Investigation of substituted-benzene dopants for charge exchange ionization of nonpolar compounds by atmospheric pressure photoionization

Solvent composition dependent signal reduction of molecular ions generated from aromatic compounds in (+) atmospheric pressure photoionization mass spectrometry

RATIONALE

The ionization process is essential for successful mass spectrometric (MS) analysis because of its influence on selectivity and sensitivity. In particular, certain solvents reduce the ionization of the analyte, thereby reducing the overall sensitivity in atmospheric pressure photoionization (APPI). Since the sensitivity varies greatly depending on the solvents, a fundamental understanding of the mechanism is required.

METHODS

Standard solutions were analyzed using a (+)-APPI Q Exactive ion trap mass spectrometer (Thermo Scientific). Each solution was infused directly into the APPI source at a flow rate of 100 μL/min and the APPI source temperature was 300°C. Other operating mass spectrometric parameters were maintained under the same conditions. Quantum mechanical calculations were carried out using the Gaussian 09 suite program.

RESULTS

Density functional theory was used to calculate the reaction enthalpies (∆H) of the reactions between toluene and other solvents. The experimental and theoretical results showed good agreement. The abundances of analyte ions were well correlated with the calculated ∆H values. Therefore, the results strongly support the suggested signal reduction mechanism. In addition, linear correlations between the abundance of toluene and analyte molecular ions were observed, which also supports the suggested mechanism.

CONCLUSIONS

A solvent composition dependent signal reduction mechanism was suggested and evaluated for the (+)-APPI-MS analysis of polyaromatic hydrocarbons (PAHs) generating mainly molecular ions. Overall, the evidence provided in this work suggests that reactions between solvent cluster(s) and toluene molecular ions are responsible for the observed reductions in signal.

Recent developments in atmospheric pressure photoionization-mass spectrometry

Recent developments in atmospheric pressure photoionization (APPI), which is one of the three most important ionization techniques in liquid chromatography-mass spectrometry, are reviewed. The emphasis is on the practical aspects of APPI analysis, its combination with different separation techniques, novel instrumental developments - especially in gas chromatography and ambient mass spectrometry - and the applications that have appeared in 2009-2014. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:423-449, 2017.

Charge Exchange Reaction in Dopant-Assisted Atmospheric Pressure Chemical Ionization and Atmospheric Pressure Photoionization

The efficiencies of charge exchange reaction in dopant-assisted atmospheric pressure chemical ionization (DA-APCI) and dopant-assisted atmospheric pressure photoionization (DA-APPI) mass spectrometry (MS) were compared by flow injection analysis. Fourteen individual compounds and a commercial mixture of 16 polycyclic aromatic hydrocarbons were chosen as model analytes to cover a wide range of polarities, gas-phase ionization energies, and proton affinities. Chlorobenzene was used as the dopant, and methanol/water (80/20) as the solvent. In both techniques, analytes formed the same ions (radical cations, protonated molecules, and/or fragments). However, in DA-APCI, the relative efficiency of charge exchange versus proton transfer was lower than in DA-APPI. This is suggested to be because in DA-APCI both dopant and solvent clusters can be ionized, and the formed reagent ions can react with the analytes via competing charge exchange and proton transfer reactions. In DA-APPI, on the other hand, the main reagents are dopant-derived radical cations, which favor ionization of analytes via charge exchange. The efficiency of charge exchange in both DA-APPI and DA-APCI was shown to depend heavily on the solvent flow rate, with best efficiency seen at lowest flow rates studied (0.05 and 0.1 mL/min). Both DA-APCI and DA-APPI showed the radical cation of chlorobenzene at 0.05-0.1 mL/min flow rate, but at increasing flow rate, the abundance of chlorobenzene M(+.) decreased and reagent ion populations deriving from different gas-phase chemistry were recorded. The formation of these reagent ions explains the decreasing ionization efficiency and the differences in charge exchange between the techniques. Graphical Abstract ᅟ.

Nucleophilic Aromatic Substitution Between Halogenated Benzene Dopants and Nucleophiles in Atmospheric Pressure Photoionization

In a preceding work with dopant assisted-atmospheric pressure photoionization (DA-APPI), an abundant ion at [M + 77](+) was observed in the spectra of pyridine and quinoline with chlorobenzene dopant. This contribution aims to reveal the identity and route of formation of this species, and to systematically investigate structurally related analytes and dopants. Compounds containing N-, O-, and S-lone pairs were investigated with APPI in the presence of fluoro-, chloro-, bromo-, and iodobenzene dopants. Computational calculations on a density functional theory (DFT) level were carried out to study the reaction mechanism for pyridine and the different halobenzenes. The experimental and computational results indicated that the [M + 77](+) ion was formed by nucleophilic aromatic ipso-substitution between the halobenzene radical cation and nucleophilic analytes. The reaction was most efficient for N-heteroaromatic compounds, and it was weakened by sterical effects and enhanced by resonance stabilization. The reaction was most efficient with chloro-, bromo-, and iodobenzenes, whereas with fluorobenzene the reaction was scarcely observed. The calculated Gibbs free energies for the reaction between pyridine and the halobenzenes were shown to increase in the order I < Br < Cl < F. The reaction was found endergonic for fluorobenzene due to the strong C-F bonding, and exergonic for the other halobenzenes. For fluoro- and chlorobenzenes the reaction was shown to proceed through an intermediate state corresponding to [M + dopant](+), which was highly stable for fluorobenzene. For the bulkier bromine and iodine, this intermediate did not exist, but the halogens were shown to detach already during the approach by the nucleophile.

Condensed Phase Membrane Introduction Mass Spectrometry with Direct Electron Ionization: On-line Measurement of PAHs in Complex Aqueous Samples

Polycyclic aromatic hydrocarbons (PAHs) are USEPA regulated priority pollutants. Their low aqueous solubility requires very sensitive analytical methods for their detection, typically involving preconcentration steps. Presented is the first demonstrated ‘proof of concept’ use of condensed phase membrane introduction mass spectrometry (CP-MIMS) coupled with direct liquid electron ionization (DEI) for the direct, on-line measurement of PAHs in aqueous samples. DEI is very well suited for the ionization of PAHs and other nonpolar compounds, and is not significantly influenced by the co-elution of matrix components. Linear calibration data for low ppb levels of aqueous naphthalene, anthracene, and pyrene is demonstrated, with measured detection limits of 4 ppb. Analytical response times (t10%–90% signal rise) ranged from 2.8 min for naphthalene to 4.7 min for pyrene. Both intra- and interday reproducibility has been assessed (<3% and 5% RSD, respectively). Direct measurements of ppb level PAHs spiked in a variety of real, complex environmental sample matrices is examined, including natural waters, sea waters, and a hydrocarbon extraction production waste water sample. For these spiked, complex samples, direct PAH measurement by CP-MIMS-DEI yielded minimal signal suppression from sample matrix effects (81%–104%). We demonstrate the use of this analytical approach to directly monitor real-time changes in aqueous PAH concentrations with potential applications for continuous on-line monitoring strategies and binding/adsorption studies in heterogeneous samples.

A Functional Group Approach for Prediction of APPI Response of Organic Synthetic Targets

Atmospheric pressure photoionization (APPI) is a technique of choice for ionization of non-polar molecules in mass spectrometry (MS). Reported APPI-based studies tend to focus on a selected compound class, which may contain a variety of functional groups. These studies demonstrate that APPI response frequently differs substantially, indicating a certain dependence on the functional group present. Although this dependence could be employed for APPI response prediction, its systematic use is currently absent. Here, we apply APPI MS to a judiciously-compiled set of 63 compounds containing a number of diverse functional groups commonly utilized in synthesis, reactive functional groups, as well as those containing boron and silicon. Based on the outcome of APPI MS of these compounds, we propose and evaluate a simple guideline to estimate the APPI response for a novel compound, the key properties of which have not been characterized in the gas phase. Briefly, we first identify key functional groups in the compound and gather knowledge on the known ionization energies from the smallest analogues containing said functional groups. We then consider local inductive and resonance effects on said ionization energies for the compounds of interest to estimate the APPI response. Finally, application of APPI MS to compounds of interest considered herein demonstrated extended upper mass ionization limit of 3.5 kDa for non-polymeric compounds.

The formation of [M-H]+ ions in N-alkyl-substituted thieno[3,4-c]-pyrrole-4,6-dione derivatives during atmospheric pressure photoionization mass spectrometry

RATIONALE

The formation of ions during atmospheric pressure photoionization (APPI) mass spectrometry in the positive mode usually provides radical cations and/or protonated species. Intriguingly, during the analysis of some N-alkyl-substituted thieno[3,4-c]pyrrole-4,6-dione (TPD) derivatives synthesized in our laboratory, unusual [M-H](+) ion peaks were observed. In this work we investigate the formation of [M-H](+) ions observed under APPI conditions.

METHODS

Multiple experimental parameters, including the type of ionization source, the composition of the solvent, the type of dopant, the infusion flow rate, and the length of the alkyl side chain were investigated to determine their effects on the formation of [M-H](+) ions. In addition, a comparison study of the gas-phase tandem mass spectrometric (MS/MS) fragmentation of [M + H](+) vs [M-H](+) ions and computational approaches were used.

RESULTS

[M-H](+) ions were observed under APPI conditions. The type of dopant and the length of the alkyl chain affected the formation of these ions. MS/MS fragmentation of [M-H](+) and [M + H](+) ions exhibited completely different patterns. Theoretical calculations revealed that the loss of hydrogen molecules from the [M + H](+) ions is the most favourable condition under which to form [M-H](+) ions.

CONCLUSIONS

[M-H](+) ions were detected in all the TPD derivatives studied here under the special experimental conditions during APPI, using a halogenated benzene dopant, and TPD containing substituted N-alkyl side chains with a minimum of four carbon atoms. Density functional theory calculations showed that for [M-H](+) ions to be formed under these conditions, the loss of hydrogen molecules from the [M + H](+)  ions is proposed to be necessary.

The ionization mechanisms in direct and dopant-assisted atmospheric pressure photoionization and atmospheric pressure laser ionization

A novel, gas-tight API interface for gas chromatography-mass spectrometry was used to study the ionization mechanism in direct and dopant-assisted atmospheric pressure photoionization (APPI) and atmospheric pressure laser ionization (APLI). Eight analytes (ethylbenzene, bromobenzene, naphthalene, anthracene, benzaldehyde, pyridine, quinolone, and acridine) with varying ionization energies (IEs) and proton affinities (PAs), and four common APPI dopants (toluene, acetone, anisole, and chlorobenzene) were chosen. All the studied compounds were ionized by direct APPI, forming mainly molecular ions. Addition of dopants suppressed the signal of the analytes with IEs above the IE of the dopant. For compounds with suitable IEs or Pas, the dopants increased the ionization efficiency as the analytes could be ionized through dopant-mediated gas-phase reactions, such as charge exchange, proton transfer, and other rather unexpected reactions, such as formation of [M + 77](+) in the presence of chlorobenzene. Experiments with deuterated toluene as the dopant verified that in case of proton transfer, the proton originated from the dopant instead of proton-bound solvent clusters, as in conventional open or non-tight APPI sources. In direct APLI using a 266 nm laser, a narrower range of compounds was ionized than in direct APPI, because of exceedingly high IEs or unfavorable two-photon absorption cross-sections. Introduction of dopants in the APLI system changed the ionization mechanism to similar dopant-mediated gas-phase reactions with the dopant as in APPI, which produced mainly ions of the same form as in APPI, and ionized a wider range of analytes than direct APLI.

Carbon disulfide as a dopant in photon-induced chemical ionization mass spectrometry

RATIONALE

The addition of a dopant to an Atmospheric Pressure PhotoIonization (APPI) source of a mass spectrometer has been shown to enhance the degree of analyte ionization. A series of different dopants has been successfully utilized; however, there has been very little published on the characteristics of a good dopant. We have proposed carbon disulfide (CS2) as a novel new dopant based on its absorption cross-section for the VUV photon's energy used and its unique gas-phase ion chemistry, notably the fact that it does not contain a proton.

METHODS

The ability of CS2 to enhance the ionization effectiveness of APPI was tested by using a group of compounds that have different proton affinities (PAs) and electron affinities (EAs). These results were compared to results obtained using the commonly used dopants, toluene and anisole. Particular attention was paid to the formation of [M](+) ions relative to [M+H](+) ions. Mass spectra were collected using a Waters Quattro Premier liquid chromatography/tandem mass spectrometry (LC/MS/MS) system equipped with a commercial Photomate™ photoionization source.

RESULTS

The results show that CS2 increases the ionization efficiency of most of the analytes studied in this work comparably to toluene and anisole. CS2 promotes both ionization routes of [M](+) and [M+H](+). In addition, due to the higher ionization energy (IE) of CS2 (10.01) compared to the IEs of toluene (8.83) and anisole (8.20), CS2 can enhance the ionization efficiency of analytes that cannot be enhanced with toluene and anisole.

CONCLUSIONS

We have determined that CS2 is a viable dopant for use in APPI sources. For some analytes, significant [M+H](+) ion signals are observed; therefore, the donated proton must come from either water clusters or solvents. In addition, CS2 promotes the ionization of analytes with low PAs and higher IEs than that of toluene and anisole.

Detection and monitoring of PAH and oxy-PAHs by high resolution mass spectrometry: comparison of ESI, APCI and APPI source detection

The objective of this work was to compare direct infusion in a Q-TOF mass spectrometer through three different atmospheric pressure sources, electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI) coupled to a high resolution Q-TOF mass spectrometer. A complex mixture of PAH and oxy-PAHs, obtained after the air oxidation of fluoranthene on mineral substrates, was used to compare the different ionization abilities of these sources. Here, we propose analytical methods for the use of all sources. Final goal was to provide background to the choice of the most appropriate source in order to analyze complex organic mixtures as those encountered in polluted soils, water, sediments, as well as in petroleum.

High-sensitivity analysis of six synthetic musks by ultra-performance liquid chromatography-atmospheric pressure photoionization-tandem mass spectrometry

The occurrence and potential toxicity of synthetic musks in environmental media have aroused concerns for the impacts of these chemicals on ecosystems and human health. Here, we present the first method using ultra-performance liquid chromatography-atmospheric pressure photoionization-tandem mass spectrometry (UPLC-APPI-MS/MS) for analysis of the six most important synthetic musks. Analysis was performed on an API 3000 triple quadrupole equipped with a PhotoSpray APPI source. Two pairs of precursor/product ions are presented that are essential for confirmation. Chromatographic separation is completed in 7 min in the positive mode and 5.1 min in the negative mode. Furthermore, three dopant solutions (0.5% anisole in toluene, 0.5% 2,4-difluoroanisole (DFA) in bromobenzene, and 0.5% DFA in chlorobenzene) are compared in terms of sensitivity, linearity, precision, and accuracy. The best sensitivity is associated with 0.5% anisole in toluene as the dopant; all LODs are below 6 pg. The linear range is 5 to 500 ppb with fairly good precision and accuracy. This analytical method has also proven its applicability by analyzing real air samples. In summary, we present a fast, sensitive, and reliable UPLC-APPI-MS/MS method for six important synthetic musks; it can be readily applied to environmental studies.

Suitability of tetrahydofuran as a dopant and the comparison to other existing dopants in dopant-assisted atmospheric pressure photoionization mass spectrometry in support of drug discovery

In this paper, we investigated the suitability of tetrahydofuran (THF) as a dopant and compared it against other common dopants for atmospheric pressure photoionization mass spectrometry (APPI-MS). In a systematic analysis of 37 drug standards and 100 Wyeth proprietary drug candidates, THF was found to increase ionization efficiency as high as 33-fold when introduced through a syringe pump at a flow rate of 20 microL/min, and as high as 114-fold when introduced through the mobile phase at 100 microL/min. As a dopant, THF is as effective as acetone, better than anisole, and slightly less effective than toluene for the majority of the test compounds. The increase in ionization efficiency by THF was found to be compound-dependent. THF was more effective in facilitating the ionization of polar compounds than of non-polar compounds. With THF, toluene and acetone as dopants, a single type of molecular ion ([M+H](+) or M(+*)) is produced for analyte molecules. However, anisole can cause the formation of an ion cluster for polar analytes. The cluster contains [M-2H+H](+), M(+*), and [M+H](+) ions with varied ratios. This complexity may make interpretation of spectra difficult for unknown compounds when complimentary data are not available. Our findings indicate that THF is a suitable dopant in the daily usage for increasing ionization efficiency, especially when THF is used as the mobile phase or as an organic modifier in the mobile phase.

Observation of molecular ions and analysis of nonpolar compounds with the direct analysis in real time ion source

Positive ions in the direct analysis in real time (DART) ion source are commonly formed by proton transfer. However, the DART source is similar to atmospheric pressure photoionization (APPI) in that it can produce molecular ions as well as protonated molecules, although the two sources differ in the initial ion formation process. This report discusses some of the factors that influence molecular ion formation in DART and shows how the DART source can be used to analyze "difficult" or nonpolar compounds such as alkanes and cholesterol. Trace reagent ions including NO(+) and O(2)(+)* formed from atmospheric gases are shown to play important roles in DART ionization. The use of the DART source as a gas chromatography/mass spectrometry (GC/MS) interface is demonstrated to show the difference between mass spectra obtained using conditions that favor proton transfer and those that favor molecular ion formation.

Comparison of dopants for charge exchange ionization of nonpolar polycyclic aromatic hydrocarbons with reversed-phase LC-APPI-MS

Atmospheric pressure photoionization (APPI) is capable of ionizing nonpolar compounds in LC/MS, through charge exchange reactions following photoionization of a dopant. Recently, several novel dopants-chlorobenzene, bromobenzene, 2,4-difluoroanisole, and 3-(trifluoromethyl)anisole-have been identified as having properties making them well-suited to serve as dopants for charge exchange ionization under reversed-phase LC conditions. Here, we report the results of experiments comparing their effectiveness to that of established dopants-toluene, anisole, and a toluene/anisole mixture, for the charge exchange ionization of model nonpolar compounds-the 16 polycyclic aromatic hydrocarbons (PAHs) identified by the US EPA as priority pollutants-when using a conventional reversed-phase LC method. Chloro- and bromobenzene were found to be much more effective than toluene for all the PAHs, due to the relatively low reactivity of their photoions with the solvent. Their overall performance was also better than that of anisole, due to anisole's ineffectiveness toward higher-IE compounds. Further, the experiments revealed that anisole's performance for higher-IE compounds can be dramatically improved by introducing it as a dilute solution in toluene, rather than neat. The two fluoroanisoles provided the highest overall sensitivity, by a slim margin, when introduced as dilute solutions in either chloro- or bromobenzene.

A dopant introduction device for atmospheric pressure photoionization with liquid chromatography/mass spectrometry

This technical note describes in detail the fabrication, operation and characterization of a pneumatically driven dopant introduction device, with a solvent reservoir capacity of 300 mL. Dopant flow rates and stability for this device are governed by the simple regulation of gas pressure rather than the progression of a stepper motor and syringe diameter, as is the case for typical infusion pumps. The device has the potential to provide days or even weeks of continuous, uninterrupted dopant flow at rates commonly adopted for atmospheric pressure photoionization (APPI) experiments without the need to replenish the dopant supply. Although not a refined instrumental design, this device was developed as an alternative cost-effective means of introducing stable dopant flow to an APPI source. The device was designed such that all components would be commercially available and easily procurable from common scientific part vendors. Figures and suggested part numbers are provided to allow those interested to fabricate similar devices to suit their individual experimental needs. Device characterization was performed while monitoring such factors as flow rate calibration, overall flow stability and reproducibility. In addition, a standard mixture of three polycyclic aromatic hydrocarbons was employed as a model sample for a typical reversed-phase liquid chromatography/atmospheric pressure photoionization mass spectrometry (LC/APPI-MS) application in order to demonstrate device performance.