A valve system in a fluidic cartridge for expelling liquid sample from a sample processing region, the apparatus comprising: a fluid pathway (C310a) for passing a liquid sample therethrough from an upstream end to a downstream end; an outlet valve (C303a) within the fluid pathway, the outlet valve configured to move between a closed position in which it prevents the liquid sample from passing through the outlet valve and an open position in which it permits the liquid sample to pass through the outlet valve; a downstream sample processing region (C304a) within the fluid pathway downstream of the outlet valve; a bypass channel coupled to the fluid pathway at a junction between the outlet valve and the downstream sample processing region, the valve system configured such that liquid sample downstream of the outlet valve may be expelled through the bypass channel when the outlet valve is in its closed position; and at least one compressible element (C306a) downstream of the downstream sample processing region, the at least one compressible element configured to become increasingly biased against fluid upstream of the compressible element as the liquid sample passes through the open outlet valve, such that the liquid sample downstream of the outlet valve may be expelled from the fluid pathway and into the bypass channel by the at least one compressible element when the outlet valve is closed, wherein the at least one compressible element (C306a) is a gas spring comprising a blind bore filled with a compressible fluid; wherein the valve system further comprises a bypass valve located within the bypass channel, the bypass valve configured to move between a closed position in which it prevents the liquid sample from passing through the bypass valve and an open position in which it permits the liquid sample to pass through the bypass valve. a fluid pathway (C310a) for passing a liquid sample therethrough from an upstream end to a downstream end; an outlet valve (C303a) within the fluid pathway, the outlet valve configured to move between a closed position in which it prevents the liquid sample from passing through the outlet valve and an open position in which it permits the liquid sample to pass through the outlet valve; a downstream sample processing region (C304a) within the fluid pathway downstream of the outlet valve; a bypass channel coupled to the fluid pathway at a junction between the outlet valve and the downstream sample processing region, the valve system configured such that liquid sample downstream of the outlet valve may be expelled through the bypass channel when the outlet valve is in its closed position; and at least one compressible element (C306a) downstream of the downstream sample processing region, the at least one compressible element configured to become increasingly biased against fluid upstream of the compressible element as the liquid sample passes through the open outlet valve, such that the liquid sample downstream of the outlet valve may be expelled from the fluid pathway and into the bypass channel by the at least one compressible element when the outlet valve is closed, wherein the at least one compressible element (C306a) is a gas spring comprising a blind bore filled with a compressible fluid; wherein the valve system further comprises a bypass valve located within the bypass channel, the bypass valve configured to move between a closed position in which it prevents the liquid sample from passing through the bypass valve and an open position in which it permits the liquid sample to pass through the bypass valve.