Contents lists available at ScienceDirect Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab Suspension assisted analysis of sulfur in petroleum coke by total-refection X-ray fuorescence Ramón Fernández-Ruiz a, ⁎ , M.J. Redrejo a , E. Josué Friedrich K. a , Nuria Rodríguez b , Ricardo Amils b,c a Servicio Interdepartamental de Investigación, Laboratorio de TXRF, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain b Centro de Astrobiología (INTA-CSIC), Crt. Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain c Centro de Biología Molecular “Severo Ochoa”, Cantoblanco, 28049 Madrid, Spain ARTICLE INFO Keywords: Sulfur Petroleum coke TXRF GI-XRF Suspension assisted analysis SAA-TXRF Particle size distribution CHNS ABSTRACT The infuence of the particle size distribution of a petroleum coke sample compared to its sulfur content was investigated. For this matrix, an optimization procedure of the Suspension Assisted Analysis (SAA) by Total- refection X-Ray Fluorescence (TXRF) quantitative method was developed. SAA-TXRF sulfur recoveries were evaluated for three particle size distributions of the same coke sample. The sulfur recovery increased when the particle size distribution was smaller. The observed behaviour was correlated and validated with CHNS ele- mental analysis and microwave-assisted digestion TXRF measurements. The results indicate that the sulfur signal is strongly infuenced by the particle size distribution and the deposition morphology of the petroleum coke material. This efect could be explained by the presence of a strong absorption efect for the sulfur signal in combination with the distortion of the X-ray Standing Waves (XSW) feld observed between the analyte, S, and the elements Ti and Co, which were used as an internal standard. The variation in sulfur observed was up to 45.5% lower than the higher recovery obtained by CHNS. This investigation suggests that for an adequate application of the SAA-TXRF method in petroleum coke or similar matrices, careful optimization of the fnal particle dispersion of the ground coke is crucial and necessary. In this case, the use of a high- power ultrasound probe was the key. 1. Introduction Total-refection X-Ray Fluorescence (TXRF) spectrometry is an analytical technique each time more consolidated. Additionally, TXRF is being increasingly used for diferent scientifc felds to solve com- plicated analytical questions in a larger number of diferent materials [1]. In this sense, the European action COST CA-18130, called EN- FORCE TXRF, was recently approved for the period (2019–2023), with the main objective to improve the knowledge and analytical potential of TXRF spectrometry in industry, research and education. The achievement of the goals of this action will increase the number of novel matrices analyzed and thus, the spectroscopic value of TXRF spectrometry [2]. It is known that TXRF spectrometry has several and quite diferent analytical capabilities. However, one of the more ex- citing features is its ability to perform direct quantitative analysis of solids without chemical distortions, that is, without acid leaching or chemical acid digestion. So far, this type of TXRF analysis has been applied to very diferent types of solid matrices, with soils [3], algae [4], ceramics [5,6], new materials [7,8] or river sediments [9] being just a few examples of its application. Several petrochemical com- pounds have been quantifed by TXRF using an innovative sample preparation method based on the application of Vapour Phase De- composition (VPD) techniques [10], but, to the author´s knowledge, this has not yet been applied to solid petroleum coke. Many groups have investigated the principles, potential sources of error and limitations of TXRF. Klockenkämper and Von Bohlen provide the basis and frst principles for the application of the SAA-TXRF technique [11], and si- milar work was done more recently by Hellin et al. [11, 12] and Fer- nández-Ruiz [13]. Further, a systematic study was recently published with regard to some of the most critical preparation parameters for the analysis of solids using the suspension-based TXRF method [14]. Petroleum coke is a solid residue from oil refning. According to global indicators, an average of 3.1% of oil is refned to petroleum coke. Depending on its content in the oil, that is, coking feedstock, petroleum https://doi.org/10.1016/j.sab.2020.105997 Received 10 July 2020; Received in revised form 30 September 2020; Accepted 7 October 2020 ⁎ Corresponding author at: Universidad Autónoma de Madrid, Facultad de Ciencias, Servicio Interdepartamental de Investigación, Laboratorio de TXRF, Avd. Francisco Tomas y Valiente, 7, Módulo 13, Cantoblanco, 28049 Madrid. Spain. E-mail address: ramon.fernandez@uam.es (R. Fernández-Ruiz). Spectrochimica Acta Part B 174 (2020) 105997 Available online 09 October 2020 0584-8547/ © 2020 Elsevier B.V. All rights reserved. T