A method of measuring a liquid sample in a well plate, the method comprising - providing an optical chip comprising o an optical sensor, for example a ring resonator or a Mach-Zehnder interferometer, that is accessible to the liquid sample at a sampling area of the chip, and o at least one free-space optical coupler (11,12) that is accessible to receive input light and/or emit output light via a coupling area of the chip, wherein the optical chip is configured to modify the output light with respect to the input light as a function of a property to be measured of the liquid sample proximate to the sampling area; - inserting a probe head of an optical instrument at least partly inside a liquid tight sleeve, wherein the liquid tight sleeve is held by the probe head inserted inside, wherein the optical chip is held by the liquid tight sleeve, wherein the liquid tight sleeve is configured to shield the coupling area from the liquid sample while keeping the sampling area exposed, and wherein an optical path extends from the coupling area through an interior free space inside the sleeve to and through a projection system of the optical instrument inside the probe head; and - dipping the probe head with the liquid tight sleeve into a well of the well plate to lower the optical chip into the liquid sample thereby submerging the sampling area in the liquid sample below a fluid level of the liquid sample while keeping the liquid sample away from the coupling area for interrogating the optical coupler (11,12) via the optical path that does not pass through the liquid sample. - providing an optical chip comprising o an optical sensor, for example a ring resonator or a Mach-Zehnder interferometer, that is accessible to the liquid sample at a sampling area of the chip, and o at least one free-space optical coupler (11,12) that is accessible to receive input light and/or emit output light via a coupling area of the chip, wherein the optical chip is configured to modify the output light with respect to the input light as a function of a property to be measured of the liquid sample proximate to the sampling area; o an optical sensor, for example a ring resonator or a Mach-Zehnder interferometer, that is accessible to the liquid sample at a sampling area of the chip, and o at least one free-space optical coupler (11,12) that is accessible to receive input light and/or emit output light via a coupling area of the chip, - inserting a probe head of an optical instrument at least partly inside a liquid tight sleeve, wherein the liquid tight sleeve is held by the probe head inserted inside, wherein the optical chip is held by the liquid tight sleeve, wherein the liquid tight sleeve is configured to shield the coupling area from the liquid sample while keeping the sampling area exposed, and wherein an optical path extends from the coupling area through an interior free space inside the sleeve to and through a projection system of the optical instrument inside the probe head; and - dipping the probe head with the liquid tight sleeve into a well of the well plate to lower the optical chip into the liquid sample thereby submerging the sampling area in the liquid sample below a fluid level of the liquid sample while keeping the liquid sample away from the coupling area for interrogating the optical coupler (11,12) via the optical path that does not pass through the liquid sample.