Analysis of 2-arachidonoylglycerol (2-AG) and
N-arachidonoylethanolamide (anandamide)
via alkali or alkaline earth metal-adduct high-energy collision-induced dissociation (CID) in fast-atom bombardment (FAB) ionization-mass spectrometry (MS) is described. The CID-MS/MS of the [2-AG+Li]
+ or [2-AG+Na]
+ ion undergoes charge-remote fragmentation (CRF), which is useful for the determination of the double-bond positions in the hydrocarbon chain, while the CID-MS/MS of the [2-AG-H+Cat]
+ (Cat = Mg
2+, Ca
2+, Ba
2+) ion provides an abundant fragment ion of the cationized arachidonic acid species, which is derived from cleaving the ester bond
via a McLafferty-type rearrangement in addition to structurally informative CRF ions in small amounts. On the other hand, the CID-MS/MS spectra of anandamide cationized with both alkali metal (Li
+ or Na
+) and alkaline earth metal (Mg
2+, Ca
2+, or Ba
2+) show CRF patterns: the spectra obtained in lithium or sodium adduct are more clearly visible than those in magnesium, calcium, or barium adduct. The McLafferty rearrangement is not observed with metal-adduct anandamide. The characteristics in each mass spectrum are useful for the detection of these endogenous ligands.
m-Nitrobenzyl alcohol (
m-NBA) is the most suitable matrix. A lithium-adduct [2-AG+Li]
+ or [anandamide+Li]
+ ion is observed to be the most abundant in each mass spectrum, since the affinity of lithium for
m-NBA is lower than that for other matrices examined.
View full abstract