Nutrient Analysis: Studying the Impacts of Freezing and Thawing Processes on Silicate Concentrations Using Programmable Flow Injection

Abstract

Oceanographic nutrient analysis is important for understanding biogeochemical cycling. However, research cruises last days, weeks, or months, requiring oceanographer to preserve collected nutrient samples when shipboard analysis equipment is unavailable. Samples are often preserved by freezing because of the simplicity of the technique. The effects of freezing on nutrient concentrations has been studied for decades, with still no consensus for freezing/thawing method. Because silicate is known to polymerize (silicate molecules bond with each other), this study focused on the effect of the freezing step in silicate behavior by investigating differences in silicate concentrations between varied storage methods and durations. Samples were preserved at three different temperatures (refrigerated, -18℃ freezer, and -80℃ freezer), then thawed at room temperature overnight and measured at three intervals post-collection and compared to shipboard measurements. Cruise and post-cruise concentration measurements were determined using programmable flow injection, a novel spectrophotometric technique via the Mini-Sequential Injection Analysis (mini-SIA) platform. Autodilution, a recently developed technique that automatically dilutes high concentration standard solutions via computer-controlled pumps, and the silicate chemical assay were optimized prior to sample measurements. In this study, results from the samples with silicate concentrations higher than 100 µM showed the most variability, and the data from the refrigerated samples showed the most stability over the entire twelve-week preservation period, indicating refrigeration is a reliable preservation method for silicate-containing samples.