XianZhen Zhao

Overcoming Field Imperfections of Quadrupole Mass Filters with Mass Analysis in Islands of Stability

We have constructed, and tested as mass filters, linear quadrupoles with added hexapole fields of 4%, 8%, and 12%, with and without added octopole fields. A hexapole field can be added to the field of a linear quadrupole by rotating the two y rods toward an x rod. This also adds an octopole field which can be removed by making the x rods greater in diameter than the y rods. In comparison to conventional quadrupole mass filters these rod sets have severely distorted quadrupole fields, with a mix of both even and odd higher spatial harmonics. They allow evaluating the performance of rod sets with strong geometric and field distortions as mass filters. Conventional mass analysis at the tip of the stability diagram has been compared to mass analysis using islands of stability. The stability islands are produced by applying an auxiliary quadrupole excitation field to the quadrupole. We show that with normal mass analysis at the tip of the stability diagram, the transmission, resolution, and peak shapes are relatively poor in comparison to a conventional rod set. However, the use of islands of stability dramatically improves the resolution and peak shape, and in some cases ion transmission, suggesting that mass analysis with islands of stability may provide a method to overcome a wide range of field imperfections in linear quadrupole mass filters.

Quadrupole excitation of ions in linear quadrupole ion traps with added octopole fields

Modeling and experimental studies of quadrupole excitation of ions in linear quadrupole traps with added octopole fields are described. An approximate solution to the equations of motion of ions trapped in a quadrupole with added octopole and dodecapole fields, with quadrupole excitation and damping is given. The solutions give the steady-state or stationary amplitudes of oscillation with different excitation frequencies. Trajectory calculations of the oscillation amplitudes are also presented. The calculations show that there can be large changes in the amplitude of ion oscillation with small changes in excitation frequency, on both the low and high-frequency sides of a resonance. Results of experiments with quadrupole excitation of reserpine ions in linear quadrupole traps with 2.0%, 2.6%, and 4.0% added octopole fields are given. It is found that as the excitation frequency is changed, two resonances are generally observed, which are attributed to the motion in the x and y directions. The two resonances can have quite different intensities. Sudden jumps or sharp sided resonances are not observed, although in some cases asymmetric resonances are seen. The calculated frequency differences between the two resonances are in approximate agreement with the experiments.

Quadrupole mass filters with added hexapole fields.

Conventional mass analysis has been investigated experimentally with six quadrupole mass filters with added hexapole fields; three with added hexapole fields of 4%, 8% and 12% with equal diameter rods, and three with added hexapole fields of 4%, 8% and 12% with unequal diameter rods to remove an added octopole field. Compared with conventional quadrupoles, these rod sets have very large field distortions. With the positive resolving dc applied to the y rods (Mathieu parameter a(x) < 0) only low resolution (10-100) and low transmission are seen. With the polarity reversed (a(x) > 0) much higher resolution (> or = 1000) and transmission are possible. Increasing the magnitude of the added hexapole field decreases the limiting resolution at m/z 609. Removing the added octopole field increases the limiting resolution. In some cases structure is formed on the peaks. For a given scan line slope, U/V(rf), the resolution decreases as the amplitude of the added hexapole field increases. These results are consistent with changes to the stability diagrams, calculated here. With a(x) > 0, adding a hexapole field causes the x stability boundary to move outward with all rod sets. With a(x) < 0, the boundaries become diffuse and the tip of the stability diagram becomes rounded, limiting the resolution to ca. 10-100. Where comparisons are possible, experiments show the rod sets with added hexapole fields have transmission 10-300 times less than a conventional quadrupole. Thus these quadrupoles are less useful for mass analysis than conventional quadrupoles. However, it is surprising, given the highly distorted fields, that some of the quadrupoles give resolution of 1000 or more.

Mass Analysis with Islands of Stability with Linear Quadrupoles Incorporating Higher Order Multipole Fields

Mass analysis with islands of stability has been investigated with three linear quadrupole mass filters: two with 4% added hexapole fields constructed with equal diameter (quadrupole 4A) and unequal diameter (quadrupole 4B) rods, and a conventional round-rod quadrupole that has apparently been slightly damaged. Islands are formed by applying auxiliary quadrupole excitation. With the Mathieu parameter, a < 0, mass analysis with both quadrupoles with hexapole fields operated normally, i.e., without islands, gives only low resolution. A factor of 10 or more increase in resolution is possible with the use of stability islands. With a > 0, when quadrupole 4A is operated normally, peak shapes similar to that of a conventional quadrupole can be obtained at resolutions higher than 850. At lower resolutions, peaks are split. When quadrupole 4B is operated without islands, resolution up to 2000 is possible, but there are low mass tails and structure is formed on the peaks. With mass analysis with an island of stability, both quadrupoles 4A and 4B show peaks free of structure and without tails. Ion transmission is also improved with some operating conditions. With the conventional round-rod quadrupole, mass analysis with islands of stability increases the limiting resolution from 2500 to 4360. At a resolution of 2500, the transmission is increased by about two orders of magnitude. These results show that the use of islands of stability improves mass analysis with quadrupoles with distorted fields, and may, in the future, allow use of quadrupoles constructed with at least some lower mechanical tolerances.