How do I explain the existence of these 2 peaks in this mass spectrometry? Question about HRMS:Phosphatidylcholines and phosphatidylethanolamines are common types of glycerophospholipids found in biological membranes. Because of the high amount of heteroatom content, electron impact mass spectrometry is a bad idea because the ions fragment too fast. For biological molecules, electrospray ionization is the most common method. A compound in the glycerophospholipid class was analyzed by low-res electrospray ionization. It produced peaks: 773 Th(100 percent abundance), and 795 Th(about 20% abundance). The question says these are not isotope peaks. The same compound was analyzed by high-rest EI mass-spec to have peaks at 772.5856 Th and 794.5680 Th. The question says to explain the 2 peaks’ presences in the high-resolution mass spectrometry.

For question #6, I came up with the molecular formula for C40H80NO8P for both classes of lipids but I was unsure of how to use that to come up with the actual molecular formula. The first lipid I found was dipalmitoylphosphatidylechloline with a molar weight of 734. But it's smaller than either of the 2 fragments. Then I added 6 methylene groups to come up with a molecular formula of C46H92NO8P. But I'm not sure how to get the molecular formula for both fragments(or if we actually need them). Can you help with this question or let me know of what I'm missing?
In electrospray ionization, a molecule can end up with an extra proton or an extra cation. Either one of these +1 charged species is accelerated and detected in the mass spectrometer (negative charged particles or neutral species are not; if there are multiple cations, they would supposedly repel each other once the solvent is gone, and not fly together).

So a molecular formula that fits the data is $\ce{C43H82NO8P}$. The lighter species detected in the spectrometer would be $\ce{C43H82NO8PH+}$, and the heavier species would be $\ce{C43H82NO8PNa+}$. The masses given by https://www.sisweb.com/referenc/tools/exactmass.htm for these species match the given values pretty well. The difference in mass is not due to fragmentation, but to a cation piggy-backing with the molecule of interest (instead of the usual proton).

The data does not distinguish between phosphatidylcholines and phosphatidylethanolamines. You can, however, infer the number of C=C double bonds in the fatty acids attached to the glycerol.

I had to look up the unit "Th" on wikipedia:


  The thomson (symbol: Th) is a unit that has appeared infrequently in scientific literature relating to the field of mass spectrometry as a unit of mass-to-charge ratio. The unit was proposed by Cooks and Rockwood naming it in honour of J. J. Thomson who measured the mass-to-charge ratio of electrons and ions.


HRMS stands for high resolution mass spectroscopy, I think.