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1. Increasing Confidence in Resolving Isomers of Emerging Per-And Polyfluoroalkyl Substances (PFASs) by Using DFT and Cosmo-Rs to Predict Chromatographic Retention Factors

   Antle, J. ; Guardin, M.G. ; Aga, D. ; Simpson, S. ( May 2022 , Annual meeting abstracts)

   Per-and polyfluoroalkyl substances (PFAS) are a class of contaminants of emerging concern frequently used in products like aqueous firefighting foams and non-stick coatings due to their stability and surfactant-like qualities. The lack of analytical standards for many emerging PFAS have severely limited our ability to comprehensively identify unknown PFAS contaminants in the environment, especially those that occur as isomers. Annotation of small molecules and identification of unknowns based only on elemental composition and mass fragmentation patterns remain major challenges in nontarget analysis employing liquid chromatography with high-resolution mass spectrometry (LC-HRMS). In this study, chromatographic retention factors (k) and mass spectral fragmentation patterns of 32 known PFAS were determined using our optimized parameters in LC-HRMS. The same method was then used to analyze previously unidentified PFAS in actual environmental samples. Using characteristic ions observed in the MS fragmentation of PFAS, the most probable isomeric structures of the detected PFAS were predicted. To increase confidence in the predicted molecular structure, Density Functional Theory and Conductor-like Screening Model for Realistic Solvents (COSMO-RS) calculations were used to predict physicochemical properties of different constitutional isomers. The DFT calculations facilitated geometric optimization, determination of polarizability, and calculation of the chemical potential the isomers. COSMO-RS uses themore »chemical potential to predict thermodynamic properties of molecules such as pKa, solubility, and Kow. These properties were then used to make a multi-variable linear regression to predict k values. The model was trained using 32 known PFAS. The properties used were log Kow of the neutral and anion species of the PFAS, and their polarizability. The model was specific enough to predict significantly different k values of unknown compounds with similar structures, which facilitated assignment of isomeric structures of PFAS.« less
2. Resolving identities of emerging per and polyfluoroalkyl substances isomers based on COSMO-RS derived retention factor and mass fragmentation patterns

   Simpson, Scott ; Guardian, M.G. ; Aga, D.S. ( December 2021 , Pacifichem)

   Chromatographic retention times and mass spectrometral fragmentation of per- and polyfluoroalkyl substances (PFASs) standards were determined using the optimized parameters obtained for liquid chromatography with tandem high-resolution mass spectrometry (LC-HRMS) analysis. Characteristic fragment ions obtained at various collision energies (MS2 fragmentation) were used for structural elucidation to predict the identities of newly discovered (emerging) PFASs detected in environmental samples. Moreover, the COnductor-like Screening MOdel for Realistic Solvents (COSMO-RS) was used to calculate the octanol-water partition coefficients (Kow) and mean isotropic polarizabilities of known PFASs, and the values were plotted against their chromatographic retention factors (k) to obtain a multivariable regression model that can be used to predict k values of unknown PFASs. Retention factor values of different structural isomers of the unknown PFASs were calculated and compared to the experimental k. For all the unknown PFASs, the predicted k value for the isomer that matches the corresponding MS2 fragmentation was found to be within 5% of the experimentally measured k value. This study demonstrates the applicability of a simple approach that combines the use of computationally-derived log Kow and polarizabilities, experimentally-determined k values, together with observed MS2 fragmentation patterns, in assigning the structures of emerging PFASs at environmentally relevant conditionsmore »when no reference standards are available.« less
3. Carbon Capture from Biogas by Deep Eutectic Solvents: A COSMO Study to Evaluate the Effect of Impurities on Solubility and Selectivity

   https://doi.org/10.3390/cleantechnol3020029  

   Quaid, Thomas ; Reza, M. Toufiq ( June 2021 , Clean Technologies)

   Deep eutectic solvents (DES) are compounds of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA) that contain a depressed melting point compared to their individual constituents. DES have been studied for their use as carbon capture media and biogas upgrading. However, contaminants’ presence in biogas might affect the carbon capture by DES. In this study, conductor-like screening model for real solvents (COSMO-RS) was used to determine the effect of temperature, pressure, and selective contaminants on five DES’ namely, choline chloride-urea, choline chloride-ethylene glycol, tetra butyl ammonium chloride-ethylene glycol, tetra butyl ammonium bromide-decanoic acid, and tetra octyl ammonium chloride-decanoic acid. Impurities studied in this paper are hydrogen sulfide, ammonia, water, nitrogen, octamethyltrisiloxane, and decamethylcyclopentasiloxane. At infinite dilution, CO2 solubility dependence upon temperature in each DES was examined by means of Henry’s Law constants. Next, the systems were modeled from infinite dilution to equilibrium using the modified Raoults’ Law, where CO2 solubility dependence upon pressure was examined. Finally, solubility of CO2 and CH4 in the various DES were explored with the presence of varying mole percent of selective contaminants. Among the parameters studied, it was found that the HBD of the solvent is the most determinant factor for themore »effectiveness of CO2 solubility. Other factors affecting the solubility are alkyl chain length of the HBA, the associated halogen, and the resulting polarity of the DES. It was also found that choline chloride-urea is the most selective to CO2, but has the lowest CO2 solubility, and is the most polar among other solvents. On the other hand, tetraoctylammonium chloride-decanoic acid is the least selective, has the highest maximum CO2 solubility, is the least polar, and is the least affected by its environment.« less
4. Solution- and gas-phase behavior of decavanadate: implications for mass spectrometric analysis of redox-active polyoxidometalates

   https://doi.org/10.1039/d1qi01618k  

   Favre, Daniel ; Bobst, Cedric E. ; Eyles, Stephen J. ; Murakami, Heide ; Crans, Debbie C. ; Kaltashov, Igor A. ( March 2022 , Inorganic Chemistry Frontiers)

   Decavanadate (V 10 O 28 6− or V10) is a paradigmatic member of the polyoxidometalate (POM) family, which has been attracting much attention within both materials/inorganic and biomedical communities due to its unique structural and electrochemical properties. In this work we explored the utility of high-resolution electrospray ionization (ESI) mass spectrometry (MS) and ion exclusion chromatography LC/MS for structural analysis of V10 species in aqueous solutions. While ESI generates abundant molecular ions representing the intact V10 species, their isotopic distributions show significant deviations from the theoretical ones. A combination of high-resolution MS measurements and hydrogen/deuterium exchange allows these deviations to be investigated and interpreted as a result of partial reduction of V10. While the redox processes are known to occur in the ESI interface and influence the oxidation state of redox-active analytes, the LC/MS measurements using ion exclusion chromatography provide unequivocal evidence that the mixed-valence V10 species exist in solution, as extracted ion chromatograms representing V10 molecular ions at different oxidation states exhibit distinct elution profiles. The spontaneous reduction of V10 in solution is seen even in the presence of hydrogen peroxide and has not been previously observed. The susceptibility to reduction of V10 is likely to be shared bymore »other redox active POMs. In addition to the molecular V10 ions, a high-abundance ionic signal for a V 10 O 26 2− anion was displayed in the negative-ion ESI mass spectra. None of the V 10 O 26 cations were detected in ESI MS, and only a low-abundance signal was observed for V 10 O 26 anions with a single negative charge, indicating that the presence of abundant V 10 O 26 2− anions in ESI MS reflects gas-phase instability of V 10 O 28 anions carrying two charges. The gas-phase origin of the V 10 O 26 2− anion was confirmed in tandem MS measurements, where mild collisional activation was applied to V10 molecular ions with an even number of hydrogen atoms (H 4 V 10 O 28 2− ), resulting in a facile loss of H 2 O molecules and giving rise to V 10 O 26 2− as the lowest-mass fragment ion. Water loss was also observed for V 10 O 28 anions carrying an odd number of hydrogen atoms ( e.g. , H 5 V 10 O 28 − ), followed by a less efficient and incomplete removal of an OH˙ radical, giving rise to both HV 10 O 26 − and V 10 O 25 − fragment ions. Importantly, at least one hydrogen atom was required for ion fragmentation in the gas phase, as no further dissociation was observed for any hydrogen-free V10 ionic species. The presented workflow allows a distinction to be readily made between the spectral features revealing the presence of non-canonical POM species in the bulk solution from those that arise due to physical and chemical processes occurring in the ESI interface and/or the gas phase.« less
5. Reductive Amination for LC–MS Signal Enhancement and Confirmation of the Presence of Caribbean Ciguatoxin-1 in Fish

   https://doi.org/10.3390/toxins14060399  

   Kryuchkov, Fedor ; Robertson, Alison ; Mudge, Elizabeth M. ; Miles, Christopher O. ; Van Gothem, Soetkien ; Uhlig, Silvio ( June 2022 , Toxins)

   Ciguatera poisoning is a global health concern caused by the consumption of seafood containing ciguatoxins (CTXs). Detection of CTXs poses significant analytical challenges due to their low abundance even in highly toxic fish, the diverse and in-part unclarified structures of many CTX congeners, and the lack of reference standards. Selective detection of CTXs requires methods such as liquid chromatography coupled to tandem mass spectrometry (LC–MS/MS) or high-resolution MS (LC–HRMS). While HRMS data can provide greatly improved resolution, it is typically less sensitive than targeted LC–MS/MS and does not reliably comply with the FDA guidance level of 0.1 µg/kg CTXs in fish tissue that was established for Caribbean CTX-1 (C-CTX-1). In this study, we provide a new chemical derivatization approach employing a fast and simple one-pot derivatization with Girard’s reagent T (GRT) that tags the C-56-ketone intermediate of the two equilibrating C-56 epimers of C-CTX-1 with a quaternary ammonium moiety. This derivatization improved the LC–MS/MS and LC–HRMS responses to C-CTX-1 by approximately 40- and 17-fold on average, respectively. These improvements in sensitivity to the GRT-derivative of C-CTX-1 are attributable to: the improved ionization efficiency caused by insertion of a quaternary ammonium ion; the absence of adduct-ions and water-loss peaks for themore »GRT derivative in the mass spectrometer, and; the prevention of on-column epimerization (at C-56 of C-CTX-1) by GRT derivatization, leading to much better chromatographic peak shapes. This C-CTX-1–GRT derivatization strategy mitigates many of the shortcomings of current LC–MS analyses for C-CTX-1 by improving instrument sensitivity, while at the same time adding selectivity due to the reactivity of GRT with ketones and aldehydes.« less