Analysis of iso-alpha-acids and reduced iso-alpha-acids in beer by direct injection and liquid chromatography with ultraviolet absorbance detection or with mass spectrometry.

A liquid chromatography (LC) method is described for the simultaneous analysis of iso-alpha-acids and reduced iso-alpha-acids in beer. Volatile mobile phase additives were selected to enable hyphenation to mass spectrometric (MS) operated in the atmospheric pressure chemical ionization (APCI) mode. Contrary to other recent LC optimization procedures for the same compounds, an alkaline pH was selected hereby improving peak shape and selectivity. Both UV and MS detection are sensitive enough to analyze beers without sample pre-concentration. All major bitter acids are separated within 65 min with exception of cis-dihydroisoadhumulone, which co-elutes with trans-isocohumulone. Due to the selectivity of the MS, these compounds could be differentiated according to their m/z value. The performance in terms of quantification of bitter acids by LC-UV and LC-MS are compared for standard solutions and a selection of 14 beers.