Multi-anode detector with increased dynamic range for...

Details Multi-anode detector with increased dynamic range for... Multi-anode detector with increased dynamic range for...

2001-12-19

2004-06-08

Lee, John R. (Department: 2881)

Radiant energy

Ionic separation or analysis

Reexamination Certificate

active

06747271

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed toward particle recording in multiple anode time-of-flight mass spectrometers using a counting acquisition technique.
BACKGROUND
Time-of-Flight Mass Spectrometry (“TOFMS”) is a commonly performed technique for qualitative and quantitative chemical and biological analysis. Time-of-flight mass spectrometers permit the acquisition of wide-range mass spectra at high speeds because all masses are recorded simultaneously. As shown in
FIG. 1
, most time-of-flight mass spectrometers operate in a cyclic extraction mode and include primary beam optics
7
and time-of-flight section
3
. In each cycle, ion source
1
produces a stream of ions
4
, and a certain number of particles
5
(up to several thousand in each extraction cycle) travel through extraction entrance slit
26
and are extracted in extraction chamber
20
using pulse generator
61
and high voltage pulser
62
. The particles then traverse flight section
33
(containing ion accelerator
32
and ion reflector
34
) towards a detector, which in
FIG. 1
consists of micro-channel plate (“MCP”)
41
, anode
44
, preamplifier
58
, constant fraction discriminator (“CFD”)
59
, time-to-digital converter (“TDC”)
60
, and computer (“PC”)
70
. Each particle's time-of-flight is recorded so that information about its mass may be obtained. Thus, in each extraction cycle a complete time spectrum is recorded and added to a historam. The repetition rate of this extraction cycle is commonly in the range of 10 Hz to 100 kHz.
If several particles of one species are extracted in one cycle, then these particles will arrive at the detector within a very short time period (possibly as short as 1 nanosecond). When using an analog detection scheme (such as a transient recorder in which the flux of charge generated by the incoming ions is recorded as a function of time), this near simultaneous arrival of particles does not cause a problem because analog schemes create a signal that is, on average, proportional to the number of particles arriving within a certain sampling interval. However, when a counting detection scheme is used (such as a time-to-digital converter in which individual particles are detected and their arrival times are recorded), the electronics may not be able to distinguish particles of the same species when those particles arrive too closely grouped in time. (A single signal is produced when a particle impinges upon the counting electronics. The signal produced by the detector is a superposition of the single signals that occur within a sampling interval.) Further, most time-to-digital converters have dead times (typically 20 nanoseconds) that effectively prevent the detection of more than one particle per species during one extraction cycle.
For example, when analyzing an air sample with twelve particles per cycle, there will be approximately ten nitrogen molecules (80% N
2
in air with mass of 28 amu) per cycle. In a time-of-flight mass spectrometer having good resolving power, these ten N
2
particles will hit the detector within two nanoseconds. Even a fast TDC with a half nanosecond bin width will not be able to detect all of these particles. Thus, the detection system will become saturated at this intense peak.
FIG. 2
shows these ten particles
6
impinging upon a detector consisting of electron multiplier
41
(with MCP upper bias voltage (
75
) and MCP lower bias voltage (
76
) as indicated), single anode
44
, preamplifier
58
, CFD
59
, TDC
60
, and PC
70
. (MCP
41
in
FIG. 2
consists of two chevron mounted multichannel plates. As would be apparent to one of skill in the art, circuitry would also be included to complete the electrical connection between the upper and lower plates. This additional circuitry is not shown in the figures.) TDC
60
will register only the first of these ten particles. The remaining nine particles will not be registered. Because only the first particle is registered, peaks for the abundant species (N
2
and O
2
) will be artificially small and will be recorded too early, resulting in an artificially sharpened peak whose centroid is shifted to an earlier and incorrect time of flight. These two undesirable effects—incorrect intensity and artificially shortened time of flight—are referred to as anode/TDC saturation effects. These anode/TDC saturation effects are therefore different from the electron multiplier gain reduction (sometimes called multiplier saturation) that occurs when too many ions impinge the electron multiplier so that the electron multiplier is no longer able to generate an electron flux that is proportional to the flux of the incoming ions.
In an attempt to overcome anode/TDC saturation effects, some detectors use multiple anodes, each of which is recorded by an individual TDC channel. (An anode is the part of a particle detector that receives the electrons from the electron multiplier.)
FIG. 3
shows such a detector with a single electron multiplier
41
and four anodes
45
of equal size. Each of the four anodes is connected to a separate preamplifier
58
and CFD
59
. Each of the four CFDs is connected to TDC
60
and PC
70
. This configuration permits the identification of intensities that are four times larger than those obtainable with a single anode detector. However, even with four anodes, the detection of the ten N
2
particles 6 leads to saturation since on average there will still be more than one particle arrival per anode. In principle, anode/TDC saturation could be avoided entirely by adding even more anodes. However, this solution is complex and expensive since each additional anode requires its own TDC channel.
Instead of using multiple anodes that each receive the same fraction of the incoming ions, one may use multiple anodes in which each anode receives a different fraction of the incoming ions. (The anode fraction is the fraction of the total number of ions that is detected by a specific anode.) By appropriately reducing this fraction, anode/TDC saturation effects can be reduced. See, for example, PCT Application WO 99/67801A2, which is incorporated herein by reference. One way to provide anodes that receive different fractions of the incoming ions is to provide electron multiplier
41
followed by anodes of different physical sizes as shown in
FIG. 4
, in which large anode
46
is located adjacent to small anode
47
. As before, each anode is connected to a separate preamplifier
58
and CFD
59
, and the CFDs are connected to TDC
60
and PC
70
. In the example of
FIG. 4
, two unequal sized anodes are provided having a size ratio of approximately 1:9. As a result, the small anode detects only one N
2
particle per cycle, which is just on the edge of saturation. Less abundant particles such as Ar (1% abundance in air and thus 0.12 particles per cycle) are detected with-out saturation on the large anode. Thus, with two anodes of unequal size it is possible to increase the dynamic range by a factor of approximately ten or more. A multi-anode detector with equal sized anodes would require ten anodes to obtain the same improvement.
In theory, the dynamic range of the unequal anode detector can be further reduced by further decreasing the size of the small anode fraction or by including additional anodes with even lower fractions. However, this theoretical increase in dynamic range is prevented by the presence of crosstalk from the larger anodes to the smaller anodes. In typical multi-anode detectors, the crosstalk from one anode to an adjacent anode ranges approximately from 1% to 10% when a single ion hits the detector. Thus, if 10 particles are detected simultaneously on a large fraction anode, the crosstalk to an adjacent small fraction anode may range from 10% to 100%. In such cases the small anode would almost always falsely indicate a single particle signal.
Bateman et al. (PCT Application WO 99/38190) disclose the dual stage detector shown in
FIG. 5
where anode
47
, in the form of a grid or a wire, is placed between MCP electron multipliers
41
and
50
. However, inst

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