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Extracted mass spectrometry chromatograms of the standards spiked in blank urine at concentrations of 250 ng/mL. Data: coumarin, in the positive ionization mode (a); 7-hydroxycoumarin, in the negative ionization mode (b); o-hydroxyphenylacetic acid, in the negative ionization mode (c); and o-coumaric acid, in the negative ionization mode (d).
