Sensitive determination of L-hydroxyproline in dairy products by capillary electrophoresis with in-capillary optical fiber light-emitting diode-induced fluorescence detection

Hydrolyzed leather proteins have been used illegally by some dairy industries in dairy products to increase the protein content in the last few years. As a specific amino acid in collagen, the detection of l-hydroxyproline (l-Hyp) can be used to indicate whether the hydrolyzed leather proteins are added to dairy products or not. A sensitive capillary electrophoretic method with in-capillary optical fiber light-emitting diode-induced fluorescence detection (ICOF-LED-IFD-CE) was developed for the determination of l-Hyp and further for identification of hydrolyzed leather proteins in dairy products. Fluorescein isothiocyanate (FITC) labelled l-Hyp (FITC-Hyp) was detected in a background electrolyte of 20 mM sodium borate at pH 9.4 and an applied voltage of 17 kV in less than 8 min. The limit of detection (LOD) for FITC-Hyp was 0.61 nM at the signal to noise ratio (S/N) of 3. The intraday precision (n = 6) of migration time and peak area was 2.65% and 1.97%, while the interday precision (n = 6) of migration time and peak area was 1.54% and 2.36%, respectively. The accuracy of the proposed method was judged by standard addition method, spectrophotometry method and commercial LIF-CE, and the results indicated that the proposed method was reliable. The ICOF-LED-IFD-CE method was successfully applied to determine l-Hyp in dairy products.