CHAPTER D4 ORTHOGONAL TIME OF FLIGHT OPTICS

Transcription

1 Back to Basics Section D: Ion Optics CHAPTER D4 ORTHOGONAL TIME OF FLIGHT OPTICS TABLE OF CONTENTS QuickGuide Summary Introduction The physical basis of orthogonal TOF Pulsedmainbeamsofions Rate of application of the pulsed field gradient. 421 Microchannel plate ion collector Resolutionbym/zvalue MS/MSoperation Advantages of orthogonal TOF arrangements. 425 Conclusion Micromass UK Limited Page 411

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3 Quick Guide Orthogonal TOF is the name commonly given to orthogonally accelerated Time-of-Flight mass spectrometry. It is sometimes referred to by the acronym oatof, especially in official publications, but it is more usual for it to be referred to simply as orthogonal TOF; this abbreviation is used here. In this Quick Guide, and purely for the purposes of illustration, orthogonal TOF optics are compared with those from magnetic sector instruments. Greater details of orthogonal TOF analysers hybridized with other kinds of ion optics are given in the relevant sections of Back-to-Basics In conventional mass spectrometry with electric and magnetic sectors arranged in-line (see Back-to-Basics, Ion Optics), an ion beam consists of a stream of ions of all m/z values, which is separated into individual m/z values by the magnetic sector before being collected by single-point or multipoint detectors (see Back-to-Basics, Point Ion Collectors and Array Collectors). In time-of-flight mass spectrometry, ions of different m/z values are detected as a function of their velocities along a flight tube (see Back-to-Basics, Time of Flight Instruments). Thus, it can be said that conventional magnetic sectors separate ions into individual m/z values by dispersion in space (spatially) and not according to their flight times. Contrarily, TOF analysers separate ions of different m/z values according to their velocities (temporally) but not spatially. These two types of analyser are frequently used alone but can be used in tandem, with ions from a first magnetic analyser passing through a region, in which there is applied an electric field at right angles to the direction of the ion beam. This orthogonal electric field is pulsed at very short time intervals. Ions accelerated from the first analyser have a velocity, which is proportional to the initial accelerating voltage in the ion source. On reaching the orthogonal zone, the pulsed electric field gives these ions a further velocity but now in a direction at right angles to their original velocity. The resultant velocity is given by the vector sum of the initial and second velocities. Micromass UK Limited Page 413

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5 After the pulsed electric field has been applied, a pulse of ions is directed into a TOF analyser placed an angle to the original ion beam. When the pulse is off, the ions have only their original velocities and continue into a different ion collector. The pulsed ions start their journeys down the TOF flight tube all at the same time; they separate in the TOF analyser according to their velocities and arrive at the TOF ion collector at different times (temporally separated). Therefore the orthogonal TOF mass spectrum is a snapshot of all the ions in the sampled ion beam at any one moment in time. The arrangement has advantages over either magnetic sectors alone or TOF instruments alone (see Back-to-Basics, Orthogonal TOF Hybrid Instruments, for further discussion). An orthogonal acceleration time-of-flight mass spectrometer can be used with continuous ion sources with a high sampling efficiency (typically 20-30%). Consequently the orthogonal TOF has a much higher duty cycle than a scanning instrument, which may have a duty cycle of only 0.1-1% when used to record a mass spectrum. This means that the sensitivity will be much higher for the orthogonal TOF mass spectrometer. Pulses of ions can be directed into the TOF analyser at the rate of about 30 KHz and therefore, more than 30,000 spectra per second can be collected and summed. There are significant improvements in signal-to-noise ratios and speed of acquisition of data. Summary In combined sector/tof analysers a beam of ions accelerated from an ion source by an electric field and sent into a sector instrument is further subjected to a second pulsed electric field applied at right angles to its initial direction. The resultant pulse of ions sets off along the flight tube of a TOF analyser, where the ions separate into m/z values and are recorded (along with their respective abundances) as a mass spectrum. The combined sector/tof analysers have several significant advantages, not least for MS/MS studies and improved signal-to-noise ratios. Micromass UK Limited Page 415

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7 ORTHOGONAL TIME-OF-FLIGHT OPTICS Introduction Ions produced in an ion source may be separated into their m/z values by a variety of analysers. The resultant set of m/z values, along with the numbers (abundances) of ions forms the mass spectrum. The separation of ions into their individual m/z values has been effected by analysers utilizing magnetic fields or RF electric fields. For example, the mass analysis of ions by instruments using a magnetic field is well-known, as are instruments having quadrupole RF electric fields (quadrupole, ion trap). Ions may also be dispersed in time, so that their m/z values are measured according to their flight times in a time-of-flight instrument. These individual pieces of equipment have their own characteristics and are commonly used in mass spectrometry. In addition, combinations of sectors have given rise to hybrid instruments. The earliest of these was the double-focusing mass spectrometer having an electric sector to focus ions according to their energies and then a magnetic sector to separate the individual m/z values. There is now a whole series of hybrid types, each with some advantage over non-hybrids. Ion collectors have seen a similar improvement in performance and any of the above analysers may be used with ion detectors based on single electron multipliers or in the case of magnetic sectors, on arrays of multipliers or, in the case of ICR, on electric field frequencies. Thus, there is a bewildering variety of instruments potentially available. However, except for very highly specialized purposes, most of the possible hybrids are not used in general mass spectrometry and only a few types are in common use. One of these is the so-called orthogonal TOF instrument. The purpose of the present Back-to-Basics guide is to describe the orthogonal arrangement and to discuss some of its advantages. Actual operation of hybrid orthogonal TOF instruments is discussed under their own headings in other sections of Back-to-Basics. Micromass UK Limited Page 417

8 a v 2 = 2z ev 2 /m v 1 = 2z ev 1 /m v= (v 1 2 +v 2 2 )= 2ze(V 1 +V 2 )/m tan a = v 2 /v 1 = V 2 /V 1 Figure 1 An ion beam is produced by accelerating ions of charge ze from an ion source through an accelerating voltage of V 1 volts so that they have kinetic energy corresponding to zev 1 and a velocity v 1 = (2zeV 1 /m). In an orthogonal acceleration chamber, the ions are subjected to a pulsed electric field with accelerating voltage of V 2 volts, which gives them additional kinetic energy zev 2 with a velocity v 2 = (2zeV 2 /m) at right angles to their original direction. The ion beam has a resultant energy of ze(v 1 +V 2 ), and a resultant velocity component v = (2ze(V 2 +V 1 )/m), in a direction α degrees from the initial direction, where tan α = (V 2 /V 1 ). Micromass UK Limited Page 418

9 The physical basis of orthogonal TOF Consider a stream of ions emitted from an ion source as a beam. The ions are produced continuously and are first accelerated through an electric field of V 1 volts. If the number of charges on an ion is z and e is the charge on the electron then the energy acquired by an ion after acceleration through a field of V 1 volts is zev 1. This energy must be equal to the kinetic energy (mv 2 /2) gained by the ion, where m is the mass of the ion and v is its final velocity. Thus, v = (2zeV 1 /m) andits momentum, mv = (2zemV 1 ). Therefore, the beam consists of a range of ions having momenta proportional to the charge, mass and accelerating voltage. As the beam is produced continuously, there is no separation of ions in time (no temporal separation). This is the beam shown as an arrow with velocity (2zeV 1 /m) infigure1. A magnetic sector, for example would separate the ions in space because the effect of the magnetic field is to bend the flight path in proportion to mass, charge and accelerating voltage (see Back-to-Basics, Ion Optics). Now consider ions emitted from an ion source not as a beam but as a pulse, so that all ions are accelerated through the potential V 1, applied as a pulse. Thus all ions start out from the ion source at exactly the same time and then pass along a flight tube of length d.the times taken for the ions to reach a collector is given by t = d/v,where v = (2zeV 1 /m), as above. This is the basis of the time-of-flight instrument (TOF), in which ions of different m/z values are separated according to t. Let there be an electrode placed so that its (pulsed) electric field gradient (direction) is at right angles to the continuous beam. If the electric potential at this point is V 2 volts, then a pulse of ions will be given additional energy zev 2,andavelocity (2zeV 2 /m) inadirection at right angles to the main beam. The vectorial resultant velocity of the ions and direction of travel of the pulsed set are shown in Figure 1. It can be seen that, effectively, a section of the main ion beam is selected and pulsed away (Figure 2). Ions in this pulsed set all start at the same instant and can be timed by a TOF instrument. The flight times give m/z values and the numbers of ions give the abundances; the two are combined to give a mass spectrum. Micromass UK Limited Page 419

10 Pulsing electrode, off Continuous ion beam Pulsing electrode, on Direction of field gradient Continuous ion beam Pulse of ions TOF drift tube zev 1 zev 1 (a) (b) Microchannel ion collector plate Figure 2 In (a), the pulsing electrode is switched off and a continuous ion beam of energy, zev 1,passesbyit. In (b), the electrode has been pulsed for a few microseconds, with a field gradient at right angles to the main beam. This has caused a section of the ion beam to travel in the direction shown, the direction being determined by the magnitudes of the voltages V 1 and V 2 (see Figure 1). The detached segment of the main beam enters the flight tube of a TOF instrument. The m/z values are determined from the times taken for the ions to reach the microchannel plate ion collector after initiation of the pulse. Micromass UK Limited Page 420

11 Pulsed main beams of ions Although the above has considered only the use of a continuous main ion beam, which is then pulsed, it is not necessary for the initial beam to be continuous; it too can be pulsed. For example, laser desorption uses pulses of laser light to effect ionization and the main ion beam already consists of pulses of ions passing the orthogonal pulsing electrode. As shown below, this is of no consequence because these pulses of ions can again be directed into a TOF flight tube just as though they had formed part of a continuous ion beam. Rate of application of the pulsed field gradient Clearly, the pulsing electrode may be turned on and off at any frequency chosen but there are some constraints on the frequencies actually used. At the fastest, there is little point in pulsing the electrode at such a rate that one lot of ions has not had time to travel the length of the flight tube before the next lot is on its way. With the physical dimensions of typical flight tubes and the magnitudes of accelerating voltages commonly used in mass spectrometers, an upper limit of about 30 kilohertz is found. The slowest rate of pulsing the electrode is almost anything! At 30 KHz, the TOF instrument can measure one mass spectrum every 33 microseconds or, put another way, in one second the TOF instrument can accumulate and sum 30,000 spectra. Little wonder that the acquisition of a spectrum appears to be instantaneous on the human time scale. Microchannel plate ion collector A fuller description of the microchannel plate forms part of a separate Back-to-Basics guide, Multipoint Collectors. Briefly, ions travelling down the flight tube of a TOF instrument are separated in time. As each m/z collection of ions arrives at the collector, it may be spread over a small area of space (Figure 3). Therefore, so as not to lose ions, rather than have a single point ion collector, the collector is composed of an array of miniature electron multipliers (microchannels), which are all connected to one electrified plate so that, no matter where an ion of any one m/z value hits the front of the array, its arrival is recorded. The microchannel plate collector could be crudely compared to a satellite TV dish receiver in that radio waves of the same frequency but spread over an area are all collected and recorded at the same time; of course, the multichannel plate records the arrival of ions not radio waves. Micromass UK Limited Page 421

12 Microchannel collector block Plate electron collector Current out m/z b m/z c m/z a Pulsed ion flow Microchannel ion collectors Figure 3 The diagram represents a flow of ions of m/z a, b, c, etc., travelling in bunches towards the front face of a microchannel array. After each ion strikes the inside of any one microchannel, a cascade of electrons is produced and moves towards the back end of the microchannel, where they are collected on a metal plate. This flow of electrons from the microchannel plate constitutes the current produced by the incoming ions (often called the ion current but actually a flow of electrons). The ions of m/z a, b, c, etc., are separated in time and reach the front of the microchannel collector array one set after another. The time at which the resulting electron current flows is proportional to (m/z) and the strength of the current represents the abundance of ions striking the microchannel plate collector. Micromass UK Limited Page 422

13 Resolution by m/z value Since the microchannel plate collector records the arrival times of all ions the resolution depends on the resolution of the TOF instrument and on the response time of the microchannel plate. A microchannel plate with a pore size of 10 µm or less has a very fast response time of less than 2 nanoseconds. The TOF instrument with microchannel plate detector is capable of unit mass resolution to beyond m/z MS/MS operation Figure 4 shows a diagrammatic representation of a typical MS/MS experiment, in which a main ion beam selected for ions (precursor ions) of mass m and having kinetic energy, zev, has been directed into a collision cell so as to cause fragmentation into two new species (products) of mass m 1 and m 2 with charges z 1 and z 2 respectively (z 1 or z 2 may be zero). The kinetic energies of the product ions can be written as z 1 ev' and z 2 ev" respectively. Without setting a pulsed electric field gradient orthogonal to the main beam, these fragment ions continue straight on. Application of a pulsed voltage to the electrode gives the ions a velocity component at right angles to their original direction. The vectorial resultant velocities form angles α 1 and α 2 to the original direction of the beam (see Figure 1). Although now directed along different paths (Figure 4), both beams of fragment ions strike the wide microchannel plate. The times of arrival at the plate are proportional to the (m/z) values of the masses involved and a mass spectrum of the product ions resulting from collisional activation is produced. It should be recalled that, after a single collision, the momenta and kinetic energies of product ions are different from the momenta or kinetic energies of the precursor ions but the velocities of the product ions are equal and equal to that of the initial precursor ions. There is no change in velocities of ions on fragmentation, only of momentum and kinetic energy. The TOF section can measure this mass spectrum in the normal fashion but, of course, it is a mass spectrum of the product ions resulting from fragmentation of precursor ions in the collision cell. Micromass UK Limited Page 423

14 Initial k inetic energy = zev Ion beam before collisional activation C ollisional ac tivation region Af ter collision, two com ponents, hav ing kinetic energy z 1 ev' and z 2 ev'' Puls ing elec trode α 1 α 2 Directions of f ragm ent ions on pulsing m 1 Direction of beam without pulsing m 2 Microchannel plate collector Figure 4 The diagram shows a mass-selected main ion beam (precursor ions) of kinetic energy, zev, entering a collisional activation region and being fragmented to produce two fragment (product) ions, having kinetic energies equal to z 1 ev and z 2 ev". If no electric field is pulsed onto the electrode, the ions continue straight on. If a pulsed electric field is applied, ions of energy z 1 ev will be deflected through an angle α 1 and the ions of energy z 2 ev" will be deflected through an angle α 2 and into a TOF analyser tube. Both deflected beams are detected at the microchannel plate collector. Micromass UK Limited Page 424

15 Advantages of orthogonal TOF arrangements As indicated above, specific orthogonal TOF instruments are covered in greater detail in the section on hybrid instruments. However, it may be noted that the orthogonal TOF instrument provides significant advantages for MS/MS operation in the examination of trace quantities of materials and as an adjunct to instruments in which the ion sources do not yield a steady ion current but rather pulsed sets of ions (laser desorption, radioactive desorption, sputtering). Even for continuous ion sources, vagaries of the ion current are smoothed out through the accumulation of, say, 30,000 spectra at 33 microsecond time intervals in a space of one second. The summed spectra are printed out as one mass spectrum. There is often a significant gain in signal-to-noise ratio for the orthogonal TOF system. Conclusion Snapshots of a beam of ions may be taken by accelerating in pulses, sections of the beam, away from the main stream. The accelerating voltage to do this is applied as electric field pulses on an electrode. The pulsed field gradient is at right angles (orthogonal) to the direction of the main beam. The pulsed ions are analysed in a time-of-flight tube and collected by a microchannel plate detector. The orthogonal TOF arrangement may be used in connection with a variety of other kinds of mass spectrometer to produce useful hybrid instruments. There are distinct advantages to these hybrids, compared with the separate instruments alone. Micromass UK Limited Page 425

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