Reference for 15N Chemical Shifts

Chemical shifts are relative parameters which can be measured with a high degree of reproductibility providing that the same reference substance is used in the same experimental conditions. Unfortunately, this goal was not achieved in the last decade, which saw the sudden rise of 15N spectroscopy, since at least thirteen different molecules in a variety of solutions were used as standards for 15N chemical shifts. Indeed, this is not really surprising if we bear in mind that about fifteen years were required for the TMS δ-scale to be universally accepted in 1H spectroscopy!