The purpose of this research is to build an inexpensive, compact, and precise spectrometer that will measure the 13C/12C ratio in a sample of carbon dioxide gas. The infrared diode laser-based apparatus operates by modulating the laser frequency as it scans. The frequency modulation is then converted to amplitude modulation in the signal. A ratio of signal intensities for 13CO2 and 12CO2 can be converted to a measurement of the samples’ isotope ratio. The analysis of the 13CO2/12CO2 ratio can be used for the detection of stomach ulcers and in various environmental applications.