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.

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-•.

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.

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.

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.

Solvent-Controlled Quadruple Catenation of Giant Chiral [8+12] Salicylimine Cubes Driven by Weak Hydrogen Bonding.

Mechanically interlocked structures are fascinating synthetic targets and the topological complexity achieved through catenation offers numerous possibilities for the construction of new molecules with exciting properties. In the structural space of catenated organic cage molecules, only few examples have been realized so far, and control over the catenation process in solution is still barely achieved. Herein, we describe the formation of a quadruply interlocked catenane of giant chiral [8+12] salicylimine cubes. The formation could be controlled by the choice of solvent used in the reaction. The interlocked structure was unambiguously characterized by single crystal X-ray diffraction and weak hydrogen bonding was identified as a central driving force for the catenation. Furthermore, scrambling experiments using partially deuterated cages were performed, revealing that the catenane formation occurs through mechanical interlocking of preformed single cages.

Dimeric and trimeric catenation of giant chiral [8 + 12] imine cubes driven by weak supramolecular interactions.

Mechanically interlocked structures, such as catenanes and rotaxanes, are fascinating synthetic targets and some are used for molecular switches and machines. Today, the vast majority of catenated structures are built upon macrocycles and only a very few examples of three-dimensional shape-persistent organic cages forming such structures have been reported. However, the catenation in all these cases was based on a thermodynamically favoured π-π-stacking under certain reaction conditions. Here, we show that catenane formation can be induced by adding methoxy or thiomethyl groups to one of the precursors during the synthesis of chiral [8 + 12] imine cubes, giving dimeric and trimeric catenated organic cages. To elucidate the underlying driving forces, we reacted 11 differently 1,4-disubstituted terephthaldehydes with a chiral triamino tribenzotriquinacene under various conditions to study whether monomeric cages or catenated cage dimers are the preferred products. We find that catenation is mainly directed by weak interactions derived from the substituents rather than by π-stacking.

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.

ESI and tandem MS for mechanistic studies with high-valent transition metal species.

The analysis of high-valent metal species has been in the focus of research for over 20 years. Mass spectrometry (MS) represents a technique routinely used for their characterization, in particular electrospray ionization mass spectrometry (ESI-MS) and cold-spray ionization-mass spectrometry (CSI-MS). The combination of both methods with tandem MS provides additional tools for understanding decay processes and reaction pathways. In this Perspective, tandem ESI-MS, an important instrument in enzyme and peptide characterization and organometallic chemistry, is discussed as a valuable tool for the elucidation of reaction mechanisms of high-valent metal species.

Poly(2-vinylpyridine) as a reference compound for mass calibration in positive-ion matrix-assisted laser desorption/ionization-mass spectrometry on different instrumental platforms.

Butyl-terminated poly(2-vinylpyridine) (P2VP), C4H9(C7H7N)nH, is evaluated for use as an external and internal mass calibrant in positive-ion matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS). P2VP oligomers covering the m/z 450-4500 range are employed to calibrate a time-of-flight (TOF) mass spectrometer in linear and reflector mode, an ion mobility-quadrupole-time-of-flight (IM-Q-TOF) mass spectrometer, and a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The proton affinity of P2VPs introduced by the numerous pyridyl groups leads to the almost exclusive formation of [M + H]+ ions with common acidic matrices like α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) as well as with the non-acidic and aprotic matrices 1,8-dihydroxy-10H-anthracen-9-on (dithranol) and 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malonitrile (DCTB). This prevalence of [M + H]+ ions evenly spaced at Δ(m/z) = 105.0578 renders butyl-terminated P2VP oligomers as convenient mass calibrants. The mass accuracies achieved across various m/z ranges with different mass analyzers and modes of operation are evaluated by using established standard compounds. Results as obtained by internal or external calibration are presented. Further, the compilation of mass reference lists tailored to suit the respective analyzer modes is discussed and those reference files are provided.

Negative-ion field desorption revitalized by using liquid injection field desorption/ionization-mass spectrometry on recent instrumentation.

Field ionization (FI), field desorption (FD), and liquid injection field desorption/ionization (LIFDI) provide soft positive ionization of gaseous (FI) or condensed phase analytes (FD and LIFDI). In contrast to the well-established positive-ion mode, negative-ion FI or FD have remained rare exceptions. LIFDI provides sample deposition under inert conditions, i.e., the exclusion of atmospheric oxygen and water. Thus, negative-ion LIFDI could potentially be applied to highly sensitive anionic compounds like catalytically active transition metal complexes. This work explores the potential of negative-ion mode using modern mass spectrometers in combination with an LIFDI source and presents first results of the application of negative-ion LIFDI-MS. Experiments were performed on two orthogonal-acceleration time-of-flight (oaTOF) instruments, a JEOL AccuTOF GCx and a Waters Micromass Q-TOF Premier equipped with LIFDI sources from Linden CMS. The examples presented include four ionic liquids (ILs), i.e., N-butyl-3-methylpyridinium dicyanamide, 1-butyl-3-methylimidazolium tricyanomethide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, and trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate), 3-(trifluoromethyl)-phenol, dichloromethane, iodine, polyethylene glycol diacid, perfluorononanoic acid, anionic surfactants, a tetraphosphazene silanol-silanolate, and two bis(catecholato)silanes. Volatile samples were delivered as vapors via the sample transfer capillary of the LIFDI probe or via a reservoir inlet. Condensed phase samples were applied to the emitter as dilute solutions via the sample transfer capillary. The compounds either yielded ions corresponding to their intact anions, A-, or the [M-H]- species formed upon deprotonation. This study describes the instrumental setups and the operational parameters for robust operation along with a discussion of the negative-ion LIFDI spectra of a variety of compounds.

Chiral Self-sorting of Giant Cubic [8+12] Salicylimine Cage Compounds.

Chiral self-sorting is intricately connected to the complicated chiral processes observed in nature and no artificial systems of comparably complexity have been generated by chemists. However, only a few examples of purely organic molecules have been reported so far, where the self-sorting process could be controlled. Herein, we describe the chiral self-sorting of large cubic [8+12] salicylimine cage compounds based on a chiral TBTQ precursor. Out of 23 possible cage isomers only the enantiopure and a meso cage were observed to be formed, which have been unambiguously characterized by single crystal X-ray diffraction. Furthermore, by careful choice of solvent the formation of meso cage could be controlled. With internal diameters of din =3.3-3.5 nm these cages are among the largest organic cage compounds characterized and show very high specific surface areas up to approx. 1500 m2 g-1 after desolvation.

Saccharose cluster ions as mass calibrants in positive-ion direct analysis in real time-mass spectrometry.

In positive-ion direct analysis in real time-mass spectrometry (DART-MS), mono-, di, and trisaccharides form [M+NH4]+ ions. Some of them, in addition, yield abundant [Mn+NH4]+ cluster ions (n = 1-6)), and thus, can serve for mass calibration. Saccharose, C12H22O11, the most common sugar, also termed sucrose, is among the [Mn+NH4]+ cluster ion forming species. Saccharose may therefore be employed as a cheap and ubiquitous mass calibration standard. The extent of saccharose cluster ion formation depends on the temperature of the DART gas, sample load, and instrumental parameters like trapping conditions of ions prior to mass analysis. This study identifies optimized experimental conditions and demonstrates the application of saccharose cluster ion-based mass calibration for accurate mass measurements in DART mode on a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer.

From the discovery of field ionization to field desorption and liquid injection field desorption/ionization-mass spectrometry-A journey from principles and applications to a glimpse into the future.

The discovery of the ionizing effect of strong electric fields in the order of volts per Ångstrom in the early 1950s eventually led to the development of field ionization-mass spectrometry (FI-MS). Due to the very low ion currents, and thus, limited by the instrumentation of the 1960s, it took some time for the, by then, new technique to become adopted for analytical applications. In FI-MS, volatile or at least vaporizable samples mainly deliver molecular ions, and consequently, mass spectra showing no or at least minor numbers of fragment ion signals. The next major breakthrough was achieved by overcoming the need to evaporate the analyte prior to ionization. This was accomplished in the early 1970s by simply depositing the samples onto the field emitter and led to field desorption-mass spectrometry (FD-MS). With FD-MS, a desorption ionization method had become available that paved the road to the mass spectral analysis of larger molecules of low to high polarity and even of organic salts. In FD-MS, all of these analytes deliver spectra with no or at least few fragment ion peaks. The last milestone was the development of liquid injection field desorption/ionization (LIFDI) in the early 2000s that allows for sample deposition under the exclusion of atmospheric oxygen and water. In addition to sampling under inert conditions, LIFDI also enables more robust and quicker operation than classical FI-MS and FD-MS procedures. The development and applications of FI, FD, and LIFDI had mutual interference with the mass analyzers that were used in combination with these methods. Vice versa, the demand for using these techniques on other than magnetic sector instruments has effectuated their adaptation to different types of modern mass analyzers. The journey started with magnetic sector instruments, almost skipped quadrupole analyzers, encompassed Fourier transform ion cyclotron resonance (FT-ICR) and orthogonal acceleration time-of-flight (oaTOF) analyzers, and finally arrived at Orbitraps. Even interfaces for continuous-flow LIFDI have been realized. Even though being niche techniques to some degree, one may be confident that FI, FD, and LIFDI have a promising future ahead of them. This Account takes you on the journey from principles and applications of the title methods to a glimpse into the future.

Self-Supplied Liquid Injection Field Desorption/Ionization Ion Source for an Orthogonal Time-of-Flight Instrument.

A new implementation of a dedicated ion source for field ionization (FI), field desorption (FD), and liquid injection field desorption/ionization (LIFDI) for the JEOL AccuTOF GC series of orthogonal-acceleration time-of-flight instruments is presented. In contrast to existing implementations, this third-party LIFDI probe and source combination does not require the exchange of the entire ion source comprising ion source block and lens stack to switch from electron ionization (EI) to LIFDI. Rather, the methods may be swapped conveniently by only exchanging the ion source block for a mechanical probe guide and inserting the LIFDI probe in place of the standard direct insertion probe (DIP) via the vacuum lock. Further, this LIFDI setup does not require any changes of the electronics or software of the AccuTOF mass spectrometer because it is self-supplied in terms of power supply, observation optics, and computer control. The setup offers advanced FI/FD/LIFDI control features such as emission-controlled emitter heating current and emitter flash baking during elongated runs as required for gas chromatography-FI-mass spectrometry (MS). The LIFDI source and probe and its operation are reported in detail. FI spectra of the volatile analytes toluene, heptane, and pentafluoroiodobenzene are presented. LIFDI operation is demonstrated for the analysis of the saturated hydrocarbon dotriacontane and the low-mass hydrocarbon polymers polystyrene 484 and polystyrene 1050. Further, the air-sensitive 2nd-generation Hoveyda-Grubbs catalyst is analyzed by LIFDI-MS. For comparison with long-established LIFDI instrumentation, some of the spectra obtained with the new setup are also compared with those from a double-focusing magnetic sector instrument.

1,1-Digoldallylium Complexes: Diaurated Allylic Carbocations Indicate New Prospects of the Coordination Chemistry of Carbon.

Reacting (NHC)(cyclopropenyl)gold(I) complexes with cationic gold complexes [(IPr)AuX] afforded extremely reactive allylium-1,1-diido-bridged digold intermediates. We prove the existence and constitution of this structure with FT-ICR-MS/MS, NMR, and UV-vis-NIR experiments and isolated the nucleophilic addition product [(Me)(Ph)(CCHC){Au(IPr)}2(SOMe2)]NTf2 with DMSO. Our computational investigation unveiled that the bonding situation of this µ-allylium-1,1-diido digold domain was best described as a three-center-four-electron bond with a π-backbond. The valence orbitals showed extreme delocalization and strong π-interactions between the three centers Au1-C1-Au2. The bridging carbon atom C1 was best described as trigonal planar sp-hybridized carbon in this structure. Excitation succeeded in UV-vis-NIR measurements with energies as low as near-IR radiation.

Basic poly(propylene glycols) as reference compounds for internal mass calibration in positive-ion matrix-assisted laser desorption/ionization mass spectrometry.

Basic poly(propylene glycols), commercially available under the trade name Jeffamine, are evaluated for their potential use as internal mass calibrants in matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry. Due to their basic amino endgroups Jeffamines are expected to deliver [M+H]+ ions in higher yields than neutral poly(propylene glycols) or poly(ethylene glycols). Aiming at accurate mass measurements and molecular formula determinations by matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry, four Jeffamines (M-600, M-2005, D-400, D-230) were thus compared. As a result, Jeffamine M-2005 is introduced as a new mass calibrant for positive-ion matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry in the range of m/z 200-1200 and the reference mass list is provided. While Jeffamine M-2005 is compatible with α-cyano-4-hydroxycinnamic acid, 2,5-dihydroxybenzoic acid, and 2-[(2 E)-3-(4- tert-butylphenyl)-2-methylprop-2-enylidene]malonitrile matrix, its use in combination with 2-[(2 E)-3-(4- tert-butylphenyl)-2-methylprop-2-enylidene]malonitrile provides best results due to low laser fluence requirements. Applications to PEG 300, PEG 600, the ionic liquid trihexyl(tetradecyl)-phosphonium tris(pentafluoroethyl)-trifluorophosphate, and [60]fullerene demonstrate mass accuracies of 2-5 ppm.