Cumulative Distribution Function and Spatially Resolved Surface-Enhanced Raman Spectroscopy for the Quantitative Analysis of Emtricitabine

Surface-enhanced Raman spectroscopy (SERS) has exceptional analytical sensitivity and selectivity. However, SERS irreproducibility presents an obstacle when using it for precise quantitative measurements. In this study, colloidal nanoparticles evaporated to dryness are used as a SERS active surface for the detection of the HIV drug emtricitabine (FTC; trade name Emtriva). Despite the irreproducibility of the SERS resulting from the stochastic process of evaporation, using a SERS scanning instrument, the SERS enhancement factors of spatially resolved spectra have a well-defined distribution of signals for a given analyte concentration. This distribution follows a power law function ranging from weak (very abundant signals) to very strong (sparse signals). By definition, a power law distribution cannot have a true mean. Hence a cumulative distribution function was used to model the concentration of emtricitabine, and calibration curves were constructed. For stochastically generated quantitative data sets, the precision of this approach is superior to methods utilizing signal averaging and improves analytical sensitivity.