Abstract: In a method for detecting a glucagon-secretin family peptide in a sample, the sample is contacted with a cleaving agent capable of digesting the glucagon-secretin family peptide by cleaving a peptide bond of at least one aspartic acid within the glucagon-secretin family peptide and thereby generating a plurality of peptide fragments, at least one of the peptide fragments containing an N-terminal end of the glucagon-secretin family peptide. The peptide fragment that contains the N-terminal end of the glucagon-secretin family peptide is then detected using liquid chromatography and mass spectrometry. The amount of peptide fragment that contains the N-terminal end of the glucagon-secretin family peptide in the sample can then be quantitated using a calibration curve.