Drive unit for a synchronous ion shield mass separator

A drive unit for a synchronous ion shield mass separator having a reference oscillator (1), a digital direct synthesizer (2), a low-pass filter (3) and a comparator (4), wherein the synchronous ion shield mass separator has a comb-shaped separation electrode (6), the reference oscillator (1) provides the direct digital synthesizer (2) with a reference frequency, the output signal generated by the direct digital synthesizer is filtered by the low-pass filter (3) and the output signal of the low-pass filter (3) is processed by the comparator (4). A drive unit that can be applied flexibly and economically is implemented in that the output signal of the comparator (4) is converted by a programmable element (11) into a number of output signals corresponding to the number of teeth (7) of the comb-shaped separation electrode (6).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a drive unit for a synchronous ion shield mass separator having a reference oscillator, a digital direct synthesizer, a low-pass filter and a comparator, wherein the synchronous ion shield mass separator has a comb-shaped separation electrode, the reference oscillator provides the direct digital synthesizer with a reference frequency, the output signal generated by the direct digital synthesizer is filtered by the low-pass filter and the output signal of the low-pass filter is processed by the comparator. The invention further relates to a method for driving a synchronous ion shield mass separator, wherein the synchronous ion shield mass separator has a comb-shaped separation electrode.

2. Description of Related Art

Mass separators of this type aid, in mass spectrometers, in separating charged particles—ions—according to mass or according to their mass/charge ratio and are thus also called analyzers. The mass separator makes up a substantial portion of the entire spatial requirements of the mass spectrometer. In the scope of miniaturizing mass spectrometers, it is thus of particular importance to develop a particularly small, yet still high-performance mass separator that further separates ions with extreme precision. Such a mass separator is described, for example, in the article “Mass spectra measured by a fully integrated MEMS mass spectrometer” by J.-P. Hauschild et al., International Journal of Mass Spectrometry, Elsevier, March 2007 and is called a synchronous ion shield mass separator there.

A synchronous ion shield mass separator consists essentially of a comb-shaped separation electrode. This comb-shaped separation electrode has a plurality of teeth, which are arranged next to one another at short distances on the comb ridge so that a small gap remains between the teeth of the separation electrode and the comb ridge. Often, the comb ridge also has small protrusions that are located opposite the teeth. The ions to be analyzed are charged with energy by an electrical field—as a function of their charge—and accelerated—as a function of their mass. After passing through the electrical field, the ions have an identical direction of movement. The electrical intensity of the field, on the one hand, and the mass and the charge of the ions, on the other hand, determine the speed of the ions after passing through the potential difference.

From one end of the gap, which is the entrance of the mass separator, the accelerated ions are placed parallel to the comb ridge in the mass separator. The mass separator is normally evacuated as far as possible, so that the ions can easily move along the gap. The requirements for the evacuation of a miniature mass separator are not as strict as that of a non-miniature mass separator, since the ions in a miniature mass separator only have to travel a very small distance and thus the possibility of impact with residual gas atoms or molecules is minimized.

By creating a voltage between one tooth and the comb ridge of the comb-shaped separation electrode, an electrical field is generated that diverts ions moving through the gap from their original direction of movement, so that they collide with the comb-shaped separation electrode and do not reach the other end of the gap, the exit of the mass separator. Depending on the charge of the ion and the direction of the electrical field, diverted ions collide either with the teeth or the comb ridge of the separation electrode. These diverted ions are no longer available for further analysis should, for example, the mass separator be inserted in a mass spectrometer.

It is known from the prior art to apply a voltage between every other tooth and the comb ridge and to apply no voltage between the teeth located between them and the comb ridge. In this way, a simple pattern of alternating applied voltage and non-applied voltage results along the teeth, called signal sequence in the following. A simplified representation of such a signal sequence occurs here with zeros and ones, wherein a one represents the presence of an electrical potential difference and a zero represents the absence of an electrical potential difference. The signal sequence described above of alternating applied voltage and non-applied voltage thus corresponds to a signal sequence of alternating zeros and ones. In a comb-shaped separation electrode having 10 teeth, the result of strictly alternating presence and absence of a potential difference is:

In order to obtain a separation of ions according to mass according to the prior art, the signal sequence is shifted by one tooth in the direction of the exits of the separation electrode with a certain cycle frequency. I.e., the following signal sequence results in the next cycle step for the above-described comb-shaped separation electrode with 10 teeth:

Only ions with a certain velocity given by the cycle frequency and the geometry of the separation electrode follow the erratic zeros of the signal sequence, i.e., the areas without a field in the separation electrode, and reach the exit of the mass separator. While moving in the gap of the separation electrode, ions with a velocity that is too high or too low arrive in areas, in which they are deflected by an electric field present between a tooth and the comb ridge. As a result, only ions having a certain mass to charge ratio are let through by the mass separator, i.e., are separated from ions having another mass to charge ratio. By changing the cycle frequency, other ion velocities and, consequently, other mass to charge ratios can be selected by the mass separator. Although the mass separator does not select according to mass, but to mass to charge ratio, it is common to speak of a mass separator.

A mass separator known from the prior art is normally driven in that the output signal of the comparator of a mass separator as described in the introduction is split into two signals and one of these signals is inverted. As a result, two complementary signals switching at the same cycle frequency are obtained. These two signals are, in turn, used for driving the teeth of the separation electrode, wherein one of the signals controls the first and every other further tooth−i.e., the uneven-numbered teeth—of the separation electrode and the other of the two signals controls the second and every other further tooth—i.e., the even-numbered teeth—of the separation electrode.

Furthermore, a method is known from the article “The novel synchronous ion shield mass analyzer” by J.-P. Hauschild et al., Journal of Mass Spectrometry, 2009, 44, in which the resolution of a synchronous ion shield mass separator is increased in that the turn-on times of the voltage on the teeth of the separation electrode are increased in relation to the turn-off times. A drive switch for implementing this method is described in “Optimierung der Ansteuerung des SIS-Massenseparators im planar integrierten Micro-Massenspektrometer”(“Optimizing the Drive of the SIS Mass Separator in a Planar Integrated Mass Spectrometer”) by G. Quiring et al., Mikrosystemtechnik Kongress, 2009, VDE Verlag GmbH. This drive switch encompasses essentially four parallel signal paths, each of which has a direct digital synthesizer, a low-pass filter and a comparator. Due to the different designs of the signal paths, this drive switch is technically elaborate and costly. Furthermore, the possible signal sequences are very limited.

SUMMARY OF THE INVENTION

Thus, a primary object of the invention is to provide a drive unit and a method for driving a synchronous ion shield mass separator that can be flexibly used and is economical.

The above object is met in that a drive unit of the type described in the introduction has the output signal of the comparator converted by a programmable element into a number of output signals corresponding to the number of teeth of the comb-shaped separation electrode. When using appropriate programming and driving of the programmable element, the use of a programmable element allows for the issue of output signals, which basically correspond to an arbitrary signal sequence. For this reason, not only the signal sequence known from the prior art, but also, even regardless of hardware, user-defined signal sequences can be used by the drive unit according to the invention. In order to create another signal sequence with the same hardware, it is sufficient to change the programming of the programmable element. Furthermore, the drive unit according to the invention is considerably simpler in terms of construction than the drive unit from the prior art, so that there is a substantial cost advantage in this case.

According to an advantageous design of the invention, it is provided that the programmable element is a programmable logic element in the form of a FPGA. A further advantageous design of the invention is wherein the programmable logic element is a CPLD. Here, FPGA is a so-called field programmable gate array, which represents a programmable integrated circuit. The complex programmable logic device, abbreviated CPLD, is also a programmable integrated circuit. FPGAs and CPLDs are widespread and thus economical microchips for implementing specific programs. Depending on the requirements of the signal sequence, the use of a FPGA or a CPLD occurs after weighing the advantages and disadvantages of the possible FPGAs and CPLDs.

Alternatively, a microcontroller can be used as a programmable element, though it is necessary to determine whether or not the requirements can be fulfilled for the signal sequence to be precisely switched in terms of time by the microcontroller and the operating system implemented there. Preferably, a digital signal processor having an operating system with real-time characteristics can be use for the present application.

The above described object is also met based on the method for driving a synchronous ion shield mass separator as described in the introduction that has been improved by the output signal of a drive unit according to the invention being used to drive the teeth of the comb-shaped separation electrode according to the above design. A particularly flexible possibility for driving a synchronous ion shield mass separator can be implemented with the method according to the invention with the drive unit as already described, since the signal sequence that can be created with the drive unit is basically arbitrary—this with a particularly simple and economical construction of the drive unit. Not every signal sequence is suitable for driving a synchronous ion shield separator. For example, a signal sequence that consists only of ones leads to the ions not being able to pass through the mass separator. A selection of particularly advantageous signal sequences is described in the following.

According to an advantageous further development of the invention, it is provided that the output signals of the driving unit have a signal sequence in which the signal sequence consists of alternating series n zeros and m ones, wherein all k cycles of the programmable element bring the signal sequence forward j steps, wherein n, m, k and j are natural numbers larger than zero and wherein n is greater than or equal to the ratio (j mod (n+m))/k. The latter requirement, that n is greater than or equal to the ratio (j mod(n+m))/k is of significant importance for such a signal frequency. Here, j mod(n+m) indicates the result of the division of j by (n+m). Foremost, this requirement guarantees that ions are even able to pass through the mass separator. This becomes particularly clear using a simple example.

For example, if n is equal to 1, m equal to 2, k equal to 1 and j equal to 2, this means that the area without a field, which is represented by zeros and in which no diversion of the ion occurs, is exactly one tooth wide. If this tooth moves exactly two teeth further at each cycle, this means that ions do not have the possibility of moving from one area without a field of a cycle to the next area without a field of the next cycle, since there is always an area that continually has an electrical field between an area without a field in one cycle and an area without a field in the next cycle. This can be seen as follows in a comb-shaped separation electrode with 10 teeth (bold represents the position that always has an electrical field):

In this example, and all following examples, it is assumed that the ions are introduced into the mass separator from the left side, i.e., in the first cycle initially reaches a tooth without a field, this corresponds to the first numeral 0 in the signal sequence of the first cycle shown above. In the second cycle, these ions do not have the possibility of reaching the next tooth without a field, since the continuous electrical field blocks the path to the next tooth without a field, which is represented by the third numeral—0—of the signal sequence of the second cycle.

An advantageous design of the invention is wherein the number n is equal to 2, the number m is equal to 2, the number k is equal to 1 and the number j is equal to 2. The first two cycles of the signal sequence are repeated in further cycles and result, for example, in the following for a comb-shaped separation electrode with 10 teeth:

According to a particularly advantageous further development of the invention, it is provided that the number 11 is equal to 2, the number m is equal to 2, the number k is equal to 1 and the number j is equal to 1. The first four cycles of this signal sequence are repeated in further cycles and result, for example, in the following for a comb-shaped separation electrode with 10 teeth:

In a further preferred design of the invention, it is provided that the number n is equal to 1, the number m is equal to 1, the number k is equal to 1 and the number j is equal to 1. This design according to the invention corresponds exactly to the signal sequence known from the prior art consisting of alternating ones and zeros, which moves one step further at each cycle. The first two cycles of this signal sequence are repeated in further cycles and result, for example, in the following for a comb-shaped separation electrode with 10 teeth:

According to a further preferred design of the invention, it is provided that the number m is greater than the number n. Here, it is of particular advantage when the number n is equal to 3 and the number m is equal to 5.

It is further provided in a preferred design that the output signals of the drive unit have a signal sequence, in which the signal sequence consists of e zeros followed by ones, wherein the signal sequence moves h steps further every g cycles of the programmable element, wherein e, g and h are natural number greater than zero and wherein e is greater or equal to the ratio h/g. The latter requirement, that e is greater of equal to the ratio h/g is of significance for such a signal sequence. Here, too, the requirement guarantees that ions can even pass through the mass separator. The signal sequence consists namely only of one single block of e zeros and otherwise only of ones, i.e., only one “package” of ions, namely in the block of e zeros, which represents an area without a field of e teeth, is accepted by the mass separator and only the ions of the package having a certain velocity and thus a certain mass to charge ratio can pass through the mass separator.

If the requirement that e is greater than or equal to the ratio hlg is not fulfilled, this means that the ions do not have the possibility of moving from the block without a field of a first cycle into the next block without a field of the following cycle, since there is always an area that has a continuous electrical field between a block without a field in one cycle and a block without a field in the next cycle.

In a particularly advantageous design of the invention, it is provided that the number e is equal to 1, the number g is equal to 1 and the number h is equal to 1. This corresponds to a signal sequence in which one, single zero moves along the teeth of the separation electrode. In a comb-shaped separation electrode with 5 teeth, the following signal sequence results:

6. and all further cycles: 11111

According to a further advantageous design of the invention, it is provided that the signal sequence is implemented by a shift register. The shift register is implemented in the programmable element. The sequence of zeros and ones stored in the storage element of the shift register moves a given number of steps further at each cycle. Values at the end of the shift register are lead back again to the beginning of the shift register. The values of the storage element of the shift register together form the output signals of the programmable element.

In a particularly advantageous design of the invention, it is provided that the signal sequence is uploaded from storage at each cycle of the element for which a change of the output signal is planned. Instead of a shift register, it is possible to provide storage in the programmable element, in which the signal sequence to be used for each cycle is stored. This signal sequence is uploaded for each cycle from the storage and issued at the exits of the programmable element.

In detail, there are a number of possibilities for designing and further developing the drive unit according to the invention as will become apparent from the following detailed description of preferred embodiments of the invention in conjunction with accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

The drive unit known from the prior art shown inFIG. 1has a reference oscillator1for creating a reference frequency signal. The reference frequency signal of the reference oscillator1is converted from a direct digital synthesizer2into a given frequency. After low-pass filtering of the frequency signal of the direct digital synthesizer2by a low-pass filter3, the frequency signal now free of unwanted frequency portions is processed by a comparator4. The comparator4issues two identical output signals, of which one is inverted by an inverter5. The inverted and the non-inverted signal aid in driving a comb-shaped separation electrode6. The separation electrode6has a plurality of teeth7on a comb ridge8. The non-inverted signal aids in driving the first and every other further tooth7of the separation electrode6. The inverted signal aids in driving the second and every other further tooth7of the separation electrode6.

The more exact function of the separation electrode6shown inFIG. 1can be seen inFIG. 2. The comb ridge8of the separation electrode6is joined to the teeth7of the separation electrode6via a voltage source9and multiple switches10. Here, each tooth7is assigned to one switch10. If all switches10are open, ions moving parallel to the comb ridge8can move forward without hindrance between the comb ridge8and the teeth7. If one of the switches10is closed, a voltage given by the voltage source9exists between the corresponding teeth7and the comb ridge8. The electrical field resulting from this voltage between the corresponding teeth7and the comb ridge8is capable of diverting ions moving parallel to the comb ridge8between the comb ridge8and the teeth7. Normally, these ions collide with the structures of the separation electrode6and are not available for further analysis.

The switches10assigned to the teeth7of the separation electrode6are, as can be seen inFIG. 1, driven by the inverted and the non-inverted signals of the comparator4. Thus, there is a voltage on every other tooth7and no voltage on the rest of the teeth7. This signal sequence of alternating applied voltage and non-applied voltage on the teeth is inverted with the frequency given by the direct digital synthesizer2. This is synonymous with the signal sequence applied to the teeth7moving one step further in the direction of the exit of the separation electrode6with each cycle of the frequency of the direct digital synthesizer2.

InFIG. 2, the exit is arranged at the upper end of the separation electrode, as can be taken from the marked arrows, the possible paths of the ions to be analyzed are described as an example. Ions that have the same velocity as the signal sequence moving along the teeth7can, when there is no voltage on the first tooth when entering the separation electrode6, i.e., they initially encounter a zero in the signal sequence, follow this area without a field represented by a zero through the separation electrode6, and thus, reach the exit of the separation electrode6. Ions having a lower or higher velocity than that of the signal sequence encounter an area within the separation electrode6, in which they are diverted by a field, which is caused by voltage applied to the teeth7in this area and do not reach the exit of the separation electrode6. A possibility not shown here for driving the teeth7comprises applying each of the inverted signal originating from the comparator4and the non-inverted signal directly to the teeth7after possible strengthening of the voltage signal. In this embodiment, the voltage source9and the switch10are not necessary.

The function of the drive unit according to the invention can be seen inFIG. 3. Similar to the drive unit known from the prior art ofFIG. 1, the drive unit according to the invention also has a reference oscillator1, a direct digital synthesizer2, a low-pass filter3and a comparator4that are switched in the same manner as inFIG. 1. The comparator4of the drive unit according to the invention, however, only issues a, single output signal, which is led to a programmable element11. The programmable element11has a number of output signals corresponding to the number of teeth7of the comb-shape separation electrode6. This means that each tooth7of the separation electrode6is assigned to one output signal of the programmable element11, and thus, each tooth7can be individually driven via the corresponding output signal of the programmable element11.

FIG. 4shows a programmable element in which the method according to the invention is implemented by a shift register. The shift register within the programmable element11has a number of storage elements12corresponding to the exits13of the programmable element11, here. The desired signal sequence is saved in the storage elements12of the programmable element11. In the present case, this is a simple sequence of alternating zeros and ones. At each cycle of the programmable element11for which a change in the output signal is planned, the value saved in each storage element12of the shift register is given further to the next storage element12of the shift register. The value saved in the last storage element12of the shift register is then given further to the first storage element12of the shift register.

FIG. 5shows a programmable storage element11that has storage14. The signal sequences to be issued by the programmable element11are stored in the storage14. At each cycle of the programmable element, in which a change in the output signal is planned, a signal sequence is uploaded from the storage14and issued via the storage element12and the exits13. In this manner, nearly any signal sequence can be issued by the programmable element11. In the present example, a simple signal sequence of alternating zeros and ones is shown which can, for example, be any of the sequences described in the Summary portion of this specification.