What are lipids and why are they important to archaeologists?

Lipid residue analysis has the capacity to advance the fundamental scientific knowledge of the interactions between human societies and their socio-cultural organization through the systematic chemical analysis of material culture. The benefits of lipid residue analysis to archaeological research fall into three broad categories: 1) the diverse technological capabilities for analysis; 2) the characteristics of lipids; and 3) the application of the lipid data to archaeological research questions. This brief discussion covers categories 1 & 2.

Fig. 1. A simple example of how lipid structure can ID source is the comparison between 2 lipid sterols, one a meat source and the other a plant sterol. As you can see, these structures have much in common, but differ at the far end where the plant sterol has 2 more carbons and 4 more hydrogens.

Technological Diversity

Organic residue analysis (ORA) is a powerful method in archaeology that continues to expand its technological capabilities (Roffet-Salque, et al., 2017). After lipid extraction, chemical techniques are utilized in lipid residue analyses that allow identification of the specific nature and source of organic remains (Evershed, 2008). Numerous combinatorial spectrometric methods are available that can analyze archaeological samples using different lipid fractions or measurements to obtain equivalent results.

Historically, in archaeological lipid research, methods of separation combined with characterization such as gas chromatography/mass spectrometry (GC-MS) and gas chromatography-isotope ratio-mass spectrometry (GC-IR-MS) have been the most frequently utilized (Cramp and Evershed, 2015), but also include tandem mass spectrometry (GC/MS-MS) (Luong, et al., 2017), pyrolysis coupled with GC/MS (pyr-GC/MS) and Fourier transform infrared spectroscopy (FTIR) (Brettell, et al., 2015; Monnier, et al., 2017). Recently, nuclear magnetic resonance spectrometry (NMR) (Lanehart, et al., 2018a; Llorens-Fons, et al., 2018; Tanasi, et al., 2017b; Tanasi, et al., 2018), metabolomics and lipidomics using liquid chromatography-tandem mass spectrometry (LC-MS-MS) (Eusebio, et al., 2017; Koelmel, 2020; Lanehart, et al., 2018b) have been incorporated into the spectrometric toolkit of archaeological lipid residue analyses.

Other analytical methods use soft-ionization instrumentation that do not require fractionated preparation of the archaeological sample, i.e., matrix assisted laser desorption/ionization-Fourier transform-ion cyclotron resonance-mass spectrometry (MALDI-FT-ICR-MS) which can be used for more degraded archaeological samples (see Oras, et al., 2017). It is possible that future major archaeological discoveries will more likely occur in the laboratory than in the field (Harper, et al., 2017).

What are Lipids?

As an analytical material in archaeology, lipids have the benefits of long-term retention in an

artifact, the ability to provide specific identification of source through its structural versatility, an abundance of biomarkers, and the diversity of archaeological materials for lipid deposition.

Lipids are organic molecules that are insoluble in water but extractable by solvents. It is the hydrophobicity of lipids that increases the likelihood of preservation within an archaeological matrix over millennia (Eglington, et al., 1991). Structurally, lipids are carbon based chemicals that bond with hydrogen, oxygen, nitrogen, sulfur, and phosphorus and have a wide range of structures due to carbon’s ability to form 4 bonds (Fig. 2).

Fig. 2. Carbon's ability to form 4 bonds provides structural diversity. (adapted from Evershed, 2007)

The structures of lipids include straight chain lipids known as hydrocarbons (n-alkanes) which are a combination of carbon & hydrogen only (Fig. 3), as well as other structural features of branched chains, aromatic rings, double bonds, and lipids containing functional groups (Fig. 4).

Fig. 3. Straight chain lipid compounds (adapted from Evershed, 2007)

Fig. 4. Other structural features of lipids (adapted from Evershed, 2007).

The total lipid matrix or extract (TLE) can be comprised of all lipid structure types and lipid fractions (Table 1) that include n-alkanes, wax and sterol esters, fatty acids, mono-, di-, tri- glycerides, fatty alcohols, sterols, and free fatty acids.

Table 1. Fractions of the Total Lipid Extract (TLE)