P NMR signals of 5h (product of reaction of 3 with ethanethiol) and 5b (product of reaction with ethanol) were surprisingly similar (117.9 and 116.9 ppm for 5h and 5b, respectively). Hence, to decipher the chemo-selectivity of the reaction of 3 with mercaptoethanol (Scheme 3), namely, the formation of either 7a or 7b, we applied high resolution tandem mass spectrometry (HR-MS/MS) technique, to determine the chemo selectivity and final structure of the reaction product. Via HR-MS/MS experiment we capture the molecule, and by using collision gas we fragment it. Analysis the spectrum reveals a signal of 354.982 Da 7b' which indicates a chemical bond of P-O rather than P-S, meaning formation of 7b. This clearly supports the chemo selectivity of the reaction (SI, Fig. ).