10.1371/journal.pone.0079736.g002 Peter Husen Peter Husen Kirill Tarasov Kirill Tarasov Maciej Katafiasz Maciej Katafiasz Elena Sokol Elena Sokol Johannes Vogt Johannes Vogt Jan Baumgart Jan Baumgart Robert Nitsch Robert Nitsch Kim Ekroos Kim Ekroos Christer S. Ejsing Christer S. Ejsing The ALEX lipid calculator. Public Library of Science 2013 alex lipid 2013-11-07 07:13:50 Figure https://plos.figshare.com/articles/figure/_The_ALEX_lipid_calculator_/943433 <p>(<b>A</b>) Representative positive ion mode FT MS spectrum of a 10:1-phase lipid extract of hippocampus from a PRG-1 knockout mouse. Note that the detection of selected lock mass ions tris(ditert-butylphenyl) phosphate (chemical background, [M+NH<sub>4</sub>]<sup>+</sup>, calculated <i>m/z</i> 680.48022, measured <i>m</i>/<i>z</i> 680.47945, <i>m</i>/<i>z</i> offset = -0.00077) and TAG 17:1/17:1/17:1 (internal standard (IS), [M+NH<sub>4</sub>]<sup>+</sup>, calculated <i>m/z</i> 860.77017, measured <i>m</i>/<i>z</i> 860.76889 and <i>m</i>/<i>z</i> offset = -0.00128). The FT MS calibration offset is estimated as the average of the <i>m/z</i> offset for both lock mass ions, i.e. the FT MS calibration offset = -0.0010. (<b>B</b>) Screenshot of the ALEX lipid calculator showing information for endogenous lipid species PC 32:0 while applying the FT MS calibration offset = -0.0010. Note that the measured <i>m/z</i> of PC 32:0 is 734.56872 and that the calculated <i>m/z</i> adjusted for the calibration offset is 734.56843 which yield a <i>m/z</i> difference of 0.00029 corresponding to a mass error of 0.4 ppm. Without applying lock mass adjustment the mass error would be 1 ppm.</p>