Cotton as a source of isotopic biases in extracted water via cryogenic vacuum extraction

Cryogenic vacuum extraction (CVE) is widely used for extracting water from solid matrices for isotopic analysis, such as δ²H and δ¹⁸O. However, materials used in sample preparation may introduce isotopic biases. These potential biases could affect isotopes applications in water source analysis, food origin tracing, and forensic studies. This research investigated degreased cotton, commonly used during CVE for filtering or preventing spills, as a potential source of δ²H and δ¹⁸O biases. The results indicated that cotton induced δ²H biases ranging from −54.47 ‰ to −10.07 ‰ and δ¹⁸O biases from −2.13 ‰ to −0.18 ‰, depending on the cotton-to-water ratios, which varied between 0.34 and 3.59. These biases significantly correlated with the cotton-to-water ratio (p < 0.0001). Infrared spectroscopy analysis indicated increased –OH and crystal water groups in cotton after CVE, suggesting these groups contribute to the biases. Polypropylene cotton, plastic, and nylon were viable alternatives to degreased cotton for use in the extraction process. This research emphasized the need to avoid using degreased cotton in CVE experiments and proposed potential alternative materials.