A digital ion trap mass spectrometer coupled with atmospheric pressure ion sources

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 ...

Section: Design Of a Linear Quadrupole Trap For Particulate Ionsmentioning

confidence: 99%

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

Koizumi

Whitten

Reilly

2008

“…If sharp sides can be produced on both the low and the high frequency sides, then high mass resolution in ion isolation will be possible, as shown for a 3-D trap by Ding et al [26]. In that work, positive or negative multipoles could be added to the field of a 3-D trap by changing the voltage on a field adjusting electrode.…”

Section: Bistable Behaviormentioning

confidence: 99%

Ion excitation in a linear quadrupole ion trap with an added octopole field

Michaud

Frank

Ding

et al. 2005

Modeling of ion motion and experimental investigations of ion excitation in a linear quadrupole trap with a 4% added octopole field are described. The results are compared with those obtained with a conventional round rod set. Motion in the effective potential of the rod set can explain many of the observed phenomena. The frequencies of ion oscillation in the x and y directions shift with amplitude in opposite directions as the amplitudes of oscillation increase. Excitation profiles for ion fragmentation become asymmetric and in some cases show bistable behavior where the amplitude of oscillation suddenly jumps between high and low values with very small changes in excitation frequency. Experiments show these effects. Ions are injected into a linear trap, stored, isolated, excited for MS/MS, and then mass analyzed in a time-of-flight mass analyzer. Frequency shifts between the x and y motions are observed, and in some cases asymmetric excitation profiles and bistable behavior are observed. Higher MS/MS efficiencies are expected when an octopole field is added. MS/MS efficiencies (N 2 collision gas) have been measured for a conventional quadrupole rod set and a linear ion trap with a 4% added octopole field. Efficiencies are chemical compound dependent, but when an octopole field is added, efficiencies can be substantially higher than with a conventional rod set, particularly at pressures of 1.4 ϫ 10 Ϫ4 torr or less. . The most widely discussed distortion is the "stretched" ion trap [2], which has the end cap electrodes moved out so that the distance to the end cap, z 0 , is increased over that of an ideal field, z 0 ϭ r 0 ⁄ ͙ 2, where r 0 is the distance from the center to the ring electrode. It has been argued that the addition of higher order multipole fields of the correct sign to 3-D traps improves MS/MS efficiency [1c, 1f, 2], and allows faster ejection at the stability boundary [2,3], to give higher scan speeds and improved mass resolution.There is increasing interest in using linear quadrupoles as ion traps, both as stand alone mass analyzers with radial [4] or axial [5] ejection, or in combination with other mass analyzers (for a recent review see [6]).There is also interest in trapping and exciting ions for MS/MS at the relatively low pressures typical for operation of the last mass analyzing quadrupole in triple quadrupole systems, ca. 3 ϫ 10 Ϫ5 torr [7]. Addition of higher multipoles to a linear ion trap might be expected to provide benefits similar to those seen with 3-D traps. Douglas and coworkers [8] have shown that an octopole field can be added to a linear quadrupole by using rod sets with rods equally spaced from the central axis but with one pair of rods different in diameter than the other pair, as shown in Figure 1. The electric potential within this rod set is given to a good approximation bywhere x is the distance from the center towards a smaller rod, y is the distance from the center towards a larger rod, r 0 is the distance from the center to any rod, and U and V rf are the amplitudes...

“…The first report of rf frequency scanning demonstrated advantages for analysis of high mass ions [9], and subsequent reports demonstrated frequency scanning in a quadrupole mass filter with ions ranging in size from mass 176 Da [10] to microparticles [11]. Frequency scanning has also been reported in specially designed digital ion traps, which do not have rf amplitude scan capabilities but which have demonstrated exceptionally fast scan rates while maintaining reasonable resolution (unit resolution for a 100 Hz scan rate, peak widths FWHM~2.0 Th for a 1000 Hz scan rate) [12][13][14]. These scan methods simplify the electronics, but there remain problems of nonlinearity in mass calibration (save for the linear calibration described in ref.…”

Section: Introductionmentioning

confidence: 99%

“…These scan methods simplify the electronics, but there remain problems of nonlinearity in mass calibration (save for the linear calibration described in ref. [14]), unexpected peaks, and poor mass resolution [15]. As a result, traditional rf amplitude sweeps have remained the favored method of ion trap operation.…”

Section: Introductionmentioning

confidence: 99%

Experimental Characterization of Secular Frequency Scanning in Ion Trap Mass Spectrometers

Snyder

Pulliam

Wiley

et al. 2016

Abstract. Secular frequency scanning is implemented and characterized using both a benchtop linear ion trap and a miniature rectilinear ion trap mass spectrometer. Separation of tetraalkylammonium ions and those from a mass calibration mixture and from a pesticide mixture is demonstrated with peak widths approaching unit resolution for optimized conditions using the benchtop ion trap. The effects on the spectra of ion trap operating parameters, including waveform amplitude, scan direction, scan rate, and pressure are explored, and peaks at black holes corresponding to nonlinear (higher-order field) resonance points are investigated. Reverse frequency sweeps (increasing mass) on the Mini 12 are shown to result in significantly higher ion ejection efficiency and superior resolution than forward frequency sweeps that decrement mass. This result is accounted for by the asymmetry in ion energy absorption profiles as a function of AC frequency and the shift in ion secular frequency at higher amplitudes in the trap due to higher order fields. We also found that use of higher AC amplitudes in forward frequency sweeps biases ions toward ejection at points of higher order parametric resonance, despite using only dipolar excitation. Higher AC amplitudes also increase peak width and decrease sensitivity in both forward and reverse frequency sweeps. Higher sensitivity and resolution were obtained at higher trap pressures in the secular frequency scan, in contrast to conventional resonance ejection scans, which showed the opposite trend in resolution on the Mini 12. Mass range is shown to be naturally extended in secular frequency scanning when ejecting ions by sweeping the AC waveform through low frequencies, a method which is similar, but arguably superior, to the more usual method of mass range extension using low q resonance ejection.