<prompt> In mass spectrometer, peptide are studied by projection of ?HeliumHydrogen ionOxygenNone  <response>: A mass spectrometer generates multiple ions from the sample under investigation, it then separates them according to their specific mass-to-charge ratio mz, and then records the relative abundance of each ion type.In positive ionisation made, a trace of formic acid is often added to aid protonation addition of proton or hydrogen ion of sample molecules. Proteins and peptides are usually analysed under positive ionisation conditions.In negative ionisation mode, a trace of ammonia solution or volatile amine is added to aid deprotonation removal of proton or hydrogen ion of the sample molecules. Saccharides and oligonucleotides are usually analysed under negative ionisation conditions. 