Final answer:
The molecule with the largest molecular weight elutes first in SEC, while the smallest elutes last. In reverse phase HPLC, phosphatidylcholines are separated based on hydrophobicity, with the most hydrophobic eluting later.
Step-by-step explanation:
SEC Elution Order and Reverse Phase HPLC Separation
In Size Exclusion Chromatography (SEC), the molecule that elutes first is typically the one with the largest molecular weight, as it cannot enter the pores of the stationary phase and thus travels more quickly through the column. Conversely, the molecule that elutes last is usually the one with the smallest molecular weight, since it enters the pores and takes longer to travel through the column. For the separation of phosphatidylcholines (PCs) using reverse phase High-Performance Liquid Chromatography (HPLC), the PC that is most attracted to the stationary phase is the one that is most hydrophobic, resulting in a slower elution. The PC that is least attracted will be the most hydrophilic, and thus, will elute faster.
Without specific details on the structure of the PCs in question, a general prediction for the order of elution from the column would be based on hydrophobicity. The more hydrophobic PCs are expected to elute later due to stronger interactions with the hydrophobic alkane chains on the stationary silica particles. The less hydrophobic ones would elute earlier.