Characterization of the polarity of natural organic matter under ambient conditions by the Polarity Rapid Assessment Method (PRAM)

The polarity rapid assessment method (PRAM) characterizes the polarity of aqueous natural organic matter (NOM) by quantifying the amount of material adsorbed onto different solid-phase extraction (SPE) sorbents. The analysis is performed under ambient conditions resulting in the elimination of pretreatment steps that may alter the chemical characteristics of the NOM, allowing an accurate representation of its polarity as it exists in the environment. Additionally, analysis only requires 200 mL of sample and can be performed in 2 h. In this paper, the underlying theory of the method is presented, followed by its optimization, with emphasis on the development of conditions for the analysis of NOM in natural waters. A series of organic probe compounds showed that the most important physicochemical property describing the interaction between the NOM and the SPE sorbents was the hydrophobic surface area, allowing for the estimation of the hydrophobic character under ambient conditions. Evaluation of the effects of chemical concentration, pH, and ionic strength show that (1) concentration did not have an effect on PRAM characterization as long as the pH and ionic strength remained constant; (2) changes in pH and ionic strength resulted in considerable changes in PRAM characterization, as a result of the changes in configuration of the NOM; and (3) PRAM characterization of NOM can be completed in the concentration range of <10 mg C/L, although this range could be expanded by evaluating the effect of concentration on a site-specific basis. Results indicate that measurement of both ultraviolet absorption and dissolved organic carbon show complementary results as they measure different aspects of NOM.