Weakness of external standard calibration curve for quantification of caffeine in several instant coffee brands by UV-VIS spectrophotometer

Abstract

Caffeine in coffee is a natural stimulant that can improve focus and energy level. Quantification of caffeine in instant coffee available in Malaysian market is important for the quality control purposes and to assist those who has low tolerance with caffeine to plan their daily coffee intake. In this work, UV-VIS spectrophotometer was used to measure the caffeine signal in the samples. The signal of caffeine measured by the UV-VIS spectrophotometer was extracted from the maximum absorbance value of the spectra. The instrument did not measure the concentration unit. Therefore, a calibration curve was plotted to obtain a relationship between the maximum absorbance and its corresponding concentration unit. In this work, an external standard calibration curve was explored for estimation of caffeine content in five brands of instant coffee. Caffeine content in Pendekar and Chek Hup were accurately quantified using the external calibration curve (% bias < 10 % when compared with the value manufacturer’s claim). However, the method inaccurately quantified caffeine content in Aik Cheong, Yit Foh and Hang Tuah as these samples had matrix interference that cannot be corrected by the external calibration method. Thus, in general external standard calibration curve can accurately quantified caffeine content in instant coffee but the sample must not consist of any matrix interferences.