The suitability of different process spectrometry techniques has been assessed, in terms of calibration requirements, accuracy, and precision, for the at-line monitoring of the sulfonation of fluorobenzene. Partial least-squares (PLS) calibration was required to analyze the spectra obtained by NIR spectrometry and low-field (29.1 MHz) <sup>1</sup>H NMR spectrometry. The low-field (27.4 MHz) <sup>19</sup>F NMR spectra contained well-resolved signals for the three fluorine containing compounds and univariate calibration was adequate. The Raman spectra of two of the compounds exhibited fluorescence and so this technique was not considered suitable for monitoring the reaction. The accuracy of the results obtained using univariate analysis of the <sup>19</sup>F NMR data and PLS analysis of NIR data were comparable (average % error of 3.5 and 2.9%, respectively, for concentrations >0.5 mol dm<sup>-3</sup> and 11.3 and 11.1%, respectively, for <0.5 mol dm<sup>-3</sup>). The least accurate results were obtained using PLS analysis of low-field <sup>1</sup>H NMR data, as the spectra of two of the components were too similar. For concentrations >0.05 mol dm<sup>-3</sup>, the most precise results were obtained with PLS analysis of NIR data (average RSD of 1.6%), although the precision of the results obtained using univariate analysis of <sup>19</sup>F NMR data was still good (average RSD of 3.7%).

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