Abstract: A labeling reagent having a distinct Raman, or surface enhanced Raman, spectral signature is used for the control and analysis samples. The labeling reagents can be fluorescent dyes with different isotopic substituents, such as the substitution of some hydrogen atoms for deuterium atoms. Such labeling does not have any detectable effect on separation retention. Raman spectroscopy is used for detection purposes. By combining SERS and SERRS, a concentration ratio prediction error of less than 3% can be obtained over four orders of magnitude of total concentration with up to a factor of 3 concentration ratio range. The method is reliable, reproducible and more sensitive than methods based on absolute SERS/SERRS intensity correlations, with no internal standard, or using a different molecule (rather than an IEIS) as a SERS/SERRS internal standard.