The Remspec low-temperature extension screws onto the end of the standard probe shaft; it includes an optical coupling crystal for good signal transfer. An overall probe diameter of 8 mm (approximately 5/16 inch) is maintained for access to very small reactors. This particular head is fitted with a ZnSe crystal suitable for use with liquid and semi-liquid samples. The Remspec low-temperature extension has been tested down to -100 C.