Article

Planar Differential Mobility Analyzer (DMA) with Resolving Power of 110

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  • Sociedad Europea de Análisis Diferencial de Movilidad S.L.
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Abstract

Planar Differential Mobility Analyzers (DMAs) have previously achieved resolving powers of 60-80 (in air or N2, at the mobility of the Tetraheptylammonium ion (THA+; ~0.97 cm2/V/s). For unclear reasons, this performance is considerably below the theoretical limit. In this work, a performance close to this ideal limit is attained in SEADM's P5 DMA via improved flow laminarization, under otherwise the same flow conditions as in prior work. The new laminarizer remains effective at unusually large gas velocities (reached with two blowers in series), yielding a resolving power of 110. The selectivity of the improved DMA combined with a mass spectrometer was assessed by analyzing a real sample of extra virgin olive oil.

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... This study revealed the great analytical capacity of the DMA, which resolved mobility peaks of isobar species with resolving powers of greater than 100. 19 The ionization source used in this work has been described in detail in a previous work. 20 It includes a desolvation stage which prevents vapors and droplets from entering the DMA, which causes mobility peak tailing. ...
... In recent years, SEADM has developed an electrospray ionization source, commercially known as D-LFSESI (desolvating low-flow secondary electrospray ionization). 20 This ionization chamber offers the possibility of ionizing samples, both liquid (ESI) 19 and gaseous (SESI), 23,24 which gives it great versatility in addressing the analysis of samples by electrospray. ...
... Its characteristics are described in previous works. 15,19,20 Except for the DMA blower and its cooler, the entire DMA circuit was heated to 140°C. The DMA voltage (V DMA ) was directly provided by the MS internal ion-spray power supply source (IS ramp mode), so the MS software (Analyst 1.6) allowed the simultaneously programming of the DMA and the MS scan parameters as well as the analysis of the DMA-MS spectra. ...
Article
Recently, the olive oil industry has been the subject of harsh criticism for false labeling and even adulterations of olive oils. This situation in which both the industry and the population are affected demands the urgent need to increase the controls to avoid fraudulent activities around this precious product. The aim of this work is to propose a new analytical platform by coupling of electrospray ionization (ESI), differential mobility analysis (DMA) and mass spectrometry (MS) for the analysis of olive oils based on the information obtained from chemical fingerprint (non-targeted analyses). Regarding the sample preparation, two approaches were proposed: (i) sample dilution and (ii) liquid-liquid extraction (LLE). To demonstrate the feasibility of the method, 30 olive oil samples of three different categories were analyzed, using 21 of them to elaborate the classification model and the remaining 9 to test it (blind samples). To develop the prediction model, principal component analysis (PCA) and orthogonal partial least squares-discriminant analysis (OPLS-DA) were used. The overall success rate of classification to differentiate extra virgin olive oil (EVOO), virgin olive oil (VOO) and lampante olive oil (LOO) was 89 % for the LLE samples and 67 % for the diluted samples. However, combining both methods, the ability to differentiate EVOO from lower quality oils (VOO and LOO), and the edible oils (EVOO and VOO) from non-edible oil (LOO) was 100 %. The results show that ESI-DMA-MS can become an effective tool for the olive oil sector.
... The electric field of a few kilovolts is applied orthogonally to that gas stream and usually spans about 1 cm. Generally speaking, DM analyzers come in two geometries, the annular, with a radial electric field between two coaxial rods, or with parallel-plate geometry, with a linear electric field gradient, see Fig. 2. 10. [112][113][114] The parallel-plate design usually provides a better resolution, which increases with the laminar-limited flow √ Re and breakdown-limited electric field ⃗ E DM because it can handle much larger sheath gas flows and the electric field gradient is constant between the two plates. Recently, a resolution of over 100 [114] was 20 CHAPTER 2. THEORY 2. 6. MOBILITY DETECTORS achieved by realizing Re as high as 9 · 10 5 while still retaining a laminar flow. ...
... Generally speaking, DM analyzers come in two geometries, the annular, with a radial electric field between two coaxial rods, or with parallel-plate geometry, with a linear electric field gradient, see Fig. 2. 10. [112][113][114] The parallel-plate design usually provides a better resolution, which increases with the laminar-limited flow √ Re and breakdown-limited electric field ⃗ E DM because it can handle much larger sheath gas flows and the electric field gradient is constant between the two plates. Recently, a resolution of over 100 [114] was 20 CHAPTER 2. THEORY 2. 6. MOBILITY DETECTORS achieved by realizing Re as high as 9 · 10 5 while still retaining a laminar flow. The annular design, on the other hand, has a larger dynamic range of up to three orders of magnitude, however at the cost of resolution. ...
Thesis
The doctoral thesis presented provides a comprehensive view of laser-based ablation techniques promoted to new fields of operation, including, but not limited to, size, composition, and concentration analyses. It covers various applications of laser ablation techniques over a wide range of sizes, from single molecules all the way to aerosol particles. The research for this thesis started with broadening and deepening the field of application and the fundamental understanding of liquid-phase IR-MALDI. Here, the hybridization of ion mobility spectrometry and microfluidics was realized by using IR-MALDI as the coupling technique for the first time. The setup was used for monitoring the photocatalytic performance of the E-Z isomerization of olefins. Using this hybrid, measurement times were so drastically reduced that such photocatalyst screenings became a matter of minutes rather than hours. With this on hand, triple measurements screenings could not only be performed within ten minutes, but also with a minimum amount of resources highlighting its potential as a green chemistry alternative to batch-sized reactions. Along the optimizing process of the IR-MALDI source for microfluidics came its application for another liquid sample supply method, the hanging drop. This demarcated one of the first applications of IR-MALDI for the charging of sub-micron particles directly from suspensions via their gas-phase transfer, followed by their characterization with differential mobility analysis. Given the high spectral quality of the data up to octuply charged particles became experimentally accessible, this laid the foundation for deriving a new charge distribution model for IR-MALDI in that size regime. Moving on to even larger analyte sizes, LIBS and LII were employed as ablation techniques for the solid phase, namely the aerosol particles themselves. Both techniques produce light-emitting events and were used to quantify and classify different aerosols. The unique configuration of stroboscopic imaging, photoacoustics, LII, and LIBS measurements opened new realms for analytical synergies and their potential application in industry. The concept of using low fluences, below 100 J/cm2, and high repetition rates of up to 500 Hz for LIBS makes for an excellent phase-selective LIBS setup. This concept was combined with a new approach to the photoacoustic normalization of LIBS. Also, it was possible to acquire statistically relevant amounts of data in a matter of seconds, showing its potential as a real-time optimization technique. On the same time axis, but at much lower fluences, LII was used with a similar methodology to quickly quantify and classify airborne particles of different compositions. For the first time, aerosol particles were evaluated on their LII susceptibility by using a fluence screening approach.
... The DMA used in this study was the DMA P5 manufactured by SEADM, Spain. 22 The DMA was operated in a counter flow mode such The Journal of Physical Chemistry A pubs.acs.org/JPCA Article that N 2 was fed to the closed sheath air flow circulation, and an excess of 0.5 L min −1 was flowing out of the DMA inlet. ...
Article
Full-text available
Highly oxygenated organic molecules (HOMs) are important sources of atmospheric aerosols. Resolving the molecular-level formation mechanisms of these HOMs from freshly emitted hydrocarbons improves the understanding of aerosol properties and their influence on the climate. In this study, we measure the electrical mobility and mass-to-charge ratio of α-pinene oxidation products using a secondary electrospray-differential mobility analyzer-mass spectrometer (SESI-DMA-MS). The mass-mobility spectrum of the oxidation products is measured with seven different reagent ions generated by the electrospray. We analyzed the mobility-mass spectra of the oxidation products C9-10H14-18O2-6. Our results show that acetate and chloride yield the highest charging efficiencies. Analysis of the mobility spectra suggests that the clusters have 1-5 isomeric structures (i.e., ion-molecule cluster structures with distinct mobilities), and the number is affected by the reagent ion. Most of the isomers are likely cluster isomers originating from binding of the reagent ion to different sites of the molecule. By comparing the number of observed isomers and measured mobilities and collision cross sections between standard pinanediol and pinonic acid to the values observed for C10H18O2 and C10H16O3 produced from oxidation of α-pinene, we confirm that pinanediol and pinonic acid are the only isomers for these elemental compositions in our experimental conditions. Our study shows that the SESI-DMA-MS produces new information from the first steps of oxidation of α-pinene.
... Due to their varying size and shape, different clusters tend to have different electrical mobilities. We use a planar-Differential Mobility Analyser (planar-DMA) (Amo-González and Pérez, 2018) to utilize this fact and select only one (known) cluster type at a time to enter the APi-TOF. We use an instrumental set-up ( Our main goals are to use this set-up to identify the clusters that are fragmented inside the APi-TOF MS, and to quantify the fragmentation. ...
Preprint
Full-text available
Sulfuric acid and dimethylamine vapours in the atmosphere can form molecular clusters, which participate in new particle formation events. In this work, we have produced, measured and identified clusters of sulfuric acid and dimethylamine using an electrospray ionizer coupled with a planar differential mobility analyser, connected to an atmospheric pressure interface time-of-flight mass spectrometer (ESI–DMA–APi-TOF MS). This set-up is suitable for evaluating the extent of fragmentation of the charged clusters inside the instrument. We evaluated the fragmentation of 11 negatively charged clusters both experimentally and using a statistical model based on quantum chemical data. The results allowed us to quantify the fragmentation of the studied clusters and to reconstruct the mass spectrum removing the artifacts due to the fragmentation.
... This DMA includes improvements over a similar instrument previously described [1]. An important practical innovation is that, when coupled to a mass spectrometer (MS), the flow-limiting orifice is no longer at the outlet slit of the DMA, hence the DMA can be quickly removed or installed without breaking the MS vacuum [2]. ...
Poster
The DMA combines an electric field and a flow field to select a narrow range of electrical mobilities. An optimized version of the P5-DMA connected to an electrometer allowed resolving powers in excess of 100 for an ion mobility of 0.97 cm2/V/s. DMAs, analogously to the quadrupole MS, separate ions in space rather than time, and may be similarly ordered in tandem to filter ions at atmospheric pressure. We have emulated the working principle of the triple quadrupole by placing an atmospheric pressure fragmentation stage (an oven) between two DMAs, resulting in a very high selectivity.
... It is noteworthy to mention that some of the points addressed in the previous paragraph are generalizations. There exist high resolution DMA systems which can reach resolutions in excess of 100 (Amo-Gonzalez & Perez, 2018;Rus et al., 2010) and there are plenty of studies on improving the transmission and reducing diffusion losses for small ions in DMAs (Downard, Dama, & Flagan, 2011;Franchin et al., 2016). Another issue that requires clari cation is that pertaining to the characterization speed of the instruments. ...
Article
Full-text available
Drift tubes (DT) are prominent tools to classify small ions in the gas phase. This is in contrast with its limited use in the aerosol field at atmospheric pressures where the differential mobility analyzer (DMA) has been the tool of choice. While the DMA has been successful, it does not normally achieve the resolution of a common DT. On the other hand, the size range of the DT is limited as well as its sensitivity. Here we propose a variation of the DT where a varying linearly decreasing field is used instead of the constant field commonly used in DT. The Varying Field Drift Tube (VFDT) has the advantage that it allows for diffusion constriction in the axial direction and thus a larger package of ions can be allowed into the system increasing its sensitivity without hampering its resolution. The VFDT also generally outperforms the DT in resolution and this is demonstrated theoretically and empirically reaching resolutions of over 90 in our data although higher resolutions are expected. The diffusion constriction capabilities are also proven theoretically and experimentally by using a mixture of tetraalkylammonium salts while injecting broad packets of ions into the system. The transformation from the raw variable arrival time distribution to Collision Cross Section or mobility diameter is linear making the transformation as simple as with a DMA.
... In our own experience with DMAs running at high Reynolds numbers, the purely diffusive limit has been approached only rarely. A recent study by Amo-Gonzalez and Perez (2018) has demonstrated the critical importance of a careful arrangement of screens in approaching ideal behavior at resolving powers as high as 110. The diffusive nonidealities observed in other studies, including the present one, may therefore be partly due to survival of free stream turbulence through the laminarization system. ...
Article
A differential mobility analyzer for high-mobility resolution (1/FWHM∼30) classification of 1–67 nm particles is designed to analyze viral particles. Inner and outer electrode radii of 1.01 and 2 cm (at the outlet slit) and a 11.6 cm long column achieve this range at a sheath gas flow (Q) and aerosol flow (q) of 30 and 1 L/min. Turbulent transition potentially resulting from this substantial length combined with high sheath gas flow rates (Q∼1000 Lit/min) required to classify 1 nm particles is avoided by stabilizing the flow via a continuous acceleration with a conical inner electrode (1° half-angle). High axisymmetry of the aerosol flow as it joins the sheath gas is achieved by injecting it through a circularizer ring with 24 symmetrically spaced orifices. The sheath flow is laminarized with two pre-laminarization schemes, three laminarization screens, and an inlet trumpet with an area ∼3 times the analyzer channel throat area. The instrument is tested with singly charged monomobile cluster ions produced by a bipolar electrospray source. A resolving power of 29 is measured at the highest flow rate reached, with a trend towards even higher resolution if either Q or the monomobile particle size could be increased. This performance indicates that the electrode concentricity is excellent and the flow highly stable. Tests carried out at limited resolution (set below 16 by a protein test aerosol) with the modest Q/q∼30 values required to classify 70 nm particles indicate that the DMA response is close to ideal at Q = 151, 110, and 47 Lit/min. Copyright © 2019 American Association for Aerosol Research
... Some of the advantages of the DMA are that it is able to work at atmospheric pressure, that its integration with any existing MS can be done very easily, that it allows only a single mobility into the MS, and that its mobility is easy to relate to the raw variables employed. While the base resolution of the DMA is set at 50-60, one can obtain resolutions larger than 110 [67]. In fact, some of the resolutions observed in this manuscript for large charge states is above 175. ...
Article
The strong synergy arising from coupling two orthogonal analytical techniques such as ion mobility and mass spectrometry can be used to separate complex mixtures and determine structural information of analytes in the gas phase. A tandem study is performed using two systems with different gases and pressures to ascertain gas-phase conformations of homopolymer ions. Aside from spherical and stretched configurations, intermediate configurations formed by a multiply charged globule and a “bead-on-a-string” appendix are confirmed for polyethylene-glycol (PEG), polycaprolactone (PCL), and polydimethylsiloxane (PDMS). These intermediate configurations are shown to be ubiquitous for all charge states and masses present. For each charge state, configurations evolve in two distinctive patterns: an inverse evolution which occurs as an elementary charge attached to the polymer leaves the larger globule and incorporates itself into the appendage, and a forward evolution which reduces the globule without relinquishing a charge while leaving the appendix relatively constant. Forward evolutions are confirmed to form self-similar family shapes that transcend charge states for all polymers. Identical structural changes occur at the same mass over charge regardless of the system, gas or pressure strongly suggesting that conformations are only contingent on number of charges and chain length, and start arranging once the ion is at least partially ejected from the droplet, supporting a charge extrusion mechanism. Configurational changes are smoother for PDMS which is attributed to the larger steric hindrance caused by protruding pendant groups. This study has implications in the study of the configurational space of more complex homopolymers and heteropolymers. Graphical Abstract
Article
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We present an electrical mobility classifier for mass–mobility measurements of atmospheric ions. Size segregation coupled with mass spectrometric detection of naturally occurring ions in the atmosphere is challenging due to the low ion concentration. Conventional electrical mobility classifying devices were not yet coupled with mass spectrometry to resolve natural ion composition. This is due to either the insufficient transmission efficiency or design concepts being incompatible with this application, e.g. using high electric fields close to the inlets to push ions from high to low electric potential. Here, we introduce an axial ion mobility classifier, termed AMC, with the aim to achieve higher transmission efficiencies to segregate natural ions at reasonable sizing resolution. Similar to the recently introduced principle of the high-pass electrical mobility filter (HP-EMF) presented by Bezantakos et al. (2015) and Surawski et al. (2017), ions are classified via an electric field that is opposed to the gas flow direction carrying the ions. Compared to the HP-EMF concept, we make use of sheath flows to improve the size resolution in the sub-3 nm range. With our new design we achieve a sizing resolution of 7 Z/ΔZ with a transmission efficiency of about 70 %.
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Sulfuric acid and dimethylamine vapours in the atmosphere can form molecular clusters, which participate in new particle formation events. In this work, we have produced, measured, and identified clusters of sulfuric acid and dimethylamine using an electrospray ionizer coupled with a planar-differential mobility analyser, connected to an atmospheric pressure interface time-of-flight mass spectrometer (ESI–DMA–APi-TOF MS). This set-up is suitable for evaluating the extent of fragmentation of the charged clusters inside the instrument. We evaluated the fragmentation of 11 negatively charged clusters both experimentally and using a statistical model based on quantum chemical data. The results allowed us to quantify the fragmentation of the studied clusters and to reconstruct the mass spectrum by removing the artifacts due to the fragmentation.
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While existing ion mobility calculators are capable of feats as impressive as calculating collision cross sections (CCS) within a few per cent and within a very reasonable time, the simplifications assumed in their estimations precludes them from being more precise, potentially overreaching with respect to the interpretation of existing calculations. With ion mobility instrumentation progressively reaching resolutions of several hundreds to thousands (accuracy in the range of ∼0.1%), a more accurate theoretical description of gas-phase ion mobility becomes necessary to correctly interpret experimental state-of-the-art separations. This manuscript entails an effort to consolidate the most relevant theoretical work pertaining to ion mobility within the ‘free molecular’ regime, describing in detail the rationale for approximations up to the two-temperature theory, using both a momentum transfer approach as well as the solution to the moments of the Boltzmann equation for the ion. With knowledge of the existing deficiencies in the numerical methods, the manuscript provides a series of necessary additions in order to better simulate some of the separations observed experimentally due to second-order effects, namely, high field effects, dipole alignment, angular velocities and moments of inertia, potential interactions and inelastic collisions among others.
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A recently described DMA designed for high resolution viral particle analysis (Perez-DMA; Perez-Lorenzo et al, 2020) is modified to decrease the relative peak full width at half maximum (FWHM) below previously achieved ≈3.3%. The electrode radii at the outlet slit (R 1 = 1.01 cm; R 2 = 2 cm) and the working length are almost unchanged (L = 114.9 vs. 116 mm). The laminarization trumpet and the radius of the curve merging the trumpet to the working section are both considerably widened to improve gas flow laminarization. DMA evaluation with salt clusters is improved by reducing the flow resistance at the gas outlet, to reach substantially larger sheath gas flow rates Q ˜ 1700 L/min. Tests with tetraheptylammonium bromide clusters with a center rod diverging at 3o demonstrate FWHM<2.7%, without indications of performance loss due to turbulence even at 1700 L/min. Correcting these high flow rate data for diffusive broadening reveals a maximal DMA FWHM in the limit of non-diffusing particles and zero sample flow, FWHM∞ = 1.8%. An uncorrected peak width approaching 2% is independently demonstrated at much lower flow rates of sheath gas with two recently described bee virus particle standards having singularly narrow size distributions at mean diameters of 38 and 17 nm. Correcting raw 38 nm particle peak widths for broadening due to diffusion and aerosol to sheath gas flow rate ratio q/Q shows an even more ideal response with FWHM∞<1%, where this value includes nonidealities in the DMA as well as possible lack of monodispersity in the viral particles.
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Interest in understanding gas-to-particle phase transformation in several disciplines such as atmospheric sciences, material synthesis, and combustion has led to the development of several distinct instruments that can measure the particle size distributions down to the sizes of large molecules and molecular clusters, at which the initial particle formation and growth takes place. These instruments, which include the condensation particle counter battery, a variety of electrical mobility spectrometers and the particle size magnifier, have been usually characterized in laboratory experiments using carefully prepared calibration aerosols. They are then applied, alone or in combination, to study the gas-to-particle transition in experiments that produce particles with a wide range of compositions and other properties. Only a few instrument intercomparisons in either laboratory or field conditions have been reported, raising the question: how accurately can the sub-10 nm particle number size distributions be measured with the currently available instrumentation? Here, we review previous studies in which sub-10 nm particle size distributions have been measured with at least two independent instruments. We present recent data from three sites that deploy the current state-of-the-art instrumentation: Hyytiälä, Beijing, and the CLOUD chamber. After discussing the status of the sub-10 nm size distribution measurements, we present a comprehensive uncertainty analysis for these methods that suggests that our present understanding on the sources of uncertainties quite well captures the observed deviations between different instruments in the size distribution measurements. Finally, based on present understanding of the characteristics of a number of systems in which gas-to-particle conversion takes place, and of the instrumental limitations, we suggest guidelines for selecting suitable instruments for various applications.
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A parallel plate differential mobility analyzer (DMA) having 100 independent current collectors is calibrated to relate the axial distances Ln between the inlet slit and the detector position to the particle mobility Z at given voltage difference V and sheath gas flow rate Q. Calibrating species are tetraheptylammonium bromide clusters (THABr) and polyethylene glycol (PEG35k, 5 nm in diameter), generated by a bipolar electrospray source, and purified in a cylindrical DMA. Gaussian fitting of the raw discrete mobility spectra in the form of ion current In versus collector position Ln, In(Ln), yield the mean value Lo of the collector position maximizing the signal for a given ion. The many (Z,V,Lo) triads obtained at given Q from many different DMA voltages and standard mobilities collapse into a single 1/(ZiVj) vs Lo curve when slight adjustments are made to the Zi. For different flow rates, Q/(ZiVj) vs. Lo curves collapse also, as long as the peaks are moderately narrow. However, for sufficiently small Q/Z, the THABr cluster peaks become broad, and the curves Q/(ZiVj) vs. Lo cease to collapse precisely. In contrast, the data for PEG show that this behavior is not a low-Q (Reynolds number) effect from the growth of the two lateral boundary layers, but is rather due to the broad and non-Gaussian peak shapes obtained at low Q or high Z. The calibration is accordingly unaffected by the Reynolds number. This simplicity was unexpected, given the three-dimensional flow in this DMA with growing lateral boundary layers. Copyright © 2020 American Association for Aerosol Research
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A tandem ion mobility spectrometer (IMS^2) built from two differential mobility analysers (DMAs) is coupled at ambient pressure with a thermal fragmenter placed in between, such that the precursor ions selected in the first DMA are thermally decomposed at ambient pressure in the fragmenter and the product ions generated are filtered in the second DMA. A thermal desorber and a multicapillary gas chromatography (GC) column are coupled to a secondary electrospray (SESI) ion source, so the adsorption sampling filters are thermally desorbed and the liberated vapours are separated in the GC column, prior to their ionization and mobility/mobility classification. The new fragmenter allows the fragmentation of the five explosives studied: RDX, PETN, NG, EGDN and TNT. The background of the analyser is evaluated for the five explosives using air samples of 500 L volume. An atmospheric background of only 2.5 pg (5 ppq) is found for TNT, being somewhat higher for the rest of explosives studied. The architecture GC-IMS^2 is compared with GC-IMS obtaining a 100-fold increase of sensitivity in the first configuration, confirming the high selectivity provided by the fragmentation cell and the second IMS stage for the product ion mobility analysis. The analyser is tested also with real explosives hidden in cargo pallets achieving successful detection of four (EGDN, NG, TNT and PETN) out of five explosives.
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This review discusses the developments in aerosol instrumentation that have led to the current vapor condensation based instruments capable of detecting sub-3 nm particles. We begin from selected reports prior to the year 1991, which have advanced the technology or understanding in condensation particle counting toward sub-3 nm sizes, and continue to more in depth review of the past efforts after 1991. We discuss how the developments in the calibration methods have progressed the development of particle counting techniques, and review briefly the sub-3 nm calibration experiments and cluster production methods used in calibration experiments. Based on these reviews, we identify several technological and scientific advances for the future to improve the accuracy, understanding, and technology of sub-3 nm particle counting. Copyright © 2019 American Association for Aerosol Research
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The objective of this work is to investigate the possible effects of plate incline on the performance of a small plate Differential Mobility Analyzer (DMA) in which two separate metal plates are installed in parallel (with constant spacing). Plate incline may occur due to improper plate DMA assembly, which raises the concern of adverse DMA performance, particularly for small DMAs with narrow plate-to-plate spacing. COMSOL Multiphysics 5.4® was used to numerically study the following parameters: the relative incline, the incline type (sided and pointed), the aspect ratio and classification length of the particle classification channel, the particle slit opening length (relative to the channel width), the sheath-to-aerosol flow rate ratio, and the total flow rate. Our study shows that the plate incline results in the deterioration of the DMA transfer function (i.e., peak mobility shifting, peak height reduction, and width-at-half-peak-height broadening), primarily attributed to the distortion of the electrical field. The effect of plate incline on the DMA transfer function manifested as either a decrease in the total DMA flow rate or an increase in the classification channel length in a plate DMA. It was concluded that the observed effects were due to increased particle residence time and the distorted electrical field acting on particles in the DMA classification channel. Additionally, the observed adverse effects were found to be sensitive to the particle-slit opening length.
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Rationale The increased frequency in the number of international terror threats has led to a corresponding increase in demand for fast, sensitive and reliable screening methods suitable for the detection of airborne explosive vapors. We demonstrate herein a workflow suitable for the determination of nitrogen‐based explosives at the picogram level in just minutes. Methods A method is described that combines Thermal Desorption (TD) sample introduction with Differential Mobility Analysis (DMA) – Tandem Mass Spectrometry (MS/MS), enabling a sensitive and accurate workflow suitable for the rapid detection of trace nitroaromatic, nitroester and nitramine explosive vapors. The methods are bridged using a novel low‐flow, field‐free Atmospheric Pressure Photoionization (APPI) source, intended specifically for the analysis of gas‐phase analytes and airborne particles. Results Limits of detection within or below the picogram range were determined for the analysis of a range of explosives standards including 2,6‐DNT, TNT, TATB, Tetryl, RDX, EGDN, PETN, HMX, and NG. Practical application of the TD‐APPI‐DMA‐MS/MS workflow was demonstrated for the detection of real trace explosive vapors produced from the volatilization of solid explosive samples stored within a sealed cardboard box. A single complete analysis was performed in less than two minutes. Conclusions The highly sensitive and accurate detection of a variety of common nitrogen‐based explosive vapors has been demonstrated, at levels suitable for practical, high‐throughput security screening applications.
Article
Two differential mobility analyzers (DMAs) acting as narrow band mobility filters are coupled in series, with a thermal fragmentation cell placed in between, such that parent ions selected in DMA1 are fragmented in the cell at atmospheric pressure, and their product ions are analyzed on DMA2. Additional mass spectrometer analysis is performed for ion identification purposes. A key feature of the tandem DMA is the short residence time (~0.2 ms) of ions in the analyzer, compared to tens of ms in drift tube IMS. Ion fragmentation within the analyzer and associated mobility tails are therefore negligible for a DMA, but not necessarily so in conventional IMS. This advantage of the DMA is demonstrated here by sharply defined product ion mobility peaks. Ambient pressure ion fragmentation has been previously demonstrated by both purely thermal means as well as rapidly oscillating intense electric fields. Our purely thermal fragmentation cell here achieves temperatures up to 700 oC measured inside the heating coil of a cylindrical ceramic heater, through whose somewhat colder axis we direct a beam of mobility-selected ions. We investigate tandem separation of chloride adducts from the explosives EGDN, nitroglycerine (NG), PETN and RDX, and from deprotonated TNT. Atmospheric pressure fragmentation of the first three ions yields one or several previously reported fragments, providing highly distinctive tandem DMA channels for explosive identification at one atmosphere. RDX ions had not been previously fragmented at ambient pressure, yet RDX-Cl- converts up to 7% (at 300 oC) into a 166 m/z product. The known high thermal resilience of TNT is confirmed here by its rather modest conversion, even when the ceramic is heated to 700 oC. At this temperature some previously reported fragments are found, but their mobilities are fairly close to each other and to the one of the far more abundant parent ion, making their identification by mobility alone problematic. We anticipate that moderately higher fragmenter temperatures will produce smaller fragments with mobilities readily separated from that of TNT-H-.
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Two fast electrometer circuits (10¹¹ and 10¹² V/A) are installed in a Faraday cage having a relatively small residence time. Removing readily distinguishable occasional spikes, the root mean square (r.m.s.) noise level at 10¹² V/A is 0.11 fA when acquiring data at 1 Hz. This value is close to the expected thermal resistor noise at room temperature (0.09 mV). Both electrometers exhibit a 20 ms flow-related delay, followed by respective half-height rise-times of ∼4 and 25 ms. Fast high-resolution mobility spectra in the 1–2 nm size range are acquired with electrosprayed tetraheptylammonium ions by combining these electrometers with a high-speed DMA. At 10¹² V/A, there is no ion mobility peak distortion when acquiring data with discrete voltage steps and dwelling 100 ms at each voltage. With the 10¹¹ V/A electrometer, the DMA voltage VDMA is continuously swept up and down over 600 V in a triangular wave, at up to 1200 V/s. A shift ΔVDMA in the peak center is apparent, with little peak shape distortion. ΔVDMA is symmetric with respect to up or down sweep, and linear with sweep frequency, corresponding approximately to a pure delay Δt = 25 ms. This peak displacement may be offset by adding the correction ΔVDMA = Δt (dVDMA/dt) to the measured peak voltage. Extrapolating the measurements made here over a mobility range Zmax/Zmin of 4 to a much wider mobility range of 300 typical of aerosol studies, we conclude that almost undistorted high-resolution mobility spectra may be acquired in 1.3 s. Copyright © 2017 American Association for Aerosol Research
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In this paper, high-resolution nano-electrospray ionization-trapped ion mobility spectrometry coupled to mass spectrometry (nESI-TIMS-MS) is used for the study of hydroxylated polybrominated diphenyl ether (OH-PBDE) metabolites. In particular, experimental ion-neutral collision cross sections (CCS) were measured for five structural OH-PBDE isomers using TIMS-MS. Candidate structures were proposed for each IMS band observed in good agreement with the experimental CCS measurements (5 % error). The analytical power of TIMS-MS to baseline and partially separate structural isomers of OH-BDE in binary and ternary mixtures is shown for single charge species with a mobility resolving power of RIMS ~ 400. This work provides the proof of concept for the analysis of low concentration OH-PBDE in environmental samples based on accurate collision cross section and mass measurements without the need for derivatization and pre-fractionation protocols, thus significantly reducing the cost and analysis time.
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As the resolution of analytical methods improves, further progress tends to be increasingly limited by instrumental parameter instabilities that were previously inconsequential. This is now the case with differential ion mobility spectrometry (FAIMS), where fluctuations of the voltages and gas pressure have become critical. A new high-definition generator for FAIMS compensation voltage reported here provides a stable and accurate output than can be scanned with negligible steps. This reduces the spectral drift and peak width, thus improving the resolving power (R) and resolution. The gain for multiply-charged peptides that have narrowest peaks is up to ~40 %, and R ~400-500 is achievable using He/N(2) or H(2)/N(2) gas mixtures.
Article
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In differential mobility spectrometry (also referred to as high-field asymmetric waveform ion mobility spectrometry), ions are separated on the basis of the difference in their mobility under high and low electric fields. The addition of polar modifiers to the gas transporting the ions through a differential mobility spectrometer enhances the formation of clusters in a field-dependent way and thus amplifies the high- and low-field mobility difference, resulting in increased peak capacity and separation power. Observations of the increase in mobility field dependence are consistent with a cluster formation model, also referred to as the dynamic cluster-decluster model. The uniqueness of chemical interactions that occur between an ion and cluster-forming neutrals increases the selectivity of the separation, and the depression of low-field mobility relative to high-field mobility increases the compensation voltage and peak capacity. The effect of a polar modifier on the peak capacity across a broad range of chemicals has been investigated. We discuss the theoretical underpinnings which explain the observed effects. In contrast to the result with a polar modifier, we find that using mixtures of inert gases as the transport gas improves the resolution by reducing the peak width but has very little effect on the peak capacity or selectivity. The inert gas helium does not cluster and thus does not reduce low-field mobility relative to high-field mobility. The observed changes in the differential mobility alpha parameter exhibited by different classes of compounds when the transport gas contains a polar modifier or has a significant fraction of inert gas can be explained on the basis of the physical mechanisms involved in the separation processes.
Article
Full-text available
A new method that allows a linear drift tube to be operated as a continuous ion mobility filter is described. Unlike conventional ion mobility instruments that use an electrostatic gate to introduce a packet of ions into a drift region, the present approach uses multiple segmented drift regions with modulated drift fields to produce conditions that allow only ions with appropriate mobilities to pass through the instrument. In this way, the instrument acts as a mobility filter for continuous ion sources. By changing the frequency of the applied drift fields it is possible to tune this instrument to transmit ions having different mobilities. A scan over a wide range of drift field frequencies for a single ion species shows a peak corresponding to the expected resonance time of the ions in one drift region segment and a series of peaks at higher frequencies that are overtones of the resonant frequency. The measured resolving power increases for higher overtones, making it possible to resolve structures that were unresolved in the region of the fundamental frequency. We demonstrate the approach by examining oligosaccharide isomers, raffinose and melezitose as well as a mixture of peptides obtained from enzymatic digestion of myoglobin.
Book
Over the last decade, scientific and engineering interests have been shifting from conventional ion mobility spectrometry (IMS) to field asymmetric waveform ion mobility spectrometry (FAIMS). Differential Ion Mobility Spectrometry: Nonlinear Ion Transport and Fundamentals of FAIMS explores this new analytical technology that separates and characterizes ions by the difference between their mobility in gases at high and low electric fields. It also covers the novel topics of higher-order differential IMS and IMS with alignment of dipole direction. The book relates the fundamentals of FAIMS and other nonlinear IMS methods to the physics of gas-phase ion transport. It begins with the basics of ion diffusion and mobility in gases, covering the main attributes of conventional IMS that are relevant to all IMS approaches. Building on this foundation, the author reviews diverse high-field transport phenomena that underlie differential IMS. He discusses the conceptual implementation and first-principles optimization of FAIMS as a filtering technique, emphasizing the dependence of FAIMS performance metrics on instrumental parameters and properties of ion species. He also explores ion reactions in FAIMS caused by field heating and the effects of inhomogeneous electric field in curved FAIMS gaps. Written by an accomplished scientist in the field, this state-of-the-art book supplies the foundation to understand the new technology of nonlinear IMS methods.
Article
The feasibility of detecting explosives in the atmosphere at concentrations as low as 0.01 ppq hinges on the poorly known question of what interfering species exist at these or higher concentrations. To clarify the issue, hundreds of samples of ambient air, either clean or loaded with explosives (from lightly contaminated environments) have been collected in fiberglass/stainless steel filters coated with Tenax-GR, thermally desorbed at variable temperature, and ionized with Cl- via secondary electrospray (SESI). They are analyzed with a narrow-band mobility filter (SEADM’s P5 DMA) and a triple quadrupole mass spectrometer (Sciex´s 5500), configured in series to transmit precursor and fragment ions of the explosives Nitroglycerin, PETN, RDX, and TNT. Blanks were sampled outdoors at a rural site (Boecillo, Valladolid, Spain), and loads were sampled at diverse locations. For RDX and TNT, atmospheric background inhibits detection below 1 part/trillion (ppt) without mobility filtering. This interference was drastically reduced by the DMA, allowing detection up to 1 part/quadrillion (ppq). Further sensitivity increase was achieved by scanning over a mobility region several percent around that of the target explosive, to separate various isobaric compounds by Gaussian deconvolution. (i) All four MS/MS channels analyzed exhibit several background peaks within the narrow mobility intervals investigated. At least one of these interferents is much stronger than the instrument background at the explosive’s mobility, making DMA separation most helpful. (ii) For Nitroglycerin and PETN the combined filtering techniques have not lowered ambient chemical noise down to 0.01 ppq. (iii) Interferents are greatly reduced for TNT and RDX, resulting in minimal chemical noise: 322 blank tests for RDX yielded mean signal of 0.0012 ppq and standard deviation sigma=0.0035 ppq (mean+3sigma detection limit of 0.01 ppq).
Article
The differential mobility analyzer (DMA) is a narrow-band linear ion mobility filter operating at atmospheric pressure. It combines in series with a quadrupole mass spectrometer (Q-MS) for mobility/mass analysis, greatly reducing chemical noise in selected ion monitoring. However, the large flow rate of drift gas (~1000 L/min) required by DMAs complicates the achievement of high gas purity. Additionally, the symmetry of the drying counterflow gas at the interface of many commercial MS instruments, is degraded by the lateral motion of the drift gas at the DMA entrance slit. As a result, DMA mobility peaks often exhibit tails due to the attachment of impurity vapors, either (1) to the reagent ion within the separation cell, or (2) to the analyte of interest in the ionization region. In order to greatly increase the noise-suppression capacity of the DMA, we describe various vapor-removal schemes and measure the resulting increase in the tailing ratio, (TR = signal at the peak maximum over signal two half-widths away from this maximum). Here we develop a low-outgassing DMA circuit connected to a mass spectrometer, and test it with three ionization sources (APCI, Desolvating-nano ESI, and Desolvating low flow SESI). While prior TR values were in the range 100-1000, the three new sources achieve TR ~ 10(5). The SESI source has been optimized for maximum sensitivity, delivering an unprecedented gain for TNT of 190 counts/fg, equivalent to an ionization efficiency of one out of 140 neutral molecules. Graphical Abstract ᅟ.
Article
A prototype is introduced based on the Transversal Modulation Ion Mobility Spectrometry (TMIMS) technique, which provides a continuous output of mobility selected ions, which greatly eases the synchronization between the different analyzing stages. In the new architecture, two stages of filtration are used to drastically reduce the background produced by one stage alone. The two-stages TMIMS was coupled with two different Atmospheric Pressure Interface Mass Spectrometer MS. The new system enables IMS-IMS-MS analysis and other modes of operation: IMS pre-filtration, IMS-IMS, and full transmission mode. It provides a resolving power R>60 in IMS mode, and R>40 in each stage in IMS-IMS mode. 2-propanol vapors were introduced in one of the stages to enhance the mobility variations, and its effect was studied on a set of tetra-alkyl ammonium ions. We found that concentrations as low as 1% (in partial pressure) produce mobility variations as high as 20%, which suggest that IMS-IMS separation using dried N2 (in one stage) and a dopant (in the other stage), could be a very powerful way to enhance the separation capacity of the IMS-IMS pre-filtration approach.
Article
Ion mobility spectrometry (IMS) as a stand-alone technique has become increasingly important for applications in security, defense, and environmental monitoring, and also in biological applications such as molecular structure and -omic analysis when combined with mass spectrometry. Yet, the majority of these devices are drift cell based and limited by low duty cycles because of ion gating. Differential Mobility Analyzers (DMAs) are attractive alternatives due to their continuous ion transmission and success in analyzing aerosol particles in real time environmental tests. But, the resolution of a DMA is low due to difficulties in achieving laminar gas flow, low sample gas flow to sheath gas flow ratio, and high velocity sheath gas using small pumps, if portability is a concern. To overcome these challenges, we will introduce a new ion mobility spectrometer that increases the amount of work done on the ions during separation by introducing an electric field opposing the gas flow direction while simultaneously preserving laminar gas flow. The development of the Periodic Focusing Differential Mobility Analyzer (PFDMA) can lead to a portable device that exhibits both high resolution and sensitivity, to meet the needs of today's expanding applications.
Article
An ion mobility spectrometer (IMS) that routinely achieves separation efficiencies in line with the theoretical diffusion limited maximum has been constructed. The theoretical maximum efficiency is a function of the total voltage drop, the temperature, the number of charges on the ion, the initial pulse width, the length of the spectrometer, and the mobility (K) of the ion of interest. Despite the fact that our current instrumental setup uses modest voltages and fairly high temperatures, we were able to achieve over 130,000 theoretical plates for a singly charged ion in less than 25 ms. In addition, selectivity can be altered in IMS by changing the drift gas or by increasing the electric field (>1000 V/cm) such that the mobility of ions is no longer a linear function of the electric field. Separation factors (α) can be altered by as much as 20% by changing the drift gas in the spectrometer. Finally leucine and isoleucine were separated in less than 23 ms with a resolution of 0.668 demonstrating the separation power of the instrument. © 2000 John Wiley & Sons, Inc. J Micro Sep 12: 172–178, 2000
Article
The electrical mobilities of multiply-charged nanodrops of the ionic liquid 1-ethyl, 3-methylimidazolium dicyanamide (EMI-N[CN]2) were accurately measured in air at 20 °C for mass-selected clusters of composition [EMI-N[CN]2] n [EMI(+)] z , with 2 ≤ n ≤ 369 and 1 ≤ z ≤ 10. We confirm prior reports that the mobility Z of a globular ion of mass m is given approximately by the modified Stokes-Millikan law for spheres, Z = Z SM,mod (d m + d g , z, m), where d m = (6m/πρ)(1/3) is the nanodrop mass-diameter based on the density ρ of the liquid (corrected for the capillary compression and electrostatic deformation of the nanodrop), and d g is an effective air molecule diameter. There is however a measurable (up to 7 %) and systematic z-dependent departure of Z from Z SM,mod . As theoretically expected at small ε (*) , this effect is accurately described by a simple correction factor of the form Z/Z SM,mod = δ(1 - βε (*)), where kTε (*) is the potential energy due to the ion-induced dipole (polarization) attraction between a perfectly-conducting charged nanodrop and a polarized neutral gas-molecule at a distance (d m + d g )/2 from its center. An excellent fit of this model to hundreds of data points is found for d g ≈ 0.26 nm, β ≈ 0.36, and δ ≈ 0.954. Accounting for the effect of polarization decreases d g considerably with respect to values inferred from earlier nanodrop measurements that ignored this effect. In addition, and in spite of ambiguities in the mobility calibration scale, the measured constant δ smaller than unity increases Millikan's drag enhancement factor from the accepted value ξ m ≈ 1.36 to the new value ξ ≈ ξ m /δ ≈ 1.42 ± 0.03.
Article
Recent progress in adding a mobility dimension to preexisting API–MS systems without modifying the MS itself is discussed, based on inserting a differential mobility analyzer (DMA) as part of the MS's atmospheric pressure ion source. Design criteria leading to high DMA resolving power R and transmission efficiency η are discussed. Various DMA prototypes have been interfaced to several triple quadrupoles, a single quadrupole and a quadrupole-TOF, all demonstrating R>50 and η>50%. We obtain two-dimensional DMA–MS spectra of the multiply charged clusters formed in electrosprays of concentrated solutions of tetrahexylammonium bromide (A+Br−). These reveal systematic loss of (ABr)A+ fragments from unstable multiply charged clusters, and provide mobility measurements in air on mass resolved (ABr)n(A+)z clusters with n>100 and z up to 10. Well-defined bands of ions not individually resolved are clearly visible at considerably larger n and z values.
Article
An improved version of the Hewitt (differential) electric mobility analyzer was developed and its classifying characteristics were determined theoretically. The central mobility of the classified aerosol was found to be (qc + qm)/4πΛV, where qc and qm are the clean air and main outlet flows, respectively, Λ is a geometric factor, and Λ is the center rod voltage. The half-width of the mobility band was found to be (qa + qs)/4πΛV, where qa and qs are the aerosol and sampling outlet flows, respectively. These expressions were verified by the tests with a monodisperse aerosol of known size and low charge.
Article
Ion mobility spectrometry (IMS) has become an important method for the detection of many compounds because of its high sensitivity and amenability to miniaturization for field-portable monitoring; applications include detection of narcotics, explosives, and chemical warfare agents. High-field asymmetric waveform ion mobility spectrometry (FAIMS) differs from IMS in that the electric fields are applied using a high-frequency periodic asymmetric waveform, rather than a dc voltage. Furthermore, in FAIMS the compounds are separated by the difference in the mobility of ions at high electric field relative to low field, rather than by compound to compound differences in mobility at low electric field (IMS). We report here the first cylindrical-geometry-FAIMS interface with mass spectrometry (FAIMS-MS) and the MS identification of the peaks observed in a FAIMS compensation voltage (CV) spectrum. Using both an electrometer-based-FAIMS (FAIMS-E) and FAIMS-MS, several variables that affect the sensitivity of ion detection were examined for two (polarity reversed) asymmetric waveforms (modes 1 and 2) each of which yields a unique spectrum. An increase in the dispersion voltage (DV) was found to improve the sensitivity and separation observed in the FAIMS CV spectrum. This increase in sensitivity and the unexpected dissimilarity in modes 1 and 2 suggest that atmospheric pressure ion focusing is occurring in the FAIMS analyzer. The sensitivity and peak locations in the CV spectra were affected by temperature, gas flow rates, operating pressure, and analyte concentration. © 1998 American Institute of Physics.
Article
Gas phase ion mobility measurements can resolve structural isomers for polyatomic ions and provide information about their geometries. A new experimental apparatus for performing high-resolution ion mobility measurements is described. The apparatus consists of a pulsed laser vaporization/desorption source coupled through an ion gate to a 63-cm-long drift tube. The ion gate is a critical component that prevents the diffusion of neutral species from the source into the drift tube. Ions travel along the drift tube under the influence of a uniform electric field. At the end of the drift tube some of the ions exit through a small aperture. They are focused into a quadrupole mass spectrometer, where they are mass analyzed, and then detected by an off-axis collision dynode and by dual microchannel plates. The apparatus is operated with a drift voltage of up to 14 000 V and a helium buffer gas pressure of around 500 Torr. The resolving power for ion mobility measurements is over an order of magnitude higher than has been achieved using conventional injected-ion drift tube techniques. Examples of the application of the new apparatus in resolving isomers of laser desorbed metallofullerenes, in studying silicon clusters generated by laser vaporization, and in following the isomerization of small nanocrystalline (NaCl)nCl− clusters as a function of temperature, are presented. © 1997 American Institute of Physics.
Article
The mass spectrometric (MS) complexity associated with the quasi-continuous distribution of mass and charge (m, z) of electrosprayed industrial polymers may be moderated by use of ion mobility spectrometry (IMS) and MS in series. However, when the high charge levels typical of polar polymers stretch the gas phase ions into linear configurations, the mobility Z tends to be closely correlated with m/z, and IMS-MS does not yield spectra more readily interpretable than pure MS spectra. Here we note that the usual high charge states observed in the ESI of polyethylene glycol (PEG) arise because the stretched gas phase chain is able to strongly bind solution cations. We weaken this binding and therefore moderate the charge level by electrospraying in negative mode (NESI). This produces exclusively globular gas phase ions. IMS-MS then readily separates into distinct bands the different z-states, enabling an unambiguous assignment of all ions and simplifying the determination of mass distributions fz(m) for each charge state. The measured probability pz(m) that a polymer ion of given mass m will carry z charges spans a surprisingly narrow z range, each mass being present at most in two charge states. PEG ions of a given charge state z become unstable at a critical mass, below which they shed just one elementary charge, evidently by ion evaporation. We argue that NESI-IMS-MS offers significant analytical advantages over alternative methods previously demonstrated, particularly at increasing masses, when individual ion peaks can no longer be discerned.
Article
The analysis of ions according to their mobility is a technique that is attracting increasing interest. The new technology presented here, which we have termed Transversal Modulation Ion Mobility Spectrometry (TM-IMS), utilizes only electric fields, operates at atmospheric pressure, produces a continuous output of mobility selected ions (according to their true mobility and not to nonlinear effects), and has a very accessible inlet and outlet. These features would make it an ideal choice for tandem IMS-MS analysis in combination with most commercial Atmospheric Pressure Interface MS (API-MS) systems. We modeled and evaluated two different TM-IMS configurations (TM-IMS, and multistage TM-IMS), and we concluded that the most promising configuration would be a two-stage TM-IMS. We developed and tested a TM-IMS, and the measured resolving power is R = 55. The TM-IMS behaves similarly to the planar Differential Mobility Analyzer, but the TM-IMS utilizes only electric fields, and no fragile flow with high Reynolds numbers is required. We tested the robustness of the TM-IMS, which proves to be a very robust and reliable analyzer: the required voltage accuracy is 5 V in 10 kV, and the mechanical precision is 1 mm in 5 cm.
Article
The mobility distributions of electrosprays from solutions of relatively large tetraalkyl ammonium halide salts A+B- produce a series of sharp peaks associated to clusters of the form (A+)z(AB)n, dominated by the bare cation A+. In order to facilitate their use as standards of mass and mobility (in air), these properties are characterized via tandem use of a differential mobility analyzer (DMA) and a mass spectrometer (MS) with an atmospheric pressure inlet. Mobility diameters (based on Millikan's conventional size vs. mobility relation) in excess of 3nm are obtained from singly and doubly charged clusters. The mobilities of the ions A+(AB)3 are anomalously large for alkyl groups from methyl to butyl, presumably due to their relatively high symmetry.
Article
Various novel mobility analyzer (MA) designs useful for the separation of gas phase ions and charged particles according to their electrical mobilities are discussed. Traditional differential mobility analyzer (DMA) designs have mostly been restricted to two-elements (electrodes) of either parallel or coaxial cylindrical geometries, with the inlet and the outlet on different elements, between which is maintained a large voltage difference. Calculations of the performance of several MA designs free from some such restrictions are presented in the case of potential flows. They include, for example, devices in which (i) the inlet and outlet are on the same element; (ii) the inlet and outlet are at the same voltage (“isopotential devices”); (iii) more than two-elements exist; (iv) a fair fraction of the sheath gas flow passes through one of the elements; (v) all ions within a finite mobility range originating at an inlet point are focused on the outlet (mobility focusing). These latter devices may be more properly described as “Ion Filters” because only ions within a tunable mobility range can reach the outlet. By restricting the “bandwidth” through additional elements or auxiliary sheath gas flow suction/injection, these filters can be used as high resolution DMAs. Isopotential aspiration counters have been used previously as atmospheric ion counters.
Article
An analysis of the effects of diffusion in differential mobility analyzers (DMAs) at high Peclet number (Pe) shows that the associated peak broadening may be greatly reduced when the axial separation between entrance and exit slits is comparable to the inter-electrode gap. This prediction is verified in a shortened version of Reischl's DMA using molecular ions from an electrospray source. Gaussian peaks with relative full width at half maximum as small as 0.066 are observed at a DMA Reynolds number of 1190 in the mobility spectra of (butyl)4N+ ions in air (Pe = 104). After correcting for the broadening effect of the aerosol flow rate and the sampling slit width, the predicted diffusive peak widths agree well with those observed, but only at the highest resolutions. The DMA described here is the first one able to measure with excellent resolution the mobilities of particles down to 1 nm in diameter, and even of molecular ions.
Article
Prior ion-mobility mass-spectrometry (IMS-MS) studies of polyethylene glycol (PEG) ions have identified only two out of many sharply different observed structures: Linear shapes with several individually solvated singly charged cations at high charge states z (beads on a string), and single multiply charged globules at low z. The present study is devoted to assign all other existing structures of PEG ions, for the first time reaching masses of 100 kDa and charge states up to z = 10. There are at most z different structures at charge state z. All involve a single globule carrying n charges, tied to one or several appendices bearing z - n separate charges in a beads-on-a-string configuration. All sharp shape transitions observed at decreasing ion mass involve ejection of one elementary charge (sometimes two) from the shrinking globule into the growing linear appendage. This picture is supported by molecular dynamics simulations and approximate calculations of electrical mobilities for computed structures.
Article
A parallel-plate differential mobility analyzer and a time-of-flight mass spectrometer (DMA-MS) are used in series to measure true mobility in dry atmospheric pressure air for mass-resolved electrosprayed GroEL tetradecamers (14-mers; ~800 kDa). Narrow mobility peaks are found (2.6-2.9% fwhm); hence, precise mobilities can be obtained for these ions without collisional activation, just following their generation by electrospray ionization. In contrast to previous studies, two conformers are found with mobilities (Z) differing by ~5% at charge state z ~ 79. By extrapolating to small z, a common mobility/charge ratio Z(0)/z = 0.0117 cm(2) V(-1) s(-1) is found for both conformers. When interpreted as if the GroEL ion surface were smooth and the gas molecule-protein collisions were perfectly elastic and specular, this mobility yields an experimental collision cross section, Ω, 11% smaller than in an earlier measurement, and close to the cross section, A(C,crystal), expected for the crystal structure (determined by a geometric approximation). However, the similarity between Ω and A(C,crystal) does not imply a coincidence between the native and gas-phase structures. The nonideal nature of protein-gas molecule collisions introduces a drag enhancement factor, ξ = 1.36, with which the true cross section A(C) is related to Ω via A(C) = Ω/ξ. Therefore, A(C) for GroEL 14-mer ions determined by DMA measurements is 0.69A(C,crystal). The factor 1.36 used here is based on the experimental Stokes-Millikan equation, as well as on prior and new numerical modeling accounting for multiple scattering events via exact hard-sphere scattering calculations. Therefore, we conclude that the gas-phase structure of the GroEL complex as electrosprayed is substantially more compact than the corresponding X-ray crystal structure.
Article
A novel ion mobility spectrometry instrument incorporating a cyclotron geometry drift tube is presented. The drift tube consists of eight regions, four curved drift tubes and four ion funnels. Packets of ions are propagated around the drift tube by changing the drift field at a frequency that is resonant with the ion's drift time through each region. The approach trims each packet of ions as it leaves and enters each new region. An electrostatic gate allows ions to be kept in the drift tube for numerous cycles, increasing the ability to resolve specified ions. We demonstrate the approach by isolating the [M + 2H](2+) or [M + 3H](3+) charge state of substance P as well as individual trisaccharide isomers from a mixture of melezitose and raffinose. Resolving powers in excess of 300 are obtainable with this approach.
Article
High-resolution ion mobility spectrometry has been combined with time-of-flight mass spectrometry for analysis of a combinatorial peptide library that is expected to contain 676 components. In this approach, the components of a mixture of three residue peptides, having the general form (D)Phe-Xxx-Xxx-CONH2 (where Xxx is randomized over 26 residues including 10 naturally occurring amino acids and 16 synthetic forms) were ionized by electrospray ionization. Ion mobility/time-of-flight distributions have been recorded for all ions using a nested drift(flight) time technique. The improvement in resolving power [(t/delta t) = 100-150 for singly charged ions] was illustrated by analysis of a mixture of tryptic digest peptides using high- and low-resolution instruments. The approach allows many components of the library (e.g., structural, sequence, and stereo isomers) that cannot be distinguished by mass spectrometry alone to be resolved. Impurities due to side reactions appear to be minimal, comprising < 10% of the total ion signal. Direct evidence for approximately 60-70% of the expected peptides is found. Variation in ion abundance for different components indicates that there are differences in solution concentrations or ionization efficiencies for the components.
Article
A novel experiment has been devised which provides direct evidence for critical point behavior in the longstanding problem of the transition to turbulence in a pipe. The novelty lies in the quenching of turbulence by reducing the Reynolds number and observing the decay of disordered motion. Divergence of the time scales implies underlying deterministic dynamics which are analogous to those found in boundary crises in dynamical systems. A modulated wave packet emerges from the long term transients and this coherent state provides evidence for connections with recent theoretical developments.
Presented at the 56th ASMS Conference
  • H Javaheri
  • Y Le Blanc
  • B A Thomson
  • J Fernandez De La Mora
  • J Rus
  • J A Sillero-Sepúlveda
A high flow rate DMA with high transmission and resolution designed for new API instruments
  • J Rus
  • D Moro
  • J A Sillero
  • J Freixa
  • J Fernández De La Mora
Method to accurately discriminate gas phase ions with several filtering devices in tandem
  • Fernández De La Mora
  • J Casado
  • A Fernández De La Mora
Method and apparatus to accurately discriminate gas phase ions with several filtering devices in tandem
  • Fernández De La Mora
  • J Casado