Quantitative analysis of antibiotics by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Comparative studies of the matrix-assisted laser desorption/ionization (MALDI) of 30 antibiotics were made using alpha-cyano-4-hydroxycinnamic acid (HCCA), 2,5-dihydroxybenzoic acid (DHB), 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphyrin and meso-tetra(N-methyl-4-pyridyl)porphyrin matrices. Most antibiotics generated intense protonated molecules in HCCA and DHB matrices, and sodium or potassium adduct ions in porphyrin matrices. Using sulfonamide antibiotics as model compounds, DHB was found to be the most suitable matrix for quantitative analysis. Detection limits were about 1 picomole. Linear correlation (R2 > 0.97), between the sample quantity over the range 1 to 100 picomole and the signal response, was obtained when ratios of the sum of peak areas of protonated molecules and sodium adduct ions, with reference to that of a structurally analogous internal standard (acetaminophen), were measured. The precision was found to be in the range of 4 to 32% relative standard deviation, dependent on the type and concentration of the analyte. A simple acylation derivatization process was developed to confirm the presence of suspected antibiotic residues. It is demonstrated that MALDI is a viable technique for the quantitative analysis of low molecular weight antibiotics.