Factors Affecting the Sensitivity of Hydroxyl Group Content Analysis of Biocrude Products via Phosphitylation and 31P NMR Spectroscopy

A useful method for quantifying concentrations of various hydroxyl group containing molecular species in biocrudes involves chemically converting species into nonionizable, P-functionalized derivatives independently observed and quantified by phosphorus-31 nuclear magnetic resonance (P-31 NMR) spectroscopy. Full validation of this method requires improved understanding of its applicability to different biocrude matrixes. The limits of detection (LODs) and limits of quantitation (LOQs) for this method are herein determined for concentrations of hydroxyl group containing analytes, grouped by molecular class, within three different biocrude matrixes: a biocrude (BC) product, a fast pyrolysis (FP) product, and an artificial matrix (AM). Matrix water content, known to affect method sensitivity, varies from <1 to 18.31 wt %. Each matrix is used to prepare a series of samples spiked to varying concentrations with three hydroxyl group containing model compounds, which represent (aliphatic) alcohol, phenolic, and carboxylic acid analyte types. Samples are then phosphitylated with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane to enable P-31 NMR spectroscopic determination of native hydroxyl analyte concentrations. LODs and LOQs are determined for each analyte type and for each matrix. Analytical precision is affected by matrix concentrations of interfering analytes and water. Among the matrixes examined, AM, which contains no hydroxyl analytes and has the lowest water content, expectedly exhibits the lowest LODs and LOQs. Matrix LODs range from 0.04 to 0.14 mmol of OH/g, while their LOQs range from 0.10 to 0.41 mmol of OH/g, with both sets of values generally increasing in the order AM < BC < FP.