Low-temperature bath/high-conductivity zone/stacking micellar electrokinetic chromatography for the analysis of phenolic acids in coffee drink

Two stacking methods in micellar electrokinetic chromatography (MEKC) were investigated in this article in an attempt to increase the amount of sample injected, as well as to focus analytes onto a small zone. One employed a “high-conductivity zone”, which was inserted between the sample zone and background solution to build an unequal conductivity gradient. The other employed a “low-temperature bath”. A portion of the capillary was immersed in a low-temperature bath, which served as a “pseudo-low-conductivity zone”. As a result, a large volume of sample injection can be achieved. Using three phenolic acids-chlorogenic acid (CGA), caffeic acid (CA) and ferulic acid (FA) in coffee as model compounds, the limit of detection (LoD) was determined to be 0.31 μg/ml (S/N = 3) for CGA by means of normal MEKC under suppressed electroosmotic flow (EOF). The LoD could be improved to 2.8 × 10−2, 5.3 × 10−3 and 6.0 × 10−3 μg/ml, respectively, when normal MEKC-stacking, high-conductivity zone MEKC-stacking and the low-temperature zone MEKC-stacking methods were applied. Furthermore, the high conductivity zone and the low-temperature bath were operated simultaneously on one capillary to investigate the synergism effect. The results showed that there did exist synergism effect for CGA and CA when the two were hyphenated. The stacking efficiency was higher than that of the single one used. However, there was not synergism effect for FA.