Effect of casting temperature on the swelling degree and water content of agarose crosslinked with oxalic acid as DGT diffusion layer

Diffusive Gradient in Thin Films (DGT) is an in situ passive sampling technique that employs a sampler comprised of a hydrogel-based diffusion layer. Agarose can be used as a substitute for polyacrylamide hydrogels. Due to its large pore, agarose should be cross-linked with oxalic acid to make the separation of analyte more selective. Agarose was cross-linked with oxalic acid by adding a solution of agarose and a solution of oxalic acid (50 percent weight of agarose) at 60 °C. The mixture was stirred, poured into a mold, and heated in an oven for 15 minutes at a casting temperature of 40, 50, and 60 °C. The gels were soaked for one, two, three, and up to twenty-four hours after being rinsed with distilled water. At each immersion period, the mass, length, width, and thickness of the gel were measured. The swelling degree of the hydrogel was calculated by comparing the mass before and after immersion. The gravimetric method was used to determine the water content. The results indicated that the swelling degree of agarose cross-linked with oxalic acid after a 24-hour immersion at temperatures of 50 and 60 °C was not significantly different. However, the swelling degree of the gel from both casting temperatures was considerably different from 40 °C. The water content of hydrogels made at 40, 50, and 60 °C differed considerably from non-crosslinked agarose.