The Analysis of Optical Brightening Agents in Paper Samples Using Liquid Chromatography with High-Resolution Mass Spectrometry

Abstract

There is limited information available on the analysis and characterization of optical brightening agents (OBA) commonly used in the pulp and paper industry. The available literature provides only a few reports on suitable chromatographic methods of analysis of OBA compounds present in paper products. The choice of suitable extraction solvents for OBA is also debatable. Some advocate using organic solvents, whereas others use water. The extracted OBAs are usually characterized by fluorescence spectroscopy or liquid chromatography with UV detection. These methods are not selective and do not yield information on the chemical structures of the compounds. We have characterized OBA compounds by liquid chromatography with high-resolution mass spectrometry, specifically liquid chromatography–quadrupole time-of-flight–mass spectrometry (LC-QTOF-MS). Preliminary extraction results show that boiling water is effective for characterizing OBAs from commercial papers. Positive-ion and negative-ion electrospray ionization allowed for observation of straightforward mass spectra for OBA standards and OBA extracts from paper products. The ESI-MS results for the most common OBA in the industry, TOBA (a tetrasulfonated stilbene derivative), gave a prominent m/z value of 1075.1797 in the negative-ion mode and 1077.1941 in the positive-ion mode corresponding to the [M − 1]− and [M + 1]+ species of tetrasulfonated optical brightening agent in the full acidic form. The LC-MS analysis of OBAs in paper extracts showed that they can exist in both cis and trans forms, an important parameter for proper quantitative analysis of OBAs. Preliminary experiments found that isomerization can actually occur when certain OBAs in solution are exposed to natural light.