Lacking linearity in quantitative stable-isotope LC-MS/MS measurement of F2-isoprostanes is an irrefutable indicator of analytical inadequacy.

In LC-MS/MS-basedquantitative analysis of biological substances such as the F2-isoprostanes including 8-iso-prostaglandin F2α (i.e., 8-iso-PGF2α) use of stable-isotope labelled analogues at matrix-relevant concentrations is indispensable, enables linearity in relevant concentration ranges, and minimizes matrix effects. The quality of the linearity of calibration curves is an indicator of the analytical reliability of the LC-MS/MS method. Poor linearity is a convincing evidence of lacking analytical reliability. Improvement of linearity is better attempted by improving the extraction of the analytes from their biological samples and/or their chromatographic separation accepting longer analysis time, rather than by constructing calibration curves using linear weighted least-squares or other mathematical means.