Mass Spectrometry and D-H Back-Exchange Experiments

deuterium oxide and (undeuterated) acetonitrile or methanol are by far the most frequently used eluent constituents employed in LC-NMR unless normal-phase material is used or NH protons [46] are studied. The use of these eluents leads to shifted masses in the mass spectrum, since all exchangeable protons of the analytes are exchanged for deuterons. 

However, by means of an additional D-H back-exchange (performed by adding water or aqueous buffer via a syringe pump to the MS eluent flow in a ratio of 1 4), information on the actual molecular ion, as well as the number of exchangeable protons, is available. 

In principle, a D-H back-exchange experiment does not necessarily need to be performed since the required structural information can also be deduced from the mass spectrum recorded under H-D exchange conditions (deuterated eluent), which is of advantage in terms of the ready distinction of hydrogen equivalents. However, it has to be noted that this approach is not compatible with the extensive use of existing MS data libraries (data bases) and it thus depends on the individual problem as to whether such spectra can be used to advantage [1]. 

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