Method of operating a secondary-electron multiplier in the ion detector of a mass spectrometer

Abstract

The disclosure relates to a method of operating a secondary-electron multiplier in the ion detector of a mass spectrometer so as to prolong the service life, wherein the secondary-electron multiplier is supplied with an operating voltage in such a way that an amplification of less than 10.sup.6 secondary electrons per impinging ion results, while the output current of the secondary-electron multiplier is amplified using an electronic preamplifier mounted close to the secondary-electron multiplier with such a low noise level that the current pulses of individual ions impinging on the ion detector are detected above the noise at the input of a digitizing unit. Further disclosed are the use of the methods for imaging mass spectrometric analysis of a thin tissue section or mass spectrometric high-throughput analysis/massive-parallel analysis, and a time-of-flight mass spectrometer whose control unit is programmed to execute such methods.

Claims

1. A method to operate a secondary-electron multiplier having at least one multichannel plate in an ion detector of a time-of-flight mass spectrometer in order to prolong the service life, comprising: supplying the secondary-electron multiplier with an operating voltage in such a way that an amplification of less than 10.sup.5 secondary electrons per impinging ion is maintained, and amplifying an output current of the secondary-electron multiplier using an electronic pre-amplifier mounted in a vacuum system of the time-of-flight mass spectrometer in which the secondary-electron multiplier is located, or on a housing of said vacuum system, wherein a pre-amplifier amplification is chosen such that a resultant noise level allows current pulses generated by individual ions impinging on the ion detector to be detected above the noise at an input of a digitizing unit.

2. The method according to claim 1, wherein the digitizing unit operates at a digitizing rate of around four giga-samples per second or more.

3. The method according to claim 1, wherein the amplification of the secondary-electron multiplier is set to a maximum of 2×10.sup.4 secondary electrons per impinging ion.

4. The method according to claim 1, wherein the preamplifier is flange-mounted on the housing of the vacuum system.

5. The method according to claim 1, wherein operation of the preamplifier is improved by cooling the preamplifier.

6. The method according to claim 5, wherein cooling is effected by a Peltier element or other suitable cooling element, which is thermally coupled to the pre-amplifier.

7. The method according to claim 5, wherein the pre-amplifier is cooled to temperatures of −50 to −20 degrees Celsius.

8. The method according to claim 1, wherein improved amplification is achieved by mounting the preamplifier less than 40 centimeters from the secondary-electron multiplier.

9. The method according to claim 1, wherein an adjustment of the amplification is implemented via the acquisition of a mass spectrum with individual ion signals at specific times of the operation of the secondary-electron multiplier.

10. The method according to claim 9, wherein the desired amplification of the secondary-electron multiplier is set via a characteristic curve which reflects the logarithm of the amplification as a function of the operating voltage.

11. The method according to claim 10, wherein two different operating voltages are used to determine the gradient of the characteristic curve and to adjust the amplification.

12. The method according to claim 1, wherein the digitizing unit is one of (i) housed in a computer of the time-of-flight mass spectrometer, which is located several meters from the time-of-flight mass spectrometer itself, and (ii) accommodated in a plug-in module in the time-of-flight mass spectrometer itself, which is located around half a meter to one meter from the secondary-electron multiplier.

13. The method according to claim 12, wherein the secondary-electron multiplier is connected to a computer by a long lead carrying the output signal of the secondary-electron multiplier to the computer.

14. The method according to claim 13, wherein the lead is a 500 coaxial cable.

15. The method according to claim 1, wherein the pre-amplifier is designed so that it can be operated in a vacuum.

16. The method according to claim 1, wherein the at least one multichannel plate is a double multichannel plate in a chevron arrangement.

17. A time-of-flight mass spectrometer whose control unit is programmed to execute a method according to claim 1.

18. The time-of-flight mass spectrometer according to claim 17, further comprising a laser desorption ion source (LDI) to which the spectrometer is coupled.

19. The time-of-flight mass spectrometer according to claim 18, wherein the laser desorption ion source is an ion source for matrix-assisted laser desorption (MALDI).

20. A method to operate a secondary-electron multiplier having at least one multichannel plate in an ion detector of a time-of-flight mass spectrometer in order to prolong the service life, during an imaging mass spectrometric analysis of a thin tissue section or a mass spectrometric high-throughput analysis/massive-parallel analysis, comprising: supplying the secondary-electron multiplier with an operating voltage in such a way that an amplification of less then 10.sup.5 secondary electrons per impinging ion is maintained, and amplifying an output current of the secondary-electron multiplier using an electronic pre-amplifier mounted in a vacuum system of the time-of-flight mass spectrometer in which the secondary-electron multiplier is located, or on a housing of said vacuum system, wherein a pre-amplifier amplification is chosen such that a resultant noise level allows current pulses generated by individual ions impinging on the ion detector to be detected above the noise at an input of a digitizing unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a fresh characteristic curve of a conventional double multichannel plate in a chevron arrangement which consists of very fine channels only two micrometers in diameter. The amplification range is somewhat limited due to the very fine channels, but even here is between 4×10.sup.4 and 1×10.sup.7. Other types of multipliers have very similar characteristic curves.

(2) FIG. 2 is a theoretical representation, supported by measurements, of the change in the characteristic curves (20) to (29) as a multiplier ages. The representation is based on the characteristic curve in FIG. 1, and takes into account the fact that the aging (a) is faster, the higher the amplification at which the SEM is operated; and (b) is faster, the higher the operating voltage which must be set for a given amplification. It was, furthermore, assumed that the characteristic curves remain straight. The characteristic curves are arranged as they each result after a specific, identical operating time, for example periods of 100 hours in each case. If the SEM is operated at an amplification of 10.sup.6 (1 M), the SEM survives only two periods (31) and (32), thus in this example only around 200 hours; at an amplification of 10.sup.5 (100 K) it survives for around four periods (41) to (44); at an amplification of only 2×10.sup.4 (20 K) it survives for nine periods (broken line). The characteristic curves necessarily change their gradient because of the assumptions (a) and (b). Hitherto it has generally been assumed that the slope of the characteristic curve remained constant as the aging progressed.

(3) FIG. 3 is a schematic diagram of a conventional MALDI time-of-flight mass spectrometer according to the Prior Art. The samples are located on the sample support plate (1), opposite the accelerating electrodes (2) and (3), and can be ionized by the beam of laser light pulses (4) supplied by the laser (5). The ions are accelerated by the accelerating electrodes (2) and (3) to create an ion beam (8), which passes through a gas cell (9) which may, if required, be filled with collision gas, a parent ion selector (10), a daughter ion post-acceleration unit (11) and a parent ion suppressor (12), and is then reflected by the reflector (13) onto the ion detector (14). The mass spectrometer housing is evacuated by a powerful vacuum pump (15). In this example illustration, the ion detector (14) has a multichannel plate and a metal cone for reflection-free matching to a 50Ω coaxial cable (16). The 50Ω coaxial cable is several meters long and feeds the output current to a computer (17) containing the high-speed digitizing unit.

(4) In FIG. 4, a preamplifier (18) is attached to the outside of the vacuum chamber, close to the detector (14), for example flange-mounted near the detector so that the SEM can be operated at a much lower amplification in order to prolong the service life. The separation between preamplifier (18) and detector (14) is preferably less than 40 centimeters, in particular less than 30 centimeters.

(5) In FIG. 5, the preamplifier (19) is located in the vacuum system of the mass spectrometer, as close as possible to the detector (14), preferably less than 40 centimeters away, particularly less than 30 centimeters, thus facilitating a very low-noise operation.

DETAILED DESCRIPTION

(6) FIG. 2 is a theoretical representation, supported by measurements, of the group of characteristic curves (20) to (29) for the aging of a multiplier. The graph shows how each characteristic curve changes after specific operating periods of the same duration, for example after a period of around 100 operating hours in each case. The representation is based on two observations: (a) The aging occurs faster, the higher the amplification at which the SEM is operated. It is highly probable that this is because the greater number of secondary electrons which impinge at the end of the SEM causes a greater change in the work function of the active surface. (b) The SEM ages faster, the higher the operating voltage which must be set for a given amplification. This is probably because the energy of the impinging electrons is higher. A higher electron density and a higher electron energy accelerate the damage to the active surfaces, so lower yields of secondary electrons are achieved. If the SEM is operated at an amplification of 10.sup.6 (1 M), the SEM only survives for two periods (31) and (32), thus in this example only around 200 hours of operation; at an amplification of 10.sup.5 (100 K) it survives for around four periods (41) to (44); at an amplification of only 2×10.sup.4 (20 K) it survives for around nine periods (broken lines). The individual characteristic curves are more or less straight, but their gradient changes.

(7) The service life of a secondary-electron multiplier (multiplier, SEM) can thus be greatly prolonged if one succeeds in operating it at a voltage which is far below the usual operating voltage for SEMs. When the multiplier is operated at an amplification of 10.sup.5, or even only 2×10.sup.4, for example, instead of the usual amplification of around 10.sup.6, it should be possible to extend the service life by a factor of three to five, since the service life depends to a great extent on the current intensity of the emitted electrons and the amplitude of the operating voltage.

(8) However, at a low operating voltage, the pulse current of secondary electrons generated by a single ion is not sufficient to produce a digital signal which clearly stands out from the noise and can be unambiguously identified at the input of the digitizing unit. The digitizing unit generates four to six digital values in one nanosecond, depending on the type. Several computing cycles are required to address and store a digital value, however, so that even in very fast computers with 2×10.sup.9 operations per second, several independent databases have to be set up, to which the measurement data are fed in turn with overlap. For these reasons, the digitizing unit is accommodated in the computer of the mass spectrometer, which can be located several meters from the mass spectrometer itself. Additional electronic noise is generated by the line carrying the output signal of the SEM to the computer, which is several meters long, usually via a 50Ω coaxial cable.

(9) As has already been explained above, it has been found that the signal-to-noise ratio at the input of the distant digitizing unit can be improved by amplifying the output signal of the SEM at a sufficiently low noise level by means of a preamplifier located close to the SEM, and by operating the SEM at a correspondingly lower operating voltage so that the service life of the SEM is prolonged many times over. Since operating a preamplifier in a vacuum is a particularly low-noise mode of operation, the preamplifier may be even located in the vacuum system of the mass spectrometer, if possible, but at least on the housing of the vacuum system. The preamplifier must, however, operate at a high enough speed so as not to distort the electron current pulses. Preamplifiers of this type with a sufficiently large bandwidth are commercially available, see for example the TA2400 model from FAST ComTech GmbH (Oberhaching, Germany). If required, the preamplifier must be designed so that it can be operated in a vacuum.

(10) The preamplifier can be operated at a particularly low noise level by cooling it to temperatures of −50 to −20 degrees Celsius, for example. To this end, a Peltier element or other suitable cooling element, thermally coupled to the preamplifier, can be used, for example.

(11) The preamplifier selected must satisfy several criteria. First, the amplifier has to have sufficient bandwidth to amplify the pulse currents of secondary electrons without any distortion. The pulses from individual ions have full width at half-maximum values below one nanosecond. Furthermore, the amplifier must operate with very little noise. Since the preamplifiers generally contribute more noise, the greater the amplification, a compromise must be made between amplification and low noise. Experiments have shown that an amplifier with twenty-fold amplification produces too much noise, while an amplifier with only five-fold amplification operates at a sufficiently low noise level. The optimum is probably an amplification of around five to ten-fold. Optimum adjustment of the electronics may allow the SEM to be operated at an amplification of only 1×10.sup.4.

(12) The operating voltage can be adjusted by a method which is explained in the aforementioned DE 10 2008 010 118 B4 patent specification (corresponding to GB 2457559 B or U.S. Pat. No. 8,536,519 B2). This method of reproducibly adjusting the amplification of a secondary-electron multiplier in a mass spectrometer essentially comprises the following steps:

(13) (a) acquisition of a mass spectrum with single ion signals;

(14) (b) calculation of the average peak height of the single ion signals;

(15) (c) adjustment of the supply voltage of the secondary-electron multiplier so that the average peak height assumes a specified value for the single ion signals. The desired amplification is set via the operating voltage with the aid of the characteristic curves.

(16) Since the characteristic curves are largely straight but, according to the findings of this disclosure, aging causes their gradient to change, it is expedient to measure the average value of the single ion signals at two different operating voltages, and to determine the slope of the characteristic curve, i.e. the ratio of the logarithmic increase in amplification to the linear increase in the operating voltage. This gradient of the characteristic curve can then be used to set the desired amplification.

(17) In order to obtain mass spectra with a sufficient number of single ion signals, it is expedient to detune the temporal and/or spatial focusing of the mass spectrometer so that its resolution becomes extremely poor and the normally well-resolved ion signals for ions of the same mass change to a broad overlapping mixture. Moreover, the number of ions reaching the detector in any mass spectrometer can be greatly reduced until predominantly only single ion signals with no overlapping appear in the mass spectrum. This can be achieved by, for example, reducing the generation rate of the ions in the ion source or restricting the ion transmission through the mass spectrometer. In mass spectrometers which operate with ion traps or temporary stores, the filling quantities can be greatly reduced. All these measures serve to reduce the mass spectrum to signals which are significantly above the electronic background noise and can be assigned to individual ions. It is irrelevant whether these single ion signals originate from ions from the usual chemical noise background or from analyte ions.

(18) It is not essential that the mass spectrum no longer contains any signals whatsoever from ion accumulations. The width of the single ion signals means that they can be identified and read out quite well.

(19) The mass spectrum is scanned in the usual way, amplified by the SEM and electronic amplifiers, digitized and digitally stored. In this digitized mass spectrum, the single ion signals can be easily recognized by their peak widths, using a suitable computer program, and their peak heights as a function of the operating voltage can be investigated. The desired amplification is then set via the average values of the peak heights and the determination of the gradient of the characteristic curve.