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Performance Evaluation of a Hybrid Linear Ion Trap/Orbitrap Mass Spectrometer

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Performance Evaluation of a Hybrid Linear Ion Trap/Orbitrap Mass Spectrometer Anal. Chem. 2006, 78, 2113-2120 Accelerated Articles Performance Evaluation of a Hybrid Linear Ion Trap/Orbitrap Mass Spectrometer Alexander Makarov,* Eduard Denisov, Alexander Kholomeev, Wilko Balschun, Oliver Lange, Kerstin Strupat, and Stevan Horning Thermo Electron (Bremen) GmbH, Hanna-Kunath-Strasse 11, Bremen 28199 Germany Design and performance of a novel hybrid mass spec- A typical example of such a mass spectrometer is the quad- trometer is described. It couples a linear ion trap mass rupole/time-of-flight (TOF) hybrid,1,2 wherein precursor ions are spectrometer to an orbitrap mass analyzer via an rf-only selected by a quadrupole mass filter and accurate mass determi- trapping quadrupole with a curved axis. The latter injects nation (including analysis of fragment ions) is carried out in an pulsed ion beams into a rapidly changing electric field in orthogonal-acceleration TOF. Though very successful for a range the orbitrap wherein they are trapped at high kinetic of applications, such hybrids nevertheless suffer from low ion energies around an inner electrode. Image current detec- transmission (resulting in poor MS/MS sensitivity and detection tion is subsequently performed after a stable electrostatic limits) and a limited intensity range over which accurate mass field is achieved. Fourier transformation of the acquired data can be acquired. transient allows wide mass range detection with high These shortcomings were overcome in a next-generation resolving power, mass accuracy, and dynamic range. The hybrid instrument, a linear ion trap/Fourier transform ion cyclo- 3 entire instrument operates in LC/MS mode (1 spectrum/ tron resonance (FTICR) mass spectrometer. Like earlier FTICR 4,5 s) with nominal mass resolving power of 60 000 and uses hybrids, this mass spectrometer accumulates ions externally to automatic gain control to provide high-accuracy mass a superconducting magnet, but in addition, it combines high n measurements, within 2 ppm using internal standards trapping capacity, MS capabilities, and automatic gain control 6 and within 5 ppm with external calibration. The maximum (AGC) of linear ion trap mass spectrometers with the unsurpassed resolving power exceeds 100 000 (fwhm). Rapid, auto- mass accuracy, dynamic range, and resolving power of FTICR mated data-dependent capabilities enable real-time ac- mass spectrometers. The unprecedented specificity and quality of data from this combination necessitates the added complexity quisition of up to three high-mass accuracy MS/MS of the superconducting magnet. This inspired the quest for a mag- spectra per second. netic field-free analyzer of comparable performance, which would be more compatible with the capacities of a typical laboratory. (1) Chernushevich, I. V.; Loboda, A. V.; Thomson, B. A. J. Mass Spectrom. Increasing speed of chromatographic separation and complex- 2001, 36, 849-865. ity of analyzed mixtures provides a continuous impetus for the (2) Morris, H. R.; Paxton, T.; Dell, A.; Langhorne, J.; Berg, M.; Bordoli, R. S.; Rapid Commun. Mass Spectrom. 10 - development of faster and simultaneously more intelligent and Hoyes, J.; Bateman, R. H. 1996, , 889 896. robust mass spectrometric detectors. Reliable separation and (3) Syka, J. E. P.; Marto, J. A.; Bai, D. L.; Horning, S.; Senko, M. W.; Schwartz, reliable identification of complex mixtures with multiple coeluting J. C.; Ueberheide, B.; Garcia, B.; Busby, S.; Muratore, T.; Shabanowitz, J.; Hunt, D. F. J. Proteome Res. 2004, 3, 621-626. compounds necessarily require higher resolving power, while (4) Senko, M. W.; Hendrickson, C. L.; Emmett, M. R.; Shi, S. D.-H.; Marshall, structural analysis by MS/MS and accurate mass determination A. G. J. Am. Soc. Mass Spectrom. 1997, 8, 970-976. become complementary confirmation tools. (5) Belov, M. E.; Nikolaev, E. N.; Anderson, G. A.; Udseth, H. R.; Conrads, T. P.; Veenstra, T. D.; Masselon, C. D.; Gorshkov, M. V.; Smith, R. D. Anal. Chem. 2001, 73, 253-261. * Corresponding author. Tel: +49(0)421-5493-410. Fax: +49(0)421-5493-426. (6) Schwartz, J. C.; Senko, M. W.; Syka, J. E. P. J. Am. Soc. Mass Spectrom. E-mail: [email protected]. 2002, 13, 659-669. 10.1021/ac0518811 CCC: $33.50 © 2006 American Chemical Society Analytical Chemistry, Vol. 78, No. 7, April 1, 2006 2113 Published on Web 02/18/2006 From its first appearance as a proof-of-principle device,7 the ejected to a pair of secondary electron multipliers.6 An important orbitrap mass analyzer was considered as a potential alternative feature of the instrument is the procedure of AGC,12 wherein a for FTICR because of its high resolution and good mass accuracy. short prescan is used to determine the ion current within the mass Using the principle of orbital trapping in electrostatic fields,8 the range of interest, hence enabling storage of a defined number of orbitrap consists of an inner and an outer electrode, which are ions (ªAGC target valueº) in the subsequent analytical scan. As shaped to create a quadro-logarithmic electrostatic potential. Ions in the LTQ FT mass spectrometer, all modes of LTQ operation rotate about the inner electrode and oscillate harmonically along remain available, but they are complemented by the ability to its axis (the z-direction) with a frequency characteristic of their analyze ions in an additional ªmass detectorº (FTICR or, as in m/z values. An image current transient of these oscillations is this work, the orbitrap). converted to a frequency spectrum using a Fourier transform In the LTQ orbitrap, a transfer octapole (300 mm long, 400 V similar to the approach used in FTICR. For a given spread of p-p rf, 5.7 mm inscribed diameter) delivers ions into a curved initial parameters of ions, the orbitrap provides the best perfor- rf-only quadrupole whose central axis follows a C-shaped arc mance within the smallest dimensions, relative to other types of (hence the name C-trap). The C-trap uses rods with hyperbolic electrostatic traps,8 thus making it the most suitable candidate surfaces and is enclosed by two flat lenses with apertures for ion for image current detection and Fourier transform mass spec- transport through them. The plate between the octapole and C-trap trometry. forms the gate electrode and the other plate forms the trap Initially, high resolving power, internal mass accuracy, and electrode. The enclosed volume is filled with nitrogen bath gas high space charge capacity were demonstrated with the orbitrap at ∼1 mTorr via an open-split interface connected to the nitrogen mass analyzer using pulsed ion sources. Because of high ion line of electrospray ion source. Nitrogen has been chosen as a velocities and the absence of any collisional cooling inside the bath gas in favor of helium due to better collisional damping and orbitrap, injection must occur in a short burst, on a time scale lower gas carryover toward the orbitrap. The C-trap assembly is well below 1 ms (actually, only a few microseconds if coherence pumped by the split-flow turbopump of the LTQ mass spectrom- of ion packets is to be provided by the injection, thereby eter, as indicated in Figure 1. eliminating the need for additional excitation9). For continuous Ions start from the back section of the linear trap held at a dc ion sources, this is possible only when additional ion storage is offset of 6-10 V (here and below voltages are given for positive introduced to store ions and then inject them rapidly into the ions). After acceleration into the transfer octapole by a 2-10 V orbitrap. Such implementation for injection of electrosprayed ions potential difference, ions are transferred into the C-trap held at a into the orbitrap was initially based on axial ejection of ions from dc offset of 0 V with an applied rf voltage of 500-2000 V p-p and a linear rf-only quadrupole.9-10 As the space charge capacity of are reflected by the trap electrode (12-15 V). The gate electrode such a trap is compromised by the need to extract ions within is always kept at a positive offset of 3-6 V. On entry into the hundreds of nanoseconds, use of large ion numbers resulted in C-trap, ions lose energy in collisions with nitrogen bath gas, these great variation of ion kinetic energies, together with angular and collisions being mild enough to avoid any fragmentation. Due to spatial spreads that limited mass range, transmission, dynamic the relatively low gas pressure and short length of the C-trap, ions range, and mass accuracy over a wide mass range. need more than one pass through the entire system to be trapped. This work involves a new approach to ion storage that is based Nevertheless, ions finally come to rest at the place where gas on orthogonal, rather than axial, ion ejection from an rf-only collisions occur with the lowest dc offset along their path, i.e., in quadrupole.11 This implementation alleviates the above problems the C-trap. and paves the way for a hybrid mass spectrometer combining the As the result of collisional cooling, the ions form a thin, long tandem mass spectrometry capability of the linear ion trap mass thread along the curved axis of the C-trap. This thread is spectrometer with the high resolution and mass accuracy capabil- compressed axially by applying 200 V to both the gate and the ity of the orbitrap. This combination allows high-quality accurate trap electrodes. After that, the rf voltage on the quadrupolar mass MSn spectra to be acquired using brief ion accumulation electrodes of the C-trap is rapidly ramped down (over 100-200 periods and relatively small ion populations. Here we characterize ns) and dc pulses are applied to the electrodes as follows: 1200 the analytical parameters and features of this hybrid instrument.
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