Multiple reflection time-of-flight mass spectrometer

Details Multiple reflection time-of-flight mass spectrometer Multiple reflection time-of-flight mass spectrometer

1999-03-30

2002-10-22

Anderson, Bruce (Department: 2881)

Radiant energy

Ionic separation or analysis

Methods

C250S287000

Reexamination Certificate

active

06469295

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to the mass spectroscopic analysis of chemical samples and more particularly to time-of-flight mass spectrometry. A means and method are described for the analysis of ionized species in a spectrometer comprising two or more reflecting devices such that ions can be reflected back and forth a plurality of times therebetween.
BACKGROUND OF THE PRESENT INVENTION
The present invention relates in general to ion beam handling in mass spectrometers and more particularly to a means of focusing ions in time-of-flight mass spectrometers (TOFMS). The apparatus and method of mass analysis described herein is an enhancement of the techniques that are referred to in the literature relating to mass spectrometry.
The analysis of ions by mass spectrometers is important, as mass spectrometers are instruments that are used to determine the chemical structures of molecules. In these instruments, molecules become positively or negatively charged in an ionization source and the masses of the resultant ions are determined in vacuum by a mass analyzer that measures their mass/charge (m/q) ratio. Mass analyzers come in a variety of types, including magnetic field (B), combined (double-focusing) electrical (E) and magnetic field (B), quadrupole (Q), ion cyclotron resonance (ICR), quadrupole ion storage trap, and time-of-flight (TOF) mass analyzers, which are of particular importance with respect to the invention disclosed herein. Each mass spectrometric method has a unique set of attributes. Thus, TOFMS is one mass spectrometric method that arose out of the evolution of the larger field of mass spectrometry.
The analysis of ions by TOFMS is, as the name suggests, based on the measurement of the flight times of ions from an initial position to a final position. Ions which have the same initial kinetic energy but different masses will separate when allowed to drift through a field free region.
Ions are conventionally extracted from an ion source in small packets. The ions acquire different velocities according to the mass-to-charge ratio of the ions. Lighter ions will arrive at a detector prior to high mass ions. Determining the time-of-flight of the ions across a propagation path permits the determination of the masses of different ions. The propagation path may be circular or helical, as in cyclotron resonance spectrometry, but typically linear propagation paths are used for TOFMS applications.
TOFMS is used to form a mass spectrum for ions contained in a sample of interest. Conventionally, the sample is divided into packets of ions that are launched along the propagation path using a pulse-and-wait approach. In releasing packets, one concern is that the lighter and faster ions of a trailing packet will pass the heavier and slower ions of a preceding packet. Using the traditional pulse-and-wait approach, the release of an ion packet as timed to ensure that the ions of a preceding packet reach the detector before any overlap can occur. Thus, the periods between packets is relatively long. If ions are being generated continuously, only a small percentage of the ions undergo detection. A significant amount of sample material is thereby wasted. The loss in efficiency and sensitivity can be reduced by storing ions that are generated between the launching of individual packets, but the storage approach carries some disadvantages.
Resolution is an important consideration in the design and operation of a mass spectrometer for ion analysis. The traditional pulse-and-wait approach in releasing packets of ions enables resolution of ions of different masses by separating the ions into discernible groups. However, other factors are also involved in determining the resolution of a mass spectrometry system. “Space resolution” is the ability of the system to resolve ions of different masses despite an initial spatial position distribution within an ion source from which the packets are extracted. Differences in starting position will affect the time required for traversing a propagation path. “Energy resolution” is the ability of the system to resolve ions of different mass despite an initial velocity distribution. Different starting velocities will affect the time required for traversing the propagation path.
In addition, two or more mass analyzers may be combined in a single instrument to form a tandem mass spectrometer (MS/MS, MS/MS/MS, etc.). The most common MS/MS instruments are four sector instruments (EBEB or BEEB), triple quadrupoles (QQQ), and hybrid instruments (EBQQ or BEQQ). The mass/charge ratio measured for a molecular ion is used to determine the molecular weight of a compound. In addition, molecular ions may dissociate at specific chemical bonds to form fragment ions. Mass/charge ratios of these fragment ions are used to elucidate the chemical structure of the molecule. Tandem mass spectrometers have a particular advantage for structural analysis in that the first mass analyzer (MS
1
) can be used to measure and select molecular ion from a mixture of molecules, while the second mass analyzer (MS
2
) can be used to record the structural fragments. In tandem instruments, a means is provided to induce fragmentation in the region between the two mass analyzers. The most common method employs a collision chamber filled with an inert gas, and is known as collision induced dissociation CID. Such collisions can be carried out at high (5-10 keV) or low (10-100 eV) kinetic energies, or may involve specific chemical (ion-molecule) reactions. Fragmentation may also be induced using laser beams (photodissociation), electron beams (electron induced dissociation), or through collisions with surfaces (surface induced dissociation). It is possible to perform such an analysis using a variety of types of mass analyzers including TOF mass analysis.
In a TOFMS instrument, molecular and fragment ions formed in the source are accelerated to a kinetic energy:
qV
=½
mv
2
  (1)
where q is the elemental charge, V is the potential across the source/accelerating region, m is the ion mass, and v is the ion velocity. These ions pass through a field-free drift region of length L with velocities given by equation 1. The time required for a particular ion to traverse the drift region is directly proportional to the square root of the mass/charge ratio:
t=L
(
m/
2
qV
)
½
  (2)
Conversely, the mass/charge ratios of ions can be determined from their flight times according to the equation:
m/e=&agr;t
2
+&bgr;  (3)
where &agr; and &bgr; are constants which can be determined experimentally from the flight times of two or more ions of known mass/charge ratios.
Generally, TOF mass spectrometers have limited mass resolution. This arises because there may be uncertainties in the time that the ions were formed (time distribution), in their location in the accelerating field at the time they were formed (spatial distribution), and in their initial kinetic energy distributions prior to acceleration (energy distribution).
The first commercially successful TOFMS was based on an instrument described by Wiley and McLaren in 1955 (Wiley, W. C.; McLaren, I. H.,
Rev. Sci. Instrumen.
26 1150 (1955)). That instrument utilized electron impact (EI) ionization (which is limited to volatile samples) and a method for spatial and energy focusing known as time-lag focusing. The simplest form of the Wiley-McLaren instrument is depicted in FIG.
1
. The instrument consists, in part, of an ion accelerator, a detector, and a field free drift region between the accelerator and the detector. At the beginning of an analysis, ions are located in the accelerator near plane P
1
—the “object plane”. The ions initially have near thermal kinetic energies. To begin the analysis, an electrical potential is applied to the accelerator. The electric field in the accelerator accelerates ions toward a detector which resides at plane P
2
—the “image plane”. For the purpose of the present discussion, it is assumed that ions are accelerated in

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