Patent Application
20080073530
In the figures which illustrate by way of example only, embodiments of the present invention,
As illustrated, ion detector 100 includes two conversion electrodes 102, 104. Conversion electrodes 102 and 104 provide collision surfaces that emit electrons in response to collisions by particles, such as molecules, ions, electrons and the like. The number of emitted electrons will be dependent on the energies of incident particles. Example conversion electrodes 102, 104 may, for example, be dynodes formed of metal or semi-conductor material. For example, conversion electrodes 102, 104 may be formed of stainless steel bars. Alternatively, conversion electrodes may be formed of alloys, or coated materials. Optional heating device may be in thermal communication with electrodes 102 and 104, to heat these to as suitable temperature to further facilitate the emission of electrons. A suitable temperature may, for example, be between 200° C. and 800° C.
An electron detector 110 is positioned downstream of conversion electrodes 102 and 104 that detects the emission of secondary electrons by electrodes 102 and 104. In the depicted embodiment, electron detector 110 includes an electron multiplier 112, having an inlet 108 and an outlet 120 connecting a channel 122 that provides electrons to a detection surface 114. Typically, a capacitor 116, transmits electron pulses emitted by electron multiplier 112 to a pulse counter, such as pulse amplifier/discriminator/counter 124. Capacitor 116 isolates the high voltage of detection surface 114 from the (usually) ground potential of the amplifier/discriminator/counter 124.
Of course, electron detector 110 could be embodied as any suitable electron detector. Electron detector 110 could, for example, accelerate the electrons (perhaps after several stages of amplification) into a photo-emissive detection surface which provides resulting photons into a photomultiplier or avalanche photodiode. Other suitable electron detectors will be apparent to those of ordinary skill.
In any event, detection surface 114 is typically a conductive or semi-conductive surface on which receives electrons to be detected. Surface 114 may, for example, be stainless steel.
Pulse amplifier/discriminator/counter 124 is an example of any suitable high sensitivity electron pulse counting apparatus. An example pulse amplifier/discriminator/counter 124 is available from ORTEC of Oak Ridge, Tenn., under model number Model Number 9302. Other suitable electron pulse counting devices will be apparent to those of ordinary skill.
Electron multiplier 112 may be a channel electron multiplier, and as such, channel 122 may be a ceramic channel, a semi-conductor channel, a glass channel, or the like. Again, the channel may be coated, with a material that facilitates emission of electrons. Alternatively, electron multiplier 112 may be a discrete dynode electron multiplier, a multi-channel plate multiplier, or any other suitable electron multiplier, known to those of ordinary skill.
Electric power supplies 118a, 118d apply DC voltages to the conversion electrodes 102 and 104, respectively. Similarly, supplies 118b and 118c apply front and rear potentials to regions proximate inlet 108 and outlet 120 of electron multiplier 112. Supply 118e provides a DC voltage to plate 114. Supplies 118a, 118b, 118c, 118d and 118e may be conventional DC supplies. Multiple ones of supplies 118a, 118b, 118c, 118d and 118e may be combined. For example, one or two physical DC power supplies and suitable resistor network may be used to provide voltages of supplies 118a, 118b, 118c, 118d and 118e.
In operation, positive and negative ions are sequentially produced by a suitable ion source upstream of detector 100. Ions (positive or negative) enter a region proximate conversion dynodes 102, 104. Positively charged ions are attracted to conversion electrode 102, at a negative voltage, and collide therewith. Conversion electrode 102 emits secondary electrons, at energies close to the voltage of power supply 118d. As the inlet 108 of electron multiplier is at a more positive potential than electrode 102, secondary electrons are accelerated to inlet 108 of electron multiplier 112.
Negative ions are similarly attracted by conversion electrode 104. Upon impact, these negative ions cause the emission of secondary electrons by conversion electrode 104. The secondary electrons, emitted by conversion electrode 104 are similarly attracted to inlet 108 of multiplier 112, which is also at a higher potential than conversion electrode 104.
Supplies 118a and 118d provide DC biases to attract incident ions. In the depicted embodiment, supplies 118a and 118d apply DC apply biases of +4 kV and −6 kV to conversion electrodes 104 and 102, respectively. Supply 118b applies a fixed voltage of +6 kV to inlet 108. As such, secondary electrons emitted by conversion electrodes 104 and 102 are respectively accelerated through potentials of 2 kV and 12 kV to inlet 108 of electron multiplier 112. Of course, other voltages could be applied to conversion electrodes 104, 102 and electron multiplier 112. For example, suitable voltages in the range of about +3 kV and +10 kV above the energies of ions to be detected, could be applied to conversion electrode 104. Similarly, voltages in the range of about −2 kV and −10 kV below the energies of ions to be detected could be applied to conversion electrode 102, depending upon the maximum mass detected. Corresponding voltages above that applied to conversion electrode 104 could be applied proximate the inlet 108 of electron multiplier 112. In the depicted embodiment, supplies 118a-118e provide the indicated voltages relative to ground. Of course, voltages would typically be provided relative to the potentials at which the ions are introduced into detector 100. For example, ions typically leave the upstream mass analyser at an elevated potential of, for example, between about 150V and −150V. Supplies 118a-118e may be biased accordingly, above the potential of the output of the mass analyser.
Power supply 118c applies a voltage higher than that proximate inlet 108. As such, secondary electrons, from both conversion electrode 102 and 104, at inlet 108, are accelerated to outlet 120 at a higher potential than inlet 108. The emission electrons, incident at inlet 108 further cause the emission of a cascade of tertiary electrons by electron multiplier 112 resulting in the electrons at output 120.
Electrons at outlet 120 are incident on detection surface 114. In order to attract electrons, detection surface 114 is maintained at a voltage higher than outlet 120. Surface 114 is maintained more positive than electrode 104 (e.g. at least +100V more positive than electrode 104, and in the depicted embodiment about +200V more positive than outlet 120), by supply 118e. Pulse detector 124, in turn, detects the output electrons. In the depicted embodiment, electron detector 110 takes the form of a pulse counting detector. As such, it may provide its output to a computing device (not shown), that in turn may tabulate counted pulses, and their masses and display measured results.
Conveniently, although the output of multiplier 112 and detection surface 114 are maintained at positive voltages, above ground, pulses may be easily detected by a pulse counting detector. Alternatively, current could be measured directly. However, high speed, sub-picoamp current detection at about the potential of outlet 120, is difficult and costly.
Conveniently, ion detector 100 allows for the detection of both positively charged and negatively charged ions. No switching of power supplies 118 is required and the sensitivity is not compromised.
Moreover, ion to electron conversion efficiencies of both conversion electrodes 102, 104 (and electron multiplier 112) are not dependent on the particular structure of incident molecules.
After ions of one polarity have been detected, ions of the opposite polarity may be introduced to detector 100, and detected.
As will be appreciated, applied voltages on electrodes 102, 104 and electron multiplier 112 (and surface 114) may be adjusted by a small amount in dependence on the polarity of ions to be detected, to aid in the formation, extraction and focusing of electrons, and remain within the scope of the invention. For example, for negative ions the voltage of electrode 104 may be made more positive by between 0 to 25% from the voltage applied for positive ions, and the voltage applied to electrode 102 may be made more negative by between 0 to 25%. For positive ions, the voltages applied to electrodes 102, 104 may again be respectively raised for electrode 104 and lowered for electrode 102.
In an alternate mode of operation, positive and negative ions may be detected concurrently by detector 100. For example, both positive and negative ions may be introduced to detector 100, as described above. Both types (i.e. positive and negative) may be detected as described above: they are attracted to one of conversion electrodes 102, 104 causing emission of secondary electrons that are attracted to and detected by electron detector 110. Discriminating detection of positive ions from negative ions may, however, not be possible as both positive and negative ions result in the detection of electrons at detection surface 114.
As will now be appreciated, conversion electrode 104 of detector 100 could actually be integrated with electron multiplier 112. In this way, detector 100 may be modified to form an alternate detector 100′ depicted in
In further embodiments, an ion detector 100″ illustrated in
In yet a further embodiment, an ion detector 100′″ illustrated in
Detectors 100′, 100″, and 100′″ of
A person of ordinary skill will now appreciate that detectors 100, 100′, 100″, and 100′″ may be used to detect particles other than ions. For example, positrons, or other charged particles could be detected.
Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.
