Determination of mercapturic acids using 1,4-dihydroxynaphthalene, a new matrix for matrix-assisted UV laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometr y (MALDI TOF-MS) was applied for the characterization of six N-acetylcystei ne conjugates (mercapturic acids) of synthetic origin. The measurement of m ercapturic acids can provide important information for the biomonitoring of exposure to electrophilic compounds. Five isomers of dihydroxynaphthalene were evaluated as a MALDI matrix, 1,4-dihydroxynaphthalene was found to be very effective and the most suitable and was therefore used for further mea surements. It has efficient absorbance at 337 nm, low background matrix ion signals and low matrix adduction,In the positive-ion mode only very weak s ignals were detected. In the negative-ion mode, the spectra exhibited depro tonated molecular ions [M - H](-) of high abundance and no adduct peaks or fragment ions were observed, This enabled reliable molecular mass determina tion of all six mercapturic acids. A detection limit of 1 pmol (signal-to-n oise ratio = 3) applied to the target was achieved, which is distinctly mor e sensitive than other detection methods. In addition, the data obtained fo r intensities of the deprotonated molecular ion [M - H](-) enabled the reco rding of the degradation kinetics of solutions with two distinct pH values during five days. Among six investigated compounds, only one, 1,2-dihydroxy -4-(N-acetylcysteinyl)butane (DIOL), was found not to be stable in aqueous solution.