Over the past few years, a number of studies have been performed that show the capability for gas-phase molecular rotational spectroscopy to perform quantitative mixture analysis. In particular, the ability of this technique to identify new compounds in a mixture on the basis of comparison to electronic structure theory is extremely powerful. For a number of reasons, however, the sample introduction and volatilization methods employed warrant new development. First, mixtures often contain components with different vapor pressures, and so care is required in extrapolating concentration information from observed signals.C. West et al., Änalysis of pear ester flavoring samples using broadband rotational spectroscopy," 2018 International Symposium on Molecular Spectroscopy, talk RH06.dditionally, operator-to-operator variability, measurement cycle time, and ease of use are factors that should be considered. We will discuss our efforts to develop sampling interfaces to enable routine quantitative mixture analysis using molecular rotational spectroscopy, as well as challenges that the field still faces.

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Footnotes:

C. West et al., Änalysis of pear ester flavoring samples using broadband rotational spectroscopy," 2018 International Symposium on Molecular Spectroscopy, talk RH06.A

As energy demands increase, more sustainable alternatives to finite fossil fuels need to be explored. Ketones are an important constituent in biomass derived liquids and small methyl ketones have high octane numbers and show low emissions of soot, yet, their combustion chemistry remains relatively unexplored. 2-hexanone is an interesting candidate as it has a long alkyl chain supporting numerous conformational isomers as well as a methyl rotor. In this study, the rotational spectrum of 2-hexanone was recorded over the 8-18 GHz region using a chirped pulse Fourier transform microwave (CP-FTMW) spectrometer. Strong field coherence breaking (SFCB) was utilized to selectively modulate rotational transitions belonging to the two lowest energy conformers of 2-hexanone, aiding the assignment. In addition, the SFCB method was applied for the first time to identify rotational transitions built off the two lowest energy hindered methyl rotor states of each conformer, 0a1 and 1e. Since these two states have rotational energy levels with different nuclear spin symmetries, their intensities could be selectively modulated by the resonant monochromatic pulses used in the SFCB method. The difference spectra, final fit and structural parameters will be discussed for all three conformers assigned.

Trace amount of water has been detected in ethanol (CH₃CH₂OH) and methanol (CH₃OH) using a K-Band BrightSpec Microwave Rotational Resonance (MRR) spectrometer in the 18-26 GHz frequency range. The design of this instrument is based on segmented Chirped Pulse Fourier Transform microwave wave (CP-FTMW) spectroscopy, which exploits recent advances in digital electronics to allow fast measurement of broadband rotational spectra of polar molecules. The analysis of the observed rotational spectra reveals the presence of a weak rotational line shape of water due to sensitivity of MRR to polar volatile organic compounds . The capability for K-band MRR to extract water in a such chemical environment has been further examined and validated by spiking samples with known small amount of water. The resulting linear curves allowed the determination of limit of detections at ppm level. These findings suggest that K-band MRR has potential to be useful as a spectroscopic tool for fast detection of water in volatile organic compounds or other raw materials.

We present the automated analysis of rotational spectra using archived experimental and ab initio data. Modern rotational spectroscopy techniques have made acquiring experimental data extremely fast, yielding many GHz of information in seconds. This is in contrast to the analysis, which still takes many hours if not weeks of an expert’s time to fit. In order to bring rotational spectroscopy to a wider audience, new applications, and enable new science, the effort and time required to analyze a spectrum needs to be reduced. To reduce the analysis time, we aggregated more than 200 experimental spectra (from local instruments and literature) and more than 16,000 ab initio calculations into a library that is leveraged by automated algorithms to screen for identifications in rotational spectra. Screening with experimental data has been shown to give both highly accurate identification and quantification of a sample within a mixture in under 10 seconds per analysis. Unmatched residual lines are presented to the user for further analysis. Work is in progress to expand the number of matchable spectra inside the database and identify unknown constants using ab initio databases.

We present a new algorithm, Robust Automated Assignment of Rigid Rotors (RAARR), for assigning rotational spectra of asymmetric tops. The RAARR algorithm can automatically assign experimental spectra under a broad range of conditions, including spectra comprised of multiple mixture components, in about 100 seconds or less. The RAARR algorithm exploits constraints placed by the conservation of energy to find sets of connected lines in an unassigned spectrum. The highly constrained structure of these sets eliminates all but a handful of plausible assignments for a given set, greatly reducing the number of potential assignments that must be evaluated. We successfully apply our algorithm to automatically assign 15 experimental spectra, including 5 previously unassigned species, without prior estimation of molecular rotational constants. In 9 of the 15 cases, the RAARR algorithm successfully assigns two or more mixture components.

The feasibility of using molecular rotational spectroscopy as an on-line monitoring system for brandy distillation has been investigated in collaboration with Monte Piccolo Farm and Distillery. The target product is brandy produced from Indian Blood peaches. A previous study identified key flavor components from 18 peach cultivars, and lactones in the C₈–C₁₂ size range appear to produce much of the characteristic peach flavor.Xi, W.; Zheng, Q.; Lu, J.; Quan, J., Horticulture Plant Journal 3, 1-12 (2017).hese lactones are present in different ratios across peach cultivars and, as a result, these ratios may help identify the ideal peach for brandy distillation. Rotational spectra of octa-, deca-, and dodecalactone isomers were collected to provide a library of possible flavor congeners. Experimental spectra of the lactones were in good agreement with dispersion corrected density functional theory calculations.S. Grimme and M. Steinmetz, Phys. Chem. Chem. Phys. 15, 16031-16042 (2013).espite the lactones possessing long alkyl tails, conformational populations observed in the pulsed-jet expansion were dominated by one or two conformations. A lab-prepared mixture of peach flavor compounds and commercial peach extracts were analyzed to test the ability of the technique to analyze complex mixtures, similar to what would be seen during distillation. These measurements use temperature-programmed broadband rotational spectroscopy to separate the full spectrum of all mixture compounds into a spectrum for each compound. Rapid monitoring of these flavor components with a cavity-enhanced Fourier transform microwave spectrometer using a pre-programmed sample profile has been demonstrated on the commercial instrument from BrightSpec that is based on the NIST compact Balle-Flygare spectrometer design.R.D. Suenram, J.U. Grabow, A. Zuban, and I. Leonov, Rev. Sci. Instrum. 70, 2127-2135 (1999).html:<hr /><h3>Footnotes: Xi, W.; Zheng, Q.; Lu, J.; Quan, J., Horticulture Plant Journal 3, 1-12 (2017).T S. Grimme and M. Steinmetz, Phys. Chem. Chem. Phys. 15, 16031-16042 (2013).D R.D. Suenram, J.U. Grabow, A. Zuban, and I. Leonov, Rev. Sci. Instrum. 70, 2127-2135 (1999).

Absolute time-resolved populations of N₂(A³Σ_u⁺) excited electronic state generated in a repetitive ns pulse discharge in nitrogen have been measured by Cavity Ring Down Spectroscopy (CRDS) and Tunable Diode Laser Spectroscopy (TDLAS). CRDS measurements of N₂(A³Σ_u⁺,v=0-2) populations are made in the discharge afterglow at a pressures of 10-40 Torr. The data reduction procedure takes into account the linewidth of the pulsed laser source, comparable with the absorption linewidth and resulting in a non-single exponential ring down decay. Peak N₂(A³Σ_u⁺,v=0,1) populations after a 10-pulse ns discharge burst are 1.5x10¹¹ cm⁻³. In the afterglow, these populations exhibit a relatively slow decay with the characteristic time of approximately 500 μs, most likely due to the quenching by the N2 molecules in the ground electronic state. TDLAS data have been taken at a higher pressure of 130 Torr. Absolute time-resolved N₂(A³Σ_u⁺,v=0,1) number densities are measured during ns pulse discharge bursts up to 25 pulses long and in the afterglow, peaking at 5x10¹² cm⁻³ and 3x10¹³ cm⁻³. The results indicate that N₂(A³Σ_u⁺) is generated after every discharge pulse on a 20-50 μs time scale, much longer compared to the discharge pulse duration, and decays between the pulses. The decay rate increases during the discharge burst. In the afterglow, N₂(A³Σ_u⁺,v=0,1) populations decay significantly more rapidly compared to the low-pressure CRDS conditions, with the characteristic time of approximately 100 μs. The results demonstrate that both CRDS and TDLAS diagnostics can be used for time-resolved, absolute N₂(A³Σ_u⁺) measurements in transient nonequilibrium plasmas and the afterglow, with the detection limit of ≈ 10¹⁰ cm⁻³. The data obtained using these two diagnostics are complementary, since TDLAS measurements can be used at the conditions when the N₂(A³Σ_u⁺) populations may be too high, or vary too rapidly for accurate CRDS measurements.

Pancreatic cancer is the third leading cause of cancer-related deaths in the United States, with the life expectancy for patients diagnosed in the late stage ranging from 6-12 months. This study is part of a group effort to examine the hypothesis that, difficult to detect, tumor initiating cells (TIC) exist in small numbers in the solid tumors and are responsible for the cancer’s progression and relapse. The pancreatic cell line MIA PaCa-2 is seeded and grown in 90 Pa 3D fibrin gels for 10 days then samples are collected and Raman spectra obtained from a 784nm laser in the 150-1800nm and 2500-3500nm ranges. These spectra are analyzed via combinations of data preprocessing, wavelength or dimension reduction and machine learning classification algorithms. We extend to other cell lines such as CFPAC-1 and PANC-1. Support vector machine (SVM) and k-nearest neighbors (kNN) supervised machine learning classifiers are applied to the raw and pre-processed data sets and with various statistical and machine learning dimension reduction protocols. These combinations are compared to determine which performed best at classifying cancer and normal cell samples, and which led to selection of the same or similar dimensions. Identification of the best performing dimensions/wavelengths is then attempted from the Raman spectra by comparing them to existing biological molecule Raman databases to identify the patterns in the spectra and any unique molecular signatures or protein expressions that could prove useful to better understanding and therefore treating pancreatic cancer.

r0pt Figure Isoprene is a highly abundant species in the atmosphere, second only to methane in hydrocarbon emissions. There are two stable conformers of isoprene, trans and gauche, and due to the fact the ground state trans is highly abundant at room temperature (97%), its microwave spectrum has been known since the 1960’s. The gauche conformer, however, has evaded microwave observation until now. We have characterized the rotational parameters for the inversion states (0⁺/0⁻) of gauche-isoprene, aided by high levels of theory and two complementary, high resolution Fourier transform microwave (FTMW) techniques: cavity enhanced FTMW coupled to a pulsed discharge nozzle, and chirped-pulse FTMW in a cryogenic buffer gas cell. Thermal enhancement (from 1.7% up to 10.3%) of the gauche population of isoprene is demonstrated with inlet temperatures ranging from 300-450 K in the cryogenic buffer gas cell. This work demonstrates for the first time that the buffer gas cell is well suited for thermochemical studies similar to matrix isolation spectroscopy, but by far simpler and more rapid analytical means.