Electrospray ionization Fourier transform mass spectrometry quantification of enkephalin using an internal standard

Fourier transform mass spectrometry (FTMS), long‐known for its capabilities in structural characterization of molecules, is an emerging tool in quantification, and quantification methods using external and internal standards with electrospray ionization (ESI) FTMS have recently been demonstrated. Here, commercial ESI‐FTMS is used to quantify the opioid pentapeptide methionine enkephalin using an internal standard. Linear working curves over three orders of magnitude are obtained using the internal standard, an improvement of one order of magnitude over the previous external standard ESI‐FTMS quantification method for enkephalins. Low coefficients of variation (generally