Correlation between secondary ion intensity and surface excess concentration of diacylphosphatidylcholines in fast‐atom bombardment mass spectrometry

The surface tension for three diacylphosphatidylcholines in glycerol was measured at different concentrations and, based on these data, the surface excess concentrations were calculated. These calculated values were then used to examine the correlation between surface excess concentration in glycerol and the observed secondary ion intensities produced during xenon bombardment of the same solutions. For the surface excess concentration of phospholipids in glycerol a behaviour, similar to that found for surfactants in water, could be established. The experimental results showed a straightforward correlation between the mass spectrometric response of the desorbed protonated molecular ions of diacylphosphatidylcholines and the calculated surface excess concentrations for the selected short time (∼1 min) of data acquisition. This finding supports a model which assumes desorption of precharged species in the first several monolayers of the liquid matrix. Furthermore, the surface excess concentration of each phospholipid in glycerol solutions of identical bulk concentration was found to be different, reflecting differences in surface activity, which explain observed suppression effects in phospholipid mixtures isolated from biological issues. Based on this observation and the existence of the same structural head group in phosphatidylcholines which are carrying the fixed positive charge, it can be assumed that differences in ionization efficiency are not responsible for the differences observed in the mass spectrometric response. An additional attempt was made to correlate surface tension, bulk concentration and secondary ion intensity of equimolar mixtures of dilauroyl‐ and distearoyl‐phosphatidylcholine in glycerol. The results indicate a nonlinear desorption behaviour for phosphatidylcholine mixtures in the combined concentration range from 9.0 × 10 −6 to 5.9 × 10 −2 M.