CHAPTER D1 ION OPTICS OF MAGNETIC/ELECTRIC SECTOR MS

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1 Back to Basics Section D: Ion Optics CHAPTER D1 ION OPTICS OF MAGNETIC/ELECTRIC SECTOR MS TABLE OF CONTENTS QuickGuide Summary Preamble MassAnalysisofIons MagneticSector ElectrostaticAnalyser(ElectricSector) Magnet / Electrostatic Analyser Combination 375 ElectricFocusingLenses Y-Focus,Z-FocusandDeflectLenses Curvature and Rotation Lenses MetastableIons EnergyFilter Conclusion Micromass UK Limited Page 367

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3 Quick Guide Substances are converted into species having positive or negative charges (ions) in the ion source. For an ion of mass (m) and a number (z) of positive or negative charges, the value m/z is an important mass spectrometric observable. Astreamofions(an ion beam) is directed out of the ion source towards a collector which records their arrival. As with a light beam and glass lenses, an ion beam can be directed and focused using electric and magnetic fields, often called lenses by analogy with their optical counterparts. The system of electric and magnetic fields or lenses is called the ion optics of the mass spectrometer. Electric lenses correct aberrations in the shape of the ion beam. Electric and magnetic fields can be used sequentially, as described in this issue. Crossed electromagnetic fields are described in the separate issue on quadrupoles. Another important property of electric and magnetic fields is their ability to separate ions according to their individual masses (m 1,m 2... m n ) or, more strictly, their mass-to-charge ratio (m 1 /z, m 2 /z... m n /z). After the ion source, the ion optics split the ion beam into its component m/z values (compare splitting white light into a spectrum of colours). By changing the strengths of the electric and magnetic fields, ions of different m/z values can be focused at just one spot (the collector). From the strengths of the electric and magnetic fields, m/z values are measured. A chart showing the number of ions (abundance) arriving at the collector and their respective m/z values is a mass spectrum. Summary The ion optics of a mass spectrometer cause the ion beam leaving the ion source to arrive at a collector after being separated into individual m/z values and focused. Micromass UK Limited Page 369

4 Field Current Direction of Deflection Figure 1 Fleming s Left Hand Rule Micromass UK Limited Page 370

5 ION OPTICS OF MAGNETIC/ELECTRIC SECTOR MASS SPECTROMETERS Preamble In the ion source, substances are converted into positive or negative ions having masses (m 1,m 2,... m n ) and a number (z) of electric charges. From a mass spectrometric viewpoint, the ratio of mass to charge (m 1 /z, m 2 /z... m n /z) is important. Generally, z =1,inwhich case, m 1 /z = m 1, m 2 /z = m 2... m n /z = m n, so that the mass spectrometer measures masses of ions. To do this a stream of ions (the ion beam) is injected into the mass analyser region, a series of electric and magnetic fields known as the ion optics. In this region, the ion beam is focused, corrected for aberrations in shape and the individual m/z ratios measured. The ion beam finally arrives at a collector which measures the number (abundance) of ions at each m/z value. The width and shape of the ion beam is controlled by a series of slits (object or source, collector, alpha etc.), situated between the ion source and the collector. A chart of m/z values and their respective abundances makes up the mass spectrum. Ion optics are considered in greater detail below. Mass Analysis of Ions Magnetic Sector In this section, magnetic and electric sectors and electric focusing lenses are discussed. When moving charged species (ions) experience a magnetic field, they are deflected. The direction of the deflection can be described by Fleming's left hand rule (Figure 1). Themagnitudeofthedeflectionisgovernedbythemomentumofthe ion and is described by the following equations (1,2). Firstly, the kinetic energy of the ion is equal to the energy gained through acceleration from the ion source (equation 1). zv = --mv2 1 2 Secondly, the centrifugal force on the ion as its path is deflected by a magnetic field is equal to the force exerted by the field on a moving charge (equation 2). mv = zb r (1) (2) Micromass UK Limited Page 371

6 Ion beam Magnet Deflected ions Figure 2 Deflection in a magnetic field of an ion beam consisting of increasing mass-to-charge ratios, m 1 /z...m 5 /z and split into different trajectories (1-5) respectively Object Slit α - Slit Focused Ion Beam (Collector Slit) Figure 3 Directional (or angular) focusing of a magnet. Micromass UK Limited Page 372

7 From equations (1,2), the velocity of the ion can be eliminated to give the relationship (3). m ---- z = B 2 r V Where: r = radius of arc of ions being deflected in the magnetic field V = accelerating potential applied to ions leaving the ion source B = magnetic field strength z = number of charges on an ion m = mass of any one ion v = velocity of an ion after acceleration through the electric field (V). If only ions with a single charge (z =1) are considered then, with a constant field strength and constant accelerating voltage, the radius of arc depends on mass and, from (3), equation (4) is obtained. r = m V (4) B 2 Thus, it is possible to separate ions of different mass (Figure 2) with ions arriving at position 1 (greater deflection) being of lower mass than those arriving at position 5 (lesser deflection). In the modern scanning mass spectrometer, it is more convenient that ions should arrive at a single point for monitoring (collection) and so r (or r 2 )is kept constant. This means that B and/or V must be varied to bring all ions to the same focus, viz., one of the relationships (5) must apply: m B 2 (V constant) m 1/V (B constant) m B 2 /V (5) From these relationships, (5), it can be seen that, if either the magnetic field (B) or the voltage (V) or both B and V are scanned, the whole range of masses of the ions may be brought into focus sequentially at a given point, the collector. Generally, a scanning magnetic sector mass spectrometer carries out mass analysis by keeping V constant and varying the magnetic field (B). A further property of the magnetic field is that a diverging ion beam entering that field leaves with the beam converging. Thus, the magnet is said to be directional (or angular) focusing (Figure 3). (3) Micromass UK Limited Page 373

8 (LOW ENERGY IONS) (HIGH ENERGY IONS) OUTER ESA PLATE SLIT INNER ESA PLATE Y FOCUS SLIT SOURCE SLIT (VARIABLE) Figure 4 Focusing and dispersion properties of an electrostatic analyser. Micromass UK Limited Page 374

9 So far, it has been assumed that all ions leaving the source have exactly the same kinetic energy but this is not really the case. In EI, the spread in kinetic energy can be as much as 1 volt and, with FAB can be as much as 4 volts. This spread results in a blurred image at the collector because the magnet has no energy focusing and ions of different kinetic energies are brought to slightly different foci. Thus a single magnetic sector has directional (or angular) focusing and therefore is said to be single focusing only. Electrostatic Analyser (Electric Sector) An electrostatic analyser (ESA) is a directional (or angular) focusing device and is also energy dispersive (Figure 4). As shown in equations (1,2), the energy gained by ions accelerated from the ion source is zv 1 = --mv 2 and in the electric sector, the 2 centrifugal force acting on the ions is given by equation (6), mv ze = (6) R where: E = electric potential (voltage) between the inner and outer ESA plates. R = radius of curvature of ion trajectory From these equations, the relationship (7) is obtained. 2V R = (7) E No mass or charge appears in this equation so that, in the electric sector, the ion flight path bends in an arc, which depends only on the accelerating voltage (V) and the ESA voltage (E). Magnet / Electrostatic Analyser Combination The ion beam is collimated when a magnetic analyser is combined with an ESA, the combination can be made both energy and mass focused, vis., the ion beam is collimated in the ESA and then properly focused in the magnetic field (Figure 5). The combination is called double focusing because it is both directional (or angular) and energy focusing. The double focusing mass spectrometer is designed such that ions of different energies (but of the same mass), converge at the collector (Figure 5). Double Focusing Forward Geometry ion optics is a combination, in which the ESA is placed before the magnet as shown in Figure 5. Micromass UK Limited Page 375

10 (HIGH ENERGY IONS) (IONS OF TUNED MASS) OUTER ESA PLATE ELECTROMAGNET (LOW ENERGY IONS) β SLIT INNER ESA PLATE COLLECTOR SLIT Y FOCUS SOURCE SLIT (VARIABLE) SOURCE SLIT (FIXED) α SLIT LENS FOCUS (BEAM CENTRE) SOURCE EI/CI Figure 5 Double focusing ion optics (forward geometry). HIGH ENERGY IONS ELECTROMAGNET OUTER ESA PLATE SOURCE SLIT (VARIABLE) 1st FFR GAS CELL Y FOCUS α SLIT LENS SOURCE SLIT (FIXED) FOCUS (BEAM CENTRE) SOURCE EI/CI LOW ENERGY IONS COLLECTOR SLIT (VARIABLE) INNER ESA PLATE CONVERSION DYNODE FARADAY COLLECTOR MULTIPLIER Figure 6 Double focusing ion optics (reverse geometry). Micromass UK Limited Page 376

11 Double Focusing Reverse Geometry ion optics is a combination, in which the magnet is placed before the ESA and is shown in Figure 6. The double focusing combination of electrostatic and magnetic sector analysers allows the inherent energy spread of the beam to be compensated for by design, and ensures that there is no spread in the beam at the collector arising from either of these sources. Electric Focusing Lenses Y-Focus, Z-Focus and Deflect Lenses It has been stated above that the focus of all masses will occur at a single position, the collector slit (Figures 5, 6). However, because the actual shape of the field within and around the pole tips of the magnet varies with changing field, especially at higher field strengths, the final focal point of the beam shifts as field strength changes. This leads to a change of focus with mass and affects the ability of the instrument to resolve small mass differences. On early mass spectrometers, the problem could be corrected by physically adjusting the position of the magnet for any given mass. On modern instruments, an electric field called the Y-focus is used to compensate for these imperfections (Figures 5, 6). The aim of this lens is to focus the ions at the same position (the collector slit) throughout the mass range. Thus, using the electric and magnetic sectors with a Y-focus lens ensures all ions are brought to the same focus and allows small differences in mass to be detected, viz., the resolution of the instrument is enhanced. On VG instruments, these lenses are sited before and after the magnetic sector. The focus and deflection lenses are used to steer the beam so that it coincides with the gap in the collector slit. The Z-focus lenses are used to change the divergence of the beam by adjusting voltages on lens plates situated on either side of the beam. The deflection lenses are used to move the whole beam; the Y-deflection lens is used to move the beam from one side to the other and the Z-deflection moves the beam up or down. The two lenses allow the ion beam to be aligned correctly with the collector slit. Voltages on the lens plates are adjusted to effect such movements of the beam (Figure 7). Micromass UK Limited Page 377

12 TYPE BEAM IN BEAM OUT Y Focus Y Deflect Z Focus Z Deflect Figure 7 Y-Focus, Z-focus and deflect lenses with their effects on the ion beam Micromass UK Limited Page 378

13 Curvature and Rotation Lenses Curvature and rotation lenses are used to correct for any imperfections (aberrations) in the cross-sectional shape of the beam before it reaches the collector slit. The curvature lens provides a means of changing any banana shaped beam cross-section into a rectangular shape (Figure 8). The rotation lens is used to rotate the beam such that the sides of the beam become parallel with the long axis of the collector slit (Figure 8). Metastable Ions Energy Filter An ion beam mainly comprises normal ions all having the same kinetic energy gained on acceleration from the ion source but there are also some ions in the beam with much less than the full kinetic energy; these are called metastable ions. An energy filter is a system of electrostatic fields which strictly has little to do with the main focusing fields but, rather, provides a means of discriminating between normal and metastable ions. The system is a filter, preventing metastable ions from being detected by the collector and it consists of a series of parallel lens plates to which is applied a decelerating voltage of 90-99% of the original accelerating potential (V). Normal ions have enough energy to pass through the filter, to reach the collector but metastable ions do not. Conclusion Through the use of sequential electric (electrostatic) and magnetic fields (sectors) and various correcting lenses, the ion beam leaving the ion source can be adjusted so that it arrives at the collector in focus and with a rectangularly shaped cross-section aligned with the collector slits. For the use of crossed electromagnetic fields, the relevant issue on quadrupole instruments should be consulted. Micromass UK Limited Page 379

14 Curvature Rotate Figure 8 Curvature and Rotate lenses and their effects on the ion beam. Micromass UK Limited Page 380