Coupling Microchip Electrospray Ionization Devices with High

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Coupling Microchip Electrospray Ionization Devices with High Pressure Mass Spectrometry William M. Gilliland, John Scott Mellors, and J. Michael Ramsey Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03484 • Publication Date (Web): 18 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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Analytical Chemistry

Coupling Microchip Electrospray Ionization Devices with High Pressure Mass Spectrometry William M. Gilliland1, Jr., J. Scott Mellors5, J. Michael Ramsey*1,2,3,4 1

Department of Chemistry, 2Department of Applied Physical Sciences, 3Department of

Biomedical Engineering, and 4Carolina Center for Genome Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States 5

908 Devices, Inc., Boston, Massachusetts, United States AUTHOR EMAIL ADDRESS [email protected]

ABSTRACT A microchip electrospray ionization source was coupled with high pressure mass spectrometry (HPMS). A continuous atmospheric inlet consisting of a stainless steel capillary and DC ion optics was designed to conduct ions into the mass spectrometer. Infusions of amino acids and peptides were performed and detected with a miniature cylindrical ion trap (mini-CIT) based mass spectrometer operated at ≥ 1 Torr with air as the buffer gas. Detection of glycine and thymopentin (separately) demonstrated the mass range of the mini-CIT detector could span from m/z 75 to 681. A microchip capillary electrophoresis (CE) separation with mini-CIT detection was performed and the results compared with detection using a commercial instrument (Waters Synapt G2). Comparable separation efficiencies were observed with both mass spectrometers as detectors, with about six times better signal-to-noise observed on the Synapt G2. Comparison of mass spectra in the two systems reveal similar features observed, but with wider peak widths in the mini-CIT than on the Synapt G2 as expected due to high pressure operation.

INTRODUCTION 1 ACS Paragon Plus Environment


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Mass spectrometry (MS) is a powerful analytical technique due to its sensitivity, versatility, and ability to provide chemical and structural information of molecules; because of this, it is often the detection method of choice for a wide range of applications. Electrospray ionization (ESI) has significantly expanded the range of mass spectrometric analysis beyond gas phase and volatile molecules to include biomolecules and other liquid-borne analytes.1 ESI provides a facile method for coupling liquid phase separations, such as liquid chromatography (LC) or capillary electrophoresis (CE) with MS detection. As a result, LC-MS has become a widely used analytical tool in fields such as proteomics,2 environmental monitoring,3,4 drug discovery and development,5 and clinical diagnostics.6,7 However, conventional LC-MS systems are usually confined to dedicated laboratories because they are large, expensive, complex, and require significant amounts of power. The development of a smaller, more portable, less expensive, and simpler liquid phase separation/MS analytical platform could be applied to many of the same fields as traditional LC-MS systems but with the potential for on-site analysis. The miniaturization of LC-MS systems is limited by the need for a rugged system of pumps, valves, and tubing, while mass spectrometers are limited by low pressure operation, which requires bulky, fragile, and expensive turbomolecular pumps. Many recent efforts have been made towards the miniaturization of both liquid phase separations and mass spectrometry,8,9 including a miniature CE separation coupled to a miniature MS.10 The combination of these systems could be a low-cost, targeted alternative to conventional instrumentation. Here, we describe the first steps in coupling microfabricated ESI devices with a miniature high pressure mass spectrometry (HPMS) system. A microfluidic-based ESI device has small sample volumes, short analysis times, flow rates ideal for ESI-MS analysis, and a small form factor that fits within the goal of a miniature analytical system.11 Many research groups have interfaced microfabricated devices and mass spectrometry with ESI either by electrospraying from a flat surface of a microchip,12,13 inserting an electrospray emitter,14–16or monolithic integration of an emitter.17–19 However, these devices are complex to fabricate or suffer from poor performance. Our lab has developed glass microchips with high performance, monolithically integrated ESI emitters that are also simple to fabricate. These devices are capable of CE for biomolecule analysis with the separation channel terminating at the 2 ACS Paragon Plus Environment

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corner of the device that acts an ESI emitter, resulting in an interface with no detectable band broadening.20–22 These microchips are controlled with only DC voltages, have a small footprint, and give high efficiency separations, making them excellent candidates for a miniature liquid phase separations based-MS platform. Miniature mass spectrometers have been developed in recent years in an attempt to reduce the cost of traditional mass spectrometry and bring its sensitivity and chemical identification ability into the field. However, the size, weight, and power (SWaP) of the vacuum components of MS systems, especially turbomolecular pumps, limit SWaP and negatively impacts cost. Systems like the Mini 1123 and the MMS 10024 have pushed the boundaries of MS miniaturization, but these systems are still limited by the turbomolecular pump needed to produce a low-pressure vacuum and require a generator for power. Recently, our lab has demonstrated HPMS at pressures up to and exceeding 1 Torr, eliminating the need for turbo pumps. HPMS has been demonstrated with helium as well as nitrogen and air buffer gases.25 The strategy for HPMS utilizes miniature cylindrical ion traps (CITs) operated at elevated RF drive frequencies compared to conventional ion traps. The resolving power (Equation 1) of an ion trap (m/∆m, ratio of ion mass to peak width) is fundamentally dependent on the buffer gas pressure (P) and the RF drive frequency (ΩRF): 26 m Ω ∝ RF . ∆m P

(1)

Increasing the operational buffer gas pressure results in a loss of resolving power, but that loss can be regained with an increase in drive frequency. At a given drive RF voltage (Vmax) and trap size (r0 and z0), the largest mass-to-charge ion that can be ejected (m/emax) decreases with increased frequency:27

8Vmax m ,   = 2 2 2  e  max q max Ω RF ( r0 + 2 z 0 )

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(2)


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where qmax = 0.908 is a trapping parameter and a constant, r0 is the radius of the trap and z0 is the axial dimension of the trap. In order to maintain practical voltages, the trap size can be decreased to maintain the mass range at higher drive frequencies. In addition, by decreasing the size of the trap, the mass range and resolution at a given pressure can be tuned by adjusting the RF drive frequency and voltage.28 Coupling microchip-ESI and HPMS offers the benefit of high separation efficiencies with MS detection in non-laboratory settings. One of the difficulties associated with coupling ESI sources with MS systems is that ions must be transported into vacuum for mass analysis.29 The transmitted ion current from an ESI source can be reduced by up to three orders of magnitude when travelling through a capillary inlet. These losses occur mostly in transfer regions from a higher pressure to lower pressure (i.e. on either side of a capillary inlet),30 and two or more of these regions are usually required for traditional ESI-MS. This presents a significant challenge for coupling ESI with MS. Operation at HPMS pressures with air as a buffer gas simplifies the ESI-MS interface. High pressure MS operation eliminates the need for differential pumping regions between the inlet and mass spectrometer necessary to perform MS analysis, given that ions may be introduced from atmospheric pressure directly into the mass analysis chamber. In addition, the use of ambient air as a buffer gas prevents dilution of analytes from the addition of other gases. These simplifications have significant potential to yield improvements in instrument sensitivity by limiting possible ion losses and dilution factors compared to conventional instruments that have one or more regions of differential pressure. In this paper, we demonstrate the coupling of a microchip ESI source with high pressure mass spectrometry via an atmospheric inlet and direct current (DC) optics. Using a miniature CIT-based mass spectrometer, we show the feasibility of a miniaturized CE-ESI-MS system. A low cost, fast, and miniature platform could provide an alternative to traditional LC-MS systems for many applications. Initial work focuses on small biomolecules including amino acids and peptides. One application of a miniaturized CE-ESI-MS system for biomolecule analysis is monitoring of amino acids for process control of bioreactors used to produce biopharmaceuticals. Monitoring amino acid concentration can be used to optimize growth conditions and monitor cellular activity in a cell culture or bioreactor.31Another possible application of this technology is 4 ACS Paragon Plus Environment

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the analysis of small peptides. Analysis of peptides could be used for QA/QC of biopharmaceuticals, identification and characterization of proteins, or to gain greater insight into cellular functions.32 Thus, amino acids and peptides were chosen as target analytes.

EXPERIMENTAL Materials and Reagents. HPLC grade acetonitrile and formic acid (99.9%) were obtained from Fisher Scientific (Fairlawn, NJ). Purified deionized water was obtained using a Nanopure Diamond water purifier (Barnstead International, Dubuque, IA). (3Aminopropyl)di-isopropylethoxysilane (APDIPES) was obtained from Gelest (Morrisville, PA). Amino acids used for analysis were obtained from Fisher Scientific. Peptides bradykinin, methionine-enkephalin, thymopentin, and angiotensin II were obtained from American Peptide Company (Sunnyvale, CA). The background electrolyte for all experiments was 50 % acetonitrile, 49.9 % water, and 0.1 % formic acid (v/v/v, pH = 3.1). Microchip Design, Fabrication, and Operation. Figure 1 shows two schematics of microchip designs used for CE-ESI (1A) and infusion-ESI (1B). The CE-ESI device consisted of four reservoirs, an injection cross, a 46-cm serpentine separation channel, an electroosmotic (EO) pumping channel, and an ESI orifice. The reservoir labels indicate sample (S), background electrolyte (BG), sample waste (SW), and electroosmotic pump (EO). The infusion device consisted of two reservoirs (sample (S) and EO pump (EO)), a 5.5-cm infusion channel, and an EO pump. For infusions, sample was loaded into both the S and EO reservoirs. Channels for both devices were isotropically etched to 10 µm deep and 70 µm full width. Microchip ESI devices were fabricated in-house from 0.5 mm thick B270 glass (Perkin Elmer, Waltham, MA) using photolithography and wet etching techniques described in detail previously.20,21 Coating procedures for the ESI devices have also been described previously.21,22 Briefly, devices were coated with APDIPES via chemical vapor deposition (CVD) using a LabKote CVD system (Yield Engineering Systems, Livermore, CA). The pumping channels were then functionalized with a 20 kDa polyethylene glycol (PEG) reagent (NanoCS, Boston, MA). The PEG reagent terminates with an 5 ACS Paragon Plus Environment


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N-hydroxysuccinimide ester that reacts with the primary amine of the APDIPES surface, forming a covalent bond between the PEG chain and the surface coating.

Both CE-ESI and infusion designs were operated by application of voltages to the reservoirs via platinum wire electrodes. Applied voltages were controlled by a custom high voltage power supply consisting of five independent voltage modules, as described previously.20,21 Three modules had a maximum output of -25 kV, and the other two had a maximum output of +10 kV (UltraVolt Inc., Ronkonkoma, NY). The power supply was connected to a computer via a SCB-68 breakout box and a PCI-6713, 8-channel analog out card (National Instruments, Austin, TX). A custom LabVIEW program was used to operate the power supply. For CE-ESI, the voltages applied to the S, B, SW, and EO reservoirs were -14, -14, -12, and +6 kV, respectively. To perform a gated injection, voltages were switched to -14, -13, -13, and +6 kV for 0.5 seconds. This produced an electric field strength of 400 V/cm with an approximate flow rate of 165 nL/min. For infusion-ESI, typical voltages were +5 kV at the S reservoir and +0.5 kV for the EO reservoir. ESI-MS. Miniature mass spectrometry (ESI-HPMS) experiments were performed with a custom atmospheric interface and a differentially pumped vacuum system. A schematic of a typical experimental setup is shown in Figure 2. The microchip-ESI device (Figure 1, CE or Infusion) was mounted on an adjustable x-y-z stage and positioned approximately 5-10 mm from the HPMS inlet capillary (Items 1-2 in Figure 2). A single sided copper clad circuit board (M.G. Chemicals, Burlington, Ontario, Canada) was used to shield the ESI orifice from the voltages applied to the reservoirs (labeled in Figure 2). The copper board reduces the influence of the voltages applied to the reservoirs on the electric field between the ESI emitter and the inlet capillary. The corner of the microfluidic devices extended about 5 mm through a slit in the board.22 The circuit board was held at +1 kV for CE experiments and GND for infusion experiments. The miniature CIT used for HPMS analysis was mounted in a custom differentially pumped vacuum chamber described previously.25 Ions were conducted into vacuum using a custom capillary interface, consisting of a stainless steel capillary (2 in Figure 2) (0.01” 6 ACS Paragon Plus Environment

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id, 1/16” od, 10 cm in length, Valco Instruments Co., Inc., Houston, TX) held in place by a Swagelok UltraTorr fitting (Swagelok, Inc., Solon, OH). Ions were typically accumulated for 5 ms before analysis. They were then scanned out of the trap and detected with an electron multiplier (6) (Detech 2300, Detector Technology, Inc., Sturbridge, MA). A typical mass spectrum was an average of 30 to 1000 individual mass scans. Two sets of pumps were used for differentially pumping the chamber (7). A dry scroll pump (SH-110, Agilent Technologies, Inc., Santa Clara, CA) was used on the mass analysis chamber (~1 Torr) and an Agilent TPS Bench turbomolecular pump (Model TV81M) backed by a dry scroll pump (SH-110) was used on the detector chamber (~10 mTorr). A turbomolecular pump and electron multiplier were used for development of ESI-HPMS because of the high sensitivity and bandwidth afforded by electron multiplier detectors. In the future, pressure tolerant detectors such as Faraday cups could be used, so the HPMS system could be operated at a single pressure with a single roughing pump. Miniature CIT electrodes were wet etched by Towne Technologies, Inc. (Somerville, NJ). Dimensions for the CITs were r0 = 250 µm, z0 = 325 µm, and endcaps with 200 µm hole diameters. Each ring electrode contained a single trap. Traps were assembled by manual alignment using alignment pins. Electrodes were mounted to a custom plate with 125 µm kapton (polyimide) spacers between them. Drive RF waveforms were applied by a Rohde and Schwarz SMB 100A Signal generator and amplified using a Mini Circuits TVA-R5-13 preamplifier and AR305 power amplifier. The signal was resonated with a tank circuit, and applied frequencies ranged from 7 to 12 MHz. Custom LabVIEW software was designed to monitor, control, and collect data. A National Instruments PXIe-1073 data acquisition chassis was used to interface the electronics and LabVIEW software. For comparison of CE separation detection, a Synapt G2 quadrupole-ion mobilitytime-of-flight mass spectrometer (Waters Corporation, Milford, MA) was used. The Synapt G2 was operated at a rate of 90 ms per summed scan with an interscan delay of 24 ms (~9 Hz). The mass range was set to 300 to 1600 m/z. MassLynx software was used to collect data and triggered by a custom LabVIEW program used to control voltages applied to the microchip. 7 ACS Paragon Plus Environment


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RESULTS AND DISCUSSION Atmospheric Interface. As the goal is to develop an easy to operate, low-cost instrument, a simple interface to conduct ions from atmosphere into vacuum for HPMS analysis was designed. Typical MS atmospheric pressure interfaces consist of capillary inlets or small apertures. Using this as a guide, a stainless steel capillary inlet was chosen for our initial atmospheric interface (2 in Figure 2). The ESI-HPMS interface developed has several advantages over conventional ESI-MS interfaces. Conventional ESI-MS interfaces consist of an atmospheric inlet, multiple regions of differential pressure, and complex ion optics – required due to the low-pressure operation of the mass analyzer. Because HPMS operates with pressures close to 1 Torr, the interface can introduce ions directly from atmosphere into the mass analyzer chamber via the capillary inlet. A simple vacuum fitting can hold the capillary. Secondly, minimal optics should be required to maximize ion transmission due to a shorter ion source-to-mass analyzer distance. Four different capillary sizes, 0.005, 0.007, 0.01, and 0.02 in. inner diameter (ID) were tested. No ions were observed with either the 0.005 and 0.007 in. ID capillaries. Pumping requirements proved too strenuous for the 0.02 in. capillary, so a capillary with 0.01 in. ID was chosen for further development. A copper electrode (3 in Figure 2) was added to align the capillary with the CIT entrance and to accelerate ions toward the trap after exiting the capillary. The capillary and accelerating electrode were in electrical contact, and a voltage between +100 and +250 V was typically applied to the combination. In addition to alignment and acceleration, a focusing element was added to further improve sensitivity. A simple DC “gate” electrode (4) was (r = 250 µm, 380 µm thick, spaced 125 µm from the endcap of the CIT) inserted between the trap and alignment electrode. The gate electrode can also be used to prevent ions from entering the trap during mass analysis. A positive voltage (+20 to +100 V) was applied to the gate electrode to focus ions into the trap, and ground or a small negative DC voltage (-5 to -30 V) was applied to stop ions. The amino acid histidine was used as the initial model analyte for the development of the microchip to MS interface. The infusion-ESI microchip was used to produce a constant source of ions. After optimization, all twenty common amino acids were 8 ACS Paragon Plus Environment

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separately infused and detected. Representative infusion-ESI-MS spectra of four amino acids (histidine, arginine, glutamic acid and proline) collected using the atmospheric interface and differential chamber setup are shown in Figure 3. Mass analysis was performed at a pressure of 1.2 Torr with ambient air as the buffer gas at a drive frequency of 10.2 MHz. Each spectrum is an average of 1000 individual mass spectral scans. The (M+H)+ peak of each amino acid is clearly detected, which provides sufficient information for identification of these amino acids in a targeted analysis scenario. In the case of histidine and glutamic acid, some fragmentation was also observed. ESI is a soft ionization technique, but operation at high pressures results in increased ion-buffer gas collisions, which can impart the energy required to induce fragmentation. In the future, these fragmentation patterns may aid in the identification of chemical species, including the differentiation of isobaric molecules. Detection of the twenty common amino acids demonstrates the ability to detect a range of analytes varying in size, polarity, and basicity. Expanding the mass range of detectable analytes would increase the utility of a miniature ESI-HPMS system. For instance, a low cost, high throughput method for the detection of peptides would be applicable to QA/QC of biopharmaceuticals. As shown in Equation 2, the mass range can be extended by decreasing the RF operating frequency for a given RF drive voltage and trap dimension. For this work, the RF drive frequency was decreased to 7.1 MHz from 10.2 MHz, giving an expected mass range of 300 – 725 m/z compared to the mass range of 75 – 225 m/z for the amino acid analysis. Using this strategy, an infusion-ESI-MS spectrum of a small peptide, thymopentin (RKDVY, (M+H)+ m/z = 681), was collected (Figure 4). Mass analysis was performed at a pressure of 1.3 Torr in ambient air as the buffer gas. The spectrum shows a large peak at m/z 341with a number of peaks between m/z 360 and 500, and finally a peak at m/z 681. Trapping and analysis of thymopentin demonstrates that the mass range of the mini-CIT can be extended to at least m/z 681. With respect to sensitivity, the signal-to-noise ratio (S/N) for thymopentin was significantly greater than the S/N observed for the amino acids (~75x greater). The smaller S/N observed for amino acids versus peptides could be due to less efficient capture of small molecules due to scattering before entering the trap. This experimental 9 ACS Paragon Plus Environment


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setup used relatively simple ion optics (one accelerating electrode and one DC “gate” lens). If critical to sensitivity, the discrepancy in S/N could be resolved with more complex optics for more efficient ion collection, such as an ion funnel.30 Despite the difference in S/N between analytes of different mass, this simple inlet interface is an effective way of introducing ions from atmospheric pressure into vacuum for HPMS analysis. CE-ESI-MS of Peptides. After demonstrating the viability of the atmospheric interface, the miniature CIT system was assessed as a potential detector for microchip CE separations and compared with a commercial MS system, the Waters Synapt G2. Figure 5 shows base peak intensity (BPI) electropherograms of a standard peptide mixture (methionine enkephalin, angiotensin II, bradykinin, and thymopentin) detected with the mini-CIT system and the Synapt G2. Fluorescein was added to the mixture as a migration time marker. The separation field strength was 400 V/cm with a flow rate of 165 nL/min. Approximately 7 fmol of peptide mixture (bulk concentration 5 µM) was injected during a 0.5 s gated injection. The mini-CIT (r0 = 250 µm) was operated at 1.2 Torr with an RF drive frequency of 7.1 MHz. The four peptides and fluorescein were separated and detected. The average S/N of the four peptides in the BPI electropherograms was 69 using HPMS and 437 using the Synapt G2. The calculated average separation efficiencies were 445,000 theoretical plates using the mini-CIT and 490,000 theoretical plates using the Synapt G2. Both mass spectrometers were able to detect these fast and highly efficient separations (average FWHM = 0.58 ± 0.04 s), with the discrepancy in calculated efficiency resulting from differences in mass spectral sampling rate. The Synapt G2 collected spectra at about 9 Hz, while the mini-CIT collected spectra at about 3 Hz. The CIT is limited by the time required to accumulate, analyze, and clear ions from the trap. With sensitivity improvements, the accumulation time can be reduced and the sampling rate increased. Detection of these peptides following CE separation shows that a miniature CIT based mass spectrometer operated at high pressure can be used for detection of fast separations with narrow peak widths. The Synapt G2 showed about 6 times better S/N, but this simple comparison demonstrates the viability of a HPMS using a mini-CIT as a chemically informing detector for the fast separation of biomolecules. 10 ACS Paragon Plus Environment

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For mixtures like these peptides, the mini-CIT system offers a simple and inexpensive alternative to a large commercial instrument such as the Synapt G2. The miniature MS system can provide useful mass spectral information for label-free detection and identification of chemical species, particularly in a known sample matrix. Sample mass spectra of bradykinin for both MS systems acquired during the CE separations are shown in Figure 6. Some similar features can be observed in the two spectra, most notably the (M+2H)2+ peaks at m/z 531. The most obvious difference is the observed peak width (12.0 m/z with HPMS; 0.026 m/z with Synapt G2, measured by full width at halfmaximum (FWHM)). Peak broadening is expected at elevated buffer gas pressures, and peak widths of about 4.5 m/z (FWHM) were reported previously for volatile organic compounds in 1 Torr of air buffer gas.33 The peaks observed here are wider than expected based on those previous results, although in that work double resonant ejection was used to improve resolution, and that technique was not used here. Also, the atmospheric inlet was coaxial to the trap, so it is possible that gas flow from the inlet disrupted mass analysis and caused the peaks to broaden In the future, peak widths could be improved with a more efficient interface and more complex optics to prevent direct ambient gas flow into the trap. In addition, peak widths can be reduced with higher RF frequency operation and the use of smaller traps to maintain the mass range, and different trap geometries or arrays of CITs would make up lost charge capacity.34,35 Finally, resolution could also be improved by applying a small RF voltage to one or both endcaps of the trap to perform resonant or double resonant ejection.36,37 Even with increased peak widths, a mass spectrum combined with CE migration time provides sufficient information for identification of many chemical species, especially for an application where the goal is detection of known target analytes.

CONCLUSION Coupling ESI with HPMS has been demonstrated. An atmospheric inlet was designed to conduct ions from atmosphere into vacuum. Microchip infusions and CE separations were performed with a mini-CIT detector operated at pressures up to and exceeding 1 Torr with air as a buffer gas, demonstrating the viability of a miniaturized CE-ESI-MS system. This type of system has the potential to replace more expensive and bulky LC11 ACS Paragon Plus Environment


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MS systems for targeted species applications. Future work will focus on improving sensitivity to maximize acquisition rate, quantification of mass accuracy and limits of detection, and exploration of high-frequency operation and helium buffer gas for improvements in resolution.

DISCLAIMER J.M. Ramsey is a scientific founder, director, consultant, and maintains a financial interest in 908 Devices, to which the research in this paper has been licensed.

ACKNOWLEDGMENTS We would like to thank the Defense Threat Reduction Agency (contract # W911NF-10-10447) for supporting this work.


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A

B

Figure 1. Schematics for capillary electrophoresis (A) and infusion (B) glass microfluidic devices. All channels were etched to a depth of 10 µm. Reservoirs are designated with circles and indicate sample (S), background electrolyte (BG), sample waste (SW), and electroosmotic pump (EO). The microchip in A consists of an injection cross, a 46-cm serpentine separation channel, and an electroosmotic pumping channel. The infusion device (B) consists of a 5.5-cm channel and an electroosmotic pumping channel, and both reservoirs are filled with the sample.



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Figure 2. Experimental setup (not to scale) for ESI-HPMS with (1) Glass microchip with electrospray, (2) stainless steel capillary and fitting, (3) accelerating electrode, (4) gate electrode, (5) trap electrodes; two endcaps (BeCu) and ring (Cu), (6) electron multiplier detector, and (7) vacuum pumps.


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Figure 3. Infusion-ESI spectra of four amino acids (100 µM) with atmospheric interface and miniature CIT (r0 = 250 µm). The drive RF was 10.2 MHz, and the buffer gas pressure was 1.2 Torr with ambient air.



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Figure 4. Infusion-ESI mass spectrum of 5 µM thymopentin obtained with mini-CIT (r0= 250 µm). The drive RF was 7.1 MHz, and the buffer gas pressure was 1.3 Torr with ambient air.


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Figure 5. BPI electropherograms of a peptide mixture with a 46 cm CE microchip and mini-CIT (black) and Synapt G2 (red). Fluorescein (*), Methionine Enkephalin (1), Angiotensin II (2), Bradykinin (3), Thymopentin (4) were the analytes. Approximately 7 fmol of peptide mixture was injected during a 0.5 s gated injection. The separation field strength was 400 V/cm.



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Figure 6. Sample mass spectra of bradykinin (peak 3 in Fig. 5) with A) mini-CIT (r0 = 250 µm) with drive RF at 7.1 MHz and buffer gas pressure at 1.3 Torr and B) Synapt G2.


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TOC FIGURE – FOR TOC ONLY


Coupling Microchip Electrospray Ionization Devices with High

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