Time-of-flight mass spectrometer and ion analysis

Details Time-of-flight mass spectrometer and ion analysis Time-of-flight mass spectrometer and ion analysis

1999-08-16

2001-10-09

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Ionic separation or analysis

Ion beam pulsing means with detector synchronizing means

Reexamination Certificate

active

06300626

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates in general to time-of-flight mass spectrometers and in particular to time-of-flight mass spectrometers.
Time-of-flight (“TOF”) analysis has found widespread application because particle velocity, momentum, and mass can be determined from an experiment by constraining the appropriate parameters for the experiment. Time-of-flight mass spectrometers (“TOFMSs”) have the very desirable characteristic of high ion transmission, high repetition rate, good resolution and modest cost, which makes them very attractive as a mass sensitive detector in analytical instrumentation. Such applications were until recently somewhat hampered by the fact that most analytical ion sources produce continuous ion beams. The pulsed operation of a conventional TOFMS causes a rather low duty-cycle and TOFMS could not live up to its promises. For more detailed description of the state of the art of TOFMS, please see “The New Time-Of-Flight Mass Spectrometry,” by Robert J Cotter,
Analytical Chemistry News and Features
, Jul. 1, 1999, pages 445A-451A.
It is desirable for an interface design between a continuous ion source and a TOFMS to overcome two problems. One is bringing the ions with as little spatial and kinetic energy spread as much as possible into the spectrometer for the purpose of achieving high mass resolution. The other is using as many of the ions supplied by the continuous source as possible without compromising on the first requirement so that a high duty-cycle can be achieved. Today, the preferred and highly refined solution to these problems is orthogonal acceleration (“OA”). See “Time-of-Flight Mass Spectrometry,” R. J. Cotter,
ACS Symposium Series
547. By OA, it is meant that the ion beam emanating from the ion source enters the TOF instrument at right angle with respect to the flight axes of the ions in the spectrometer. This geometry allows a low spatial and kinetic energy spread to be achieved. The duty-cycle objective is met by expanding the width of the extraction region so that a larger fraction of the ion beam coming from the source can be sampled. Active ion storage can be achieved by accumulation of ions in an ion guide connecting ion source and extraction region during the time an extracted ion packet disperses in the instrument.
In U.S. Pat. No. 5,396,064, Myerholtz et al. describe a multiplexing procedure using a conventional TOF instrument in which an extraction region involving a pair of grids is pulsed and a cross-correlation is carried out numerically. This scheme, however, is still seriously impaired in practice by the difficulty of implementing a procedure using a pair of grids and parameters allowing for space focusing. A conventional space-focusing type of TOFMS is difficult to operate in a full multiplexing mode over an extended mass range. The pair of grids cannot be pulsed sufficiently rapidly to accomplish this objective because of the time it takes for ions to drift into the region between the grids. Moreover, this drift, of course, is mass dependent. For this reason, space focusing, which requires an extraction region defined by more than one grid, is undesirable.
None of the above-described TOFMS schemes are entirely satisfactory for measuring ions. It is therefore, desirable to provide an improved TOFMS technique where the above-described difficulties are avoided.
SUMMARY OF THE INVENTION
A continuous beam of ions is modulated so that the beam is passed substantially unaltered during on periods or portions thereof but is affected during off periods according to a binary sequence to encode the beam with phase information of the binary sequence. When the beam is passed substantially unaltered, the beam has a substantially constant flux. The ions in the beam reach a detector where the times of arrival of the ions in the modulated beam are detected. The output signal of the detector is demodulated using the phase information to obtain an ion mass spectrum.
In one embodiment, during the off periods, the beam is deflected so that it does not reach the detector or reaches a different area of the detector. This may be accomplished by deflecting the beam during the off periods electrically. Alternatively, the beam can be simply stopped during off periods but let through during the on periods, such as by means of a mechanical chopper. Still other possible modulation techniques may be used, such as a separate particle or photon beam which would deflect or otherwise interrupt the beam during off periods but leave the beam substantially unaltered during on periods.
Since the beam is first encoded according to a binary sequence and the detector output demodulated using phase information from the sequence, and ions from consecutive on periods may overlap, it is possible to achieve a duty cycle close to or equal to 50%. Furthermore, since the beam that is passed during the on periods has substantially constant flux, the demodulation process is simple and can be achieved quickly, unlike conventional modulation and demodulation schemes such as the Fast Fourier Transform.
It is also possible to deflect the beam during off periods towards a detector different from that used for detection during the on periods or to a different active area of the same detector. This can improve the duty-cycle to 100% or close to it.
Preferably, multiple TOFMS may share a common modulator and chamber housing the different ion beams to reduce space and cost.


REFERENCES:
patent: 4707602 (1987-11-01), Knorr
patent: 5396065 (1995-03-01), Myerholtz et al.
“The New Time-of-flight Mass Spectrometry,” R.J. Cotter,Analytical Chemistry News&Features, Jul. 1, 1999, pp. 445 A—461 A.
“Fourier Transform Ion Mobility Spectrometry,” F.J. Knorr et al.,Analytical Chemistry, vol. 57, No. 2, Feb. 1985, pp. 402-406.
“Beam compression and beam multiplexing in a time-of-flight mass spectrometer,” I. Riess,Rev. Sci. Instrum., vol. 58, No. 5, May 1987, pp. 785-787.
“Fourier Transform Time-of-Flight Mass Spectrometry,” F.J. Knorr et al.,Analytical Chemistry, vol. 58, No. 4, Apr. 1986, pp. 691-694.
A miniature time of flight mass spectrometer, C.A. Bailey et al.,Vacuum, vol. 21, No. 10, Jul. 19, 1971, pp. 461-464.
“Hadamard Transform Time-of-Flight Mass Spectrometry,” A. Brock et al.,Analytical Chemistry, vol. 70, No. 18, Sep. 15, 1998, pp. 3735-3741.
“On the origin of spurious peaks in pseudorandom time-of-flight analysis,” P. Zeppenfeld et al.,Rev. Sci. Instrum., vol. 64, No. 6, Jun. 1993, pp. 1520-1523.
“Use of a Correlation Chopper For Time of Flight Neutron Scattering(*), Part I: Theory of the Deconvolution,” J.L. Buevoz et al.,Revue de Physique Appliquée, vol. 12, Apr. 1977, pp. 591-596.
“Use of a Correlation Chopper For Time of Flight Neutron Scattering(*), Part II: Deconvolution in the Experimental Case,” J.L. Buevoz et al.,Revue de Physique Appliquée, vol. 12, Apr. 1977, pp. 597-602.

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