## 1 Introduction  
The analysis of historical objects is often difficult due to restricted sampling. The value of historical substance and its integrity is rarely outweighed by the information we gain through destructive sampling and subsequent chemical analysis. In the analysis of paper, we are often interested in the parameter, which is crucial for its durability, i.e. pH [1]. The degradation pathway and the rate of chain scission depend on this parameter: in an acidic environment, cellulose degrades according to acid-catalysed hydrolysis, while in a moderately alkaline environment, it degrades through autoxidation. The latter process is much slower and the lifetime of traditionally produced moderately alkaline paper (produced prior to \(\sim\)1850) is thought to be several millennia. This is in a stark contrast with acidic paper produced between 1850 and 1990: some documents from this period are not expected to survive another century. The catastrophic circumstance is that it is estimated that more than 70% of documents in Western libraries are acidic [2].  
This is why the determination of acidity in historical paper is a major analytical task [3]. Most of the techniques presently available are based on traditional pH sensors, such as flat surface pH electrode. Unfortunately, the material surface needs to be wetted for the purpose, which is unacceptable for valuable objects. Micro-pH sensors have been developed, with which only a few micrograms of fibres are analysed since micro-sampling is occasionally permitted [4].  
There is a source of information which has been largely overlooked in development of analytical methods for cultural heritage: emissions of volatile organic compounds (VOCs). Ever since [PERSON] had summarised his pioneering work on pyrolysis of biomass in 1982 [5], it has been known that VOCs are information-rich: using HPLC and IEC with cryo-trap sampling differences were shown in degradation pathways of paper in different reaction conditions [6]. Following the advent of solid-phase micro-extraction (SPME) [7-9], the use of this technique was recently explored also for characterisation of paper [10,11] and selection of the appropriate fibre material and different sampling techniques were studied. It was found out that the DVB/CAR/PDMS SPME fibre was the most appropriate for extraction of the variety of VOCs emitted from paper. Contact sampling was carried out for 15 days at room temperature and a wide array of volatiles was identified. The concept of contact SPME was developed for a variety of purposes, including studies of emissions from live insects [12].  
The presence of VOCs, especially acetic acid, formic acid and formaldehyde in museum environments can have corrosive effects on a variety of materials. SPME in combination with GC-MS has been proposed as the method of choice for such analyses [13]. While paper is a source of VOCs, it is also a potent adsorbent [14] and during quantitative work this effect has to be taken into account.  
It was our primary interest to relate the information on VOC emission, which can be obtained using contact-SPME sampling with chemical information on the emitting material. VOCs can be produced in a variety of cellulose degradation reactions. It is known that low-MW organic acids form during end-wise degradation of polysaccharide chains, especially acetic and formic acid and similarly, furfural is a well-known product of acid-catalysed hydrolysis of cellulose [15]. It might be expected that during natural ageing, these products accumulate in a book and are emitted only slowly [16]. Thus, the degradation pathway may influence the VOC profile of a degrading paper to an appreciable extent.  
In this paper, we used two SPME techniques: static headspace sampling at an elevated temperature and well-defined conditions, and contact sampling at room conditions. The extracted VOCs were analysed using GC-MS and the relative abundances were compared with paper pH. It turned out that contact SPME can be used to estimate the pH of paper in a non-destructive manner.