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Abstract

A method and apparatus for selecting a subset of a predetermined number of test sample aliquots to mix with respective reagents using a shear valve having a maximum predetermined number of aliquot loops in an analytical instrument, including without limitation a clinical hematology or a flow cytometer instrument. Each aliquot loop can be filled with a test sample aliquot. Selected ones of the aliquot loops can be filled with a rinse solution and, using bypass passages and valves, closed off from the rinse source and a vacuum source used to empty the rinse from the non-closed off aliquot loops. The vacuum source also is used to fill the non-closed off aliquot loops with the sample to be aliquotted, so that the closed off aliquot loop remains filled with rinse. The shear valve can then be shifted to a second position, where the reagents for the non-closed off aliquot loops are pumped through those aliquot loops to mix the selected sample aliquot loops with their respective reagents. Preferably, the reagent for the closed-off aliquot loop is not pumped and thereby conserved. The selective shear valve is usable in an analytical instrument, such as a clinical hematology instrument where it is possible to select from a standard panel of tests a particular one test or a particular combination of tests to perform. It is particularly useful when it is frequently desirable to perform only some of the full panel of tests, e.g., three or four of five standard blood chemistry tests, rather than always performing the full panel of tests.

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