Ion supply system and method to control an ion supply system

Abstract

Disclosed herein is an ion supply system, having an ion source emitting ions into a fore vacuum chamber, an ion transport device having stacked electrodes arranged in the fore vacuum chamber, a control system supplying an oscillatory voltage to the electrodes of the ion transport device and a vacuum chamber, arranged downstream from the ion transport device. A vacuum gauge is arranged in the vacuum chamber. The pressure signal of the vacuum gauge is supplied to the control system supplying the oscillatory voltage to electrodes of the ion transport device. The control system adjusts the amplitude of the oscillatory voltage in accordance with the pressure signal.

Claims

1. An ion supply system, comprising: an ion source emitting ions into a fore vacuum chamber; an ion transport device having stacked electrodes arranged in the fore vacuum chamber; a control system supplying a oscillatory voltage to the electrodes of the ion transport device; and a vacuum chamber, arranged downstream from the ion transport device, in which a vacuum gauge is arranged, the vacuum gauge generating a pressure signal representative of a measured pressure; wherein the pressure signal is supplied to the control system supplying the oscillatory voltage, and the control system is configured to set an amplitude of the oscillatory voltage in accordance with a known correlation between a pressure in the vacuum chamber and an amplitude range for optimized ion transmission.

2. The ion supply system of claim 1, wherein the amplitude of the oscillatory voltage supplied by the control system to the electrodes of the ion transport device is changed when a change of pressure is detected by the vacuum gauge.

3. The ion supply system of claim 2, wherein the control system is configured to change the amplitude of the oscillatory voltage supplied by the control system to the electrodes of the ion transport device according to a calibration curve relating the pressure signal and the amplitude.

4. The ion supply system of claim 1, wherein the vacuum gauge is arranged in the vacuum chamber close to a device exit of the ion transport device.

5. The ion supply system of claim 1, wherein the vacuum gauge is arranged in the vacuum chamber close to a lens aperture of an extraction lens downstream from the ion transport device, which is arranged between the fore vacuum chamber and the vacuum chamber.

6. The ion supply system of claim 1, wherein the spacing of adjacent electrodes of the ion transport device is increased in the direction of the ion travel.

7. The ion supply system of claim 1, wherein the electrodes of the ion transport device are grouped into a first electrode set positioned adjacent to a device entrance, and a second set of electrodes positioned adjacent to a device exit, and the electrodes of the first electrode set have apertures that are greater in size relative to apertures of electrodes of the second electrode set.

8. The ion supply system of claim 1, wherein the electrodes of the ion transport device have apertures that decrease in size from an entrance of the ion transport device to an exit of the ion transport device.

9. An ion supply system, comprising: an ion source emitting ions into a fore vacuum chamber; an ion transport device having stacked electrodes arranged in the fore vacuum chamber; a control system supplying an oscillatory voltage to the electrodes of the ion transport device; and a vacuum chamber, arranged downstream from the ion transport device; wherein a vacuum gauge is arranged in the fore vacuum chamber and wherein the pressure signal of the vacuum gauge representative of a pressure measured by the vacuum gauge is supplied to the control system supplying the oscillatory voltage, and the control system adjusts an amplitude of the oscillatory voltage in accordance with a known correlation between a pressure in the vacuum chamber and an amplitude range for optimized ion transmission.

10. Method for supplying ions into a vacuum system, comprising the steps: (i) creating ions in an ion source; (ii) emitting the ions into an ion channel of an ion transport device having stacked electrodes, which is arranged in a fore vacuum chamber; (iii) applying an oscillation voltage to the electrodes of the ion transport device, so that the ions travelling through the ion transport device are radial confined to an aperture arranged behind the ion transport device, through which they travel into a vacuum chamber; (iv) measuring with a vacuum gauge the pressure in the vacuum chamber downstream of the ion transport device; and (v) submitting a pressure signal of the vacuum gauge to a control unit, controlling at least the oscillation voltage applied to the electrodes of the ion transport device, wherein the control unit is applying an oscillation voltage to the electrodes, which is correlated to the pressure measured in the vacuum chamber.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a mass spectrometer with an ion supply system according to the prior art.

(2) FIG. 2 shows a first embodiment of an ion supply system according to the invention.

(3) FIG. 3 shows the measurement of the ion transmission of a specific ion supplied by different ion sources.

(4) FIG. 6 shows the experimental conditions of the measurements of FIG. 3.

(5) FIG. 4 shows the correlation of the cut off amplitudes of the oscillating voltage supplied to the electrodes of an ion transport device with the pressure in the vacuum chamber downstream from the ion transport device.

(6) FIG. 5 shows another embodiment of an ion supply system according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(7) FIG. 1 is a schematic depiction of a mass spectrometer 100 incorporating an ion transport device 105 constructed known from the prior art, e.g. from US 2009/0045062, which is hereby incorporated to the description in total by reference. Analyte ions may be formed by electrospraying a sample solution into an ionization chamber 107 via an electrospray probe 110. For an ion source that utilizes the electrospray technique, ionization chamber 107 will generally be maintained at or near atmospheric pressure. The analyte ions, together with background gas and partially desolvated droplets, flow into the inlet end of a conventional ion transfer tube 115 (e.g., a narrow-bore capillary tube) and traverse the length of the tube under the influence of a pressure gradient. In order to increase ion throughput from ionization chamber 107, multiple ion flow channels may be provided by substituting multiple capillaries or a divided flow path ion transfer tube for the single channel ion transfer tube depicted herein. Analyte ion transfer tube 115 is preferably held in good thermal contact with a block 120 that is heatable by cartridge heater 125. As is known in the art, heating of the ion/gas stream passing through ion transfer tube 115 assists in the evaporation of residual solvent and increases the number of analyte ions available for measurement. The analyte ions emerge from the outlet end of ion transfer tube 115, which opens to an entrance 127 of the ion transport device 105 located within fore vacuum chamber 130. As indicated by the arrow, chamber 130 is evacuated to a low vacuum pressure by a mechanical pump or equivalent. Under typical operating conditions, the pressure within fore vacuum chamber will be in the range of 1-5 millibar, but it is believed that an ion transport device according to embodiments of the present invention may be successfully operated over a broad range of low vacuum pressures, e.g., between 0.1 millibar and 50 millibar.

(8) It should be understood that the electrospray ionization source depicted and described herein is presented by way of an illustrative example, and that the ion transport device of the present invention should not be construed as being limited to use with an electrospray or other specific type of ionization source. Other ionization techniques that may be substituted for (or used in addition to) the electrospray source includes chemical ionization, photo-ionization, and laser desorption or matrix-assisted laser desorption/ionization (MALDI).

(9) The analyte ions exit the outlet end of ion transfer tube 115 as a free jet expansion and travel through an ion channel 132 defined within the interior of ion transport device 105. As will be discussed in further detail below, radial confinement and focusing of ions within ion channel 132 are achieved by application of oscillatory voltages to apertured electrodes 135 of ion transport device 105. As is further discussed below, transport of ions along ion channel 132 to device exit 137 may be facilitated by generating a longitudinal DC field and/or by tailoring the flow of the background gas in which the ions are entrained. Ions leave ion transport device 105 as a narrowly focused beam and are directed through aperture 140 of extraction lens 145 into the vacuum chamber 150. The ions pass thereafter through ion guides 155 and 160 and are delivered to a mass analyzer 165 (which, as depicted, may take the form of a conventional two-dimensional quadrupole ion trap) located within chamber 170. Chambers 150 and 170 may be evacuated to relatively low pressures by means of connection to ports of a turbo pump, as indicated by the arrows. While ion transport device 105 is depicted as occupying a single chamber, alternative implementations may utilize an ion transport device that bridges two or more chambers or regions of successively reduced pressures.

(10) The ion transport device shown in FIG. 1 shows one embodiment of an ion transport device which can be used in the invention as described below. The shown ion transport device 105 is formed from a plurality of generally planar electrodes 135 arranged in longitudinally spaced-apart relation (as used herein, the term longitudinally denotes the axis defined by the overall movement of ions along ion channel 132). Devices of this general construction are sometimes referred to in the mass spectrometry art as stacked-ring ion guides. Each electrode 135 is adapted with an aperture through which ions may pass. The apertures collectively define an ion channel 132, which may be straight or curved, depending on the lateral alignment of the apertures. To improve manufacturability and reduce cost in the shown embodiment all of the electrodes 135 may have identically sized apertures (in contradistinction to the device disclosed in the aforementioned U.S. Pat. No. 6,107,628 to Smith et al., wherein each electrode possesses a uniquely sized aperture). An oscillatory (e.g., radio-frequency) voltage source 210 applies oscillatory voltages to electrodes 135 to thereby generate a field that radially confines ions within ion channel 132. According to a preferred embodiment, each electrode 135 receives an oscillatory voltage that is equal in amplitude and frequency but opposite in phase to the oscillatory voltage applied to the adjacent electrodes. As depicted, electrodes 135 may be divided into a plurality of first electrodes interleaved with a plurality of second electrodes, with the first electrodes receiving an oscillatory voltage that is opposite in phase with respect to the oscillatory voltage applied to the second electrodes. In a typical implementation, the frequency and amplitude of the applied oscillatory voltages to the electrodes are 0.5-1 MHz and 20-400 Vp-p (peak-to-peak), the required amplitude being strongly dependent on frequency. It should be noted that the number of electrodes 135 depicted in the figures has been chosen arbitrarily, and should not be construed to limit the invention to any particular number of electrodes. Typical implementations of an ion transport device having a length of 50 mm will have between 12 and 24 electrodes. Due to the increased inter-electrode spacing near the device exit, an ion transport device constructed in accordance with this embodiment of the invention will generally utilize fewer electrodes relative to the conventional ion funnel device described in U.S. Pat. No. 6,107,628 to Smith et al. and the related publications cited above. The ion transport device 105 shown in FIG. 1 in detail depicted (in rough cross-sectional view) in FIG. 2 of US 2009/0045062.

(11) In FIG. 2 is now shown an improved ion supply system of a mass spectrometer shown in FIG. 1. In the shown ion supply system, the same reference numbers are used for the same components as in FIG. 1. It is provided an additional vacuum gauge 200, which is measuring the pressure in the vacuum chamber 150, which is typically in a range of 0.05 mbar up to 0.5 mbar, preferably in a range of 0.08 mbar up to 0.3 mbar and particularly preferably in a range of 0.10 mbar up to 0.25 mbar. Ions leaving the ion transport device 105 as narrow focused beam are directed through aperture 140 into the vacuum chamber 150. The pressure signal measured by the pressure gauge 200 is provided by the signal line 205 to a control system 210.

(12) This control system 210 is supplying at least the oscillating voltage to the electrodes 135 of the ion transport device 105 via the supply lines 220 and 220. For simplicity of the drawing it is not shown how each electrode 135 of the ion transport device 105 is supplied with the oscillating voltage. Details about this are well known be skilled persons and can be found the documents mentioned before about ion transport devices with stacked electrodes. The control system 210 is supplying the oscillatory voltage to the electrodes with an amplitude, which is correlated to the pressure measured by the vacuum gauge 200 in the vacuum chamber 150.

(13) Due to this improved supply of oscillating voltage to the stacked electrodes 135 of the ion transport device 105 the transmission efficiency of the ions supplied by the transfer tube 115 of the ion source can be increased. This is, because to each pressure value measured in the vacuum chamber 150 an optimal amplitude of the oscillating voltage can be correlated to achieve the maximum transmission efficiency of the ions, which shall be investigated.

(14) The invention can be applied to all known ion transport devices with stacked electrodes, to which an oscillatory voltage has to be applied. This can be an ion transport device shown in FIG. 1 and also an ion transport device shown in FIG. 2.

(15) The ion transport device 105 is constructed from a plurality of apertured electrodes 135 that are grouped into a first electrode set 230 positioned adjacent to device entrance, and a second set of electrodes 231 positioned adjacent to device exit 235. First electrode set 230 has apertures that are greater in size relative to apertures of second electrode set 231. Ions are introduced to the entrance via an ion transfer tube 115. In both sets of electrodes 231, 232 the spacing of adjacent electrodes is increased in the direction of the ion travel to focus the ions into the center of the ion channel 132 given by the aperture of the electrodes according teaching of the US patent US 2009/0045062 A1.

(16) In another embodiment the ion transfer tube 115 may have an outlet that is laterally offset with respect to the center of aperture of the initial electrode of first electrode set 231. Ion transfer tube 115, or a terminal segment thereof, then has a central flow axis that is angularly offset (typically by about 5) with respect to the central flow axis defined by the centers of apertures of the first electrode set 230.

(17) In another not shown embodiment of the ion transport device 105, the centers of the apertures of second electrode set 231 may be laterally offset with respect to each other and the centers of apertures of the first electrode set 230, such that no line-of-sight path exists between the outlet of ion transfer tube 115 and the central aperture of exit lens 145. In this manner, analyte ions must follow an arcuate path to traverse the length of ion transport device and pass through the lens aperture of the extraction lens 145.

(18) In FIG. 3 the transmission of ions of a specific mass to charge ratio m/z of the ion transport device 105 is shown, when in the ion supply system of FIG. 2 ions of different ion sources are supplied with different ion flow rates and temperatures to the ion transport device 105.

(19) In Table 1 of FIG. 6 the detailed parameters of the different experiments are shown. Two type of ion sources have been investigated using the ionization methods nanoelectrospray ionization (nESI) and heated electrospray ionization (HEST). Different ion flow rates through the ion transfer tube 115 have been applied and different ion temperatures, resulting from different source gas temperatures in the electrospray probe 110. Also in table 1 is shown for each experiment the pressure, which was detected in the vacuum chamber 150 by the vacuum gauge 200.

(20) In FIG. 3 the transmission of ions of a specific mass to charge ratio m/z is shown, which is m/z=195. Only these ions have been detected by the mass analyzer 165 of a mass spectrometer equipped with an ion supply system according to the invention, when only ions of these mass to charge ratio have been filtered by a quadrupole mass filter after passing the ion transport device and before the ions arrive in the mass analyzer. The transmission of the ions has been detected in dependence on the amplitude of the oscillatory voltage, which is in this case a RF voltage, applied to the electrodes of the ion transport device 150.

(21) For each experimental condition a voltage range can be defined, in which a maximum ion transmission is possible. The limits of this range, the flanks of the mass peak in FIG. 3 depend on the experimental conditions. Additional it can be observed that they are nearly equal for certain experiments. The same limits can be found for experiments, when the same pressure has been detected by the pressure gauge 200 in the vacuum chamber 150. So, the limits of the range of oscillating voltages, which can applied to the stacked electrodes of the ion transport device 105 to achieve the transmission of an ion through the ion transport device 105 are correlated to the pressure measured in the vacuum chamber 150. In other words, the minimum and maximum value of the oscillating voltages applied to the stacked electrodes of the ion transport device 105 is correlated with the pressure measured by the pressure gauge 200, which is provided by the invention in the vacuum chamber 150.

(22) So, the correlation of the minimum and maximum value of the oscillating voltages applied to the stacked electrodes of the ion transport device 105 can be detected for different pressure values measured in the vacuum chamber 150, arranged downstream the ion transport device.

(23) Such a correlation of the minimum and maximum value, the low mass cut off value (LMCO) and high mass cut off value (HOMO) of the oscillating voltages applied to the stacked electrodes of the ion transport device 105 with the pressure in the vacuum chamber is shown in FIG. 4. The shown cut off values are defined by that value of the applied RF voltage, when the transmission of the investigated ion has been reduced to 60% of the maximum transmission. As shown in FIG. 4, the correlation of the minimum and maximum value with the measured pressure in the chamber 150 is at least nearly linear. Very often a linear approach can be used to define a calibration curve.

(24) Such a calibration curve correlating the maximum value or minimum value of the oscillating voltages applied to the stacked electrodes of the ion transport device 105 with the pressure in the vacuum chamber 150, in which the ions are transferred, if they have passed the ion transport device, can be determined for each ion supply system or all instruments using such a ion supply system in common.

(25) These calibration curves can be used by the control system 210, which is supplying the oscillating voltage to the electrodes of the ion transport device 105. The control system is receiving the pressure signal of the pressure in the vacuum chamber 150 via the vacuum gauge 200, provided in the vacuum chamber 150. It is advantageous if the vacuum gauge is provided close to the device exit 137, 235 and the extraction lens 145.

(26) If now the pressure gauge 200 is detecting a pressure change in the vacuum chamber 150, the control system can adjust the oscillating voltage to the electrodes of the ion transport device 105 according to the calibration curve. From the calibration curve it can be derived, which change of the supplied oscillating voltage is necessary that a full ion transmission is possible independent on the pressure change. In general, the calibration curve of the maximum value or minimum value of the oscillating voltages applied to the stacked electrodes of the ion transport device 105 can be used or the mean value of the maximum value or minimum value of the oscillating voltages applied to the stacked electrodes of the ion transport device 105 (the middle) as calibration curve. So, the inventive ion supply system is now flexible to guarantee an ion transmission, in particular an optimized ion transmission, though a pressure change has happened in the vacuum chamber 150, which is correlated with a pressure change in the ion channel of the ion transport device. In this was there is an online correction of the oscillating voltage supplied to the electrodes of the ion transport device 105 for the first time possible.

(27) Origin of possible pressure changes can be a clogging in the ion inlet device, e.g. in the ion transfer tube 115 or an intentional or unintentional change of the experimental setup, e.g. changes of temperature of the sample, the temperature of the ion inlet device, the setting of the ion source, the orientation or shape of the ion inlet device, the sample flow or the ion flow.

(28) A calibration curve might be defined for one specific mass to charge ratio and then applied to all pressure changes. It can also be defined for several ions of specific mass to charge ratio. Then the medium slope of all calibration curves can be used for the correction of the oscillating voltage supplied to the electrodes of the ion transport device 105 due to pressure change.

(29) It has been described how the oscillating voltage supplied to the electrodes of the ion transport device 105 can be adapted to a pressure change in the vacuum chamber 150. If the used calibration curve has been determined for an ion supply system in common, this calibration curve can be adapted to each individual device e.g. by measuring the ion transmission according to FIG. 3 one time of ions of the mass to charge for which the calibration curve is determined at a specific pressure value p.sub.0. If the detected maximum value or minimum value of the oscillating voltages applied to the stacked electrodes of the ion transport device 105 does not fit with the common calibration curve, then the individual calibration curve of the individual ion supply device can be defined by adding to the common calibration curve the difference between the detected maximum value or minimum value and the maximum value or minimum value of the common calibration curve at the pressure p.sub.0. Then this individual calibration curve can be used by the control system 210 to adapt the oscillating voltage supplied to the electrodes of the ion transport device 105 based on the pressure measured by the pressure gauge 200 in the vacuum chamber. This adaptation is now related to the measured pressure and not a measured pressure change. So now the control unit is able to correct the supplied oscillatory voltage directly, when after the start of an experiment it is detected by the pressure gauge 200, that the pressure in the chamber 150 deviates from the pressure expected for the experiment.

(30) In general, it is to say, that the detection of the pressure in the vacuum chamber 150, in which the ions move directly after they have passed the ion transport device 150 makes it possible, that always the appropriate oscillating voltage is applied to the stacked electrodes 135 of the ion transport guide resulting in an improved ion transmission of the ion transport device 105.

(31) In FIG. 5 is shown another embodiment of the invention, which is also solving the problem addressed before. The only difference between the ion supply systems of the FIGS. 2 and 5 is, that in the vacuum gauge 200 is now not arranged in the vacuum chamber 150 downstream of the ion transport device. The vacuum gauge 200 is now arranged in the fore vacuum chamber 130. Also in the embodiment the pressure signal 205, detected by the vacuum gauge is supplied to the control system 210 supplying the oscillatory voltage to the electrodes of the ion transport device. But with this further solution the improvement of the ion transmission of an ion transport device is limited compared to the embodiment, when the pressure is detected in the vacuum chamber 150 downstream of the ion transport device.