To study transmetalation effects of the gadolinium-based contrast agent Magnevist (Gd-DTPA), the first analytical method for the simultaneous determination of Gd-DTPA and its transmetalation products in complex clinical samples was developed. The high separation efficiency of capillary electrophoresis (CE) was employed to separate Gd-DTPA, Fe-DTPA, Cu-DTPA, Zn-DTPA, and the free DTPA (diethylenetriaminepentaacetic acid) ligand. The coupling of CE with electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) provided the required sensitivity and excellent selectivity for the analysis of complex samples, such as blood plasma and whole blood. Separation and detection parameters were optimized, and crucial steps for CE/MS method development are pointed out. Limit of detection (LOD) is 5 x 10(-7) mol/L, limit of quantification (LOQ) is 1.7 x 10(-6) mol/L, and the linear range comprises 2 decades, starting at the limit of quantification. To determine recovery rates, precision, and accuracy of the method, blank plasma samples were spiked with Gd-DTPA in three different concentrations. Blood plasma samples from 10 patients with normal renal function, having received Magnevist, were analyzed for Gd-DTPA and possible transmetalation products by CE/ESI-TOF-MS. The method was validated by determination of the total Gd concentration using inductively coupled plasma optical emission spectroscopy (ICP-OES). Transmetalation assays of Magnevist with and without supplementary iron were carried out in incubated whole blood samples.