Estimation of the average molecular weight of humic-like substances isolated from fine atmospheric aerosol

Rural fine aerosol collected in Hungary was analysed with different techniques to estimate the average molecular weight (AMW) of humic-like substances (HULIS). First, ultrafiltration was applied to the total water-soluble organic fraction. Virtually all the water-soluble organic matter passed through the ultrafiltration membrane having a 500 Da nominal molecular weight (MW) cut-off. Then a significant portion (more than half) of the water-soluble organic matter was isolated in pure form and further analysed by liquid chromatography coupled to mass spectrometry. These analyses confirmed that the isolated organic matter was mostly composed of HULIS, which had been suggested on the basis of elemental analysis and different spectroscopic studies earlier. Furthermore, mass spectrometry indicated that the AMW of these substances could be expected in the 200–300 Da range. As detailed in the paper, this estimate may be negatively biased to some extent as a consequence of unwanted processes in the mass spectrometer. In order to reduce uncertainties in the MW estimates another independent technique—vapour pressure osmometry—was also applied and resulted in number AMW estimates in the range of 215–345 Da.