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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. Examining the mass loss and thermal properties of 3D printed models produced by fused deposition modeling and stereolithography under elevated temperatures

   https://doi.org/10.1108/RPJ-01-2022-0007  

   Hsieh, Shu-An ; Anderson, Jared L. ( June 2022 , Rapid Prototyping Journal)

   Purpose This paper aims to study the mass loss of three-dimensional (3D) printed materials at high temperatures. A preconcentration and analysis technique, static headspace gas chromatography-mass spectrometry (SHS-GC-MS), is demonstrated for the analysis of volatile compounds liberated from fused deposition modeling (FDM) and stereolithography (SLA) 3D printed models under elevated temperatures. Design/methodology/approach A total of seven commercial 3D printing materials were tested using the SHS-GC-MS approach. The printed model mass and mass loss were examined as a function of FDM printing parameters including printcore temperature, model size and printing speed, and the use of SLA postprocessing procedures. A high temperature resin was used to demonstrate that thermal degradation products can be identified when the model is incubated under high temperatures. Findings At higher printing temperatures and larger model sizes, the initial printed model mass increased and showed more significant mass loss after thermal incubation for FDM models. For models produced by SLA, the implementation of a postprocessing procedure reduced the mass loss at elevated temperatures. All FDM models showed severe structural deformation when exposed to high temperatures, while SLA models remained structurally intact. Mass spectra and chromatographic retention times acquired from the high temperature resin facilitated identification of eight compoundsmore »(monomers, crosslinkers and several photoinitiators) liberated from the resin. Originality/value The study exploits the high sensitivity of SHS-GC-MS to identify thermal degradation products emitted from 3D printed models under elevated temperatures. The results will aid in choosing appropriate filament/resin materials and printing mechanisms for applications that require elevated temperatures.« less
3. Determination of size and porosity of chars during combustion of biomass particles

   https://doi.org/https://doi.org/10.1016/j.combustflame.2022.112182  

   Yao, Yuan ; Panahi, Aidin ; Schiemann, Martin ; Levendis. Yiannis A. ( August 2022 , Combustion and flame)

   Egolfopoulos, Fokion (Ed.)

   This research focused on the size and overall porosity (pore volume) of carbonaceous chars, originating from high-heating rates and high-temperature pyrolysis and/or combustion of biomass. Emphasis was given to torrefied biomass chars. First, the porosity of char residues of single biomass particles of known mass was determined, based on an assumed value of skeletal density and by comparing experimentally observed temperature-time histories with numerical predictions of their burnout times. The average char porosities (effective porosities) of several raw and torrefied biomass particles were calculated to be in the range of 92–95%. Thereafter, these deduced porosity values were input again to the model to calculate the size of chars of other biomass particle precursors, whose initial size and mass were not known. Such biomass particles were sieve-classified to different nominal size ranges. This time, besides the porosity, representative time-temperature profiles of biomass particles in the aforementioned size ranges were also input to the model. Biomass particles are highly irregular with large aspect ratios and, in many cases, they melt and spherodize under high heating rates and elevated temperatures. Knowledge of the initial size of the chars, upon extinction of the volatile flames, is needed for modeling their heterogeneous combustion phase. Formore »this purpose, numerical predictions were in general agreement with measurements of char size obtained from both scanning electron microscopy of captured chars and real-time high-speed, high- magnification cinematographic observations of their combustion. Results showed that the generated chars of the examined biomass types were highly porous with large cavities. The average initial dimension of the chars, upon rapid pyrolysis, was in the range of 50–60% the mid-value of the mesh size of the sieves used to size-classify their highly irregular parent biomass particles.« less
4. Quantitative Structure Retention Relationships to Identify Per-/Poly-Fluoroalkyl Substances and Other Contaminants of Emerging Concern

   Simpson, S. ; Antle, J.l Halwatura ; LaRock, M. ; Dickman, R. ; Aga, D. ( May 2022 , Annual meeting abstracts)

   Nontarget analysis using liquid chromatography–high resolution mass spectrometry (LC–HRMS) is a valuable approach in characterizing for contaminants of emerging concern (CECs) in the environment. However, identification of these analytes can be quite costly or taxing without proper analytical standards. To circumvent this problem we utilize Quantitative structure-retention relationships (QSRR) models to predict elution order and retention times. Properties calculated from density functional theory (DFT) and the conductor-like screening model for real solvents (COSMO-RS) theory are used to produce our QSRR models, which can be calculated for virtually any analyte. We show that this methodology has been successful in identification of per- /poly-fluoroalkyl substances (PFAS) and other contaminants. Nontarget analysis using liquid chromatography– high resolution mass spectrometry (LC–HRMS) is a valuable approach in characterizing for contaminants of emerging concern (CECs) in the environment. However, identification of these analytes can be quite costly or taxing without proper analytical standards. To circumvent this problem we utilize Quantitative structureretention relationships (QSRR) models to predict elution order and retention times. Properties calculated from density functional theory (DFT) and the conductor-like screening model for real solvents (COSMO-RS) theory are used to produce our QSRR models, which can be calculated for virtually any analyte. We show thatmore »this methodology has been« less
5. Elemental fluorine detection by dielectric barrier discharge coupled to nano-ESI mass spectrometry for non-targeted analysis of fluorinated compounds

   https://doi.org/10.1021/acs.analchem.0c02141  

   Zheng, Kunyu ; Lesniewski, Joseph E ; Dolan Jr., Michael J ; Li, Wanqing ; Metallo, Tyler ; Jorabchi, Kaveh ( June 2020 , Analytical Chemistry)

   The growing use of fluorochemicals has elevated the need for non-targeted detection of unknown fluorinated compounds and transformation products. Elemental mass spectrometry coupled to chromatography offers a facile approach for such analyses by using fluorine as an elemental tag. However, efficient ionization of fluorine has been an ongoing challenge. Here, we demonstrate a novel atmospheric-pressure elemental ionization method where fluorinated compounds separated by GC are converted to Na2F+ for non-targeted detection. The compounds are first introduced into a helium dielectric barrier discharge (DBD) for breakdown. The plasma products are subsequently ionized by interaction with a nano-ESI plume of sodium-containing aqueous electrolytes. Our studies point to HF as the main plasma product contributing to Na2F+ formation. Moreover, the results reveal that Na2F+ is largely formed by the ion-neutral reaction between HF and Na2A(NaA)n+, gas-phase reagent ions produced by nano-ESI where A represents the anion of the electrolyte. Near-uniform fluorine response factors are obtained for a wide range of compounds, highlighting good efficiency of HF formation by DBD regardless of chemical structure of the compounds. Detection limits of 3.5 to 19.4 pg fluorine on-column are obtained using the reported GC-DBD-nano-ESI-MS. As an example of non-targeted screening, extractions from oil-and-water-repellent fabrics are analyzedmore »via monitoring Na2F+, resulting in detection of a fluorinated compound on a clothing item. Notably, facile switching of the ion source to atmospheric-pressure chemical ionization with the exact same chromatographic method allows identification of the detected compound at the flagged retention time.« less