The IUPAC recommends the following internal/external references ( 98JB1-12):

DSS (2,2-dimethyl-2-silapentane-5-sulfonic acid) for ¹H (xi factor of 1.000000000) and ¹³C internal referencing (xi factor of 0.251449530).

External anhydrous liquid ammonia for ¹⁵N referencing (xi factor of 0.101329118).

External 100% trifluoroacetic acid for ¹⁹F referencing (xi factor of 0.940867196).

Internal 10% trimethylphosphate for ³¹P referencing (xi factor of 0.404808636).

The xi factor allows the referencing of other nuclei than proton using the absolute proton frequency of internal DSS. For instance, the absolute frequency of liquid ammonia can be directly obtained from the absolute frequency of the DSS signal in our proton spectrum multiplied by the xi factor.

Methods for chemical shift referencing: ( 05JB143-31)

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