Quantitative determination of erythromycin 2'-ethylsuccinate in human plasma by fast atom bombardment mass spectrometry.

A method for simultaneous quantitative determination of erythromycin 2'-ethylsuccinate and erythromycin in human plasma is described. After extracting the deuterium-labelled internal standards and analytes from alkalinized plasma to diethyl ether the extracts were analysed by fast atom bombardment mass spectrometry. The quantification limit for 2'-ethylsuccinate ester was 50 ng ml-1 and for erythromycin 100 ng ml-1. The calibration curves were linear up to 5 micrograms ml-1 for both. The precision of the method at higher concentration was less than 2% and at the quantification limit approximately 6% for both analytes.