An assay apparatus for determining properties of a sample. The apparatus has first and second chambers for receiving a sample. The first and second chambers are linked by a conduit along which at least part of the sample may travel so that the sample can move from the first chamber to the second chamber in series. The apparatus also has a slot extending from the conduit and a valve body comprising magnetic or magnetically susceptible material which is housed at least substantially within the slot. The valve body is movable between a first position in which the valve body is arranged at least substantially outside the conduit and a second position in which the valve body extends across the conduit thereby restricting a sample from travelling along the conduit from the first chamber to the second chamber. The valve body is slidably movable between the two positions by an external magnetic field.