A flow cell for use in highly corrosive environments is constructed from an extended metal body containing a smooth longitudinal circular center bore of uniform cross-section. The metal body contains a circular opening at each end extending perpendicularly into the center bore, adapted to receive a sample into the center bore and discharge the sample from the center bore. A circular connector having a larger diameter than the center bore is contained in each of the opposing ends of the center bore, abutting the solid body to form a right angle shoulder between the inside of the connector and the solid body. Opposing fiber optic probes each with an external sapphire window which is sealed into a metal tube contained in each probe with melted glass and an optional plastic seal over the melted glass are contained in the circular connectors, each said window extending into the center bore a sufficient distance so that the face of said window is in vertical alignment with the nearest internal surface of the circular opening into the center bore. An o-ring gasket is positioned between the right angle shoulder and metal tube end to form a tight cover over the glass seal (and plastic seal) when the probes are assembled in the flow cell, thereby protecting the glass seal (and plastic seal) from the corrosive environment.