Trapping of intact, singly-charged, bovine serum albumin ions injected from the atmosphere with a 10-cm diameter, frequency-adjusted linear quadrupole ion trap

Abstract

High-resolution real-time particle mass measurements have not been achievable because the enormous amount of kinetic energy imparted to the particles upon expansion into vacuum competes with and overwhelms the forces applied to the charged particles within the mass spectrometer. It is possible to reduce the kinetic energy of a collimated particulate ion beam through collisions with a buffer gas while radially constraining their motion using a quadrupole guide or trap over a limited mass range. Controlling the pressure drop of the final expansion into a quadrupole trap permits a much broader mass range at the cost of sacrificing collimation. To achieve high-resolution mass analysis of massive particulate ions, an efficient trap with a large tolerance for radial divergence of the injected ions was developed that permits trapping a large range of ions for on-demand injection into an awaiting mass analyzer. The design specifications required that frequency of the trapping potential be adjustable to cover a large mass range and the trap radius be increased to increase the tolerance to divergent ion injection. The large-radius linear quadrupole ion trap was demonstrated by trapping singly-charged bovine serum albumin ions for on-demand injection into a mass analyzer. Additionally, this work demonstrates the ability to measure an electrophoretic mobility cross section (or ion mobility) of singly-charged intact proteins in the low-pressure regime. This work represents a large step toward the goal of high-resolution analysis of intact proteins, RNA, DNA, and viruses. (J Am Soc Mass Spectrom 2008, 19, 1942-1947) © 2008 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry E ver since the mass spectrometer was introduced, scientists have looked for ways to increase the mass range. The greatest advances came with the introduction of electrospray ionization (ESI) [1] and matrix-assisted laser desorption ionization (MALDI) [2]. These techniques permitted the production of massive ions in vacuum for subsequent mass analysis. Yet even with the advance of large ion production into the megadalton (MDa) range, the working range of mass spectrometers remained in the tens of kilodalton (kDa) range. The working range defines the range of mass-tocharge ratios where the spectrometer yields the most useful information; in other words, the range with the highest resolution, sensitivity, and mass accuracy.The working range limitation results from the kinetic energy imparted to the large ions during expansion into vacuum. Liu et al. [3] measured the centerline velocity of particles exiting their aerodynamic lens system through a 2-Torr expansion into vacuum. By extrapolating their results to zero particle diameter (mass), the kinetic energy of the particles traveling along the centerline of the expansion was found to be roughly linear yielding ϳ13.5, 85.3, and 530 eV for 10, 100, and 1000 kDa molecules. This result shows that even a lowpressure expansion can impart enough kinetic energy into large ions to compete ...

Controlling the expansion into vacuum—the enabling technology for trapping atmosphere-sampled particulate ions

“…The illustration of circular concave electrodes to create hyperbolic fields found in Dawson's text inspired our large radius design. The LR-LQIT was first demonstrated in a previous publication using singly-charged bovine serum albumin ions and an aerodynamic lens inlet system [8].…”

“…The second quadrupole configuration used to examine the inlet was a quadrupole with a 5.42 cm radius [8]. This design uses a circularly concave electrode structure to minimize the cross section of the device while maximizing the radius.…”

“…This configuration was used to create a large-radius linear quadrupole ion trap (LR-LQIT). The operation of the LR-LQIT was demonstrated in an earlier publication [8]. In these experiments, the ions were collected, held in a 5-mTorr buffer gas (air), and then ejected ondemand into a conversion dynode/channeltron detector.…”

“…The first two segments involved the proof of principle for trapping massive singly charged ions. From our work with aerodynamic lens inlet systems [8], we knew that the kinetic energy induced through a mere 2-Torr supersonic expansion into a vacuum in the mTorr pressure range or lower was shown to be significant and capable of thwarting trapping of ions in the 100 kDa mass range. Consequently, the pressure of the final supersonic expansion into vacuum should be less than 2 Torr and adjustable if possible.…”

“…For this reason, our group designed a large-radius, frequencyadjusted linear quadrupole ion trap [8] (LR-LQIT) that is capable of accepting dispersive injection of ions. The basic idea was to markedly increase the radius of a linear ion trap to increase the trapping efficiency of ions with relatively large off-axis velocity components.…”