δ¹³C (delta-13 C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. δ¹³C isotopic analysis, also referred to as an isotopic signature, is used for source attribution of carbon-based molecules. For example, stable carbon isotopes in carbon dioxide (CO₂) are utilized differentially by photosynthesis. Therefore, δ¹³C isotopic signatures can be used directly as a source attribution for plants—such as place of origin and authenticity of plant-derived foods and beverages. It can also be used indirectly for analysis of the diets of plant-eating animals. This enables an understanding of animal eating patterns as well as the authenticity of food of animal origin.

δ¹³C isotopic analysis can also be used for source attribution of methane (CH₄). Determining whether methane is biogenic or anthropogenic is essential to identifying its origin in groundwater near a hydraulic fracturing well, monitoring fugitive methane emissions from a landfill, or analyzing the biochemical pathways that produce and consume methane in wetlands.

Picarro has designed and produced several isotope analyzers that precisely measure δ¹³C in carbon dioxide and methane for laboratory and field studies.