The ALEX lipid calculator.
(A) 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+NH4]+, calculated m/z 680.48022, measured m/z 680.47945, m/z offset = -0.00077) and TAG 17:1/17:1/17:1 (internal standard (IS), [M+NH4]+, calculated m/z 860.77017, measured m/z 860.76889 and m/z offset = -0.00128). The FT MS calibration offset is estimated as the average of the m/z offset for both lock mass ions, i.e. the FT MS calibration offset = -0.0010. (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 m/z of PC 32:0 is 734.56872 and that the calculated m/z adjusted for the calibration offset is 734.56843 which yield a m/z 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.