High-resolution nitrogen gas profiles in sediment porewaters using a new membrane probe for membrane-inlet mass spectrometry

A probe inlet for membrane-inlet mass spectrometry (MIMS) was developed to measure dissolved nitrogen and argon gas in sediment porewaters. This technique represents a significant improvement in the measurement of dissolved gas ratios in sediment cores and in small volume samples (<5 ml). The probe is stable, inexpensive and has relatively fast equilibration times (∼ 4-5 min) for dissolved N₂/Ar ratio analyses. The membrane probe is mounted at the end of a stainless steel capillary (0.75 mm OD) connected directly to the inlet system of a quadrupole mass spectrometer. The membrane is ∼ 1 mm x 0.5 mm resulting in profiles that have a depth resolution of < 2 mm. Nitrogen/argon ratios measured with the probe inlet have a precision of < 0.2%. Porewater N₂/Ar ratios measured in cores collected from Raritan Bay, NJ, indicated that N₂ was in equilibrium with the atmosphere in overlying waters and that nitrogen was up to 8% supersaturated by a depth of 1 cm. The increase in N₂ is consistent with nitrogen production from denitrification of ∼ 2 mmol N₂ m^-2 day^-1. High-resolution nitrate profiles also provide evidence for denitrification, with nitrate being depleted at the same depth horizon where N₂ gas concentrations are increasing.