Ethyl centralite

Optima® LC/MS grade methanol (MeOH), acetonitrile (ACN), and water (H 2 O), all containing 0.1% formic acid (FA), were obtained from Thermo-Fisher Scientic (Watham, MA) and used either as extraction solvents or mobile phases. Standard (neat) organic constituents used in this study included: akardite II (AK2), ethyl centralite (EC), methyl centralite (MC), diphenylamine (DPA), N-nitrosodiphenylamine (N-NDPA), 2-nitrodiphenylamine (2-NDPA), and 4-nitrodiphenylamine (4-NDPA), all obtained from Sigma-Aldrich and Accustandard® (New Haven, CT). Calibration curves were generated from working stock solutions and diluted in MeOH ($95%). Complexing agents 18-crown-6-ether (18C6) and tartaric acid (TT) were purchased from Sigma-Aldrich (St. Louis, MO 99% purity) and diluted using MeOH and water, respectively. Metal IGSR standards used to form complexes utilized ICP-MS metal standards (VHG Laboratories, Manchester, NH) and methanol. Micro-bulk digestions utilized nitric (HNO 3 ) and hydrochloric (HCl) acids (ultrapure grade, Thermo-Fisher Scientic, Waltham, MA). Two internal standards were used throughout the study including deuterated diphenylamine-d 10 (D 10 -DPA) which was purchased from CDN Isotopes (Quebec, Canada) and used as the internal standard for the organic molecules and thallium (Tl) for the IGSR (VHG Laboratories, Manchester, NH) and complexing agents.

LC-MS grade water containing 0.1 % formic acid, methanol, formic acid, and acetonitrile were purchased from Sigma-Aldrich (Buchs, Switzerland). Diphenylamine was from Fluka (Buchs, Switzerland). Ethylcentralite, N-nitrosodiphenylamine, 4-nitrodiphenylamine, akardite II, 1,3diphenylurea and N,N-diphenylformamide were obtained from Sigma-Aldrich (Buchs, Switzerland).
2-nitrodiphenylamine was from Alfa Aesar (Karlsruhe, Germany). MEthylcentralite was purchased from MP Biomedicals (Illkirch, France).