Fig. 1.

Normalization by reference pool measurements. In a GC–MS sequence plan, every 8th position a reference pool sample is measured. For pool normalization, the average of the 2 nearest pools is calculated for each metabolite i (${\displaystyle \overline{X_i}}$). Then, for each sample the metabolite intensity of each metabolite i is divided by the corresponding average ${\displaystyle \overline{X_i}}$ of the pools: $\frac{X_i\left(Sample\right)}{{{\displaystyle \overline{X}}}_i\left(Pool\right)}$.