Mixed-Mode On-Line SPE-LC Broadens Analytical Coverage for Non-Target Discovery of Mobile Chemicals in Water

Discovery and quantification of hazardous mobile substances in environmental waters are challenged by their poor retention in solid-phase extraction (SPE) or reversed-phase (RP) liquid chromatography (LC). Here we employed a test-mixture of 218 diverse analytes (log P -4.8 to 7.9) and tested 13 columns to optimize and validate two complementary on-line SPE-LC workflows to maximize analytical coverage for a range of hydrophobic and hydrophilic analytes in target and non-target analysis. When combining the on-line SPE-RPLC and -mixed-mode (MM)LC methods developed here, each consuming only 1-2 mL sample per injection, we detected 206 of 218 analytes with median absolute recovery of 96% and median method detection limit of 1 ng/L. The SPE MMLC method enabled expanded chemical-space coverage for smaller, more polar and ionic analytes. When applied to 21 urban stormwater samples, 46 target analytes were quantified, including 1,3-diphenylguanidine (42-5200 ng/L), diphenyl phosphate (5.6-550 ng/L) and perfluorohexanoic acid (5.0-11 ng/L) in all samples. Many of the non-target annotations (level 2a) were unique to the RPLC (N=100) or MMLC methods (N=95), demonstrating broader analytical coverage by their complementarity application. Annotations included many known anthropogenic chemicals from traffic and urban activity, as well as novel discoveries such as the tire-wear compound isomer N-(4-methylpentan-2-yl)-N-phenylbenzene-1,4-diamine.