Pressure Interface Research Articles

Multiphysical description of atmospheric pressure interface chemical ionisation in MION2 and Eisele type inlets

Abstract. Chemical ionisation inlets are fundamental instrument components in chemical ionisation mass spectrometry (CIMS). However, the sample gas and reagent ion trajectories are often understood only in a general and qualitative manner. Here, we evaluate two atmospheric pressure interface chemical ionisation inlets (MION2 and Eisele type inlet) with 3D computational fluid dynamics physicochemical models regarding the reagent ion and sample gas trajectories and estimate their efficiencies of reagent ion production, reagent ion delivery from the ion source volume into the ion–molecule mixing region, and the interaction between reagent ions and target molecules. The models are validated by laboratory measurements and quantitatively reproduce observed sensitivities to tuning parameters, including ion currents and changes in mass spectra. The study elucidates how the different transport and chemical reactions proceed within the studied inlets, where space charge can already be relevant at ion concentrations as low as 107 cm−3, and compares the two investigated inlet models. The models provide insights into how to operate the inlets and will help in the development of future inlets that further enhance the capability of CIMS.

Desorption of Positive and Negative Ions from Areoles of Opuntia microdasys Cactus at Atmospheric Pressure: Cactus-MS.

The areoles and spines of cacti can be used to desorb ions of ionic liquids (ILs) by the mere action of an electric field into the atmospheric pressure (AP) interface of a mass spectrometer. The small cactus species Opuntia microdasys bears numerous very fine hairs on its areoles and tiny sharp spines that appeared suited to serve as needle electrodes sharp enough for field desorption of ions to occur. In fact, positive and negative ions of four ILs could be desorbed by a process analogous to AP field desorption (APFD). In contrast to APFD where activated field emitters are employed, the ILs were deposited onto one or two adjacent areoles by applying 1-3 µL of a dilute solution in methanol. After evaporation of the solvent, the cactus was positioned next to the spray shield electrode of a trapped ion mobility-quadrupole-time-of-flight instrument. Desorption of IL cations and IL anions, respectively, did occur as soon as the electrode was set to potentials in the order of ±4.5 kV, while the cactus at ground potential was manually positioned in front of the entrance electrode to bring the areole covered with a film of the sample into the right position. Neither did mixing of ILs occur between neighboring areoles nor did the cactus suffer any damage upon its use as a botanical field emitter.

Extending the Range of Detectable Trace Species with the Fast Polarity Switching of Chemical Ionization Orbitrap Mass Spectrometry.

Chemical ionization (CI) atmospheric pressure interface mass spectrometry is a unique analytical technique for its low detection limits, softness to preserve molecular information, and selectivity for particular classes of species. Here, we present a fast polarity switching approach for highly sensitive online analysis of a wide range of trace species in complex samples using selective CI chemistries and high-resolution mass spectrometry. It is achieved by successfully coupling a multischeme chemical ionization inlet (MION) and an Orbitrap Fourier transform mass spectrometer. The capability to flexibly combine ionization chemistries from both polarities effectively extends the detectability compared to using only one ionization chemistry, as commonly used positive and negative reagent ions tend to be sensitive to different classes of species. We tested the performance of the MION-Orbitrap using reactive gaseous organic species generated by α-pinene ozonolysis in an environmental chamber and a standard mixture of 71 pesticides. Diethylammonium and nitrate are used as reagent ions in positive and negative polarities. We show that with a mass resolving power of 280,000, the MION-Orbitrap can switch and measure both polarities within 1 min, which is sufficiently fast and stable to follow the temporal evolution of reactive organic species and the thermal desorption profile of pesticides. We detected 23 of the 71 pesticides in the mixture using only nitrate as the reagent ion. Facilitated by polarity switching, we also detected 47 pesticides using diethylammonium, improving the total number of detected species to 59. For reactive organic species generated by α-pinene ozonolysis, we show that combining diethylammonium and nitrate addresses the need to measure oxygenated molecules in atmospheric environments with a wide range of oxidation states. These results indicate that the polarity switching MION-Orbitrap can promisingly serve as a versatile tool for the nontargeted chemical analysis of trace species in various applications.

Direct Measurements of Covalently Bonded Sulfuric Anhydrides from Gas-Phase Reactions of SO3 with Acids under Ambient Conditions.

Sulfur trioxide (SO3) is an important oxide of sulfur and a key intermediate in the formation of sulfuric acid (H2SO4, SA) in the Earth's atmosphere. This conversion to SA occurs rapidly due to the reaction of SO3 with a water dimer. However, gas-phase SO3 has been measured directly at concentrations that are comparable to that of SA under polluted mega-city conditions, indicating gaps in our current understanding of the sources and fates of SO3. Its reaction with atmospheric acids could be one such fate that can have significant implications for atmospheric chemistry. In the present investigation, laboratory experiments were conducted in a flow reactor to generate a range of previously uncharacterized condensable sulfur-containing reaction products by reacting SO3 with a set of atmospherically relevant inorganic and organic acids at room temperature and atmospheric pressure. Specifically, key inorganic acids known to be responsible for most ambient new particle formation events, iodic acid (HIO3, IA) and SA, are observed to react promptly with SO3 to form iodic sulfuric anhydride (IO3SO3H, ISA) and disulfuric acid (H2S2O7, DSA). Carboxylic sulfuric anhydrides (CSAs) were observed to form by the reaction of SO3 with C2 and C3 monocarboxylic (acetic and propanoic acid) and dicarboxylic (oxalic and malonic acid)-carboxylic acids. The formed products were detected by a nitrate-ion-based chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (NO3--CI-APi-TOF; NO3--CIMS). Quantum chemical methods were used to compute the relevant SO3 reaction rate coefficients, probe the reaction mechanisms, and model the ionization chemistry inherent in the detection of the products by NO3--CIMS. Additionally, we use NO3--CIMS ambient data to report that significant concentrations of SO3 and its acid anhydride reaction products are present under polluted, marine and polar, and volcanic plume conditions. Considering that these regions are rich in the acid precursors studied here, the reported reactions need to be accounted for in the modeling of atmospheric new particle formation.

Influence of the Capillary Design and Conditions upon the Transmission Efficiency of a Portable Ion Trap Mass Spectrometer

The capillary is a widely used atmospheric pressure interface in mass spectrometers that may be heated to enhance the desolvation of ions. In this study, the ion transmission efficiency of a portable ion trap mass spectrometer was investigated using capillaries with different inner diameters and temperatures, different vacuum pressures, and different ion optics designs. Due to the incomplete desolvation and the space charge effect inside the miniature ion funnel, the increase of ion intensities with the inner diameter of the capillary leveled off when the inner diameter of the capillary was larger than 0.37 mm. With the increase of capillary temperature, the ion intensities first increase due to better desolvation and then decrease due to greater diffusion loss. The optimal temperature of the capillary increases with the inner diameter of the capillary and the mass-to-charge ratio of ion. In addition, the pressures of different vacuum stages also exerted remarkable effects on the sensitivity of the portable mass spectrometer. From the perspective of instrument miniaturization, optimal transmission efficiency can be obtained when the first-stage vacuum pressure is maintained from 4 to 6 mbar. For a capillary with an inner diameter of 0.37 mm, the background of mass spectrum can be significantly reduced by replacing the pinhole behind the ion funnel with an off-axis skimmer.

A Miniature Orthogonal Injection Ion Funnel (MO-IF) Providing Enhanced Performance for the Miniature Mass Spectrometer.

The sensitivity of the miniature mass spectrometer (mini-MS) is largely restricted by the ion transmission in rough vacuum region. Even though various "in-line" ion transfer devices have improved mini-MS sensitivity, the severe dynamic gas is still weakening the efficiency of ion transmission in this region. Inspired by the "off-axis" ion funnel design in the lab-scale mass spectrometers, a miniature orthogonal injection ion funnel (MO-IF) was developed in this study for the mini-MS with a continuous atmospheric pressure interface. Capable of directing injected ions by 90° and then transport them forward to the downstream skimmer, the MO-IF enabled the separation of ions from the dynamic gas flow jetted out of the inlet capillary. The key factors were optimized for the MO-IF, including the effects of RF amplitude, DC electric fields, and the position of the repeller. Under optimized conditions, the MO-IF minimized the negative effects of dynamic gas and improved the ion transmission efficiency by ∼2-fold in comparison with the in-line injection ion funnel. As a result, a lower limit of detection of 0.5 ng/mL were obtained with good linearity for hypaconitine. Additionally, the MO-IF further decreased the buffer gas pressure in the second vacuum chamber and improved the mass resolution by 1.1-1.5 times at different scan rates.

Discontinuous Atmospheric Pressure Interface in-Trap Single Photon Ionization Ion Trap Mass Spectrometry (DAPI in-Trap SPI ITMS)

A compact in-trap single photon ionization (SPI) system, termed discontinuous atmospheric pressure interface (DAPI) ion trap mass spectrometry (ITMS), for the rapid determination of trace volatile organic compounds (VOCs). DAPI enables the maintenance of high analyte pressure in the SPI source while preserving the vacuum. Utilizing vacuum ultraviolet (VUV) lamp as the ion source, we ionize the analyte within an enclosed miniature 3D ion trap for efficient analysis. Experimental findings demonstrate that the system achieves a mass resolution of less than 0.5 atomic mass units for full width at half maxima (FWHM), providing unit resolution. For the response for benzene, the system achieves almost instantaneous response, with rise and fall times of only 2 s, which is a significant advantage compared to the membrane inlet (MI) approach necessary for on-site detection. Additionally, tandem mass spectrometry (MS/MS) with unit m/z isolation window was achieved and resolved dimethyl disulfide and phenol with similar molecular weights. Our results demonstrate that the system provides excellent performance and we anticipate its application in rapidly determining VOCs.

Atmospheric pressure field desorption-trapped ion mobility-mass spectrometry coupling

While field ionization (FI) and field desorption (FD) are established soft vacuum ionization methods in mass spectrometry (MS), the technique of atmospheric pressure field desorption (APFD) has only recently been added to the repertoire. Similar to FI and FD, APFD can yield both positive even-electron ions of highly polar or ionic compounds and positive molecular ions, M+•, e.g., of polycyclic aromatic compounds. Thus, a dedicated APFD source assembly has been constructed and demonstrated to allow for robust APFD operation. This device also enabled observation of the emitter during operation and allowed for resistive emitter heating, thereby speeding up the desorption of the analytes and expanding the range of analytes accessible to APFD. While initial work was done using a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer, the new APFD source offered the flexibility to also be used on a trapped ion mobility-quadrupole-time-of-flight (TIMS-Q-TOF) instrument, and thus, it would be possible to be mounted to any Bruker mass spectrometer featuring an atmospheric pressure (AP) interface. Operating an APFD source at a TIMS-Q-TOF instrument called for the exploration of the combined use of APFD and TIMS. Here, operation, basic properties, and capabilities of this new atmospheric pressure field desorption-trapped ion mobility-mass spectrometry (APFD-TIMS-MS) coupling are described. APFD-TIMS-MS is employed for the separation of individual components of oligomers and for the accurate determination of their collision cross section (CCS). This work describes the application of APFD-TIMS-MS on poly(ethylene glycol) forming [M + Na]+ ions by cationization and on an amine-terminated poly(propylene glycol) yielding [M + H]+ ions. Some compounds forming molecular ions, M+•, by field ionization such as [60]fullerene and a mixture of four polycyclic aromatic hydrocarbons (PAHs) are examined. In APFD-TIMS-MS, the limits of detection (LODs) of fluoranthene and benzo[a]pyrene M+• ions are determined as ≈100 pg and <1 pg, respectively. Finally, [60]fullerene is analyzed by negative-ion APFD-TIMS-MS where it yields a molecular anion, M−•.Graphical abstract

Development and characterization of discontinuous atmospheric pressure interface - Dual pressure chamber miniature mass spectrometer

Miniaturized mass spectrometers have become increasingly prevalent for real-time detection and analysis, owing to their compact size and portability. The pursuit of performance enhancement in these instruments is a pivotal objective within the domain of mass spectrometry miniaturization. This study introduces a novel miniature mass spectrometer featuring a discontinuous atmospheric pressure interface and a dual pressure chamber. Compared to conventional single-chamber, discontinuous sampling interface mass spectrometers, the newly developed instrument demonstrates a more than tenfold improvement in detection efficiency. This significant enhancement is achieved without the need for complex control of switch coupling time series, thereby streamlining the circuit design and improving the instrument's fault tolerance. Furthermore, by capitalizing on the benefits of discontinuous sampling, the instrument reduces the operational pressure relative to traditional continuous sampling in differential pressure vacuum chambers. It accommodates larger inlet capillary (0.38 mm) and skimmer (0.5 mm) diameters, leading to a ninefold increase in response strength for risperidone and lowering the detection limit to 0.5 ppb. The instrument's capacity for rapid drug detection, along with enhanced resolution and detection limits, underscores its potential utility. Additionally, it facilitates the use of smaller mechanical pumps, significantly diminishing both the instrument's volume and power consumption. This presents a promising avenue for further miniaturization of mass spectrometers.

Measurement report: Insights into the chemical composition and origin of molecular clusters and potential precursor molecules present in the free troposphere over the southern Indian Ocean: observations from the Maïdo Observatory (2150 m a.s.l., Réunion)

Abstract. New particle formation (NPF) in the free troposphere (FT) is thought to be a significant source of particles over the oceans. The entrainment of particles initially formed in the marine FT is further suspected to be a major contributor to cloud condensation nuclei (CCN) number concentrations in the marine boundary layer (BL). Yet, little is known about the process and, more broadly, about the composition of the marine FT, which remains poorly explored due to access difficulties. Here we report measurements performed in April 2018 at the Maïdo Observatory with a nitrate-based chemical ionization atmospheric pressure interface time-of-flight mass spectrometer, which have allowed the first molecular-level characterization of the remote marine FT composition. A number of molecules and clusters were identified and classified into nine groups according to their chemical composition; among the identified species, the groups containing methanesulfonic acid (MSA) and C2 amines show signals that are on average significantly higher when the site is under conditions representative of the marine FT (compared to the BL). The correlation analysis revealed apparent connections between the signals of the identified compounds and several variables concurrently measured at the site (under FT conditions) or related to air mass history, suggesting that oxalic acid, malonic acid, and observed C2 amines could be of terrestrial origin, with, in addition, a possible marine source for oxalic acid and amines, while iodic acid, sulfur species, and maleic acid have a dominant marine origin. Identification of FT conditions at the site was based on the analysis of the standard deviation of the wind direction; this parameter, which can easily be derived from continuous measurements at the site, is shown in the first part of the study to be a relevant tracer when compared to predictions from the Meso-NH atmospheric model. Similar to other high-altitude sites, FT conditions are mainly encountered at night at Maïdo; therefore, the link to NPF could not be established, and further research is needed to assess the composition of precursors to nanoparticle formation in the marine FT.

A study of the transient gas flow affected ion transmission in atmospheric pressure interfaces based on large eddy simulation for electrospray ionization mass spectrometry

Modern atmosphere pressure interface (API) enables high-efficiency coupling between mass analyzers in high vacuum and atmosphere ionization sources such as electrospray ionization (ESI) source. The transient gas flow entering API possesses strong compressibility and turbulent characteristics, which exerts a huge impact on ion transmission. However, the instantaneous nature and vortical morphology of the turbulence in API and its affection in ion transmission were hardly covered in the reported research. Here we conduct a transient turbulent flow-affected ion transmission evaluation for two typical APIs, the ion funnel and the S-lens, based on scale-resolving large eddy simulation and electro-hydrodynamical ion tracing simulation. In our simulation, the transient properties of the gas flow in the two APIs are illustrated and analyzed in-depth. After experimentally validated on a homemade ESI-TOF-MS platform, the results suggest that the ion funnel can achieve a higher droplet desolvation rate by introducing a unique droplet recirculation mechanism. Meanwhile, the less-dispersed gas flow in S-lens is beneficial in actuating ions axially. In conclusion, the application of the scale-resolving turbulence model helps us to understand the complicated fluid-ion interaction mechanism in APIs and is promising in the development of mass spectrometry instruments of higher performance.

Robust and versatile assembly for emitter positioning, observation, and heating in atmospheric pressure field desorption mass spectrometry.

Atmospheric pressure field desorption (APFD) mass spectrometry (MS) has recently been introduced as a new variant of field desorption (FD) mass spectrometry. The development aimed at providing the basic characteristics of FD-MS in combination with instruments equipped with an atmospheric pressure (AP) interface. Hitherto, APFD has been demonstrated to yield both positive and negative even electron ions of highly polar or ionic compounds as well as to enable the generation of positive molecular ions, M+•, of polycyclic aromatic compounds. The prototype setup for APFD was based on a nano-electrospray ionization (nanoESI) source slightly modified to allow for emitter positioning in front of the AP interface of a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer. The entrance electrode of the interface was set to negative or positive high voltage with respect to the emitter at ground potential, thereby permitting the formation of positive or negative ions, respectively. This work describes a custom-built device for quicker and more reproducible sample loading on and positioning of field emitters at the entrance electrode of the atmospheric pressure interface of a mass spectrometer. In addition, the device provides means for observation of the emitter during operation and for resistive emitter heating as employed in traditional FD-MS. Emitter heating both speeds up the desorption of the analytes and allows for the desorption/ionization of analytes of higher molecular weight than without emitter heating. In some cases, the signal-to-noise ratio of APFD mass spectra is improved due to higher ion currents effected by compressing the entire process into shorter periods of spectral acquisition. The new setup enables robust and reliable operation in APFD-MS. Moreover, it has been designed as to allow for use on a range of instruments as it can either be used on an FT-ICR mass spectrometer or in combination with a trapped ion mobility-quadrupole-time-of-flight (TIMS-Q-TOF) instrument.

Characterization of the planar differential mobility analyzer (DMA P5): resolving power, transmission efficiency and its application to atmospheric relevant cluster measurements

Abstract. The planar differential mobility analyzer (DMA), functioning as a particle sizer, exhibits superior transmission and selection accuracy at ambient pressure relative to its cylindrical counterparts. It also presents integration potential with atmospheric pressure interface mass spectrometry (API-MS) for enhanced cluster detection with an additional ion mobility dimension. In this study, the performance of a commercially available planar DMA (DMA P5) was evaluated. The device is capable of sizing particles below 3.9 nm, with larger sizes measurable through a sheath gas flow restrictor. The resolving power was appraised under various recirculation arrangements, including suction and counterflow modes along with different sheath flow rates, using electrosprayed tetra-alkyl ammonium salts. The peak resolving powers for tetrahexylammonium (THA+) achieved in suction and counterflow modes were 61.6 and 84.6, respectively. The DMA P5 offers a sizing resolution that is 5 to 16 times greater than that of cylindrical DMAs. Resolving power displayed a near-linear relationship with the square root of the applied voltage (VDMA) in counterflow mode. Conversely, the resolving power for THA+ ceased its linear enhancement with VDMA beyond a VDMA of 3554.3 V, entering a plateau which is ascribed to the perturbations in sample flow impacting the laminar nature of sheath flow. The DMA P5 transmission efficiency reaches 54.3 %, markedly surpassing that of conventional DMAs by nearly 1 order of magnitude. Moreover, the mobility spectrum of various electrosprayed tetra-alkyl ammonium salts and the mass-to-charge versus mobility 2D spectrum of sulfuric acid clusters were characterized using the DMA P5 MS system.

Application of atmospheric pressure field desorption for the analysis of anionic surfactants in commercial detergents

Recent work has shown that field desorption (FD) and field ionization (FI) using activated field emitters may be performed at atmospheric pressure, too. While some limitations apply to atmospheric pressure field desorption (APFD) mass spectrometry (MS), the method can deliver both positive and negative even electron ions of highly polar or ionic compounds. Furthermore, APFD even permits the generation of positive molecular ions of polycyclic aromatic compounds. Here, an application of negative-ion APFD for the analysis of anionic surfactants contained in commercial detergent products for body care, household, and technical uses is presented. The samples include liquid soaps and shower gels, dishwashing liquids, and cooling lubricants. Surfactant solutions in methanol/water or pure methanol at 2–10 µl ml−1 were deposited on commercial 13-µm activated tungsten emitters. The emitters were positioned in front of the atmospheric pressure interface of a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer by means of a slightly modified nano-electrospray ionization (nanoESI) source. The entrance electrode of the interface was set to positive high voltage with respect to the emitter at ground potential. Under these conditions, negative-ion desorption was achieved. The surfactant anions, organic sulfates and organic sulfonates, were characterized by accurate mass-based formula assignments, and in part, by tandem mass spectrometry. The negative-ion APFD spectra were compared to results by negative-ion electrospray ionization (ESI) either obtained using the FT-ICR mass spectrometer or by using a trapped ion mobility-quadrupole-time-of-flight (TIMS-Q-TOF) instrument when product ions of low m/z needed to be detected in tandem MS.Graphical

Molecular identification of organic acid molecules from α-pinene ozonolysis

Oxidized organic molecules from ozonolysis of α-pinene are believed to participate in atmospheric new particle formation in forest regions. However, the detailed process of nucleation originated from α-pinene ozonolysis remains unclear. In this study, an atmospheric pressure interface time-of-flight mass spectrometer coupled with a planar differential mobility analyzer (DMA-MS) was applied to investigate the gas-phase reaction of α-pinene and ozone. Ion mobility and mass to charge ratio of the oxidized molecules were simultaneously measured. The ion mobility distribution of these oxidation products highlighted three mobility diameter ranges: 0.89–0.99 nm, 1.16–1.24 nm and 1.31–1.38 nm, where the dominating molecules were CH2O2, C7-10H10-20O3-7 and C19-20H30-32O6-9, respectively. Combining the DMA-MS measurements, quantum chemistry calculations, and ion mobility modeling, the dominant monomer (C10H16O3) was confirmed as pinonic acid while the primary dimer (C20H32O6) was considered as pinonic acid cluster ((C10H16O3)2). Considering the yield of pinonic acid produced from α-pinene ozonolysis (2–4%), (C10H16O3)2 potentially accounts for part of the nocturnal C20 dimers and ungrowable sub-2-nm particles in forests. Our work provides a new method to facilitate the understanding of atmospheric nucleation and initial growth.

Revealing the sources and sinks of negative cluster ions in an urban environment through quantitative analysis

Abstract. Atmospheric cluster ions are important constituents in the atmosphere, and their concentrations and compositions govern their role in atmospheric chemistry. However, there is currently limited quantitative research on atmospheric ion compositions, sources, and sinks, especially in the urban atmosphere where pollution levels and human populations are intense. In this study, we measured the compositions of negative cluster ions and neutral molecules using an atmospheric pressure interface high-resolution time-of-flight mass spectrometer (APi-TOF) and a chemical ionization mass spectrometer in urban Beijing. Quantitative analysis of cluster ions was performed by their comparison with condensation sink (CS), reagent ions, and neutral molecules. We demonstrate the feasibility of quantifying cluster ions with different compositions using in situ-measured ion mobility distributions from a neutral cluster and air ion spectrometer (NAIS). The median concentration of negative cluster ions was 85 (61–112 for 25 %–75 %) cm−3 during the measurement period, which was negatively correlated with CS. The negative cluster ions mainly consisted of inorganic nitrogen-containing ions, inorganic sulfur-containing ions, and organic ions in the form of adducts with NO3- or HSO4-. The CHON-related organic ions accounted for over 70 % of the total organic ions. Although the molecules clustered with NO3- and HSO4- had similar compositions, we found that HSO4- clustered more efficiently with CHO and CHONnonNPs species (CHON excluding nitrated phenols), while NO3- clustered more efficiently with nitrated phenols (CHONNPs). Additionally, most organic ions were positively correlated with neutral molecules, resulting in similar diurnal cycles of organic ions and neutral molecules. However, an exception was found for CHONNPs, the concentration of which is also significantly influenced by the reagent ions NO3-. The charge fractions are generally higher for molecules with higher molecular weight and a higher oxidation state, and the opposite diurnal variations in charging fractions between H2SO4 and organic species indicate a charging competition between them. Finally, we choose HSO4- and C3H3O4- as representatives to calculate the contribution of different formation and loss pathways. We found their losses are condensational loss onto aerosol particles (73 %–75 %), ion–molecule reaction losses (19 %), and ion–ion recombination losses (6 %–8 %).

Molecular ion formation on activated field emitters in atmospheric pressure field desorption mass spectrometry

Atmospheric pressure field desorption (APFD) mass spectrometry (MS) has recently been explored as a new contribution to the field of ambient desorption/ionization (ADI). Depending on the selected polarity applied to the field emitter, ionic and polar analytes were demonstrated to deliver positive as well as negative ions. Whereas this recent study solely reported on the formation of even-electron ions of either polarity, the present work on APFD-MS demonstrates the abundant formation of positive molecular ions, M+•, from polycyclic aromatic compounds. Molecular ions were formed on and desorbed from standard 13-µm activated tungsten wire emitters at atmospheric pressure. The commercial field emitters were positioned at about 2 mm distance in front of the atmospheric pressure interface of a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer and the entrance electrode of the interface was set to –4.5 to –5.5 kV with respect to the emitter. Emitter-disrupting electric discharges did normally not occur under these conditions. The electric field strengths achieved at the dendritic microneedles were sufficient to allow for the abundant formation of M+• ions of various polycyclic aromatic compounds such as benzo[a]pyrene, anthracene, fluoranthene, 1,1,4,4-tetraphenyl-butadiene, and 1-aza-[6]helicene. In case of the extremely basic 1-aza-[6]helicene protonation strongly competed with molecular ion formation and tended to suppress the field ionization process. All molecular ion compositions were assured by accurate mass-based formula assignments.Graphical

Field-Gradient-Focusing Ion Guide for Enhanced Transfer Efficiency of Low-Mass Ions.

Efficient transmission of low-mass ions in a rough vacuum pressure region has always been a challenging issue in mass spectrometry (MS). In this study, a novel ion guide, namely, field-gradient-focusing ion guide (FGF-IG), was proposed to improve the transfer efficiency of ions, especially low-mass ions in a rough vacuum region. The FGF-IG has 12 electrodes whose surfaces gradually narrowed and tilted inward, and its electric field gradually varies from dodecapole (or multipole) to quadrupole along the ion transfer route. The field radius was gradually decreased from 6 to 2 mm in the multipole region (65 mm in length) and finally remained unchanged as 2 mm in the quadrupole region (20 mm in length). By integrating into a miniature mass spectrometer (mini-MS) with a continuous atmospheric pressure interface, this ion guide was optimized in terms of inlet capillary position, radio frequency amplitude, and direct current voltage applied on it. Results showed that a reduced low-mass discrimination effect and improved efficiency of simultaneously transferring mid and low m/z ions were achieved for FGF-IG compared with a conventional ion funnel. Under optimized conditions, a limit of detection of 1 ng/mL was obtained for both reserpine (m/z 609) and arginine (m/z 175) ions by integrating FGF-IG into the mini-MS. The sensitivity of smaller arginine ions using FGF-IG was enhanced by ∼10 times than that obtained using the conventional ion funnel (10 ng/mL) in comparative experiments. The idea of smooth transfer from dodecapole to quadrupole fields could be extended to other multipole fields, as well as in lab-scale MS instruments.

Hexapole-Assisted Continuous Atmospheric Pressure Interface for a High-Pressure Photoionization Miniature Ion Trap Mass Spectrometer.

Miniature mass spectrometers are powerful tools for on-site chemical analysis in the fields of homeland security, personal healthcare, and environmental monitoring. This study presents a novel hexapole-assisted continuous atmospheric pressure interface for a high-pressure photoionization miniature ion trap mass spectrometer (HA-HPPI-IT). Efficient ion transmission was achieved by combining radial focusing by an RF electric field and axial driving by gas flow, which was demonstrated by SIMION simulation and experimental verification. The pressure in the ionization-transmission chamber and the inner diameter of the skimmer were optimized, which helped in determining the number density of product ions and affected the ion transmission in the hexapole, respectively. After systematic optimizations, about 16-fold increase in signal intensity was achieved as the RF amplitude was varied from 140 to 400 Vpp, and a limit of detection of 1 ppbv was obtained. In addition, the HA-HPPI-IT exhibited high stability and the relative standard deviation was as low as 5.47%. Finally, the apparatus was applied for discovering the simulated spot for illicit drug synthesis by detecting toluene and propiophenone released to air and monitoring the evolutions of perchloroethylene residues from dry-cleaned clothes.

Desorption of positive and negative ions from activated field emitters at atmospheric pressure.

Field desorption (FD) traditionally is an ionization technique in mass spectrometry (MS) that is performed in high vacuum. So far only two studies have explored FD at atmospheric pressure or even superatmospheric pressure, respectively. This work pursues ion desorption from 13-µm activated tungsten emitters at atmospheric pressure. The emitters are positioned in front of the atmospheric pressure interface of a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer and the entrance electrode of the interface is set to 3-5 kV with respect to the emitter. Under these conditions positive, and for the first time, negative ion desorption is achieved. In either polarity, atmospheric pressure field desorption (APFD) is robust and spectra are reproducible. Both singly charged positive and negative ions formed by these processes are characterized by accurate mass-based formula assignments and in part by tandem mass spectrometry. The compounds analyzed include the ionic liquids trihexyl(tetradecyl) phosphonium tris(pentafluoroethyl) trifluorophosphate) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, the acidic compounds perfluorononanoic acid and polyethylene glycol diacid, as well as two amino-terminated polypropylene glycols. Some surface mobility on the emitter is prerequisite for ion desorption to occur. While ionic liquids inherently provide this mobility, the desorption of ions from solid analytes requires the assistance of a liquid matrix, e.g. glycerol.