Design and characterization of a multisource hand-held tandem mass spectrometer

A wireless-controlled miniature rectilinear ion trap mass spectrometer system, total weight with batteries 5.0 kg, consuming less than 35 W of power, and having dimensions of 22 cm in length by 12 cm in width by 18 cm in height, is characterized. The design and construction of the mass spectrometer including mass analyzer, vacuum system, electronics system, and data acquisition and processing systems, is detailed. The mass spectrometer is compatible with various types of ionization sources including a glow discharge electron impact ionization source used in the internal ionization mode, and various atmospheric pressure ionization sources, including electrospray ionization, atmospheric pressure chemical ionization, and desorption electrospray ionization, which are employed for external, atmospheric pressure ionization. These external sources are coupled to the miniature mass spectrometer via a capillary interface that is operated in a discontinuous fashion (discontinuous atmospheric pressure interface) to maximize ion transport. The performance of the mass spectrometer for large and small molecules is characterized. Limits of detection in the parts-per-billion range were obtained for selected compounds examined using both the internal ionization and external ionization modes. Tandem mass spectrometry and fast in situ analysis capabilities are also demonstrated using a variety of compounds and ionization sources. Protein molecules are analyzed as the multiply protonated molecules with mass/charge ratios up to 1500 Da/charge.     

Sampling wand for an ion trap mass spectrometer

A new sampling wand concept for ion trap mass spectrometers equipped with discontinuous atmospheric pressure interfaces (DAPI) has been implemented. The ion trap/DAPI combination facilitates the operation of miniature mass spectrometers equipped with ambient ionization sources. However, in the new implementation, instead of transferring ions pneumatically from a distant source, the mass analyzer and DAPI are separated from the main body of the mass spectrometer and installed at the end of a 1.2 m long wand. During ion introduction, ions are captured in the ion trap while the gas in which they are contained passes through the probe and is pumped away. The larger vacuum volume due to the extended wand improves the mass analysis sensitivity. The wand was tested using a modified hand-held ion trap mass spectrometer without additional power or pumping being required. Improved sensitivity was obtained as demonstrated with nano-electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and low temperature plasma (LTP) probe analysis of liquid, gaseous, and solid samples, respectively.     

Subambient pressure ionization with nanoelectrospray source and interface for improved sensitivity in mass spectrometry

A nanoelectrospray ionization mass spectrometry (ESI-MS) source and interface has been designed that enables efficient ion production and transmission in a 30 Torr pressure environment using solvents compatible with typical reversed-phase liquid chromatography (RPLC) separations. In this design, the electrospray emitter is located inside the mass spectrometer in the same region as an electrodynamic ion funnel. This avoids the use of a conductance limiting ion inlet, as required by a conventional atmospheric pressure ESI source, and allows more efficient ion transmission to the mass analyzer. The new subambient pressure ionization with nanoelectrospray (SPIN) source improves instrument sensitivity and enables new electrospray interface designs, including the use of multi-emitter approaches. Performance of the SPIN source was evaluated by electrospraying standard solutions at 300 nL/min and comparing results with those obtained from a standard atmospheric pressure ESI source that used a heated capillary inlet. This initial study demonstrated an approximately 5-fold improvement in sensitivity when the SPIN source was used compared to a standard atmospheric pressure ESI source. The importance of desolvation was also investigated by electrospraying at different flow rates, which showed that the ion funnel provided an effective desolvation region to aid the creation of gas-phase analyte ions.     

Ambient mass spectrometry with a handheld mass spectrometer at high pressure

The first coupling of atmospheric pressure ionization methods, electrospray ionization (ESI) and desorption electrospray ionization (DESI), to a miniature hand-held mass spectrometer is reported. The instrument employs a rectilinear ion trap (RIT) mass analyzer and is battery-operated, hand-portable, and rugged (total system: 10 kg, 0.014 m(3), 75 W power consumption). The mass spectrometer was fitted with an atmospheric inlet, consisting of a 10 cm x 127 microm inner diameter stainless steel capillary tube which was used to introduce gas into the vacuum chamber at 13 mL/min. The operating pressure was 15 mTorr. Ions, generated by the atmospheric pressure ion source, were directed by the inlet along the axis of the ion trap, entering through an aperture in the dc-biased end plate, which was also operated as an ion gate. ESI and DESI sources were used to generate ions; ESI-MS analysis of an aqueous mixture of drugs yielded detection limits in the low parts-per-billion range. Signal response was linear over more than 3 orders of magnitude. Tandem mass spectrometry experiments were used to identify components of this mixture. ESI was also applied to the analysis of peptides and in this case multiply charged species were observed for compounds of molecular weight up to 1200 Da. Cocaine samples deposited or already present on different surfaces, including currency, were rapidly analyzed in situ by DESI. A geometry-independent version of the DESI ion source was also coupled to the miniature mass spectrometer. These results demonstrate that atmospheric pressure ionization can be implemented on simple portable mass spectrometry systems.     

Atmospheric pressure ionization in a miniature mass spectrometer

A miniature cylindrical ion trap mass spectrometer featuring an atmospheric pressure interface allowing atmospheric pressure chemical ionization and electrospray ionization is described together with its analytical performance characteristics. The vacuum system, ion optics, mass analyzer, control electronics system, and detection system have all been designed and built in-house. The design is based upon a three-stage, differentially pumped vacuum system with the instrument capable of being interfaced to many types of atmospheric pressure ionization sources. Ions are transferred through home-built ion optics, and instrument control is achieved through custom-designed electronics and LabView control software. Corona discharge ionization and electrospray ionization sources are implemented and used to allow the analysis of both gaseous- and solution-phase samples during the characterization of the instrument. An upper mass/charge limit of approximately 450 Th with unit resolution was achieved using a 2.5-mm-internal radius cylindrical ion trap as the mass analyzer. The specificity of the instrument can be increased by employing the MS/MS capabilities of the ion trap and has been demonstrated for nitrobenzene. Limits of detection for the trace analysis in air of the chemical warfare agent simulant methyl salicylate (1.24 ppb) and for nitrobenzene (629 pptr) are achieved. The dynamic range of the instrument is currently limited to approximately 2 orders of magnitude by saturation of the detection electronics. Isolation and collision-induced dissociation efficiencies in MS/MS experiments both greater than 50% are reported. Electrospray/nanospray data are presented on solutions including 100 microM (D,L)-arginine, 10 microM (-)-ephedrine, and 10 microM lomefloxacin.     

Preparative linear ion trap mass spectrometer for separation and collection of purified proteins and peptides in arrays using ion soft landing

A preparative mass spectrometer for microarray fabrication is reported. The instrument includes an atmospheric pressure ionization source, a linear ion trap mass analyzer, an ion collection surface positioning system, and a surface loading chamber with independent vacuum pumping. It was designed for the production of protein arrays using the ion soft-landing technique to collect ions on a surface after separation by mass/charge ratio. Small microarrays have been prepared by isolating and soft landing individual protein or peptide ions after electrospray ionization of mixtures. The composition and purity of the separated materials has been confirmed using independent external mass spectrometric analysis of rinse solutions of the collected spots, either by the new method of electrosonic spray ionization MS or by nanospray ionization MS. The ability to retain bioactivity in the mass-selected and collected biomolecules has been demonstrated in particular cases. The reported instrument has also been characterized as an analytical mass spectrometer.     

A quadrupole ion trap mass spectrometer with three independent ion sources for the study of gas-phase ion/ion reactions

An instrument for the study of gas-phase ion/ion reactions in which three independent sources of ions, namely, two electrospray ionization sources and one atmospheric sampling glow discharge ionization source, are interfaced to a quadrupole ion trap mass analyzer is described. This instrument expands the scope of gas-phase ion/ion reaction studies by allowing for manipulation of the charge states of multiply charged reactant and product ions. Examples are provided involving the formation of protein-protein complexes in the gas phase. Complexes with charge states that cannot be formed from reactant ion charge states present in the normal electrospray charge state distributions can be formed in the new apparatus. Strategies that rely on both reactant ion charge state manipulation and product ion charge state manipulation are demonstrated. In addition, simplification of product ion spectra generated from dissociation of complexes formed via ion/ion reactions can be effected by using the discharge source to reduce the charge state of the product ions to primarily 1+.     

Comparison of atmospheric pressure chemical ionization, electrospray ionization, and atmospheric pressure photoionization for the determination of cyclosporin A in rat plasma

Atmospheric pressure chemical ionization was compared with electrospray ionization and atmospheric pressure photoionization (APPI) as an interface of high-performance liquid chromatography (HPLC)-tandem mass spectrometry (MS/MS) for the determination of cyclosporin A (CsA) in biological fluids in support of in vivo pharmacodynamic studies. These ion sources were investigated in terms of their suitability and sensitivity for the detection of CsA. The effects of the eluent flow rate and composition as well as the nebulizer temperatures on the photoionization efficiency of CsA in the positive ion mode under normal-phase HPLC conditions were explored. The ionization mechanism in the APPI environment with and without the use of the dopant was studied using two test compounds and a few solvent systems employed for normal-phase chromatography. The test compounds were observed to be ionized mainly by proton transfer with the self-protonated solvent molecules produced through photon irradiation. Furthermore, ion suppression due to sample matrix interference in the normal-phase HPLC-APPI-MS/MS system was monitored by the postcolumn infusion technique. The applicability of these proposed HPLC-API-MS/MS approaches for the determination of CsA at low nanogram per milliliter levels in rat plasma was examined. These proposed methods were then compared with respect to specificity, linearity, detection limit, and accuracy.     

High-throughput miniature cylindrical ion trap array mass spectrometer

A fully multiplexed cylindrical ion trap (CIT) array mass spectrometer with four parallel ion source/mass analyzer/detector channels has been built to allow simultaneous high-throughput analysis of multiple samples. A multielement external chemical ionization/electron ionization source was coupled to a parallel array of CITs each of equal size (internal radius 2.5 mm), and the signal was recorded using an array of four miniature (2-mm inner diameter) electron multipliers. Using external electron ionization, the spectra of four separate samples were recorded simultaneously in real time using a four-channel preamplifier system and a data acquisition program written using LabVIEW software. These experiments mark the first demonstration of externally generated ions being successfully trapped in a miniature CIT mass analyzer. The instrument currently provides mass/charge range of approximately m/z 50-500. Average peak width is m/z 0.3, corresponding to a resolution of 1000 at m/z 300. The four-channel mass spectrometer is housed in a single vacuum manifold and operated with a single set of control electronics. The modular design of this instrument allows scale-up to many more channels of analysis for future applications in the areas of industrial process monitoring and combinatorial analysis and in the fields of proteomics and metabolomics.     

Rotation planar chromatography coupled on-line with atmospheric pressure chemical ionization mass spectrometry

The coupling of a rotation planar preparative thin-layer chromatography system on-line with mass spectrometry is demonstrated using a simple plumbing scheme and a self-aspirating heated nebulizer probe of a corona discharge atmospheric pressure chemical ionization source. The self-aspiration of the heated nebulizer delivers approximately 20 microL/min of the 3.0 mL/min eluate stream to the mass spectrometer, eliminating the need for an external pump in the system. The viability of the coupling is demonstrated with a three-dye mixture composed of fat red 7B, solvent green 3, and solvent blue 35 separated and eluted from a silica gel-coated rotor using toluene. The real-time characterization of the dyes eluting from the rotor is illustrated in positive ion full-scan mode. Other self-aspirating ion source systems including atmospheric pressure photoionization, electrospray ionization, and inductively coupled plasma ionization, for example, might be configured and used in a similar manner coupled to the chromatograph to expand the types of analytes that could be ionized, detected, and characterized effectively.     

Atmospheric pressure photoionization. 1. General properties for LC/MS

In this work, we describe the performance of an atmospheric pressure photoionization (APPI) source for sampling liquid flows. The results presented here primarily focus on the mechanism of direct photoionization (PI), as compared to the dopant mechanism of PI. Measured detection limits for direct APPI were comparable to atmospheric pressure chemical ionization (APCI; e.g., 1 pg for reserpine). The ion signal is linear up to 10 ng injected quantity, with a useful dynamic range exceeding 100 ng. Evidence is presented indicating that APPI achieves significantly better sensitivity than APCI at flow rates below 200 microL/min, making it a useful source for capillary liquid chromatography and capillary electrophoresis. Results are presented indicating that APPI is less susceptible to ion suppression and salt buffer effects than APCI and electrospray ionization (ESI). The principal benefit of APPI, as compared to other ionization sources, is in efficiently ionizing broad classes of nonpolar compounds. Thus, APPI is an important complement to ESI and APCI by expanding the range and classes of compounds that can be analyzed. In this paper, we also discuss the role of direct APPI vs PI-induced APCI using dopants.     

Microchip sonic spray ionization

The first microchip version of sonic spray ionization (SSI) as an atmospheric pressure ionization source for mass spectrometry (MS) is presented. The microchip used for SSI has recently been developed in our laboratory, and it has been used before as an atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) source. Now the ionization is achieved simply by applying high (sonic) speed nebulizer gas, without heat, corona discharge, or high voltage. The microchip SSI was applied to the analysis of tetra-N-butylammonium, verapamil, testosterone, angiotensin I, and ibuprofen. The limits of detection were in the range of 15 nM to 4 microM. The technique was found to be highly dependent on the position of the chip toward the mass spectrometer inlet, and on the gas and the sample solution flow rates. The microchip SSI provided dynamic linearity following a pattern similar to that used with electrospray, good quantitative repeatability (RSD=16%), and long-term signal stability.     

Characterizing electrospray ionization using atmospheric pressure ion mobility spectrometry

Reduced flow rate electrospray ionization has been proven to provide improved sensitivity, less background noise, and improved limits of detections for ESI-MS analysis. Miniaturizing the ESI source from conventional electrospray to microelectrospray and further down to nanoelectrospray has resulted in higher and higher sensitivity; however, when effects of flow rate were investigated for atmospheric pressure ESI-IMS using a nanospray emitter, a striking opposite result was observed. The general tendency we observed in ESI-IMS was that higher flow rate offered higher ion signal intensity throughout a variety of conditions investigated. Thus, further efforts were undertaken to rationalize these contradictory results. It is well accepted that decreased flow rate increases both ionization efficiency and transmission efficiency, thus improving ion signal in ESI-MS. However, our study revealed that decreased flow rate results in decreased ion signal because ion transfer is constant, no matter how flow rate changes in ESI-IMS. Since ion transfer is constant in atmospheric pressure ESI-IMS, ionization efficiency can be studied independently, which otherwise is not possible in ESI-MS in which both ionization efficiency and transmission efficiency vary as conditions alter. In this article, we present a systematic study of signal intensity and ionization efficiency at various experimental conditions using ESI-IMS and demonstrate the ionization efficiency as a function of flow rate, analyte concentration, and solvent composition.     

Rectilinear ion trap mass spectrometer with atmospheric pressure interface and electrospray ionization source

A rectilinear ion trap (RIT) mass analyzer was incorporated into a mass spectrometer fitted with an electrospray ionization source and an atmospheric pressure interface. The RIT mass spectrometer, which was assembled in two different configurations, was used for the study of biological compounds, for which performance data are given. A variety of techniques, including the use of a balanced rf, elevated background gas pressure, automatic gain control, and resonance ejection waveforms with dynamically adjusted amplitude, were applied to enhance performance. The capabilities of the instrument were characterized using proteins, peptides, and pharmaceutical drugs. Unit resolution and an accuracy of better than m/z 0.2 was achieved for mass-to-charge (m/z) ratios up to 2000 Th at a scan rate of approximately 3000 amu/(charge.s) while reduced scan rates gave greater resolution and peak widths of less than m/z 0.5 over the same range. The mass discrimination in trapping externally generated ions was characterized over the range m/z 190-2000 and an optimized low mass cutoff value of m/z 120-140 was found to give equal trapping efficiencies over the entire range. The radial detection efficiency was measured as a function of m/z ratio and found to rise from 35% at low m/z values to more than 90% for ions of m/z 1800. The way in which the ion trapping capacity depends on the dc trapping potential was investigated by measuring the mass shift due to space charge effects, and it was shown that low trapping potentials minimize space charge effects by increasing the useful volume of the device. The collision-induced dissociation (CID) capabilities of the RIT instrument were evaluated by measuring isolation efficiency as a function of mass resolution as well as measuring peptide CID efficiencies. Overall CID efficiencies of more than 60% were easily reached, while isolation of an ion with unit resolution at m/z 524 was achieved with high rejection (>95%) of the adjacent ions. The overall analytical capabilities of the ESI-RIT instrument were demonstrated with the analysis of a mixture of pharmaceutical compounds using multiple-stage mass spectrometry.     

Achieving 50% ionization efficiency in subambient pressure ionization with nanoelectrospray

Inefficient ionization and poor transmission of the charged species produced by an electrospray from the ambient pressure mass spectrometer source into the high vacuum region required for mass analysis significantly limits achievable sensitivity. Here, we present evidence that, when operated at flow rates of 50 nL/min, a new electrospray-based ion source operated at ～20 Torr can deliver ～50% of the analyte ions initially in the solution as charged desolvated species into the rough vacuum region of mass spectrometers. The ion source can be tuned to optimize the analyte signal for readily ionized species while reducing the background contribution.     

Atmospheric pressure MALDI/ion trap mass spectrometry

A new sample ionization technique, atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI), was coupled with a commercial ion trap mass spectrometer. This configuration enables the application-specific selection of external atmospheric ionization sources: the electrospray/APCI (commercially available) and AP MALDI (built in-house), which can be readily interchanged within minutes. The detection limit of the novel AP MALDI/ion trap is 10-50 fmol of analyte deposited on the target surface for a four-component mixture of peptides with 800-1700 molecular weight. The possibility of peptide structural analysis by MS/MS and MS3 experiments for AP MALDI-generated ions was demonstrated for the first time.     

Alternately pulsed nanoelectrospray ionization/atmospheric pressure chemical ionization for ion/ion reactions in an electrodynamic ion trap

The alternate operation of nanoelectrospray ionization and atmospheric pressure chemical ionization, using a common atmosphere/vacuum interface and ion path, has been implemented to facilitate ion/ion reaction experiments in a linear ion trap-based tandem mass spectrometer. The ion sources are operated in opposite polarity modes whereby one of the ion sources is used to form analyte ions while the other is used to form reagent ions of opposite polarity. This combination of ion sources is well-suited to implementation of experiments involving multiply charged ions in reaction with singly charged ions of opposite polarity. Three analytically useful ion/ion reaction types are illustrated: the partial deprotonation of a multiply protonated protein, the partial protonation of a multiply deprotonated oligonucleotide, and electron transfer to a multiply protonated peptide. The approach described herein is attractive in that it enables both single proton-transfer and single electron-transfer ion/ion reaction experiments to be implemented without requiring major modifications to the tandem mass spectrometer hardware. Furthermore, a wide range of reactant ions can be formed with these ionization methods and the pulsed nature of operation appears to lead to no significant compromise in the performance of either ion source.     

Simple coupling of gas chromatography to electrospray ionization mass spectrometry

A simple method for direct coupling of gas chromatography (GC) with electrospray ionization mass spectrometry (ESI/MS) has been developed. The outlet of the GC capillary column was placed between the ESI needle and the atmospheric pressure ionization (API) source of a mass spectrometer. The ionization occurs via dissolution of neutral compounds into the charged ESI droplet followed by ion evaporation or via a gas-phase proton transfer reaction between a protonated solvent molecule and an analyte. The mass spectra of organic volatile compounds showed abundant protonated molecules with little fragmentation, being very similar to those produced by normal liquid ESI. The quantitative performance of the system was evaluated by determining the limit of detection (LOD), linearity ( r (2)), and repeatability (RSD). The GC-ESI/MS method was shown to be stable, providing high sensitivity and good quantitative performance.     

Producing highly charged ions without solvent using laserspray ionization: a total solvent-free analysis approach at atmospheric pressure

First examples of highly charged ions in mass spectrometry (MS) produced from the solid state without using solvent during either sample preparation or mass measurement are reported. Matrix material, matrix/analyte homogenization time and frequency, atmospheric pressure (AP) to vacuum inlet temperature, and mass analyzer ion trap conditions are factors that influence the abundance of the highly charged ions created by laserspray ionization (LSI). LSI, like matrix-assisted laser desorption/ionization (MALDI), uses laser ablation of a matrix/analyte mixture from a surface to produce ions. Preparing the matrix/analyte sample without the use of solvent provides the ability to perform total solvent-free analysis (TSA) consisting of solvent-free ionization and solvent-free gas-phase separation using ion mobility spectrometry (IMS) MS. Peptides and small proteins such as non-β-amyloid components of Alzheimer's disease and bovine insulin are examples in which LSI and TSA were combined to produce multiply charged ions, similar to electrospray ionization, but without the use of solvent. Advantages using solvent-free LSI and IMS-MS include simplicity, rapid data acquisition, reduction of sample complexity, and the potential for an enhanced effective dynamic range. This is achieved by more inclusive ionization and improved separation of mixture components as a result of multiple charging.     

Secondary ionization of chemical warfare agent simulants: atmospheric pressure ion mobility time-of-flight mass spectrometry

For the first time, the use of a traditional ionization source for ion mobility spectrometry (radioactive nickel ((63)Ni) beta emission ionization) and three alternative ionization sources (electrospray ionization (ESI), secondary electrospray ionization (SESI), and electrical discharge (corona) ionization (CI)) were employed with an atmospheric pressure ion mobility orthogonal reflector time-of-flight mass spectrometer (IM(tof)MS) to detect chemical warfare agent (CWA) simulants from both aqueous- and gas-phase samples. For liquid-phase samples, ESI was used as the sample introduction and ionization method. For the secondary ionization (SESI, CI, and traditional (63)Ni ionization) of vapor-phase samples, two modes of sample volatilization (heated capillary and thermal desorption chamber) were investigated. Simulant reference materials, which closely mimic the characteristic chemical structures of CWA as defined and described by Schedule 1, 2, or 3 of the Chemical Warfare Convention treaty verification, were used in this study. A mixture of four G/V-type nerve simulants (dimethyl methylphosphonate, pinacolyl methylphosphonate, diethyl phosphoramidate, and 2-(butylamino)ethanethiol) and one S-type vesicant simulant (2-chloroethyl ethyl sulfide) were found in each case (sample ionization and introduction methods) to be clearly resolved using the IM(tof)MS method. In many cases, reduced mobility constants (K(o)) were determined for the first time. Ion mobility drift times, flight times, relative signal intensities, and fragmentation product signatures for each of the CWA simulants are reported for each of the methods investigated.