The in-line sampler separator utilizes an improved method of measuring the gas, oil and water components in a fuel material mixture being processed at high pressure in a processing system. A sampling loop obtains mixture samples to be collected in a sampler. The sampler releases the collected sample to a separator tank when a certain volume has been reached. The tank operates at or below a pressure that allows the gas component to be released in gaseous form. The oil and water components separate within the tank, and their respective volumes are determined.