Kinematic viscosity prediction of jet fuels and alternative blending components via comprehensive two-dimensional gas chromatography, partial least squares, and Yeo-Johnson transformation.

This work presents an accurate yet simplified partial least squares model to predict the kinematic viscosity of conventional and alternative jet fuels at -20°C using comprehensive two-dimensional gas chromatography coupled to a flame ionization detector (GC × GC/FID). Three different normalization methods (mean-centering, logarithmic, and Yeo-Johnson) were evaluated to identify their impact in the prediction of middle distillates' physical properties. Results using Yeo-Johnson transformation exhibited improved viscosity prediction capabilities over the validation set with a mean absolute percentage error of 5.3%, a root-mean-squared error of 0.23, and a coefficient of determination (R2 ) of 0.9404 using only 10 latent variables. Unlike previously reported correlations, this model allowed the identification of specific hydrocarbon groups and carbon numbers that drive jet fuel viscosity at low temperatures. The presence of even small amounts of large branched-alkanes (C15 -C17 ), dicyclic-alkanes (C10 ), and cycloaromatics (C11 ) have the potential to strongly increase the kinematic viscosity of jet fuels. Contrastingly, light monocycloalkanes and branched-alkanes (≤ C10 ) were associated with lower viscosity values. Novelly, this model suggests the implementation of Yeo-Johnson transformations to predict the physical properties of middle distillates to further improve the performance metrics of partial least squares models based on GC data.

Proton Affinities of Alkanes.

Chemical characterization of complex mixtures of large alkanes is critically important for many fields, including petroleomics and the development of renewable transportation fuels. Tandem mass spectrometry is the only analytical method that can be used to characterize such mixtures at the molecular level. Many ionization methods used in mass spectrometry involve proton transfer to the analyte. Unfortunately, very few proton affinity (PA) values are available for alkanes. Indeed, previous research has shown that most protonated alkanes (MH+) are not stable but fragment spontaneously via the elimination of a hydrogen molecule to form [M - H]+ ions. Here, the PAs of several n-alkanes and alkylcyclohexanes containing 5-8 carbon atoms, n-pentane, n-hexane, n-heptane, n-octane, cyclohexane, methylcyclohexane, and ethylcyclohexane, were determined via bracketing experiments by using a linear quadrupole ion trap mass spectrometer. Monitoring the formation of the [M - H]+ ions in reactions between the alkanes and protonated reference bases with known PAs revealed that the PAs of all the alkanes fell into the range 721 ± 20 kJ mol-1. In order to obtain a more accurate estimate of the relative PAs of different alkanes, two alkanes were introduced simultaneously into the ion trap and allowed to react with the same protonated reference base. Based on these experiments, the longer the alkyl chain in an n-alkane or alkylcyclohexane the greater the PA. Further, when considering alkanes with the same number of carbon atoms, the PAs of those with a cyclohexane ring were found to be greater than those with no such ring.

Determination of jet fuel system icing inhibitor by GC×GC-FID.

Jet fuel usually contains a small amount of dissolved water, which can separate out at high altitudes and low temperatures. This can bring along serious clog issues as water can freeze in fuel pumps, lines, or filters; blocking the fuel flow which can even cause engine shut down. To prevent such a disaster, an additive called fuel system icing inhibitor (FSII) is added to jet fuels. The amount of FSII is regulated in both civil and military jet fuels by pertinent standards. A method for quantification of FSII: diethylene glycol monomethyl ether (DiEGME) by comprehensive two-dimensional gas chromatography with flame ionization detector (GC × GC-FID) was developed. The method allows the determination of DiEGME from a very small quantity of samples (0.5 µL) and is very fast with a mean absolute error of 0.001 vol% and a correlation coefficient of 0.9997. The DiEGME content (in the range of 0.07-0.12 vol%) in 23 fuel samples was analyzed via GC × GC-FID. The accuracy of the proposed method was evaluated by the ASTM standard D5006. The procedure that utilizes a refractometer, outlined in D5006, is currently the only available standard for determining the DiEGME concentration in fuel. Results were within the repeatability of the D5006 method (0.009 vol%). Since the D5006 method is accepted as an accurate technique for DiEGME content determination, the GC × GC method proposed in this study can be considered precise and accurate.

Identification and Quantitation of Linear Alkanes in Lubricant Base Oils by Using GC×GC/EI TOF Mass Spectrometry.

Linear alkanes are a class of compounds known to negatively affect the physical performance of lubricant base oils. The ability to rapidly identify and quantify linear alkanes in lubricant base oils would enable oil companies to more effectively evaluate their refinery methods for converting crude oil to lubricant base oils. While mass spectrometry is a powerful method for elucidation of the structures of compounds in complex mixtures, it is not innately quantitative. An approach is presented here for the identification and quantitation of linear alkanes in base oil samples by utilizing GC×GC/EI TOF MS. Identification of the linear alkanes in base oils was achieved based on their retention times in both GC columns as well as their EI mass spectra. Linear alkane model compound mixtures were used to generate calibration plots for quantitation of the linear alkanes in the base oils. The accuracy of this method was greater than 83.8%, within-day precision lower than 6.2%, between-day precision lower than 16.2%, and total precision lower than 17.2%. All noted figures of merit surpass the acceptable limits for a new validated quantitative method, where accuracy must be better than 80% and precision lower than 20% at the lower limit of quantitation. The n-alkane content in both base oil samples was further validated using a GC×GC/FID method (the gold standard for quantitation), which provided nearly identical results to those obtained using the GC×GC/EI TOF MS method. Therefore, GC×GC/EI TOF MS can be used to both identify and quantitate linear alkanes.

Differentiation of Deprotonated Acyl-, N-, and O-Glucuronide Drug Metabolites by Using Tandem Mass Spectrometry Based on Gas-Phase Ion-Molecule Reactions Followed by Collision-Activated Dissociation.

Glucuronidation, a common phase II biotransformation reaction, is one of the major in vitro and in vivo metabolism pathways of xenobiotics. In this process, glucuronic acid is conjugated to a drug or a drug metabolite via a carboxylic acid, a hydroxy, or an amino group to form acyl-, O-, and/or N-glucuronide metabolites, respectively. This process is traditionally thought to be a detoxification pathway. However, some acyl-glucuronides react with biomolecules in vivo, which may result in immune-mediated idiosyncratic drug toxicity (IDT). In order to avoid this, one may attempt in early drug discovery to modify the lead compounds in such a manner that they then have a lower probability of forming reactive acyl-glucuronide metabolites. Because most drugs or drug candidates bear multiple functionalities, e.g., hydroxy, amino, and carboxylic acid groups, glucuronidation can occur at any of those. However, differentiation of isomeric acyl-, N-, and O-glucuronide derivatives of drugs is challenging. In this study, gas-phase ion-molecule reactions between deprotonated glucuronide metabolites and BF3 followed by collision-activated dissociation (CAD) in a linear quadrupole ion trap mass spectrometer were demonstrated to enable the differentiation of acyl-, N-, and O-glucuronides. Only deprotonated N-glucuronides and deprotonated, migrated acyl-glucuronides form the two diagnostic product ions: a BF3 adduct that has lost two HF molecules, [M - H + BF3 - 2HF]-, and an adduct formed with two BF3 molecules that has lost three HF molecules, [M - H + 2BF3 - 3HF]-. These product ions were not observed for deprotonated O-glucuronides and unmigrated, deprotonated acyl-glucuronides. Upon CAD of the [M - H + 2BF3 - 3HF]- product ion, a diagnostic fragment ion is formed via the loss of 2-fluoro-1,3,2-dioxaborale (MW of 88 Da) only in the case of deprotonated, migrated acyl-glucuronides. Therefore, this method can be used to unambiguously differentiate acyl-, N-, and O-glucuronides. Further, coupling this methodology with HPLC enables the differentiation of unmigrated 1-ß-acyl-glucuronides from the isomeric acyl-glucuronides formed upon acyl migration. Quantum chemical calculations at the M06-2X/6-311++G(d,p) level of theory were employed to probe the mechanisms of the reactions of interest.

Middle distillates hydrogen content via GC×GC-FID.

Liquid transportation fuels in the middle distillate range contain thousands of hydrocarbons making the predictions and calculations of properties from composition a challenging process. We present a new approach of hydrogen content determination by comprehensive two-dimensional gas chromatography with flame ionization detector (GC×GC-FID) using a weighted average method. GC×GC-FID hydrogen determination precision was excellent (0.005 wt% repeatability). The method accuracy was evaluated by high-resolution nuclear magnetic resonance (NMR) technique, which is non-biased, measures the H signal directly and was independently validated by controls in the current study. The hydrogen content (in the range of 12.72-15.54 wt%) in 28 fuel samples were determined using GC×GC-FID. Results were within ±â€¯2% of those obtained via NMR. Owing to the fact that NMR is accepted as an accurate technique for hydrogen content determination, the GC×GC method proposed in this study can be considered precise and accurate.