Volume 140 Number 9 7 May 2015 Pages 2891–3292
Analyst www.rsc.org/analyst
ISSN 0003-2654
COMMUNICATION Julia Laskin et al. Design and performance of a high-flux electrospray ionization source for ion soft landing
Analyst COMMUNICATION
Cite this: Analyst, 2015, 140, 2957 Received 2nd February 2015, Accepted 15th March 2015
Published on 23 March 2015. Downloaded on 24/04/2015 09:14:12.
DOI: 10.1039/c5an00220f
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Design and performance of a high-flux electrospray ionization source for ion soft landing K. Don D. Gunaratne,†a Venkateshkumar Prabhakaran,†a Yehia M. Ibrahim,b Randolph V. Norheim,b Grant E. Johnsona and Julia Laskin*a
www.rsc.org/analyst
We report the design and evaluation of a new high-intensity electrospray ionization source for ion soft-landing experiments. The source incorporates a dual ion funnel, which enables operation with a higher gas load through an expanded diameter heated inlet into the additional first region of differential pumping. This capability allowed us to examine the effect of the inner diameter (ID) of the heated stainless steel inlet on the total ion current transmitted through the dual funnel interface and, more importantly, the mass-selected ion current delivered to the deposition target. The ion transmission of the dual funnel is similar to the transmission of the single funnel used in our previous soft landing studies. However, substantially higher ion currents were obtained using larger ID heated inlets and an orthogonal inlet geometry, in which the heated inlet was positioned perpendicular to the direction of ion propagation through the instrument. The highest ion currents were obtained using the orthogonal geometry and a 1.4 mm ID heated inlet. The corresponding stable deposition rate of ∼1 μg of mass-selected ions per day will facilitate future studies focused on the controlled deposition of complex molecules on substrates for studies in catalysis, energy storage, and self-assembly.
Introduction Soft and reactive landing of mass-selected ions onto surfaces is ideally suited for the highly controlled preparation of novel materials and purification of compounds present in complex mixtures. This approach allows the isolation of selected species on substrates and subsequent characterization of their structure and reactivity.1–11 In addition, precise control of the composition, ionic charge state, kinetic energy, and shape of the ion beam facilitates fundamental studies of the chemistry and physics of ion–surface interactions that are not complia Pacific Northwest National Laboratory, Physical Sciences Division, P.O. Box 999, MSIN K8-88, Richland, Washington 99352, USA. E-mail: [email protected] b Biological Sciences Division and Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 3335 Innovation Avenue (K8-98), P.O. Box 999, Richland, Washington 99352, USA † These authors contributed to this work equally.
This journal is © The Royal Society of Chemistry 2015
cated by the presence of residual reactants, counterions, and solvent molecules. Applications of mass-selected ion deposition include the preparation of protein arrays12–14 and carbon-based surfaces,15–17 studies of how surfaces impact protein conformations,18 preparation of conformationallyselected peptide layers on surfaces,19,20 chiral enrichment of enantiomers,21,22 deposition of redox-active molecules23,24 and molecular magnets,25 covalent modification of polymer films,26,27 covalent immobilization of organic and biological molecules on surfaces,28–32 deposition of catalytic organometallic complexes,33–35 molecular clusters,36,37 as well as metal and metal alloy clusters and larger nanoparticles.2,38–45 These diverse applications of ion soft landing will all benefit directly from the development of instrumentation capable of delivering intense focused beams of mass-selected ions to surfaces. Historically, soft and reactive landing of mass-selected ions have been used primarily for the preparation of relatively low coverage (∼10% of a monolayer) samples dispersed over small areas (deposition area ∼5 mm2) for fundamental studies in surface science and catalysis.3,10,46–49 The main objective in many of these experiments has been to isolate well-defined molecules and clusters on supports at low enough coverages that they do not interact with their neighbors or agglomerate into larger poorly-defined aggregates.8,50 Similar low surface coverages are also desirable for high resolution microscopy studies of the structure and magnetic properties of individual proteins and molecular clusters soft landed from solution using electrospray deposition.18,25 It is, therefore, extremely fortunate that the high sensitivity of many surface analytical techniques such as scanning tunneling microscopy (STM),18,51 atomic force microscopy (AFM),52 electron microscopy,53,54 and secondary ion mass spectrometry (SIMS)55,56 has enabled soft landed materials to be characterized even at these relatively low coverages (∼1011–1012 total ions on 1014 ions) over larger (∼1 cm2) areas for advanced applications in specialty devices such as chemical sensors,58 electrochemical capacitors,59 and molecular cluster batteries,60 as well as preparation of high density microarrays of proteins on surfaces for applications in high-throughput biological screening.12 It is expected that numerous unforeseen applications will also emerge as the high-flux ionization source technology matures. The ion current delivered to surfaces is the key factor determining the rate of ion deposition. Electron impact (EI) ionization typically generates intense ion beams making it possible to deliver up to 20 nA of mass-selected ions to a deposition target.62,63 High ion currents have also been obtained using high frequency laser vaporization sources.64,65 The source, reported by Heiz and co-workers, generated currents up to 1 nA of mass-selected Nbn+ (n = 1–18) cluster ions.66 Ion currents in the range of 5–8 nA were also reported by Judai et al.67 for vanadium-benzene cluster ions produced using a similar cluster ion source. Similar ion currents have been reported for deposition instruments equipped with magnetron sputter gas aggregation and pulsed arc discharge cluster ion sources.68 For example, notable ion currents of 6.3 nA and 2 nA were recently obtained by Vajda and co-workers for massselected Ag3+ and Cu4+ ions, respectively, produced using this technique.69 Similarly, high ion currents of 3–6 nA were reported for small Sin+ (n = 4–10) clusters produced using pulsed arc discharge68 and 4–8 nA for mass-selected 3–4 nm Cu nanoparticles generated using magnetron sputtering.70 Electrospray ionization (ESI) is also a widely used method of ionization for ion soft-landing experiments. ESI provides access to a broad range of precursor ions with the capability to generate stable continuous ion beams over extended time periods with relatively little source maintenance required from the operator. Ion currents of 1–5 nA without mass selection56,71–73 and up to 600 pA with mass selection56,74 have been previously reported for ion deposition systems equipped with ESI sources. In this study, we describe the design and performance of a new high-intensity ESI source for ion soft-landing experiments
2958 | Analyst, 2015, 140, 2957–2963
Analyst
that combines several key developments in instrumentation. Specifically, mass-selected ion currents of up to 2.5 nA were obtained for the triply charged Keggin polyoxometalate (POM) anion, PMo12O403−, corresponding to a deposition rate of ∼1 μg per day. This deposition rate is at least five times higher than what may be achieved using the single-funnel instrument described elsewhere.32,75 The higher ion currents were obtained by combining a wide-bore stainless steel heated inlet in an orthogonal geometry for desolvation and droplet transfer into the first region of differential pumping with a tandem ion funnel developed by Smith and co-workers for efficient transfer of ions through two stages of differential pumping to vacuum conditions for subsequent mass-selection and deposition.76
Experimental setup A tandem ion funnel, shown schematically in Fig. 1, was installed on an existing ion deposition system described in detail elsewhere.32,75 Briefly, the instrument is equipped with an ESI source, a tandem ion funnel system (described in more detail later in the text), an RF-only collision quadrupole (CQ), a quadrupole mass filter (Extrel CMS, Pittsburgh, PA), and three einzel lenses which focus the ion beam onto a deposition target. Ions produced using ESI are transferred into vacuum through a 100 mm long resistively heated stainless steel (SS) inlet tube maintained at a temperature of 150 °C. The instrument is equipped with both a direct inlet76 and an orthogonal inlet geometry.77 It has been demonstrated that orthogonal ion injection efficiently decouples ion transport from the gas flow dynamics, eliminates neutral molecules and solvent droplets from the instrument axis, and reduces electrode contamination.77 The heated tube in the direct inlet is located ∼6 mm off the center instrument axis and positioned close to the entrance plate of the first ion funnel. The orthogonal inlet is introduced through a 20 mm × 20 mm cutout in the middle of the first section (F1) of the outer funnel, as shown in Fig. 1. The benefit of having the orthogonal inlet and a direct inlet with an offset from the ion beam is the reduced transfer of neutral droplets into the high vacuum region of the instrument, preventing neutral contaminants from reaching the surface during soft landing experiments. The tandem ion funnel consists of two RF ion funnels fabricated using printed circuit board technology described in detail elsewhere77,78 and housed in two differentially pumped vacuum regions. The first ion funnel, which is denoted as the outer funnel in Fig. 1, is comprised of a total of 128 ring electrodes. The first 64 electrodes have the same inner diameter (ID) of 50.8 mm (2 inches) while the ID of the last 64 electrodes decreases linearly from 50.8 mm to 2.5 mm. The second ion funnel, referred to hereafter as the inner funnel, is comprised of 68 ring electrodes. The first 34 electrodes have the same ID of 25.4 mm (1 inch) while the ID of the last 34 electrodes decreases linearly from 25.4 mm to 2.5 mm. Both ion funnels are powered by home-built high-Q heads; the outer funnel resonates at a frequency of ∼2080 kHz while the inner
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 23 March 2015. Downloaded on 24/04/2015 09:14:12.
Analyst
Communication
Fig. 1 Schematic drawing of the tandem ion funnel interface showing the orthogonal and direct heated inlets, the first and second funnels, and the collisional quadrupole. The first funnel is separated into two regions denoted as F1 and F1#. Typical pressures and ion currents are listed in Tables 1 and 2, respectively.
funnel resonates at ∼860 kHz. In each funnel, an RF voltage of the same amplitude and frequency is applied to all electrodes with a phase shift of 180 degrees between neighboring electrodes. Typical peak-to-peak voltages are ∼148 V and ∼122 V in the outer and inner funnel, respectively. A series of 0.5 MΩ resistors connected between all adjacent electrodes of the ion funnel generates a DC potential gradient when voltages are applied to the front and the back plates of the funnel. The outer funnel is separated into two DC regions, the first stack having 42 electrodes (F1) and the second stack (F1#) with 86 electrodes. This arrangement provides better control over the DC gradient applied to the funnel, which is particularly useful when ions are introduced orthogonally. The inner funnel has a single DC power supply which provides a gradient from the beginning to the final ring electrode. The following representative potentials were applied to generate DC gradients in the inner and outer funnel: F1 front plate, −394 V; F1 back plate, −367 V; F1# front plate, −366 V; F1# back plate, −126 V; F2 front plate, −175 V; F2 back plate, −44 V. The chamber housing the outer funnel is differentially pumped by an Edwards 80 two stage rotary vane mechanical pump (56 cubic feet per minute (cfm)) to an operating pressure of 1–10 Torr, while the vacuum chamber housing the inner funnel is differentially pumped by a Leybold TriVac® D25B rotary vane mechanical pump (20 cfm) to an operating pressure of 0.4–1.5 Torr. The last plate of the outer funnel serves as the conductance limit between the first and second regions of differential pumping, while the last plate of the inner funnel serves as the conductance limit between the second and the third differentially pumped vacuum regions of the instrument. Typical operating pressure in the CQ located in the third differentially pumped region of the system is in a range of 10–50 mTorr. The quadrupole mass filter and the deposition target are located in the fourth differentially pumped region of the instrument maintained at 7 × 10−5–1 × 10−4 Torr during deposition by a Varian TV-301 Navigator
This journal is © The Royal Society of Chemistry 2015
turbomolecular pump (250 L s−1) backed by another Leybold TriVac® D25B rotary vane mechanical pump (20 cfm). Sodium phosphomolybdate hydrate (Na3[PMo12O40]·xH2O CAS: 1313-30-0, Sigma-Aldrich, St. Louis, MO.) was dissolved in methanol to a final concentration of 150 μM and introduced into the ESI source using a syringe pump (Legato 180, KD Scientific, Holliston, MA) at a flow rate of 65–80 μL h−1. Charged microdroplets were produced by applying a ∼3 kV negative potential to the stainless steel union connected to the ESI emitter. Three types of ESI emitters were used in this study; (i) single bore (ii) double-bore (iii) four-bore emitters. Single bore emitters (Polymicro Technologies, Phoenix, AZ) are polyimide-coated fused silica capillaries with an ID of 100 µm and an outer diameter (OD) of 360 µm. Double bore and four bore emitters were prepared by pulling 500 µm OD boro-silicate glass tubes (VitroCom, NJ, USA) having two and four bores (ID of each bore: 125 µm), respectively, to a final OD of 130 µm using a micropipette puller (P-2000, Sutter Instrument Company, Novato, CA). Typical pulling parameters (in relative units used by the instrument software) were as follows: heat, 350; filament, 4; velocity, 30; delay, 200; pull strength, 0. PMo12O403− ions, the dominant species observed after droplet desolvation, were used for the optimization and characterization of the high-intensity ESI interface. Ion current measurements were performed using a picoammeter (model 9103, RBD Instruments, Bend, OR).
Results and discussion The ability to operate at higher pressure in the outer funnel region while maintaining
Analyst www.rsc.org/analyst
ISSN 0003-2654
COMMUNICATION Julia Laskin et al. Design and performance of a high-flux electrospray ionization source for ion soft landing
Analyst COMMUNICATION
Cite this: Analyst, 2015, 140, 2957 Received 2nd February 2015, Accepted 15th March 2015
Published on 23 March 2015. Downloaded on 24/04/2015 09:14:12.
DOI: 10.1039/c5an00220f
View Article Online View Journal | View Issue
Design and performance of a high-flux electrospray ionization source for ion soft landing K. Don D. Gunaratne,†a Venkateshkumar Prabhakaran,†a Yehia M. Ibrahim,b Randolph V. Norheim,b Grant E. Johnsona and Julia Laskin*a
www.rsc.org/analyst
We report the design and evaluation of a new high-intensity electrospray ionization source for ion soft-landing experiments. The source incorporates a dual ion funnel, which enables operation with a higher gas load through an expanded diameter heated inlet into the additional first region of differential pumping. This capability allowed us to examine the effect of the inner diameter (ID) of the heated stainless steel inlet on the total ion current transmitted through the dual funnel interface and, more importantly, the mass-selected ion current delivered to the deposition target. The ion transmission of the dual funnel is similar to the transmission of the single funnel used in our previous soft landing studies. However, substantially higher ion currents were obtained using larger ID heated inlets and an orthogonal inlet geometry, in which the heated inlet was positioned perpendicular to the direction of ion propagation through the instrument. The highest ion currents were obtained using the orthogonal geometry and a 1.4 mm ID heated inlet. The corresponding stable deposition rate of ∼1 μg of mass-selected ions per day will facilitate future studies focused on the controlled deposition of complex molecules on substrates for studies in catalysis, energy storage, and self-assembly.
Introduction Soft and reactive landing of mass-selected ions onto surfaces is ideally suited for the highly controlled preparation of novel materials and purification of compounds present in complex mixtures. This approach allows the isolation of selected species on substrates and subsequent characterization of their structure and reactivity.1–11 In addition, precise control of the composition, ionic charge state, kinetic energy, and shape of the ion beam facilitates fundamental studies of the chemistry and physics of ion–surface interactions that are not complia Pacific Northwest National Laboratory, Physical Sciences Division, P.O. Box 999, MSIN K8-88, Richland, Washington 99352, USA. E-mail: [email protected] b Biological Sciences Division and Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 3335 Innovation Avenue (K8-98), P.O. Box 999, Richland, Washington 99352, USA † These authors contributed to this work equally.
This journal is © The Royal Society of Chemistry 2015
cated by the presence of residual reactants, counterions, and solvent molecules. Applications of mass-selected ion deposition include the preparation of protein arrays12–14 and carbon-based surfaces,15–17 studies of how surfaces impact protein conformations,18 preparation of conformationallyselected peptide layers on surfaces,19,20 chiral enrichment of enantiomers,21,22 deposition of redox-active molecules23,24 and molecular magnets,25 covalent modification of polymer films,26,27 covalent immobilization of organic and biological molecules on surfaces,28–32 deposition of catalytic organometallic complexes,33–35 molecular clusters,36,37 as well as metal and metal alloy clusters and larger nanoparticles.2,38–45 These diverse applications of ion soft landing will all benefit directly from the development of instrumentation capable of delivering intense focused beams of mass-selected ions to surfaces. Historically, soft and reactive landing of mass-selected ions have been used primarily for the preparation of relatively low coverage (∼10% of a monolayer) samples dispersed over small areas (deposition area ∼5 mm2) for fundamental studies in surface science and catalysis.3,10,46–49 The main objective in many of these experiments has been to isolate well-defined molecules and clusters on supports at low enough coverages that they do not interact with their neighbors or agglomerate into larger poorly-defined aggregates.8,50 Similar low surface coverages are also desirable for high resolution microscopy studies of the structure and magnetic properties of individual proteins and molecular clusters soft landed from solution using electrospray deposition.18,25 It is, therefore, extremely fortunate that the high sensitivity of many surface analytical techniques such as scanning tunneling microscopy (STM),18,51 atomic force microscopy (AFM),52 electron microscopy,53,54 and secondary ion mass spectrometry (SIMS)55,56 has enabled soft landed materials to be characterized even at these relatively low coverages (∼1011–1012 total ions on 1014 ions) over larger (∼1 cm2) areas for advanced applications in specialty devices such as chemical sensors,58 electrochemical capacitors,59 and molecular cluster batteries,60 as well as preparation of high density microarrays of proteins on surfaces for applications in high-throughput biological screening.12 It is expected that numerous unforeseen applications will also emerge as the high-flux ionization source technology matures. The ion current delivered to surfaces is the key factor determining the rate of ion deposition. Electron impact (EI) ionization typically generates intense ion beams making it possible to deliver up to 20 nA of mass-selected ions to a deposition target.62,63 High ion currents have also been obtained using high frequency laser vaporization sources.64,65 The source, reported by Heiz and co-workers, generated currents up to 1 nA of mass-selected Nbn+ (n = 1–18) cluster ions.66 Ion currents in the range of 5–8 nA were also reported by Judai et al.67 for vanadium-benzene cluster ions produced using a similar cluster ion source. Similar ion currents have been reported for deposition instruments equipped with magnetron sputter gas aggregation and pulsed arc discharge cluster ion sources.68 For example, notable ion currents of 6.3 nA and 2 nA were recently obtained by Vajda and co-workers for massselected Ag3+ and Cu4+ ions, respectively, produced using this technique.69 Similarly, high ion currents of 3–6 nA were reported for small Sin+ (n = 4–10) clusters produced using pulsed arc discharge68 and 4–8 nA for mass-selected 3–4 nm Cu nanoparticles generated using magnetron sputtering.70 Electrospray ionization (ESI) is also a widely used method of ionization for ion soft-landing experiments. ESI provides access to a broad range of precursor ions with the capability to generate stable continuous ion beams over extended time periods with relatively little source maintenance required from the operator. Ion currents of 1–5 nA without mass selection56,71–73 and up to 600 pA with mass selection56,74 have been previously reported for ion deposition systems equipped with ESI sources. In this study, we describe the design and performance of a new high-intensity ESI source for ion soft-landing experiments
2958 | Analyst, 2015, 140, 2957–2963
Analyst
that combines several key developments in instrumentation. Specifically, mass-selected ion currents of up to 2.5 nA were obtained for the triply charged Keggin polyoxometalate (POM) anion, PMo12O403−, corresponding to a deposition rate of ∼1 μg per day. This deposition rate is at least five times higher than what may be achieved using the single-funnel instrument described elsewhere.32,75 The higher ion currents were obtained by combining a wide-bore stainless steel heated inlet in an orthogonal geometry for desolvation and droplet transfer into the first region of differential pumping with a tandem ion funnel developed by Smith and co-workers for efficient transfer of ions through two stages of differential pumping to vacuum conditions for subsequent mass-selection and deposition.76
Experimental setup A tandem ion funnel, shown schematically in Fig. 1, was installed on an existing ion deposition system described in detail elsewhere.32,75 Briefly, the instrument is equipped with an ESI source, a tandem ion funnel system (described in more detail later in the text), an RF-only collision quadrupole (CQ), a quadrupole mass filter (Extrel CMS, Pittsburgh, PA), and three einzel lenses which focus the ion beam onto a deposition target. Ions produced using ESI are transferred into vacuum through a 100 mm long resistively heated stainless steel (SS) inlet tube maintained at a temperature of 150 °C. The instrument is equipped with both a direct inlet76 and an orthogonal inlet geometry.77 It has been demonstrated that orthogonal ion injection efficiently decouples ion transport from the gas flow dynamics, eliminates neutral molecules and solvent droplets from the instrument axis, and reduces electrode contamination.77 The heated tube in the direct inlet is located ∼6 mm off the center instrument axis and positioned close to the entrance plate of the first ion funnel. The orthogonal inlet is introduced through a 20 mm × 20 mm cutout in the middle of the first section (F1) of the outer funnel, as shown in Fig. 1. The benefit of having the orthogonal inlet and a direct inlet with an offset from the ion beam is the reduced transfer of neutral droplets into the high vacuum region of the instrument, preventing neutral contaminants from reaching the surface during soft landing experiments. The tandem ion funnel consists of two RF ion funnels fabricated using printed circuit board technology described in detail elsewhere77,78 and housed in two differentially pumped vacuum regions. The first ion funnel, which is denoted as the outer funnel in Fig. 1, is comprised of a total of 128 ring electrodes. The first 64 electrodes have the same inner diameter (ID) of 50.8 mm (2 inches) while the ID of the last 64 electrodes decreases linearly from 50.8 mm to 2.5 mm. The second ion funnel, referred to hereafter as the inner funnel, is comprised of 68 ring electrodes. The first 34 electrodes have the same ID of 25.4 mm (1 inch) while the ID of the last 34 electrodes decreases linearly from 25.4 mm to 2.5 mm. Both ion funnels are powered by home-built high-Q heads; the outer funnel resonates at a frequency of ∼2080 kHz while the inner
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 23 March 2015. Downloaded on 24/04/2015 09:14:12.
Analyst
Communication
Fig. 1 Schematic drawing of the tandem ion funnel interface showing the orthogonal and direct heated inlets, the first and second funnels, and the collisional quadrupole. The first funnel is separated into two regions denoted as F1 and F1#. Typical pressures and ion currents are listed in Tables 1 and 2, respectively.
funnel resonates at ∼860 kHz. In each funnel, an RF voltage of the same amplitude and frequency is applied to all electrodes with a phase shift of 180 degrees between neighboring electrodes. Typical peak-to-peak voltages are ∼148 V and ∼122 V in the outer and inner funnel, respectively. A series of 0.5 MΩ resistors connected between all adjacent electrodes of the ion funnel generates a DC potential gradient when voltages are applied to the front and the back plates of the funnel. The outer funnel is separated into two DC regions, the first stack having 42 electrodes (F1) and the second stack (F1#) with 86 electrodes. This arrangement provides better control over the DC gradient applied to the funnel, which is particularly useful when ions are introduced orthogonally. The inner funnel has a single DC power supply which provides a gradient from the beginning to the final ring electrode. The following representative potentials were applied to generate DC gradients in the inner and outer funnel: F1 front plate, −394 V; F1 back plate, −367 V; F1# front plate, −366 V; F1# back plate, −126 V; F2 front plate, −175 V; F2 back plate, −44 V. The chamber housing the outer funnel is differentially pumped by an Edwards 80 two stage rotary vane mechanical pump (56 cubic feet per minute (cfm)) to an operating pressure of 1–10 Torr, while the vacuum chamber housing the inner funnel is differentially pumped by a Leybold TriVac® D25B rotary vane mechanical pump (20 cfm) to an operating pressure of 0.4–1.5 Torr. The last plate of the outer funnel serves as the conductance limit between the first and second regions of differential pumping, while the last plate of the inner funnel serves as the conductance limit between the second and the third differentially pumped vacuum regions of the instrument. Typical operating pressure in the CQ located in the third differentially pumped region of the system is in a range of 10–50 mTorr. The quadrupole mass filter and the deposition target are located in the fourth differentially pumped region of the instrument maintained at 7 × 10−5–1 × 10−4 Torr during deposition by a Varian TV-301 Navigator
This journal is © The Royal Society of Chemistry 2015
turbomolecular pump (250 L s−1) backed by another Leybold TriVac® D25B rotary vane mechanical pump (20 cfm). Sodium phosphomolybdate hydrate (Na3[PMo12O40]·xH2O CAS: 1313-30-0, Sigma-Aldrich, St. Louis, MO.) was dissolved in methanol to a final concentration of 150 μM and introduced into the ESI source using a syringe pump (Legato 180, KD Scientific, Holliston, MA) at a flow rate of 65–80 μL h−1. Charged microdroplets were produced by applying a ∼3 kV negative potential to the stainless steel union connected to the ESI emitter. Three types of ESI emitters were used in this study; (i) single bore (ii) double-bore (iii) four-bore emitters. Single bore emitters (Polymicro Technologies, Phoenix, AZ) are polyimide-coated fused silica capillaries with an ID of 100 µm and an outer diameter (OD) of 360 µm. Double bore and four bore emitters were prepared by pulling 500 µm OD boro-silicate glass tubes (VitroCom, NJ, USA) having two and four bores (ID of each bore: 125 µm), respectively, to a final OD of 130 µm using a micropipette puller (P-2000, Sutter Instrument Company, Novato, CA). Typical pulling parameters (in relative units used by the instrument software) were as follows: heat, 350; filament, 4; velocity, 30; delay, 200; pull strength, 0. PMo12O403− ions, the dominant species observed after droplet desolvation, were used for the optimization and characterization of the high-intensity ESI interface. Ion current measurements were performed using a picoammeter (model 9103, RBD Instruments, Bend, OR).
Results and discussion The ability to operate at higher pressure in the outer funnel region while maintaining
