The ALEX lipid calculator.

<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>