Long freeze-drying times are not necessary during the preparation of thin sections for X-ray microanalysis

The temperature profile that occurs when a brass block warms up in a vacuum evaporation unit was determined. Freshly drawn human blood was concentrated by centrifugation, and the pellet was cryofixed and cryosectioned. The cryosections were subject to different freeze-drying protocols, using a freeze-drier with a temperature-controlled stage, to determine the effect of freeze-drying time on element distribution. Spectra were collected by spot analyses at various distances across the interface between red cells and plasma. Concentrations of sodium were high and variable outside the cell and low in the cell interior, with potassium showing the reverse distribution. The number of counts under the iron peak closely followed the potassium distribution. The concentration of sodium was higher than expected at 40 nm inside the cell membrane. This was attributed to the formation of ice crystals at the interface between the cells and plasma during cryofixation and the use of a wide probe size.