On page 1935, left, line 2, the injected sample amount for analysis was given incorrectly as a result of a miscalculation. The correct injected amount of Leu-enkephalin is 13 pg (23 fmol) and not 13 ng (23 fmol).
The original article to which this Erratum refers was published in Rapid Commun. Mass Spectrom. 2006; 20: 1932–1938.