arginine and ADMA and SDMA, respectively. The addition of l-arginine, ADMA, or SDMA did not influence the measurement of the other analytes. We also determined within-and between-run imprecision (CVs; n ϭ 10). The within-run CVs were 3.3% for l-arginine, 2.6% for ADMA, and 2.5% for SDMA. Between-run CVs were 4.7% for l-arginine, 4.1% for ADMA, and 3.9% for SDMA We also measured ADMA in plasma samples from healthy human volunteers (n ϭ 22) by this LC-MS/MS method. Shown in Fig. 1 is a typical chromatogram of a human plasma sample. Samples were aliquoted (50 L) and frozen at Ϫ20°C before analysis. The mean (SD) concentrations of l-arginine, ADMA, and SDMA were 65.6 (23.4), 0.55 (0.14), and 0.69 (0.23) mol/L, respectively. The mean (SD) molar ratio of l-arginine to ADMA was 132 (55).Our LC-MS/MS-based method allows for the simultaneous determination of ADMA, SDMA, and l-arginine and has been validated for the measurement of these 3 analytes in human plasma. Values obtained for l-arginine, ADMA, and SDMA in human plasma are within previously reported ranges (11)(12)(13)16 ). Major advantages over previous reported methods for the measurement of ADMA include a minimized sample volume and reduction in laboratory work. In particular, in comparison with other LC-MS-based methods (13,14,16,17 ), sample run time has been reduced to 4 min including reequilibration. Chromatographic separation required only slightly more than 2 min. This considerable reduction in analysis time was achieved by use of the specific fragmentation patterns of the ester derivatives of ADMA and SDMA. Identical MS spectra were obtained from the ester derivatives of ADMA and SDMA, but the major fragments in the daughter ion spectra for ADMA and SDMA were at m/z 214 and m/z 228, respectively. These ions were specific for ADMA and SDMA: they were absent in the corresponding spectra of SDMA and ADMA, respectively. In contrast to previous reports, the proposed method allows, for the first time, spectrometric instead of chromatographic separation of these methylated arginines (see the Data Supplement that accompanies the online version of this Technical Brief at http://www.clinchem.org/content/ vol51/issue7/). When a fetus is subjected to a massive perinatal hypoxicischemic insult, it may suffer sufficient damage to cause intrauterine death and stillbirth (1)(2)(3)(4)(5). In less severe,
