CHARACTERIZATION OF PAPER BY NEAR INFRARED SPECTROSCOPY
Dirk Lichtblau, Manfred Anders
ZFB Zentrum für Bucherhaltung, Leipzig, Germany
lichtblau@zfb.com
coresponding author: manders@zfb.de
Conservation and preservation decisions take into account some general strategic guidelines and are
based on condition of the collection in question. The condition can generally be defined by
physical, (micro)biological and chemical state of paper preservation. Since chemical damage is
usually not visually detectable, it thus represents an especially difficult and critical parameter
contributing to well-informed collection management. Since most classical and widely available
methods of analysis are either too expensive, too time-consuming, require extensive sample
preparation, are destructive, or even require a large amount of sample, their application to originals,
even for microanalysis, is hardly conceivable.
For these reasons, an NIR (near infrared) spectroscopic analytical technique was developed
potentially enabling us to evaluate whole collections within a reasonable timeframe.' The technique
does not require any sample preparation; collection and evaluation of a spectrum is a
straightforward procedure, and it is non-destructive. Using an additionally developed software tool
NIR-PR (NIR Paper Rating), safe evaluation of the condition of an artefact or even a collection is
possible. The use of NIR spectroscopy for characterization of paper is well-known and has already
been used in paper degradation studies.2
The NIR-PR tool makes use of standardized measurement categories (parameters) which are used to
describe the state of degradation.' Among these, mechanical properties, degree of polymerization
(DP), lignin content and pH are undoubtedly the most important, although other parameters such as
carbonyl group content can also be determined. The basis for any quantitative NIR method
development is a good set of reference data. These must be obtained from a large and well
representative set of sample papers of historical origin.
The data of chemical; and predominantly mechanical characterization were subsequently correlated
with NIR spectra using advanced chemometric (statistical and mathematical) methods, e.g.
multivariate statistics. From the obtained correlations, the state of degradation of a certain sample
can be evaluated.
In this contribution, we will shortly review the state of the art of the methodology and explore some
future needs and prospects.
1. Method development
As a result of a considerable number of analyses, a classification model has been developed and
implemented in the NIR-PR tool.
The classification according to the NIR-PR tool is done in the following way. After spectroscopic
characterization of a sample, evaluation of the state of degradation according to the determined
physical and chemical parameters is performed on the basis of preset criteria. These criteria are
independent of the general strategic preservation plan, and are selected in order to mirror the
decision-making system on the basis of which a sample is considered more or less stable. An
inclusion of a proposal of a treatment strategy in the NIR-PR is optional. The recommendation
regarding the applicability of mass deacidification can also be included (Fig. 1).
Figure 1: Classification model for the rating of paper condition.
On the basis of such a model three ratings are given: States 1, 3 and 5. With an increased set of data
in future investigations a finer division and better definition of the thresholds 1-3 and 3-5 is
planned.
In order to obtain a satisfactory set of workable data, more than 100 different wood pulp and
cellulose papers produced between 1886 and 1993 were analyzed in a destructive way. The real
historical samples were obtained from a variety of sources in Germany - in the phase of method
development, we have not attempted to obtain a sample set representative for a variety of paper
sources and geographical origins. Among the more important parameters, tensile strength (Fmax) in
the machine direction and intrinsic viscosity were determined. The definition of limits of the
Groups and States in Figure 1 was done according to the obtained data. Alter the limits were
defined, the data obtained from NIR spectra by chemometric evaluation were grouped accordingly.
Additionally, as the individual properties are not all equally significant for the definition of paper
condition, statistical weights had to be associated with each property.
In order to minimise the error associated with NIR-PR, separate methods were developed for the
two sets of samples: wood pulp and bleached cellulose/cotton. This was necessary due to the widely
different spectral characteristics. Besides, the intervals of measured chemical and mechanical
parameters depended on the paper variety very much. E.g., lower tensile strength Fmaxwas
determined in wood pulp papers than in celluloses as a rule.
In order to enable this division into two categories, the lignin content of papers was characterized
during spectroscopic evaluation first: if lignin content was found to be lower than 10%, the sample
was characterised as bleached cellulose/cotton paper.
2. Experimental
NIR is the spectral area between visible and mean infrared light (12,000-4,000 cm-1, i.e. 850-2,500
nm). Absorption of light in this interval of wavelengths corresponds to a great variety of
combinalion and overtones, which increases the information content of the spectrum. Its exact
analytical interpretation is therefore hardly possible. However, using the chemometric approach, the
complex interrelated information within a spectrum is reduced to a few specific basic patterns
(absorption bands), the parameters of which can then be correlated with known properties of the
measured system, i.e. paper. Such treatment of NIR spectra allows for easy, fast and chemical-free
quantitative analyses.
The spectra arc obtained on the basis of trans-flection (transmission and reflection): the sample is
irradiated with non-destructive NIR light from an integrated source and the reflected spectrum is
recorded using an array detector. High reproducibility can be achieved if good resolution and
thermal stability can be assured.
The NIR system developed by our group consists of a portable dispersive NIR spectrometer with a
specially constructed external "sampling head" for solid materials, which is connected to the
spectrometer by means of an optical fibre and is equipped with a sample carrier and a plate to gently
press the sample down on the sampling head during a measurement. A computer, which is an
integral part of the instrument, allows for immediate display of the results (Figure 2).
The paper to be characterized is placed on the outlet of the external light source. In order to assure
high precision, at least three consecutive measurements are carried out per evaluation in three or
more different
Figure 2: NIR spectrometer with the periphery.
parts of the sample (measurement spots). The spots should be selected in a statistical manner in
order to take into account possible sample inhomogeneities. The recorded spectra are automatically
averaged, followed by an instant display of the rating according to the NIR-PR tool. The required
time of analysis per object depends on the required sample handling: for the actual recording of
spectra and calculating the results, milliseconds are needed.
Due to the large penetration depth of NIR light, measurements have to be carried out on several
layers of paper or with a ceramic background such as the one used for calibration. Text on the paper
docs not interfere with measurements; however, sections with high colour saturation (e.g.
illustrations with a high percentage of black pigments) are to be avoided. While only minimal
pressure against the sampling head is sufficient for a book, a metal plate should be used for loose
sheets in order to obtain higher reproducibility.
3. Software tool
The elaborated method was integrated into a Microsoft-Windows-based software application with
an appropriate user interface. The combinalion of the spectrometer and application software enables
the operator to start evaluation with a mouse click. After having chosen a specific file and entering
an appropriate sample description, internal calibration of the system and measurement is started by
the software, automatically. The evaluation progress is displayed on the monitor. As a result, the
paper condition is displayed, described by the state (Figure 1).
For the evaluation of large collections, the results are compiled in a table. The number of
characterized samples is not limited. In addition to sample description, a commentary, as well as the
date and time arc optional parameters. Certainly, the results of all individual measurement
categories are also compiled in the file, which can be easily converted into a Microsoft Excel file or
other similar application.
4. Outlook
Wood pulp and bleached cellulose containing acidic papers account for a greater part of endangered
paper collections in libraries, museums and archives. NIR-PR enables us to survey the condition of
such collections on a scientific basis in a repeatable manner. It is also useful to monitor the ageing
behaviour of collections by performing repeated evaluations. The non-destructive NIR-PR
evaluation represents a powerful tool which enables the collection manager to plan efficient
preservation strategies.
In the EC 6th Framework research project SwveNIR we will considerably expand the sample sets
with regard to the geographical and material characteristics (dale and type of production) thus
extending the possibilities of the NIR-PR for surveying extremely large and variable collections.