A spectrometer cell having an internal volume for containing a sample gas, the spectrometer cell comprising: a spacer, the spacer at least partially defining the internal volume and comprising a first contact end; an end cap (304, 324), the end cap positioned proximate to and contacting the first contact end and at least partially enclosing the internal volume; and a mirror (104, 106, 302, 322), the mirror comprising a reflective surface for receiving and redirecting a beam of light at least once along an optical path length that originates from at least one light source, the reflective surface having an optical axis and wherein the reflective surface is formed of a material comprising at least one of stainless steel, copper, aluminium, alumino-silicate alloys, ceramic, and a composite material, the optical path length passing at least once through the internal volume, the mirror (104, 106, 302, 322) being secured in place by a mechanical attachment, the mechanical attachment holding the optical axis in a fixed orientation relative to other components of the spectrometer cell and of a spectrometer device that comprises the spectrometer cell, characterised in that a direct and stable physical contact exists between the end cap (304, 324) and the spacer secured by at least one attachment device, the direct and stable physical contact ensuring at least one of a reproducible alignment and a reproducible orientation of the optical axis relative to the beam of light when the spacer and the end cap are assembled, wherein the attachment device comprises screws, clamps or bolts, to form the mechanical attachment including the direct and stable physical contact, wherein the reflective surface is disposed on a detachable mirror part, the detachable mirror part being mechanically connectable to a face of the end cap (304, 324), the detachable part and an inner face of the end cap (304, 324) having mating reference surfaces that ensure at least one of a specific alignment and a specific orientation of the optical axis relative to the beam of light when the detachable mirror part and the end cap (304, 324) are joined and the end cap (304, 324) is assembled to the spacer, wherein the mating reference comprises a recess being machined into one side or face of the end cap to tight tolerances such that the detachable mirror fits snugly into the recess, wherein the spacer and at least one of the end cap (304, 324) and the mirror (104, 106, 302, 322) have a similar thermal expansion coefficient. a spacer, the spacer at least partially defining the internal volume and comprising a first contact end; an end cap (304, 324), the end cap positioned proximate to and contacting the first contact end and at least partially enclosing the internal volume; and a mirror (104, 106, 302, 322), the mirror comprising a reflective surface for receiving and redirecting a beam of light at least once along an optical path length that originates from at least one light source, the reflective surface having an optical axis and wherein the reflective surface is formed of a material comprising at least one of stainless steel, copper, aluminium, alumino-silicate alloys, ceramic, and a composite material, the optical path length passing at least once through the internal volume, the mirror (104, 106, 302, 322) being secured in place by a mechanical attachment, the mechanical attachment holding the optical axis in a fixed orientation relative to other components of the spectrometer cell and of a spectrometer device that comprises the spectrometer cell, characterised in that a direct and stable physical contact exists between the end cap (304, 324) and the spacer secured by at least one attachment device, the direct and stable physical contact ensuring at least one of a reproducible alignment and a reproducible orientation of the optical axis relative to the beam of light when the spacer and the end cap are assembled, wherein the attachment device comprises screws, clamps or bolts, to form the mechanical attachment including the direct and stable physical contact, wherein the reflective surface is disposed on a detachable mirror part, the detachable mirror part being mechanically connectable to a face of the end cap (304, 324), the detachable part and an inner face of the end cap (304, 324) having mating reference surfaces that ensure at least one of a specific alignment and a specific orientation of the optical axis relative to the beam of light when the detachable mirror part and the end cap (304, 324) are joined and the end cap (304, 324) is assembled to the spacer, wherein the mating reference comprises a recess being machined into one side or face of the end cap to tight tolerances such that the detachable mirror fits snugly into the recess, wherein the spacer and at least one of the end cap (304, 324) and the mirror (104, 106, 302, 322) have a similar thermal expansion coefficient.