Using Laser-Induced Breakdown Spectroscopy to Assess Preservation Quality of Archaeological Bones by Measurement of Calcium-to-Fluorine Ratios

Abstract

We determined calcium-to-fluorine (Ca/F) signal ratios at the surface and in the depth dimension in approximately 6000-year-old sheep and cattle bones using Ca I 671.8 and F I 685.6 emission lines. Because the bones had been previously analyzed for collagen preservation quality by measurement of C/N ratios at the Oxford Radiocarbon Accelerator Unit, we were able to examine the correlation between our ratios and quality of preservation. In the bones analyzed in this experiment, the Ca I 671.8/F I 685.6 ratio was generally lower and decreased with successive laser pulses into poorly preserved bones while the ratio was generally higher and increased with successive laser pulses into well-preserved bones. After 210 successive pulses, a discriminator value for this ratio (5.70) could be used to distinguish well-preserved and poorly preserved bones regardless of species.

Use of micro-computed tomography imaging and porosity measurements as indicators of collagen preservation in archaeological bone

“…Centering the spectral window at 680 nm enabled simultaneous observation of the F I 685.6 and Ca I 671.8 emissions, with no interference. The experiment, including a schematic diagram, is described in detail by Rusak, et al (2011).…”

“…The average Ca/F ratio for ten pulses beginning with pulse 210 are given in Table 2 for the analyzed samples. In their publication, Rusak, et al (2011) provide Ca/F ratios for all of the laser pulses, and a reanalysis of these results, plotted in Figure 7, is instructive. Figure 7 shows a significant difference of Ca/F ratios between bones that contain no collagen (black lines), and those with >1% collagen (light gray lines).…”

“…LIBS is a minimally destructive technique in that the laser does not bore deeply into the bone. After an analysis comprising approximately 220 laser pulses, the finished bone has a barely-visible channel with a crater depth on the order of 10 2 microns (Rusak et al 2011).…”

“…The identification of peaks specific to collagen, such as the amide I peak at 1636 cm -1 , hold more promise but are more difficult to detect using the portable hand-held instruments (Chadefaux, et al 2009;France, et al 2014;Lebon, et al 2016;Madden, et al 2018). Other methods for predicting collagen preservation, such as trace element analysis (King, et al 2011;Fernandes, et al 2013), laser-induced breakdown spectroscopy (Rusak, et al 2011), X-ray and neutron radiography (Sołtysiak, et al, 2018), tensile strength (Turner-Walker & Parry 1995), and differential scanning calorimetry have met with mixed success.…”

“…Ca 671.8 / F 685.6 ratio measured by LIBS, average of ten pulses beginning with pulse 210. Details of the experimental methods and results are reported inRusak, et al (2011).…”