Mass range extension and the analysis of peptides with a quadrupole ion trap mass spectrometer

Raymond Elmer Kaiser, Purdue University

Abstract

Previously, the principal disadvantage of the commercial ion trap mass spectrometer was its limited mass range of 650 dalton. Three different methods for extending the mass range of the quadrupole ion trap beyond this restraint is described. The first method for mass range extension involved a reduction in the trapping volume by designing and machining three sets of electrodes having half, third and quarter the dimensions of the commercial set. Using this method, the mass range of the ion trap could be extended a factor of 4, 9 and 16 times that of the commercial system, respectively. The second method involved a reduction in the drive frequency of the ion trap from 1.1 MHz to 0.6 MHz, resulting in a mass range which was 3.4 times higher than the commercial system. The final and most successful method for mass range extension involved using a novel method for reducing the q$\sb{\rm eject}$ value which was termed "axial modulation." This method allowed ions to cross from stability to instability on the Mathieu stability diagram much earlier than normal, resulting in mass ranges near 50,000 Da, a factor of 77 times that of the commercial system. Parameterization and aspects of operating the ion trap at high mass is also described. The second disadvantage of the ion trap was that there was previously no way to ionize non-volatile species, thereby limiting its applicability. To lift this restriction, the implementation of Cs$\sp{+}$ secondary ion mass spectrometry and matrix assisted laser desorption to the ion trap is described to allow for ionization and ion injection of higher mass, thermally labile species. The excellent sensitivity of the ion trap, consequence of its ability to accumulate ions from an external source, and the high efficiencies for collisionally dissociating ions in tandem experiments is demonstrated for the analysis of peptides. Both MS and MS$\sp{\rm n}$ experiments have been demonstrated on femtomole levels of peptides below 4000 dalton. MS and MS/MS spectra on subfemtomole levels of Gramicidin S, a cyclic peptide, were also obtained.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry

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