Hybrid spe precipitation cartridge h ppt

The reference standards of 18 analytes examined in this study (Table 1) were obtained from Sigma (St. Louis, MO). The deuterium-labeled internal standards (ISs) used were: 2,2,3,4,4,6-d₆-cholesterol and 2,2,3,4,4,6-d₆-cholesteryl stearate (C/D/N Isotopes, Pointe-Claire, Quebec, Canada) for cholesterol and CEs, respectively; and 25,26,26,26,27,27,27-d₇-4β-OHC and 25,26,26,26,27,27-d₆-27-OHC (Avanti Polar Lipids, Alabaster, AL) for three cholesterol precursors and eight OHCs. The trimethylsilylating (TMS) agents, N-methyl-N-trifluorotrimethylsilyl acetamide (MSTFA), ammonium iodide (NH₄I), and dithioerythritol (DTE), were purchased from Sigma. The hybrid SPE-precipitation cartridge (H-PPT) (1 ml, 30 mg) was supplied by Supelco (Bellefonte, PA). All organic solvents were of analytical and HPLC grade and were purchased from Burdick and Jackson (Muskegon, MI).

All reference standards were obtained from Steraloids (Newport, RI, USA). Other chemicals included the deuterium-labeled internal standards (ISs), 2,2,3,4,4,6-d₆-cholesterol and 2,2,3,4,4,6-d₆-cholesteryl stearate (C/D/N Isotopes, Pointe-Claire, Quebec, Canada) for cholesterol, plant sterols and cholesteryl esters (CEs), respectively, and 25,26,26,26,27,27,27-d₇-4β-hydroxycholesterol and 25,26,26,26,27,27,27-d₇-27-hydroxycholesterol (Avanti Polar Lipids, Alabaster, AL, USA) for 3 cholesterol precursors and 8 hydroxycholesterols (OHCs). For trimethylsilylation (TMS), N-methyl-N-trifluorotrimethylsilyl acetamide (MSTFA), ammonium iodide (NH₄I), and dithioerythritol (DTE) were purchased from Sigma (St. Louis, MO, USA). The hybrid SPE-precipitation cartridge (H-PPT, 1 mL, 30 mg) was supplied by Supelco (Bellefonte, PA, USA). All organic solvents were of analytical and HPLC grades and were purchased from Burdick & Jackson (Muskegon, MI, USA).