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

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Abstract
Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF-MS) was applied for the characterization of six N-acetylcysteine conjugates (mercapturic acids) of synthetic origin. The measurement of mercapturic 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 measurements. It has efficient absorbance at 337 nm, low background matrix ion signals and low matrix adduction. In the positive-ion mode only very weak signals were detected. In the negative-ion mode, the spectra exhibited deprotonated molecular ions [M – H]− of high abundance and no adduct peaks or fragment ions were observed. This enabled reliable molecular mass determination of all six mercapturic acids. A detection limit of 1 pmol (signal-to-noise ratio = 3) applied to the target was achieved, which is distinctly more sensitive than other detection methods. In addition, the data obtained for intensities of the deprotonated molecular ion [M – H]− enabled the recording 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.

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