Application Note: PFAS in drinking water by fully automated SPE-LC-MS/MS

PFAS is determined by automated weak anion exchange SPE coupled to LC-MS/MS. A 1mL sample provides LOQs below 1ng/L. Adsorption loss is eliminated.

The perfluorinated carbonic and sulfonic acids listed in the EU Drinking Water Directive (EU 2020/2184) are determined by a fully automated method based on weak anion exchange SPE coupled directly to LC-MS/MS. Limits of quantification (LOQs) were determined from calibrations in the range of 0.2 – 2.0 ng/L according to DIN 32645. These are all below 1 ng/L, enabling monitoring of the sum of 20 PFAS at the 0.1 μg/L limit set by the EU Drinking Water Directive. The method accuracy was demonstrated based on analysis of spiked 1 mL water samples from different sources. Relative standard deviations were below 10% and trueness mainly between 80 and 110%. LC/MS systems are becoming ever more sensitive, in principle making direct injection of water samples a competitive alternative for low level analysis of PFAS. But the analysis of long chain acids is challenging. These are adsorbed on surfaces leading to low, irreproducible recovery. To overcome this, water samples must be diluted with methanol and filtered prior to analysis. Online SPE enables quantitative transfer of analytes to the analysis system, resulting in improved limits of detection and quantitation, even when sample volumes are significantly reduced – such as the 1 mL sample volume used here. In this work, the efficiency of SPE is combined with the simplicity of direct injection. Elution is performed with ammonia in methanol, which cannot be transferred directly onto the LC column. Automated cartridge exchange as well as automated rinsing of the entire sample flow path between injections is performed. The rinse solution is then included in the analysis, reducing analyte loss to an absolute minimum. All steps of a typical SPE workflow are performed automatically including conditioning, loading, rinsing, and eluting the cartridge. Following the elution step, the cartridge is removed from the HPLC mobile phase flow path and the next sample prepared during the ongoing LC-MS/MS analysis without adding to the overall analysis time ensuring highest possible throughput.