Abstract

Although solution-state NMR is frequently used in metrologically-​traceable quantification studies, this is not the case for solid-state NMR. However, solid-​state NMR allows quantification of substances without the need of dissolution, providing a truly non-​destructive approach, and extending metrologically-​traceable quantitative NMR to sample classes that are difficult to characterize in solution. In this contribution we present a thorough and rigorous protocol for ¹⁹F quantitative solid-state NMR employing a certified reference material as external calibrant to provide metrological traceability to absolutely quantify the content of trifluoroacetic acid (TFA) in a peptide sample, typically the major impurity in synthetic peptides. The protocol includes determining the quantitative volume of the solid-​state NMR sample holder (rotor), the ERETIC (Electronic REference To access In vivo Concentrations) method [Akoka et al. 1999 Anal. Chem. 71 2554] to compensate for variations in the sensitivity of the radio frequency resonant circuit when an external calibrant is used, and the EASY (Elimination of Artefacts in NMR Spectroscopy) method [Jaeger & Hemmann 2014 Solid State Nucl. Magn. Reson. 57-58 22] to effectively suppress the ¹⁹F NMR background signal from the probe head. We applied the protocol to quantify the amount of TFA in a candidate NRC certified reference material of the peptide angiotensin II. The results obtained by ¹⁹F quantitative solid-​state NMR are in excellent agreement with those obtained by quantitative NMR in solution employing an internal calibrant.