Substantially simultaneous resonance-enhanced multiphoton and laser desorption ionization for single particle mass spectroscopy

Abstract

Devices and methods for mass spectroscopic analysis of particles are disclosed herein. An example device includes: a first irradiation unit configured to irradiate a particle with electromagnetic radiation to cause components of the particle to detach from the particle. The example device further includes a second irradiation unit configured to irradiate substantially simultaneously i) at least a part of the detached components, and optionally a residual core of the particle, with a first beam of electromagnetic radiation the first beam of electromagnetic radiation exhibiting a first intensity, and ii) at least a part of the residual core, of the particle with a second beam of electromagnetic radiation. The second beam of electromagnetic radiation exhibiting a second intensity, which is preferably larger than the first intensity. The example device further includes a mass spectrometer comprising an ion source region, a first detection channel, and optionally a second detection channel.

Claims

1. A device for mass spectroscopic analysis of particles, the device comprising: a first irradiation unit comprising an IR laser, the first irradiation unit configured to irradiate a particle with electromagnetic radiation to cause components of the particle to detach from the particle, the detached components of the particle being located in proximity of a residual core of the particle, a second irradiation unit comprising a single UV laser and irradiating substantially simultaneously at least a part of the detached components and the residual core of the particle, with a first beam of electromagnetic radiation from the single UV laser to cause a resonance-enhanced multiphoton ionization (REMPI) or single-photon ionization (SPI) of at least a part of the detached components, the first beam of electromagnetic radiation exhibiting a first intensity, and at least a part of the residual core of the particle with a second beam of electromagnetic radiation from the single UV laser to cause a laser desorption and non-resonant ionization (LDI) of at least a part of the components of the residual core of the particle, the second beam of electromagnetic radiation exhibiting a second intensity, which is larger than the first intensity, wherein the second irradiation unit comprises a focusing optical element configured to generate the second beam of electromagnetic radiation by focusing at least a part of the first beam, the focusing optical element comprising i) a focusing mirror or ii) a planar mirror and a focusing lens, the first beam generated by the single UV laser impinging on both the detached components and the residual core of the particle, the second beam focused by the focusing optical element impinging only on the residual core of the particle, so that the irradiation of the detached components of the particle with the first beam and the irradiation of the residual core of same particle with the second beam occurs substantially simultaneously and both the REMPI or SPI of the detached components of the particle and the LDI of the components of the residual core of the particle is caused by electromagnetic radiation from the single UV laser directed towards the particle as the first beam and reflected by the focusing optical element back towards the particle as the second beam, wherein the single UV laser is directed towards the particle as the first beam by a deflection mirror or the single UV laser is oriented towards the particle, and a mass spectrometer comprising an ion source region configured to accommodate positive ions (+) and negative ions () of the detached components and/or of the components of the residual core, a first detection channel configured to detect the positive ions (+) and a second detection channel configured to detect the negative ions ().

2. The device according to claim 1, wherein the first beam of electromagnetic radiation is a substantially parallel beam.

3. The device according to claim 1, wherein the second irradiation unit is arranged such that the first beam of electromagnetic radiation impinges at a first side of the detached components and the residual core of the particle, and the focusing mirror is located at a second side of the detached components and the residual core of the particle, wherein the second side is opposite to the first side.

4. The device according to claim 1, wherein the second irradiation unit is configured such that a time difference between the irradiation of the detached components and the residual core of the particle, with the first beam and the irradiation of the residual core of the particle with the second beam is less than 20 ns.

5. The device according to claim 4, wherein the time difference between the irradiation of the detached components and the residual core of the particle, with the first beam and the irradiation of the residual core of the particle with the second beam is less than 5 ns.

6. The device according to claim 4, wherein the time difference between the irradiation of the detached components and the residual core of the particle, with the first beam and the irradiation of the residual core of the particle with the second beam is less than 1 ns.

7. The device according to claim 1, wherein the first detection channel is configured to detect the positive ions (+) with a first detection sensitivity, and/or the second detection channel is configured to detect the negative ions () with a second detection sensitivity.

8. The device according to claim 1, wherein the first detection channel is configured to record a first mass spectrum of the detected positive ions (+), and/or the second detection channel is configured to record a second mass spectrum of the detected negative ions ().

9. The device according to claim 1, wherein the detachment of the components of the particle comprises desorption, ablation, evaporation, or combinations thereof.

10. The device according to claim 1, wherein the second irradiation unit is arranged such that the first beam of electromagnetic radiation impinges at a first side of the particle, and both the planar mirror and the focusing lens are located at a second side of the particle, wherein the second side is opposite to the first side, so that at least a part of the first beam is reflected by the planar mirror and subsequently focused by the focusing lens, whereby the focused beam is impinging on the residual particular core.

11. The device according to claim 1, wherein the second irradiation unit is arranged such that the first beam of electromagnetic radiation impinges at a first side of the particle, and the planar mirror is located at a second side of the particle, wherein the second side is opposite to the first side, whereas the focusing lens is located at the first side of the particle, so that at least a part of the first beam is first focused by the focusing lens and subsequently reflected by the planar mirror such that the focus point of the focused and reflected beam hits the residual particular core.

12. The device according to claim 1, wherein the first beam and second beam of electromagnetic radiation from the single UV laser have a wavelength of 248 nm, so that the first beam ionizes polycyclic aromatic hydrocarbons (PAHs) contained in the detached components, while the second beam from the single UV laser causes, substantially simultaneously, LDI of components of the residual core of the particle resulting in a positive mass spectrum showing inorganic substances at low masses including iron and PAHs in a higher mass range.

13. The device according to claim 1, wherein the first beam and second beam of electromagnetic radiation from the single UV laser have a wavelength of 157 nm, so that the first beam causes SPI of the detached components, while the second beam from the same UV laser causes, substantially simultaneously, LDI of components of the residual core of the particle.

14. The device according to claim 1, wherein the first beam is deflected by the deflection mirror and the electromagnetic radiation from the single UV laser is directed by the deflection mirror towards the particle as the first beam.

15. A method for mass spectroscopic analysis of particles, the method comprising the following steps: a) with a first irradiation unit comprising an IR laser, irradiating a particle with electromagnetic radiation to cause components of the particle to detach from the particle, the detached components of the particle being located in proximity of a residual core of the particle, b) with a second irradiation unit comprising a single UV laser, irradiating substantially simultaneously at least a part of the detached components and the residual core of the particle, with a first beam of electromagnetic radiation from the single UV laser to cause a resonance-enhanced multiphoton ionization (REMPI) or single-photon ionization (SPI) of at least a part of the detached components, the first beam of electromagnetic radiation exhibiting a first intensity, and at least a part of the residual core of the particle with a second beam of electromagnetic radiation from the single UV laser to cause a laser desorption and non-resonant ionization (LDI) of at least a part of the components of the residual core of the particle, the second beam of electromagnetic radiation exhibiting a second intensity, which is larger than the first intensity, wherein positive ions (+) and negative ions () of the detached components and/or of the components of the residual core are accommodated in an ion source region, wherein the second irradiation unit comprises a focusing optical element configured to generate the second beam of electromagnetic radiation by focusing at least a part of the first beam, the focusing optical element comprising i) a focusing mirror or ii) a planar mirror and a focusing lens, the first beam generated by the single UV laser impinging on both the detached components and the residual core of the particle, the second beam focused by the focusing optical element impinging only on the residual core of the particle, so that the irradiation of the detached components of the particle with the first beam and the irradiation of the residual core of same particle with the second beam occurs substantially simultaneously and both the REMPI or SPI of the detached components of the particle and the LDI of the components of the residual core of the particle is caused by electromagnetic radiation from the single UV laser directed towards the particle as the first beam and reflected by the focusing optical element back towards the particle as the second beam, wherein the single UV laser is directed towards the particle as the first beam by a deflection mirror or the single UV laser is oriented towards the particle, and c) detecting the positive ions (+) by a first detection channel and detecting the negative ions () by a second detection channel.

16. The method according to claim 15, wherein the detachment of the components of the particle comprises desorption, ablation, evaporation, or combinations thereof.

Description

(1) Further advantages, features and examples of the present invention will be apparent from the following description of following figures:

(2) FIG. 1 shows an example of a device for spectroscopic analysis of particles at a first point in time;

(3) FIG. 2 shows an example of a device for spectroscopic analysis of particles at a second point in time;

(4) FIG. 3 shows an example of a device for spectroscopic analysis of particles at a third point in time;

(5) FIG. 4 shows a first alternative example of an optical unit of the device;

(6) FIG. 5 shows a second alternative example of an optical unit of the device;

(7) FIG. 6 shows a first example of a positive and negative mass spectrum;

(8) FIG. 7 shows a second example of a positive and negative mass spectrum; and

(9) FIG. 8 shows a third example of a positive and negative mass spectrum.

(10) FIG. 1 shows an example of a device for spectroscopic analysis of particles at a first point in time t.sub.1. The device comprises a first irradiation unit 4, e.g. an infrared (IR) laser, which generates a light beam 4, also referred to as desorption beam, which is directed towards a single particle 1 to cause components of the particle 1 to detach from the particle 1, as indicated by radially extending arrows, whereby a cloud or plume 2 of detached components of the particle 1 is formed around a residual particle core 3.

(11) Alternatively or additionally to using an IR laser to cause components of the particle 1 to desorb from the particle 1, it is possible to use different laser types, in particular a laser type configured to generate ultra-short optical pulses, to cause an ablation of components from the particle 1. Same applies accordingly to laser types configured to cause an evaporation of components from the particle 1.

(12) Preferably, the described desorption of components of the particle 1 is performed in an ion source region 5 of a mass spectrometer, which comprises a first detection channel 6 by which positive ions can be detected, and a second detection channel 9 by which negative ions can be detected. Each of the detection channels 6 and 9 comprises extraction electrodes 7 or 10, respectively, by which positive or negative, respectively, ions are extracted from the ion source region 5 and accelerated towards a detector 8 or 11, respectively, where positive or negative, respectively, ions are detected. Within present disclosure, the detection channels 6 and 9 are also referred to as flight tubes.

(13) The detection channels 6 and 9, including extraction electrodes 7 and 10 and detectors 8 and 11, are arranged at opposing sides of the ion source region 5 of the mass spectrometer.

(14) The device further comprises a second irradiation unit 14 to 16 which is configured to irradiate both the plume 2 of detached components and the residual particle core 3. This will be described in more detail in the following.

(15) FIG. 2 shows an example of a device for spectroscopic analysis of particles at a second point in time t.sub.2, which is preferably 6 to 8 s, in particular approximately 7 s, later than t.sub.1. At the second point in time t.sub.2, the first irradiation unit 4 is preferably in an off state, while an irradiation source 14, for example an ultraviolet (UV) laser, of the second irradiation unit 14 to 16 generates a first beam 17 of, preferably pulsed, radiation which is directed, e.g. by means of deflection element 15, towards the plume 2 of detached components and the residual particle core 3 surrounded by the plume 2 and, after having passed the plume 2, towards optical element 16.

(16) The optical element 16, preferably a focusing mirror, focuses the deflected first beam 17 into a focused second beam 18 which is directed towards the residual particle core 3. Preferably, the focus of the second beam 18 coincides with the residual particle core 3. As a result, the intensity of the second beam 18 impinging on the residual particle core 3 is considerably, preferably at least 10 times, higher than the intensity of the first beam 17 impinging on the plume 3.

(17) Alternatively to generating the focused second beam 18 by focusing a part of the deflected first beam 17 towards the residual particle core 3, the second beam 18 can be generated by another irradiation source 19, for example another ultraviolet (UV) laser, which generates a beam which is focused by optical element 16, e.g. a focusing lens in this case, towards the particle core 3. In this alternative embodiment, The irradiation sources 14 and 19 are preferably configured to generate beams of radiation at different wavelengths, e.g. at 248 nm and 193 nm, respectively.

(18) The deflected first beam 17 and the focused second beam 18 impinge on the plume 2 and the particle core 3, respectively, simultaneously or substantially simultaneously, whereby a possible small time difference of preferably less than 1 ns may result from different light propagation times of the first beam 17 and the second beam 18 prior to impinging on the plume 2 or on particle core 3, respectively.

(19) When impinging on the plume 2 and the residual particle core 3, the deflected first beam 17 causes a resonance-enhanced multiphoton ionization (REMPI) of detached components contained in the plume 2, whereby predominantly positive ions (+), preferably positive ions of PAHs, are generated. Apart from positive ions (+), however, also negative ions (not shown) of components contained in the plume 2 and/or by other ionization processes may be generated.

(20) At the same time or substantially the same time, the focused second beam 18 impinges mainly on the residual particle core 3 and causes a non-resonant desorption and ionization, also referred to as laser desorption and ionization (LDI), of components contained in the particle core 3, whereby both positive ions (+) and negative ions () are generated (see dashed lines illustrating that these ions emerge from the particle core 3 rather than from the plume 2).

(21) The ions generated by REMPI (i.e. predominantly positive ions (+)) and LDI (i.e. positive ions (+) and negative ions ()) are detected by detector 8 of the first detection channel 6 or detector 11 of the second detection channel 9, respectively.

(22) Due to the different ionization mechanisms (i.e. REMPI and LDI) induced by different intensities of the first and second beam 17 and 18, the detection signals generated by the respective detector 8, 11 when detecting ions generated by REMPI of the plume 2 are considerably smaller than the detection signals generated when detecting ions of the particle core 3 generated via LDI.

(23) Yet, in order to ensure a particularly accurate and reliable detection of the ions generated by the different mechanisms, preferably the sensitivity of the respective detection channel 6, 9 is adapted as described in the following.

(24) FIG. 3 shows an example of a device for spectroscopic analysis of particles at a third point in time t.sub.3, which is preferably only few s later than the second point in time t.sub.2. At the third point in time t.sub.3, both the first irradiation unit 4 and the irradiation source 14 (see FIG. 2, not shown in FIG. 3) are in an off state, and the ions that were generated in the ion source region 5 of the mass spectrometer have further propagated towards the detector 8 for positive ions (+) and the detector 11 for negative ions (). For illustration purposes, ions predominantly generated by REMPI of components contained in the plume 2 (see FIGS. 1 and 2) and/or by another ionization process are denoted with reference sign 20, while ions predominantly generated by LDI of components of the residual particle core 3 (see FIGS. 1 and 2) are denoted with reference sign 21.

(25) In order to account for lower detection signals to be expected for ions 20 generated by REMPI or another ionization process compared to detection signals to be expected for ions 21 generated by LDI of the particle core, it is preferred to increase the sensitivity of at least one of the detection channels 6, 9 for heavier ions, e.g. ions having a mass-to-charge ratio of at least 100 being predominantly generated by REMPI, and/or to decrease the sensitivity of at least one of the detection channels 6, 9 for lighter ions, e.g. ions having a mass-to-charge ration of less than 100 being predominantly generated by LDI.

(26) This is preferably achieved by providing a first sensitivity modulating element 12 in the first detection channel 6 and/or a second sensitivity modulating element 13 in the second detection channel 9. Preferably, the sensitivity modulating element 12, 13 has a transmissivity for ions depending on the mass or mass-to-charge ratio of the ions and/or is configured to, preferably quickly, vary its transmissivity with time. Preferably, a control unit 24 is provided which is configured to control at least one of the sensitivity modulating element 12, 13 to vary its transmissivity for ions accordingly.

(27) For example, the sensitivity modulating element 12, 13 is configured as a Bradbury-Nielsen gate exhibiting an attenuated transmission for lighter ions compared to heavier ions. Alternatively, the sensitivity modulating element 12, 13 may comprise an ion optics, also referred to as attenuation ion optics, configured to laterally deflect ions dependent on a time-dependent and/or modulated voltage applied thereto.

(28) In this way, the transmissivity of the elements 12, 13 for ions and, therefore, the sensitivity of the detection channel 6, 9 can be modulated and adapted to the mass of different ions 20, 21 to be detected. Preferably, the lighter ions 21 are detected with a first sensitivity value which is smaller than a second sensitivity value with which the heavier ions 20 are detected.

(29) In the examples of the device shown in FIGS. 1 to 3, the deflection element 15, which is in particular a deflection mirror, and the optical element 16, which is preferably a focusing or concave mirror, form an optical unit by which at least a part of the first beam 17 is converted into a focused second beam 18 directed towards the residual particle core 3. Advantageously, the optical unit according to this embodiment is robust and compact and allows for easily adjusting the LDI intensity of the focused second beam 18 while a parallel first beam for REMPI is maintained.

(30) FIG. 4 shows a first alternative example of an optical unit of the device, wherein the optical element 16 of the optical unit comprises, instead of a focusing and/or concave mirror (see FIGS. 1 to 3), a preferably moveable planar mirror 16a and a preferably moveable focusing lens 16b. Both the mirror 16a and the lens 16b are located (with respect to the deflected first beam 17) behind the particle 1 so that at least a part of the deflected first beam 17 is reflected by the mirror 16a and subsequently focused by the lens 16b, whereby a focused second beam 18 directed towards and/or impinging on the residual particle core 3 is obtained. Advantageously, this alternative embodiment of the optical unit works very well with simple and cheap components 16a, 16b, making e.g. a concave mirror dispensable.

(31) FIG. 5 shows a second alternative example of an optical unit of the device, wherein the optical element 16 of the optical unit also comprises a preferably moveable planar mirror 16a and a preferably moveable focusing lens 16b. In distinction to the example shown in FIG. 4, however, the mirror 16 is located behind the particle 1, whereas the lens 16b is located in front of the particle 1. At least a part of the deflected first beam 17 is first focused by the lens 16b and subsequently reflected by the mirror 16a such that the focus point of the focused and reflected second beam 18 hits the residual particle core 3. Likewise, this alternative embodiment of the optical unit works very well with simple and cheap components 16a, 16b, making e.g. a concave mirror dispensable. Further, the intensity of the first beam 17 impinging on the plume 2 and preferably causing REMPI in the plume 2 as well as the intensity of the second beam 18 impinging on the particle core 3 and preferably causing LDI can be easily adjusted. Last but not least, a narrow beam at the vacuum chamber exit is achieved so that there is less scattering light inside.

(32) Preferably, the device further comprises a processing unit 25 which is configured to analyze a first mass spectrum of the detected positive ions (+) and/or a second mass spectrum of the detected negative ions () by performing a Fourier transformation of the first mass spectrum to obtain a first Fourier spectrum and/or to perform a Fourier transformation of the second mass spectrum to obtain a second Fourier spectrum, and identifying one or more first amplitudes of one or more components of the first Fourier spectrum and/or one or more second amplitudes of one or more components of the second Fourier spectrum. Preferably, the processing unit 25 is further configured to derive information regarding the identity and/or substance class and/or amount, in particular relative amount, of one or more components of the particle based on the one or more first amplitudes and/or one or more second amplitudes. Preferably, applying a Fourier transformation to single mass spectra of particles or other compositions allows for identifying proportions of different classes of molecules within the particle and/or for estimating a proportion of (poly-) aromatic substances without requiring exact knowledge of their exact composition and distribution. In this way, the distribution of PAHs and their derivatives on individual particles can be assessed and their importance in the formation of secondary aerosols and their contribution to the health effects of air pollution can be much better determined.

(33) FIG. 6 shows a first example of a positive and negative mass spectrum of a single particle from ambient air. The positive and negative mass spectra correspond to spectra from conventional ATOF-MS method. As apparent form the figure, the mass spectra of the particle are dominated by inorganics, while only little organic molecular, e.g. with a regularity of m/z of 12 or 14, signals are present, corresponding to small amplitudes of the fast Fourier-transformed (FFT) signals (see inset).

(34) FIG. 7 shows a second example of a positive and negative mass spectrum of a single particle from ambient air. The mass spectra correspond to a combination of conventional ATOF-MS spectra and single-particle PAH spectra. As apparent from the figure, the positive mass spectrum of the particle is dominated by PAHs, wherein organic signals from (alkylated) PAHs yield FFT signals (see inset) of positive ions which are dominated by a regularity of 12.

(35) FIG. 8 shows a third example of a positive and negative mass spectrum of a single particle from ambient air. As apparent from the figure, the mass spectra of the particle are dominated by organics from many fragments (in particular for m/z<100), PAHs, PAH-derivatives, possible oligomers etc. Accordingly, PAHs and derivatives are reflected by FFT signals (see inset) with a regularity of 12 for positive ions, while other organics are reflected with a regularity of 14 for positive, and, in particular, for negative ions.

(36) In the exemplary mass spectra shown in FIGS. 6 to 8 different sensitivities sensitivity 1 and sensitivity 2 of each of the detection channels 6 and 9 (see FIGS. 1 to 3) are indicated. Preferably, positive and negative ions with lower masses m or mass-to-charge ratio values m/z, e.g. below approximately 105, are detected with a first sensitivity sensitivity 1 of the first and second detection channel 6 and 9, respectively, whereas positive and negative ions with higher masses m or mass-to-charge ratio values m/z, e.g. above approximately 105, are detected with a second sensitivity sensitivity 2 of the first and second detection channel 6 or 9, respectively, wherein the second sensitivity sensitivity 2 is preferably higher than the first sensitivity sensitivity 1.