The present Invention relates to the monitoring of biological substances, such as non-invasive monitoring of such substances in animal, for examples biomarkers and metabolites. Specifically, the invention further relates to such monitoring using rare earth tagged marker compounds. The invention further relates to such monitoring using laser spectroscopy or Raman spectroscopy. The invention further relates to the use of such monitoring in disease states, such as stroke, neurological disorders and cardiovascular disorders. The invention further relates to novel rare-earth conjugated marker compounds and processes for preparing said rare-earth conjugated compounds
