TIME-OF-FLIGHT MASS SPECTROMETER WITH SPATIAL FOCUSING OF A BROAD MASS RANGE

[0001] 1. Field of the Invention

[0002] The invention relates to measurement methods for time-of-flight mass spectrometers which operate with pulsed ionization of superficially adsorbed analyte substances and with an improvement in the mass resolution by means of a time-delayed start of the ion acceleration; in particular with ion-accelerating voltages which change over time after a delayed start in order to obtain a rather constant mass resolution over broad mass ranges.

[0003] 2. Description of the Related Art

[0004] Time-of-flight mass spectrometers are often operated with pulsed ionization of superficially adsorbed analyte substances; methods for the ionization of samples by matrix-assisted laser desorption (MALDI) are known in particular. A plasma cloud, which expands and thus produces a distribution of the velocities of the plasma particles, is generated in the laser focus, said distribution being wider the further the plasma particles (ions and molecules) are from the surface. The velocity distribution means that the mass resolution can be improved by temporally delaying the start of the ion acceleration. Ions of a higher velocity then only pass through a portion of the accelerating field, and thus receive a lower additional acceleration, so the originally slower ions can catch up with them in a temporal focal point. Unfortunately, ions of different mass do not have exactly the same focal point. The focal points for ions of different mass can, however, be made to approach one another if ion-accelerating voltages are used which vary over time after a delayed start, particularly if they continuously increase or decrease (depending on polarity). In combination with a Mamyrin reflector, it is possible to obtain a high mass resolution which is approximately constant over large mass ranges (cf. documents DE 196 38 577 C1, GB 2 317 495 B or U.S. Pat. No. 5,969,348 A, J. Franzen, 1996).

[0005] The international patent application WO 2005/114699 A1 describes a standard ion lens system as a corrective ion optic element.

[0006] The invention is based on the finding that the accelerating field in the space in front of the sample support plate produces a lens effect in the typically round aperture of the accelerating electrode, and thus slightly defocuses the ion beam. Since fast ions with low masses leave this acceleration space quickly, the increasing accelerating field strength has a greater effect on the slow ions with large masses than on faster ions with low masses. This produces a broadening of the ion beam at right angles to the direction of flight, and the inventor has observed that this broadening increases with ion mass. The invention now proposes to compensate, to the desired extent, for the broadening of the ion beam with the aid of an additional ion-optical lens whose voltage is also varied over time. The lens can be an einzel lens, or more precisely an element of an einzel lens, or an acceleration lens, for instance.

[0007] For ions of a very broad mass range, it is quite possible to keep the ion beam at a diameter of approximately four millimeters (or less) by focusing with this additional lens while the ions pass through the first flight path, the reflector and the second flight path.

[0008] For some other operating modes, a diameter slightly above this minimum can be optimal. For example, at the point of reversal of the ions in the reflector, where the ions fly very slowly, the mass resolution may be reduced by the effect of the space charge if the ion beam is too narrow. Or the ion detector may be saturated by an ion density which is too high at some points. An optimum for the mass resolution and dynamic measuring range can thus be achieved by suitable variation of the function for the variable lens voltage. In any case, the beam diameter can be significantly reduced compared to an operating mode with static lens voltage.

[0009] In general, the reduction and homogenization of the beam diameter over a broad mass range produces better quantifiability of the ions because without these steps, the ion beam would broaden too much for it to be completely accepted or received by the geometry of the reflector and/or detector over a large mass range. The outer ions, especially at high charge-related masses m/z, would be lost to the measurement and thus also diminish its sensitivity.

[0015] While the invention has been shown and described with reference to a number of embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the scope of the invention as defined by the appended claims.

[0016] As has been set out before, since the varying accelerating voltage in the acceleration space produces a broadening of the ion beam at right angles to the direction of flight, and this broadening increases with the ion mass, the invention proposes to compensate, to the desired extent, for the broadening of the ion beam with the aid of an additional ion-optical lens whose voltage is also varied over time.

[0017] A greatly simplified schematic diagram of a MALDI time-of-flight mass spectrometer (MALDI-TOF) and a more detailed view of a corresponding ion source are shown in FIGS. 1 and 2. The samples on the sample support plate (1), which together with the accelerating electrode (2) is initially at a constant high voltage of around 20 to 30 kilovolts, are bombarded with nanosecond light pulses (12) of 1 to 10 nanoseconds duration from a pulsed UV laser (11). Each laser pulse creates a tiny plasma cloud at the impact location, and this cloud expands unhindered in the initially field-free space between sample support plate (1) and accelerating electrode (2). After a delay t_v of a few tenths of a microsecond, for example, the voltage on the accelerating electrode (2) is switched so that the ions are accelerated, whereby temporal focusing for ions of the same mass is achieved at a selectable location, for example location (14), in the known way. Most of the acceleration does not, however, usually take place between the sample support plate (1) and the accelerating electrode (2), but between the accelerating electrode (2) and the base electrode (3), which is at ground potential in normal operation. This is of no consequence for the invention, however. The different field strengths on either side of the accelerating electrode (2) produce a lens effect in the aperture of the accelerating electrode (2), causing the ion beam to become slightly divergent. An einzel lens (4, 5, 6) focuses the slightly divergent ion beam (7), which enters the Mamyrin-type reflector (8) after the first straight flight path, is reflected there and impinges on the ion detector (10) after a second flight path (9).

[0018] The location (14) for the temporal focus of the ions can be selected at will via the time delay and amplitude of the accelerating voltage. It is usual to select a location which, as shown in FIG. 1, is not too far away from the ion source. This location (14) for the temporal focus, through which ions of the same mass pass simultaneously but with slightly different energies, is imaged onto the detector (10) by the energy-focusing reflector (8) so as to be temporally focused again.

[0019] Unfortunately, the location (14) for the first temporal focusing of the ions is not at exactly the same position for ions of different mass. In fact, the focal length depends slightly on the mass of the ions. In order to make the location of the temporal focus approximately the same for ions of all masses, there is an operating mode in which the accelerating voltage is continuously varied after the delayed start of acceleration of the ions. The temporal variation of the accelerating voltage between sample support plate (1) and accelerating electrode (2) is depicted in the diagram of FIG. 3, by way of example. This ensures that the focal length for the temporal focusing of the ions becomes rather constant over a broad mass range, with the consequence that the mass resolving power is also consistently high over a large mass range, as desired. It is to be noted that, without delayed acceleration such as illustrated by curve (22) in FIG. 5, the temporal resolution as one of the most significant figures of merit for a TOF mass spectrometer is too low for most contemporary applications.

[0020] As has already been mentioned, the typically round aperture of the accelerating electrode (2) acts like a lens because the field strengths on either side of the accelerating electrode (2) are different. This causes the ion beam (7) to become slightly defocused. Since fast ions with low masses leave this acceleration space quickly, the increasing accelerating field strength has a greater effect on the slow ions with large masses than on faster ions with low masses. This produces a broadening of the ion beam at right angles to the direction of flight, and this broadening increases with ion mass; as depicted by the curve (20) in the diagram of FIG. 5.

[0021] The invention now proposes to compensate, to the desired extent, for the mass-dependent broadening of the ion beam by temporally varying the voltage of the middle element (5) of the einzel lens (4,5,6), which is used here by way of example. The lens voltage is varied during the spectral acquisition as a function of the time of flight and hence of the mass. As illustrated in FIGS. 1 and 2, the lens can be an einzel lens, but it is also possible to use an accelerating lens which does not have the same potential on both sides of the lens and represents part of the whole acceleration system. The lens voltage of an einzel lens is applied commonly only to the center diaphragm. An example of the temporal variation of the lens voltage is shown in the diagram of FIG. 4. The variation starts after a time delay at the lens of t_L. The time delay at the lens t_L can, in particular, be identical to the time delay t_v for the accelerating voltage. After the mass spectrum has been acquired, the lens voltage returns to the initial value again in preparation for the next laser pulse.

[0022] Different functions can be selected for the variation of the lens voltage. An exponential variation is simple to generate electrically, for example

$U_L=V_1+W_1\times \left\{1-\exp \left(-\frac{t-t_L}{t_1}\right)\right\},$

[0023] where the lens voltage U_L at time t_L starts with the base voltage V₁ and approaches the limit value (V₁+W₁) with the time constant t₁. As has already been mentioned, the time t_L can be identical to the time delay t_v. A curve of this type is shown in the time diagram in FIG. 4.

[0024] The time-of-flight mass spectrometer used, which is provided with ionization of the ions by matrix-assisted laser desorption, having a power supply for a delayed start and a varying accelerating voltage for the ions, and having a lens for spatial focusing of the ion beam, must therefore have a power supply for the lens which can supply a variable voltage on a short timescale, in the order of microseconds, during the spectral acquisition.

[0025] It should be noted here that a varying lens voltage requires a new mass calibration of the mass spectrometer, since a changed lens voltage has the effect of changing the dwell time of the ions in the lens. Such an adjustment is considered to be easily within the routine skill of a practitioner in this field, so no further explanation is required here.

[0026] The diagram in FIG. 5 shows the diameters of the ion beam as a function of the mass of the ions for three operating modes, as are produced from a simulation with the SIMION™ program. The bottom curve (22) shows the development of the beam diameter as obtained without applying the delayed acceleration, when the lens voltage is set correctly, for comparison purposes. The top curve (20) shows the increase in the beam diameter as the accelerating voltage increases after a delayed switch-on, but with a constant lens voltage. As can be seen, there is a comparatively narrow range of minimal beam diameter between about 1000 and 2000 atomic mass units. The middle curve (21), in contrast, which is obtained by optimum variation of the lens voltage, keeps the diameter of the ion beam at significantly less than four millimeters for ions of all masses by focusing with this additional lens while the ion beam passes through the first flight path, the reflector and the second flight path. This setting can be useful especially for applications which generate many spontaneously decaying ions in the ion source (also known as in-source decay: ISD).

[0027] For some operating modes, an ion beam diameter that is (slightly) larger than this minimum may be optimal. If, for example, high ion currents exist at the point of reversal of the ions in the reflector, where the ions fly very slowly, the effect of the space charge may cause the ions to mutually interfere, which leads to a reduction in the mass resolution. On the other hand, an ion detector, for example a multichannel plate, may be overloaded by too high an ion density at a particular point. In such cases, an optimum mass resolution, dynamic measuring range and/or sensitivity can be achieved by varying the temporal characteristic of the variable lens voltage. In any event, this achieves a significant improvement compared to the beam diameter as shown as curve (20) in FIG. 5, which results from an operating mode without temporal variation of the lens voltage.

[0028] In some commercial time-of-flight mass spectrometers, it is possible to reflect a slightly divergent ion beam in the reflector onto the ion detector by solid angle focusing (cf. documents U.S. Pat. No. 6,740,872 B1 or GB 2 386 750 B; A. Holle, 2001). To this end, the equipotential surfaces in the reflector, near the ions' point of reversal, are slightly curved. The focusing is ideal only for ion beams of a limited diameter, however. Setting of the lens voltage variation according to the invention can be used here to illuminate the reflector in an ideal way. An optimum setting can be found by measuring the mass resolution and the sensitivity under varied conditions.

[0029] A time-of-flight mass spectrometer can also be operated without a reflector (or with the reflector switched off) in linear mode. In FIG. 1, a second ion detector (13) is provided for this operating mode, and the ion beam travels on to this second detector when the operating voltage of the reflector (8) is switched off. The variation of the lens voltage according to the invention can be used here to optimally illuminate the ion detector for ions of all masses (or at least a large range of masses).

[0030] Many time-of-flight mass spectrometers with reflectors are also equipped for measuring daughter ions of selected parent ions. The parent ions are selected by a “parent-ion selector” (not shown) at the location of the first temporal focus (14). It is a fast deflector which deflects ions of all masses and removes them from the ion path, the only exception being the selected parent ions. Here too, a lens voltage varying according to the invention can improve mass resolution and sensitivity.

[0031] The invention has been shown and described with reference to a number of different embodiments thereof. It will be understood, however, that various aspects or details of the invention may be changed, or various aspects or details of different embodiments may be arbitrarily combined, if practicable, without departing from the scope of the invention. Generally, the foregoing description is for the purpose of illustration only, and not for the purpose of limiting the invention which is defined solely by the appended claims.