Analyzing Mummies' Hair Using LA-ICP-MS: An Interview with Dula Amarasiriwardena

Andean mummies have fascinated archaeologists and scientists for years.

These mummies provide a snapshot into pre-Colombian civilization. Dating back thousands of years, Andean mummies offer valuable insights into the beliefs, rituals, and daily lives of pre-Colombian civilizations inhabiting the Andean region of South America, such as the Inca, Moche, and Chachapoya. Because they are often found well preserved in mountain caves, high-altitude tombs, or ceremonial sites, Andean mummies were often interred with elaborate offerings, tools, and textiles. This reflects the reverence and importance placed on the afterlife in early civilizations. Their remarkably well-preserved bodies, attributed to the cold, dry climate of the Andes, continue to shed light on the rich cultural history of the Andean people and their enduring legacies.

Dulasiri Amarasiriwardena, emeritus professor of chemistry at Hampshire College, Amherst, Massachusetts, and his team have been conducting research using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to investigate trace metal nutrition and exposure to toxic metal(loid) pollutants by studying Andean mummy remains, in particular hair (1–5,10, 12). Spectroscopy recently spoke to Prof. Amarasiriwardena about his team’s findings and what they can tell us about ancient Andean culture.

Prof. Dula Amarasiriwardena of Hampshire College. Photo Credit: © Prof. Dula Amarasiriwardena.

Can you provide an overview of the methodology used in this study to investigate exposure to environmental arsenic in ancient Andeans?

We used laser ablation (LA) to extract a tiny sample material from a well-cleaned ancient Andean hair strand. The resulting material is then introduced into an excitation source, an inductively coupled plasma (ICP), vaporized and atomized to form singly charged ions of arsenic. These ions are then extracted by the mass spectrometer (MS) and subsequently separated according to their mass-to-charge ratios (m/z) to confirm the presence of As in the hair. By measuring the intensity of these ions, we can determine the concentration of As in the hair using external calibration. Thus, LA-ICP-MS is an elegant method for determining As exposure in antiquity (1,2).

How does laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) contribute to analyzing arsenic levels in human hair, and why was this method chosen for the study?

LA-ICP-MS is an ideal archaeometric method for analyzing ancient Andean hair samples, where the sample amounts and sizes are minimal due to the accessibility and availability of these samples of archaeological interest. This semi-destructive technique needs only a few micrograms of hair samples (1,2). Hair is an incremental tissue. The LA-ICP-MS also offers the ability to look at the tissue-level arsenic distribution in a hair strand, thus serving as a unique window to investigate exposure during the last stages of their lifetimes.

Your study mentions ancient Andeans from two river valleys (Azapa and Camarones) (3). Can you elaborate on the selection criteria for the individuals included in the study and the significance of choosing these specific river valleys?

We considered anthropological and archaeological information about these pre-Columbian Andeans while selecting samples for this study, the river valleys where these Andeans were buried, and geogenic environmental chemical factors. This interdisciplinary project was done with my collaborator, Prof. Bernardo Arriaza, Universidad de Tarapaca, Chile, who selected these subjects for this study. The Andean mummies we selected are subject to the availability of archaeological studies and permission from the National Monuments Council in Chile (3).

The two river valleys, Azapa and Camarones, are in northern Chile in the Atacama Desert and are home to ancient Andeans, at least since the Chinchorros’ time (that is, ancient hunter-gatherer coastal tribe, circa ~ 7000 BC) (2,4,5). Modern environmental chemical data indicated that water in Camarones River is contaminated with dissolved natural arsenic. This river was the primary source of drinking water for ancient Andeans and was utilized to grow crops. Further north, the Azapa River valley was settled by the Morro and Azapa people. The water in this river is not rich in environmental arsenic compared to the Camarones River. Also, many ancient mummies are found in both river valleys.

The results indicate that arsenic is mostly bioaccumulated in the medulla and cortex regions. Could you explain the relevance of understanding the spatial distribution of arsenic within a hair strand and its implications for assessing exposure levels?

In addition to the many functions of human hair, it also acts as an excretory tissue. A hair stand has three central tissue regions: the outer cuticle and inner cortex, followed by the medulla tissue region. The latter two tissues are associated with the internal blood and nervous system. Any endogenous elements will appear first in the medulla region. Thus, understanding the spatial distribution of contaminants or nutrients in the medulla is essential. So, looking at the tissue level arsenic distribution in hair tissue is necessary. Also, any diagenetic incorporation of hair arsenic is immediately associated with outer cuticle tissue. Our LA-ICP-MS approach does just that (1–3).

The study notes elevated levels of arsenic in children and older adults (3). What potential explanations or hypotheses can be drawn from these age-related findings, and how might they relate to the exposure pathways in the ancient Andean populations?

We see elevated levels of As exposure among children (5–10 years) and older adults (>50 years) or seniors. As described previously, dissolved As levels are several orders of magnitude higher in the Camarones River compared to uncontaminated water bodies. The main arsenic exposure pathways appear via drinking water and food grown on irrigated fields with arsenic-rich water. The frequency and intensity of exposure likely contributed to those observed arsenic concentrations for children and seniors. In the case of older adults, arsenic accumulation during the terminal weeks of their lives is demonstrated, perhaps exhibiting intense and more frequent arsenic exposure (3).

The research highlights significant differences in arsenic exposure between the Camarones and Azapa valleys (3). What factors could contribute to these regional disparities, and how might they be linked to the consumption of arsenic-rich water in specific areas?

Of the two river valleys under consideration in this study, the Camarones River has very high dissolved As levels, and some villages along the river have reported about a hundred times greater than acceptable As for potable water (the U.S. EPA and WHO standard for As in drinking water is 10 mg/L) (3,4,6). On the other hand, the concentration of dissolved As in the Azapa River is low. This is reflected in hair As concentrations found among Camarones individuals.

The study mentions a calibration process with a certified reference human hair standard (3). How was the accuracy of the LA-ICP-MS method validated, and what measures were taken to ensure the reliability of the arsenic measurements?

We used the pelletized human hair-certified reference material (CRM-GBW 07604) to calibrate the LA-ICP-MS instrument. The resulting response factors are used to determine the hair concentration. The response factors were updated frequently. The internal standard ¹³C was used, and counts were normalized to account for any laser ablation variations. A 1000-μm linear laser scans along the hair at the root and mid-regions were analyzed along the well-cleansed hair to measure the As concentration. The average of the three scans were used to determine hair arsenic concentrations. The accuracy of +7.5% and the detection limit of 0.14 μg/g were achieved (3).

Considering the exposure levels observed in ancient Andeans, what implications does this study have for understanding the historical consumption of arsenic-rich water in the Atacama Desert? Additionally, what areas of research or further investigation do you believe are warranted based on these findings?

The ancient Andeans who settled in the river valleys had to deal with chronic hydroarsenicism due to the high levels of dissolved arsenic in the water. While the abundance of water in these valleys was an oasis for these settlers and ideal for growing maize and other crops, the arsenic-contaminated irrigation waters posed a significant threat to their health and daily lives. Although these valleys were seemingly attractive places to settle for the ancient Andeans, they were unknowingly poisoned with geogenic arsenic––a “silent killer.” This might have affected the health of their fetuses, newborns, children, and adults (2–5).

Apart from arsenic (3,4,7-9), the river valleys in this region also contain elevated concentrations of other geogenic elements like boron (10), lithium (11), and manganese (12). Understanding how these elements collectively affected the health and lifestyles of the ancient Andeans would be beneficial. It is also vital to investigate the exposure to the multiple elements from their diet, environment, and occupational activities.

References and Further Readings

(1) Steely, S.; Amarasiriwardena, D.; Jones, J.; Yáñez J. A rapid approach for assessment of arsenic exposure by elemental analysis of a single strand of hair using laser ablation-inductively coupled plasma-mass spectrometry. Microchem. J. 2007, 86, 235. https://doi.org/10.1016/j.microc.2007.03.009

(2) Byrne, S.; Amarasiriwardena, D.; Bandak, B.; Bartkus, L.; Kane, J.; Jones, J.; Yáñez, J.; Arriaza B.; Cornejo L. Were Chinchorros exposed to arsenic? Arsenic determination in Chinchorro mummies' hair by laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). Microchem J. 2010, 94, 28. https://doi.org/10.1016/j.microc.2009.08.006

(3) Amarasiriwardena, D.; Ahmed, M.; Arriaza, B. Environmental Arsenic Exposure by Ancient Andeans: Measurement of As in Mummy Hair Using LA-ICP-MS. Journal of Archaeological Science: Reports 2023, 48, 103883. https://doi.org/10.1016/j.jasrep.2023.103883

(4) Arriaza, B.; Amarasiriwardena, D.; Standen, V.; Yáñez, J.; Van Hoesen, J.; Figueroa, L. Living in poisoning environments: Invisible risks and human adaptation. Evol. Anthropol. 2018, 27, 188. https://doi.org/10.1002/evan.21720

(5) Arriaza, B.; Amarasiriwardena, D.; Starkings, J.; Ogalde, J. P. Use of LA-ICP-MS to Evaluate Mercury Exposure or Diagenesis in Inca and Non-Inca Mummies from Northern Chile. Archaeol. Anthropol. Sci. 2022, 14, 76. https://doi.org/10.1007/s12520-022-01547-w

(6) Yáñez, J.; Fierro, V;Mansilla, HD.;Figueroa, L.; Cornejo, L.; Barnes, RM. Arsenic speciation in human hair: a new perspective for epidemiological assessment in chronic arsenisism. J. Environ. Monit. 2005, 7 (12),1335. https://doi.org/10.1039/B506313B

(7) Kakoulli, I.; Prikhodko, S.V.; Fischer, C.; Cilluffo, M.; Uribe, M.; Bechtel, H.A.; Fakra, S.C., Marcus, M. A. Distribution and chemical speciation of arsenic in ancient human hair using synchrotron radiation. Anal. Chem. 2014. 86, 52. https://doi.org/10.1021/ac4024439

(8) Fresnais, M.; Richardin, P.; Gimat, A.; Sepúlveda, M.;Leize-Wagner, E.; Charrié, A. Recent advances in the characterization of hair of mummies from the Chilean Andean coast. Forensic Sci. Int. 2015. 249, 25. https://doi.org/10.1016/j.forsciint.2015.01.005

(9) Swift, J.; Cupper, M.; Greig, A.; Westaway, M.; Carter, C.; Santoro, C.; Wood, R.; Jacobsen, G.; Bertuch, F.Skeletal arsenic of the pre-Columbian population of Caleta Vitor, northern Chile. J. Archaeol. Sci. 2015. 58, 31. http://dx.doi.org/10.1016/j.jas.2015.03.024

(10) Arriaza, B.; Blumenstiel, D.;Amarasiriwardena, D.; Standen, VG.; Vizcarra, A. Five thousand years of bellyaches: Exploring boron concentration in ancient populations of the Atacama Desert. Am J Phys Anthropol. 2021, 174, 254.

(11) Blumenstiel, D.; McDonald, M.; Arriaza, B.;and Amarasiriwardena, D. Exposure to geogenic lithium in ancient Andeans: Unraveling lithium in mummy hair using LA-ICP-MS. J. Archaeol. Sci. 2020, 113 (1), 105062, https://doi.org/10.1016/j.jas.2019.105062

(12) Arriaza, B.; Figueroa,; L., Ogalde, J.P.; Vivien Standen,V.; Halcrow, S.; Amarasiriwardena, D.;Van Hoesen. J.; Castelleti, J.; An archaeometric approach to biocontamination with manganese pigments in ancient marine hunter-gatherers of the Atacama Desert: health, ideological, and socioeconomic considerations. Archaeol. Anthropol. Sci. 2023, 15, 188. https://doi.org/10.1007/s12520-023-01884-4