Determination of amino acid isotope ratios by electrospray ionization-mass spectrometry

Electrospray ionization-mass spectrometry has been shown capable of measuring the isotope ratios in the amino acids, proline, leucine, and arginine with standard deviations of around 0.1%, obviating the need for derivatization and GC separation. The efficiency of electrospray ionization coupled with the sensitivity of the quadrupole mass filter and ion multiplier allows the isotope ratio to be measured with less than 50 nmol of amino acid. The resolution of a standard commercial quadrupole mass filter is capable of providing sufficient resolution such that there is minimal contribution of the primary molecular mass ion, [M+H]+, to either the isotopically important [M+ H + 1)+ or [M+ H - 1]+ peak. The isotope dilution curve between 0 and 25% is linear, with a correlation coefficient of 0.993. It is shown that the precision is great enough that the addition of 0.85 mol% of a single 13C-labeled isotopomer was easily detected and quantified.