Microfluidics is a well-established technique to process, synthesize and analyse small amounts of materials for chemical, biological, medical, and environmental applications. Typically, it involve the use of reagents with volume that is smaller than ~1 microliter – ideally even nano- or pico-liter. Conventional electron spin resonance (ESR), is typically carried out with ~ 1 ml of sample, thereby making it incompatible with most microfluidics applications. Here we show that by using a new class of miniature surface resonators, combined with photolithography to prepare microfluidics patterns, ESR can be applied to measure small samples, down to picoliter volume, without scarifying concentration sensitivity. Our experiments with resonators having mode volume of ~ 1 nano-liter can obtain good signal from solutions with ~ 1 mM spin concentration ,while smaller resonators can be used to measure even smaller volumes, but for higher spin concentrations. All our experiments are performed at room temperature, making our technique compatible with future microfluidics applications that would employ compact resonators, microfluidic chips, miniature magnet and a compact ESR-on a chip spectrometer. This could result in a complete new approach to process and measure paramagnetic liquid samples, applicable for a verity of chemical, biological, medical and environmental applications.