Total water-soluble sugars quantification in bio-oil using the phenol–sulfuric acid assay

• Bio-oil total water-soluble sugars were quantified using an easy AOAC method.
• Furans showed interference and a correction factor was established for bio-oil.
• Results using the AOAC method were comparable to total sugars by HPLC.
• Uncertainty of measurement was ±2.0% at the 95% confidence level.

Bio-oil produced from fast pyrolysis of biomass contains sugars originating from cellulose. Traditional quantification of sugars in bio-oil is accomplished by gas chromatography/mass spectroscopy (GC/MS) via derivatization, high performance liquid chromatography (HPLC), ion chromatography (IC), or nuclear magnetic resonance (NMR) methodologies. These techniques are highly specific for each sugar, tedious to perform, expensive, and involve the use of hazardous solvents. We have evaluated a standardized test method developed for food and agriculture applications, the Association of Analytical Communities, Inc. (AOAC) Method 988.12 (44.1.30) Phenol–Sulfuric Acid Assay for Total Carbohydrate Determination, to quantify total sugars in the water-soluble fraction of bio-oil. This study investigates accuracy relative to matrix effects caused by non-sugar compounds using positive and negative controls. Positive controls included levoglucosan, d-glucose, d-mannose, d-xylose, d-fructose, d-galactose, l-arabinose, l-fucose, and cellobiosan. Negative controls included phenol, acetic acid, formic acid, propionic acid, glycolic acid, acetol, furfural, 5-hydroxymethylfurfural (5HMF), furfuryl alcohol, 2-methylfuran and 2(5H)-furanone. Potential interference with the quantification of total water-soluble sugars by the AOAC Method 988.12 (44.1.30) was calculated for all positive and negative controls by using data obtained when adding the contributor (positive controls) and the interferent (negative controls) into the water-soluble fraction of bio-oil with typical concentrations found in bio-oil. It was found that furfural, 2(5H)-furanone, 5HMF, and furfuryl alcohol influenced results with a range of potential errors of 9.56–29.7%, 9.52–29.8%, 2.91–24.8%, and 1.34–11.9%, respectively. A correction factor of 0.76 wt% was established to reduce or eliminate this influence. Total water-soluble sugars content in bio-oil detected by AOAC Method 988.12 (44.1.30) was comparable to the quantity of sugars detected using hydrolysis with quantification by HPLC. The uncertainty of measurement of water-soluble sugars in bio-oil at 95% confidence was ±2.0% using AOAC Method 988.12 (44.1.30) when the correction factor was employed.