An on-line liquid chromatography/tandem quadrupole (LC/MS/MS) instrument was used to monitor aromatic acids in a complex mixture. This mixture contains components known to exist as humic acid degradation products. The use of the tandem quadrupole removed LC solvent interferences and selected the protonated molecular ion of a particular acid to undergo collisional activation. The resulting fragments provide the specificity to detect target acids; the CA spectrum is “clean” and resembles the reference spectrum. This specificity is unobtainable by LC/MS alone. Present LC/MS/MS detection limits are 5 μg injected on column (50 ng in source) for a full scan detecting fragment ions greater than 5% relative intensity.