FIGURE 3.

Schematic illustration of the inter-relationship among the MS/MS techniques for the analysis of individual molecular species of a class of interest. We only illustrate the analysis of three species (M₁, M₂, and M₃) of a class for simplicity, whereas there exist up to hundreds of individual molecular species within a class. We assume that this class of lipid species, similar to a class of phospholipids or sphingolipids possesses one common neutral-loss fragment with mass of m_a (i.e., M₁ − m_1a = M₂ − m_2a = M₃ − m_3a = m_a (a constant)), one common fragment ion at m/z m_c (i.e., m_1c = m_2c = m_3c = m_c), and a specific ion to individual species at m/z m_1b, m_2b, and m_3b, respectively, which might not be identical to each other. Specifically, the common neutral fragment and the common fragment ion both result from the head group of the class; the individual species-specific ions represent the fatty acyl moieties of the species; and thus the residual part of each individual species can be derived from these fragments in combination with the m/z of each molecule ion. Panel A shows a simplified full-mass scan; Panel B illustrates the product-ion analysis of these molecule ions; Panel C demonstrates the scanning of the individual neutral-loss fragment between a specific molecule ion and its individual fragment ion; and Panel D represents the scanning of each individual fragment ion. It should be emphasized that, although the analyses of fragments with either neutral-loss scanning (NLS) or precursor-ion scanning (PIS) are much more complicated than those in product-ion scanning in this simplified case, the analyses by NLS or PIS are much simpler than that with product-ion scanning in reality as discussed in the text.