Single point calibration for semi-quantitative screening based on an internal reference in thin layer chromatography-SERS: the case of Rhodamine B in chili oil

Thin layer chromatography (TLC) has been used in combination with surface enhanced Raman spectroscopy (SERS) for onsite screening of various analytes. In this work, we propose a novel concept for future field semi-quantitative SERS screening applications, where calibration curves were pre-built in the lab but subjected to onsite single point known standard amendment. Rhodamine B (RhB) in chili oil, a case of food scandal reported in China, was chosen as our model sample. A standard calibration curve of RhB was built using melamine as an internal standard and used throughout the assay. Before the analysis of samples, one single known RhB standard mixed with melamine was tested and used to calibrate the previously built standard calibration curve. RhB in chili oil was separated through the TLC method. Then, it was extracted and mixed with melamine. The signal of the mixture was recorded and compared with the single point calibrated calibration curve, instead of building a new curve each time. A limit of quantification (LOQ) of RhB from chili oil by SERS, 1.00 x 10(-7) M, was realized for the first time, and the recovery range was from 66.1% to 110%, despite the fact that the cheapest and non-uniform SERS substrate was used. We expect such a protocol could be used for future fast onsite food quality assurance inspection.