An apparatus for analyzing a vapor comprising: a module assembly, the module assembly including: a module assembly housing, with a first faceplate having a first optical port (374a, 374b), and contained in the housing an analysis unit rigidly attached to the first faceplate, the analysis unit being configured to receive a sample to be analyzed, the sample attenuating light introduced into the analysis unit via the first optical port; and a main assembly, the main assembly including: a main assembly housing, with a second faceplate having a second optical port (474a, 474b), and contained in the housing an engine unit having a detector, wherein the engine unit is rigidly attached to the second faceplate and is configured to generate light that passes to the analysis unit via the first and second optical ports wherein the module assembly, when attached to the main assembly, causes the first and second faceplates to act as a single mechanical unit; the apparatus further comprising: a first alignment pin attached to the second faceplate; a second alignment pin attached to the second faceplate; a first hole in the first faceplate; and a second hole in the first faceplate, wherein the first alignment pin mates with the first hole and the second alignment pin mates with the second hole when the main assembly is attached to the module assembly. a module assembly, the module assembly including: a module assembly housing, with a first faceplate having a first optical port (374a, 374b), and contained in the housing an analysis unit rigidly attached to the first faceplate, the analysis unit being configured to receive a sample to be analyzed, the sample attenuating light introduced into the analysis unit via the first optical port; and a module assembly housing, with a first faceplate having a first optical port (374a, 374b), and contained in the housing an analysis unit rigidly attached to the first faceplate, the analysis unit being configured to receive a sample to be analyzed, the sample attenuating light introduced into the analysis unit via the first optical port; and a main assembly, the main assembly including: a main assembly housing, with a second faceplate having a second optical port (474a, 474b), and contained in the housing an engine unit having a detector, wherein the engine unit is rigidly attached to the second faceplate and is configured to generate light that passes to the analysis unit via the first and second optical ports wherein the module assembly, when attached to the main assembly, causes the first and second faceplates to act as a single mechanical unit; the apparatus further comprising: a first alignment pin attached to the second faceplate; a second alignment pin attached to the second faceplate; a first hole in the first faceplate; and a second hole in the first faceplate, wherein the first alignment pin mates with the first hole and the second alignment pin mates with the second hole when the main assembly is attached to the module assembly. a main assembly housing, with a second faceplate having a second optical port (474a, 474b), and contained in the housing an engine unit having a detector, wherein the engine unit is rigidly attached to the second faceplate and is configured to generate light that passes to the analysis unit via the first and second optical ports wherein the module assembly, when attached to the main assembly, causes the first and second faceplates to act as a single mechanical unit; the apparatus further comprising: a first alignment pin attached to the second faceplate; a second alignment pin attached to the second faceplate; a first hole in the first faceplate; and a second hole in the first faceplate, wherein the first alignment pin mates with the first hole and the second alignment pin mates with the second hole when the main assembly is attached to the module assembly. a first alignment pin attached to the second faceplate; a second alignment pin attached to the second faceplate; a first hole in the first faceplate; and a second hole in the first faceplate, wherein the first alignment pin mates with the first hole and the second alignment pin mates with the second hole when the main assembly is attached to the module assembly.