1. A method of diagnosing a health problem in a patient comprising: administering a 13 C-isotope-labeled compound which decomposes at a specific site in the body to a patient; using a Raman analyzer to determine an isotope ratio of 12 CO 2 / 13 CO 2 in breath exhaled from the patient comprising the steps of: splitting light emitted from the Raman cell into a first beam and a second beam; filtering the first beam to remove a Raman scattered line; interrupting periodically the first and second beam; converting light from the first beam into a first electrical signal; converting light from the first beam into a second electrical signal; comparing the first electrical signal to the second electrical signal to normalize the first electrical signal, and recovering normalized signal intensity by electronic digital Fourier analysis; and monitoring the isotope ratio of 12 CO 2 / 13 CO 2 as a function of time.