The methods and apparatus described may be used for the discrimination of two closely spaced spectral lines. In certain embodiments the methods and apparatus may be used for the detection of isotopes and isotopic ratios by Raman spectroscopy. In certain embodiments the conversion of a labeled substrate to a labeled product may be detected. In particular embodiments the conversion of a <13>C labeled substrate to a <13>C labeled product may be detected. In other embodiment the conversion of <13>C urea to <13>CO2 is detected. Embodiments described include the diagnostic determination of <13>CO2/<12>CO2 ratio in a test sample.
