Citation: Johnson, J.B.; Farquhar, H.; Ismay, M.; Naiker, M. Infrared Spectroscopy for the Quality Control of a Granular Tebuthiuron Formulation. Eng 2022, 3, 596–619. https://doi.org/10.3390/ eng3040041 Academic Editor: Antonio Gil Bravo Received: 14 November 2022 Accepted: 30 November 2022 Published: 2 December 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Infrared Spectroscopy for the Quality Control of a Granular Tebuthiuron Formulation Joel B. Johnson 1, * , Hugh Farquhar 2 , Mansel Ismay 3,† and Mani Naiker 1 1 School of Health, Medical & Applied Sciences, Central Queensland University, Bruce Highway, Rockhampton, QLD 4701, Australia 2 Cirrus Ag, 171 Alexandra St, Kawana, Rockhampton, QLD 4701, Australia 3 Independent Researcher, Gladstone, QLD 4680, Australia * Correspondence: joel.johnson@cqumail.com † Previous affiliation: Cirrus Ag, 171 Alexandra St, Kawana, Rockhampton, QLD 4701, Australia. Abstract: Tebuthiuron is a selective herbicide for woody species and is commonly manufactured and sold as a granular formulation. This project investigated the use of infrared spectroscopy for the quality analysis of tebuthiuron granules, specifically the prediction of moisture content and tebuthiuron content. A comparison of different methods showed that near-infrared spectroscopy showed better results than mid-infrared spectroscopy, while a handheld NIR instrument (MicroNIR) showed slightly improved results over a benchtop NIR instrument (Antaris II FT-NIR Analyzer). The best-performing models gave an R 2 CV of 0.92 and RMSECV of 0.83% w/w for moisture content, and R 2 CV of 0.50 and RMSECV of 7.5 mg/g for tebuthiuron content. This analytical technique could be used to optimise the manufacturing process and reduce the costs of post-manufacturing quality assurance. Keywords: process analytical technology; quality assurance; non-destructive assessment; NIRS 1. Introduction Tebuthiuron is a thiadiazolyl urea herbicide (Figure 1) primarily used for the control of woody plants. Application is typically via pellet-type (granular) formulations containing tebuthiuron (200–400 mg/g), which may be applied from the ground (either by hand or mechanically), or aerially dispersed if a large area is to be treated. As tebuthiuron is highly water-soluble [1], it leaches from the granules into the soil [2], where it is subsequently absorbed by the roots and translocated to the leaf tissue [3]. Its mode of action is through inhibition of Photosystem II, thus preventing photosynthesis in the affected plant [4]. The degradation and persistence of tebuthiuron is still an area under investigation, with studies reporting half-lives between 20 days [5] and 16–22 days [6], to as high as one year [7], 12.9 months [8], ‘considerably greater’ than 15 months [9] and even 2–7 years [10]. du Toit and Sekwadi [11] reported that tebuthiuron residue remained active in soil for 8 years after application. Figure 1. The chemical structure of tebuthuiron. Retrieved from http://www.chemspider.com/ (accessed on 28 September 2022) under Creative Commons 4.0 license. Eng 2022, 3, 596–619. https://doi.org/10.3390/eng3040041 https://www.mdpi.com/journal/eng