An Alternate and Simplified GC–MS Approach for Characterization of Volatile Fatty Acids (VFAs) in Liquid Anaerobic Digestate Samples

The anaerobic digestion process generally generates volatile fatty acids (VFAs) depending upon the metabolic pathway. VFAs are short chain fatty acids containing from one up to eight interlinked carbon atoms. Separation, identification, and quantification of VFAs can enhance metabolic process especially biogas production during anaerobic digestion. Therefore, the continuous monitoring of VFAs is mandatory and is typically performed by liquid chromatography combined with mass spectrometry (LC–MS) and gas chromatography equipped with flame ionization detector (GC–FID) that require sample derivatization and larger sample size of 10–30 mL. Therefore, the current study develops a simple and readily applicable protocol to determine and quantify VFAs (C1–C7) in liquid digestate samples by gas chromatography combined with mass spectrometry (GC–MS) with a reduced sample size of only 0.5 mL. The protocol utilized mix VFAs standard solution for calibration and solvent extraction technique to extract and dissolve VFAs from digestate samples to a volatile solvent dimethyl carbonate (DMC). The results indicate the effective and well separated peaks of all analyte acids from C1 to C7 with regression coefficients r2 ≥ 0.98 (C1–C4) and ≥ 0.93 (C5–C7) during calibration. The digestate samples analyzed showed abundance of acetic acid from 4.6 to 11 mM concentration in all four samples followed by propanoic acid (C3) to hexanoic acid (C6) in a range from 0.3 to 4.5 mM concentration. The method is suitable for quantifying all VFAs from C1 to C7 analyte acids as well as the isomers of C4 to C6 analyte acids. Moreover, the method is applicable to a variety of liquid samples from water treatment to energy valorization.