Resonance-enhanced multiphoton ionization time-of-flight mass spectrometry for direct analysis of liposome-encapsulated compounds.

The direct measurement of compounds encapsulated into liposomes without pretreatment allows verification of both the encapsulation efficiency and the release rate of liposomes in their original state. In the present study, the direct analysis of liposomes was conducted via resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS). When analyte species (2-phenoxyethanol) encapsulated in liposomes were measured online, spike signals appeared in a time profile of the peak area for 2-phenoxyethanol, which suggested a dispersion of the compound in this sample. In addition, the spikes disappeared when the liposomes collapsed following the addition of a Triton X-100 aqueous solution. These results strongly suggest that the appearance of spikes arises from the compound encapsulated into the dispersed liposomes. REMPI-TOFMS has an inherent characteristic of superior selectivity, which suggests that this process would be useful for achieving a precise evaluation of the release properties of target compounds even in a liposome sample containing a large variety of components.