Determination of β-carboline alkaloids in foods and beverages by high-performance liquid chromatography with electrochemical detection at a glassy carbon electrode modified with carbon nanotubes

Simple and sensitive methods for the separation and quantification of β-carboline alkaloids in foods and beverages by HPLC with electrochemical detection at carbon nanotubes–modified glassy carbon electrodes (CNTs–GCE) are reported. Electrode modification with multi-wall CNTs produced an improved amperometric response to β-carbolines, in spite of the working medium consisting of methanol:acetonitrile: 0.05 mol L−1 Na2HPO4 solution of pH 9.0 (20:20:60). On the contrary to that observed at a bare GCE, a good repeatability of the amperometric measurements carried out at +900 mV versus Ag/AgCl (R.S.D. of 3.2% for ip, n = 20) was achieved at the CNTs–GCE. Using an Ultrabase C18 column and isocratic elution with the above mentioned mobile phase, a complete resolution of the chromatographic peaks for harmalol, harmaline, norharmane, harmane and harmine, was achieved. Calibration graphs over the 0.25–100 μM range with detection limits ranging between 4 and 19 ng mL−1, were obtained. The HPLC–ED at CNTs–GCE method was applied to the analysis of beer, coffee and cheese samples, spiked with β-carbolines at concentration levels corresponding to those may be found in the respective samples. The steps involved in sample treatment, such as extraction and clean-up, were optimized for each type of sample. Recoveries ranging between 92 and 102% for beer, 92 and 101% for coffee, and 88 and 100% for cheese, at sub-μg mL−1 or g−1 analytes concentration levels were achieved.