Abstract: A method of positioning a laminar flow of a sample fluid on a surface within a flow cell is disclosed. The method comprises providing a laminar flow of a first guiding fluid adjacent to a laminar flow of a second guiding fluid different from the first guiding fluid such that the two fluids flow together over the surface with a detectable interface to each other; detecting the interface between the two different guiding fluids; optionally adjusting the interface laterally to a desired position by adjusting the relative flow rates of the two guiding fluids; and introducing a laminar flow of the sample fluid between the laminar flows of the first and second guiding fluids such that the flow of the sample fluid is sandwiched between the guiding fluids. An analytical system for carrying out the method, a computer program, a computer program product and a computer system for performing the method are also disclosed.