Applied_Spectroscopy_Practica_01_01 - 19

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Applied Spectroscopy Practica 1(1)
included in the analysis. This model accounted for 73.6% of
the variance in the data within the first two principal components
(PCs).
The cluster along the upper half of Fig. 3a consists of a
majority of the data points for loci 149, 36, 25, 22, and 3
with Si and Fe identified as markers for these loci based on
more elements than XRF. In addition to the detection of transition
metals for which it is ideal, LIBS is highly sensitive to
low atomic mass elements such as Li, Be, C, O, N, Na, Mg,
and Al, which are challenging or impossible to detect using
XRF.
Roman Wall Mortars
A representative handheld LIBS spectrum of Trench C mortar
with labeled emission lines for all identified elements is
provided in Fig. 2. Elements found to be present in the
Coriglia Trench C wall mortars are typical of ancient
Roman mortars.21-23
The PCA of the entire mortar data set (750+ analyses)
was unsuccessful in that no clear groupings among loci
were found. A similar observation was made with previous
mortar phasing work using XRF data and was remedied by
instead selecting a data subset based on the physical location
in the trench. PCA of eight loci in the eastern section of
Trench C was performed next with the resulting twodimensional
score plot in Fig. 3a together with a labeled
trench map in Fig. 3b indicating the locations of all loci
loading plot results; these wall sections, consisting of three
parallel walls and their corresponding perpendicular attached
walls, are shaded blue in Fig. 3b. Si is likely associated with
quartz sand mixed into the lime mortar as aggregate.
Elevated Fe in mortars is associated with a hydraulic character
from volcanic pozzolan.23 The chemical similarities among
loci 149, 36, 25, and 22 were previously established with
handheld XRF analyses16 and are now further supported by
the handheld LIBS data. The addition of locus 3 to this grouping
establishes a more complete construction picture for this
section of the trench in that the mortars for complete rooms
are demonstrated to be associated. The cluster along the
bottom half of Fig. 3a consists of a majority of the data points
for loci 579, 270, and 30 and is shaded in pink in Fig. 3b;
examination of the loading plot indicates Sr and Ba as markers
for these loci. Sr is likely associated with dendric material
in the aggregate24 and has been reported to show great variation
in relative concentrations across different mortar samples.
Ba may be associated with feldspars.23 Loci 579 and 270
form an " L " at the northern edge of the trench, while locus
30 is part of a drainage system that bisects the trench.
Although the grouping of loci 579 and 270 with each
other and separate from the other eastern wall loci is in
agreement with the handheld XRF findings, the association
of locus 30 with the loci 579 and 270 grouping is not in
4
widely accepted by researchers in the field.20 As was the case
in the two studies presented here, minimal surface damage
from laser precleaning shots followed by LIBS analysis was
allowed and likely led to improved elemental analysis data
compared to surface-based analysis using XRF. LIBS spectra
are more complex and require more analysis time compared
to XRF. Calibration for quantitative work is a challenge for
both techniques often requiring matrix-specific standards.
Lastly, a significant advantage to LIBS is its ability to detect
Figure 2. Representative handheld LIBS spectrum for wall mortar.
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