Chemical stabilization of glass microfiber type F filters for measuring particulate phosphorus using the extra high‐temperature dry combustion method

Abstract Accurate quantification of particulate phosphorus is critical for understanding biogeochemical processes in aquatic systems. Extra high‐temperature dry combustion at 800°C improves phosphorus recovery by ~ 11% compared to lower‐temperature methods. Glass microfiber type F (GF/F) filters are often used for particulate carbon, nitrogen, and phosphorus analysis but melt at 800°C, entrapping phosphorus within the melted matrix and leading to poor phosphorus recovery, thereby limiting their compatibility with the extra high‐temperature dry combustion method. To address this limitation, we show that adding 3200 μ mol of magnesium sulfate (MgSO 4 ) yields statistically equivalent recoveries of total particulate phosphate from both laboratory‐grown phytoplankton and seawater particulate matters collected on 25 mm GF/F filters, comparable to recoveries from 25 or 47 mm polycarbonate filters. We hypothesize that MgSO 4 reacts with SiO 2 in the GF/F filter matrix to form crystalline MgSiO 3 , increasing the thermal stability of the filters (melting point >1000°C). This transformation preserves filter structure during the 800°C combustion and enables complete recovery of KH 2 PO 4 and particulate phosphorus. Structural and chemical changes to the GF/F filter due to MgSO 4 addition were confirmed by scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and X‐ray diffraction. The addition of MgSO 4 to GF/F increases their thermal stability, making them suitable for the extra high‐temperature dry combustion method; consequently, particulate carbon, nitrogen and phosphorus samples can all be collected and analyzed on 25 mm GF/F filters, improving the consistency of particulate C : N : P estimates in aquatic biogeochemistry.