ION SOURCE AND METHOD FOR GENERATING ELEMENTAL IONS FROM AEROSOL PARTICLES

Abstract

The invention relates to an ion source (50) for generating elemental ions and/or ionised metal oxides from aerosol particles, comprising: a reduced pressure chamber (61) having an inside; an inlet (56) and a flow restricting device (60) for inserting the aerosol particles in a dispersion comprising the aerosol particles dispersed in a gas, in particular in air, into the inside of the reduced pressure chamber (61), the inlet (60) fluidly coupling an outside of the reduced pressure chamber (61) via the flow restricting device (60) with the inside of the reduced pressure chamber (60); a laser (62) for inducing in a plasma region (63) in the inside of the reduced pressure chamber (61) a plasma in the dispersion for atomising and ionising the aerosol particles to elemental ions and/or ionised metal oxides; wherein the reduced pressure chamber (61) is adapted for achieving and maintaining in the inside of the reduced pressure chamber (61) a pressure in a range from 0.01 mbar to 100 mbar. The invention further relates to a method for generating elemental ions and/or ionised metal oxides from aerosol particles, comprising the steps of inserting aerosol particles in a dispersion comprising the aerosol particles dispersed in a gas, in particular in air, through an inlet (56) via a flow restricting device (60) into an inside of a reduced pressure chamber (61), while maintaining in the inside of the reduced pressure chamber (61) a pressure in a range from 0.01 mbar to 100 mbar, preferably from 0.1 mbar to 100 mbar or from 1 mbar to 100 mbar, particular preferably from 0.1 mbar to 50 mbar or from 1 mbar to 50 mbar, most preferably from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar; and inducing with a laser (62) in a plasma region (63) in the inside of the reduced pressure chamber (61) a plasma in the dispersion for atomising and ionising the aerosol particles to elemental ions and/or ionised metal oxides, wherein the laser (62) is adapted for inducing in the plasma region (63) in the inside of the reduced pressure chamber (61) the plasma in the gas of the dispersion for atomising and ionising the aerosol particles to elemental ions.

Claims

1. An ion source for generating elemental ions and possible ionised metal oxides from aerosol particles, comprising: a) a reduced pressure chamber having an inside; b) an inlet and a flow restricting device for inserting said aerosol particles in a dispersion comprising said aerosol particles dispersed in a gas, in particular in air, into said inside of said reduced pressure chamber, said inlet fluidly coupling an outside of said reduced pressure chamber via said flow restricting device with said inside of said reduced pressure chamber; c) a laser for inducing in a plasma region in said inside of said reduced pressure chamber a plasma in said dispersion for atomising and ionising said aerosol particles to elemental ions and possible ionised metal oxides, wherein said laser is adapted for inducing in said plasma region in said inside of said reduced pressure chamber said plasma in said gas of said dispersion for atomising and ionising said aerosol particles to elemental ions; wherein said reduced pressure chamber is adapted for achieving and maintaining in said inside of said reduced pressure chamber a pressure in a range from 0.01 mbar to 100 mbar, preferably from 0.1 mbar to 100 mbar or from 1 mbar to 100 mbar, particular preferably from 0.1 mbar to 50 mbar or from 1 mbar to 50 mbar, most preferably from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar.

2. The ion source according to claim 1, wherein said ion source comprises a denuder for removing contaminations in said dispersion, said denuder fluidly coupling said inlet with said flow restricting device for inserting said dispersion through said denuder and subsequently through said flow restricting device into said inside of said reduced pressure chamber.

3. The ion source according to claim 1, wherein said ion source comprises a gas exchange device for exchanging said gas, in particular said air, in said dispersion by a clean plasma gas before inserting said dispersion comprising said aerosol particles into said inside of said reduced pressure chamber.

4. The ion source according to claim 1, wherein said ion source comprises an aerodynamic lens or an acoustic lens for focussing said aerosol particles to a focus region in said inside of said reduced pressure chamber.

5. The ion source according to claim 1, wherein said ion source comprises a fragmenting device, in particular a collision cell, for fragmenting ionised debris, in particular ionised molecules, originating from said aerosol particles, and possible ionised metal oxides, wherein the metal originates from the aerosol particles, into elemental ions, wherein said fragmenting device is fluidly coupled to said plasma region in said inside of said reduced pressure chamber for transferring ionised debris, in particular ionised molecules and possible ionised metal oxides, of said aerosol particles generated in said plasma through the fragmenting device for fragmenting said ionised debris, in particular ionised molecules, originating from said aerosol particles, and possible ionised metal oxides, wherein the metal originates from the aerosol particles, into elemental ions.

6. An apparatus for analysing an elemental composition of aerosol particles, comprising: a) an ion source according to claim 1; and b) a first mass analyser for analysing said elemental ions and possible ionised metal oxides, wherein said inside of said reduced pressure chamber is fluidly coupled with said first mass analyser.

7. The apparatus according to claim 6, wherein said apparatus comprises a differentially pumped interface comprising at least one differentially pumped stage, preferably at least two differentially pumped stages, particular preferably at least three differentially pumped stages, said differentially pumped interface fluidly coupling said inside of said reduced pressure chamber with said first mass analyser for transferring said elemental ions and possible ionised metal oxides from said reduced pressure chamber to said first mass analyser.

8. The apparatus according to claim 6, wherein said apparatus comprises a multipole ion guide, in particular a quadrupole ion guide, for resonant excitation of said elemental ions and possible ionised metal oxides, said multipole ion guide fluidly coupling said inside of said reduced pressure chamber with said first mass analyser for transferring said elemental ions and possible ionised metal oxides from said reduced pressure chamber to said first mass analyser.

9. The apparatus according to claim 6, wherein said apparatus comprises a second mass analyser for analysing said elemental ions and possible ionised metal oxides, wherein said inside of said reduced pressure chamber is fluidly coupled with said second mass analyser for transferring said elemental ions and possible ionised metal oxides from said reduced pressure chamber to said second mass analyser.

10. The apparatus according to claim 9, wherein said first mass analyser is adapted for analysing positive ions and said second mass analyser is adapted for analysing negative ions.

11. The apparatus according to one of claims 6, wherein said apparatus comprises an ionised aerosol particle mobility analyser for separating ionised aerosol particles according to their mobility, wherein said ionised aerosol particle mobility analyser is fluidly coupled with said inlet of said ion source for inserting said dispersion comprising said aerosol particles via said aerosol particle mobility analyser to said ion source.

12. The apparatus according to claim 6, wherein said apparatus further comprises an electronic data acquisition system for processing signals provided by said first mass analyser, whereas said electronic data acquisition system comprises a) at least one analogue-to-digital converter producing digitised data from said signals obtained from said first mass analyser; b) a fast processing unit receiving said digitised data from said analogue-to-digital converter; wherein c) said fast processing unit is programmed to continuously, in real time inspect said digitised data for events of interest measured by said first mass analyser; and d) said electronic data acquisition system is programmed to forward said digitised data representing mass spectra relating to events of interest for further analysis and to reject said digitized data representing mass spectra not relating to events of interest.

13. The apparatus according to claim 6, whercin said apparatus further comprises an aerosol particle detection unit for detecting aerosol particles when they enter said plasma region, and a control unit for synchronising said laser and said first mass analyser with said aerosol particle detection unit in order to enable single aerosol particle analysis.

14. A method for generating elemental ions from aerosol particles, comprising the steps of: a) inserting aerosol particles in a dispersion comprising said aerosol particles dispersed in a gas, in particular in air, through an inlet via a flow restricting device into an inside of a reduced pressure chamber, while maintaining in said inside of said reduced pressure chamber a pressure in a range from 0.01 mbar to 100 mbar, preferably from 0.1 mbar to 100 mbar or from 1 mbar to 100 mbar, particular preferably from 0.1 mbar to 50 mbar or from 1 mbar to 50 mbar, most preferably from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar; and b) inducing with a laser in a plasma region in said inside of said reduced pressure chamber a plasma in said dispersion for atomising and ionising said aerosol particles to elemental ions and possible ionised metal oxides, wherein said laser is adapted for inducing in said plasma region in said inside of said reduced pressure chamber said plasma in said gas of said dispersion for atomising and ionising said aerosol particles to elemental ions.

15. A method for analysing an elemental composition of aerosol particles, comprising the steps of: a) generating elemental ions and/or ionised metal oxides from aerosol particles with the method according to claim 14, b) transferring said elemental ions and/or ionised metal oxides to a first mass analyser and c) analysing said elemental ions and/or ionised metal oxides with said first mass analyser.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0088] The drawings used to explain the embodiments show:

[0089] FIG. 1 a schematic view of a known, prior art apparatus for analysing the elemental composition of aerosol particles based on an inductively coupled plasma ion source,

[0090] FIG. 2 a schematic view of a known, prior art ATOFMS type instrument for analysing the elemental composition of aerosol particles,

[0091] FIG. 3 a schematic view of an apparatus for analysing an elemental composition of aerosol particles using an ion source according to the invention for generating elemental ions and possible ionised metal oxides from aerosol particles,

[0092] FIG. 4 a schematic view of another apparatus for analysing an elemental composition of aerosol particles, the apparatus comprising another ion source according to the invention for generating elemental ions and possible ionised metal oxides from aerosol particles,

[0093] FIG. 5 a schematic view of a more space saving configuration of the apparatus shown in FIG. 4, and

[0094] FIG. 6 a schematic view with reduced details of a modified apparatus for analysing the elemental composition of aerosol particles.

[0095] In the figures, the same components are given the same reference symbols.

[0096] Preferred Embodiments

[0097] FIG. 1 shows a schematic view of a known, prior art apparatus 501 for analysing the elemental composition of aerosol particles, the apparatus being based on an inductively coupled plasma ion source. The apparatus 501 comprises a gas exchange device 502, a plasma ion source 503, an atmospheric pressure interface 504 and a mass analyser 505. Aerosol particles dispersed in a dispersion comprising the aerosol particles dispersed in air are inserted through an inlet 506 into the gas exchange device 502. In the gas exchange device 502, the air in the dispersion is exchanged by a clean plasma gas which is in the present case argon. Thus, after having passed the gas exchange device 502, the dispersion comprises the aerosol particles dispersed in argon instead of air. This dispersion is then transferred into the plasma ion source 503 where the aerosol particles are atomised and ionised by an inductively coupled plasma as described for example in US 2015/0235833 A1 of Bazargan et al. The resulting elemental ions are then transferred through the atmospheric pressure interface 504, where the gas pressure is reduced, to the mass analyser 505 where they are analysed. The mass analyser 505 is a known time-of-flight mass analyser and provides mass spectra which are spectra of values of mass per charge of the elemental ions.

[0098] FIG. 2 shows a schematic view of a known, prior art ATOFMS type instrument for analysing the elemental composition of aerosol particles. In this apparatus 601, a laser 609 is used for vaporising the aerosol particles and ionising the vaporised substances under high vacuum. This apparatus 601 comprises an aerodynamic lens 607 which focuses the aerosol particles to the centre of the airstream inserted through the inlet 606 of the apparatus 601. From the aerodynamic lens 607, the aerosol particles are transferred through a differentially pumped interface 608 into a high vacuum or ultra-high vacuum with a pressure of approximately 10.sup.7 mbar in mass analyser 605. There, the aerosol particles are hit by a laser beam generated by laser 609 such that the aerosol particles are atomised and ionised. Subsequently, the resulting elemental ions are analysed by the mass analyser 605. Instead of the aerodynamic lens 607, the apparatus 601 may for example comprise an acoustic lens.

[0099] FIG. 3 shows a schematic view of an apparatus 1 for analysing an elemental composition of aerosol particles, the apparatus 1 comprising an ion source 50 according to the invention for generating elemental ions and possible ionised metal oxides from aerosol particles. The apparatus 1 further comprises a differentially pumped interface 8, a mass analyser 5 and a data acquisition system 10. The ion source 50 comprises an inlet 56, a denuder 64, a gas exchange device 52, an aerodynamic lens 57, a flow restricting device 60 which is formed in the present example by an orifice, a reduced pressure chamber 61 and a laser 62.

[0100] A dispersion comprising the aerosol particles dispersed in air is inserted through inlet 56 into the denuder 64, where the air is scrubbed from gaseous trace gases by passing the denuder 64. Thus, gaseous contaminants in the air like for example trace gases, in particular VOC are greatly reduced which reduces the background in the elemental analysis of the aerosol particles otherwise caused by such gaseous contaminants. From the denuder 64, the dispersion is transferred through the gas exchange device 52, where a clean plasma gas is substituted for the air in the dispersion. The clean plasma gas is in the present example argon. It could however be any other noble gas or even any inert gas like for example nitrogen. From the gas exchange device 52, the dispersion comprising the aerosol particles now dispersed in argon instead of air is transferred through the aerodynamic lens 57 and inserted through the flow restricting device 60 into the reduced pressure chamber 61.

[0101] In a variant to the embodiment shown in FIG. 3, the apparatus 1 may go without denuder, without gas exchange device or the succession of the denuder 64 and the gas exchange device 52 may be swapped such that the denuder 64 is located downstream of the gas exchange device 52.

[0102] In the embodiment shown in FIG. 3, the pressure in the reduced pressure chamber 61 is reduced as compared to atmospheric pressure. More precisely, the pressure in the reduced pressure chamber 61 is in the range from 0.01 mbar to 100 mbar. In a variant, the pressure in the reduced pressure chamber 61 however is in the range from 0.1 mbar to 100 mbar. In another variant, the pressure in the reduced pressure chamber 61 is in the range from 1 mbar to 100 mbar. In another variant, the pressure in the reduced pressure chamber 61 is in the range from 0.1 mbar to 50 mbar. In another variant, the pressure in the reduced pressure chamber 61 is in the range from 1 mbar to 50 mbar. In another variant, the pressure in the reduced pressure chamber 61 is in the range from 0.1 mbar to 40 mbar. In yet another variant, the pressure in the reduced pressure chamber 61 is in the range from 1 mbar to 40 mbar. In order to achieve and maintain the indicated pressure in the reduced pressure chamber 61, the reduced pressure chamber 61 may comprise some means for achieving and maintaining the pressure in a range from 0.01 mbar to 100 mbar, from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively, in the inside of the reduced pressure chamber 61. Such a means may for example be a vacuum pump. In the present example however, the reduced pressure chamber 61 is the first chamber of a differentially pumped interface 8 which comprises three differentially pumped chambers 8.1, 8.2, 8.3. Thus, the means for achieving and maintaining this pressure in the reduced pressure chamber 61 is a vacuum pump (not shown here) of the differentially pumped interface 8.

[0103] As the dispersion is inserted into the inside of the reduced pressure chamber 61, the aerosol particles are focused by the aerodynamic lens 57 to a focus region which is located in the inside of the reduced pressure chamber 61 in a region where the dispersion is inserted into the inside of the reduced pressure chamber 61 by the flow restricting device 60. Thus, the focus region is located in a region where the dispersion which is inserted into the inside of the reduced pressure chamber 61 is expanding into the reduced pressure chamber 61. Consequently, the focus region is located inside of the reduced pressure chamber 61 where the gas pressure is larger than in other parts of the inside of the reduced pressure chamber 61 which are further distanced from where the dispersion is inserted into the inside of the reduced pressure chamber 61 by the flow restricting device 60.

[0104] Since the pressure in the inside of the reduced pressure chamber 61 is inhomogeneous, the above indicated value of the pressure in the above indicated range from 0.01 mbar to 100 mbar, from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 10 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively, refers to the pressure measured in the inside of the reduced pressure chamber 61 at a first measurement position located where the dispersion is inserted into the inside of the reduced pressure chamber 61 by the flow restricting device 60. Thus, the first measurement position is distanced by maximally 2 cm and thus 2 cm or less from the inlet 56. Thereby, the apparatus 1 may go with or without a first pressure sensor located at the first measurement position for determining the pressure. In a variant however, the above indicated value of the pressure in the above indicated range from 0.01 mbar to 100 mbar, from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively, in the inside of the reduced pressure chamber 61 is the pressure measured by a second pressure sensor in the inside of the reduced pressure chamber 61 at a second measurement position where a gradient of the pressure is less than 10%, preferably less than 5%, particular preferably less than 2% of the maximum gradient of the pressure in the focus region. As a consequence, the second measurement position is distanced from the region where the dispersion is inserted into the reduced pressure chamber 61 and distanced from a position where the means for achieving and maintaining the indicated pressure in the reduced pressure chamber 61 is connected to the reduced pressure chamber 61. Thereby, the second measurement position is distanced by more than 2 cm from the insert 56.

[0105] In another variant, the pressure in the above indicated range refers to the pressure measured at the first measurement position and at the second measurement position, wherein the pressure measured at the respective position is within the indicated range. Thus, in a first variant, the pressure measured at the first measurement position is in the range from 0.01 mbar to 100 mbar, while the pressure at the second measurement position is in the range from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively. In a second variant, the pressure measured at the first measurement position is in the range from 0.1 mbar to 100 mbar, while the pressure at the second measurement position is in the range from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively. In a third variant, the pressure measured at the first measurement position is in the range from 1 mbar to 100 mbar, while the pressure at the second measurement position is in the range from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively. In a fourth variant, the pressure measured at the first measurement position is in the range from 10 mbar to 100 mbar, while the pressure at the second measurement position is in the range from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively. In a fifth variant, the pressure measured at the first measurement position is in the range from 0.1 mbar to 50 mbar, while the pressure at the second measurement position is in the range from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively. In a sixth variant,the pressure measured at the first measurement position is in the range from 1 mbar to 50 mbar, while the pressure at the second measurement position is in the range from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively. In a seventh variant, the pressure measured at the first measurement position is in the range from 0.1 mbar to 40 mbar, while the pressure at the second measurement position is in the range from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively. In an eighth variant, the pressure measured at the first measurement position is in the range from 1 mbar to 40 mbar, while the pressure at the second measurement position is in the range from 0.1 mbar to 100 mbar, from 1 mbar to 100 mbar, from 0.1 mbar to 50 mbar, from 1 mbar to 50 mbar, from 0.1 mbar to 40 mbar or from 1 mbar to 40 mbar, respectively.

[0106] In the inside of the reduced pressure chamber 61, a laser beam of the laser 62 is focused to a spot within the focus region. In this example of FIG. 3, this spot is distanced by 2 cm from the inlet 56. In variations however, this spot is distanced by more than 2 cm or by less than 2 cm from the inlet 56. Independent of the precise distance of the spot from the inlet 56, the parameters of the laser 62 are optimised to induce a plasma in the argon of the dispersion which is inserted via the flow restricting device 60 into the reduced pressure chamber 61. Thus, an argon plasma is generated and maintained in a plasma region 63 around the spot of the laser beam. Due to this argon plasma, the aerosol particles entering the plasma region 63 are atomised and ionised to elemental ions and possible ionised metal oxides.

[0107] In the present example, where the ion source 50 comprises the gas exchange device 52 which substitutes argon for the air in the dispersion, possible metal atoms comprised in the aerosol particles are rather unlikely to become oxidised to ionised metal oxides. Thus, for simplicity reasons, in the following, the explanations are limited to the case of elemental ions.

[0108] Nonetheless, in case the aerosol particles comprise metal atoms, at least some of these metal atoms become oxidised and ionised to ionised metal oxides. These ionised metal oxides can be separated into elemental ions of the metal as described above for example by a fragmenting device. Furthermore they can be analysed by the analysers described below in the same way as described in the summary of the invention.

[0109] In order to optimise the efficiency of the atomisation and ionisation to elemental ions, the parameters of the laser 62, the pressure in the plasma region 63 and the size of the focus region are chosen such that the plasma region 63 is larger than the focus region and that the focus region is located within the plasma region 63. Additionally, these parameters are chosen such that the plasma is steady maintained, wherein a temperature of the plasma is high, up to 10000 K or even higher. Since the plasma is induced in the gas of the dispersion, the gas not only serves as the plasma gas but also enables a collisional cooling of the elemental ions generated from the atomised aerosol particle material.

[0110] Since the plasma region 63 can be chosen to be relatively small, a considerably smaller laser is sufficient as compared to the lasers required in ATOFMS type instruments like apparatus 601 described above in the context of FIG. 2. Thus, considerably less energy is required to power the plasma in the ion source 50 according to the invention.

[0111] There are many types of lasers known in the art which are suitable for laser 62 to generate and maintain the plasma. In an example, the laser 62 is a passive locking mode Nd:YAP laser with a wavelength of 1078 nm with a laser pulse duration of 80 ns and a pulse frequency of 3 kHz. However, any other laser suitable for generating and maintaining the plasma can be employed. In particular, the dispersion inserted into the inside of the reduced pressure chamber 61 comprises another gas than argon, another laser may be better suited.

[0112] The elemental ions resulting from the atomised and ionised aerosol particles are transferred sequentially through the chambers 8.1, 8.2, 8.3 of the differentially pumped interface 8 to the mass analyser 5 for obtaining mass spectra from the elemental ions. In the present example, the mass analyser 5 is a time-of-flight mass analyser. It may however be any other type of mass analyser, too.

[0113] Upon detection of an ion, the mass analyser 5 provides a signal to the electronic data acquisition system 10 for processing the signals received from the mass analyser 5. This electronic data acquisition system 10 comprises at least one analogue-to-digital converter 10.1 producing digitised data from signals obtained from the mass analyser 5 and a fast processing unit 10.2 receiving the digitised data from the analogue-to-digital converter 10.1. The fast processing unit 10.2 is a field programmable gate array and is programmed to continuously, in real time inspect the digitised data for events of interest measured by the mass analyser 5. Furthermore, the electronic data acquisition system 10 is programmed to forward the digitised data representing mass spectra relating to events of interest for further analysis to a computer (not shown) and to reject the digitised data representing mass spectra not relating to events of interest. Thus, the apparatus 1 enables event triggering. How this event triggering works in detail, is known and described in WO 2016/004542 A1 of Tofwerk AG.

[0114] The ion source 50 of apparatus 1 shown in FIG. 3 comprises a collision cell 65 as fragmenting devices for fragmentation of molecules into elements, or for removing molecules by collisions.

[0115] This collision cell 65 is located downstream of the plasma region 63. Within the collision cell 65, ionised debris, in particular ionised molecules, originating from the aerosol particles are fragmented into elemental ions, wherein the collision cell 65 is fluidly coupled to the plasma region 63 in the inside of the reduced pressure chamber 61 for transferring ionised debris, in particular ionised molecules, of the aerosol particles generated in the plasma through the collision cell 65 for fragmenting the ionised debris, in particular ionised molecules, originating from the aerosol particles to elemental ions. Herein, ionised debris comprises anything ionised originating from the aerosol particles. Thus, ionised debris includes the elemental ions as well as other ionised debris like for example ionised molecules or ionised clusters of atoms which have not been atomised in the plasma.

[0116] In the second chamber 8.2 of the differentially pumped interface 8, a quadrupole ion guide 11 is arranged such that elemental ions passing the second chamber 8.2 pass through the quadrupole ion guide 11. This quadrupole ion guide 11 serves as a mass filter. It provides in its inside two superimposed electric fields. A first field is used for transporting the elemental ions from the entrance to the exit of the quadrupole ion guide 11. For this, the field direction is essentially parallel to the quadrupole ion guide 11's main axis, and the field can be static. By tuning the strength of this field, the ions can be accelerated or deaccelerated when being transferred from the entrance to the exit of the quadrupole ion guide 11. A second electric field is applied for confining the elemental ions close to the axis. This second electric field is a radio frequency (RF) quadrupole field with low amplitudes on the chamber axis and larger amplitudes away from the axis. The frequency of the RF quadrupole field is chosen to filter for a specific range of mass per charge ratios: Ions having a mass per charge ratio within the filtered range are transferred through the quadrupole ion guide 11 while ions having another mass per charge ratio are rejected. This range is selected such that elemental ions originating from the aerosol particles are transferred through the quadrupole ion guide 11, while most other ions are rejected. Furthermore, the frequency of the RF quadrupole field is chosen such that argon ions originating from the plasma gas are thrown out of the quadrupole even in case they are within the filtered range of mass per charge ratios.

[0117] The elemental ions which are passed through the quadrupole ion guide 11 are focused by the quadrupole ion guide 11 into an ion beam with a thin diameter. From the quadrupole ion guide 11, they are passed through the differentially pumped interface 8 into the mass analyser 5, where they are analysed.

[0118] In a variant, the quadrupole ion guide 11 extends into the first chamber 8.1 of the differentially pumped interface 8 around the collision cell 65 such that the plasma region in the inside of the reduced pressure chamber is created very close to, or within an ion focusing device like the quadrupole ion guide 11 in order to focus the elemental ions close to the axis after and during the collisional cooling and further atomisation of debris from the aerosol particles within the collision cell 65 mentioned above.

[0119] In a further variant, the ion source 50 comprises a test gas line (not shown) for fluidly coupling a test gas source via the denuder 64 and the flow restricting device 60 with the inside of the reduced pressure chamber 61. The test gas contains known particles with known metal content. Thus, the apparatus 1 for analysing the elemental composition of aerosol particles can be calibrated in a simple way by analysing the test gas.

[0120] In yet a further variant, the ion source 50 comprises a clean gas line (not shown) for fluidly coupling a clean gas source via the denuder 64 and the flow restricting device 60 with the inside of the reduced pressure chamber 61. This clean gas is preferably Argon or Nitrogen.

[0121] In yet a further variant, the ion source 50 may go with an acoustic lens instead of the aerodynamic lens 57.

[0122] FIG. 4 shows a schematic view of another apparatus 101 for analysing an elemental composition of aerosol particles, the apparatus 101 comprising another ion source 150 according to the invention for generating elemental ions from the aerosol particles.

[0123] In the example shown in FIG. 4, the ion source 150 is constructed similar to the ion source 50 shown in FIG. 3. However, the ion source 150 of FIG. 4 does not provide a denuder and does not provide a collision cell as fragmenting device. Otherwise, the aerosol particles are treated by the ion source 150 of FIG. 4 the same as described above in the context of the ion source 50 shown in FIG. 3. Even though not shown in FIG. 4, the ion source 150 comprises as well a laser for inducing the plasma in the plasma region as the ion source 50 shown in FIG. 3 does. Thereby, the plasma is induced in the gas of the dispersion for atomising and ionising the aerosol particles to ions.

[0124] The apparatus 101 shown in FIG. 4 comprises a differentially pumped interface 108 which is somewhat different to the differentially pumped interface 8 of the apparatus 1 shown in FIG. 3. The details of these differences are described below. Furthermore, the apparatus 101 shown in FIG. 4 comprises a dual polarity mass analyser 105 instead of the mass analyser 5 of apparatus 1 shown in FIG. 3. This dual polarity mass analyser 105 comprises two mass analysers within the same mass analysing unit. It enables the analysis of negative ions and of positive ions and provides for both types of ions separate mass spectra. In order to enable the analysis of both types of ions, the mass analyser 105 provides two inlets 106.1, 106.2. One of these inlets 106.1 is for inserting negative ions into the dual polarity mass analyser 150, while the other of these inlets 106.2 is for inserting positive ions into the dual polarity mass analyser 150. Instead of this dual polarity mass analyser 105, the apparatus 101 can also comprise two separated mass analysers, wherein one is adapted for analysing negative elemental ions, while the other one is adapted for analysing positive elemental ions.

[0125] After the aerosol particles are atomised and ionised by the ion source 150 to elemental ions, the elemental ions are separated according to their polarity. Negative elemental ions are transferred into a first bent quadrupole ion guide 112.1, while positive elemental ions are transferred into a second bent quadrupole ion guide 112.2. These two bent quadrupole ion guides 112.1 are both arranged in the first chamber 108.1 of the differentially pumped interface 108 and direct the negative and positive elemental ions, respectively, in opposite directions away from the plasma region to separate orifices to the second chamber 108.2 of the differentially pumped interface 108. Thereby, the negative and positive elemental ions are transferred away from the plasma region in directions different to a direction in which the aerosol particles enter the plasma region. Both the first bent quadrupole ion guide 112.1 and the second bent quadrupole ion guide 112.2 are each adapted for holding two superimposed electric fields, wherein a first electric field of the two superimposed electric fields is a static electric field and wherein a second field of the two superimposed electric fields is a RF multipole field with low amplitudes on an axis of the multipole ion guide and larger amplitudes away from the axis. Furthermore, for both the first bent quadrupole ion guide 112.1 and the second bent quadrupole ion guide 112.2, a strength of the respective first electric field is tuneable.

[0126] After being transferred into the second chamber 108.2, the negative and positive elemental ions are filtered by a first quadrupole ion guide 111.1 and second quadrupole ion guide 111.2, respectively, as described for the quadrupole ion guide 11 shown in FIG. 3. Subsequently, the negative and positive elemental ions are passed through the third chamber 108.3 of the differentially pumped interface 108 into their respective inlet 106.1, 106.2 of the dual polarity mass analyser 105, where they are analysed. Thereby, a pressure in the dual polarity mass analyser 105 is less than 0.0001 mbar. In a variant however, the pressure in the dual polarity mass analyser 105 is less than 0.00001 mbar.

[0127] FIG. 5 shows a schematic view of a more space saving configuration of the apparatus 101 shown in FIG. 4. Here, the differential pumping interfaces and the mass analysers of the two polarities are arranged behind each other.

[0128] FIG. 6 shows a schematic view with reduced details of a modified apparatus 201 for analysing the elemental composition of aerosol particles. This apparatus comprises 201 an aerosol particle ionisation source 230 for ionising the aerosol particles and an ionised aerosol particle mobility analyser 231 for separating ionised aerosol particles according to their mobility. The aerosol particle ionisation source 230 is adapted for ionising aerosol particles without atomising and even without fragmenting the aerosol particles. Furthermore, the ionised aerosol particle mobility analyser 231 can be any ion mobility analyser suitable for analysing the mobility of ionised aerosol particles. In the apparatus 201, the aerosol particle ionisation source 230 and the aerosol particle mobility analyser 231 are arranged upstream of the ion source 50. Thus, the aerosol particles inserted into the apparatus 201 are first ionised by the aerosol particle ionisation source 230 and then separated according to their mobility by the aerosol particle mobility analyser 23. Subsequently, the aerosol particles are atomised and ionised to elemental ions by ion source 50 and the resulting elemental ions are forwarded to detector 5 for being analysed.

[0129] With apparatus 201, the mobility of the aerosol particles can be determined which provides information on the size and cross section of the aerosol particles. Furthermore, with apparatus 201, the aerosol particles are separated according to their mobility when reaching the ion source 50. Thus, analysis of the elemental ions from the aerosol particles can be achieved in single aerosol particle mode where the elemental ions originating from a specific aerosol particle are knowingly analysed as originating from one and the same specific aerosol particle. In order to facilitate this single aerosol particle mode, the above described event triggering can be employed. However, the ion source 50 can also be modified to comprise an aerosol particle detection unit which detects an aerosol particle when entering the plasma region. This aerosol particle detection unit can for example be an optical unit. Furthermore, the ion source 50 can also comprise a control unit. With this control unit, the laser of ion source 50 can be triggered upon detection of an aerosol particle to induce the plasma in the plasma region for atomising and ionising the aerosol particle. Furthermore, with the control unit, the mass analyser 5 can be triggered to analyse the elemental ions originating from the respective aerosol particles. Thus, the laser of the ion source 50 and the mass analyser 5 can be synchronised by the control unit.

[0130] In a variant, the aerosol particle ionisation source and the ionised aerosol particle mobility analyser may be arranged within ion source 50. For example, they may be arranged between the gas exchange unit and the flow restricting device.

[0131] The invention is not limited to the embodiments described above. Various variations of the described embodiments are possible besides the variants which are already described above.

[0132] In summary, it is to be noted that an ion source and a method for generating elemental ions from aerosol particles is created which is suitable for an apparatus and a method for analysing the elemental composition of aerosol particles pertaining to the technical field initially mentioned that enables precise and reliable analysis of the elemental composition of aerosol particles and which can be employed for different types of analysis of the elemental composition of aerosol particles, like for example on-line and real-time analysis in monitoring applications or field applications.