Selected Ion Monitoring for LC-MS

Lipid quantitation with this methodology on a large scale is quite limited [63], although targeted analysis of a small number of lipids whose standard curves can be generated is quite common [64], 

It should be recognized that the identity of an individual extracted ion is not definitively identified in the approach. 

The specificity of the extracted ion to the compound of interest is generally a concern with SIM due to multiple interferences that are commonly present. 

To reduce any artificial ion extraction, very high mass accuracy/high mass resolution instrumentation is always preferable. Alternatively, analysis of individual lipid species may be limited to one lipid class that is pre-fractionated. A good example of this alternative is the analysis of human skin ceramide species. Masukawa and colleagues quantified ceramide species in human stratum corneum with LC-MS after SPE separation of the ceramide fraction [65]. Over 182 molecule ions corresponding to the diverse ceramide species in the stratum corneum based on m/z were measured by using the SIM method. 

2 Selected/Multiple Reaction Monitoring for LC-MS Selected/multiple reaction monitoring (SRM/MRM), which is the special case of either PIS or NLS (see Chapter 2), becomes the preferred method for detection of a particular ion since it can be done in a very short time frame. SRM/MRM could be very specific if the monitored fragment ion is specific to the precursor in combination with an LC separation and no interfering transitions are concomitantly present. 

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