an apparatus and method for detecting the presence and measuring the concentrations of molecules through a barrier and / or at a distance utilizes the principle of chemical / electrostatic attraction , i . e ., the affinity between charged atoms and molecules that cause their chemical interactions and reactions , to infer , based on the behavior of molecules on one side of the barrier , the presence and concentration of corresponding molecules on the other side of the barrier . the invention is useful , by way of example and not limitation , in non - invasively measuring glucose levels of diabetic patients .