Back to Basics Ion Optics of Magnetic/Electric Sector MS Back to Basics Section D: Ion Optics CHAPTER D1 ION OPTICS OF MAGNETIC/ELECTRIC SECTOR MS TABLE OF CONTENTS QuickGuide ...........................369 Summary..............................369 Preamble..............................371 MassAnalysisofIons....................371 MagneticSector.......................371 ElectrostaticAnalyser(ElectricSector).....375 Magnet / Electrostatic Analyser Combination 375 ElectricFocusingLenses .................377 Y-Focus,Z-FocusandDeflectLenses .....377 Curvature and Rotation Lenses . 379 MetastableIons.........................379 EnergyFilter .........................379 Conclusion ............................379 Micromass UK Limited Page 367 Back to Basics Ion Optics of Magnetic/Electric Sector MS This page is intentionally blank. Micromass UK Limited Page 368 Back to Basics Ion Optics of Magnetic/Electric Sector MS 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 (m1,m2 ..... mn) or, more strictly, their mass-to-charge ratio (m1/z, m2/z ..... mn/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 Back to Basics Ion Optics of Magnetic/Electric Sector MS Field Current Direction of Deflection Figure 1 Fleming’s Left Hand Rule Micromass UK Limited Page 370 Back to Basics Ion Optics of Magnetic/Electric Sector MS ION OPTICS OF MAGNETIC/ELECTRIC SECTOR MASS SPECTROMETERS Preamble In the ion source, substances are converted into positive or negative ions having masses (m1,m2, ..... mn) and a number (z) of electric charges. From a mass spectrometric viewpoint, the ratio of mass to charge (m1/z, m2/z ....... mn/z) is important. Generally, z =1,inwhich case, m1/z = m1,m2/z = m2 ..... mn/z = mn, 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 In this section, magnetic and electric sectors and electric focusing of Ions lenses are discussed. Magnetic Sector 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). 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 (2) r Micromass UK Limited Page 371 Back to Basics Ion Optics of Magnetic/Electric Sector MS 5 4 3 Ion beam Magnet 2 1 Deflected ions Figure 2 Deflection in a magnetic field of an ion beam consisting of increasing mass-to-charge ratios, m1/z....m5/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 Back to Basics Ion Optics of Magnetic/Electric Sector MS From equations (1,2), the velocity of the ion can be eliminated to give the relationship (3). m B2r2 ----- = ------------ (3) z 2V 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. 2V rm= ------- (4) B2 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 r2)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 ∝ B2 (V constant) m ∝ 1/V (B constant) m ∝ B2/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). Micromass UK Limited Page 373 Back to Basics Ion Optics of Magnetic/Electric Sector MS (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 Back to Basics Ion Optics of Magnetic/Electric Sector MS 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 An electrostatic analyser (ESA) is a directional (or angular) focusing (Electric Sector) device and is also energy dispersive (Figure 4). As shown in equations (1,2), the energy gained by ions accelerated 1 from the ion source iszV = ---mv2 and in the electric sector, the 2 centrifugal force acting on the ions is given by equation (6), mv2 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 The ion beam is collimated when a magnetic analyser is combined Analyser Combination 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 Back to Basics Ion Optics of Magnetic/Electric Sector MS (HIGH ENERGY IONS) (IONS OF TUNED MASS) COLLECTOR SLIT OUTER ESA PLATE ELECTROMAGNET (LOW ENERGY IONS) β SLIT INNER ESA PLATE Y FOCUS α SLIT SOURCE SLIT (VARIABLE) SOURCE SLIT LENS (FIXED) FOCUS (BEAM CENTRE) SOURCE EI/CI Figure 5 Double focusing ion optics (forward geometry).