Research Article Received: 10 July 2013
Revised: 14 October 2013
Accepted: 15 October 2013
Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2014, 28, 230–238 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6768
Efficient injection of low-mass ions into high magnetic field Fourier transform ion cyclotron resonance mass spectrometers Behrooz Zekavat1, Jan E. Szulejko2, David LaBrecque3, Abayomi D. Olaitan1 and Touradj Solouki1* 1
Department of Chemistry and Biochemistry, Baylor University, Waco, TX 76706, USA Department of Environment & Energy, Sejong University, Seoul 143-747, Republic of Korea 3 Department of Chemistry, University of Maine, Orono, ME 04469-5706, USA 2
RATIONALE: Low-mass cut-off restrictions for injecting ions from external ion sources into high magnetic fields impose
limitations for wide mass range analyses with Fourier transform ion cyclotron resonance (FTICR) instruments. Radiofrequency (RF)-only quadrupole ion guides (QIGs) with higher frequencies can be used to overcome low-mass cut-off in FTICR instruments. METHODS: RF signals (1.0 MHz to 10.0 MHz) were applied to QIGs to transfer externally generated ions from either electron ionization (EI) or electrospray ionization (ESI) sources into ICR cells of 9.4 T FTICR mass spectrometers. Efficiencies of QIGs were evaluated using externally generated ions from: EI of acetone, air, and perfluorotributylamine mixture, EI of gas chromatography (GC)-separated components of a standard sample mixture, and ESI of complex mixtures such as petroleum and fulvic acid samples. RESULTS: We were able to transfer ions with m/z as low as 26 from an external EI source into the ICR cell of a 9.4 T FTICR mass spectrometer and extend the operational low-mass range for ESI-FTICR analyses. High mass resolving power and mass measurement accuracy of GC/FTICR mass spectrometry were utilized to discriminate between oxygenated and non-oxygenated compounds in a ’Grob’ sample. Ion losses based on SIMION ion trajectory predictions were consistent with experimental findings. CONCLUSIONS: We demonstrated that the use of high-frequency QIGs can extend the operational lower m/z range for both external EI- and ESI-FTICR mass spectrometers. By considering both ICR and Mathieu equations of motions to describe ion trajectories, theoretical ion ejection thresholds (consistent with our experimental findings) could be predicted. Copyright © 2013 John Wiley & Sons, Ltd.
In Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) experiments, high background pressure or ion-neutral collisions can limit the achievable mass resolving power (m/Δm50%).[1] To minimize high background pressures for optimal FTICR analyses, ions can be generated in external high-pressure sources and transferred into differentially pumped ICR cells, housed within high magnetic field regions, for detection. The presence of high magnetic fields can complicate the ion transfer process. Various ion guide devices have been used to overcome the ’magnetic mirror effect’[2] during ion injection (from external ion sources into ICR cells). For instance, the following devices and approaches have been previously employed in external ion source FTICR instruments: (a) electrostatic ion guide lenses,[3] (b) hybrid quadrupole/segmented hexapole ion guides,[4] (c) stray magnetic field lines,[5] (d) off-axis ion injection into ICR
230
* Correspondence to: T. Solouki, Department of Chemistry and Biochemistry, Baylor University Sciences Building, One Bear Place #97348, Waco, TX 76706, USA. E-mail: [email protected]
Rapid Commun. Mass Spectrom. 2014, 28, 230–238
cells,[6] (e) RF-only multipole (e.g., quadrupole) ion guides,[2] (f) stacked-ring electrostatic ion guides,[7] and (g) wire ion guides.[8] Among the aforementioned approaches, RF-only quadrupole ion guides (QIGs) are the most widely used ion guide devices because of the simplicity of their operations. To efficiently utilize QIGs in FTICR instruments, the following three factors must be considered and addressed: (a) ’time-of-flight (TOF) effect’,[9–11] (b) ’magnetic mirror effect’,[2] and (c) radial excitation and ejection of ions.[12,13] The TOF effect is one of the causes of mass discrimination in external ion source FTICR instruments. The TOF effect results from acceleration of all ions in a constant direct current (DC) electric field between the external ion source and the ICR cell, where ions of different masses and same charge travel with different velocities and reach the ICR cell at different times. Such a mass-dependent velocity dispersion allows for a smaller ion to reach the ICR cell faster than a heavier ion that has the same charge. Because all ions are simultaneously pulsed from an external ion source into the ICR cell, only a limited time slice of ion packages can be trapped inside the ICR cell and, as a result, mass discrimination is commonly observed in external ion source FTICR instruments. In addition to the TOF effect, kinetic
Copyright © 2013 John Wiley & Sons, Ltd.
Injection of low-mass ions for high magnetic field FTICRMS energy distributions of ions generated in external ion sources can contribute to biased ion trapping in FTICR instruments. Two experimental approaches including: (i) double voltage ramps on multipole ion accumulation devices and ’front ICR cell trap electrode’[10] and (ii) application of an axial DC electric field gradient in the multipole ion accumulation devices[11] can be utilized to reduce the TOF effect in FTICR instruments. The magnetic mirror effect is among the major causes of ion losses in external ion source FTICR instruments equipped with QIGs. The magnetic mirror effect prevents ions from reaching the ICR cells by reversing ions’ directions away from their intended destination.[2] Theoretical calculations reported by McIver predicted that application of higher RF amplitudes to QIGs and/or injection of ions into QIGs with minimum radial kinetic energy could reduce the magnetic mirror effect.[2] The radial ion excitation results from the presence of an axial magnetic field gradient around QIGs. As an ion travels towards an ICR cell, in the presence of an axial magnetic field gradient, at some point along the ion’s pathway its natural ICR frequency matches the frequency applied to QIGs. Power absorption from the applied RF signal increases the cyclotron radii of ions and leads to ion losses.[12,13] The radial excitation of ions in QIGs imposes a low-mass cut-off for injection of ions from external ion sources into ICR cells. Our electrospray ionization (ESI) and gas chromatography (GC)/FTICR mass spectrometers share a common 9.4 T superconducting magnet. Both of these instruments utilize QIGs for ion transfer from external ion sources (electron ionization (EI) or ESI) into the ICR cell. At 9.4 T magnetic field strength, the original manufacturer’s ~1.1 MHz (with an optional ~2.5 MHz for the GC side) RF power supply, at optimized RF voltages, imposes a low-mass cutoff limit of m/z ~300 (and m/z ~70 at ~2.5 MHz). Low-mass cut-off constraints make it impracticable to use external source FTICR instruments for detection of low m/z ions (e.g., in metabolomics studies).[14] In this paper, we evaluate (both experimentally and theoretically) the possibility of using high-frequency QIG RF signals to overcome low-mass cut-off limitations in external ion source FTICR mass spectrometers. In a systematic approach, we compare complementary results from external ion source EI 7.0 T and 9.4 T FTICR instruments. We also show GC/FTICRMS results from the analysis of a Grob test mixture and the ESI-FTICRMS characterization of complex sample mixtures.
EXPERIMENTAL AND THEORETICAL Samples
Rapid Commun. Mass Spectrom. 2014, 28, 230–238
Instrumentation Direct introduction For direct sample introduction external EI-FTICRMS experiments, test mixtures were introduced from a sample reservoir (pressure of ~25 Torr and room temperature) into an external EI ion source[14] through a needle valve (Swagelok, Solon, OH, USA). Gas chromatography (GC) An SRI gas chromatograph (model 8610C; SRI Instruments, Las Vegas, NV, USA) was used to separate the components of a Grob sample (Table S1, Supporting Information) in GC/external EIFTICRMS experiments. The Grob sample (0.5 μL of liquid phase) was injected into a 60-m (0.28 mm i.d., 1.0-μm cross-bonded 100 % poly(dimethyl siloxane) stationary phase coating) MTX-1 capillary column (Restek Corp., Bellefonte, PA, USA). A homebuilt post-column splitter was utilized to restrict the carrier gas flow and direct the GC effluent into an external EI ion source[14] using a heated (~200 °C) transfer line made of an MTX guard column (i.d. 0.25 mm, o.d. 0.5 mm). The splitter was constructed by using low-volume Swagelok fittings (Swagelok, Solon, OH, USA) and a pre-existing needle valve of SRI GC split/splitless injector. The split ratio of the post-column splitter was adjusted (estimated to be ~1:11 viz., one part was introduced into the external EI ion source and 11 parts were vented to atmosphere) for optimal detection of GC eluting compounds in GC/ FTICRMS experiments. The GC injection port temperature was kept at 150 °C and the GC transfer line (from GC to external EI ion source) was maintained at 185 °C. The GC column temperature programming for analysis of Grob sample was: 1 min at 40 °C, ramp at 5 °C/min to 180 °C, 51 min at 180 °C. The GC He gas head pressure was set at 15 psi. Electrospray ionization (ESI) In ESI experiments, ions were generated using an Analytica ESI source (Analytica of Branford, Inc., Branford, CT, USA) equipped with an in-house-built spraying setup.[19] The ion
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/rcm
231
A test mixture containing acetone (Fisher Scientific, Pittsburgh, PA, USA), ambient laboratory air, and perfluorotributylamine (FC-43; Scientific Instrument Services, Inc., Ringoes, NJ, USA) was used for direct introduction in the external EIFTICRMS experiments. A Grob[15] standard mixture (Supelco Analytical, Bellefonte, PA, USA) containing 12 chemical components (Table S1, Supporting Information) was used for GC/external EIFTICRMS experiments. Helium gas (99.999% purity; The BOC Group, Inc., Murray Hill, NJ, USA) was used as a GC carrier gas. The Grob sample is a mixture of 12 organic
compounds with different functional groups that was originally introduced by Grob et al.[15] to test the quality of GC glass capillary columns under temperatureprogrammed conditions. A previously characterized northern coniferous fulvic acid (NCFA)[16] sample was used for ESI-FTICRMS experiments discussed in the first section of the "Results and Discussion". Approximately 3 mg of the dried FA sample was dissolved in 10 mL of an electrospray solvent containing methanol/ water/acetic acid (49.95:49.95:0.1) and used for positive ion mode ESI experiments. A Naftalan oil sample (from the Naftalan region, near the lesser Caucasus Mountains in Azerbaijan, former USSR) which contains hundreds of low-mass species (
Revised: 14 October 2013
Accepted: 15 October 2013
Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2014, 28, 230–238 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6768
Efficient injection of low-mass ions into high magnetic field Fourier transform ion cyclotron resonance mass spectrometers Behrooz Zekavat1, Jan E. Szulejko2, David LaBrecque3, Abayomi D. Olaitan1 and Touradj Solouki1* 1
Department of Chemistry and Biochemistry, Baylor University, Waco, TX 76706, USA Department of Environment & Energy, Sejong University, Seoul 143-747, Republic of Korea 3 Department of Chemistry, University of Maine, Orono, ME 04469-5706, USA 2
RATIONALE: Low-mass cut-off restrictions for injecting ions from external ion sources into high magnetic fields impose
limitations for wide mass range analyses with Fourier transform ion cyclotron resonance (FTICR) instruments. Radiofrequency (RF)-only quadrupole ion guides (QIGs) with higher frequencies can be used to overcome low-mass cut-off in FTICR instruments. METHODS: RF signals (1.0 MHz to 10.0 MHz) were applied to QIGs to transfer externally generated ions from either electron ionization (EI) or electrospray ionization (ESI) sources into ICR cells of 9.4 T FTICR mass spectrometers. Efficiencies of QIGs were evaluated using externally generated ions from: EI of acetone, air, and perfluorotributylamine mixture, EI of gas chromatography (GC)-separated components of a standard sample mixture, and ESI of complex mixtures such as petroleum and fulvic acid samples. RESULTS: We were able to transfer ions with m/z as low as 26 from an external EI source into the ICR cell of a 9.4 T FTICR mass spectrometer and extend the operational low-mass range for ESI-FTICR analyses. High mass resolving power and mass measurement accuracy of GC/FTICR mass spectrometry were utilized to discriminate between oxygenated and non-oxygenated compounds in a ’Grob’ sample. Ion losses based on SIMION ion trajectory predictions were consistent with experimental findings. CONCLUSIONS: We demonstrated that the use of high-frequency QIGs can extend the operational lower m/z range for both external EI- and ESI-FTICR mass spectrometers. By considering both ICR and Mathieu equations of motions to describe ion trajectories, theoretical ion ejection thresholds (consistent with our experimental findings) could be predicted. Copyright © 2013 John Wiley & Sons, Ltd.
In Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) experiments, high background pressure or ion-neutral collisions can limit the achievable mass resolving power (m/Δm50%).[1] To minimize high background pressures for optimal FTICR analyses, ions can be generated in external high-pressure sources and transferred into differentially pumped ICR cells, housed within high magnetic field regions, for detection. The presence of high magnetic fields can complicate the ion transfer process. Various ion guide devices have been used to overcome the ’magnetic mirror effect’[2] during ion injection (from external ion sources into ICR cells). For instance, the following devices and approaches have been previously employed in external ion source FTICR instruments: (a) electrostatic ion guide lenses,[3] (b) hybrid quadrupole/segmented hexapole ion guides,[4] (c) stray magnetic field lines,[5] (d) off-axis ion injection into ICR
230
* Correspondence to: T. Solouki, Department of Chemistry and Biochemistry, Baylor University Sciences Building, One Bear Place #97348, Waco, TX 76706, USA. E-mail: [email protected]
Rapid Commun. Mass Spectrom. 2014, 28, 230–238
cells,[6] (e) RF-only multipole (e.g., quadrupole) ion guides,[2] (f) stacked-ring electrostatic ion guides,[7] and (g) wire ion guides.[8] Among the aforementioned approaches, RF-only quadrupole ion guides (QIGs) are the most widely used ion guide devices because of the simplicity of their operations. To efficiently utilize QIGs in FTICR instruments, the following three factors must be considered and addressed: (a) ’time-of-flight (TOF) effect’,[9–11] (b) ’magnetic mirror effect’,[2] and (c) radial excitation and ejection of ions.[12,13] The TOF effect is one of the causes of mass discrimination in external ion source FTICR instruments. The TOF effect results from acceleration of all ions in a constant direct current (DC) electric field between the external ion source and the ICR cell, where ions of different masses and same charge travel with different velocities and reach the ICR cell at different times. Such a mass-dependent velocity dispersion allows for a smaller ion to reach the ICR cell faster than a heavier ion that has the same charge. Because all ions are simultaneously pulsed from an external ion source into the ICR cell, only a limited time slice of ion packages can be trapped inside the ICR cell and, as a result, mass discrimination is commonly observed in external ion source FTICR instruments. In addition to the TOF effect, kinetic
Copyright © 2013 John Wiley & Sons, Ltd.
Injection of low-mass ions for high magnetic field FTICRMS energy distributions of ions generated in external ion sources can contribute to biased ion trapping in FTICR instruments. Two experimental approaches including: (i) double voltage ramps on multipole ion accumulation devices and ’front ICR cell trap electrode’[10] and (ii) application of an axial DC electric field gradient in the multipole ion accumulation devices[11] can be utilized to reduce the TOF effect in FTICR instruments. The magnetic mirror effect is among the major causes of ion losses in external ion source FTICR instruments equipped with QIGs. The magnetic mirror effect prevents ions from reaching the ICR cells by reversing ions’ directions away from their intended destination.[2] Theoretical calculations reported by McIver predicted that application of higher RF amplitudes to QIGs and/or injection of ions into QIGs with minimum radial kinetic energy could reduce the magnetic mirror effect.[2] The radial ion excitation results from the presence of an axial magnetic field gradient around QIGs. As an ion travels towards an ICR cell, in the presence of an axial magnetic field gradient, at some point along the ion’s pathway its natural ICR frequency matches the frequency applied to QIGs. Power absorption from the applied RF signal increases the cyclotron radii of ions and leads to ion losses.[12,13] The radial excitation of ions in QIGs imposes a low-mass cut-off for injection of ions from external ion sources into ICR cells. Our electrospray ionization (ESI) and gas chromatography (GC)/FTICR mass spectrometers share a common 9.4 T superconducting magnet. Both of these instruments utilize QIGs for ion transfer from external ion sources (electron ionization (EI) or ESI) into the ICR cell. At 9.4 T magnetic field strength, the original manufacturer’s ~1.1 MHz (with an optional ~2.5 MHz for the GC side) RF power supply, at optimized RF voltages, imposes a low-mass cutoff limit of m/z ~300 (and m/z ~70 at ~2.5 MHz). Low-mass cut-off constraints make it impracticable to use external source FTICR instruments for detection of low m/z ions (e.g., in metabolomics studies).[14] In this paper, we evaluate (both experimentally and theoretically) the possibility of using high-frequency QIG RF signals to overcome low-mass cut-off limitations in external ion source FTICR mass spectrometers. In a systematic approach, we compare complementary results from external ion source EI 7.0 T and 9.4 T FTICR instruments. We also show GC/FTICRMS results from the analysis of a Grob test mixture and the ESI-FTICRMS characterization of complex sample mixtures.
EXPERIMENTAL AND THEORETICAL Samples
Rapid Commun. Mass Spectrom. 2014, 28, 230–238
Instrumentation Direct introduction For direct sample introduction external EI-FTICRMS experiments, test mixtures were introduced from a sample reservoir (pressure of ~25 Torr and room temperature) into an external EI ion source[14] through a needle valve (Swagelok, Solon, OH, USA). Gas chromatography (GC) An SRI gas chromatograph (model 8610C; SRI Instruments, Las Vegas, NV, USA) was used to separate the components of a Grob sample (Table S1, Supporting Information) in GC/external EIFTICRMS experiments. The Grob sample (0.5 μL of liquid phase) was injected into a 60-m (0.28 mm i.d., 1.0-μm cross-bonded 100 % poly(dimethyl siloxane) stationary phase coating) MTX-1 capillary column (Restek Corp., Bellefonte, PA, USA). A homebuilt post-column splitter was utilized to restrict the carrier gas flow and direct the GC effluent into an external EI ion source[14] using a heated (~200 °C) transfer line made of an MTX guard column (i.d. 0.25 mm, o.d. 0.5 mm). The splitter was constructed by using low-volume Swagelok fittings (Swagelok, Solon, OH, USA) and a pre-existing needle valve of SRI GC split/splitless injector. The split ratio of the post-column splitter was adjusted (estimated to be ~1:11 viz., one part was introduced into the external EI ion source and 11 parts were vented to atmosphere) for optimal detection of GC eluting compounds in GC/ FTICRMS experiments. The GC injection port temperature was kept at 150 °C and the GC transfer line (from GC to external EI ion source) was maintained at 185 °C. The GC column temperature programming for analysis of Grob sample was: 1 min at 40 °C, ramp at 5 °C/min to 180 °C, 51 min at 180 °C. The GC He gas head pressure was set at 15 psi. Electrospray ionization (ESI) In ESI experiments, ions were generated using an Analytica ESI source (Analytica of Branford, Inc., Branford, CT, USA) equipped with an in-house-built spraying setup.[19] The ion
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/rcm
231
A test mixture containing acetone (Fisher Scientific, Pittsburgh, PA, USA), ambient laboratory air, and perfluorotributylamine (FC-43; Scientific Instrument Services, Inc., Ringoes, NJ, USA) was used for direct introduction in the external EIFTICRMS experiments. A Grob[15] standard mixture (Supelco Analytical, Bellefonte, PA, USA) containing 12 chemical components (Table S1, Supporting Information) was used for GC/external EIFTICRMS experiments. Helium gas (99.999% purity; The BOC Group, Inc., Murray Hill, NJ, USA) was used as a GC carrier gas. The Grob sample is a mixture of 12 organic
compounds with different functional groups that was originally introduced by Grob et al.[15] to test the quality of GC glass capillary columns under temperatureprogrammed conditions. A previously characterized northern coniferous fulvic acid (NCFA)[16] sample was used for ESI-FTICRMS experiments discussed in the first section of the "Results and Discussion". Approximately 3 mg of the dried FA sample was dissolved in 10 mL of an electrospray solvent containing methanol/ water/acetic acid (49.95:49.95:0.1) and used for positive ion mode ESI experiments. A Naftalan oil sample (from the Naftalan region, near the lesser Caucasus Mountains in Azerbaijan, former USSR) which contains hundreds of low-mass species (
