Chemical shift reference

The standard for referencing chemical shifts is TMS 0 ppm (1H).  The 2008 paper by Zilm describes the simple method for calculating the chemical shift of any other nuclei (13C, 31P, 29Si, etc.) without the use of an internal or external standard.  IUPAC recommends this method over typical methods using secondary standards.  Authors should note their method of referencing in all publications.

doi:10.1016/j.ssnmr.2008.02.004

Summary:   https://www.chem.wisc.edu/~cic/nmr/Guides/Other/Xi_chem_shift_scale.pdf