A non-derivatization Analysis of Fatty Acids from C6 to C20 in PM 2.5 Aerosols using GC/FID and GC/MSD

Fatty acids make large contributions to aerosol mass. In particular, palmitic acid (C₁₆) and stearic acid (C₁₈) are dominant in atmosphere and in many sources of aerosols. Their analysis after sampling is usually based on methylation or silanization into methyl esters or silanes for analysis on GC/FID or GC/MSE. In this paper, a non-derivatization analysis method of fatty acids ranged from C₆ to C₂₀ in PM_2.5 aerosols is discussed. The entire analysis method includes the processes of extraction using Accelerated Solvent Extraction (ASE), cleanup using silica gel column chromatography, concentration by blow down, and GC/FID and GC/MSD analysis/identification with the DB-FFAP capillary column.

The calibration curves of the fatty acids on the GC/FID basically go through the origin and have highly linear response (r² from 0.99862 to 0.99979) for the concentrations ranged from 0.6 to 128 ng/μl. Detection limits of fatty acids on the GC/FID are from 0.2 to 0.6 ng/μl depending on the compounds. The linear response of the calibration curves on the GC/FID is from 0.92 to 0.99. The detect limits on the GC/MSD are around 0.8 ng/μl. The relative standard deviations (RSD) of quantitation on the GC/FID are smaller than on the GC/MSD. Using the GC/FID, the recoveries of fatty acids using the entire method with spiked internal standards (surrogates) are between 80 and 104% with the RSD <8% except C₆ and C₇. The recoveries of fatty acids using the entire method without the surrogates are 50 to 61% for C₁₀ to C₂₀, and are lower than 40% for C₆ to C₉, with the RSD <8% for all compounds. The temperature is a key factor for the stability of the compounds on the ASE performance. The compounds through the ASE at 40°C are very stable for at least 3 months and very less contamination. However, when extracted at 80°C on the ASE the compounds were not stable and later turned into their methyl esters within one month, as well as creating some contamination peaks that rendered the quantitative analysis useless.