CHAPTER D3 TOF ION OPTICS

Transcription

1 Back to Basics Section D: Ion Optics CHAPTER D3 TOF ION OPTICS TABLE OF CONTENTS QuickGuide Summary Background EquationsofMotionofIons Resolution Reflectron Comparison with other mass spectrometers Conclusion Micromass UK Limited Page 397

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3 Quick Guide In a Time-Of-Flight (TOF) mass spectrometer, ions formed in an ion source are extracted and accelerated to a high velocity by an electric field into an analyser consisting of a long straight drift tube. The ions pass along the tube until they reach a detector. After the initial acceleration phase, the velocity reached by an ion is inversely proportional to its mass (strictly, inversely proportional to the square root of its m/z value). Since the distance from the source to the detector is fixed, the time taken for an ion to traverse the analyser in a straight line is proportional to its velocity and hence its mass (strictly, proportional to the square root of its m/z value). Thus, each m/z value has its characteristic time-of-flight from the source to the detector. In effect, the ions race each other along the drift tube but the winners are always the ions of smallest m/z value since these have the shortest flight times. The last to arrive at the detector are always those of greatest mass which have the longest flight times. However, as in a race, for there to be a separation at the finish line (the detector), the ions must all start from the ion source at the same time (no handicapping allowed!). The times taken for ions of differing m/z values to reach the detector are of the order of a few microseconds and the separation in times of arrival for ions of differing m/z value is less than this. Thus, if ions of different mass are to be separated adequately in a time domain (good resolution), they should all start from the ion source at exactly the same time or, more practically, within a few nanoseconds of each other. Ions for TOF mass spectrometry must be extracted from the ion source in instantaneous pulses. Therefore, either ions are produced continuously but are extracted from the source in pulses or ions are produced directly in pulses. TOF mass spectrometry is ideally suited to those ionization methods that inherently produce ions in pulses, as with pulsed laser desorption or Cf-radionuclide ionization. Micromass UK Limited Page 399

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5 There is no theoretical upper limit on m/z that can be examined and TOF mass spectrometry is useful for substances having very high molecular mass. In practice, the current upper limit is about 350,000. Unfortunately, ions even of the same m/z value do have a spread of velocities after acceleration and therefore the resolution achievable with TOF is not very high because bunches of ions of one m/z value overlap those at the next m/z value. By use of an electrostatic ion mirror, called a reflectron the velocities of ions of the same m/z value can be made more nearly equal, thereby making their arrival times at the detector more nearly equal and so improving resolution. After reflection in the reflectron, the ions must pass down a second length of analyser set at a small angle to the first so as to reach the detector. The improvement in resolution with the reflectron is achieved at the expense of some loss in overall sensitivity due to loss of ions in the reflectron and in the second length of analyser. For very high mass, when sensitivity is frequently critical, the reflectron is not used and lower resolution is accepted. The mass spectrum gives the abundances of ions for different times of arrival at the detector. Since the times are proportional to the square root of the m/z values, it is simple to convert the arrival times into m/z values. Summary After acceleration through an electric field, ions pass ( drift ) along a straight length of analyser under vacuum and reach a detector after a time which depends on the square root of their m/z values. The mass spectrum is a record of the abundances of ions and the times (converted to m/z) they have taken to traverse the analyser. TOF mass spectrometry is valuable for its fast response time, especially for substances of high mass which have been ionized in pulses. Micromass UK Limited Page 401

6 Ion source Acceleration region Drift region Detector (a) (b) (c) (d) (e) Figure 1 The essentials of time-of-flight optics. In (a), a pulse of ions is formed and then accelerated out of the source (b) into the drift region (c). After a short time (d), the ions have separated along the drift region according to m/z value. In (e), the ions with smallest m/z value (fastest moving) begin arriving first at the detector, to be followed by the ions of gradually increasing m/z. Micromass UK Limited Page 402

7 TIME-OF-FLIGHT ION OPTICS Background Whereas understanding the fundamentals of the ion optics for magnetic and electric sector, quadrupole, ion trap and FT ion cyclotron resonance mass spectrometers ranges from fairly simple to difficult, the basic ion optics of Time-Of-Flight (TOF) instruments is very straightforward. Basically, ions are extracted from the ion source in short pulses and directed down an evacuated straight tube to a detector. The time taken to travel the length of the drift or flight tube depends on the mass of the ion and its charge. For singly charged ions (z = 1;m/z= m), the time taken to traverse the distance from the source to the detector is proportional to a function of mass, the greaterthemassoftheionthesloweritisinarrivingatthedetector. Thus, there are no electric or magnetic fields to constrain the ions into curved or complicated trajectories. After initial acceleration, the ions pass in a straight line, at constant speed, to the detector. The arrival of the ions at the detector is recorded in the usual way as a trace of ion abundance against time of arrival, the latter being converted into a mass scale to give the final mass spectrum. Equations of Motion of Ions A short pulse of ions is extracted from the ion source (Figure 1). It is necessary to use a pulse because, using only time to differentiate amongst the masses, it is important that the ions all leave the ion source at the same instant (like runners in a race on hearing a starting pistol). The first step is acceleration through an electric field (E volts). With the usual nomenclature (m = mass, z = number of charges on an ion, e = thechargeonanelectron,v= the final velocity reached on acceleration), the kinetic energy (mv 2 /2) of the ion is given by equation (1). mv 2 /2 = z.e.e (1) Equation (2) follows by simple rearrangement. v = (2z.e.E/m) (2) Micromass UK Limited Page 403

8 Slowest ions Single m/z value (a) Number of ions Fastest ions Spread in velocity Number of ions m/z Overlap (m+1)/z (b) Spead in velocity Number of ions m/z (m+1)/z (c) Increased overlap Spead in velocity (d) m/z (m+200)/z Number of ions Increased overlap Spead in velocity Figure 2 (a) Each set of ions at any one m/z value will have a small spread in speed because they are not all formed at exactly the same place in the ion source. (b) Two sets of ions separated by one mass unit (m, m+1) overlap because the slower ions of smaller mass overlap the faster ions of greater mass. (c) For larger m/z values, this effect leads to almost total disappearance of unit resolution. (d) At still greater m/z values, even mass differences of 200 or more may not be separated. Micromass UK Limited Page 404

9 If the distance from the ion source to the detector is d, then the time (t) taken for an ion to traverse the drift tube is given by equation (3). t = d/v = d/ (2z.e.E/m) = d.[ (m/z)]/ (2e.E) (3) In equation (3), d is fixed, E is held constant in the instrument and e is a universal constant. Thus, the flight time of an ion t is directly proportional to the square root of m/z (equation 4). t = (m/z) x a constant (4) Equation (4) shows that an ion of m/z 100 will take twice as long to reach the detector as an ion of m/z 25: (t 100 /t 25 = 100/ 25 = 10/5 = 2) Resolution Generally, the attainable resolving power of a TOF instrument is limited, particularly at higher mass. There are two major reasons for this, one inherent in the technique, the other a practical problem. First, the flight times are proportional to the square root of m/z. The difference in the flight times (t m and t m+1 ) for two ions separated by unit mass is given by equation (5). t m - t m+1 = t = [( m/z)-( (m+1)/z] x a constant (5) As m increases, t becomes progressively smaller (compare the difference between the square roots of 1 and 2 (= 0.4) with the difference between 100 and 101 (= 0.05). Thus, the difference in arrival times of ions arriving at the detector become smaller and smaller and more difficult to differentiate. This inherent problem is a severe restriction even without the second difficulty which is due to the fact that not all ions of any one given m/z value reach the same velocity after acceleration nor are they all formed at exactly the same point in the ion source. Therefore, even for any one m/z value, the ions reach the detector over an interval of time instead of all at one time. Clearly, where separation of flight times is very short, as with TOF instruments, the spread for individual ion m/z values means there will be overlap in arrival times between ions of closely similar m/z values. This effect (Figure 2) decreases available (theoretical) resolution but it can be ameliorated by modifying the instrument to include a reflectron (see below). Micromass UK Limited Page 405

10 Ion source Acceleration region Drift region Reflection (a) Detector (b) (c) (d) (e) (f) Figure 3 In (a), a pulse of ions is formed but, for illustration purposes, all with the same m/z value. In (b), the ions have been accelerated but, because they were not all formed in the same space, they are separated in time and velocity, some ions having more kinetic energy than others. In (c), the ions approach the ion mirror or reflectron which they then penetrate to different depths, depending on their kinetic energies (d). The ones with greater kinetic energy penetrate furthest. In (e), the ions leave the reflectron in a bunch (i.e., no longer separated in time) and travel on to the detector (f). The path taken by the ions is indicated by the dotted line in (f). Micromass UK Limited Page 406

11 Reflectron A homogeneous electrostatic field is placed at the end of the flight path of the ions and has a polarity the same as that of the ions, viz., positive (negative) ions experience a retarding positive (negative) potential. The ions come to a stop and are then accelerated in the opposite direction. The ions are reflected by the ion mirror or reflectron. The ion mirror is often at a slight angle (Figure 3) to the line of flight of the ions and, when reflected, the ions do not travel back along the same path but along a slightly deflected line (some instruments reflect ions back along the path they took to the reflection). Consider ions of any one given m/z value. The faster ions, having greater kinetic energy, travel further into the electrostatic field before being reflected than do slower ions. As a result, the faster ions spend slightly more time within the reflectron than do the slower ones. This causes the faster and slower ions to bunch up so that they leave the reflectron closer together and travel onto the detector, where the ions arrive close together. There is a similar effect for all other individual m/z values and overall resolution is greatly improved. Instead of a typical TOF resolution of about 1000, resolutions of around 10,000 can be achieved. There is a disadvantage in using a reflectron in that the sensitivity of the instrument is decreased through ion loss by collision and dispersion from the main beam. It is a more severe problem for ions of large mass and, for these, the reflectron is often not used despite the better resolution attainable with its use Comparison with other mass spectrometers TOF mass spectrometers are very robust and usable with a wide variety of ion sources and inlet systems. Having only simple electrostatic and no magnetic fields, their construction, maintenance and calibration are usually straightforward. There is no upper theoretical mass limitation; all ions can be made to proceed from source to detector. In practice, there is a mass limitation in that it becomes increasingly difficult to discriminate between times of arrival at the detector as the m/z value becomes large. This effect coupled with the spread in arrival times for any one m/z value means that discrimination between unit masses becomes difficult at say m/z At m/z 50,000 overlap of 50 mass units is more typical, i.e., mass accuracy is no better than about mass units (Figure 2). Nevertheless, the ability of a TOF instrument to measure routinely and simply masses this large gives it a decided advantage over many other types of mass analyser. Micromass UK Limited Page 407

12 Ion source Acceleration region Drift region Detector (a) (b) (c) (d ) (e) Figure 4 In this mode, ions are formed continuously in the ion source (a) but the electrostatic accelerating potential is applied in pulses (b). Thus, a sample of ions is drawn into the drift region (c) with more ions being formed in the source. As shown in figure 1, the ions separate according to m/z values (d) and arrive at the detector (e), the ions of largest m/z arriving last. Micromass UK Limited Page 408

13 A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ionsneedtoleavetheionsourceatthesametimeifthereistobeno overlap between m/z values at the detector. In turn, this means that, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utilized in pulses by switching the ion extraction field on and off very quickly (Figure 4). On the other hand, there are some ionization techniques that are very useful, particularly at very high mass, but only produce ions in pulses. For these sources, the ion extraction field can be left on continuously. Two prominent examples are Californium radionuclide and laser desorption ionization. In the former, nuclear disintegration occurs within a very short time frame to give a short pulse of ions; the same disintegration is used to start the timer ( stopwatch ) for the race of the ions down the flight tube. Similarly, a laser pulse lasting only a few nanoseconds produces a bunch of ions and acts to start the timer also. Conclusion By accelerating a pulse of ions into a flight or drift tube, the ions achieve different velocities depending on their individual m/z values; the bigger the m/z value, the slower the ion travels down the tube. A detector at the end of the tube records the times of arrival of the ions and hence their flight times. The flight times are easily converted into m/z information by a simple formula. The Time-of-Flight mass spectrometer is particularly useful for ions produced in pulses and where fast response times are needed. It can be used to detect ions of very large m/z values. Micromass UK Limited Page 409

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