QC Shotgun Parameters
- Mass accuracy
- Averaged accuracy calculated for masses of detectable standards, in ppm. ISTD References is a comprehensive list of internal standards, including m/z and detection mode.
- Resolution drop
- Mass resolution (FWHM) drop (in %) is calculated by comparing the resolution reported for low m/z standards (at m/z 571.5789 ± 5ppm in positive mode; m/z 531.3476 ± 5ppm in negative mode) and high m/z standards (at m/z 947.8793 ± 5ppm in positive mode; m/z 823.6411 ± 5ppm in negative mode).
- Isotopic Concordance
- Indicates the fidelity of reported isotope distributions. It is calculated as the ratio (expressed in %) between the measured and theoretical relative intensity of the first (+1 Th) isotopic peaks reported for a few lipids. The average value is determined for both high (m/z 841.8016 ± 5ppm in positive; m/z 823.6219 ± 5ppm in negative) and low m/z standards (m/z 571.5790 ± 5ppm in positive; m/z 559.3766 ± 5ppm in negative). A value of 100% indicates a perfect match between measured and expected intensities.
- Class Response
- Relative standard deviation (RSD) is calculated for peak intensities of each lipid within the same class, after normalisation to their concentrations. This assessment is performed for nine triacylglycerol (TG) peaks in positive ion mode and five phosphatidylethanolamine (PE) peaks in negative ion mode. RSD value of zero indicates no variation between normalised peak intensities within the same lipid class. Higher RSD values indicate greater variability in peak intensities, suggesting a less uniform response within the lipid class.
- In-source fragmentation
- It is assessed by calculating the ratio (expressed in %) of the intensity of a characteristic fragment ion to the total intensity of all potential precursor ions in the FT-MS spectrum.
(i) Positive ion mode:
(ii) Negative ion mode:
An ideal value is zero, indicating that no in-source fragmentation occurs. Higher ratio reflects pronounced in-source fragmentation, possibly leading to artefact lipid (i.e. lyso-lipid fragments)
- Sensitivity
- The ratio of summed intensities of all peaks of lipid standards (Ist) to intensities of other peaks (I all – I st) in FT MS spectrum.
Higher values suggest improved sensitivity, as it reflects a greater proportion of the standard signal relative to the background.
- MS2 Fragmentation
- The fragmentation efficacy is expressed as the ratio of intensities of the lipid precursor (acquired at NCE 10) and fragment (acquired at NCE 30) peaks in the MS2 spectrum. Fragmented lipids are PC-d5 33:1 and its head group (m/z 184.0736 ± 5ppm) in positive, and PE-d5 33:1 and carboxylate anion FA 16:0 (253.217 ± 5ppm) in negative modes. The value of zero indicates that no fragment was detected in the FT MS/MS spectrum.