In many experiments utilizing a Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS) pressure is of little concern. That is, as long as conditions are isobaric and pressure is low enough for signal, an absolute pressure measurement is not needed. In many cases, however, (e.g. rate constant measurements or measuring cross sections using the CRAFTI technique) absolute pressure is critical. Unfortunately, pressure is usually measured at a point remote from the trapping cell. Pressures measured in this way can be unreliable and will differ from the absolute pressure inside the cell. Accurate measurements will make absolute CRAFTI cross section measurements possible.We have seen that CRAFTI cross sections of polyatomic ions generally increase with increasing kinetic energy. This behavior is counter-intuitive; we would expect increasing kinetic energy to lower a molecule's collision cross section. We explain this by noting that polyatomics can dephase not only by scattering, but also by dissociation, which increases with increasing collision energy. This was verified by measuring the CRAFTI cross sections of Cs+ (which obviously has no dissociation pathways). We have seen that the cross section of Cs+ does indeed decrease with increasing kinetic energy. Using crystallography data for the expected cross section of Cs+, we can determine the absolute pressure inside the trapping cell.

Absolute pressure in FTICR-MS using the "CRAFTI" technique for measuring collision cross sections

248th National Meeting (2014)

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