An essential prerequisite for successful cryo-substitution is starting with good cryo-fixation such as double-sided propane jet freezing, metal mirror freezing or high pressure freezing. Successfully cryofixed samples, typically stored in LN2 are then transferred into the FS7500 where the substitution solvent is pre-programmed to a temperature below that where secondary ice crystal growth can occur (typically 190K for most biological samples) and above the melting point temperature of the solvent being used.

The FS7500 uses liquid nitrogen as a coolant to achieve a base operating temperature of –140C (133K) which is well below the ice recrystallization temperature in any biological structure. The temperature of the substitution chamber is set & regulated by the system controller to an accuracy of better than +/- 0.1C over the range from -140C to + 70C

The FS7500 is a free standing, caster wheel mounted, system with a large, hard acrylic, worktop mounted on to its 35 liter high efficiency LN2 Dewar. The temperature controlled substitution chamber is conveniently located in the center of the worktop for easy access. The chamber is provided with a viewing port/cover, which is simply lifted off to gain access into the substitution workspace. As it is lifted off, the ventilation system is automatically turned on to extract fumes and the chamber internal illumination is turned on.

Low temperature embedding in methacrylates, such as Lowicryl K4M, is commonly employed in freeze substitution and is accommodated in the FS7500 system with appropriate embedding molds and holders. Polymerization of the resin is generally by long wave UV irradiation for several hours at 240K. It is important during this process to minimize the presence of O2 in the chamber since O2 is a very strong inhibitor of the polymerization of the resin. This is achieved in the FS7500 by an automatic N2 gas generator, which is turned on whenever the chamber cover is removed. An ultraviolet lamp, with power supply, is provided as standard with each FS7500 system.

The substitution media is a solvent such as acetone or methanol, which slowly dissolves away and replaces the ice in the frozen specimen. The solvent is placed in commonly used laboratory items such as Eppendorf tubes, scintillation vials, glass cryo-vials, Beem capsules, gelatin capsules, etc, which in turn are placed in the FS7500 specimen holder. A universal specimen holder is provided with the system which accommodates most of these commonly used cryo-vials and containers, and a "start up" kit of these consumables is provided with each FS7500 system.

The specimen holders used with the FS7500 are designed with a large thermal mass to provide good temperature uniformity and stability.

These features combine to make the FS7500 an extremely convenient to use and versatile Freeze Substitution System.