Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
1999-09-13
2003-09-30
Berman, Jack (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
active
06627877
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a time of flight analysis device and in particular to an inductively coupled plasma time of time of flight mass spectrometer. However, the invention has application to time of flight instruments in which a sample is produced by methods other than ICP techniques, such as MALDI techniques, electro spray or a APCI and elemental analysis by glow discharge or DC plasma means.
BACKGROUND OF THE INVENTION
Time of flight mass spectrometers analyses a sample by first vaporising and then ionising the sample using a number of possible techniques such as those described above. Once formed, the ionised components of a sample are directed towards an electrostatic accelerator usually by some form of ion optics which collimates or focuses the ion beam. The accelerator imparts a specific kinetic energy to all the ions having the same charge, producing a pulse of ions in which the individual ion velocities are inversely proportional to the square root of the mass to charge ratio. The ion pulse is then directed towards an ion detector a well defined distance away. In travelling to the detector the original ion pulse will be dispersed as a result of the different velocities of the different ion masses. The distribution of ionic masses in the initial pulse and hence the sample can then be determined by measuring the time each ion or group of ions takes to travel the known distance to the detector.
SUMMARY OF THE INVENTION
The object of the present invention is to improve existing time of flight analysis devices.
A first aspect of the invention may be said to reside in a time of flight analysis device, including:
means for producing an ionised sample for analysis;
a time of flight cavity;
means for receiving the ionised sample and for directing the ionised sample into the time of flight cavity;
a detector for detecting the ionised sample after travel through the time of flight cavity;
an analysis means coupled to the detector for receiving an output from the detector, said analysis means including;
(a) time to digital conversion circuit means for receiving the output; and
(b) an integrating transient recorder circuit means coupled in parallel with the time to digital conversion circuit means, also for receiving the output from the detector.
By coupling both a time to digital conversion circuit and integrating transient recorder circuit to the detector the dynamic range of the device is greatly improved. The time to digital conversion circuit means can determine results for very small concentrations of sample in which very few ions of the desired species may be present in the ion flow, and the integrating transient recorder circuit means analysing much higher abundant species which may be present to thereby give the increased dynamic range of the device.
Time to Digital Conversion (TDC) uses a system of accurate timers to accurately record the actual arrival time of each pulse induced by arrival of individual ions at the detector. By summing many consecutive spectra a picture of the relative quantities and mass of the constituents is built up. While very fast and accurate, a key limitation of this approach derives from the fact that the system detects ion pulses as logical events (ie. ion present or ion not present). If then two or more ions arrive within one time bin, or if the pulse produced by the detector is induced by arrival of a short multi ion packet, count from only one event is recorded and the other ions are lost. State of the art multiple stop time analysers for TOF MS have now days excellent timing resolution (fractions of nS) and dynamic range, but still suffer from high dead time (>50 nS). The dynamic range of such a system depends only on the upper counting limit of the counters used, providing the threshold of the discriminator usually installed between analogue and digitising parts eliminates all the analogue noise sources (like preamplifier noise etc.) At a rate of 10-50 kHz (typical repetition frequency of single scan in TOF MS), having <0.5 ions per peak per scan, the dynamic range of 1 e6, achievable in quadruple ICPMS within 1 min of acquisition, would require 7-33 min of acquisition, making “TDC only” technique not very practical. In the Integrating Transient Recorder (ITR) a transient signal of a single waveform from a repetitive bunch is digitised by a very high speed ADC (typically at less than 5 ns sampling rate), and then data representing the magnitude of signal waveform at each sampling point is temporarily stored in some buffer memory for further summation with the set of data representing the next waveform. After predetermined number of summations integrated data are outputed in the form of magnitude-time array. ITR based techniques always employ some data reduction via summation before storage, as real time spectra acquired at very high repetition frequency of 10-50 kHz can not be stored individually on line. Moreover, ion detectors usually have very high standard deviations on the gain resulting in single ion pulse height distribution with up to 100% RSDs, so ITR can not be used for quantitative analysis when less than a 100 integrated ion pulses represent mass peak. As a result of that and due to limited noise figures of analog signal processing means, the dynamic range of the technique is limited to about 1e4 value. Thus incorporating both a multistop TDC and a Integrating Transient Recorder in parallel, allowing acquisition and processing of TOF mass spectra where any mass peak can contain 1e-5 to 1e3 ions, said limits are given as an example only are defined by ion extraction pulse repetition frequency (100 kHz) and linearity range of ion detector.
Preferably the analysis means also includes a logarithmic preamplifier for extending the dynamic range of the integrating transient recorder circuit means and pulse stretching after a discriminator to increase the effectiveness of the time to digital conversion circuit means.
The second aspect of the invention relates to protection and extension of the lifetime of detectors used in time of flight analysis devices. Modern ion detectors suffer from low dynamic range, being unable to withstand high input currents which may destroy the detector or significantly reduce its lifetime. Discrete dynode electro multipliers have been demonstrated to have higher range of acceptable incoming ion current at which the gain is not declining, in comparison to continuous dynode detectors. However, at extremely high count rates even discrete dynode detectors age quickly. This results in two problems. The overall lifetime of detectors becomes unpractically short and maintenance of a constant gain as the detector wears out requires a change in the voltage applied across the dynodes. The change in voltage is conventionally done by either changing detector entrance DC potential with the anode kept at virtual ground or by applying a DC potential to the anode, which is capacitively decoupled with a preamplifier or the last dynode. Changing the ion detector entrance potentials in time of flight mass spectrometers necessarily implies changing liner voltage. This means the average ion energy changes which means the mass scale has to be recalibrated for every value of detector voltage. Supplying DC potentials to the anode or the last dynode means capacitively coupling DC high voltage supply to the input of a preamplifier so that the AC ripple of the supply affects noise of the detection system.
A second aspect of the invention may be said to reside in a detector for a time of flight mass spectrometer, including:
an ion sensitive surface for receiving a flow of ions;
a plurality of discrete dynodes arranged in series for receiving a secondary emission from the ion sensitive surface when ions impact on the ion sensitive surface, the secondary emission being amplified by the discrete dynodes; and
means for varying the voltage between the ion sensitive surface and an adjacent dynode or between any pair of adjacent dynodes in this series of discrete dynodes so that a voltage between the ion sens
Bandura Dmytro
Davis Stephen C
Hoffman Andy D
Makarov Alexander A
Berman Jack
GBC Scientific Equipment Pty Ltd.
Pillsbury & Winthrop LLP
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