Rt-to-RI: A Python Tool for Automated Conversion of Retention Time (Rt) to Retention Index (RI) in Gas Chromatography and Instrumental Aroma Analysis

Abstract In gas chromatography (GC), and particularly in GC olfactometry (GC-O), the retention times (Rt) of independent measurements are only comparable to a limited extent, as they depend strongly on the equipment, temperature programme and measurement conditions. Although retention indices (RI) are routinely used to identify individual substances, there is currently no freely available and easily accessible solution for converting complete chromatograms into RI-scaled data sets. This work presents a Python-based open-source tool ( Rt-to-RI ) that enables the automated conversion of complete GC chromatograms from retention time to retention index. The tool processes standard raw data formats (.csv, .txt), calculates RI values according to Kováts (Helv Chim Acta 41(7):1915-1932, 1958) and van den Dool & Kratz (J Chromatogr 11:463-471, 1963) for temperature-programmed GC, and provides the results as reusable data sets and interactive visualisations. The applicability of the tool was demonstrated using a standard mixture of eight odour-active substances by GC-MS and GC-O measurements. The RI-based presentation allowed for improved overlaying of chromatographic and sensory information as well as direct comparison with literature and database values. The tool presented contributes to the standardisation of chromatographic evaluations and is a practical tool for aroma and food analysis as well as for GC-O-based studies. The programming code, including test files for testing purposes, and the link to the freely accessible online tool can be found at https://github.com/mue-li/Rt-to-RI .