“…Nearly half of the published studies used SPE for sample preparation (Baumann et al, 2010; Byrd et al, 2005; Dobrinas et al, 2011; Earla et al, 2014; Fischer et al, 2015; Gray et al, 2008; Heavner et al, 2005; Kim & Huestis, 2006; Kim et al, 2020; Marchei et al, 2008; Miller et al, 2010; Moyer et al, 2002; Pacifici et al, 1993; Paniagua‐Gonzalez et al, 2018; Piller et al, 2014; Quintana et al, 2019; Scheidweiler et al, 2012; Shakleya & Huestis, 2009a, 2009b, 2009c; Shu & Wang, 2013; Tretzel et al, 2016; Vieira‐Brock et al, 2011; Xia et al, 2011; Xu et al, 2004; You et al, 2016; Yue et al, 2010). These researchers chose water, acid (hydrochloric acid, acetic acid, formic acid, and trichloroacetic acid), and alkaline (NH 4 OH) and organic solvents (methanol, ethanol, hexane and hexane‐ethyl acetate, acetonitrile, chloroform, and acetone) to purify the samples loaded on the cation‐exchange SPE cartridge (Baumann et al, 2010; Dobrinas et al, 2011; Earla et al, 2014; Fischer et al, 2015; Miller et al, 2010; Piller et al, 2014; Shakleya & Huestis, 2009b; Shu & Wang, 2013; Xia et al, 2011; You et al, 2016).…”