Abstract:
This invention relates to the field of mass spectrometry, and more specifically to a vacuum ultraviolet photoionization and chemical ionization combined ion source, which consists of a vacuum ultraviolet light source and an ion source chamber. An ion acceleration electrode, an ion repulsion electrode, an ion extraction electrode, and a differential interface electrode positioned inside the ion source chamber are arranged along the exit direction of the vacuum ultraviolet light beam in sequence and spaced, coaxial, and parallel from each other. The ion acceleration electrode, the ion repulsion electrode, the ion extraction electrode, and the differential interface electrode are all plate structures with central through holes. The vacuum ultraviolet light beam passes through the central through holes of the electrodes along the axial direction. By utilizing a single vacuum ultraviolet light source, the ion source is feasible to switch between two ionization modes, vacuum ultraviolet photoionization (VUV PI) and chemical ionization (CI), under suitable ion source pressure, thus greatly expanding the range of detectable samples.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of mass spectrometry, and more specifically to a vacuum ultraviolet photoionization and chemical ionization combined ion source. By utilizing a single vacuum ultraviolet light source, the ion source is feasible to switch between two ionization modes, vacuum ultraviolet photoionization (VUV PI) and chemical ionization (CI), under suitable ion source pressure, thus greatly expanding the range of detectable samples. 
     BACKGROUND OF THE INVENTION 
     Electron ionization (EI) source is the most widely used ion source in traditional organic mass spectrometry. EI uses 70-eV electrons to impact organic molecules, and ionizes them to obtain characteristic spectrum for each organic compound, thus realizing accurate qualitative analysis. However, the mass peaks are seriously overlapped while analyzing complex organic mixtures, because of the high degree of fragmentation of organic compounds with EI. Therefore, it&#39;s difficult for EI to achieve rapid and online analysis. Vacuum ultraviolet (VUV) light can be used as a soft ion source for organic compounds with ionization energies (IEs) lower than the photon energy, and is especially propitious for rapid qualitative and quantitative analysis, due to its high molecular ion yield and very low degree of fragmentation. Keyong Hou et. al. (Chinese invention patent No. 200610011793.2) and Peichao Zheng et. al. (Chinese invention patent No. 200810022557.X) have reported the online mass spectrometry with VUV PI ion source. The achieved mass spectra in the patents are simple, including only the molecular ion peaks of the investigated organic compounds, and these features are beneficial to rapid qualitative and quantitative analysis. 
     The energies of photons emitted from VUV light sources are limited by the optical window materials used in the light sources. The highest photon energy that can be transmitted by the currently available optical window material is 11.8 eV, by using the optical window material of LiF. Thus, organic molecules with IEs lower than 11.8 eV can be effectively ionized by VUV PI. However, some organic and inorganic compounds, such as methane (CH 4 , IE=12.61 eV), acetonitrile (CH 3 CN, IE=12.20 eV), sulfur dioxide (SO 2 , IE=12.35 eV), and nitrous oxide (N 2 O, IE=12.89 eV), cannot be ionized by the VUV photons emitted from the existing VUV light sources. Therefore, the applications of VUV PI mass spectrometry subject to certain restrictions. 
     SUMMARY OF THE PRESENT INVENTION 
     An object of the present invention is to provide a VUV PI and CI combined ion source for mass spectrometry, which combines CI mode into VUV PI source and ionizes molecules with IEs higher than the photon energy. 
     Accordingly, in order to accomplish the above objective, the present invention provides a VUV PI and CI combined ion source for mass spectrometry, wherein the combined ion source consists of a vacuum ultraviolet light source and an ion source chamber. The ion source chamber includes a gas vent on its side wall. A vacuum gauge is connected to a through hole on the side wall of the ion source chamber by means of a vacuum line. 
     An ion acceleration electrode, an ion repulsion electrode, an ion extraction electrode, and a differential interface electrode are positioned inside the ion source chamber. The vacuum ultraviolet light source emits vacuum ultraviolet light beam inside the ion source chamber. The ion acceleration electrode, the ion repulsion electrode, the ion extraction electrode, and the differential interface electrode are arranged along the exit direction of the vacuum ultraviolet light beam in sequence and spaced, coaxial, and parallel from each other. 
     A reagent gas sampling tube passes through the outer wall of the ion source chamber and inserts into the inner side of the ion source chamber. The gas outlet of the reagent gas sampling tube is located in the interspace between the ion acceleration electrode and the ion repulsion electrode and is orthogonal to the vacuum ultraviolet light beam, and the gas inlet of the reagent gas sampling tube is connected to a reagent gas source. 
     A sample gas sampling tube passes through the outer wall of the ion source chamber and inserts into the inner side of the ion source chamber. The gas outlet of the sample gas sampling tube is located in the interspace between the ion repulsion electrode and the ion extraction electrode and is orthogonal to the vacuum ultraviolet light beam, and the gas inlet of the sample gas sampling tube is connected to a sample gas source. 
     The ion acceleration electrode, the ion repulsion electrode, the ion extraction electrode, and the differential interface electrode are all plate structures with central through holes. The ion extraction electrode is one or more than one plate structure which is spaced, coaxial, and parallel from each other. The vacuum ultraviolet light beam passes through the central through holes of the electrodes along the axial direction of the electrodes. 
     A reactant ion region is located in the central zone between the ion acceleration electrode and the ion repulsion electrode, where reactant ions necessary for chemical ionization are generated. The length of said reactant ion region is in the range of 0.1 to 2 cm. 
     An sample ion region is located in the central zone between the ion repulsion electrode and the differential interface electrode, where vacuum ultraviolet photoionization and chemical ionization of sample molecules occur. The length of the sample ion region is in the range of 0.1 to 10 cm. 
     A series of voltages in the sequence from high to low are applied to the ion acceleration electrode, the ion repulsion electrode, the ion extraction electrode, and the differential interface electrode. The ion acceleration electric field formed along the axis direction of the reactant ion region is in the range of 5 to 500 V/cm. The ion extraction electric field formed along the axis direction of the sample ion region is in the range of 1 to 50 V/cm. 
     There is a restriction orifice in the central part of the ion repulsion electrode. The vacuum ultraviolet photons and ions in the reactant ion region are transferred into the sample ion region through the restriction orifice. The diameter of the restriction orifice is in the range of 0.5 to 5 mm. 
     The vacuum ultraviolet light source is a gas discharge lamp, or a laser-based light source, or a synchrotron light source. 
     There is a differential interface pinhole on the differential interface electrode and connected to a mass analyzer. The ions in the ion source chamber are transferred into the mass analyzer through the differential interface pinhole. 
     The mass analyzer is a time-of-flight mass analyzer, or a quadrupole mass analyzer, or an ion trap mass analyzer. 
     A gas vent is placed on the side wall of the ion source chamber and connected to an outlet valve through a vacuum line. A mechanical vacuum pump is connected to the other side of the outlet valve through another vacuum line. 
     The outlet valve is a flow adjustable vacuum valve. The outlet valve is a vacuum flapper valve, or a vacuum butterfly valve, or a vacuum needle valve. 
     The sample load and the pressure inside the ion source chamber can be adjusted by regulating the flow of the outlet valve, and the inner diameters and lengths of the reagent gas sampling tube and the sample gas sampling tube. The inner diameters of the reagent gas sampling tube and the sample gas sampling tube are in the range of 50 to 530 μm, and the lengths of the reagent gas sampling tube and the sample gas sampling tube are in the range of 5 to 500 cm. The sample load is in the range of 0.1 to 200 mL/min. The pressure inside the ion source chamber is in the range of 10 −3  to 10 mbar. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a VUV PI and CI combined ion source according to the invention. 
         FIG. 2  shows the VUV PI mass spectrum of a benzene, toluene and p-xylene gas mixture with a respective concentration of 10 ppm in Example 1. 
         FIG. 3  shows the VUV PI-CI mass spectra of O 2   +  reactant ions (a), 10 ppm propenenitrile (b) and 10 ppm chloroform (c), while using O 2  as the reagent gas in Example 2. 
         FIG. 4  shows the VUV PI-CI mass spectra of NO reactant ions (a), 10 ppm propenenitrile (b) and 10 ppm acetic acid (c), while using NO as the reagent gas in Example 2. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a schematic diagram of the presented invention is illustrated. A combined ion source provided in the present invention consists of a reagent gas sampling tube  1 , a sample gas sampling tube  2 , a vacuum ultraviolet light source  3 , an ion acceleration electrode  4 , an ion repulsion electrode  5 , an ion extraction electrode  7 , a differential interface electrode  8 , and an ion source chamber  13 . 
     The ion acceleration electrode  4 , the ion repulsion electrode  5 , the ion extraction electrode  7 , and the differential interface electrode  8  are positioned inside the ion source chamber  13 . The vacuum ultraviolet light source  3  emits vacuum ultraviolet light beam  19  inside the ion source chamber  3 . The ion acceleration electrode  4 , the ion repulsion electrode  5 , the ion extraction electrode  7 , and the differential interface electrode  8  are arranged along the exit direction of the vacuum ultraviolet light beam  19  in sequence and spaced, coaxial, and parallel from each other. 
     The reagent gas sampling tube  1  passes through the outer wall of the ion source chamber  13  and inserts into the inner side of the ion source chamber  13 . The gas outlet of the reagent gas sampling tube  1  is located in the interspace between the ion acceleration electrode  4  and the ion repulsion electrode  5  and is orthogonal to the vacuum ultraviolet light beam  19 , and the gas inlet of the reagent gas sampling tube  1  is connected to a reagent gas source  17 . 
     The sample gas sampling tube  2  passes through the outer wall of the ion source chamber  13  and inserts into the inner side of the ion source chamber  13 . The gas outlet of the sample gas sampling tube  2  is located in the interspace between the ion repulsion electrode  5  and the ion extraction electrode  7  and is orthogonal to the vacuum ultraviolet light beam  19 , and the gas inlet of the sample gas sampling tube  2  is connected to a sample gas source  18 . 
     The ion acceleration electrode  4 , the ion repulsion electrode  5 , the ion extraction electrode  7 , and the differential interface electrode  8  are all plate structures with central through holes. The ion extraction electrode  7  is one or more than one plate structure which is spaced, coaxial, and parallel from each other. The vacuum ultraviolet light beam  19  passes through the central through holes of the electrodes along the axial direction of the electrodes. 
     A reactant ion region  11  is located in the central zone between the ion acceleration electrode  4  and the ion repulsion electrode  5 , where reactant ions necessary for CI are generated. The length of said reactant ion region  11  is in the range of 0.1 to 2 cm. An sample ion region  12  is located in the central zone between the ion repulsion electrode  5  and the differential interface electrode  8 , where VUV PI and CI of sample molecules occur. The length of the sample ion region  12  is in the range of 0.1 to 10 cm. 
     A series of voltages in the sequence from high to low are applied to the ion acceleration electrode  4 , the ion repulsion electrode  5 , the ion extraction electrode  7 , and the differential interface electrode  8 . The ion acceleration electric field formed along the axis direction of the reactant ion region  11  is in the range of 5 to 500 V/cm. The ion extraction electric field formed along the axis direction of the sample ion region  12  is in the range of 1 to 50 V/cm. 
     There is a differential interface pinhole  9  on the differential interface electrode  8  and connected to a mass analyzer  10 . The ions in the ion source chamber  13  are transferred into the mass analyzer  10  through the differential interface pinhole  9 . 
     Ambient sample gas and reagent gas are directly introduced into the ion source chamber  13  through two sampling tubes. The analytes are ionized by VUV PI or CI, and the product ions are directly transferred into the mass analyzer  10  through the differential interface pinhole  9  on the differential interface electrode  8 . A mechanical vacuum pump  15  is connected to the rear end of an outlet valve  14  (flow adjustable vacuum valve) through a vacuum line. The excess gas molecules in the ion source chamber  13  are pumped away by the mechanical vacuum pump  15  through the outlet valve  14 . The gas sample load is controlled by regulating the flow of the outlet valve  14  and the inner diameters and lengths of the reagent gas sampling tube  1  and the sample gas sampling tube  2 . The pressure in the ion source chamber  13  is adjusted to induce enough collisions between ions and molecules in the ion source region, which meets the requirement of ion-molecule reactions in CI. The inner diameters of the reagent gas sampling tube  1  and the sample gas sampling tube  2  are in the range of 50 to 530 μm, and the lengths of the reagent gas sampling tube  1  and the sample gas sampling tube  2  are in the range of 5 to 200 cm. The gas sample load is in the range of 0.1 to 100 mL/min. The vacuum degree in the ion source chamber  13  is maintained at 10 −3  to 10 mbar. 
     The combined ion source provided by the present invention is capable of fast switching between VUV PI mode and VUV PI-CI mode through simply regulating the sampling gas and the operation voltage of the ionization region. In VUV PI mode, only the sample gas is introduced to the sample gas sampling tube  2 , and the molecules in the sample gas with IEs lower than the photon energy are ionized by VUV PI. The molecular ions of the analytes in the reactant ion region  11  and sample ion region  12  are extracted to the mass analyzer  10  through the differential interface pinhole  9 , under the propulsion of weak ion acceleration electric field and ion extraction electric field. 
     In VUV PI-CI mode, the reagent gas and the sample gas are introduced to the reagent gas sampling tube  1  and the sample gas sampling tube  2 , respectively. While the IE of the reagent gas is higher than the photon energy, the ion acceleration electric field in the reactant ion region  11  is enhanced, and the photoelectrons generated by irradiation of VUV light on the surfaces of metal electrodes are accelerated to high kinetic energies to impact with the reagent gas molecules and ionize them via EI. While the IE of the reagent gas is lower than the photon energy, the reagent gas molecules in the reactant ion region  11  are ionized by VUV PI under the propulsion of weak ion acceleration electric field. The reactant ions generated in the reactant ion region  11  enter the sample ion region  12  through a restriction orifice  6  in the central part of the ion repulsion electrode  5 . The collisions between the reactant ions and the sample molecules in the sample ion region  12  lead to ion-molecule reactions under certain ion source pressure. The molecules in the sample gas with IEs higher than the photon energy are ionized by CI, while some of the molecules in the sample gas with IEs lower than the photon energy are ionized by VUV PI and the others by CI. The product ions obtained eventually are transferred into the mass analyzer  10  through the differential interface pinhole  9  and analyzed by the mass analyzer  10 . 
     EXAMPLE 1 
     To investigate the performance of the combined ion source in VUV PI mode, a commercial krypton (Kr) lamp with the photon energy of 10.6 eV was used as the VUV light source. The combined ion source was coupled with a time-of-flight mass analyzer. Two 250 μm i.d., 100 cm long fused silica capillaries were used as the reagent gas sampling tube and the sample gas sampling tube, respectively. In VUV PI mode, the pressure in the ion source chamber was adjusted to 0.3 mbar, and the sample load was 30 mL/min. The strengths of the ion acceleration electric field and the ion extraction field were both set at 6 V/cm.  FIG. 2  shows the VUV PI mass spectrum of a benzene (IE=9.24 eV), toluene (IE=8.83 eV) and p-xylene (IE=8.44 eV) gas mixture with a respective concentration of 10 ppm. As can be seen from the figure, the achieved mass spectrum is simple, containing mainly the molecular ion peaks of the organic constituents with IEs lower than the VUV photon energy by VUV PI, which is in favor of rapid and online qualitative and quantitative analysis of complex mixtures. 
     EXAMPLE 2 
     To investigate the performance of the combined ion source in VUV PI-CI mode, oxygen (O 2 , IE=12.07 eV) with IE higher than 10.6 eV and nitric oxide (NO, IE=9.26 eV) with IE lower than 10.6 eV were chosen as the reagent gas. Two 250 μm i.d., 100 cm long fused silica capillaries were used as the reagent gas sampling tube and the sample gas sampling tube, respectively. The pressure inside the ion source chamber was adjusted to 0.3 mbar, and the sample load was 30 mL/min. While O 2  was used as the reagent gas, high-purity O 2  (99.999%) was introduced into the reagent gas sampling tube, and a sample gas containing 10 ppm propenenitrile (CH 3 CN, IE=12.20 eV) or 10 ppm chloroform (CHCl 3 , IE=11.37 eV) was simultaneously introduced into the sample gas sampling tube. The strengths of the ion acceleration electric field and the ion extraction field were set at 100 V/cm and 6 V/cm, respectively. The acquired mass spectra are shown in  FIG. 3 .  FIG. 3( a )  shows mass spectrum of O 2   +  reactant ions obtained by introducing O 2  reagent gas alone, and  FIG. 3( b )  and  FIG. 3( c )  are the CI mass spectra of propenenitrile and chloroform with O 2   +  reactant ion peaks subtracted. It can be clearly seen that pure O 2   +  reactant ions with high intensity could be obtained from the combined ion source of the present invention, and molecules with IEs higher than the VUV photon energy were effectively ionized by CI by using O 2   +  as the reactant ions. The acquired mass spectra of the analytes contained only a few fragment ion peaks. 
     While NO was used as the reagent gas, 3% NO diluted in high-purity He was introduced into the reagent gas sampling tube, and a sample gas containing 10 ppm propenenitrile (CH 3 CN, IE=12.20 eV) or 10 ppm acetic acid (CH 3 COOH, IE=11.65 eV) was simultaneously introduced into the sample gas sampling tube. The pressure inside the ion source chamber was adjusted to 0.3 mbar, and the strengths of the ion acceleration electric field and the ion extraction field were both set at 6 V/cm. The acquired mass spectra are shown in  FIG. 4 . It can be clearly seen that pure NO +  reactant ions with high intensity could be obtained from the combined ion source of the present invention either, and molecules with IEs higher than the VUV photon energy were effectively ionized by CI by using NO +  as the reactant ions. The acquired mass spectra of the analytes M contained mainly peaks of quasi-molecular ion [MNO] + .