Quantification of [Dmt1]DALDA in ovine plasma by on-line liquid chromatography/quadrupole time-of-flight mass spectrometry

The synthetic peptide [Dmt(1)]DALDA (Dmt-D-Arg-Phe-Lys-NH(2); Dmt = 2',6'-dimethyltyrosine; 'super-DALDA') is a mu opioid-receptor agonist. On-line liquid chromatography/quadrupole time-of-flight mass spectrometry and the corresponding stable isotope-incorporated synthetic peptide internal standard were used to quantify [Dmt(1)]DALDA that had been extracted from ovine plasma samples. The [M+2H](2+) ion was used to construct the calibration curve, and the product ion was used for verification of the peptide. The detection sensitivity for the [Dmt(1)]DALDA [M+2H](2+) ion was 12.5 fmol and 50 fmol for the m/z 432.3 product ion. The concentration profile of [Dmt(1)]DALDA was determined from a set of ovine plasma samples. The molecular specificity of the peptide quantification was confirmed by tandem mass spectrometry (MS/MS).