Determination of gentamicin after trimethylsilylimidazole and trifluoroacetic anhydride derivatization using gas chromatography and negative ion chemical ionization ion trap mass spectrometry

A gas chromatographic method for the determination of gentamicin, an aminog lycoside antibiotic, was developed. A two step derivatization method utiliz ing trimethylsilylimidazole for silylation of the hydroxyl groups and trifl uoroacetic anhydride for acetylation of the amino groups was used. Chemical ionization and negative ion monitoring resulted in greatly improved sensit ivity and more informative fragmentation of the gentamicin components compa red with electron ionization and positive ion monitoring. Tandem mass spect rometry was used for the identification of the gentamicin components and fo r characterization of the derivatized groups. The optimized method also all owed the use of three ions for selected ion monitoring. The method signific antly improved the determination of gentamicin compared with previously des cribed gas chromatographic methods. Chemical ionization and negative ion mo nitoring with the use of an ion trap analyzer appeared superior to other de tection modes.