Fig. 3: Xenobiotics are detected in a time-dependent manner in sweat from the fingertips after coffee consumption.

a Levels of normalised areas under the curve (nAUCs) for caffeine, chlorogenic acid and trigonelline, before (0) and 15 min (15) after coffee consumption are shown, demonstrating a significant increase in all participants (n = 13 × 2 time-points) after 15 min. D’Agostino & Pearson test was performed to check normality of the data. Paired two-tailed Wilcoxon signed rank tests were performed for 13 volunteer profiles, delivering a p-value = 0.0002 for caffeine, chlorogenic acid and trigonelline. The mean nAUCs (boxes) and standard deviations are the following: for caffeine 4.8 ± 4.4 at 0 min and 56 ± 35 at 15 min, for chlorogenic acid 0.03 ± 0.04 at 0 min and 0.8 ± 0.7 at 15 min, for trigonelline 1.0 ± 1.7 at 0 min and 12 ± 16 at 15 min. b The temporal evolution of metabolite profiles is exemplarily shown for one participant (Volunteer 3, study A). The sampling time-point 1440 min represents the time-point before consumption on the second sampling day. Whereas caffeine (violet) and chlorogenic acid AUCs (orange) were found to increase quickly after coffee consumption followed by rapid clearance, the levels of theobromine, paraxanthine and theophylline (green) increased more slowly within the observation period. c Similarity of extracted ion chromatograms (EIC) of chlorogenic acid and its isomers from coffee extracts and from sweat of the fingertips 15 min after coffee consumption. The corresponding sample collected just before coffee consumption (0 min) served as negative control.