Online profiling of living rat brain extracellular pH using a pH-Dependent solid phase extraction scheme coupled with microdialysis sampling and inductively coupled plasma mass spectrometry.

In this study, a novel analytical method was developed for online profiling of living rat brain extracellular pH. It involved microdialysis (MD) sampling, introduction of metal ions (e.g., Mn, Co, Ni, Cd) as acid/base indicators, and a pH-dependent solid phase extraction (SPE) based on polymer-metal ion interactions as a sample pretreatment scheme prior to analysis through inductively coupled plasma mass spectrometry (ICP-MS). With a 10-μL microdialysate, optimization of the system provided a working pH range from 5.8 to 8.2 and allowed differentiation of tiny changes in pH, down to a resolution of 0.015 pH units. Standard deviations of measured pH for 6 h of continuous measurement were below 0.05, which was sufficient for evaluating the homeostatic status and the dynamic fluctuations in living rat brain extracellular pH. The method's applicability was verified through measurement of the basal extracellular pH in living rat brains and through monitoring of its changes in response to perfusion of a high-K medium. By applying metal ions as acid/base indicators and coupling a pH-dependent SPE scheme with MD sampling and ICP-MS, this method presents an alternative strategy for reliable online profiling of the living rat brain extracellular pH.