A method of performing an optical analysis of a liquid containing dissolved gas includes transferring an amount of the liquid containing the dissolved gas from a reservoir into a holder of a flow system of the optical analyser, holding the amount of the liquid in the holder at around ambient pressure for a period such that a portion of the dissolved gas is expelled from the amount of liquid held in the holder while the holder is open to a waste reservoir, transferring at least a portion of the amount of the liquid containing the dissolved gas held in the holder under a pressure above ambient into a measurement cell of the optical analyser as a liquid sample, and performing the optical analysis of the liquid sample from a detection of optical radiation by an optical detector after its interaction with the liquid sample in the measurement cell.