Sensitive Quantitative Analysis of C-Peptide in Human Plasma by 2-Dimensional Liquid

established for quantification of metabolites or small drug molecules in biological fluids. Because of their increased specificity, IDAs are an alternative to immu-noassays for measuring C-peptide. Methods:We evaluated a 2-dimensional liquid chroma-tography–mass spectrometry (2D LC/MS) IDA method. Sample preparation was by off-line solid-phase extrac-tion, and C-peptide separation was performed on an Agilent 1100 2D LC system with a purification method based on high-pressure switching between 2 high-reso-lution reversed-phase columns. Because of the low frag-mentation efficiency of C-peptide, multiple-reaction monitoring analysis was omitted and selective-ion mon-itoring mode was chosen for quantification. Native and isotope-labeled ([M18] and [M30]) C-peptides wer