Efficient creation, transport, containment, and detection of ions are issues at the core of mass spectrometry. Improvement in any of these areas leads to improved overall performance of the mass spectrometer. As in most sequences of operations, maximum performance benefits usually are obtained with improvements at the beginning of the sequence. As a result, improvements in the efficiency of ionization often result in large rewards in instrument sensitivity. As electrospray ionization (ESI) progressed through its first few generations of source design, the instrument performance increases were substantial. Once the ions are created, it is the role of ion lenses to extract ions from the source efficiently and focus them as they pass through the mass spectrometer. As one might expect, ion lenses of diverse designs have evolved to match ionization sources of different types. Lens designs also vary with the masses, charges, and velocities of the ions to be focused. Advanced texts and computer programs are available for a broad overview (1—4). The tutorial of Wollnick (5), the review by Burgoyne and Hieftje for mass spectrographs (6), and the detailed study of ion deceleration lenses by O'Connor and colleagues (7) are especially useful for mass spectrometrists interested in instrument design.

Kenneth L. Busch

In "beam" mass spectrometers, ions are first created in the ionization source, then moved elsewhere in the instrument for sorting by mass (the mass analyzer), and are finally sent off to the detector. Thus, beam instruments physically transport either a continuous stream, or a series of packets of ions, through the components of the instrument, proceeding from ionization source through the mass analyzer to the detector. In contrast, "in time" instruments can store ions within a cell or a trap, but only after the ions are transported there from the separate ionization source. Mass analysis occurs as the instrument monitors the behavior of this trapped ion population with time. However, transport of ions out of the cell might still be required for ion detection. If the detection is done in situ, then the elements of the holding cell also function as ion lenses. The trapped ions never cease their movement, and scattering collisions with neutral molecules reduce the trapping efficiency.

Figure 1

The set of coordinates used to chart ion movement within a mass spectrometer is shown in Figure 1, which shows that "x" usually is chosen to correspond to the direction of ion movement from the source toward the detector, and "y" and "z" represent the mutually orthogonal axes. In a magnetic sector instrument, "y" traditionally was set to be the axis orthogonal to the magnet pole faces; in Figure 1, this is set as the vertical axis from the point of view of the instrument operator. In a beam instrument, therefore, ions move along the x-axis from source to detector, motivated by the accelerating potential. Additionally, of course, the ion path can be curved (as through a magnet), or oscillating (as through a quadrupole mass filter), and therefore, the ions move in both directions on the y- and z-axes. In a reflecting time-of-flight mass spectrometer, the ion path turns back on itself, and ions therefore move in both +x and –x directions to reach the detector. The goal of ion lenses is to maximize transmission and shape the ion beam in (x, y, and z) as it passes from the source to the detector. Ion lenses can include both potential or magnetic elements, reflecting the fact that moving ions will respond with a change in direction to either an applied potential field or an applied magnetic field, following the fundamental laws of physics.

We concentrate here exclusively on potential-driven lenses. Mass spectrometers usually contain multiple sets of ion lenses, and more complex lens structures often are assembled from combinations of simpler elements. In this overview, we will describe four simple types of ion lenses, starting with source lenses, which act primarily to extract ions from the ionization source. Source lenses influence the divergence of an ion beam in the "z" or "y" directions, and pften are designated by that coordinate name (z-lens or y-lens) when they are used immediately after the ionization source. Next, we will describe the chicane lens, which acts to remove the line-of-sight from an ion beam. Removal of a line-of-sight is often important in the reduction of background signals. We'll provide a short description of the einzel lens, commonly used to restrict the energy width of an ion beam. Finally, we will describe a multicomponent ion funnel used to transport ions from a relatively high-pressure electrospray ionization (ESI) source into the lower-pressure mass analyzer of an instrument.

The potentials applied to potential lenses can be static or dynamic, or even a combination of both. Static operation means that the device potentials are fixed; a dynamic mode means that the applied potentials can vary with time. One reason to adopt a dynamic mode of operation is to maintain lens efficiency across the full range of ion masses in the beam that we wish to focus. For example, in an inductively coupled plasma–mass spectrometry (ICP-MS) system, the analytical range of atomic ion masses varies over about 1.3 orders of magnitude. Therefore, a dynamic ion lens maximizes ion transmission in the full ion mass range. An organic mass spectrometer deals with an even broader mass range (encompassing about 2.5 orders of magnitude), and the need for dynamic operation might be even more pronounced. Ionization sources might also produce multiply charged ions, and the ion lens system can even be designed and operated to discriminate between various charge states. Finally, remember that there is a fundamental limit to how tightly an ion beam can be focused, as the Coulombic repulsion between same-charged ions will always tend to cause spatial divergence in the ion beam.