Temperature optimization for improved determination of phosphatidylserine species by micro liquid chromatography with electrospray tandem mass spectrometric detection

A sensitive method for determination of disaturated phosphatidylserine species in the presence of their monounsaturated analogs has been developed, using micro liquid chromatography coupled to electrospray ionization tandem mass spectrometry. The hydrophobic nature of the phosphatidylserine species required a combination of low-eluting sample solvents and sub-ambient temperatures in order to focus large sample volumes up to 20 microL. The samples were dissolved in 2-propanol:hexane:water (20:10:4, v/v/v) prior to 1:9 dilution with ammonium formate buffer:2-propanol:tetrahydrofuran (30:55:15, v/v/v) and final 1:4 dilution with ammonium formate buffer (10 mM):2-propanol: tetrahydrofuran (55:37.5:7.5, v/v/v). The analytical column was a 0.5 x 150 mm stainless steel column packed with 5 microm C30 particles, while the mobile phase contained ammonium formate buffer (10 mM): 2-propanol:tetrahydrofuran (30:55:15, v/v/v). A temperature program from 5 degrees C (hold for 3 minutes) to 75 degrees C at 8 K/min provided separation of the disaturated phosphatidylserine species from their monounsaturated analogs, making available a sensitive determination of the isobaric species. The mass limit of detection for dipalmitoyl phosphatidylserine was 100 pg, corresponding to a concentration limit of detection of 5 pg/microL when using an injection volume of 20 microL. This is an improvement by a factor of 20 as compared to previously reported numbers obtained with conventional LC columns. The within-assay precision of dipalmitoyl phosphatidylserine was 11.9% RSD (n = 3), while the retention time precision was 4.1% RSD (n = 6).