Carbon content (%) measurement for determination of ¹⁴C specific activity (Bq kg⁻¹C) in environmental matrices based on thermal oxidation of the sample using pyrolyser is discussed. The carbon content in biota samples (N = 574) of the tropical region were in the range 30–45%, 28–47%, 31–46%, 32–45%, 43–49%, and 37–50% with corresponding mean values of 38 ± 3.1%, 38 ± 4.4%, 41 ± 3.1%, 41 ± 3.7%, 46 ± 1.5%, and 44 ± 5.1% respectively for wild plant leaves, vegetables, fruits, cereals, wood, and fish samples. The present study shows that carbon content within the species and category (fruit, leaves, vegetables, etc.) of the sample may differ up to ~ 20% points.