech Laboratories, Inc., Chantilly, Va.; Skatron AS, Lier, Norway; SLT-Labinstruments, Groeding, Austria; Tomtec, Orange, Conn.; and Flow Laboratories, Inc., McLean, Va.
Although such equipment is efficient in carrying out the washing operation, the equipment is complicated and costly. Careful design must occur if the aspirating pipettes are to remove the maximum amount of liquid for efficient washing without disturbing reagents attached to the walls of the microtiter plate.
Simplified equipment is available for washing individual items, such as cuvettes, using an upward-directed spray. However, such devices typically use vacuum to pull wash liquid into the cuvette and then withdraw the washing liquid. Vacuum is not appropriate for a relatively large flat object such as a microtiter plate, since the resulting relatively large force (from the vacuum acting on the large area of the plate) would make manipulation of the microtiter plate difficult.
Accordingly, simplified equipment of lower cost and few design constraints is desirable.
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