A resonator apparatus and a method for electron spin resonance (ESR) measurements are disclosed. The resonator apparatus comprises a dielectric resonator and a sample vessel extending through the resonator. The sample vessel is configured as one single flexible tube. Means are provided for conveying a liquid sample substance through the flexible tube. According to the method a liquid sample substance is guided through the sample vessel, wherein the sample substance is gated by cyclically conveying and stopping, resp., a flow of the sample substance. A measurement is conducted within the resonator when the flow of sample substance is stopped.