Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
2000-06-15
2003-05-27
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S281000, C250S282000, C250S3960ML, C250S286000, C250S288000
Reexamination Certificate
active
06570152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to time-of-flight mass spectrometers that incorporate an ion mirror or reflector to increase the effective length of the drift region focusing. More particularly it relates to such spectrometers having more than one ion reflector and in which the number of reflections undergone by the ion packets can be varied to adjust the resolution of the spectrometer.
2. Discussion of the Prior Art
In a time-of-flight-mass spectrometer, mass-to-charge ratio of an ion is determined by accelerating it through to a given energy by means of an electrostatic field and measuring its subsequent flight time through a field-free drift region. The mass resolution of such a spectrometer is obviously dependent on the length of the drift region, because increasing its length will increase the separation in time between ions of adjacent mass-to-charge ratios. However, drift region length is in practice limited by the physical size of the spectrometer and certain observations, discussed below, limit the maximum resolution obtainable, irrespective of the drift region length. The two most important aberrations arise from:
1) variations in the position in the accelerating field at which the ion is generated; and
2) variations in the velocity imparted to the ion during its creation.
The first of these aberrations may be at least in part corrected by the space focusing technique, first taught by Wiley and McLaren in Rev. Sci. Instrum. 1955 vol 26 (12) pp 1150-1157, and in many subsequent papers and patents. The second aberration requires to be corrected by velocity focusing. The technique known as delayed extraction, also first suggested by Wiley and McLaren (ibid.) is a commonly used method of providing some degree of velocity focusing.
Another way of reducing the effect of different initial ion velocities is to provide an ion reflector at the end of the drift region to reflect the ions back towards the source through the drift region to a detector located close to the source. (See, for example, Mamyrin, Karatev et al, Sov. Phys. JETP, 1973, vol 37 (1) pp 45-48). Ions that leave the source region with a high velocity in the direction of the drift region will penetrate further into the ion reflector before being turned around than will ions of a lower initial velocity.
Consequently, ions with high initial velocity will travel a greater distance between the source and the detector than will ions of lower initial velocity. It is therefore possible to arrange the instrumental parameters so that the “initially fast” and the “initially slow” ions arrive at the detector at the same time.
Another advantage that results from the use of a reflecting analyser is that the distance travelled by the ions in the drift region is approximately double that it would be in a linear analyser of the same physical size, which results in improved resolution.
By the provision of multiple reflectors, it is possible to reduce the physical size of a reflection time-of-flight mass spectrometer still further merely by reflecting the ion packets backwards and forwards along a short drift region. However, each reflection results in a transmission loss (typically between 10% and 50%), and mass peaks tend to be broadened (and therefore reduced in intensity) as the path length is increased.
Several different versions of prior multiple-reflection time-of-flight mass spectrometers are known. That described by Chen and Su in Hezi Kexue (Nucl. Sci. J) 1991, vol 28 (3) pp 183-189 is a spectrometer having two parallel ion mirrors which reflect the ion packets a fixed number of times before they pass beyond the edge of one mirror to be received by an ion detector.
The spectrometers described in DE4418489, and U.S. Pat. No. 5,880,466 are essentially ion traps in which a packet of ions is repeatedly reflected between two parallel ion mirrors and does not enter a conventional ion detector. Instead, the oscillating ion packet is caused to induce a signal in sensing electrodes, which signal can be measured and processed by suitable electronic data processing equipment. GB 2080021 discloses in its FIG. 6 embodiment a multiple reflection time-of-flight mass spectrometer comprising an ion mirror that can be electronically tilted to reflect the incoming ion packets at different angles. At one such angle, the reflected packets pass into an ion detector, thereby enabling a moderate resolution spectrum to be recorded. At another such angle the reflected packets are directed to a second ion mirror and then to another ion detector, thereby providing increased path length and enabling a higher resolution spectrum to be recorded at lower sensitivity.
Multiple-reflection time-of-flight mass spectrometers incorporating switchable ion mirrors are taught by Wollnik and Przewloka in Int. J. Mass Spectrorn. and Ion Proc. 1990 vol 96 pp 267-274. The potentials applied to these mirrors can be switched off so that instead of being reflected, ion packets merely pass through the mirror undeflected, typically to an ion detector. Various configurations may be used in conjunction with suitable electronic timing and control circuiting to provide spectrometers with different path lengths.
Soviet Inventors Certificate SU 1725289 teaches a multiple-reflection spectrometer in which the ion source and the detector can be physically moved along an axis midway between the two reflectors. The distance between the source and detector controls the number of reflections of the ion packets and hence the resolution of the spectrometer. Piyadasa, Hakansson et. al. in Rapid Comm. in Mass Spectrom, 1999 vol 13 pp 620-624 describe a multiple reflection time-of-flight mass spectrometer in which two parallel ion mirrors are used to trap the ions, in a manner similar to that taught in U.S. Pat. No. 5,880,466. However, Piyadasa detects the ions by switching off one of the mirrors after a predetermined time to allow the ion packets to pass through the mirror and impact on a conventional ion detector. In this device, the number of ion reflections may be varied by adjustment of the time interval between the generation of an ion packet and the moment the ion mirror is switched off.
Hohl, Wurz, Scherer et.al. in Int. J. Mass Spectrom. 1999 vol 188 pp189-197 describe an instrument comprising two ion mirrors, the second of which is disposed between the ion source and the detector. In the “triple reflection” mode, ion packets pass from the source to the first mirror where they are reflected towards the second mirror. The second mirror returns them to the first mirror, which in turn reflects them to the ion detector. An electrostatic lens is located at the entrance of the first mirror. When a relatively low potential is applied to this lens, the spectrometer operates in a single-reflection mode wherein ion packets entering the first ion mirror are reflected directly into the ion detector. A higher potential applied to the lens results in the reflected ion packets being reflected by the second mirror so that the spectrometer operates in the triple reflection mode. These prior switchable mode spectrometers all require additional components such as lenses, power supplies and/or ion detectors.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a switchable mode multi-reflection time-of-flight mass spectrometer which is capable of at least first order velocity focusing in both single and multi-reflection modes and which requires fewer additional components than prior types. It is another object of the present invention to provide a time-of-flight mass analyzer having variable resolution into which ions are orthogonally injected. It is a further objective to provide a reflecting time-of-flight mass analyzer having orthogonal ion injection in which the resolution may be selected by varying the number of reflections undergone by the ions. Another objective is to provide such a time-of-flight mass analyzer in which the number of reflections can be changed more easily than is possible with prior multiple-reflection s
Diederiks & Whitelaw PLC
Lee John R.
Micromass Limited
Vanore David A.
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