Multivariate optimization of headspace solid-phase microextraction followed by gas chromatography-mass spectrometry for the determination of methylpyrazines in cocoa liquors

Abstract

A central composite design was used for the multivariate optimization of an analytical method for the quantitative determination of methylpyrazines in cocoa liquors using headspace-SPME followed by GC-MS. The optimal conditions for the three experimental parameters affecting SPME performances were: equilibration time of 5. min, extraction time of 65. min, and extraction temperature of 60. °C. The internal standard method was used for the quantitative determination of four pyrazines (2,3-dimethylpyrazine, 2,5-dimethylpyrazine, 2,3,5-trimethylpyrazine and tetramethylpyrazine). Good figures of merit were obtained: limits of quantitation ranged from 0.7 to 1.5. ng/g, repeatability intraday varied between 9 and 10% and interday between 17% and 24%. The optimized method was then applied to cocoa liquor samples of Ghana and Ecuador, produced with different roasting temperatures in an Italian industrial plant. Ghana cocoa liquors showed higher methylpyrazine content, with tetramethylpyrazine being the most abundant in all samples. With regard to differences related to roasting temperatures, dimethylpyrazine and trimethylpyrazine showed higher content in samples treated at 145/140. °C than those roasted at lower temperatures, while tetramethylpyrazine remained rather constant.