Thermally Sensitive Behavior Explanation for Unusual Orthogonality Observed in Comprehensive Two-Dimensional Gas Chromatography Comprising a Single Ionic Liquid Stationary Phase

Exploring thermal isomerisation in gas chromatography analyses using natural pyrethrins: Comparison of comprehensive two-dimensional and one-dimensional gas chromatography

This study aims to assess and qualitatively compare the visual presentation of chromatographic data from the isomerisation of natural pyrethrins - a group of pesticides derived from Chrysanthemum flowers - using one-dimensional gas chromatography (1DGC) and comprehensive two-dimensional gas chromatography (GC×GC). Molecular structural changes, such as thermal isomerisation in this case, occur during gas chromatography injection and separation, to provide characteristic patterns which may not be routinely recognised on the 1D chromatogram. To demonstrate the influence of analytical method parameters on isomerisation processes, variations in oven temperature (isothermal vs. temperature programmed analysis), inlet mode (split vs. splitless), inlet temperature, and carrier gas flow rate were investigated. Increasing oven temperature was the most significant factor affecting isomerisation. Splitless injection mode and increasing inlet temperature promoted isopyrethrin formation, while the effect of inlet temperature appeared minimal with a split injection technique, most likely due to the short residence time in the inlet. Increased carrier gas flow rates in a temperature programmed analysis reduced retention time and minimised isomerisation. The unique presentation of isopyrethrin peaks on a GC×GC contour plot allows for facile recognition of isomerisation especially at low concentrations, simplifies chromatogram interpretation, and aids in analyte identification. It also confirms that the isomerisation process is irreversible since the pyrethrin I and II compounds are absent throughout the bridge formation. These benefits support the use of GC×GC over 1DGC to study isomerisation. Additionally, due to limited data in the literature, Kováts retention indices and linear retention indices of the natural pyrethrins, including isopyrethrins, were experimentally determined on four columns: DB-5 ms UI, Rxi-17Sil MS, SLB-IL60i, and SLB-IL111i.

Tutorial for the Characterization of Fatty Acid Methyl Esters by Gas Chromatography with Highly Polar Capillary Columns

The fatty acid composition of fats and oils is commonly determined by gas chromatography after preparing fatty acid methyl esters (FAME). Capillary columns coated with polyethylene glycol emerged as the preferred separation tool for the quantification of the polyunsaturated fatty acids contained primarily in marine oils. However, their selectivity is inadequate for measuring the trans fatty acids (TFA) contained in refined vegetable oils, dairy fats, and marine oils. Highly polar 100% poly(biscyanopropyl siloxane) capillary columns provide the necessary selectivity, but small differences in the phase polarity caused by column age, conditioning, or manufacturing variations affect the reproducibility of their separations of these complex samples. In this study, a simple procedure is described to compensate for small variations in column selectivity by adjusting the elution temperature. The balance between the dipole-induced dipole interactions and dispersive interactions was determined by measuring selectivity factors [SF(i)] corresponding to the elution of an unsaturated FAME such as 18:3n-3 relative to two saturated FAME such as 20:0 and 22:0. Knowing the SF(i) provided by the installed capillary column at a given elution temperature, and the SF(i) of the target separation, we propose a simple calculation to determine the necessary elution temperature adjustment to achieve (or restore) the desired separation. After determining the SF(i) which provides the optimal separation of TFA, the novel methodology was applied to the separation of refined vegetable oils, butter fats, and marine oils.

Strategies towards simpler configuration and higher peak capacity with comprehensive multidimensional gas chromatography

Experimental and data analysis approaches in multidimensional gas chromatography (MDGC) comprising comprehensive multiple heart-cut (H/C) and comprehensive two dimensional GC (GC × GC) were developed with an example application illustrated for analysis of a technical glycol precursor sample. The GC × GC system employed a long ¹D (30 m) and a short ²D (5 m) column with a flow modulator and a Deans switch (DS) as a splitter; meanwhile. The H/C system was applied solely as a DS located between long ¹D (30 m) and ²D (60 m) columns without use of cryogenic trapping devices. The effects of injection time and ²D column flow in GC × GC and the impacts of H/C window and number of injections (total analysis time) in H/C analysis were investigated. The analysis performance for each condition was evaluated according to peak capacity and number of separated compounds. The continuum between the two techniques was then established via the relationship between analysis time and analysis performance. The separation performances were improved with longer analysis time so that the suitable condition was selected within this compromise. Under the selected conditions, volatile compounds in the technical glycol precursor sample were identified according to the match between the experimental MS spectra and first dimensional retention indices (¹I) with that from the NIST2014 database and literature. An hour analysis with GC × GC resulted in a total peak capacity of 798, number of separated peaks of 61 and average MS match score of 887 ± 35; meanwhile, the corresponding numbers were improved to be 9198, 107 and 898 ± 24, respectively, with the 25 h comprehensive H/C analysis.

Experimental and data analysis approaches in MDGC comprising comprehensive H/C and GC × GC were developed with an example application illustrated for analysis of a technical glycol precursor sample.

Cryogen-free comprehensive heartcut multidimensional gas chromatography using a Deans switch for improved analysis of petrochemical products derived from palmitic acid oxidation

Comprehensive heartcut multidimensional gas chromatography was applied with example application for analysis of a sample obtained from palmitic acid oxidation in a Rancimat instrument. The system utilized a single Deans switch (DS) located between first dimensional semi-standard nonpolar (30 m) and second dimensional polar (60 m) columns. A cyclic multiple heartcut strategy consisting of 150 heartcuts with a 0.2 min window was applied offering comprehensive analysis and injection of a narrow band of compounds onto the second column without use of cryogenic trapping devices. Untargeted compound analysis of the sample prepared by solid phase micro-extraction was performed based on match between the experimental MS spectra and first dimensional retention indices with that from the NIST library. The sample contained the major compounds of 2-octanone, 1-methylcyclohexanol, 2,3,6-trimethylphenol, 3-phenylpropanol and 2-nonanone. This approach was then evaluated based on peak capacity and the number of identified compounds. Compared with one dimensional gas chromatography providing a total peak capacity of 172 and 43 identified compounds, the analysis performance was much more improved with a capacity of 5840 and 235 identified compounds by using comprehensive heartcut multidimensional gas chromatography with the total analysis time of 15.3 h. By comparison within the same set of identified compounds, the one dimensional and multidimensional approaches provided the MS match scores of 769 ± 81 and 836 ± 88, respectively. In addition, the nonlinear relationship between the analysis time and number of identifiable peaks was calculated according to the set of 235 compounds. This revealed that the analysis time could be shortened with the compensation of lower separation performance, where application of a 2.5 min heartcut window with the total analysis time of 1.2 h could result in the total peak capacity of 390 with 150 identifiable compounds.

Ionic liquids as gas chromatographic stationary phases: how can they change food and natural product analyses?

The volatile fraction of natural products often consists of complex mixtures of isomeric and/or homologous components with similar structural and physical characteristics (e.g. mono- and sesquiterpenoids) that are not easy to separate simultaneously with conventional GC stationary phases, even when used with multidimensional systems. The introduction of ionic liquids (ILs) as stationary phases has opened up new perspectives in this field as their unique solvation properties result in uncommon selectivity, which is completely different to that of classic polydimethylsiloxane (PDMS)- and polyethyleneglycol (PEG)-based columns. Because of their peculiar selectivity and high inertness, IL-based columns have already found several applications in the natural product field in mono- and multidimensional GC and preparative GC, leading to the exhaustive analysis of complex samples (including aqueous solutions), and the separation of challenging pair(s) of compounds. This article provides an overview of how IL-based columns can be exploited in the fields of food and natural products, explores the wide range of applications that have already been developed and highlights the main features of these promising stationary phases, which are expected to be further extended in the near future, in particular, for routine use. Graphical abstract.

Comprehensive 2D gas chromatography-time-of-flight mass spectrometry with 2D retention indices for analysis of volatile compounds in frankincense (Boswellia papyrifera)

Frankincense gum resin secreted from Boswellia papyrifera was analysed by comprehensive 2D gas chromatography hyphenated with accurate mass time-of-flight mass spectrometry (GC×GC-accTOFMS). Direct multiple injection experiments with stepwise isothermal temperature programming were then performed to construct isovolatility curves for reference alkane series in GC×GC. This provides access to calculation of second dimensional retention indices (²I). More than 500 peaks were detected and 220 compounds mainly comprising monoterpenes, sesquiterpenes, diterpenes and oxygenated forms of these compounds were identified according to their ¹I, ²I and accurate mass data. The study demonstrates the capability of GC×GC-accTOFMS with retention data on two separate column phases, as an approach for improved component identification. A greater number of identified and/or tentatively identified terpenoids in this traditional Chinese medicine allow for a more comprehensive coverage of the volatile composition of frankincense.

Recent advances on ionic liquid uses in separation techniques

The molten organic salts with melting point below 100°C, commonly called ionic liquids (ILs) have found numerous uses in separation sciences due to their exceptional properties as non molecular solvents, namely, a negligible vapor pressure, a high thermal stability, and unique solvating properties due to polarity and their ionic character of molten salts. Other properties, such as viscosity, boiling point, water solubility, and electrochemical window, are adjustable playing with which anion is associated with which cation. This review focuses on recent development of the uses of ILs in separation techniques actualizing our 2008 article (same authors, J. Chromatogr. A, 1184 (2008) 6-18) focusing on alkyl methylimidazolium salts. These developments include the use of ILs in nuclear waste reprocessing, highly thermally stable ILs that allowed for the introduction of polar gas chromatography capillary columns able to work at temperature never seen before (passing 300°C), the use of ILs in liquid chromatography and capillary electrophoresis, and the introduction of tailor-made ILs for mass spectrometry detection of trace anions at the few femtogram level. The recently introduced deep eutectic solvents are not exactly ILs, they are related enough so that their properties and uses in countercurrent chromatography are presented.

Ionic liquids as stationary phases for fatty acid analysis by gas chromatography

The present paper provides an overview of the application of ionic liquid (IL) columns for GC analysis of fatty acid methyl esters (FAMEs).