# Determination of Cellulose Degree of Polymerization in Historical Papers with High Lignin Content
## 1 Introduction  
Regardless of the complexity of paper material, the two main chemical degradation pathways are acid hydrolysis and oxidation. Both processes result in cellulose depolymerization, which ultimately leads to a loss of mechanical strength of the paper. The depolymerization of cellulose is primarily followed by determination of the weight average molecular mass (Mw) or the average degree of polymerization (DPv) of cellulose [1]. Determinations are performed either by a standardized procedure using viscometry or size-exclusion chromatography (SEC). Both techniques require sample dissolution, which limits their application. In the pulp and paper industry, conventional viscometry is still the standard approach for the quality control of bleached pulp. It is also quite commonly used by laboratories interested in paper degradation. Indeed, the solution viscosity of pulps allows a simple and easily applicable calculation of the cellulose molecular mass. The most commonly used nonderivatizing cellulose solvents for viscometry include copper(II) complexes with ethylenediamine (CED) or with ammonia (cuoxam) and cadmium ethylenediamine hydroxide (cadoxen) [2]. The three sources of fibrous materials primarily used for historical European paper include groundwood fibers produced directly from wood by thermomechanical processes,rags, or native cotton fibers, and partially delignified cellulose fibers derived from wood [3]. The influence of the lignin content in cellulose pulp on paper durability was recently demonstrated [4]. Contrary to the previous opinions that lignin causes changes in paper due to the hydrolysis of glycoside bonds and oxidation, the authors observed no effects of the lignin content on the degradation rate of paper produced at neutral pH [4]. A different correlation between the decrease in cellulose DP and the loss of mechanical properties was found for contemporary papers compared to the historical groundwood papers subjected to accelerated thermal degradation [5]. The relationship between the DP decrease of cellulose and loss of mechanical properties in the contemporary model groundwood pulp paper sample during accelerated degradation was determined to be linear, whereas an exponential correlation was observed for the historical groundwood pulp paper samples [5]. However, the methods for DP determination of cellulose in papers with high lignin content still represent a challenge for researchers in the field of cultural heritage. The high lignin content in groundwood pulps makes such papers insoluble in the most common nonderivatizing cellulose solvents. For the native lignin-containing papers for which viscometry is not suitable due to insolubility [3,6,7], i.e., actually for most real archival objects, SEC becomes the only method of choice [1,4,8]; unfortunately, it is not broadly available. SEC of pure cellulose can be carried out after dissolution in suitable solvents, LiCl-DMAC being the most common [9]. It is often necessary to derivatize the cellulosic sample to obtain a soluble material for SEC analysis in a moderately polar solvent, such as tetrahydrofuran (THF) [10]. Hydroxyl groups of the cellulose molecule can be converted to a variety of ethers and esters. These derivatives are often soluble and can theoretically be used for SEC analysis of cellulose derivatives. Thus, it is assumed that the information obtained on the molecular mass distribution of the cellulose derivative accurately reflects that of the original cellulose after appropriate adjustment for increased molecular mass due to a derivative formation. In the past, cellulose trinitrate and to a lesser extent cellulose acetate have been used as derivatives of choice. The tricarbanulate derivative of cellulose is now commonly used for SEC analyses and has several advantages over the nitrate derivative.