Identification of darkened pigments in cultural objects by graphite
furnace atomic absorption spectroscopy and inductively coupled
plasma-mass spectrometryw
Douglas M. Goltz,*aKimberly Charleton,
abEdward Cloutis,
aPatricia Grinberg
c
and Cathy Collinsd
Received 10th August 2006, Accepted 3rd November 2006
First published as an Advance Article on the web 20th November 2006
DOI: 10.1039/b610488f
Inorganic pigments in paint have been used by artists throughout history, primarily because of
their resistance to chemical change over long periods of time. Many of the historically-important
pigments consist of transition metal oxides, sulfides or carbonates. The determination of metals in
pigments is a powerful tool that can be used by conservators and researchers to identify pigments.
In this study, pigments were sampled using a dry cotton bud that was contacted lightly with the
surface of painted objects to remove a small quantity of pigment (o1 mg). This sampling
approach causes no visible damage to a paint surface. Evidence of contact cannot be detected
visually, even with the aid of a magnifying glass. Prior to analysis, pigment metals were extracted
from the cotton with concentrated HNO3 in a 2 ml polystyrene beaker. The pigment metals were
determined using graphite furnace atomic absorption spectroscopy (GFAAS) and electrothermal
vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). The small sample
size requirement and high sensitivity of these instruments make them very suitable for analyzing
pigment metals. Solution nebulization SN-ICP-MS was also used for obtaining a mass scan for
most elements in the periodic table. The application of both GFAAS and ICP-MS is described
for determining the identity of the metals in different pigments of two cultural objects (a painting
and a map), which had darkened over time. Raman spectroscopy was used for confirming the
identity of a darkened red pigment in a map as Red Lead (Pb3O4).
Introduction
Throughout time, inorganic pigments have been used success-
fully in paint for their opacity or for providing colour. One
property that most historically-important pigments share is
their resistance to changes in appearance or chemical degrada-
tion over time. The colour permanence of pigments is essen-
tially due to their chemical stability, as the most stable
pigments often occur naturally as minerals. Since the appear-
ance and chemical composition of most historically-important
pigments are well known, pigment identification is possible by
determining the metals that are present in them. Pigment
identification can be an important step prior to the preserva-
tion and conservation of historical objects. Pigment identifica-
tion may also be a tool that art historians can use for
attribution or dating of artwork.1 Numerous approaches are
available to the conservator for identifying metals or pigments
used in paintings, illuminated manuscripts and other cultural
objects, including textiles.
Spectroscopic techniques have become some of the most
important tools used by conservators for studying cultural
objects.2 For example, Raman spectroscopy has emerged as an
important technique for identifying the chemical structure of
pigments in paint. When combined with microscopy, Raman
spectroscopy is an extremely powerful tool for non-destruc-
tively examining paint, pigments and binding agents in cultur-
al objects; numerous papers have been published on its use.3–7
Libraries of Raman spectra of various painting materials,
including pigments, have been published by Clark et al.8,9
Atomic spectroscopy has also been used for identifying the
metals in pigments because most metals can be identified with
very high sensitivity. One potential drawback is that the
analysis may result in damage to the sample surface. Laser
induced breakdown spectroscopy (LIBS) has emerged as a
useful technique for identifying metals in pigments. A
Nd-YAG laser is used to excite the atoms on the surface of
a cultural object and the excited atoms emit an emission
pattern, which is indicative of the metals present.10 Damage
to the painted surface can be minimized by focusing the laser
on a small area (1 mm2).11–15 An added benefit of LIBS is that
depth profiling of different paint layers can be achieved.16,17
X-ray spectroscopy has also become established as one of the
most important tools for non-destructively identifying metals
a C-SCAPE-Centre for Scientific and Curatorial Analysis of PaintingElements, Dept. of Chemistry, University of Winnipeg, 515 PortageAve, Winnipeg, MB, Canada R3B 2E9
bDept. of Chemistry, University of Manitoba, Winnipeg, MB, CanadaR3T 2N2
cChemical Metrology, Institute for National Measurement Standards,National Research Council of Canada, Ottawa, Ontario, CanadaK1A 0R9
dWinnipeg Art Gallery, 300 Memorial Blvd, Winnipeg, MB, CanadaR3C 1V1
w Electronic supplementary information (ESI) available: photographsof painting Untitled (Arctic Spring) and a 1669 map. See DOI:10.1039/b610488f
140 | J. Anal. At. Spectrom., 2007, 22, 140–146 This journal is �c The Royal Society of Chemistry 2007
PAPER www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry
in pigments. Unfortunately, portable instruments that use X-
ray fluorescence (XRF) spectroscopy can suffer from poor
sensitivity for some elements. To overcome this problem, total
reflection XRF (TXRF) has been developed and applied to the
analysis of pigments in the paint and pigments of illuminated
manuscripts18 and oil paintings.19–22 The most important
advantage of TXRF is that it is a very sensitive, non-destruc-
tive technique which requires very small quantities of sample.
Central to the success of TXRF is the sampling approach used
for works of art such as paintings. Typically, a very small
amount of pigment is gently removed using a cotton bud by
rubbing the paint surface. Since only mg-quantities of pigment
are required, there is minimal, if any, physical damage to the
paint surface. When approaches using atomic spectroscopy are
combined with molecular spectroscopy (i.e. Raman spectro-
scopy) they become extremely powerful tools for identifying
most inorganic pigments that are used in paint.18,19 Although
most of the previous work on pigment sampling with a Q-tips
used XRF for metal analysis, some work has been carried out
using graphite furnace atomic absorption spectroscopy
(GFAAS).23 The disadvantage of using GFAAS is that ana-
lysis can be slow as only one element at a time can be
determined. Fortunately, for most samples, there are a limited
number of elements that correspond to a specific pigment.
Furthermore, one potential advantage of GFAAS over TXRF
is that a greater sensitivity of measurement may be achieved
for lighter elements.
One objective of this work was to test the feasibility of
sampling paint using a dry cotton bud prior to analysis by a
number of approaches including: GFAAS, solution nebuliza-
tion (SN-ICP-MS), electrothermal vaporization (ETV-ICP-
MS) and Raman spectroscopy. One application of this meth-
odology is demonstrated for identifying pigments in two
different cultural objects that have darkened over time.
Experimental
The general procedure for extracting pigment metals is to
gently contact a dry cotton bud (similar to a Q-tips) to
the surface of a painted object. Since only a small quantity
(B1 mg) of pigment is required for analysis, it is usually
impossible to detect any pigment on the dry cotton bud with
the unaided eye. Furthermore, if too much pigment is removed
with the cotton bud, a 10–100-fold dilution step is required
prior to analysis with GFAAS or ICP-MS. After sampling, the
cotton bud was placed in a 2 ml polystyrene beaker and 500 mlof high purity HNO3 (Mallinckrodt, AR Select Plus) was
added. The beaker was then placed on a hotplate at a low
temperature to evaporate most of the acid. After 1 h, de-
ionized water (Milli-Q 18 MO cm�1) was added and the
undigested cotton was removed from the beaker.
To avoid potential problems with undissolved cellulose
fibres, the sample could have been filtered or centrifuged:
however for simplicity and to minimize the risk of contamina-
tion, the sample was allowed to sit for 1 h. This allowed the
undigested material to settle to the bottom of the beaker. The
choice of acid used in the digestion of the cotton may have a
significant impact on the analysis. For example, if HCl is used
the presence of large amounts of Cl� in the sample can cause
matrix effects for some elements with GFAAS. If SN-ICP-MS
analysis is performed, Cl� in the sample can also cause spectral
interferences. For example Cl� can recombine to form40Ar35Cl+, which overlaps the only isotope of 75As+. Since
GFAAS and ICP-MS analyses (both SN and ETV) were
performed on the same solutions, compromise conditions were
required for sample preparation. For this reason only HNO3
was used for the digestion of all the samples.
The ICP-MS used in this work was a PerkinElmer Elan 6000
that was equipped with a quartz concentric nebulizer and a
cyclonic spray chamber for solution nebulization. The
ICP-MS was also interfaced to a PerkinElmer HGA 600
graphite furnace using 50 cm of PTFE tubing. Small volumes
of sample (10–40 ml) were pipetted into the electrothermal
vaporizer (ETV) using a PerkinElmer AS 60 autosampler. The
graphite furnace AAS was a PerkinElmer AS800 that was
equipped with an AS800 autosampler and Zeeman effect
background correction. Experimental parameters for the
ICP-MS are indicated in Table 1. For GFAAS, the recom-
mended temperature program for each element was used.
However, with ETV-ICP-MS, the multi-elemental analysis
required a temperature program, as shown in Table 2, which
required compromise conditions for vaporizing more than one
element. To reduce spectral interferences from ArO, Fe was
also analyzed in DRC mode using NH3 with a gas flow rate of
0.3 ml min�1.
A portable Raman spectrometer was used for confirming
pigment identification. A Delta Nu Raman spectrometer
equipped with a 3 mW 633 nm HeNe laser was used. The
spectral range of this instrument was 200–3400 cm�1 with a
resolution of 10 cm�1. Spectra were typically obtained by
placing the paint surface 1 cm from the source and the
measurement time for each spectrum was 1–5 accumulations
of 10–30 s each.
Table 1 Instrumental operating and data acquisition parameters
ICP mass spectrometerRF power 1100 WCoolant Ar flow 15.0 l min�1
Auxiliary Ar flow 1.4 l min�1
Carrier Ar flow 1.0 l min�1
ICP-MS data acquisitionDwell time 50 msScan mode Peak hoppingNumber of masses (m/z) monitored 3–10Signal measurement Average counts (SN)
Peak area (ETV)Resolution 0.7 amu at 10% peak height
Table 2 Instrumental operating and data acquisition parameters ofthe ETV-ICP-MS
T/1C Ramp/s Hold/sAr flowrate/ml min�1
StepDry 120 10 60 300Char 400–800 10 30 300Atomization 2000–2400 0 5 150a
Clean out 2650 0 5 300
a Read step.
This journal is �c The Royal Society of Chemistry 2007 J. Anal. At. Spectrom., 2007, 22, 140–146 | 141
Discussion
Perhaps the most important property associated with sam-
pling a painted surface using a cotton bud is the mass of metal
or pigment removed. Clearly, there is a relationship between
the amount of pigment removed and the amount of damage
sustained by the painted surface. For painted surfaces of
valuable artwork, any damage to the surface would be unac-
ceptable. In a previous study,23 painted surfaces were sampled
manually with a cotton bud and it was estimated that the
amount of pigment removed with a cotton bud is approxi-
mately 1 mg. To acquire a better estimate of the mass of
pigment that is typically removed with a cotton bud during
sampling, an experiment was designed to quantify the mass of
pigment removed with respect to the contact force. In this
experiment a series of microscope slides were painted using a
stainless steel spatula and allowed to dry overnight. The paint
was sampled by placing the microscope slide on a pan balance.
A cotton bud was secured with a clamp and carefully lowered
onto the paint surface. The force on the paint surface was
determined by mass. The pan of the balance was rotated for a
predetermined number of times, in both clockwise and counter
clockwise directions. This allowed the pigments on the paint
surface to be removed in a reproducible manner. Periodically,
the clamp securing the cotton bud was adjusted to ensure that
the mass remained within 10% of the desired value. Some
fluctuations were inevitable, since the cotton bud would com-
press somewhat as more force was exerted on it from the
painted surface.
Fig. 1 summarizes the relationship between contact force of
the cotton bud and the mass of pigment metal extracted for
different pigments. Since a fresh paint surface was sampled for
each determination, error bars on the vertical axis represent
the measurement uncertainty of the instrument (i.e. � 10%).
The absence of a good fit of the continuous curve in these
experiments can probably be attributed to variations in the
paint surfaces between samples, as well as between different
cotton buds. In Fig. 1(a) the relationship between the mass of
pigment and contact force is shown for the watercolour paint
‘olive green’. This paint contains chromium oxide, Cr2O3, as
the pigment and therefore, the metal used to estimate the
amount of pigment sampled was Cr. This pigment dried to a
smooth, hard finish, which meant that smaller amounts of
pigment were removed when contacted with the cotton bud,
especially when compared with the oil paint. For contrasting
surface properties a commercial oil paint ‘flake white’ was
chosen, which is shown in Fig. 1(b). This paint contains basic
lead carbonate, 2PbCO3 �Pb(OH)2, as the pigment and there-
fore the metal used to estimate the quantity of pigment
sampled was Pb. This paint required a longer period of time
(47 days) to dry because of the linseed oil. Compared to the
watercolour paint, the oil paint surface was much rougher and
more susceptible to damage when contacted with the cotton
bud. As a result, much higher masses of Pb were obtained,
even when gentle pressure (o0.5 N) was applied to the paint
surface.
Although this experiment differs in many ways from the
manual approach to pigment sampling of paint, it is useful for
understanding how the surface of a painting may be affected.
Recording the mass with a pan balance made it possible to
estimate the amount of pressure applied to the paint surface.
The amount of pressure applied is certainly subjective, how-
ever it is possible to make an estimate of the pressure exerted
on the surface of a painting. With a microscope slide on the
pan balance, for comparison, the estimated force on the paint
surface is approximately 0.05–0.1 N of force per 0.25 cm2. For
most paints this amount of force will have a negligible effect on
the surface. A larger amount of force (40.3 N) can cause a
dent to the paint surface, which is both harmful to the
appearance of the paint and unnecessary for obtaining a
representative paint sample.
The first object that was examined in this study was the
painting ‘untitled—Arctic Spring’ (1950) by George Swinton.
This abstract painting is a watercolour and it has been
described as a ‘‘hopeful presentation of springtime in the
Canadian Arctic’’. A colour image of this painting is available
as Electronic Supplementary Information (ESI)w. Swinton
(1917–2002) was a respected Canadian artist and collector of
Inuit artwork in the province of Manitoba, Canada. The
problem with ‘Arctic Spring’ is that it has darkened noticeably
since it was created. Areas of paint that were originally white
in appearance have become either grey or dark grey over time.
Therefore, identifying the white pigments in this painting is a
logical first step towards understanding why darkening has
occurred.
Fig. 1 (a) Relationship between the mass of Cr extracted from the
pigment ‘olive green’ and the applied force of the cotton bud. (b)
Relationship between the mass of Pb extracted from the pigment ‘flake
white’ and the applied force of the cotton bud.
142 | J. Anal. At. Spectrom., 2007, 22, 140–146 This journal is �c The Royal Society of Chemistry 2007
A number of areas in ‘Arctic Spring’ were sampled using a
cotton bud, with the goal of detecting the pigment metals for
the purpose of identifying both the white pigment and possibly
the cause of the darkening of the paint in this piece. Fig. 2(a)
shows transient signals obtained from a graphite furnace
atomic absorption spectrophotometer (GFAAS). This can be
a time-consuming approach because the signals for each metal
must be collected separately. Absorbance signals for a number
of metals that are typically associated with white pigments
were measured, including: Ti, Ba, Zn, Pb and Ca. Of these, the
largest signals were for Ba and Zn, which indicates that the
important white pigment in this piece is probably lithopone
(BaSO4, ZnS).
A second approach that was explored for metal identifica-
tion was electrothermal vaporization (ETV) inductively
coupled plasma mass spectrometry (ICP-MS). With this tech-
nique, the sample is vaporized in much the same manner as
GFAAS. The vaporized pigment metals are ionized in the ICP
prior to entering the mass spectrometer. The advantage of
using this approach is that multiple signals can be acquired in
a single firing of the furnace. Although we typically monitored
6–7 masses per ETV firing, other workers have shown that as
many as 20–25 masses can be measured with quadrupole based
instruments.24,25
Fig. 2(b) shows the transient signals of metals from a
darkened area of ‘Arctic Spring’. As with the GFAAS deter-
minations, the presence of large amounts of 64Zn+ and 138Ba+
were detected using ICP-MS, again indicating that the dar-
kened pigment is lithopone. With the ETV-ICP-MS, trace
amounts of other metals such as Cd and Ti could be detected.
Surprisingly large signals for Pb were also observed using both
GFAAS and ETV-ICP-MS. The higher sensitivity with which208Pb+ can be detected using ETV-ICP-MS is also illustrated
in the figure.
Solution nebulization SN-ICP-MS is extremely useful for
obtaining information regarding the elemental abundance for
most of the elements in the periodic table. For example, a
more complete estimation of the elemental distribution of
metals in different pigments is easily achieved by performing
a mass scan. This can provide useful information on both
pigment metals and other metals which may be present either
as impurities or as a result of mixing two or more pigments.
Fig. 3 shows the elemental composition of different areas
(dark, light, paper) of ‘Arctic Spring’. Concentrations of each
metal were quantified with an external calibration curve.
Compositions are expressed as a mole % of the detected
elements only, and therefore the actual concentrations of
metals in the pigment would be lower than those shown in
Fig. 3. All metals were background-subtracted from blanks to
correct for traces of metals that were present in the cotton
buds. The darkest areas of lithopone are labelled as ‘dark’ and
areas that had darkened, but were lighter in appearance are
labelled as ‘light’. For comparison, a small area on the
painting that was white in appearance is also shown as ‘paper’
in this figure. This particular area did not appear to have a
visible layer of paint on the surface, except possibly a thin
ground layer of white paint. In Fig. 3 the presence of large
amounts of 64Zn+ and 138Ba+ were detected in all areas of the
darkened white pigment, which is consistent with the pigment
lithopone. The proportion of Ba:Zn was approximately 2:1 in
each of the sampled areas. Low concentrations of 114Cd+,88Sr+ and 208Pb+ were also detected in the lithopone, possibly
as impurities. The small area that was white in appearance
(‘paper’) contained significant amounts of 44Ca+ and much
less 64Zn+ or 138Ba+, which may indicate that a Ca-containing
pigment (e.g. CaCO3) was used as a ground layer.
Fig. 2 (a) GFAAS signals of pigment extracted from darkened area
of ‘Arctic Spring’. (b) ETV-ICP-MS signals of pigment extracted from
darkened area of ‘Arctic Spring’.
Fig. 3 Mass scan of white and darkened areas of Swinton’s ‘Arctic
Spring’.
This journal is �c The Royal Society of Chemistry 2007 J. Anal. At. Spectrom., 2007, 22, 140–146 | 143
The questionable permanence of lithopone has been known
for a long time and its problems as a modern pigment are well
documented.26,27 Exposure to sunlight and atmospheric levels
of oxygen are known to darken this pigment. The darkening of
lithopone is attributed to the photochemical reduction of the
ZnS to elemental Zn, which results in a darkened appearance
on the paint surface. Lithopone was originally developed
primarily as a pigment for household paint. Early in its
history, manufacturers became aware of this problem, as well
as its reversibility when stored in the dark. A number of
approaches have been developed to prevent the darkening,
such as adding magnesium or even coating it with varnish.28
Apart from positively identifying the darkened pigment, an
interesting outcome of this work is the identification of other
metals such as Pb and Fe in the lithopone. The concentrations
of Pb and Fe were further quantified using GFAAS, to
determine if the concentrations of these metals varied with
the extent of darkening. The concentrations of both Pb and Fe
were found to be approximately the same in areas of the
painting that were darker as shown in Table 3. In this
experiment, the concentrations of the metals were quantified
using the method of standard addition. Because of their low
concentrations relative to Ba and Zn, it would be difficult to
suggest that Pb or Fe would contribute significantly to the
darkening of the lithopone. At present, we have no experi-
mental evidence to confirm why the lithopone is darkened in
‘Arctic Spring’ other than the photochemical reduction of Zn.
The ‘Mapp or Generall Carte of the World’ (circa 1669)
provided a similar challenge in terms of pigment identification.
A colour image of this map is also available as Electronic
Supplementary Information (ESI)w. This map of the world is
part of the holdings in the Hudson Bay Archives (Accession
Number: 1987/199) and is housed in the Archives of Manitoba
(Winnipeg, Canada). This map is an engraved map that was
executed in black ink on laid paper during the time of King
Charles II (1660–1685). It is a map of the world, drawn in ink
by Monsieur Sanson as two hemispheres. It was rendered into
English and illustrated with figures by Richard Blome. As
such, some portions of the map are coloured in blue, yellow
and red pigments. The problem of darkening of the red
pigments became apparent while the map was undergoing
cleaning by a conservator. Pigments that were initially red in
appearance, such as the decorative flowers, appear to have
darkened to a brown colour. In some places the red appear-
ance of this pigment is still evident, but most areas containing
the red pigment have a darkened appearance, to some degree.
Solution nebulization-ICP-MS was used to obtain mass scans
of the blue and red pigments found in the map. Electrothermal
vaporization-ICP-MS was again used to detect a number of
metals in the blue pigment of the oval-shaped seal, as well as in
the red pigment used to colour the decorative flowers. Tran-
sient signals for the most important metals detected are shown
in Fig. 4(a). This figure shows the ETV-ICP-MS signals
obtained from the red pigment. Other than trace amounts of52Cr+, the element that was in the highest concentration was208Pb+. In this experiment, the large 208Pb+ signal, relative
to the blank, clearly indicates that this is a Pb-containing
pigment. Fig. 4(b) shows transient signals obtained from
ETV-ICP-MS for the blue-coloured pigment. It is apparent
from this experiment that relatively high concentrations of63Cu+ were detected, as well as other metals including 138Ba+,52Cr+ and 75As+. The transient signals obtained using ETV-
ICP-MS are useful for identifying the metals present: however,
quantifying the relative concentration of metals is more useful
for pigment identification.
The quantitative results of the metals in the blue and red
pigments are summarized in Fig. 5. Analysis of these pigments
was carried out in the same manner as the pigments in Arctic
Spring (Fig. 3). The relative purity of the pigments used to
colour portions of this map becomes evident, by the absence of
metals other than the pigment metals. Apart from some degree
of fading with the blue pigment, the colour of this pigment is
still apparent and has not degraded. The relative absence of Cr
and the high concentration of Cu, suggests that this pigment is
most likely to be copper carbonate. The basic form of copper
carbonate, CuCO3 �Cu(OH)2 is a pigment obtained from the
mineral Azurite.
Table 3 Summary of quantitative analysis of selected elements in‘Arctic Spring’
Concentration/nM
Pb Ba Zn Fe
Dark grey 0.04 � 0.01 6.77 � 3.77 2.61 � 0.45 0.18 � 0.02Light grey 0.06 � 0.06 7.77 � 10.1 3.22 � 2.68 0.28 � 0.12
Note: � denotes the average deviation (n = 3).
Fig. 4 (a) ETV-ICP-MS signals of darkened red pigment in the
‘Generall Carte of the World’. (b) ETV-ICP-MS signals of blue
pigment in the ‘Generall Carte of the World’.
144 | J. Anal. At. Spectrom., 2007, 22, 140–146 This journal is �c The Royal Society of Chemistry 2007
The absence of Fe or Hg indicates that neither Red Ochre
(Fe2O3) nor Cinnabar (HgS) were used in this object. Since it
had a very high concentration of Pb, the most likely identity
of the red pigment could be Chrome Red (PbO �PbCrO4),
Molybdate Red (7PbCrO4 � 2PbSO4 �PbMnO4) or Red Lead
(Pb3O4). A portable Raman spectrometer was used to confirm
the structure of the darkened red pigment. This was necessary
because both Cr and Mn were detected in measurable quan-
tities with the ICP-MS. Fig. 6 shows a Raman spectrum of the
darkened red pigment. For comparison, a Raman spectrum of
pure Pb3O4 is also shown and indicates that the darkened
pigment is consistent with Red Lead. The absence of CrO42�
or MnO4� in the Raman spectrum indicates that Cr and Mn
are present as impurities in the darkened pigment.
Red lead is a pigment that is known to darken over time as a
result of oxidation of the Pb(II) atoms in Pb3O4 to PbO2, which
is dark brown in appearance.29 Platernite (PbO2) appears to
have been detected in wall paintings using X-ray diffraction:
however, a more recent study used the laser of Raman spectro-
scopy for identifying degradation products of lead pigments.30
This work presented a reasonable degradation pathway for
Red Lead, Pb3O4, to PbO (litharge, tetragonal), which will
ultimately form PbO (massicot, orthorhombic). Unfortunately
the detection of PbO2 in painted objects using Raman spectro-
scopy is difficult at best, since PbO2 is a weak Raman scatterer.
The most important advantage of using the cotton bud is
the relative ease with which a surface layer of paint is sampled,
and it works equally well when either oil or water is used as the
binding agent. This approach is limited to sampling only the
surface, and a protective layer of varnish would certainly
diminish the usefulness of microsampling with a cotton bud.
For older cultural objects that have not been stored properly,
dirt or soot accumulated on the surface would certainly affect
the elemental composition that is observed by the analyst. The
magnitude of the AAS or ICP-MS signals, obtained from the
metal-containing pigments, are usually large enough that small
amounts of dirt or soot should not interfere with pigment
identification. This may not be the case for metals that are
present in pigments in low concentrations, such as when
pigments are mixed with one another.
Conclusion
Sampling with a cotton bud allows the possibility of analyzing
very small quantities of paint, with minimal damage to the
surface of an object. When compared to solution nebulization
(SN), the small sample size requirement (1 mg or less) of
GFAAS and ETV-ICP-MS makes them very powerful tech-
niques for detecting small quantities of metals. In order to
perform a mass scan, samples had to be diluted to provide a
sufficient volume prior to analysis by solution nebulization. As
a result, sensitivity is sacrificed for some metals compared with
ETV-ICP-MS. In many ways the multi-technique approaches
used in this study were complimentary to each other. The most
important advantage of using SN-ICP-MS was obtaining a
survey of the elements present in the pigment samples quickly.
The GFAAS was useful for determining As, Fe and Cr, which
have spectral interferences with 40Ar16O+ and 40Ar12C+, and
can be problematic for the ETV-ICP-MS user. Whilst GFAAS
is capable of measuring only single elements, the most im-
portant advantage of ETV-ICP-MS is its capability to deter-
mine multiple elements simultaneously. The Raman
spectrometer was excellent for identifying the primary pigment
but lacked the sensitivity to determine if another pigment was
present as well. The high sensitivity of ETV-ICP-MS and
GFAAS, for most metals, allows the user to determine if more
than one pigment is present. Since only detecting the metals
present is often not sufficient for identifying a pigment, the use
of Raman spectroscopy can be essential for confirming the
chemical structure of the pigment.
Acknowledgements
Funding for this research was provided by the Natural
Sciences and Engineering Research Council of Canada. The
authors gratefully acknowledge the Winnipeg Art Gallery and
the Manitoba Archives for access to material. R. Ridgen and
A. Walton of the Manitoba Archive are also thanked for their
technical assistance. The National Research Council of
Fig. 5 Mass scan of blue and darkened red pigments in the ‘Generall
Carte of the World’.
Fig. 6 Raman spectrum of darkened pigment of the ‘Generall Carte
of the World’.
This journal is �c The Royal Society of Chemistry 2007 J. Anal. At. Spectrom., 2007, 22, 140–146 | 145
Canada is also thanked for their expertise and use of their
research facilities in Chemical Metrology. K. Charleton is a
graduate student at the University of Manitoba.
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