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A specific and sensitive liquid chromatography-electrospray ionization tandem mass spectrometry method (LC-ESI- MS/MS) was developed and validated for the simultaneous quantification of porphyrins (coproporphyrin, pentacarboxylporphy- rin, hexacarboxylporphyrin, heptacarboxylporphyrin, and uroporphyrin) in human plasma and urine. Acidified plasma samples and urine samples were prepared by using liquid-liquid extraction using ethyl acetate and protein precipitation with acetonitrile, respectively. The separation was achieved onto a Synergi Fusion RP column (150 mm × 2.0 mm, 4 µm) with a gradient elution of mobile phase A (0.1% formic acid in 2 mmol/L ammonium acetate, v/v) and mobile phase B (20% methanol in acetonitrile, v/ v) at a flow rate of 450 µL/min. Porphyrins and the internal standard (IS), coproporphyrin I- 15 N4, were detected by a tandem mass spectrometer equipped with an electrospray ion source operating in positive ion mode. Multiple reaction monitoring (MRM) transitions of the protonated precursor ions and the related product ions were optimized to increase selectivity and sensi- tivity. The proposed method was validated by assessing selectivity, linearity, limit of quantification (LOQ), precision, accuracy, recovery, and stability. The calibration curves were obtained in the range of 0.1-100 nmol/L and the LOQs were estimated as 0.1 nmol/L for all porphyrins. Results obtained from the validation study of porphyrins showed good accuracy, precision, recov- ery, and stability. Finally, the proposed method was successfully applied to clinical studies on the autism spectrum disorder (ASD) diagnosis of 203 Korean children.