Designing performance into the quadrupole ion trap mass spectrometer: Mechanical and ion optical methodologies

Lynn Anne Gill, Purdue University

Abstract

The performance of the quadrupole ion trap has continued to be developed since its commercial introduction in 1984. Both the geometry of the ion trap and the motion of the ions in the ion trap are critical to its operation. This thesis introduces a new mechanical methodology for investigating and optimizing the performance of the quadrupole ion trap. Additionally, an ion optical methodology is utilized to better understand ion motion in the quadrupole ion trap. The performance of the quadrupole ion trap is dependent upon the quadrupolar electric fields used for ion trapping. Imperfections in these fields arise from truncated electrodes, machining errors and apertures in the endcaps. To compensate for these imperfections, an axial $\rm (z\sb0)$ stretch of 10.7%, from the theoretical value of $\rm z\sb0 = 0.707cm,$ has been incorporated into most commercial instruments. Because no systematic study of performance as a function of $\rm z\sb0$ has been previously reported, a system was designed that allowed for in situ adjustment of the electrode geometry under operating conditions. This method showed that both resolution and signal intensity were improved while maintaining mass measurement accuracy. The ability to change the geometry in situ also allowed investigation of compound-dependent mass shifts and optimization of these shifts allowed resolution of two different sets of isobars. Understanding ion motion in the quadrupole ion trap is another means of improving performance. The method used to map the ion motion in these experiments is DC tomography which utilizes DC pulses to investigate ion motion. Ions undergo collisional cooling with the helium buffer gas, which is present in the ion trap. DC tomography was used to measure cooling times for different compounds with similar mass but different elastic scattering cross-sections. Cooling times for krypton, $\rm d\sb6$-benzene and 1-hexene were obtained and the data correlated well with results from both calculated cross-sections and traditional ion mobility experiments.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry|Chemistry

COinS