A. It is important for several reasons. First it must be reasonably thick to ensure that the rate of water flow has a negligible effect on the measurement. Second it must be sufficiently strong to ensure a robust device that can be readily manufactured and deployed in demanding conditions. Thirdly it must not interact with the analyte, as this can affect the measurement. Research has shown that the diffusive gel we use in most products, a polyacrylamide gel crosslinked with an agarose derivative, known as APA, best meets these requirements. Charged species, such as trace metal cations and anions, interact with gels that themselves have a charge. When correctly prepared, there is negligible charge on the APA gel, even at ionic strengths of 1 mM, and so DGT measurements can be interpreted with confidence. This is not the case if agarose is used as the binding layer. This gel has a distinct negative charge at ionic strengths less than 10 mM, and consequently DGT measurements in freshwaters of di or trivalent cations, using this as the diffusion layer, are overestimated, while anions are underestimated. Some of our competitors supply such devices without pointing out these limitations. We only use agarose as the diffusive layer for analytes with little charge where it will not affect the measurement, particularly for organic compounds and uni-valent species.