A Marriage Made in MS - ACS Publications - American Chemical Society

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Report

A Marriage Made in MS

T

he time-of-flight (TOF) mass spectrometer has become a widely used tool for mass analysis of large biomolecules (1). It can rapidly analyze an entire mass distribution simultaneously, which is particularly important when the ionization source has a low duty cycle. The instrument's greatest advantage, however, is that it is a relatively simple mass analyzer, with no moving parts, scanning electricfields,or slits. It is inexpensive, easy to build and maintain, and can analyze large ions with reasonable resolution and mass accuracy. The quadrupole ion trap is a powerful tool for mass analysis and storage of ions over a wide mass range with excellent detection limits. It has been used with many ionization sources, including electron impact, chemical ionization, photoionization, and matrix-assisted laser desorption/ionization (MALDI) (2), which

M a r k G. Qian David M. Lubman The University of Michigan 234 A

A hybrid instrument that combines TOF with the ion trap yields excellent sensitivity for small samples can create ions directly inside the trap. In addition, techniques that require the introduction of ions from an external source, such as atmospheric pressure sampling glow discharges and electrospray (3), have been interfaced to ion traps. The trap also has been used to store and analyze high-mass ions and recently has been used to achieve extraordinarily high resolution (4). However, the key feature of the ion trap is its ability to obtain high sensitivity through ion storage and integration of the signal over an extended period. One instrumental configuration that

Analytical Chemistry, Vol. 67, No. 7, April 1, 1995

could allow exploitation of the advantages of TOF and the ion trap is a combination quadrupole ion trap (IT)/reflectron (re) TOF mass spectrometer (5). Ions stored in the trap could be ejected using the pulsed dc mode into the reTOF half of the instrument for mass analysis. This configuration results in a hybrid nonscanning mass spectrometer that combines the selective storage and MS/MS capabilities of the ion trap with the speed, resolution, and high mass capabilities of the reflectron TOF spectrometer. The capabilities for long-term storage and ejection of unwanted background can provide excellent sensitivity for small samples. In addition, the storage capabilities of the trap provide a convenient way to interface a low-intensity continuous ion source such as electrospray to a reTOF spectrometer. This configuration allows detection of electrospray ionization (ESI)- produced ions over a wide mass range in a nonscanning mass spectrometer so that rapid analysis of chromatographic eluents can be achieved. The trap also can operate at elevated pressures to cool MALDI-pro0003-2700/95/0367-234A/$09.00/0 © 1995 American Chemical Society

duced ions and provide for enhanced resolution and extensive fragmentation; this allows structural analysis of MALDIactivated ions by long-term storage in the trap. In addition, there is the possibility of developing a compact, simple version of this instrument that may find use in biomedical studies and the biotechnology industry. In this Report, we will describe the instrument built in our laboratory and demonstrate its usefulness in a variety of applications. Background

TOF mass spectrometers can be used with ion reflectors to obtain excellent resolution for biomolecules in the mass range < 3000 u (6) ; they can be complemented with jet cooling (7, 8), post-source pulse focusing (9), and delayed extraction methods (10), which have been used to obtain resolution in excess of several thousand using various ionization sources. TOF instruments do have a major limitation in that they cannot do selective ion storage before mass analysis. Ion storage is important for trace analysis via ion integration using trapping methods (3,11,12). In addition, the ability to selectively store target ions in ion traps and eject unwanted background ions is important for improving the spectrum's S/N and ultimately as a means of selecting ions for structural analysis by MS/MS. Although there have been several attempts either to store or to selectively eject ions in TOF instruments by using dc fields, success has been limited. A number of TOF configurations have been developed, however, to obtain structural information, including a tandem TOF instrument to achieve MS/MS (13). An alternative instrument for achieving both ion storage and tandem MS is the ion trap mass spectrometer. In the ion trap mass spectrometer, se-

lective ejection and storage of target ions can be achieved by applying various auxiliary rffieldsto the end caps of the trap in a technique called axial modulation. This has led to the unique ability to perform multiple stages of tandem MS in combination with collision or photodissociation fragmentation techniques inside the trap (14). This is unlike tandem MS using a triple quadrupole, where multiple MS stages are required, with transmission limitations due to scanning the multiple quads in tandem. Although tandem MS can be done with FT-ion cyclotron resonance instruments, the trap can operate at the high

The reflectron compensates for the difference in the TOF of ions with different energies to focus them at the detector pressure (10~3 torr) critical for effective collision-induced dissociation (CID) and for interfacing to GC and LC. The quadrupole ion trap does have several disadvantages. Although it can store high-mass ions, when it is used in the mass-selective instability mode (used for ejecting ions out of the trap as a function of mass) it may be difficult to scan the rf voltage at a sufficiently large value to scan out high-mass ions. A number of other scan modes, such as axial modulation (11), can be used to scan high-mass ions out of the trap. The drawback of using this method, however, is that some of the ac-

curacy of the mass calibration is lost. In addition, very high resolution (> 100,000) has been achieved in the trap by scanning the rf voltage very slowly (4). However, the rate at which the mass range is scanned to achieve this high resolution is impractical for many applications, and the resolution and mass accuracy will be affected by the density of ions in the trap and the effects of space charge as the density increases. Although various scan methods have been developed to correct for some of the drawbacks of scanning quadrupole ion traps, an alternative detection method is to use pulsed dc ejection to move the ions from the trap to the detector. The dc pulse is applied to the exit end cap, destabilizing the trap and ejecting the trap contents for analysis. Pulsed dc ejection has been used for more than 20 years to interface ion traps to quadrupole and sector mass spectrometers (15-17). Some of these studies have focused on the fundamental aspects of dc ejection whereas others have used TOF analysis to study the energy distribution and mean kinetic energy of the ion cloud in the trap. More recent hybrid instruments have ion traps interfaced to other ion traps and hybrid BEQ-type devices (17). Instrumentation

Figure 1 is a schematic of an ESI source interfaced to an IT/reTOF instrument (5). Here the ion trap has replaced the acceleration region of a standard Mamyrin reflectron source (R. M. Jordan Co.). The key to the operation of the IT/reTOF spectrometer is that ions stored in the trap are not scanned out sequentially, as in the usual ITMS configuration. Instead, they are pulsed out of the trap and into the reTOF instrument for analysis. The ions are ejected by placing a dc pulse on an end cap. Following ejection,

Analytical Chemistry, Vol. 67, No. 7, April 1, 1995 235 A

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