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If we assume that the optimal stationary phase has been chosen, the length and internal diameter ID of a capillary column as well as its stationary phase film thickness (d_f) must also be taken into account. The conditions for a good separation of the analytes need to be found without increasing the analysis time. From a practical standpoint, in GC, we can only change the temperature and the carrier gas flow rate. In both cases, retention factors k and selectivities α are not much changed. For the carrier gas, we choose a flow rate such that its speed ū is close to the optimal value of the Van Deemter curve. For volatile compounds, we choose a column with a weak phase ratio (β < 100), hence with a thick film. Inversely, we choose a thin‐film column for less volatile compounds.

However, we must not forget that coupled GC‐MS does not necessarily require greatly optimized separation of chromatographic peaks (sufficient resolution), which is a significant time gain for the chromatographer. Moreover, in a series of analyses, if we can avoid the use of a temperature gradient, we can thus eliminate the time necessary to return to initial conditions in order to conduct the following analysis.

2D GC. Optimization sometimes involves dual chromatography on two different stationary phases (for example, polar and nonpolar) with a single set‐up including a carrier gas inlet shared by the two columns. The first chromatograph is used to isolate a peak, possibly corresponding to an unresolved mixture, which then goes through a second column for another separation.