Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
1999-09-22
2002-04-16
Anderson, Bruce (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S282000
Reexamination Certificate
active
06373052
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods and apparatus for correcting errors in the measured mass-to-charge ratios of ions determined by time-of-flight mass spectrometry, especially those due to the dead-time of an ion-counting detector.
In a time-of-flight mass spectrometer, bunches of ions are caused to enter a field-free drift region with essentially the same kinetic energy. In the drift region ions of different mass-to-charge ratios in each bunch travel with different velocities and therefore arrive at an ion detector disposed at the exit of the drift region at different times. Measurement of the ion transit-time therefore determines mass-to-charge ratio of that ion.
Currently, the ion detector most commonly employed in time-of-flight mass spectrometers is a single-ion counting detector which produces an electrical pulse signal in response to an ion impact on its detecting surface. In practice, such a detector may comprise one or more channelplate electron multipliers which produce a bunch of electrons in response to an ion impact. These electrons are collected on one or more collection electrodes which are connected to a charge-sensing discriminator. The discriminator generates an electrical signal in response to the electrons arriving at the collection electrode. The signal produced by the discriminator is used to determine the transit time of the ion which struck the detector, typically by means of a multistop time-to-digital converter which is started as a bunch of ions enters the drift region.
In order to acquire a complete mass spectrum, bunches of ions are repetitively generated from a sample and the transit times (as determined by the time-to-digital converter) of the detected ions are used to produce a histogram of the number of ion arrivals against mass-to-charge ratio. Typically, about 1,000 ion bunches may be analyzed to obtain a complete spectrum during a total time period of a few mS. The chief advantage of this form of time-of-flight mass spectroscopy is therefore that every ion which enters the drift region is in theory detected in contrast with scanning mass analyzers in which only a small proportion of the ions entering the analyzer can be detected in any instant. However, this theoretical advantage is only realised in practice if the ion bunches can be produced very quickly, otherwise a complete mass spectrum may take longer to acquire than it would with a scanning mass analyzer.
Partly for this reason, and also because of the limited mass resolution of prior types of time-of-flight mass spectrometers, time-of-flight mass analyzers have not been extensively used in organic mass spectroscopy until recently, despite their ability to analyze ions of very high mass-to-charge ratios. However, the availability of fast and cheap digital computers which are capable of processing the large quantity of data produced sufficiently quickly, and the development of techniques for improving resolution such as orthogonal acceleration time-of-flight mass spectrometers have resulted in time-of-flight mass spectrometers having become the analyzer of choice for high mass organic molecules.
As explained, in order to obtain maximum advantage from a time-of-flight analyzer in organic applications the ion detector must be capable of very fast operation. Typically, ion arrival times are recorded at 1 nS intervals, but in practice all detectors exhibit a certain dead-time following an ion impact during which the detector cannot respond to another ion impact A typical detector dead-time may be of the order of 5 ns and it is quite likely that during acquisition of a typical spectrum ions will arrive during detector dead-time and will consequently fail to be detected. As discussed below, failure to detect ions has a distorting effect on the resultant mass spectra which can only be avoided by reducing the rate at which ions reach the detector or by applying a dead-time correction. Particularly in the case of organic mass spectrometry, reducing the ion arrival rate is unacceptable for the reasons explained above, so that an effective and practical method of dead-time correction becomes very important.
The correction of dead-time for single-particle detectors or counters was first addressed by Schiff in Phys. Rev, 1936 vol 50 pp 88-96. Schiff assumed that the number of particles arriving at a detector in a given time-interval was governed by Poisson's law and derived analytical expressions which allowed an observed count rate to be corrected once the counter dead-time had been determined. Since then there have been many publications which give expressions for dead-time correction for a variety of different detectors and experimental conditions. Coates (in J. Phys. E, 1968 vol 1 pp 878-, J. Phys. E. vol 5 pp 150- and Rev. Sci. Instrum 1972 vol 43 pp 1855-) and Donohue and Stern in Rev. Sci. Instrum. 1972 vol 43 pp 791-) describe analytical solutions of the problem of reconstructing the time distribution of detectors which have an extendable dead-time (that is, detectors wherein the arrival of a second particle during the dead-time already triggered by the earlier arrival of another particle causes that dead-time to extend).
Esposito et al, in Rev. Sci. Instrum. 1991 vol 62 (11) pp 2822- has extended this analysis to include the situation where more than one particle is expected to be detected in any one measurement cycle and for the more complicated case of detectors with non-extendable dead-times. Coates, in Rev. Sci. Instrum. 1992 vol 63 (3) pp 2084 -, points out that in most practical cases the corrections can be achieved using simpler formulae, and gives as an example a simulated time-of-flight mass spectrum corrected for dead-time distortions using an iterative procedure. Stephan, Zehnpfenning and Benninghoven, in J. Vac. Sci. Technol. A, 1994 vol 12 (2) pp 405- specifically discuss the correction of time-of-flight mass spectra for detector dead-time effects using a procedure similar to those suggested by Esposito et al and Coates et al.
In order to apply these prior methods to data representing a complete time-of-flight mass spectrum, bunches of ions are generated repetitively from a sample and allowed to enter the drift region. The ion transit times are determined and allocated to time channels so that after all the bunches have been generated, each time channel contains a count equivalent to the number of ions which had that particular transit time. The value of the count in each time channel is then corrected using the appropriate equation from the prior publications discussed above. It will be appreciated that the correction applied to the count in each channel is dependent on the counts contained in at least some of the earlier channels so that to correct the entire spectrum it is necessary to apply the correction equations to every channel in sequence. This requires that the raw data is stored to enable the corrections to be carried out before the spectral information can be processed and acquisition of the next spectrum can begin. Because the correction calculations require significant computational time, the correction process will limit the rate at which spectra can be acquired and processed, which as explained may seriously limit the usefulness of time-of-flight analysis for organic mass spectrometry. The only way in which this delay in starting the acquisition of the next spectrum can be avoided is by storing all the raw data in fast memory for subsequent processing, which is an equally unattractive option.
SUMMARY OF THE INVENTION
It is an objective, therefore, of the present invention to provide a method of correcting the distortion of a time-of-flight mass spectrum due to detector dead-time without the need to either store or process the acquired data immediately following acquisition. It is another object of the invention to provide a method of correcting the error in the mass-calibration of time-of-flight mass spectra which sometimes results from detector dead-time and in particular a method which can be applied once the data has been proc
Cottrell Jonathan C.
Hoyes John B.
Alix Yale & Ristas, LLP
Anderson Bruce
Micromass Limited
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