High performance quadrupole ion trap mass spectrometry

Jon David Williams, Purdue University

Abstract

When resonance ejection is performed to extend the mass-to-charge range of the quadrupole ion trap, typically, less than unit mass resolution and poor mass measurement accuracy result when ions with m/z ratios greater than 1,000 Da/charge are analyzed. This thesis shows methods to improve the performance of the high mass quadrupole ion trap. Mass resolution is shown to be dependent on the scan rate of the mass-selective instability scan when resonance ejection is performed. With very slow scans, mass resolutions exceeding 10$\sp6$ are observed. Better than unit resolution is achieved to select precursor ions and record the product ion spectrum for MS/MS experiments. A split tip probe has been devised to allow salt cluster ions and peptide ions to be simultaneously injected into the trap. This internal calibration method improves mass accuracy because calibrant and analyte ions are subjected to identical trapping conditions. With peak matching and internal calibration, the accuracy of mass measurement is improved from $\le$0.1% to $<$ 0.01%. Other factors that affect mass resolution and mass measurement accuracy were explored. Mass resolution achieved by forward and reverse resonance scans are compared. Poorer mass resolution is achieved when ions are scanned from the trap by a reverse RF scan and is attributed to a minor octapolar field component in the trapping field. Simultaneous storage of positive and negative ions is shown to improve mass resolution when ions are mass analyzed by the mass-selective instability scan. The addition of counterions reduces spacecharging of the trap. Using spatially resolved photodissociation, the motion of ions inside the ion trap can be monitored. Studies elucidating how collisions with helium and other buffer gases alter the trajectories of ions are shown. Heavier buffer gases such as Ne and Ar cool the ion cloud to the center of the trap more effectively than He when comparable pressures are used. Cooling of the ion trajectories after RF/DC isolation and resonance excitation are presented. The extent of cooling of the cloud is determined by (1) pressure, (2) length of cooling time and (3) value of the Mathieu parameter, q$\sb{\rm z}$.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry

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