Quality control (QC) pooled samples were used to condition the LC-MS analytical system so as to assess the reliability and reproducibility of the analysis, and for non-linear signal correction . Sample acquisition was randomized and the QC sample (six injections) was analyzed every 10 injections to monitor and correct changes in the instrument response, with each sample being injected three times. Furthermore, six QC runs were performed at the beginning and end of the batch to ensure system equilibration. Such sample randomization provides stochastic stratification in sample acquisition so as to minimize measurement bias. In the PCA space, the QC samples were clustered closely to each other (results not shown, as it is not the focus of this study), thus confirming the stability of the LC-MS system used, the reliability, and reproducibility of the analysis.