|
Chapter 4.5 Analytical Methods Based on Laser Ablation
Sampling Matija Strlič1, Vid
Simon Šelih2, Jana Kolar3 1 University College London, Centre for
Sustainable Heritage, The Bartlett School of Graduate Studies,
London, U.K. 2 University of 3 National and University Library,
Contents 4.5.1
Introduction
Some analytical methods utilize laser ablation
(LA) as a “light chisel” in order to take a tiny
amount of sample from the artefact surface for chemical analysis. Most often, the sample is transported into an
analyser consisting of inductively-coupled plasma and a mass
spectrometer (ICP-MS). In this way, the elemental composition of the
sampled spot is obtained. The information is useful for identification
of pigments, metals, alloys, ceramics, glasses, and for trace analysis
of organic materials, e.g. varnishes, paper, textiles etc.
Occasionally, other types of analyzers are used, although ICP-MS is by
far the most common one and will be described in more detail in this
Chapter. Scanning mode allows the information to be
obtained along a line or any other path scanned by the laser. Since a
minute amount of the artefact is removed, the method can be classified
as micro-destructive. Whether the trace left by the laser is visible or
not depends on the substrate material and laser parameters: materials
such as stones, glass, and ceramics are less sensitive than organic
materials. LA-ICP-MS combines the micrometer-scale
resolution of a laser probe with the speed, sensitivity and
multi-elemental capability of ICP-MS, and rivals other micro-beam
techniques such as the proton microprobe and secondary ion mass
spectrometry (i.e. the "ion probe"). Compared to other micro-sampling
analytical techniques, LA-ICP-MS has several advantages: - laser probing utilizes light rather than
charged particles and can, therefore, analyze both conducting and
non-conducting materials without the need for e.g., a conductive
coating; -
no vacuum is required in the sample chamber; - the analysis step is divided from the sampling
step - therefore, both steps can be independently controlled and
optimized; However, quantification of data is difficult
and requires well-defined standard samples. Also, the technique is not
portable. The detection limit for most analytes (from Li on, excluding
noble gasses, C, N, O, F, and Cl) is in the range of 0.01 - 0.1
µg/g. For the more scientific reader, a review of
LA-ICP-MS was recently published.1
4.5.2.1
Sample Preparation and the Sampling Chamber In order to collect and transport the ablated
material, the sample is put in a chamber flushed with a carrier gas.
There is no sample preparation needed. The sample surface is not
necessarily flat. Fragments of frescoes, plasters, paint layers,
already prepared in resin mounts for microscopic investigation can
readily be used. The usual sample dimension is cca. 1-2 cm, the sampled
area is limited by the laser beam diameter, which can be focussed down
to a few micrometers, if necessary. Special samplers can be constructed
allowing in-situ sampling of whole artworks.2
The principal components of the sampling part
include a high-power laser, beam steering and focusing optics, an
ablation chamber and interface connections that ensure an efficient
transport of aerosol. A general block diagram of the LA setup is shown
in Figure 4.5.1. The sample size must be adapted to the sample
chamber, which is equipped with a quartz glass lid transparent to laser
light (Figure 4.5.2). A stream of carrier gas enters the sample cell
and picks up fine sample particles produced by the ablation process and
transports them into the analyser (ICP-MS). The carrier gas can be Ar
or He. The sample cell is mounted on a stage that
allows the sample to be moved relative to the laser beam in all three
directions. The laser is focused and the ablation process can be
observed on a monitor showing the images captured by a video camera.
The lasers initially used for the purpose were
ruby lasers. Other lasers also in use were carbon dioxide and nitrogen
lasers and recently also excimer lasers. The most common type of laser
is the Q-switched (pulsed) Nd-YAG laser. The fundamental wavelength of
this laser is 1064 nm, however, since UV laser light interacts more
efficiently with most solids, the Nd-YAG lasers in LA-ICP-MS
instruments commonly use the fourth harmonic (266 nm) or even fifth
harmonic (213 nm). The interaction between UV laser light and most
solids tends to involve photochemical phenomena (bond scission) and
physical ablation, whereas IR laser light often causes unwanted sample
heating and melting. The basic requirement for the laser is to
deliver energies of sufficiently high power densities (typically 1-20
J/cm2, 10-20 Hz, ~5 ns) at the lens focus to
ablate and vaporise diverse sample matrices. The laser ablation
instrument requires an accurate optical system of lenses, prisms and
mirrors that conducts and focuses the laser beam onto the sample. The
optical system may also include a system of apertures of different
diameters that can be used to vary the beam diameter and/or shape. The laser beam can be thought of as a "light
chisel" which interacts with the solid sample to generate very fine
solid particles leaving behind a minute ablation crater (in the order
of tens of µm in diameter).
Inductively coupled plasma - mass spectrometry
requires the sample to be introduced into a high temperature argon
plasma (approx. 8000 °C), which dissociates molecules and
ionizes atoms. The positive ions are passed into vacuum via an
interface, where electrostatic lenses focus and accelerate the ion beam
into a mass spectrometer. Here, the ions are sorted by mass and
detected using an ion detector, usually an electron multiplier. The plasma torch consists of an assembly of
three concentric quartz tubes, providing flow path for plasma, sample
and outer Ar gas. At the end of the torch, Ar plasma is formed and
sustained with constant delivery of high power, high frequency RF
energy via the load coil (1 to 2 kW, 27 MHz). The energy is transferred
by interaction of ionized argon with the electromagnetic field of the
induction coil. Both positive argon ions and electrons are accelerated
by the high-frequency field of the coil. Due to constant movement and
friction, particles are heated to very high temperatures.
4.5.2.4
Quadrupole Mass Spectrometer A quadrupole mass spectrometer consists of four
hyperbolically shaped parallel metal rods, onto which a potential is
applied (Figure 4.5.3). The applied voltages affect the ions travelling
down the flight path centred between the four rods. For given applied
voltages, only ions of a certain mass-to-charge ratio pass through such
a quadrupole filter while all other ions are thrown out, not reaching
the detector. A mass spectrum is obtained by monitoring the ions
passing through the quadrupole filter as the voltages on the rods are
varied. Other types of mass spectrometers are also in use, with the
same aim, to separate and detect ions on the basis of their
mass-to-charge ratio.
There are a number of interesting examples of
the use of laser ablation methods described in the literature. The aim
of the following paragraphs is not to provide a literature review but
rather to highlight some interesting applications on different
materials. An interesting approach without the use of
ICP-MS was applied by Colombini et al.3 The
group used sampled micro-amounts of mainly organic materials from thin
layers of painting using a pulsed Er:YAG laser system operating
at 2.94 μm assisted by water/ethanol mixtures. The ablated
materials condensed on glass coverslips and were characterised by
Fourier transformed infrared spectrometry (FT-IR) and gas
chromatography mass spectrometry (GC-MS). In this way, a 17th
century canvas copy of Caravaggio’s “Christ crowned
with thorns” and a 13th century panel
“Virgin with Child” by Anonymous were examined and
the presence of overpainting consisting of egg and Venice turpentine in
one case, and of “beverone” over a varnish (linseed
oil and Venice turpentine) in the other one was highlighted. A very similar approach was used by Feely et al,4
who studied health risks associated with the use of Nd:YAG lasers for
the removal of black sulphation crusts. The ablated material was collected on glass
plates and analysed using scanning electron microscopy (SEM) with
energy dispersive X-ray spectroscopy (EDS). The presence of
Al, Si, Fe in addition to Ca, S, and O was indicated. Research on marble crusts by Ulens et al.5 using
LA-ICP-MS showed that this technique can be used in authentication
studies. The depth profiles of Mg, Al, Mn, Fe, Zn, Sr, Ba, La and Pb in
weathered marble crusts were obtained. Hogan et al.6 used laser
ablation mass spectrometry (without the ICP part) to characterize the
surface composition of a daguerreotype. Using their instrument, consisting of an
Nd:YAG laser and a home-built mass spectrometer, they were able to
determine the elemental composition of the surface. By ablating and
analyzing the ablated material, they were able to follow the process of
laser cleaning. On a tarnished nineteenth century daguerreotype, they
found Na+, Ca2+, Hg2+,
Agn+, and AgnS+
ions. The analyses of the daguerreotype after laser cleaning showed
that silver sulphide was greatly reduced or eliminated. LA-ICP-MS was applied also for the
characterization of historical glass samples.7 Iridescent
Art Nouveau lead crystal glass samples were analyzed and it was proven
that the material of single layers originated from different glass
sources. The analysis of glass fragments from the archaeological site
in A very interesting application was developed to
recognize art forgeries.2 The authors applied
LA-ICP-MS to the analysis of artist paints from different manufacturers
to identify variation in the elemental composition. In this way, a
comparison of the paints to assist provenancing was facilitated.
Preliminary trials of a prototype sample collection device designed to
reduce damage and allow for in-situ sampling of artworks were also
undertaken. The device allowed direct laser-based sampling of a
complete painting. Devos et al. investigated early-medieval archaeological iron finds.8 The analysis of
elemental impurities in the iron provided useful archaeometallurgical
information on the production process and the provenance of the iron.
Preliminary results from the analysis of archaeological iron samples
from several excavations in Roman mints were analysed by Ponting et al.9
using Pb isotope analysis. They showed that Pb isotope compositions
gave isotope fingerprints despite the likely reworking of the metal
during coin production. The analysis of gold coins issued from the 7th to
the 12th century AD in the Arab Empire by
Gondonneau and Guerra10 using LA-ICP-MS showed
that several different supplying sources were used, according to the
region and to the period. The identification of different gold ores by
means of characteristic trace elements indicated recycling of the
ancient coinage and, after AD 750, the minting in the entire Arab
Empire of different new gold ores: Egyptian type, North Eastern type
and West African type. Islamic glazed ceramic ware were analysed by Hill et al.11
In a survey of 175 artefacts from Sasanian and Early Islamic period
sites located on the Deh Luran Plain in south-western Iran were
examined to identify the constituents of the ceramic glazes. The
results of the analyses revealed that alkaline-based glazed ceramics
have paste compositions that are distinct from contemporary and later
ceramics decorated with alkaline-low-lead and lead-based glazes. Non-conservation oriented applications of the
LA-ICP-MS technique include other materials of conservation interest,
e.g. wood12, corals13 etc. Two case studies are presented here carried out
at the University of Ljubljana, using the instrumental set-up
consisting of the New Wave Research UP-213 Deep UV YAG laser ablation
workstation with the standard sample cell, He carrier gas, coupled with
Agilent 7500ce ICP-MS instrument, 1500 W Ar plasma.
4.5.3.1
Case Study 1: Iron Gall Ink on Paper The LA-ICP-MS technique could be useful for
determination of metals in the paper material, however, it can also
provide valuable information on the composition of various inks
deposited on its surface. The infamous iron gall inks, for instance,
usually contain an appreciable amount of transition metals, such as
iron, copper, manganese etc.,14 and reports on
the use of LA-ICP-MS are already available in the literature.15
Here, we present an LA-ICP-MS scan of an
iron-gall ink fragment (Figure 4.5.5a). The laser light wavelength used
was 213 nm, pulse length 4 ns, beam diameter 25 μm.
Without proper calibration, the results can be
given on a relative scale, meaning that we know only the distribution
of a particular element (Figure 4.5.5b), and not its actual content.
Nevertheless, the distributions are of enormous interest if we want to
evaluate, e.g. migration processes during a particular conservation
treatment.
4.5.3.2
Case Study 2: Paint Layers Paint layers or façade fragments can
readily be studied even after inclusion into a resin for routine
microscopic investigations (Figure 4.5.6b). Here, we present an
LA-ICP-MS scan of a paint fragment taken from an 18th-century
statue of St. Zacharias from the
The results (Figure 4.5.6c) clearly indicate
the calcium-rich ground layer, onto which lead-rich pigment layer was
painted, probably lead white (2PbCO3·Pb(OH)2).
This is a face fragment, the pink colouration was obtained using a
mercury-rich pigment, e.g. vermillion (HgS) possibly mixed with the red
minium (Pb3O4). The top
layer contains some barium-containing pigment – this could
well be a consequence of a later conservation intervention.
Laser ablation (LA) sampling is a simple and
often micro-destructive technique for sampling of artefacts. Following
the sampling, analysis can proceed using a variety of analytical
techniques, e.g. IR spectroscopy, gas chromatography or X-ray
spectroscopy. By far the most often used analytical techniques
associated with laser ablation sampling is inductively coupled plasma
with mass spectrometry (ICP-MS). Using the latter technique, which is not
available as a portable instrument, multielemental analysis can be
performed, however, the sample has to be introduced into a sample
compartment, thus limiting its size. There are new developments in this
area, promising in-situ sampling of whole artefacts. Studies of artefacts include metals, glass,
ceramics, paintings, daguerrotypes, paper, stone, providing information
on authenticity, provenance, or composition of pigment, patina and
crust. In comparison to other techniques, LA-ICP-MS offers trace
analysis of samples (elements from Li on, excluding noble gasses, C, N,
O, F, and Cl) at atmospheric pressure with the spatial resolution of a
few μm, at a moderate cost.
The authors acknowledge the financial support
of the Ministry of Higher education, science and technology of the
1
J. S. Becker: “Applications of inductively coupled plasma
mass spectrometry and laser ablation inductively coupled plasma mass
spectrometry in materials science”, Spectrochim. Acta B 57
(2002) 1805–1820. 2
K. Smith, K. Horton, R. J. Watling, N. Scoullar: “Detecting
art forgeries using LA-ICP-MS incorporating the in situ application of
laser-based collection technology”, Talanta 67 (2005) 402-413. 3
M. P. Colombini, A. Andreotti, G. Lanterna, M. Rizzi: “A
novel approach for high selective micro-sampling of organic painting
materials by Er:YAG laser ablation”, J. Cult. Herit. 4 (2003)
355s–361s. 4
J. Feely, S. Williams, P. S. Fowles: “An initial study into
the particulates emitted during the laser ablation of sulphation
crusts”, J. Cult. Herit. 1(2000) S65–S70. 5
K. Ulens, L. Moens, R. Dams, S. Vanwinckel, L. Vandevelde:
“Study of element distribution in weathered marble crusts
using laser-ablation inductively-coupled plasma-mass
spectrometry”, J. Anal. Atom. Spectr. 9 (1994) 1243-1248. 6
D. L. Hogan, V. V. Golovlev, M. J. Gresalfi, J. A. Chaney, C. S.
Feigerle, J. C. Miller, G. Romer, P. Messier: “Laser ablation
mass spectroscopy of nineteenth century daguerreotypes”,
Appl. Spectrosc. 53 (1999) 1161–1168; V. V. Golovlev, M. J.
Gresalfi, J. C. Miller, G. Romer, P. Messier: “Laser
characterization and cleaning of nineteenth century
Daguerreotypes”, J. Cult. Herit. 1 (2000) S139–S144. 7
G. Schultheis, T. Prohaska, G. Stingeder, K. Dietrich, D.
Jembrih-Simburger, M. Schreiner: “Characterisation of ancient
and art nouveau glass samples by Pb isotopic analysis using laser
ablation coupled to a magnetic sector field inductively coupled plasma
mass spectrometer (LA-ICP-SF-MS)”, J. Anal. Atom. Spectrom.
19 (2004) 838-843. 8
W. Devos, M. Senn-Luder, C. Moor, C. Salter: “Laser ablation
inductively coupled plasma mass spectrometry (LA-ICP-MS) for spatially
resolved trace analysis of early-medieval archaeological iron
finds”, Fresen. J. Anal. Chem. 366 (2000) 873-880. 9
M. Ponting, J. A. Evans, V. Pashley: “Fingerprinting of Roman
mints using laser-ablation MC-ICP-MS lead isotope analysis”,
Archaeometry 45 (2003) 591-597. 10 A. Gondonneau, M. F.
Guerra: “The circulation of precious metals in the Arab
Empire: the case of the near and the 11 D. V. Hill, R. J.
Speakman, M. Glascock: “Chemical and mineralogical
characterization of Sasanian and Early Islamic glazed ceramics from the
Deh Luran Plain, southwestern 12 T. Prohaska, C.
Stadlbauer, R. Wimmer, G. Stingeder, C. Latkoczy, E. Hoffmann, H.
Stephanowitz: “Investigation of element variability in tree
rings of young Norway spruce by laser-ablation-ICPMS”, Sci.
Tot. Environ. 219 (1998) 29-39. 13 D. J. Sinclair, L. P. J.
Kinsley, M. T. McCulloch: “High resolution analysis of trace
elements in corals by laser ablation ICP-MS”, Geochem.
Cosmochem. Acta, 62 (1998) 1889-1901. 14 J. Kolar, M. Strlic
(Eds.): “Iron Gall Inks: On the Manufacture,
Characterisation, Degradation and Stabilisation”, National
and University Library, 15 B. Wagner, S. Garbos, E.
Bulska, A. Hulanicki: “Determination of iron and copper in
old manuscripts by slurry sampling graphite furnace atomic absorption
spectrometry and laser ablation inductively coupled plasma mass
spectrometry”, Spectrochim. Acta B 54 (1999) 797-804.
Laser ablation solid sampling http://www.new-wave.com/1nwrProducts/LaserAblation.htm http://www.cetac.com/prods/laser/laser_overview.html http://www.tuilaser.com/markets/science/ablation.htm ICP-MS http://www.chem.agilent.com/Scripts/PCol.asp?lPage=513 http://www.thermo.com/com/cda/product/detail/1,1055,15265,00.html http://las.perkinelmer.com/Catalog/default.htm?CategoryID=ICP+Mass+Spectrometry+%5bICP-MS%5d
More on LA-ICP-MS http://www.agu.org/revgeophys/neal00/node11.html 30-min guide to ICP-MS http://las.perkinelmer.com/Content/RelatedMaterials/D-6355A-30Min.pdf
Matija
Strlič Vid
Simon Šelih Jana
Kolar |
