New Strategies for the Physico-chemical Characterization of Nitrocellulose with Capillary Electrophoresis

Nitrocellulose (NC) is a nitrate cellulose ester polymer whose nitrogen content determines its physical and chemical properties, and its industrial applications. Indeed, NCs with a nitrogen content less than 12.5% are widely used as raw material in daily use products (printing inks, paints, lacquers, varnishes and filter membranes), whereas highly-nitrated NCs (nitrogen content > 12.5%) are employed in the manufacturing of explosive materials (smokeless gunpowders and dynamites). Therefore, and also for regulation reasons, methods for the determination of nitrogen content in NCs are needed , and there is a strong need in developing more modern, simpler, faster, and safer methods. In a first part, the denitration yield obtained after alkaline hydrolysis of NCs was optimized by means of an experimental design [1,2]. Capillary electrophoresis (CE) was used to follow the reaction through the quantitation of nitrite and nitrate ions released during hydrolysis. Then, a new method to determine the nitrogen content of NCs, based on a linear relationship between it and the molar ratio of nitrite to nitrate ions released after alkaline hydrolysis was developed [3]. These methods were successfully applied to various explosive and non-explosive NC-containing samples. The second part aimed at studying the products obtained after partial acid depolymerization in an attempt to generate a mixture of oligosaccharides retaining information on the initial NC sample and/or the cellulose used to prepare it, which would be of forensic interest. To this end, NC fingerprinting was achieved by CE with laser induced fluorescence detection (LIF) after acid depolymerization, and derivatization with 8aminopyrene-1,3,6-trisulfonic acid (APTS) [4]. The resulting derivatized oligomers were identified using APTS-derivatized cellodextrin standards. These depolymerization products can also be analyzed with a new CE method with a high-alkaline background electrolyte containing sodium chloride and detection was performed by mid-UV absorbance after photo-oxidation in the detection window [5]. Finally, the advantages of CE instrumentation (miniaturization of the assay and automation) were used to determine the intrinsic viscosity of various NC samples [6]. Thanks to the calculation of Mark Houwink parameters, the weight-average molecular weight of unknown NC samples could be determined.