Patent Publication Number: US-11640005-B2

Title: Daly detector operable in negative ion and positive ion detection modes

Description:
TECHNICAL FIELD 
     The present disclosure relates to mass spectrometry. More particularly, the present invention relates to ion detectors used in mass spectrometry. 
     BACKGROUND 
     The ion detector configuration which comprises a conversion dynode in front of an electron multiplier (EM) is very popular for use in various types of mass spectrometer systems, such as state-of-the-art single and triple quadrupole, ion trap systems, and hybrid mass spectrometers. For example,  FIG.  1    schematically illustrates a triple-quadrupole mass spectrometer system, as generally designated by the reference numeral  1  that may employ an electron multiplier detector  10 . The operation of mass spectrometer  1  can be controlled and data  11  can be acquired by a control and data system (not depicted) of various circuitry of one or more known types, which may be implemented as any one or a combination of general or special-purpose processors (digital signal processor (DSP)), firmware, software to provide instrument control and data analysis for mass spectrometers and/or related instruments. A sample containing one or more analytes of interest can be ionized via an ion source  5  operating at or near atmospheric pressure. The resultant ions are directed via predetermined ion optics that often can include tube lenses, skimmers, and multipoles, e.g., reference characters  6  and  7 , so as to be urged through a series of chambers, e.g., chambers  2 ,  3  and  4 , of progressively reduced pressure that operationally guide and focus such ions to provide good transmission efficiencies. The various chambers communicate with corresponding ports  12  (represented as arrows in  FIG.  1   ) that are coupled to a set of vacuum pumps (not shown) to maintain the pressures at the desired values. 
     The example mass spectrometer system  1  of  FIG.  1    is illustrated with a triple-stage configuration  8  within a high vacuum chamber  9 , the triple-stage configuration having sections labeled Q 1 , Q 2  and Q 3  electrically coupled to respective power supplies (not shown). The Q 1 , Q 2  and Q 3  stages may be operated, respectively, as a first quadrupole mass filter, a fragmentation cell, and a second quadrupole mass filter. Ions that are either filtered, filtered and fragmented or fragmented and filtered within one or more of the stages are passed to the detector  10 . Such a detector is beneficially placed at the channel exit of the quadrupole (e.g., Q 3  of  FIG.  1   ) to provide data that can be processed into a rich mass spectrum  11  showing the variation of ion abundance with respect to m/z ratio. During conventional operation of a multipole mass filter, such as the quadrupole mass filter Q 3  shown in  FIG.  1   , to generate a mass spectrum, the detector  10  is used to measure the quantity of ions that pass completely through the mass filter as a function of time while the RF and DC voltage amplitudes are scanned. 
     Generally, the detector  10  ( FIG.  2   ) comprises a conversion dynode and an electron multiplier. The conversion dynode converts a stream of primary ions, as received from an ion outlet of a mass analyzer, into a stream of electrons when used in “positive-ion-detection” mode or else a stream of positively charged particles, when used in the “negative ion detection mode”. The electrons or positively-charged particles emitted by the conversion dynode are received by the electron multiplier. In response to the reception of the charged particles, a cascade of multiple secondary electron emission events is stimulated within the electron multiplier in known fashion. The multiple secondary electron emission events that are generated with the electron multiplier ultimately yield an output electron current, at a receiving anode, which may be several orders of magnitude greater than the magnitude of the input current. 
     The classic channel electron multipliers suffer from fast aging effects. Over the course of operation, the active surface or surfaces of the electron multiplier dynode(s) may rapidly lose their ability to emit secondary electrons, especially when the electron multiplier is operated in a poorly evacuated environment or in an environment that contains organic vapor. Although the discrete dynode type of electron multipliers have been demonstrated to age somewhat more slowly than the continuous dynode type, the loss of gain stability and reduction in operational lifetime of discrete dynode EM detectors is nonetheless problematical. Consequently, frequent calibrations and eventually detector replacement are required to maintain appropriate gain for adequate performance. This is an inconvenience causing downtime and cost to mass spectrometer users. 
     It has hypothesized that the primary aging mechanism of EM detectors is the so-called “stitching” effect, in which a layer of organic particles adheres to or is embedded in the multiplier&#39;s active surface and increases in thickness over time. Among the improvements that have been made in an attempt to overcome the stitching process, one popular trend has been to improve the isolation of the electron multiplier from the rest of the vacuum system that contains the organic molecules. Nonetheless, it is a fundamental challenge to completely seal the multiplier while still allowing the electrons and positive particles from the conversion dynode to effectively penetrate through to be detected and multiplied. 
     The Daly detector (e.g., see U.S. Pat. No. 3,041,453 and Daly, N. R. “Scintillation type mass spectrometer ion detector.”  Review of Scientific Instruments  31, no. 3 (1960): 264-267) has been found to be a very promising alternative to the electron multiplier in terms of its better gain stability and longer lifetime.  FIG.  2    is a schematic diagram of a conventional Daly detector  20 . The apparatus  20  includes a conversion dynode  34 , a scintillator  31 , a thin-film scintillator electrode  32  disposed on a face of the scintillator and a photomultiplier tube (PMT)  27 . The conversion dynode  34  is housed within the evacuated interior of a vacuum vessel  21 . The vessel  21  has a flanged extension  22  for attachment to a mass spectrometer (not shown). Extension  22  contains a plate  23  having a slit  24  for receiving ions from the mass spectrometer and pumping holes  25  by which the interior is evacuated. The conversion dynode  34  is held in position at a distance, s, from the scintillator  31  with its electrode  32  by an electrified support arm  35  that is coupled to a high-voltage supply. A wall  26  of the vacuum vessel has a recess that supports the PMT  27 . 
     In operation of the conventional Daly detector ( FIG.  2   ), a stream  28  of positively-charged ions is introduced, through slit  24 , into the interior of the evacuated vessel  21  at a substantially right angle to the direction in which the conversion dynode is displaced from the scintillator  31 . A kilovolt-range negative electrical potential bias applied to the conversion dynode  34  causes the primary positive ions  28  to be deflected onto the conversion dynode. Secondary electrons  29  are generated when the primary positive ions hit the conversion dynode. The electrons are then accelerated to impact the scintillator and thereby generate photons by an appropriate electrical potential difference between the scintillator electrode  32  and the conversion dynode  34 . The photons emitted by the scintillator are converted by the PMT into electrical pulses which can be amplified and counted as desired. 
     Because the dynodes of the PMT are contained in a completely sealed vacuum tube, particles of organic vapor and other contaminants emitted from the mass spectrometer cannot reach the active surface area of these dynodes. The “stitching” effect is thus minimized or eliminated and the overall detector life span is extended, relative to the conventional EM detector. Unfortunately, the PMT scintillator is unresponsive to impacts of positively charged particles, as would conventionally be expected to be emitted from a conversion dynode that receives negative ions from an ion outlet of a mass analyzer. Therefore, the Daly detector has only been designed for, and demonstrated to work only in, the positive ion detection mode of mass spectrometer operation. Attempting to detect negative ions using a conversion dynode as a front end to a Daly detector results in no useful signal. 
     SUMMARY 
     In order to address the above-noted deficiencies in the art of ion detection, this disclosure teaches a novel detector that has the advantages of the PMT-based Daly detector but can also work in both the positive and the negative Ion detection modes. Furthermore, this novel detector is small in size and therefore is more suitable for a compact mass spectrometer system. 
     According to a first aspect of the present teachings, an ion detector that can detect either positive or negative ions is disclosed, the ion detector comprising: an ion inlet comprising an ion focusing lens; a dynode having a surface configured to intercept, within a zone of interception, a stream of ions passing through the ion focusing lens, wherein a plane that is tangent to the dynode surface at the zone of interception is disposed at an angle to a line that passes through the center of the dynode surface and the center of the focusing lens; a scintillator having a surface that is configured to receive secondary electrons emitted from the zone of interception; a scintillator electrode affixed to the scintillator surface; a photodetector configured to receive photons emitted by the scintillator and to generate an electric signal in response thereto; and one or more power supplies electrically coupled to the focusing lens, the dynode, the scintillator electrode and the photodetector. The photodetector may comprise any device that may suitably detect wavelengths of light emitted by the scintillator. Accordingly, the photodetector may comprise, without limitation, a photomultiplier tube or any suitable semiconductor light sensor device, either as a standalone sensor or as a plurality of individual sensors in an array format. Suitable semiconductor light sensor devices include, without limitation, solid-state silicon photomultipliers, charge-coupled devices, active-pixel sensors and photodiodes of various types. 
     The one or more power supplies may be configured to provide a first set of voltages to the ion lens, the dynode and the scintillator electrode, the first set of voltages corresponding to the detection of positively charged ions by the ion detector, and to provide a second set of voltages to the ion lens, the dynode and the scintillator electrode, the second set of voltages corresponding to the detection of negatively charged ions by the ion detector. 
     According to various embodiments, the angle is within the range of twenty degrees to thirty degrees, inclusive. According to various embodiments, the dynode surface is a concave reentrant surface. According to various embodiments, the scintillator has a thin-film metallic coating on the surface of the scintillator. According to various embodiments, a line that passes through the center of the dynode surface and the center of the scintillator surface makes an angle of between 80 degrees and 110 degrees with the line that passes through the center of the dynode surface and the center of the focusing lens. According to various embodiments, the dynode is disposed such that no portion of the dynode is closer than three millimeters from the scintillator electrode and also such that no portion of the dynode surface is further than seven millimeters from the scintillator electrode. 
     According to various embodiments, the ion lens comprises a tube lens or a ring lens that is configured to intercept ions emitted axially from a multipole mass analyzer or mass filter. According to various other embodiments, the ion lens comprises a pair of electrodes, each electrode having an edge, the edges aligned parallel to one another and defining a gap therebetween and wherein the gap is configured to intercept ions emitted radially from a linear ion trap. 
     According to a second aspect of the present teachings, a method of ion detection comprises: (a) setting electrical potentials of a dynode and a scintillator electrode of a Daly detector and of a focusing lens disposed at an ion inlet of the Daly detector so as to detect negatively charged ions received at the ion inlet from a mass analyzer or mass filter; (b) transferring the negatively charged ions from the mass analyzer or mass filter to the Daly detector through the focusing lens and detecting a first signal that corresponds to detection of the negatively charged ions by a photodetector of the Daly detector; (c) setting electrical potentials of the dynode, the scintillator electrode and the focusing lens of the Daly detector so as to detect positively charged ions received at the ion inlet from the mass analyzer or mass filter; and (d) transferring the positively charged ions from the mass analyzer or mass filter to the Daly detector through the focusing lens and detecting a second signal that corresponds to detection of the positively charged ions by the photodetector of the Daly detector. The photodetector may comprise any device that may suitably detect wavelengths of light emitted by the scintillator. Accordingly, the photodetector may comprise, without limitation, a photomultiplier tube or any suitable semiconductor light sensor device, either as a standalone sensor or as a plurality of individual sensors in an array format. Suitable semiconductor light sensor devices include, without limitation, solid-state silicon photomultipliers, charge-coupled devices, active-pixel sensors and photodiodes of various types. 
     According to third aspect of the present teachings, a method of ion detection comprises: (a) setting electrical potentials of a dynode and a scintillator electrode of a Daly detector and of a focusing lens disposed at an ion inlet of the Daly detector so as to detect positively charged ions received at the ion inlet from a mass analyzer or mass filter; (b) transferring the positively charged ions from the mass analyzer or mass filter to the Daly detector through the focusing lens and detecting a first signal that corresponds to detection of the positively charged ions by a photodetector of the Daly detector; (c) setting electrical potentials of the dynode, the scintillator electrode and the focusing lens of the Daly detector so as to detect negatively charged ions received at the ion inlet from the mass analyzer or mass filter; and (d) transferring the negatively charged ions from the mass analyzer or mass filter to the Daly detector through the focusing lens and detecting a second signal that corresponds to detection of the negatively charged ions by the photodetector of the Daly detector The photodetector may comprise any device that may suitably detect wavelengths of light emitted by the scintillator. Accordingly, the photodetector may comprise, without limitation, a photomultiplier tube or any suitable semiconductor light sensor device, either as a standalone sensor or as a plurality of individual sensors in an array format. Suitable semiconductor light sensor devices include, without limitation, solid-state silicon photomultipliers, charge-coupled devices, active-pixel sensors and photodiodes of various types. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above noted and various other aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings, not necessarily drawn to scale, in which: 
         FIG.  1    is a schematic illustration of a triple-quadrupole mass spectrometer system; 
         FIG.  2    is a schematic depiction of a conventional Daly detector for a mass spectrometer system; 
         FIG.  3    is a pair of voltage profiles, as taught in U.S. patent application Ser. No. 16/941,124, as may be applied between a mass analyzer, a metal-channel conversion dynode (MCD) and a luminescent phosphor component of a mass spectrometer (upper diagram) for the detection of positive ions emitted from the mass analyzer (uppermost voltage profile) and the detection of negative ions emitted from the mass analyzer (lowermost voltage profile); 
         FIG.  4 A  is a schematic elevation view of a modified Daly detector in accordance with the present teachings showing an envelope of calculated trajectories of positively-charged ions incident into the detector from an outlet slit of a linear ion trap; 
         FIG.  4 B  is the schematic Daly detector depiction of  FIG.  4 A , showing an envelope of calculated trajectories of secondary electrons emitted from a dynode of the detector onto a metallized scintillator; 
         FIG.  5 A  is the schematic Daly detector depiction of  FIG.  4 A , showing an envelope of calculated trajectories of negatively-charged ions incident into the detector from an outlet slit of a linear ion trap; 
         FIG.  5 B  is the schematic Daly detector depiction of  FIG.  4 A , showing an envelope of calculated trajectories of secondary electrons emitted from a dynode of the detector onto a metallized scintillator; 
         FIG.  6 A  is schematic depiction of an ion lens of the Daly detector of  FIGS.  4 A- 4 B and  5 A- 5 B ; 
         FIG.  6 B  is schematic depiction of an alternative ion lens of the Daly detector of  FIGS.  4 A- 4 B and  5 A- 5 B ; 
         FIG.  7 A  is a schematic top view of a portion of the Daly detector of  FIGS.  4 A- 4 B and  5 A- 5 B  showing an envelope of calculated ion trajectories of positively-charged ions incident onto a dynode of the detector from an outlet slit of a linear ion trap; 
         FIG.  7 B  is a schematic elevation view of a Daly detector that differs from the Daly detector of  FIGS.  4 A- 4 B and  5 A- 5 B  in that the focusing lens is removed, the view also showing representative calculated trajectories of positively-charged ions incident into the detector from an outlet slit of a linear ion trap; 
         FIG.  8    is a flow diagram of a first method of operating a mass spectrometer including a Daly detector in accordance with the present teachings; and 
         FIG.  9    is a flow diagram of a second method of operating a mass spectrometer including a Daly detector in accordance with the present teachings. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments and examples shown but is to be accorded the widest possible scope in accordance with the features and principles shown and described. To fully appreciate the features and advantages of the present invention in greater detail, please refer to  FIGS.  1 - 3 ,  4 A- 4 B,  5 A- 5 B,  6 A- 6 B,  7 A- 7 B,  8  and  9    in conjunction with the following discussion. 
     In the description of the invention herein, it is understood that a word appearing in the singular encompasses its plural counterpart, and a word appearing in the plural encompasses its singular counterpart, unless implicitly or explicitly understood or stated otherwise. Unless otherwise stated, the word “substantially”, when used in this document so as to indicate that a quantity X is “substantially” a quantity Y, should be understood to mean that the quantity X is equal to the quantity Y within a five percent tolerance. Furthermore, it is understood that, for any given component or embodiment described herein, any of the possible candidates or alternatives listed for that component may generally be used individually or in combination with one another, unless implicitly or explicitly understood or stated otherwise. It will be understood that any list of candidates or alternatives is merely illustrative, not limiting, unless implicitly or explicitly understood or stated otherwise. Moreover, it is to be appreciated that the figures, as shown herein, are not necessarily drawn to scale, wherein some of the elements may be drawn merely for clarity of the invention. Also, reference numerals may be repeated among the various figures to show corresponding or analogous elements. 
     As used herein, the term “DC”, when referring to a voltage applied to one or more electrodes of a mass spectrometer component (such as an ion funnel), does not necessarily imply the imposition of or the existence of an electrical current through those electrodes but is used only to indicate that the referred-to applied voltage either is static or, if non-static, is non-oscillatory and non-periodic. The term “DC” is thus used herein to distinguish the referred-to voltage(s) from applied periodic oscillatory voltages, which themselves may be referred to as either “RF” or “AC” voltages. 
     In commonly-assigned, co-pending U.S. patent application Ser. No. 16/941,124 titled “Ion-to-Electron Conversion Dynode for Ion Imaging Applications,” which was filed on Jul. 28, 2020, Silivra et al. describe conversion dynode devices that can emit an analytically-sufficient quantity of secondary electrons when impacted by either positively-charged or negatively-charged ions. The conversion dynode apparatus described by Silivra et al. is referred to as a micro-Venetian-blind metal channel dynode (or micro-Venetian-blind MCD). Previous general conventional operation of conversion dynodes for the purpose of detecting negative ions has relied on the conversion of a stream of primary negative ions to a stream of secondary positive ions that are subsequently detected. The discovery of the emission of analytically sufficient quantities of secondary electrons, when operated in the negative ion mode as described in Silivra et al. opens up the possibility of implementing a Daly-detector-like detector system that is operable in both the positive-ion and negative-ion modes without the need for additional conversion stages. 
     Silivra et al. hypothesized that it is generally the case that both secondary protons and secondary electrons are generated upon initial impact of ions (either positive or negative) with any metal channel dynode, but with protons usually being generated in excess of electrons. Silivra et al. further hypothesized that, as a result of inter-particle collisions in the immediate vicinity of the dynode active surface, essentially all of the electrons and some proportion of the more-abundant protons are generally neutralized. If true, this hypothesis can explain why, in general, only a weak beam of protons is observed during operation of metal dynode in conventional negative ion detection mode. Moreover, Silivra et al. further hypothesized that, under the application of an appropriate electric field on the detector side of the conversion dynode, the less-abundant secondary electrons may be successfully separated from the more abundant secondary protons and subsequently be detected. 
     The topmost portion of  FIG.  3    is a schematic depiction of the use of a so-called micro-Venetian MCD, as taught by Silivra et al. In this diagram, ions, I, are depicted as emitted, for example, from an exit end of a mass analyzer  79 , such as a quadrupole mass analyzer. However, the use of the micro-Venetian-blind MCD  60  is not limited to this or any other particular mass analyzer configuration. As another example, ions may be emitted in a direction that is transverse to the quadrupole rods of the mass analyzer  79 , such as by means of a slot  44  in one of the rods, in which case, the ions are emitted along trajectory  45 . In such an alternative case, the micro-Venetian-blind MCD  60 , scintillator  31  and scintillator electrode  31  would be disposed along the trajectory  45  and the exemplary electrical potential profiles depicted in the mid-portion and lowermost portion of  FIG.  3    will still be applicable. 
     The middle and lowermost diagrams of  FIG.  3    are schematic depictions of voltage profiles as may be employed for the detection of positive and negative ions, respectively, emerging from a mass analyzer when employing a micro-Venetian-blind MCD  60 , as described by Silivra et al. The uppermost voltage profile of  FIG.  3    is appropriate for the detection of positively charged ions. A negative electrical potential bias of the MCD relative to the mass analyzer  79 , by an amount ΔV c , causes the positive ions to be attracted to and to impact the MCD device. Secondary electrons generated by the impact energy are then extracted from the MCD  16  and towards the scintillator electrode  32  and scintillator  21  as a result of the positive bias of scintillator electrode  32  by an amount, ΔV e , relative to the micro-Venetian-blind MCD  60 . Conversely, when using the micro-Venetian-blind MCD  60  to detect negative ions, it is necessary to apply a voltage profile of the form shown in the lowermost portion of  FIG.  3   , in which the MCD is positively biased relative to the mass analyzer and the phosphor electrode is further positively biased relative to the MCD by an amount, ΔV e . 
     In this document, the present inventors apply the teachings of Silivra et al. to an expansion of the use of Daly detectors.  FIGS.  4 A- 4 B and  5 A- 5 B  schematically depict a modified Daly detector  80  in accordance with the present teachings that may be operated in both positive-ion and negative-ion detection modes. Components  41  and  42 , shown with dashed outlines, are not portions of the detector apparatus  80  but are, instead, components of an ion trap apparatus that may supply ions, either positive or negative, to the detector. Specifically, component  41  is a slotted quadrupole rod electrode (depicted in cross section) of a linear ion trap mass analyzer and component  42  is an extraction electrode (also depicted in cross section) of the same mass analyzer. In known fashion, application of an alternating-current (AC) resonant excitation voltage to rods of the linear ion trap mass analyzer may cause ions to be selectively ejected from the mass analyzer through the slot  44  in the quadrupole rod electrode  41 . Application of an extraction voltage to the extraction electrode  42  urges the ejected ions along ion path  45  (cf. topmost portion of  FIG.  3   ). Although the modified Daly detector  80  is depicted as receiving ions from an ion trap mass analyzer, this detector may be utilized to detect ions derived from any suitable provider of ions. 
     The modified Daly detector  80 , proper, comprises a metal casing  81   a , two of the walls of which are illustrated in  FIGS.  4 A- 4 B and  5 A- 5 B . The interior of the casing  81   a  is under the same high vacuum as the mass analyzer. Upon approaching the modified Daly detector  80 , ions that are ejected from the linear ion trap mass analyzer are caused to closely follow trajectory  45  by the action of focusing-lens  83 . Note that, since ions may be emitted from the linear ion trap at any point along a slotted aperture  44 , the ion paths are not constrained to follow a single line, as might be suggested by the line  45  in  FIGS.  4 A and  5 A  but, instead, are essentially constrained by the focusing lens  83  to remain within a plane that is perpendicular to the plane of the drawings and includes the line  45 . Each of  FIGS.  4 A,  4 B,  5 A and  5 B  depicts the focusing lens  83  as comprising pair of separate electrodes  89  that are structurally supported by one or more electrically insulating support structures  81   b . The two electrode components  89  may, in some embodiments, be joined as a single-piece electrode. The two electrodes  89 , if present, are electrically coupled to one another and are maintained at the same electrical potential. An electrical potential difference between the slotted quadrupole rod electrode  41  and a dynode  160  causes the ions to continue along trajectories that cause them to impact the dynode  160 . Secondary electrons emitted from the dynode  160  are then urged towards a scintillator  31  by a suitable applied electrical potential difference between the dynode  160  and a scintillator electrode  32 . Preferably, but not necessarily, the scintillator electrode is formed as a thin-film metal coating on a face of the scintillator  31  that is sufficiently thin to allow electrons to penetrate into the scintillator and stimulate the emission of photons from the scintillator material. A number of suitable scintillator materials are well known to those of ordinary skill in the art and include various synthetic rare-earth-doped single crystals, polycrystalline materials and glasses. The emitted photons are detected by photodetector  27 . The photodetector may comprise any device that may suitably detect wavelengths of light emitted by the scintillator. Accordingly, the photodetector may comprise, without limitation, a photomultiplier tube or any suitable semiconductor light sensor device, either as a standalone sensor or as a plurality of individual sensors in an array format. Suitable semiconductor light sensor devices include, without limitation, solid-state silicon photomultipliers, charge-coupled devices, active-pixel sensors and photodiodes of various types. 
     Preferably, the dynode of the modified Daly detector  80  is fabricated from a metal, such as stainless steel. The shape of the dynode  160  is not restricted to any particular form. However, the inventors have found that optimal performance is achieved if the surface of the dynode  161  onto which the incident ions are directed is a concave, reentrant surface, thereby causing the dynode to be in the form of a shallow bowl or cup. In order to make the detector  80  as compact as possible, the normal line to the electron-receiving surface of the scintillator is preferably disposed at an angle of substantially ninety degrees relative to the trajectory  45  of the incoming ions. Alternatively, a line passing through both the center of the electron-receiving surface and the center of the surface  161  is preferably disposed at an angle of substantially ninety degrees to a line that passes through both the center of the surface  161  and the center of the ion focusing lens  83 . 
     Charged-particle trajectory calculations (discussed further below) indicate that, for optimum results, the dynode  160  should preferably disposed at a tilt angle, a, that is in the range of 20-30 degrees, inclusive. In the illustrated example, the tilt angle, a, is the angle that is made between the incoming ion trajectory  45  and a plane that is tangent to the concave surface  161  at the center of the surface. If the surface of the dynode onto which the incident ions are directed is convex, then the tilt angle would be defined in terms of the tangent plane at the center of that surface; if the surface is flat, then the tangent plane is identical to the plane of the surface. 
     If the photodetector  27  comprises a photomultiplier-tube detector, then, preferably, the interior of the photomultiplier-tube detector  27  should be isolated from the vacuum environment inside the casing  81   a . Most commercial photomultiplier-tube detectors are provided within a hermetically sealed package having a transparent window for reception of photons. In such instances, the hermetically sealed package can isolate the photomultiplier-tube components from contaminants within the casing  81   a . Otherwise or in addition, the photomultiplier-tube detector  27  may be disposed completely outside of the casing  81   a . Regardless of whether the photodetector  27  comprises a photomultiplier tube or some other type of detector, one or more optically transmissive components  91 , such as a window, may be provided within or as part of the casing in order to provide an optical line of sight between the scintillator  31  and a photodetector  27  that is disposed outside of the casing  81   a . The one or more optically transmissive components  91  may be provided in order to facilitate optical coupling regardless of whether the photodetector  27  is disposed inside of or outside of the casing  81   a . The optically transmissive components may include any number of windows, lenses, fiber optics, prisms, filters, anti-reflection coatings, gratings, etc. 
     One way in which the modified Daly detector  80  ( FIGS.  4 A,  4 B,  5 A,  5 B ) differs from the conventional Daly detector (e.g.,  FIG.  2   ) is through the provision of a focusing-lens electrode or electrodes at the ion inlet of the detector  80 . The configuration of the focusing-lens electrode or electrodes  89  may vary depending on the particular configuration of a mass analyzer ion outlet from which the detector receives ions. For example, if the mass analyzer comprises a quadrupole from which the ions are emitted axially, similar to trajectory  43  illustrated in  FIG.  3   , then the focusing-lens  83  may comprise a simple ring electrode, a tube lens, or a plate electrode having an aperture, preferably a circular aperture. Alternatively, if the ions are emitted radially from a slot  44 , similar to trajectory  45  illustrated in  FIGS.  3 ,  4 A and  5 A , then the focusing-lens  83  may be configured under the assumption that ions may be ejected from the mass analyzer along the length of the slot. 
       FIGS.  6 A- 6 B  show different possible shapes of the electrodes  89  of the focusing lens  83 . The electrodes  89  shown in  FIG.  6 A  are plates that are separated by a gap  88  that is aligned parallel with the slot  44  of the rod electrode  41  from which ions are emitted. The electrodes  89  depicted in  FIG.  6 B  comprise an alternative configuration having parallel bar-like or rod-like protrusions  89   b  that have a gap  88  therebetween that is aligned parallel with the slot  44 . The electrodes  89  as shown in  FIG.  6 B  may be wires or metallic deposits that are affixed to a printed circuit board (not shown) that has a slot with edges that essentially align with the edges of the gap  88 . 
     The simulated trajectories of a large number of individual ions that are emitted from the electrode slot  44  along its entire length been modeled using Simion® ion optics simulation software. The envelope of calculated simulated trajectories of incoming positive ions, as projected onto the plane of the drawing (the x, y plane; cf.  FIGS.  6 A  and  6 B), is represented by spatial region  84  in  FIG.  4 A . Likewise, the envelope of calculated simulated trajectories of incoming negative ions is, as projected onto the plane of the drawing, is represented by spatial region  86  in  FIG.  5 A . The outline of each of the envelopes  84 ,  86  encloses more than ninety percent of the calculated trajectories of the respective ions. The modelling of the trajectories of the positive ions ( FIG.  4 A ) and of the resulting emitted secondary electrons ( FIG.  4 B ) was performed assuming applied voltages of −12000 V at the focusing-lens electrode(s), −12000 V at the dynode  160  and +8000 V at the scintillator electrode  32 . The modelling of the trajectories of the negative ions ( FIG.  5 A ) and of the resulting emitted secondary electrons ( FIG.  5 B ) was performed assuming applied voltages of +12000 V at the focusing-lens electrode(s), +12000 V at the dynode  160  and +16000 V at the scintillator electrode  32 . Separate simulations of ion trajectories were performed assuming the absence of any focusing-lens electrodes. 
     The simulated trajectories of incoming ions indicate that, under the influence of the electrical potential applied to the focusing-lens electrode(s)  83 , essentially all of the incoming ions, either positive or negative, strike the dynode  160  within its central region. The beneficial effect of the focusing lens may be observed by comparing the simulation results depicted in  FIGS.  4 A and  7 A , which account for the presence of the focusing lens, with the simulated results in  FIG.  7 B , which assume the absence of a focusing lens. Note that  FIGS.  4 A and  7 A  both pertain to the same set of simulation data, relating to the introduction of positively-charged ions into the apparatus  80 .  FIG.  4 A  shows the envelope of calculated trajectories in side elevational view (i.e., as projected onto the x, y plane) and  FIG.  7 A  shows the envelope in top view (i.e., as projected onto the x, z plane).  FIG.  7 B  illustrates a few representative calculated ion trajectories that were calculated under the assumption that no focusing lens is present. In this case, the full envelope of all simulated trajectories is too complex to easily display. Note that, since the trajectories are projected onto the plane of the drawing, the ion trajectories that appear to pass “through” certain components represent trajectories that are either in front of or behind the particular component, in three dimensions. Generally speaking, the simulations indicate that, when using the negative ion detection mode in the absence of the focusing lens, only a small proportion of the incoming ions impact upon the dynode  160 . The remainder of the incoming ions are deflected away from the dynode by the attractive potential of the positively biased scintillator. This result is due to: (a) the influence of the electric field of the scintillator, which is otherwise partially shielded by the focusing lens; and (b) the lower kinetic energy and velocity of the incoming negatively charged ions in the absence of the focusing lens. Because of the latter factor, the ions lack sufficient kinetic energy, in the absence of the focusing lens, to maintain their trajectories when they see the potentials from the dynode and the scintillator. 
     The focusing lens  83  of the illustrated detector configuration provides two functions. Firstly, it serves as the incoming ion accelerator and helps to partition the field as described above. Secondly, it acts as a beam-shaping ion optic that converges the spread of incoming primary ions to a well-focused beam, thereby resulting in higher detection efficiency. Because of these functions provided by the focusing lens, the dynode may be moved much closer to the mass analyzer than would otherwise be possible while still achieving adequate focusing. 
     After identifying impact zones of both positive and negative ions on the dynode  160  by ion trajectory simulations, the inventors utilized the identified impact zones as to set boundary conditions for calculations of the trajectories of secondary electrons emitted from the dynode. The envelopes of the calculated electron trajectories are shown as envelope  85  ( FIG.  4 B ) for electrons emitted in response to a flux of positive ions and envelope  87  ( FIG.  5 B ) for electrons emitted in response to a flux of negative ions. The outline of each of the envelopes  85 ,  87  encloses a substantial portion of the calculated trajectories of the corresponding respective secondary electrons. The electrode voltages that were assumed for purposes of the calculations are given in the discussion above. 
     Additional ion-trajectory and electron-trajectory simulations (not shown) using different inter-electrode spacings indicate that the scintillator needs to be placed close enough to the dynode (e.g., within about 7 mm, depending on the applied voltages) to ensure minimum field penetrations from the other electrodes that would otherwise distort the extraction field. At the same time, in order to prevent electrical arcing, there is a minimum allowable minimum distance (approximately 3-4 mm, depending on the applied voltages) at the point of closest approach (i.e. the left side of dynode  160 , as depicted in the drawings) of the dynode to the scintillator electrode  32 . Further, the results of the simulations indicate that optimal results are achieved when the dynode and focusing lens are maintained at substantially a same electrical potential and when the scintillator electrode is maintained at a potential that is slightly more “positive” than (or slightly less negative than) the potential of the dynode. By meeting these constraints, the simulation indicate that this detector can perform effectively for detection of both positive and negative ions. 
       FIGS.  8  and  9    are flow diagrams of methods for detecting ions emitted from a mass analyzer in accordance with the present teachings. Initial Step  102  of the method  100  ( FIG.  8   ) comprises setting electrical potentials applied to a conversion dynode, a scintillator electrode and a focusing lens or electrode of a Daly detector so as to detect negative ions, wherein the Daly detector further comprises a scintillator and a photodetector and wherein the conversion dynode comprises a physical configuration that causes it to emit secondary electrons upon impingement thereon of either positive ions or negative ions, depending upon the applied voltages. In the subsequent step  104 , negatively-charged ions emitted from a mass analyzer are transferred into the Daly detector through the focusing lens while, at the same time, the photodetector detects photons emitted from the scintillator in response to the reception of secondary electrons emitted from the conversion dynode in response to impingement of negative ions onto the conversion dynode. The photodetector may comprise any device that may suitably detect wavelengths of light emitted by the scintillator. Accordingly, the photodetector may comprise, without limitation, a photomultiplier tube or any suitable semiconductor light sensor device, either as a standalone sensor or as a plurality of individual sensors in an array format. Suitable semiconductor light sensor devices include, without limitation, solid-state silicon photomultipliers, charge-coupled devices, active-pixel sensors and photodiodes of various types. 
     The method  100  may optionally terminate after execution of the Step  104 . However, in an optional extension of the method, positive ions emitted from the mass analyzer may be detected in optional Steps  106 - 108 . In optional Step  106 , the electrical potentials on the electrodes and on the conversion dynode of the Daly detector are reconfigured so as to enable the detector to detect positive ions. Then, in Step  108 , positive ions emitted from the mass analyzer are transferred into the Daly detector through the focusing lens while, at the same time, the photodetector detects photons emitted from the scintillator in response to the reception of secondary electrons emitted from the conversion dynode in response to impingement of negative ions onto the conversion dynode. 
     A related method  110 , in accordance with the present teachings, for detecting ions emitted from a mass analyzer is outlined in  FIG.  9   . The method  110  is similar to the optionally extended version of method  100  ( FIG.  8   ) except that the detection of positive ions occurs prior to the detection of negative ions. Thus, steps  112 - 114  of the method  110  are analogous to the steps  106 - 108  of the method  100  and steps  116 - 118  of the method  110  are analogous to the steps  102 - 104  of the method  100 . Of course, any of the methods outlined in  FIGS.  8 - 9    (i.e., the method  110 , the abridged form of method  100  and the extended form of method  100 ) may be repeated any number of times. Likewise, any sequence of execution of these methods may be performed in any order. 
     Any patents, patent applications, patent application publications or other literature mentioned herein are hereby incorporated by reference herein in their respective entirety as if fully set forth herein, except that, in the event of any conflict between the incorporated reference and the present specification, the language of the present specification will control.