A rapid and simple on-line solid-phase extraction coupled with isotope dilution-liquid chromatography-tandem mass spectrometry (SPE-ID-LC-MS/MS) method was developed to quantitate maleic acid in serum and urine of Sprague Dawley (SD) rats. The aforementioned biological samples were spiked with 13C2-maleic acid, vigorously vortexed, added with acetonitrile to precipitate proteins, and then injected into the on-line SPE LC-MS/MS system for quantification. Upon validation, this method demonstrated excellent feasibility and sensitivity: Calibration curves for maleic acid in serum and urine display excellent linearity with the coefficient of determination (R2) greater than 0.999, the limits of detection and quantitation (LOD and LOQ) for maleic acid were determined at 0.2 and 0.5 μg L?1, respectively. Additionally, intra-day accuracy for maleic acid in serum and urine samples ranged from 94.0 to 100.2% and 101.3 to 104.4%, respectively. Furthermore, inter-day accuracy ranged from 93.6 to 101.0% and from 102.3 to 111.4% in serum and urine samples, respectively. Intra-day precision %RSD of maleic acid in serum and urine samples was 13.8% or less, whereas the inter-day precision was 6.1% or less. The matrix effects were not found to be statistically significant (p = 0.9145 and p = 0.5378, respectively) based on the calculations of recovery functions. The collected serum and urine samples were analyzed using SPE-ID-LC-MS/MS. Our results reveal trace levels of maleic acid in the control rats; demonstrating that this method is capable of analyzing background levels of contaminants in biofluids with excellent sensitivity and specificity at parts-per-billion levels concentrations in complex matrices.
