Analysis of caffeine,theobromine and theophylline in coffee by near infrared spectroscopy (NIRS) compared to high-performance liquid chromatography (HPLC) coupled to mass spectrometry

摘要

Coffee is one of the most important raw materials within the international trade,for which highest quality is demanded.Due to the high number of samples to be analysed,new analytical techniques providing fast and reliable data about the quality are essential.Therefore,we established a new analytical method based on near infrared spectroscopy (NIRS) for the quantitation of the three main alkaloids caffeine (Caf),theobromine (Tbr) and theophylline (Tph) in roasted coffee after discrimination of the rough green beans into Arabic and Robusta.This validated method was compared to the most commonly used liquid chromatography (LC) connected to UV and mass spectrometric (MS) detection.In this course non-porous silica-C18 showed to enable the analysis of the three alkaloids with high robustness within approx2.5 min,compared to 20min employing porous silica-C18 as a stationary phase.As analysis time plays an important role in choosing a reference method for the calibration of the NIR-spectrometer,the non-porous silica-C18 phase offers a very fast method.Coupling of the optimised LC method to a mass spectrometer (MS) via an electrospray ionisation (ESI) interface not only allowed to identify Caf,Tbr and Tph by their characteristic fragmentation pattern using collisionally induced dissociation (CID),but also to quantitate the content of the three analytes,which was found to be 6% higher compared to UV-detection.The validated LC-UV method was chosen as a reference method for the calibration of the NIRS system.Analysis of 83 liquid coffee extracts in random order resulted for Caf and Tbr in values for S.E.E.(standard error of estimation) of 0.34,0.40 g/100 g,S.E.R (standard error of prediction) of 0.07 and 0.10 g/100 g with correlation coefficients of 0.86 and 0.85 in a concentration range between 0.10 and 4.13 g/100 g.Compared to LC the lower limit of detection (LOD) of the NIRS-method is found at 0.05 g/100 g compared to 0.244-0.60 ng/100 g in LC,which makes it impossible to analyse Tph by NIRS.Therefore,main focus is put on the NIRS analysis of caffeine and theobromine.Nevertheless,NIRS offers a serious alternative to LC for the coffee producing industry because of the short analysis time of a few seconds and a guaranteed high sample throughput.