Research article

Received: 23 December 2011 Revised: 16 February 2012 Accepted: 17 February 2012 Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/jrs.4055

Some ideas on the definition of Ramanspectroscopic detection limits for the analysisof art and archaeological objects†

Peter Vandenabeelea* and Luc Moensb

Raman spectroscopy is increasingly often used for the investigation of archaeological and art objects. Often, questions areasked concerning achievable limits of detection. However, the definition of Raman spectroscopic detection limits is notstraightforward, especially in art analysis where the investigation of solid particles, often dispersed in a solid matrix, isinvolved. Moreover, apart from the definition of the limit of detection, as often qualitative analysis is performed, it is equallynecessary to establish a definition for the limit of identification of a product. Moreover, some ideas are discussed on thedescription of relative Raman band intensities and on how to include this in the definition of the limit of identification. Inthis paper, some topics are raised, and we hope to initiate with this paper future discussions in the Raman spectrometrycommunity on this topic. Copyright © 2012 John Wiley & Sons, Ltd.

Keywords: Raman spectroscopy; archaeometry; art analysis; limit of detection

* Correspondence to: Peter Vandenabeele, Department of Archaeology, GhentUniversity, Sint-Pietersnieuwstraat 35, B-9000 Ghent, Belgium E-mail: Peter.Vandenabeele@UGent.be

† This article is part of the Journal of Raman Spectroscopy special issue entitled“Raman spectroscopy in art and archaeology” edited by Juan Manuel MadariagaMota and Danilo Bersani.

a Department of Archaeology, Ghent University, Sint-Pietersnieuwstraat 35,B-9000 Ghent, Belgium

b Department of Analytical Chemistry, Ghent University, Krijgslaan 281 (S-12),B-9000 Ghent, Belgium

Introduction

Raman spectroscopy is an analytical technique that is increasinglyoften used for cultural heritage studies.[1] Indeed, Raman spectros-copy is a versatile technique, which is basically non-destructive (ifthe laser power is not too high, there is no permanent damage tothe artefact or the sample). Mobile Raman spectrometers arecoming increasingly more available for this research, allowing usto perform in situmeasurements, directly on the artefact.[2,3] Whenusing a micro-Raman spectrometer, it is possible to record spectraof particles in samples, down to ~1mm. Depending on the samplepreparation (e.g. when using embedded samples), it is possible toevaluate the stratigraphy of the sample. Raman spectroscopy is amolecular spectroscopic technique; therefore, spectra can berecorded of inorganic as well as of organic components. As aconsequence, the technique can be applied for the investigationof antique as well as of modern objects (e.g. objects painted withsynthetic pigments).

Raman spectroscopy is increasingly often used to study degra-dation phenomena that are taking place. Often, these studiesevaluate the influence of environmental stresses on the artwork.These include, amongst others, the occurrence of aerosols in theambient atmosphere, which are attacking construction materialsor wall paintings.[4] In literature, software has been used toevaluate possible degradation pathways that can happen, basedon thermodynamic constants, to evaluate possible equilibriumstates.[5] This approach can help the interpretation of complexmixtures, but apart from the thermodynamic aspects, also kineticeffects and phase transitions should be taken into account.

To preserve objects under optimal conditions, it is highly impor-tant to monitor the air quality in museum enclosures (e.g. displaycases and storage rooms). Traditionally, parameters such as relativehumidity, temperature and light intensity are monitored, but itwould be good to be able to monitor other parameters as well.Currently, the MEMORI dosimeter, which contains a passive air

J. Raman Spectrosc. (2012)

sampler that is placed in the enclosure and a dosimeter reader,is developed. This dosimeter measures the combined degradationeffect of climate, inorganic and organic vapours to which an objectis exposed in an enclosure. Moreover, possible degradation pro-cesses of different materials (varnish, pigments, leather, parch-ment, paper and textiles) are studied.[6]

However, when degradation products are being studied, eitheron artificially degraded test samples in the laboratory or on objectsby means of direct analysis, a question arises: What is the minimalamount of degradation product that can be detected? In otherwords, the detection limits for these products should be determined.However, as will be discussed in this paper, the definition (hence thedetermination) of the Raman spectroscopic limit of detection (LOD)is not straightforward in the case of cultural heritage objects. It isclear that the discussion on hand is closely related to the (quantita-tive) determination of constituting components in mixtures.

The issue on the definition of Raman spectroscopic LODs isimportant in the investigation of objects of art and antiquities,as well as in other related research domains.[7] Although Ramanspectroscopy is often considered as being not very sensitive,its unique characteristic of being able to provide a molecularspectrum of a single micrometre-sized grain makes it extremely

Copyright © 2012 John Wiley & Sons, Ltd.

a.

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P. Vandenabeele and L. Moens

sensitive and particularly suited in several research areas, such asarchaeometry, forensics and geosciences. Indeed, the technique isoften used for the identification of traces of drugs of abuse ontextile fibres,[8,9] explosives[10] or biomarker molecules in ageological matrix.[11,12] Apart from focussing solely on thedefinition of the LOD, we will point out that the experimental setup is equally important. Mobile and handheld spectrometers arefrequently used for in situ investigations, but good positioningequipment to focus the laser beam on the object is highly impor-tant. Handheld spectrometers have usually a larger spot size (hencesampling volume) as well as a lower spectral resolution, which maycause that, in some cases, a lower LOD can be reached.[7]

The aims of this research paper are to discuss the difficultiesthat rise with the definition of LODs for Raman spectroscopicmeasurements of solids in a solid matrix and to focus on particu-lar problems in the field of cultural heritage. We will point outsome possible pitfalls and difficulties, and we will indicate somepossible approaches towards solutions. This paper, however, willbarely provide definitive answers, but it should rather beconsidered as a starting point or a thinking path towards furtherdiscussion in working groups of specific organisations, such asthe International Union of Pure and Applied Chemistry (IUPAC).

Figure 1. (a) Microscopic image of a mixture of orange (red lead, Pb3O4)and yellow (Hansa yellow, C.I. PY1) pigments. (b) The same image aftercomputer processing to clearly mark the difference between the differ-ently coloured pigments. The projected circles (diameter of 5mm) onrandom locations illustrate that different measurement positions mayresult in different intensity ratios in the obtained Raman spectra.

Results and discussion

Dispersions of solids in solids

Most Raman experiments in the field of cultural heritage involvethe measurement of solid particles in a solid matrix, such asmixtures of pigment grains in a binding medium. When Ramanspectroscopic measurements are performed, the laser beam isfocussed in a small volume, typically of a few cubic micrometreswhen performing micro-Raman measurements. This favourablecharacteristic allows the user to identify minute inhomogeneitiesand to obtain often a spectrum of only one of the components ina mixture, avoiding the interference of the other materials thatare present. As typical pigment sizes are usually in the micro-metre range, micro-Raman spectroscopy is extremely well suitedto examine the different grains separately. In the past, thisapproach has been applied to study relative concentrations inmixtures of vermilion (HgS) and red lead (Pb3O4).

[13] Oppositeto measurements of liquid solutions, in our cases, on the micro-metre-level, the samples are usually not homogeneous andthe meaning of quantitative measurements on a microscale isarguable. However, when it comes to measuring (relative) con-centrations in inhomogeneous mixtures, the positioning of thesample in the laser beam, as well as the beam size (which isrelated to the selected objective lens), is critical.This is illustrated in Fig. 1, where, as a test case, a microscopy

picture of a mixture of two pigments (red lead (Pb3O4) and aHansa yellow pigment (C.I. PY1)) is presented. The upper pictureis a greyscale image, where, clearly, the two pigments can bedistinguished, whereas the lower part represents the same areabut after image processing to enhance the distinction of thetwo pigments. In Fig. 1(b), circles with a diameter of 5 mm arerandomly drawn, to illustrate how different measurements mayresult in different ratios between the two components. We areaware that this illustration oversimplifies the problems of quanti-fication and disregards, for instance, different Raman crosssections of the pigments, partial absorption and enhancementeffects, the focal depth, inhomogeneity of the laser beam andso on. However, this approach illustrates the point that the

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Raman spot size and positioning influence the detected intensityratios – hence the achievable LOD.

When defining detection limits for Raman spectroscopic mea-surements of cultural heritage materials, it is critical to be awareof the measured volume in relation to the inhomogeneity ofthe sample. Apart from the spot size, also the focal depth is ofimportance, especially as archaeometrical samples are often notentirely flat. When confocal measurements are performed, withthe size of the confocal pinhole changed, the focal depth canbe influenced. However, although the theoretical description ofthe sampled volume is based on a non-absorbing medium, inreality, the shape of the sampling volume is influenced byabsorption; thus, matrix effects influence the LOD. Moreover,absorption as well as penetration depth is dependent of the laserwavelength. In objects of cultural heritage, often, multilayermaterials, such as a varnish layer covering different paint layers,are involved. As a consequence, not only absorption of the laserand the Raman-scattered light by (matrix of) the layer understudy determines the sampled volume but the interaction withthe upper layers should also be taken into account.

As the use of mobile instruments has increased for in situ inves-tigations of art objects, it should be remarked that the samplingvolume under these non-laboratory conditions is usually poorlydefined. Spot sizes are usually larger than by using laboratoryinstrumentation, and good positioning equipment is very impor-tant. This positioning equipment should be able to easily focusthe laser and to keep the probe head stable during the measure-ment. Apart from this, good positioning equipment should alsobe versatile (i.e. easily adaptable to the in situ conditions on hand)

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The definition of Raman spectroscopic detection limits in art and archaeology

and should allow easy movement from one spot to another pointto analyse.[14]

So, when measuring concentrations or ratios of particles embed-ded in a solid matrix or when establishing the corresponding LODs, itis important to adapt the measured volume to the size of the parti-cles. Using appropriate objective lenses is one approach that can beused, or using multiple measurements on different locations (andaveraging)[15] or rotating the sample[16] might be useful. However,this approach makes that one of the advantageous properties ofRaman spectroscopy (its high spatial resolution) is neglected. Onthe other hand, if the particles can be visualised and discriminated un-der themicroscope, it is often possible to record a Raman spectrumofa single micrometre-sized pigment grain – which might be consid-ered as a very low overall LOD. The difficulty is to find that particularspot to focus the laser beamon; therefore, it is very important to com-bine micro-Raman measurements with a good microscopic survey.

Apart from these considerations about the analysed volume,the LOD is also product dependent. Different products havedifferent Raman cross sections: some pigments are extremelygood Raman scatterers, whereas other products intrinsically yielda lower Raman signal and therefore have a higher LOD.

Raman spectroscopic LOD

The LOD of a product is the lowest concentration of that productin the same matrix that can be detected, given a certain analyticalapproach. For instance, this can be the lowest concentration of apigment degradation product that can be detected in a paintlayer, by using Raman spectroscopy under well-defined experi-mental conditions. Note that, although this approach is relatedto concepts from quantitative analysis, the LOD is usually lowerthan the limit of quantification (i.e. the lowest concentration ofa product in a specific matrix that can be quantitatively deter-mined with sufficient precision, given specific experimentalconditions). Experimental conditions should be defined, as theLOD is dependent of several factors, including, amongst others,the laser wavelength, instrumental properties (type of detector,optics, etc.), measurement time and number of accumulations.When a dispersive Raman spectrometer is used, the sensitivityis inversely proportional to the spectral resolution.[17,18]

So, when determining the limit concentration when a certainproduct still is detected, one can determine the lowest intensityof the most intense Raman band, which is still detectable. In otherwords, usually, the LOD of a product equals the LOD of themost in-tense Raman band. Obviously, if, specifically, themost intense bandcannot be detected, e.g. because of coincidence with bands of ma-trix molecules, another band should be selected to define the LOD.

Commonly, in spectroscopy, in order to detect a peak, its inten-sity should at least equal two or three times the noise. In the mostcommon situations in Raman spectroscopy, the detector darknoise, as well as other sources of noise, is low compared withthe shot noise of the measured signal.[17] The shot noise equalsthe square root of the measured signal; therefore, the LOD of aRaman band can be defined as in Eqn (1):

LOD�� 3� √ Itot�� 3� √ IRaman þ Ibackground� �

(1)

where Itot is the measured total intensity at that point in thespectrum. This total intensity equals the sum of the Raman signal(IRaman) and the background signal (Ibackground).

In dispersive Raman spectroscopy, the noise is not homoge-neous over the whole spectrum but is dependent on the

J. Raman Spectrosc. (2012) Copyright © 2012 John Wiley

intensity on that position in the spectrum. Shot noise is theuncertainty on the measured (total) signal intensity, and accord-ing to the laws of statistics, it equals the square root of the overallsignal. Therefore, we can conclude that shot noise is caused notonly by the analyte itself but also by all components that contrib-ute to the measured signal. Therefore, stray light and backgroundfluorescence radiation also contribute to the measured noise andtherefore cause a higher LOD. Mathematical procedures, such asbaseline subtraction, can correct for the offset of the baseline (i.e.the background) but not for the noise that is introduced by thebackground signal.

The two or three times criterion is illustrated in Fig. 2, on thebasis of simulated data. In Fig. 2(a,b), a Raman band is superim-posed on a polynomial (background). Figure 2(c ,d) contains thesame information as in Fig. 2(a,b), respectively, but shot noise issimulated on the curves. The simulated noise has a standarddeviation that equals the square root of the total signal intensity.The Raman band in Fig. 2(b,d) is slightly more intense than inFig. 2(a,c), so that the intensity in Fig. 2(a,c), on the one hand,and in Fig. 2(b,d), on the other hand, exactly equals two or threetimes the standard deviation of the simulated noise, respectively.This clearly illustrates that the two times criterion is ratheroptimistic, whereas the three times criterion for Raman banddetection at first sight seems more realistic.

The obtained LOD can be considered as a function of threedifferent groups of variables, namely variables that are relatedto the sample (a), the experimental set up (b) and the algorithmto determine the LOD from the spectrum (g) (Eqn (3)).

LOD = f a; b; gð Þ (2)

Sample-related parameters (a) include, amongst others, thenature of the analyte (e.g. its intrinsic Raman scattering efficiency)and matrix effects (absorption, fluorescence, interferences, etc.).The experimental set up (b) consists of a large group of parameters,such as the experimental settings (e.g. measuring time, number ofaccumulations, laser wavelength, laser power, aperture size andselected grating), themeasurement geometry (e.g. 90� or backscat-tering geometry, focussing, stability, spot size and focal depth) andthe spectrometer efficiency (e.g. detector sensitivity and transmis-sion efficiency through the spectrometer). The last group ofvariables contains all parameters that are related to the heuristicsof calculating the LOD from the recorded Raman spectrum (g).These include, amongst others, decisions whether band intensitiesor band areas are used, the concentration units used for the expres-sion of the detection limit, the selection of the Raman band to usefor the determination of the detection limit and the option whetherthe three or two times criterion is used. It should be noted thatthese three parameters all may contribute to the achievable LODbut that they are not independent from each other. The intrinsicRaman scattering efficiency of a product (in parameter a) is, forinstance, dependent on the selected laser wavelength (b).

If there can be found an agreement to standardise thealgorithm to determine the LOD (g), the consequence of Eqn 3is that there are still instrumental as well as sample-relatedfactors that determine the achievable LOD. If the same sample,or a standard, can be analysed, the LOD can be used as an objec-tive criterion to compare two different experimental approaches.

The main disadvantage of this approach towards the determi-nation of the LOD is that this is an a posteriori approach; i.e. thepredictive value of this approach is very limited. Once a spectrum

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Figure 2. Simulated Raman spectra (a,b) without noise and (c,d) with noise. The Raman band present in spectra (a) and (b) has the same intensity as inspectra (c) and (d), respectively. For spectra (c) and (d), the band intensity exactly meets the 2� √Itot and 3� √Itot criteria, respectively.

P. Vandenabeele and L. Moens

is recorded, the intensity, hence the noise, can be measured;thus, the LOD can be determined. There is no way to predictthe minimum concentration of a degradation product thatshould be present, so that the analyst is sure that he or she willpick up the Raman signal. In practice, when performing in situexperiments, the spectroscopist usually starts by measuring atlow laser power for a short time. During the experiment, severalspectra are recorded on the same position, gradually increasingthe measurement time and laser power, until an acceptablesignal-to-noise ratio is reached.The LOD of a product, as defined here, corresponds with the

LOD of the most intense Raman band. Thus, a product is detectedas soon as one of its bands is detected in the spectrum. However,it should be clear that, this way, we can detect whether there is aproduct present in the sample that gives rise to a Raman band onthat position, but we cannot be sure what product exactly iscausing this Raman band. It is not possible to identify a producton the basis only of the presence of a single Raman band.Therefore, it is necessary to define a limit of identification (LOI).As the LOD is a function of the sample composition (Eqn (3)), itwould be a good practice to use this term only for samples inwhich the analyte and matrix components are identified, as thisis the only way that we can be sure that the detected Ramanband is originated by the analyte.[19]

Raman spectroscopic LOI

When products are identified by their Raman spectrum, thespectrum of the unknown is compared with the spectra from areference collection. Different collections of reference spectraare available today.[20–22] So, the question on the LOI can berephrased as to determine the limit when we all can agree thattwo spectra are the same. When studying Raman spectra, a spec-troscopist usually examines different features of the spectrum,such as band position and relative band intensities.Similar spectra have their Raman bands in the same position. In

order to have a good agreement in band positions of the

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spectrum of the unknown and the spectrum of the referenceproduct, good instrumental calibration is an obvious and neces-sary condition.[23] For optimal comparison of the spectra, bothof the unknown and from the reference materials should berecorded by using similar instrumentation (with the same laserwavelength). However, even if the spectrometers are properlycalibrated, slight shifts can occur. On the one hand, shifts canbe caused by slightly different chemical compositions (e.g. differ-ent crystalline phases) or degradation effects that might happen.On the other hand, the determination of the exact band positionmight be difficult, especially when very broad bands or weakRaman bands are considered (Fig. 2). A typical example of apigment with very broad Raman bands is carbon black. Thereason for these broad bands is the coexistence of differentphases that might cause slight shifts in band position. However,their exact band position is often not exactly reported.

When using an identification algorithm that tries to identify theproducts on the basis of their band position, it should be pointedout how similar band positions should be to agree that the sameRaman bands are considered.[24] There, the influence of spectralresolution is obvious. Generally, in spectroscopy, the Rayleighcriterion is used to determine whether two bands are overlap-ping or not.[18] We could think about using a similar criterion todetermine whether two Raman bands are coinciding or not –

although the Rayleigh criterion is purely based on the bandshape and does not take into account possible reasons for slightband shifts [e.g. different matrices, slightly different chemicalcomposition or impurities, pressure effects (e.g. when studyingmineral inclusions) and different crystalline phases].

Apart from the presence and position of Raman bands, theintensity of a Raman band is also examined by spectroscopists,to evaluate whether a certain product is present or not. Thedetermination of absolute Raman scattering coefficients forproducts is quite cumbersome, especially when measuring solidsin solid matrices, where the exact sampling volume is unknown(as discussed earlier). Therefore, in practice, usually, indicationsof relative band intensities are mentioned in literature. Typically,

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The definition of Raman spectroscopic detection limits in art and archaeology

a coding is used, with relative intensities ranging from verystrong (vs) to very weak (vw). In literature, it appears that twopossible systems are used: a seven-level system and a five-levelsystem. Unfortunately, although these systems are very com-monly used, in literature, to our best knowledge, there seemsto be no standard definition for these annotations available;therefore, with the examination of band intensities, as reportedin literature, we propose to use the intensity intervals aspresented in Table 1.[7]

When comparing relative band intensities as reported in litera-ture or when working with reference databases, care has to betaken that the spectra are recorded with a laser with the samewavelength: (relative) Raman band intensities are dependenton the used laser wavelength, because of, amongst others,resonance effects.[25] As a consequence, apart from mentioningband positions and relative Raman band intensities (Table 1), alsothe used excitation wavelength should be mentioned. Moreover,when anisotropic solids are measured, the observed relativeRaman intensities are highly orientation dependent. It shouldalso be remarked that, in mixtures, the evaluation of the relativeintensities of the Raman bands can be hampered, because of thepresence of multiple series of Raman bands (from differentproducts) as well as because of coincident Raman bands.

When trying to establish a definition for the LOI of a product, ata certain stage, it will be necessary to define how many Ramanbands of a product should be present, in order to agree on apositive identification. Note that we define a band that is presentas a band that is detected (more intense than the LOD), which iscoincidental with a band of the reference product and which is ofsimilar relative intensity as the reference. One could think ofdefining a minimum number of Raman bands that should bepresent to positively identify a product. However, some materialshave much more Raman bands than others. As an example,indigo or 20th-century synthetic pigments have much moreRaman bands than vermilion (HgS), red lead (Pb3O4) orcalcite (CaCO3).

Alternatively, the LOI could be set in such a way that the verystrong and strong bands of a certain product should be present.A disadvantage of this approach is that the most intense bandsare not necessarily the most discriminative or most uniqueRaman bands. Many organic pigments with a benzene ring in

Table 1. Overview of relative Raman band intensity indications, ascommonly used in literature

Indication Relative intensity (relative to most intense bandintensity) (%)

Very strong 100–90

Strong 90–75

Medium to strong 75–65

Medium 65–35

Medium to weak 35–25

Weak 25–10

Very weak 10–0

Very strong 100–90

Strong 90–70

Medium 70–30

Weak 30–10

Very weak 10–0

J. Raman Spectrosc. (2012) Copyright © 2012 John Wiley

their structure have an intense Raman band around 1000 cm�1.Products with a highly similar chemical structure, such aspigments belonging to the same class, can often only be distin-guished from each other on the basis of some Raman bands ofweak intensity.[26,27] This is the case for organic pigments, butfor some inorganic pigments, this can also be observed. In thecase of the orthorhombic yellow pigment massicot (PbO), threeRaman bands are detected at 143 (vs), 289 (s) and 385 (w) cm�1,whereas its red polymorph litharge (tetragonal PbO) also hasthree Raman bands at 145 (vs), 285 (vw) and 336 (w) cm�1..[22] Itis clear that the distinction between both pigments is notstraightforward, especially when in situ investigations undernon-laboratory conditions are involved.[28]

Chemometrical approach

A totally different approach towards the definition of the LOD orLOI of a product could be established by using chemometricaltechniques, opposite to the here described more classical ap-proach. Chemometry is often used for searching databases orfor identifying small changes between a series of Raman spectra.In common approaches, such as principal component analysis orlinear discriminant analysis, often, a rescaling of the raw spectra isperformed: spectra are plotted in a multidimensional space, anddifferent weight factors are given to the different variables (wave-numbers) in the spectrum. As a consequence, the response is notalways linear as a function of concentration; therefore, theconcept of the LOD (in a multidimensional space) is difficult tounderstand or to visualise.

Moreover, when using chemometrical techniques, the user hasto select many parameters (e.g. type of scaling and number oflatent variables), and the results may be dependent on boththe selected parameters as well as the complete dataset (e.g. allthe spectra in the database). As a consequence, when using achemometrical approach, it is of the utmost importance thatthe algorithms as well as all the variables that are used areexplicitly mentioned (factor g in Eqn (3)).

Conclusions

In this paper, we have pointed out some peculiarities on thedefinition of Raman spectroscopic LODs. We have providedexamples and specific pitfalls from the practice in archaeometry.It is not only interesting to establish a way to determine the LODof a product but it is also equally important to define the LOI of aproduct. In this context, different aspects, such as the relativeRaman band intensity and the Rayleigh criterion for coincidentRaman bands, are included in the discussion. We explained thatthe LOD, and implicitly also the LOI, is a function of three groupsof variables, namely those related to the sample (a), the experi-mental set up (b) and the algorithm to determine the LOD (g).It was pointed out that the LOD of a Raman band equals two orthree times the noise of the spectrum (where the 3� √(Itot)criterion is the most secure). In this paper, we discussed that itwill be necessary to provide objective criteria for the Ramanspectroscopic LOI, but little definite answers could be given.However, we pointed out some possible approaches and pitfalls,which might form the basis of a more profound discussion onthis topic in the Raman spectroscopic community and in interna-tional organisations such as the IUPAC.

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P. Vandenabeele and L. Moens

Acknowledgements

The authors wish to acknowledge the MEMORI project for theirfinancial support and for the interesting discussions with thecolleagues. The MEMORI, ‘Measurement, Effect Assessment andMitigation of Pollutant Impact on Movable Cultural Assets. Innova-tive Research for Market Transfer’, project is supported through theSeventh Framework Programme of the European Commission.

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