Vacuum Ultraviolet Lamp Based Magnetic Field Enhanced

Vacuum Ultraviolet Lamp Based Magnetic Field Enhanced Photoelectron Ionization and Single Photon Ionization Source for Online Time-of-Flight Mass Spec...
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Vacuum Ultraviolet Lamp Based Magnetic Field Enhanced Photoelectron Ionization and Single Photon Ionization Source for Online Time-of-Flight Mass Spectrometry Qinghao Wu,†,‡ Lei Hua,†,‡ Keyong Hou,† Huapeng Cui,†,‡ Wendong Chen,†,‡ Ping Chen,†,‡ Weiguo Wang,† Jinghua Li,† and Haiyang Li*,† † ‡

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China Graduate School of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

bS Supporting Information ABSTRACT:

A magnetic field enhanced photoelectron ionization (MEPEI) source combined with single photon ionization (SPI) was developed for an orthogonal acceleration time-of-flight mass spectrometer (oaTOFMS). A commercial radio frequency (rf) powered vacuum ultraviolet (VUV) lamp was used as SPI light source, and the photoelectrons generated by photoelectric effect were accelerated to induce electron ionization (EI). The MEPEI was obtained by applying a magnetic field of about 800 G with a permanent annular magnet. Compared to a nonmagnetic field photoelectron ionization source, the signal intensities for SO2, SF6, O2, and N2 in MEPEI were improved more than 2 orders with the photoelectron energy around 20 eV, while most of the characteristics of soft ionization still remained. Simulation with SIMION showed that the sensitivity enhancement in MEPEI was ascribed to the increase of the electron moving path and the improvement of the electrons transmission. The limits of detection for SO2 and benzene were 750 and 80 ppbv within a detection time of 4 s, respectively. The advantages of the source, including broad range of ionizable compounds, reduced fragments, and good sensitivity with low energy MEPEI, were demonstrated by monitoring pyrolysis products of polyvinyl chloride (PVC) and the intermediate products in discharging of the SF6 gas inpurity.

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ingle photon ionization (SPI) is one of the most popular ionization methods used for online monitoring, due to its high molecular ion yield and simple spectrum interpretation.1 7 However, molecules with ionization energy (IE) higher than photon energy cannot be ionized. Therefore, SPI is seldom used for detecting inorganic compounds because their IEs are usually higher than 11 eV. There are two available methods to expand the range of analytes for SPI. One is using light sources with higher photon energy, such as VUV synchrotron radiation (as high as 13 eV)8 11 and tunable VUV laser (as high as 11.5 eV limited by window materials).12 However, the fixed location and expensive cost of the VUV synchrotron radiation obviously limit its applications to mass spectrometry. The disadvantages for tunable VUV laser are the complication and high cost of laser components. The other r 2011 American Chemical Society

method to expand the ionization ability is combining a hard ionization source with soft SPI. Zimmermann and co-workers reported a SPI source coupling with a typical hot-filament EI source to ionize compounds through either SPI or EI.13 However, in most EI sources, the pressure around the filament was kept as low as possible to prolong the lifetime of the heated filament, which limited the molecular density in the ion source and consequently restricted further improvement of the sensitivity. Moreover, hot filament was readily damaged by oxidative gas; thus, it was difficult to analyze air samples directly. Furthermore, fragments and overlapped peaks in standard 70 eV EI Received: July 12, 2011 Accepted: October 21, 2011 Published: October 21, 2011 8992

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Figure 2. Mass spectra in MEPEI (a) and PEI (b) for 1000 ppmv SO2 and 500 ppmv benzene.

Figure 1. Schematic diagram of the MEPEI source coupled with an orthogonal acceleration time-of-flight mass spectrometer.

spectra make it extremely difficult to identify molecular species in a complex mixture without a prior separation step, such as gas or liquid chromatography. Photoelectrons generated through photoelectric effect can avoid some disadvantages caused by the hot filament, especially for the working pressure limitation.6,14 19 Zenobi and co-workers discussed the mass spectra with EI characteristics using photoelectrons generated by a VUV lamp in an ion trap mass spectrometer.20 Our previous work reported a photoelectron ionization (PEI) source which can generate spectra with characteristics of EI under pressure of 0.2 Pa, but the sensitivity for high IE compounds was relatively poor (20 ppmv for SO2) due to the low density of photoelectron emitted (20 30 nA).21 As is known, the ionization efficiency of EI can be increased by using a magnetic field.22,23 Compared to the perpendicular magnetic field geometry used in a typical EI source, better sensitivity for EI had been achieved with an axial magnetic field ion source, where both the electron beam and ion beam were on the same axis as the magnetic field.24 27 However, the electron generation assembly using a hot filament in ion sources hampered the applications in detecting highly reactive compounds. In this study, we designed a magnetic field enhanced photoelectron ionization (MEPEI) source combined with SPI for an orthogonal acceleration time-of-flight mass spectrometer (oaTOFMS). A commercial VUV lamp was used to generate photoelectrons and induce SPI. To understand the mechanism of the magnetic field enhancement, the ion source was simulated by using the SIMION program. The advantages of MEPEI source was illustrated by online monitoring of the thermal decomposition products of polyvinyl chloride (PVC) and the intermediate components in the discharging of impure SF6 gas. Concept and Instrumentation of the MEPEI Source. The MEPEI and SPI combined ion source coupled to an oaTOFMS are shown in Figure 1. The ion source consists of three parts: a VUV light source, an MEPEI ion extraction assembly, and ion lenses. A commercial radio frequency (rf) excited krypton discharge lamp (PKR106, Heraeus Noble light Analytics Ltd.,

Cambridge, UK) was set outside the ionization chamber and sealed with an O-ring. The lamp emitted two kinds of photons, 116.9 nm (10.6 eV) and 123.9 nm (10.0 eV), and the output photon flux of 123.9 nm was 3-fold higher than that of 116.9 nm. The MEPEI ions extraction assembly consisted of repelling electrodes, a permanent annular magnet, a focusing lens, and a skimmer. Two sets of repelling electrodes (stainless steel, 21.8 mm in diameter) were fixed under the VUV lamp. A NdFeB magnet, which offered a maximum energy product (BHmax) of 38 MG Oe (megagauss oersteds), was fixed on the focusing lens to obtain a strong magnetic field. The dimensions of the annular magnet were 21.8 mm in inner diameter, 37.4 mm in outer diameter, and 2.9 mm in thickness. A nonmagnetic electrode with the same dimensions was used to keep the electric field constant when comparing the ion signal intensity between magnetic and nonmagnetic electrode. A skimmer with a hole of 1 mm in diameter was used to control the electron energy and maintain a pressure drop of about 2 orders between the ion source and the mass analyzer. Three sets of lenses (stainless steel, 9 mm in diameter and 5 mm thick) were fixed on the chamber of the mass analyzer to control the ions passing through skimmer. Gas sample was introduced into the ionization region through a fused silica capillary (100 μm i.d., 30 cm long). The working pressure in the source was kept at about 0.1 Pa with a 110 L/s turbo-molecular pump (KYKY Technology Development Ltd., Beijing, China). The homemade oaTOFMS analyzer was reported in detail previously.2,21 The electrons were generated by the photoelectric effect when VUV light irradiated on electrodes and consequently accelerated in electric field to the energy high enough to ionize molecules. The electron energy in the ionization process depended on the initial energy of electron ejected and energy obtained from the electric field. The maximum initial energy of an ejected electron is 5.6 eV [the dominant photon energy (10.0 eV) minus the working function of stainless steel (4.4 eV)]; however, the most probable energy along the acceleration axis is around 2.8 eV (5.6/2 eV). The energy that the electron obtained in the electric field can be approximately calculated by subtracting the voltage on the skimmer from the voltage on focusing lens. According to the optimized voltages in the study (12 V on focusing lens and 6 V on skimmer), the dominant electron energy was around 20 eV. 8993

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Figure 3. Simulation of the trajectories for the ion source with SIMION 8.04. The trajectories of electrons, ions, and isopotential lines (black lines) are shown. The blue and black rectangles are magnet poles with magnetic potentials of 1200 and 1200 Mags, respectively. The magnet field intensity in the labeled area is 890 G, detected with a Hall effect sensor. (a) Electrons trajectories in YX axial direction view with nonmagnetic electrode. (b) Electrons trajectories in YX axial direction view with magnetic electrode. (c) Electrons trajectories in ZY axial direction view with nonmagnetic electrode. (d) Electrons trajectories in ZY axial direction view with magnetic electrode. (e) Ions trajectories in ZY axial direction view with nonmagnetic electrode. (f) Ions trajectories in ZY axial direction view with magnetic electrode.

As SPI and PEI took place simultaneously in the ion source, analytes with IEs higher than photon energy (10.6 eV in this setup) can only be ionized by PEI; otherwise, it can be ionized by both PEI and SPI. A mixture of SO2 (IE 12.35 eV, 1000 ppmv) and benzene (IE 9.24 eV, 500 ppmv) was used to test the performance of the MEPEI.

’ RESULTS AND DISCUSSION Performance of the MEPEI. The performance of the MEPEI source was tested and compared with nonmagnetic electrode PEI source. The voltage applied to repelling electrode, focusing lens, and skimmer was 15, 12, and 6 V, respectively. At such voltages, the dominant electron energy was around 20 eV, a relative low energy compared to typical EI source but high enough to ionize almost all of the inorganic and organic compounds. The mass spectra taken in MEPEI with magnetic electrode and PEI with nonmagnetic electrode are shown in Figure 2. Adding a magnetic field in the ionization source can not only increase the ion signals for compounds of IE > 10 eV but also increase the ion

signals for compounds of IE < 10 eV. The ion intensities for SO2 in MEPEI and PEI were 796 and 21 counts, respectively, presenting an enhancement of about 38-fold, while for benzene the signal intensity in MEPEI increased about 2-fold from 2154 to 4112 counts. Because the photon flux of the VUV lamp and the electron energy were kept the same during all the experiments, the increase of the ion intensities should be attributed to the increase of the EI efficiency. This deduction was proved by the calculation from electron impact ionization cross section at an electron energy of 19 eV, which agreed well with the electron energy evaluated (Supporting Information, Figure S-1). SIMION Simulation Analysis for the MEPEI. In order to understand the MEPEI, the electron trajectories and equipotential lines in the source were calculated by using the program SIMION 8.04 (Figure 3). The magnetic field was defined with magnetic scalar potentials.28 The magnetic potentials of the two magnetic poles (blue and black rectangles in Figure 3d,f) were set to the value measured by a Hall effect sensor (HT20, Hengtong Magnet Tech Ltd. Shanghai, China). The magnetic field intensity in the labeled area was 890 G, detected with Hall effect sensor 8994

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Figure 5. Ion signal ratios of magnet and nonmagnetic electrode for SF6, O2, N2, and H2O molecules as function of voltages applied to the skimmer.

Figure 4. The ion intensity with magnetic and nonmagnetic electrode as a function of voltages applied to the skimmer electrode for benzene (a) and SO2 (b). The blue lines represent the signal ratios for magnetic electrode/nonmagnetic electrode.

(Figure 2d), which was very close to the simulated value of 850 G. The permeability of the magnet was ignored because the electrodes, composed of austenitic stainless steel (type 304), have little response to the magnetic field (magnetic permeability is 1.003 1.005). The direction of the magnetic field around the skimmer and focusing lens in the simulation was also verified by using the Hall effect sensor. All of the voltages applied to electrodes were the same as those in the experiment. The simulation results indicated that the EI efficiency increased through two ways with the magnetic electrode. On one hand, the electrons spiraled in the magnetic field (Figure 3b,d) instead of going straight in the nonmagnetic field (Figure 3a,c), which resulted in a great improvement of the electron path. On the other hand, about 24.4% of photoelectrons collided with the focusing lens with a nonmagnet electrode (Figure 3c), while all the photoelectrons passed through the focusing lens along helical trajectories with the magnet electrode (Figure 3d). Although the magnetic field strongly affected the electron trajectories, it had little effect on the transmission of ions (46.7% with magnetic electrode and 48.0% with nonmagnetic electrode, Figure 3e,f), due to the mass of ions being more than 3 orders heavier than that of electrons. The Characteristics of MEPEI Source. The energy of photoelectron, adjusted by the voltage applied to the skimmer, is one of the main factors to optimize the ion signal intensity. The mass spectra of SO2 and benzene with different voltages applied to the skimmer are showen in Figure 4a,b. The ion intensities of both SO2 and benzene increased rapidly when the skimmer voltage changed from 0 to 10 V and remained almost constant as the skimmer voltage fell below 20 V. This observation can be attributed to the positive correlation between the electron ionization

cross section and the electron energy.29 The ion intensity ratios enhanced by magnetic field, calculated by dividing ion intensity with magnetic electrode by ion intensity with nonmagnetic electrode, are also shown in Figure 4a,b with blue lines to reveal the extent of magnetic enhancement. A remarkable improvement as high as 120-fold for SO2 was achieved at a skimmer voltage of 4 V, while the improvement for benzene was only about 2-fold. The results can be well explained by the improvement of EI efficiency: sulfur dioxide are ionized through EI; therefore, the signal intensity increased significantly with the improvement of EI efficiency, whereas benzene was ionized by both SPI and EI, the 2-fold improvement of signal intensity revealing that the benzene ionized by EI was approximately equal to that ionized by SPI. The magnetic enhancement ratios of SF6 (IE 15.32 eV), O2 (IE 12.07 eV), N2 (IE 15.58 eV), and H2O (IE 12.62 eV) were also tested to confirm the signal intensity enhancement with magnetic electrode. As shown in Figure 5, the best improvements of these molecules appeared at the voltages between 2 and 8 V and declined steadily below 10 V, which were similar to those of SO2. In general, more than 2 orders of enhancement in signal intensity can be obtained with low-energy MEPEI. The limits of detection (LODs) (S/N = 3) were calculated on the basis of mass spectra accumulated for 4 s with a repetition rate of 25 kHz. The voltages applied to the repelling electrode, the focusing lens, and the skimmer were 15, 12, and 6 V, respectively. The mass spectra of SO2 (10 ppmv) and benzene (5 ppmv) are shown in Figure 6. According the ion counts and background noise observed in Figure 6, the LODs for SO2 and benzene are evaluated to be 750 and 80 ppbv, respectively. The sensitivity in low-energy MEPEI for SO2 has been improved by more than 20-fold over that reported in our previous work.21 A similar TOFMS with hot-filament EI and electron beam pumped rare gas excimer lamp source had achieved LODs of 700 ppb in EI and 35 ppb in SPI for toluene with 65 000 extractions based on S/N = 2.13 The low-energy EI such as 20 eV was extensively studied previously.30 32 Low-energy EI can reduce fragments and 8995

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Figure 6. Evaluation of the limits of detection of SO2 and benzene with MEPEI source. The voltages applied to the repelling electrodes, focusing lens, and the skimmer were 15, 12, and 6 V, respectively.

enhance molecular ions, but the sensitivity decreases very much due to much smaller collision cross section in low-electron energy comparing to standard 70 eV EI. Acrylonitrile was a good example to show the characteristic of fragments with different electron energy, because it cannot be ionized by SPI due to the IE of 10.91 eV and has a main fragment of CN+ at m/z = 26 in 70 eV EI spectrum (Supporting Information, Figure S-3). The intensity of the molecular ion of C3H3N+ increased only 50% when the electron energy increased from 20 to 70 eV, while the intensity of fragment ion CN+ increased more than 20-fold. Therefore, the electron energy in MEPEI at 20 eV is a good choice to balance the sensitivity and fragmentation. Application on Monitoring Products in Pyrolysis of PVC. PVC is one of the most widely used plastics, because it is cheap and durable and has low reactivity. However, at temperature above 200 °C, it liberates many toxic pollutants, including HCl, aromatic hydrocarbons, chlorinated aromatic compounds, and even dioxins.33 Measuring intermediate products in the pyrolysis of PVC gives clues toward the understanding of the decomposition mechanism in the pyrolysis process. Online mass spectrometry with soft ionization is one of the important tools to study the process, because of the speediness and ease with which spectrum are interpreted. Zimmermann and co-workers observed benzene (IE 9.24 eV), toluene (IE 8.83 eV), and chlorinated aromatic compounds with SPI and resonanceenhanced multiphoton ionization (REMPI), which provided new information on the compositions of the pyrolysis products than previous studies. However, as one of the main products in the pyrolysis process, HCl (IE 12.74 eV) was not detected because of its high IE.34 With an improved photon energy of 13 eV, Qi and co-workers detected a great amount of HCl in the same process using a VUV synchrotron radiation source.11 Parts a and b of Figure 7 show the mass spectra for pyrolysis products of PVC powder at temperatures of 261 and 430 °C, respectively. Compounds with wide IEs, such as HCl, alkenes, dienes, benzene, toluene, and naphthalene, were well-detected. The low-energy MEPEI source, being much simpler and cheaper than those mentioned above, can provide more information due to its wide ionization potential and good sensitivity. Figure 7c shows the relative intensities of H35Cl, C4H8, C6H6 (benzene), C7H8 (toluene), and C8H10 (xylene or ethylbenzene)

Figure 7. Mass spectra for pyrolysis products of PVC powder at temperatures of (a) 261 °C and (b) 430 °C. (c) The ion intensities for H35Cl, C4H8, C6H6 (benzene), C7H8 (toluene), and C8H10 (xylene or ethylbenzene) as a function of the temperatures of the pyrolysis tube. The PVC powder was placed in a quartz tube with the rate of temperature increase of 10 °C min 1 and a pure N2 flux of 50 mL min 1.

as a function of the pyrolysis temperature. It can be seen that there were two mass loss regions at temperature between 230 to 330 °C and above 350 °C, which agreed well with previous study using thermogravimetric analysis.35 At the first mass loss region, HCl and benzene were the dominant products (Figure 7a). At the temperature above 350 °C, the composition of the components in the spectrum changed immediately: benzene and HCl became minor components, whereas toluene, xylene, and 8996

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and SiF4 were easily distinguished without peak overlap. Figure 8b shows the component profiles monitored in the discharging process with an accumulated time of 8 s for each point. The concentration of SO2 decreased with the increase of the time of discharge, while concentrations for SOF2, SO2F2, and SOF4 (SOF2+, SO2F2+, and SOF3+ in mass spectrum) had the opposite trends. SiF4 (SiF3+ in mass spectrum) may be generated from active compounds reacting with the glass chamber in discharging process. These results demonstrated that the low-energy MEPEI source is capable of measuring toxic and highly reactive compounds with high IEs.

’ CONCLUSIONS A VUV lamp based MEPEI source has been developed for online oaTOFMS. The ion intensities of molecules with IEs higher than 10.6 eV were improved over 2 orders. The simulation showed that the permanent magnet improved the ionization efficiency, mainly through increasing the electron path and reducing the electron loss. Direct component monitoring in PVC pyrolysis and SF6 discharging has revealed some attractive advantages of MEPEI, such as wide ionization ability, acceptable sensitivity, and reduced fragments. Benefiting from these advantages, the low-energy MEPEI source has various potential applications in process analysis and environmental pollutants monitoring. ’ ASSOCIATED CONTENT

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Supporting Information. Figures S-1 S-3.This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Components monitored in the SF6 discharging process. (a) The mass spectrum taken after discharging 1500 s. (b) The concentration profiles for SiF4, SOF2, SO2F2, and SOF4 as a function of discharge time.

polyaromatics made up the major components. Benefitting from fragment-reduced spectrum and high sensitivity, the low-energy MEPEI ionization source coupled with TOFMS has characteristics of easy interpretation and flexible accumulated time for each spectrum, which were in favor of fast monitoring of complicated mixtures without prior separation. Application on Monitoring Components in Discharging of SF6. SF6 (IE 15.32 eV) is an important gas used in the electrical industry as a dielectric medium for high-voltage circuit breakers, switchgear, and other electrical equipment. The impurities in SF6 will greatly reduce its performance; thus, the monitoring of components gives a way to prevent the discharging.36 In addition, the monitoring can also serve as an effective way for online fault detection and diagnosis in SF6 insulated equipment, because of the characteristic products in different discharge modes.37 In order to demonstrate the potential of MEPEI in rapid analysis of the impurity compounds, an apparatus was set up to simulate the SF6 discharging process (Supporting Information, Figure S-2). The impure SF6 (99.956%, Relation Electronics Ltd. Zhengzhou, China) was filled in a glass chamber, in which a pinto-pin discharge setup with an alternating voltage of 9 kHz was installed. Figure 8a is the mass spectrum obtained at the time of 1500 s during the discharging process. In the spectrum, the lowconcentration compounds such as SO2, CF4, SOF2, SO2F2, SOF4

’ AUTHOR INFORMATION Corresponding Author

*Fax: +86-411-84379517. E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the NSF of China (Grant Nos. 20877074, 20907052, and 40637036) and the Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YZ200907) are gratefully acknowledged. ’ REFERENCES (1) Tonokura, K.; Kanno, N.; Yamamoto, Y.; Yamada, H. Int. J. Mass Spectrom. 2010, 290, 9–13. (2) Hou, K. Y.; Wang, J. D.; Li, H. Y. Rapid Commun. Mass Spectrom. 2007, 21, 3554–3560. (3) Saraji-Bozorgzad, M. R.; Eschner, M.; Groeger, T. M.; Streibel, T.; Geissler, R.; Kaisersbeiger, E.; Denner, T.; Zimmermann, R. Anal. Chem. 2010, 82, 9644–9653. (4) Schramm, E.; Kurten, A.; Holzer, J.; Mitschke, S.; Muhlberger, F.; Sklorz, M.; Wieser, J.; Ulrich, A.; Putz, M.; Schulte-Ladbeck, R.; Schultze, R.; Curtius, J.; Borrmann, S.; Zimmermann, R. Anal. Chem. 2009, 81, 4456–4467. (5) Hanley, L.; Zimmermann, R. Anal. Chem. 2009, 81, 4174–4182. (6) Wang, L.; Li, H. Y.; Bai, J. L.; Hua, X. Q.; Lu, R. C. Int. J. Mass Spectrom. 1998, 181, 43–50. (7) Vanbramer, S. E.; Johnston, M. V. J. Am. Soc. Mass Spectrom. 1990, 1, 419–426. (8) Gasper, G. L.; Takahashi, L. K.; Zhou, J.; Ahmed, M.; Moore, J. F.; Hanley, L. Anal. Chem. 2010, 82, 7472–7478. 8997

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