There are some cases where collecting a sample and processing it on board is just not good enough.  This is the case when tasked with determining the exposure of benthic invertebrates and eggs to porewater solutes such as metals.  It has been shown that commonly used methods of centrifugation and pressurization can shift the equilibrium between dissolved and particulate species, even when performed in N2-purged environments.  Even in a controled atmosphere and a decrease in O2 from 21% to 1%, the redox potential is still high enough to oxidize ferrous iron and shift equilibria.  Further, these approaches generally use fairly thick intervals of sediment, and lose the finer resolution that may be needed for the receptor of interest.  Diffusive gels and diffusive equilibrators require multiple trips to the field and are limited in the quantity of sample collected.
The solution in many cases, and for some species of elements, such as iron (III), manganese (II), sulfide, lead, copper, cadmium and chromium, is in situ measurement with micro-electrodes.  This is not a commonly used method due to the high cost of equipment and required expertise of the operator.  Micro-electrodes have low-ppb detection limits and sub-millimeter vertical resolution, so can be used to assess conditions at any interval of interest.   Measurements can be made in situ with remotely-operated landers and micro-manipulators.  An alternative is to bring a large and intact sample to the surface where direct observation of the interface is possible prior to measurement.  Benthica is equipped and experienced in this specialized sampling and analysis.

Above is a 10 micron diameter gold micro-electrode encased in a pasteur pippette, which is used to profile sediments.  To the right is a sediment microcosm that shows nearly vertical invertebrate burrows that irrigate the sediment, the accumulation of fecal pellets at the surface, as well as some light and dark zonation that is of interest.  Below, a vertical profile of the zones adjacent to a burrow is in progress  (micro-electode on the right, and counter and reference electrodes to the left.  A 3-axis micro-manipulator holding the electrode is used to advance the electrode at increments as small as 10 microns.