Characteristics of a rf-only hexapole ion-guide interface for plasma-source time-of-flight mass spectrometry

Abstract

A hexapole collision cell has been designed and incorporated into an orthogonal extraction plasma-source time-of-flight mass spectrometer (TOFMS). The ion-guide assembly was constructed from 20 cm long, 3.175 mm diameter stainless steel rods. The cell is positioned between the second and third vacuum stages of the mass spectrometer. The device was characterized with an inductively coupled plasma ionization source (ICP). Pressurization of the cell with a buffer gas (such as helium or hydrogen) lowers the axial energy of the ions, and produces a beam having a most probable energy of 1.3 eV and an energy spread of less than 2 eV. The signal levels were found to be strongly influenced by pressure in the cell, rf voltage applied to the rods, and operating frequency of the ion guide. Lighter ions (m/z 7) were transmitted more efficiently at higher frequencies and reduced rf voltages, while signals for heavier ions (m/z 238) were greater at lower frequencies and elevated voltages. Detection limits with the conventional static ion optics and hexapole collision cell were comparable, in the range of 0.4–2 ng ml−1 and similar when either helium or hydrogen collision gas was used. Precision for the peak height measurement was 5.4% RSD with the hexapole ion guide, slightly better than when static optics were employed (7.3% RSD). The strong signals from 40Ar+ could be reduced by four orders of magnitude through use of hydrogen as a collision gas because of efficient and selective charge-exchange reactions. Isotope ratio precision was found to be counting-statistics limited, with the best precision obtained for the isotopes of silver at 0.043% RSD (n = 7) for 10 million sequentially summed spectra. The mass resolving power (FWHM) of the spectrometer was improved through the use of the hexapole ion-guide assembly (2915) relative to that obtained with static ion optics (1510).