Liquid chromatography–atmospheric pressure photoionization-mass spectrometry analysis of triacylglycerol lipids—Effects of mobile phases on sensitivity

In this work, we evaluate the performance of liquid chromatography–atmospheric pressure photoionization-mass spectrometry (LC–APPI-MS) for non-aqueous reversed phase analysis of six triacylglycerol model compounds using six binary mobile phases including MeOH/iPrOH, MeOH/CHCl3, MeOH/CH2Cl2, CH3CN/iPrOH, CH3CN/CHCl3, and CH3CN/CH2Cl2. All mobile phases give comparably good separation performance on a Gemini C18 column with carefully adjusted gradient elution programs. APPI sensitivity varies from one mobile phase to the other without dopants; however use of dopants brings sensitivity to comparable levels for all mobile phases. MeOH/iPrOH offers high sensitivity without dopants due to self-doping effect and dopants are not necessary for this mobile phase. Dopants enhance analyte sensitivity to a varying degree for each of the mobile phases tested. Photo-induced chemical ionization (PCI) of solvent may play a significant role in achieving high sensitivity. Two critical parameters affecting sensitivity are photoabsorption cross-sections and ionization potentials of mobile phase solvents. How these mobile phase solvents affect APPI sensitivity and their dependency on dopant use are discussed. All six mobile phases offer comparable overall limits of detection for the analytes tested. These results indicate that LC–APPI-MS is a successful tool for neutral lipid analysis, giving high sensitivity with a variety of non-aqueous mobile phases.