The invention relates to a valve, in particular a sample injection valve, for a device (1) for synthesizing, analyzing, and/or separating, comprising at least three liquid connections (3′, 4′, 5′, 6′, 7′, 7″), a housing (8) as a valve part, and a valve body (9) as another valve part for selectively connecting the liquid connections (3′, 4′, 5′, 6′, 7′, 7″) by means of at least one flow channel (10, 11, 12) bounded at least partially by sealing surfaces (10′, 11′, 12′) between the housing (8) and the valve body (9), wherein the housing (8) and/or the valve body (9) are supported in such a way as to be movable relative to each other. In order to create a one-way valve, at least one valve part (8 or 9) adjacent to the sealing surface (10′, 11′, 12′) and made of a plastic material can be plastically deformed according to a relative position (9′), in particular of the valve body (9), in order to be able to withstand the elevated pressure loads in the flow channel (10, 11, 12) in a liquid-tight manner in the relative position (9′), in particular of the valve body (9).