Establishing 18O‑Labeled Inositol Phosphates for Quantitative Capillary Electrophoresis-Mass Spectrometry: Fragmentation Pathways and Comparison with 13C‑Labeled Analogs

Capillary electrophoresis mass spectrometry (CE-MS) allows for the rapid and accurate quantitative analysis of inositol phosphates (InsPs) and inositol pyrophosphates (PP-InsPs). The recent discovery of new InsPs and PP-InsPs isomers in plants and mammals necessitates new heavy isotope references for quantitative analysis of complex cellular extracts. Here, we evaluate 18O-labeled InsPs and PP-InsPs as alternatives to 13C labeled internal standards for quantitation by CE-MS. In contrast to 13C labels, the 18O labels are introduced at the end of a synthetic campaign and not at the beginning, rendering 18O much more accessible and affordable as a label. A series of 18O-labeled InsPs and PP-InsPs with different numbers and positions of 18O atoms were synthesized, enabling systematic investigation of MS2 fragmentation pathways. We propose two major dissociation pathways to elucidate the 18O redistribution of the dominant product ion (the loss of H3PO4). Based on these insights, we identified the loss of HPO3 as a suitable transition for minimizing isotope redistribution in MS2 analysis. The ratios of this alternative product ion and dominant product ion were reproducible across replicates, concentration, and measurement days, supporting the use of this alternative product ion as a reliable product ion for quantitative analysis. Application to Saccharomyces cerevisiae, HCT116 cells, and Arabidopsis thaliana extracts confirmed accurate quantitation and precision comparable to 13C-based methods.