Water content from analysis of freeze-dried thin sections

The analytical electron microscope and related electron beam instruments are uniquely able to measure the water content of biological tissue at a subcellular level. An analysis of the Rick procedure for quantifying this water content (Dörge et al., 1978; Rick et al., 1979) reveals a hidden assumption; namely, that the sample and standard densities, in the hydrated state, must be equal. In general this will not be valid. Equations are derived that do not invoke this hidden assumption. These equations show that errors in using the Rick technique for measuring water content should be small, a few per cent, unless the sample contains a very heterogeneous distribution of mass or the water content of the standard is a poor representation of the sample. For freeze-dried samples with extensive mass heterogeneity, such as skin, the equations for calculating water content presented in this paper should be used.