Applied_Spectroscopy_Practica_01_01 - 18

Donais et al.
36
Figure 1. Digital images of (a) Roman wall mortar Locus 22 and (b) pottery sherds in gray (left) and black (right).
2.5 × 1.5 mm in size. Specific, detailed data collection protocols
used by our research team for in situ data collection
have been described elsewhere.19
The intensity data of each spectrum was analyzed with the
SciAps Utility application to identify elements present in samples.
The emission wavelengths and corresponding elements
selected for each study are listed in Table I. Sodium and lithium
were identified in many samples but were attributed to
soil contamination and thus are not included in the table
nor included in subsequent statistical analyses. All spectra
were five-point Savitzky-Golay smoothed, integrated, and
normalized. Elements for normalization were selected, in
this case, calcium for the mortar spectra and silicon for the
pottery spectra, based on the presence of that element in
all samples and the availability of strong emission lines within
each spectrometer wavelength range for that element.
The normalized integration results were then imported
into The Unscrambler chemometrics software (Aspentech,
USA). Within The Unscrambler, data were evaluated with
principal components analysis (PCA) with random segmented
cross-validation. For each PCA, data were meancentered
and divided by standard deviation weighting was
used for each variable.
Results and Discussion
Utility of Handheld LIBS for Field Archaeology
Considering the pervasiveness of XRF within archaeometry
research, a comparison to LIBS and a discussion specific to
field applications is warranted. Physical dimensions, weights,
battery life, cost, and general ease of operation are fairly
comparable among commercially available handheld LIBS
and XRF instruments. For safety considerations, XRF
requires care regarding radiation exposure, whereas laser
safety is important with LIBS operation. Most consumables
and general supplies specific to fieldwork can be easily transported.
The handheld LIBS used for these studies has an
argon purge option that requires small, compressed gas
Table I. Spectral lines used for data analysis.
Element Ionization Wavelength (nm) Wall mortar Pottery
Ca
Si
Al
Mn
Fe
Ca
Sr
Ba
Ti
Si
Mg
Mg
K
O
II
I
I
I
I
I
I
II
I
II
I
I
I
I
317.93
390.55
403.08
✓
✓
394.40 ✓✓
✓
438.35 ✓✓
445.49 ✓✓
460.73 ✓✓
493.41 ✓✓
498.17 ✓✓
504.10 ✓✓
✓
✓
517.27
518.36
769.69 ✓✓
777.19 ✓✓
cylinders; these cylinders cannot be transported via air travel
together with other supplies. Analysis times are much
shorter with handheld LIBS at a few seconds for a single analysis
compared to handheld XRF at typically 30 s to a few minutes
per analysis. Using the same data collection protocols
for in situ measurements, on average over three times the
number of analyses, or approximately 150 analyses per day
were performed using handheld LIBS compared to about
40 analyses per day using handheld XRF. This difference is significant
and provides archaeology researchers the opportunity
to collect more LIBS data within one study and/or
conduct more studies within one excavation season compared
to XRF.
Accounting for LIBS' minimally destructive nature, care
and planning must be taken when using this technique for culturally
valuable objects. Choice of analysis location and
instrument settings are vital to avoid unintended damage to
archeological materials. With care, a single ablation spot
can be undetectable by the naked eye, and, similar to laser
ablation inductively coupled plasma mass spectrometry, the
application of LIBS for cultural heritage and archaeology is

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