Analytical Sciences

Low-temperature Electronic Paramagnetic Resonance Measurements of TEMPO and 4-Hydroxy-TEMPO Benzoate for Purity Analyses by the Effective Magnetic-moment Method

Nobuhiro MATSUMOTO and Takuya SHIMOSAKA

National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), AIST Central-3, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan

The purities of two high-purity organic compounds with a nitroxyl radical moiety were quantified based on their free-radical content using the effective magnetic-moment method. Magnetic moments were measured and X-band electron paramagnetic resonance (EPR) spectra were obtained using a superconducting quantum interference device and X-band EPR spectrometer, respectively, over a wide temperature range of near-room temperature to near-liquid-helium temperature. Concerning measurements of effective g-values using an EPR spectrometer, both the sweep direction and sweep speed were taken into account to obtain accurate g-values. The purities of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl benzoate (4-hydroxy-TEMPO benzoate) were close to 1 kg kg⁻¹ with relative uncertainties of 1%, which represented improved values compared to those obtained by us previously. These results show a possibility for both compounds to act as reference materials in providing reliable quantification of free radicals per unit mass using this analytical method.

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