Determination of caffeine in coffee products by dynamic complexation with 3,4-dimethoxycinnamate and separation by CZE.
A method based on the formation of pi-complexes with chlorogenate-like species was proposed for the determination of caffeine in regular (nondecaffeinated) and decaffeinated coffee. Both caffeate and 3,4-dimethoxycinnamate were able to transform caffeine--a neutral species in aqueous solutions--into an anionic species. The usage of 3,4-dimethoxycinnamate in the running electrolyte is advantageous, because of its greater chemical stability and the improved resolution of the peaks of caffeine, theobromine, and theophylline. Negative peaks were registered with a capacitively coupled contactless conductivity detector when solutions of these alkylxanthines were analyzed with a BGE composed of 20 mmol/L 3,4-dimethoxycinnamic acid and pH adjusted to 8.5 with Tris. This behavior was expected, because the complex is larger and thus should move slower than the free anion. Caffeine was determined in ground and instant coffee with precision and accuracy that meet Brazilian norms about such products. The LOD was estimated as 33 mg/L, which corresponds to 0.8 and 0.3 mg of caffeine per gram of dry instant coffee and ground coffee, respectively. For the case of decaffeinated coffee, ten times preconcentration with dichloromethane was carried out to allow the quantitation of caffeine, which should not exceed the concentration of 1 mg/g in dry matter.
Nogueira
Thiago
T
Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Avenida Prof. Lineu Prestes 748, CEP 05508-000 São Paulo, SP, Brazil.
do Lago
Claudimir Lucio
CL
D000838:Anions; D001628:Beverages; D001665:Binding Sites; D002110:Caffeine / Q000032:analysis* / Q000737:chemistry*; D002726:Chlorogenic Acid / Q000737:chemistry; D002934:Cinnamates / Q000737:chemistry*; D003069:Coffee / Q000737:chemistry*; D004553:Electric Conductivity; D019075:Electrophoresis, Capillary / Q000379:methods*; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013048:Specimen Handling; D013056:Spectrophotometry, Ultraviolet / Q000379:methods*; D013805:Theobromine / Q000737:chemistry; D013806:Theophylline / Q000737:chemistry
D000838:Anions; D002934:Cinnamates; D003069:Coffee; D002726:Chlorogenic Acid; D002110:Caffeine; C065784:3-(3,4-dimethoxyphenyl)propenoic acid; D013806:Theophylline; D013805:Theobromine