Abstract :
[en] Retention indices (RIs) are commonly used in both GC and GC×GC as chromatographic parameters to support compound identification, especially in non-targeted analysis. Predicting RIs, particularly for compounds not found in databases, can significantly aid in these analyses. However, in order to develop an effective prediction model, it's essential to first understand the factors that influence RIs.
In GC×GC, retention indices tend to be less consistent and exhibit greater variability. This variability is closely tied to the chromatogram reconstruction step and the additional parameters involved in the technique. In this context, a study was conducted to identify the key parameters affecting RI values using the Century Mix (CM), a standardized reference mixture designed to evaluate and compare the performance of chromatographic systems. The CM includes approximately one hundred compounds from various chemical families, such as alkanes, alkenes, alkynes, alcohols, ketones, esters, and aromatics. Its chemical diversity allows for the simulation of complex mixtures, providing a solid basis for testing GC×GC separation capabilities2.
An interlaboratory study was first carried out using various GC×GC systems (including thermal and flow modulators) and different column configurations (normal, reverse, hybrid) with different type of detector (TOF MS, Quadrupole MS, FID). Operational conditions (such as carrier gas flow, temperature ramp, detector acquisition parameters, etc) were standardized across systems to ensure comparability. The primary objective was to assess the performance of different GC×GC systems under controlled conditions and to identify which components of the system have the most significant impact on RI values. A Principal Component Analysis (PCA) was performed using the retention indices of all compounds of the CM. The PCA plot effectively captured variations in system performance, grouping systems based on their retention characteristics. The results revealed clear clusters, with the primary column’s stationary phase emerging as the dominant factor influencing overall separation behavior in GC×GC.
Subsequently, a full factorial design of experiments was implemented to investigate the influence of parameters such as temperature ramp, carrier gas flow, modulation period, and oven offset. The goal was to evaluate their impact on RI values using a reverse column configuration (mid-polar × apolar) and to determine their influence on individual compounds in the CM. The results showed that for 33 compounds, the temperature ramp had the most significant effect on RI variation, followed by flow rate. For the 44 remaining compounds, the tested operational parameters had minimal impact, the stationary phase had a much greater influence on retention behavior.
