Comparison of isolation and quantification methods to measure humic-like substances (HULIS) in atmospheric particles

Humic-like Substances (HULIS) comprise a significant fraction of the water-soluble organic aerosol mass and influence the cloud microphysical properties and climate effects of aerosols in the atmosphere. In this work, the most frequently used HULIS isolation and quantification methods including ENVI-18, HLB, XAD-8 and DEAE were comparatively characterized with two model standards, ten interfering compounds, and five ambient aerosol samples. Quantification of HULIS is performed with a TOC analyzer, complemented by an investigation of the chemical structure of the extracted fractions by UV–Vis spectroscopy. The results show that the four isolation methods were all characterized by high reliability, high reproducibility, and low limit of detection (LOD), indicating that each method can be used to efficiently recover Suwannee River Fulvic Acid (SRFA) and be applied to the quantification of the lower amount of HULIS in atmospheric particles. The analytical results of the UV–Vis spectra of HULIS fractions isolated also indicate that they are all favorable for extraction of compounds of high UV absorbance, high MW, and high aromaticity and that the DEAE protocol is the most significant one. Compared with the DEAE method that favors extraction of highly UV-absorbing and more aromatic compounds, SRFA isolated by the ENVI-18, HLB, and XAD-8 protocols were more representative of the global matrix. Each method has its own advantages and disadvantages and is suitable for a particular application. No single method is ideal for both isolation and quantification of HULIS in atmospheric samples.