Septum Sorption: A Hidden Bias in Headspace Analysis of Neutral PFAS and Its Implications for Air–Water Partition Coefficients

Abstract

Reliable quantification of neutral per- and polyfluoroalkyl substances (nPFAS) via headspace analysis remains hindered by unrecognized analytical biases. We reveal a severe, previously unquantified artifact arising from sorptive interaction between nPFAS and commonly used polytetrafluoroethylene (PTFE)/silicone septa, resulting in time-dependent signal loss exceeding 90% within 12 h. Incorporating septum sorption into the headspace equilibrium model shows that this artifact systematically distorts air–water partition coefficients (Kaw), artificially masking true nPFAS volatility and compromising environmental fate modeling. A simple aluminum-foil barrier successfully eliminates septum artifacts, restoring analyte stability and enabling reliable Kaw determination. These findings reveal a critical but underappreciated source of systematic error in headspace workflows, highlighting the need to account for sorptive artifacts to understand nPFAS environmental behavior.