  Magnetic resonance spectroscopy is universally regarded as one of the most
important tools in chemical and bio-medical research. However, sensitivity
limitations typically restrict imaging resolution to length scales greater than
10 \mu m. Here we bring quantum control to the detection of chemical systems to
demonstrate high resolution electron spin imaging using the quantum properties
of an array of nitrogen-vacancy (NV) centres in diamond. Our quantum magnetic
resonance microscope selectively images electronic spin species by precisely
tuning a magnetic field to bring the quantum probes into resonance with the
external target spins. This provides diffraction limited spatial resolution of
the target spin species over a field of view of ~50x50 \mu m^2. We demonstrate
imaging and spectroscopy on aqueous Cu2+ ions over microscopic volumes (0.025
\mu m^3), with detection sensitivity at resonance of 104 spins/voxel, ~100
zeptomol (10^-19 mol). The ability to image, perform spectroscopy and
dynamically monitor spin-dependent redox reactions and transition metal
biochemistry at these scales opens up a new realm of nanoscopic electron spin
resonance and zepto-chemistry in the physical and life sciences.
