Abstract: A metering valve assembly for use in an analytical system includes fixed and movable valve members that have slidably engaged faces. One valve member includes a first metering chamber in the form of a through passage having an axis perpendicular to the direction of relative valve motion and a second metering chamber of elongated shape that has a length at least three times its width and that extends along the direction of relative valve motion. In a first position, the metering chambers are connected in series with a source of the sample liquid to be analyzed; and in a second valve assembly position, the first metering chamber is connected in series between a source of auxiliary fluid and a first analysis unit and the second metering chamber is concurrently connected in series between a source of auxiliary fluid and a second analysis unit.