PERFORMANCE-CHARACTERISTICS OF GEL PROBES USED FOR MEASURING THE CHEMISTRY OF PORE WATERS

Polyacrylamide gel probes provide a rapid, simple procedure for measuring iron and manganese in pore waters at sub-millimeter resolution by diffusional equilibration in thin films (DET). Fe(II) and Mn(II) diffuse freely within the gel matrix, and with a 0.4 mm thick gel, complete equilibration is achieved in 6 min. Gels must be deoxygenated prior to deployment, and exposure to air limited to 15 s to avoid overestimation of iron. Treatment with 10 mmol L(-1) NaOH or KOH quantitatively immobilizes Fe and Mn as their insoluble oxyhydroxides and brings about a diffusional transfer of Fe and Mn to the edges of the gel. The time from removal from the sediment to fixing or slicing the gel is the greatest limit to resolution. Fixing within 20 s should allow good resolution at a millimeter scale, but to achieve 250-mu m resolution, fixing within 1-2 s in a 250 mu m thick gel is required.