HPLC-PDA Method Development, Validation, Optimization, and Comparison with GC-FID for The Quantification of Organic Acids

ESTHER OLONIMOYO, Naresh Kumar Amradi, Stephanie Lansing, Akua Asa-Awuku, Candice M. Duncan, University of Maryland

     Abstract Number: 76
     Working Group: Instrumentation and Methods

Abstract
Substantial uncertainties are associated with aerosol chemical composition and secondary organic aerosol formation mechanisms. Current models only consider acetic and formic acids, thereby underestimating organic acid atmospheric concentrations by 50%. In this work, we developed and validated a fast analytical method for a High-Pressure Liquid Chromatograph coupled with a Photodiode Array detector (HPLC-PDA) for quantifying six monocarboxylic organic acids: formic, acetic, propionic, butyric, isovaleric, and valeric acids. Analyte separation was performed on an XBridge C18 (4.6 x 150 mm, 3.5 µm) column using a mobile phase solution composed of 10% acetonitrile and 10 mM potassium dihydrogen phosphate buffer solution (KH₂PO₄) acidified with phosphoric acid (H₃PO₄). We optimized existing methods by investigating elution modes, mobile phase pH, and column temperature to improve peak resolution and shorten analysis time. Our proposed method is characterized by a gradient elution mode with flow rates ranging from 1 to 2 mL min ^-1 and a short analysis time (˂ 8 mins). The calibration curves showed excellent correlation (R² ˃ 0.999) and peak resolution (˃ 1.5). The limits of determination for all six acids (LOD) ranged from 0.87 to 1.51 ppm, while the limit of quantification (LOQ) ranged from 2.63 to. 4.59 ppm. We also compared the optimized HPLC method and a Gas Chromatograph coupled with a Flame Ionization Detector (GC-FID) method to investigate technique suitability for organic acid analysis based on sensitivity, precision, and dynamic range. Our preliminary results indicate greater sensitivity and precision at lower concentrations, thus showing that the HPLC system is better suited for monocarboxylic organic acids. Lastly, the HPLC method was applied to fog and rainwater samples collected from a building rooftop in an urban environment.