Sulfuric acid h2so4 acs reagent

Pure standards of n-alkanes (from n-C7 to n-C30) for system evaluation and linear retention index (I^T) determination were purchased from Merck (Milan, Italy).
Process internal standards (P-ISs) heptadecanoic acid (C17:0 (C₁₇H₃₄O₂)) and glyceryl tripentadecanoate (C₄₈H₉₂O₆) were used for recovery evaluation. Analytical internal standards (A-ISs) normal pentadecane (n-C15 (C₁₅H₃₂)) and normal heptadecane (n-C17 (C₁₇H₃₆)) were used for quality control purposes (i.e., response normalization) and for quantitation through FID response factors (RFs). All ISs were from Sigma Aldrich (Merck, Milan Italy).
The reference mixture of fatty acid methyl esters (FAMEs) for identification and external calibration (Supelco 37 Component FAME Mix), for which detailed composition is provided in Supplementary Table S1, was purchased from Supelco-Merck (Milan, Italy).
Solvents (n-hexane and methanol (MeOH)) of LC-MS purity grade were from Merck (Milan, Italy).
Potassium hydroxide (KOH) ACS reagent, ≥ 85%, and sulfuric acid (H₂SO₄) ACS reagent, 95.0–98.0%, were from Sigma Aldrich (Merck, Milan, Italy).

Methanol (MeOH, LC–MS grade, ≥ 99.9%) was purchased from Honeywell (Charlotte, NC, USA). Chloroform (CHCl₃ stabilized with ethanol) and hexane (both for analysis, 99+%), ethanol absolute (EtOH, laboratory regeant grade), acetonitrile (ACN, LC–MS grade, ≥ 99.9%) and formic acid (FA, LC–MS grade) were purchased from Thermo Fisher Scientific (Waltham, MA, USA). Sulfuric acid (H₂SO₄, ACS reagent, 95.0–98.0%) and sodium chloride (NaCl, ≥ 99.5%) were purchased from Sigma Aldrich (St. Louis, MO, USA). Ultra-pure water was obtained from a Milli-Q system (Merck, Darmstadt, Germany) with a resistance of 18.2 MΩ.cm⁻¹ at 25 °C. Standard substances used for identification were of analytical grade and purchased from different suppliers.