## 3 Results and Discussion
### Static Headspace Sampling  
Using this methodology, the sample (paper, pulp or cellulosic) is inserted into a glass vial, which is then closed and thermostated at 80 \({}^{\circ}\)C for 18 h. The mass of sample, temperature and duration of this sample preparation procedure were optimised in order to obtain appropriate signal-to-noise (S/N) ratios for compounds of interest and thus well repeatable results. For this purpose, sample no. 14 was chosen randomly. The sample size may vary, although it has to be taken into account that smaller samples lead to less emitted VOCs and as a consequence, less reliable results (Figure 1). With the proposed procedure, the typical uncertainty of determination of chromatographic peak areas was less than 10%, which was considered satisfactory. Figure 1: Comparison of chromatographic peak areas for acetic acid and furfural emitted by sample no. 14 in a glass vial at 80 \({}^{\circ}\)C in 18 h. Conditions of extraction using SPME fibre: 40 \({}^{\circ}\)C, 1 h. The intervals denote standard deviation of triplicate experiments. A certain period of time is needed for the paper sample to equilibrate with the surrounding atmosphere. A prolonged period of heating as used in the proposed procedure leads to accelerated degradation of sample and as a consequence, it leads to increased emission of volatiles. This procedure of sample pre-treatment is important, as we may assume that the emissions from sample predominantly mirror the degradation, which occurred during the 18-h heating period and not during previous natural degradation. Thus, better reproducibility may be expected. Certainly, this procedure is not suitable for valuable objects. As can be seen in Figure 2, release of acetic acid is almost independent of the temperature during the sample preparation (degradation) phase, and an increased content is only determined at 250 \({}^{\circ}\)C. The detected emissions of furfural increase considerably with temperature. Although at higher temperatures we detect more degradation products, leakage of VOCs from vials needs to be taken into account. In addition, the degradation of paper at temperatures \(>\)100 \({}^{\circ}\)C is likely to be less representative of the degradation observed at \(<\)100 \({}^{\circ}\)C, where water undoubtedly plays an important role and the conditions more closely resemble those of natural ageing. Optimisation of this step is thus not possible in view of production of VOCs. We thus decided for the most reasonable procedure, both from the viewpoint of duration and from the aspect of degradation reactions we are interested in (i.e. temperature). Thus, the conditions of 80 \({}^{\circ}\)C for 18 h were finally chosen. In view of the temperature, the experimental conditions also reflect the ISO 5630-3:1996 standard on accelerated degradation of paper. The fact that the content of emitted VOCs in a vial reflects degradation during the 18-h sample preparation period is important in comparative studies of freshly prepared samples made of filter paper, impregnated with phosphate buffer solutions of various pH (Figure 3). The results clearly show that the area of the peak representing acetic acid does not vary with pH in the observed pH range,  
Figure 2: Comparison of chromatographic peak areas for acetic acid and furfural emitted by sample no. 14 in a glass vial at different conditions. Conditions of extraction using SPME fibre: 40 \({}^{\circ}\)C, 1 h. The intervals denote standard deviation of triplicate experiments. while that of furfural does. In fact, a satisfactory correlation between furfural peak area and pH has been obtained. It is of interest to see that the emission of acetic acid is almost independent of the sample pH - this must be a consequence of not only the degradation chemistry but also of the fact that above pH 5 acetic acid exists mainly in the form of acetate. Since acetate readily absorbs at pH above 5 emissions of volatile acetic acid will be expected only from a highly acidic sample (pH \(<\) 4).