One-dimensional and comprehensive two-dimensional gas chromatography coupled to soft photo ionization time-of-flight mass spectrometry: a two- and three-dimensional separation approach

Soft laser photo-ionization mass spectrometry is presented as a separation dimension hyphenated with gas chromatographic techniques. Single photon ionization (SPI) is a universal soft ionization method which ionizes organic molecules with an ionization potential below 10.5 eV if 118 nm laser radiation is used. The inherently soft ionization of photo ionization techniques can further be utilized together with gas chromatography as a comprehensive two-dimensional separation method (GC x MS), using the GC retention time as first separation dimension and the molecular mass as second separation dimension. Some GC x MS chromatograms of diesel petroleum samples using SPI are presented and discussed. Finally, it is demonstrated that the coupling of soft SPI mass spectrometry with comprehensive two-dimensional gas chromatography (GC x GC) provides a three-dimensional separation technique (GC x GC x SPI-MS).     

Characterization of silicones by direct-deposition GC/FT-IR

Materials used in the manufacture of silicone breast prostheses were analyzed by supercritical fluid chromatography (SFC) and high temperature gas chromatography (GC) coupled with direct deposition FT-IR detection. The chromatographic resolution and deposition quality were found to be unsatisfactory for SFC/FT-IR; however, high temperature GC/FT-IR produced excellent results. The chromatograms and spectra obtained allowed for the identification of poly-dimethyl silicones and mono-, di-, and tri-diphenylsubstituted poly-dimethyl silicones. An automated method of baseline correction was developed that allowed functional group chromatograms due to molecules containing Si-phenyl bonds to be constructed without significant interference from Si-methyl bonds despite the fact that the Si-C stretching mode of Si-phenyl bands overlapped significantly with the strong bands due to Si-CH₃ groups.     

Analysis of pyrethrins using capillary supercritical fluid chromatography and capillary gas chromatography with fourier transform infrared detection

The six insecticidally active components in a commercial preparation of pyrethrin extract are separated by capillary gas chromatography (GC) and by capillary supercritical fluid chromatography (SFC). Thermal degradation of pyrethrin I and II are observed under the GC conditions required to separate the pyrethrin components. The use of shorter columns and thinner stationary phase coatings reduce the amount of degradation but cannot eliminate degradation of pyrethrin II. The SFC chromatograms, obtained under thermally mild conditions, show that all components including pyrethrin II elute without degradation. Infrared spectra of cinerin I & II, jasmolin I & II, and pyrethrin I & II are obtained using a flow through SFC/IR detection cell. Spectra clearly reveal the structural differences needed to distinguish and identify the components in the extract.     

Analysis of long-chain polyprenols using supercritical fluid chromatography and matrix-assisted laser desorption ionization time-of-flight mass spectrometry

The separation of long-chain polyprenols was successfully achieved using supercritical fluid chromatography (SFC). Each 100-mer greater component was separated using tetrahydrofuran as a mobile phase modifier. The molecular mass distributions derived from SFC analyses agreed with the results of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analyses. The number-average molecular mass calculated by MALDI-TOF-MS data were also in accord with the results of quantitative 1H-NMR analysis of terminal groups. A combination of SFC and MALDI-TOF-MS analyses is a powerful tool for the elucidation of the complicated structures of natural polyprenols.     

GC/MS analysis of chlorinated paraffins with negative ion chemical ionization

Although chlorinated paraffins (CP) are produced in large amounts (300 000 tonnes per year), little is known about their occurrence in the environment due to the lack of specific and sensitive analytical methods. The present paper describes the GC/MS analysis of different CP's using capillary gas chromatography with on-column injection and negative ion chemical ionization (NCI) mass spectrometry. Chromatographic resolution of groups of isomers and homologues was obtained. The chromatograms and mass spectra are discussed. The suitability of this method for trace analysis of a CP sample using multiple ion detection (MID) is described.     

Determination of molecular mass distribution of silicone oils by supercritical fluid chromatography, matrix-assisted laser desorption ionization time-of-flight mass spectrometry and their off-line combination

Silicone oil samples were characterized by supercritical fluid chromatography (SFC), matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI--TOF MS), and their off-line combination. SFC was used to separate samples of silicone oils on micropacked capillary columns. The fractions for the identification studies were obtained from SFC runs at defined time intervals, when the restrictor was pulled out from the chromatographic flame ionization detector (FID) and inserted into a glass vial with acetone. MALDI--TOF MS was used for the identification of individual oligomers in the fractions separated. The molecular mass distributions determined based on SFC and MALDI--TOF MS measurements were compared. From this comparison, it follows that the results are in good agreement. However, certain differences were observed: MALDI--TOF MS was capable of detecting somewhat larger oligomers than the SFC-FID, but the lower molecular mass oligomers were not present in the MALDI spectra. Differences in the region of lower molecular masses can be explained by evaporation of the more volatile low molecular mass oligomers resulting from heating of the sample during the MALDI--TOF MS measurements as a result of the absorption of the laser shot energy. The fact that no high mass discrimination effects of the MALDI--TOF MS measurements, compared with SFC, were observed is very promising for further applications of MALDI--TOF MS in characterizing synthetic polymers of moderate polydispersity.     

Comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry peak sorting algorithm

We report a novel peak sorting method for the two-dimensional gas chromatography/time-of-flight mass spectrometry (GC x GC/TOF-MS) system. The objective of peak sorting is to recognize peaks from the same metabolite occurring in different samples from thousands of peaks detected in the analytical procedure. The developed algorithm is based on the fact that the chromatographic peaks for a given analyte have similar retention times in all of the chromatograms. Raw instrument data are first processed by ChromaTOF (Leco) software to provide the peak tables. Our algorithm achieves peak sorting by utilizing the first- and second-dimension retention times in the peak tables and the mass spectra generated during the process of electron impact ionization. The algorithm searches the peak tables for the peaks generated by the same type of metabolite using several search criteria. Our software also includes options to eliminate non-target peaks from the sorting results, e.g., peaks of contaminants. The developed software package has been tested using a mixture of standard metabolites and another mixture of standard metabolites spiked into human serum. Manual validation demonstrates high accuracy of peak sorting with this algorithm.     

Comparison by capillary SFC and SFC-MS of soxhlet and supercritical fluid extraction of hamster feces

Components of hamster feces ranging from low molecular weight fatty acids through the expected range of triglycerides have been eluted in a single SFC run with simultaneous pressure and temperature programming. Temperature programming from 140°C to 240°C was required to provide optimum conditions for separation of the fatty acids and to move the elution region of the sterol esters away from that of the triglycerides. Data from chemical ionization and electron impact mass spectrometry of compounds separated by SFC were used to confirm identities suggested by retention measurements and to provide tentative identities of unknown compounds. SFC with flame ionization detection was used to compare Soxhlet extraction, off-line supercritical fluid extraction (SFE), and on-line SFE of the feces. Although samples obtained by Soxhlet extraction and SFE produced very similar chromatograms, SFE required far less time and consumed much smaller quantities of organic solvent.     

Supercritical fluid chromatography hyphenated with twin comprehensive two-dimensional gas chromatography for ultimate analysis of middle distillates

This paper reports the conditions of online hyphenation of supercritical fluid chromatography (SFC) with twin comprehensive two-dimensional gas chromatography (twin-GC × GC) for detailed characterization of middle distillates; this is essential for a better understanding of reactions involved in refining processes. In this configuration, saturated and unsaturated compounds that have been fractionated by SFC are transferred on two different GC × GC columns sets (twin-GC × GC) placed in the same GC oven. Cryogenic focusing is used for transfer of fractions into the first dimension columns before simultaneous GC × GC analysis of both saturated and unsaturated fractions. The benefits of SFC–twin-GC × GC are demonstrated for the extended alkane, iso-alkane, alkene, naphthenes and aromatics analysis (so-called PIONA analysis) of diesel samples which can be achieved in one single injection. For that purpose, saturated and unsaturated compounds have been separated by SFC using a silver loaded silica column prior to GC × GC analysis. Alkenes and naphthenes are quantitatively recovered in the unsaturated and saturated fractions, respectively, allowing their identification in various diesel samples. Thus, resolution between each class of compounds is significantly improved compared to a single GC × GC run, and for the first time, an extended PIONA analysis of diesel samples is presented.     

气相色谱-表面电离检测器分析汽油中含氮化合物的分布

采用液液萃取的方法分别从90#、93#、97#汽油中提取了含氮化合物,并将气相色谱(GC)和作者所在研究组研制的表面电离检测器(SID)联用对含氮化合物进行了分析。结合GC-氢火焰离子化检测器(FID)、GC-氮磷检测器(NPD)和GC-质谱(MS)的分析结果,可鉴定出GC-SID谱图中的峰基本为含氮化合物,且大部分为NPD和FID未检出的峰,说明SID的选择性和灵敏度更好。分析结果表明,这3种汽油含氮化合物种类相似,含量有所差异;所提取的含氮化合物主要是苯胺类化合物;SID能从汽油样品中检出多种痕量的高沸点含氮组分,对于检测含氮组分而言,SID具有优于商品NPD的灵敏度和选择性。SID为GC分析提供了一种性能优异的选择性检测器。     

Identification of chlorinated fatty acids in fish by gas chromatography/mass spectrometry with negative ion chemical ionization of pentafluorobenzyl esters

This paper reports the development of a technique for identifying and confirming chlorinated fatty acids previously detected in fish by gas chromatography (GC) with halogen-specific detection (XSD). Fatty acid methyl esters (FAMEs) including chlorinated FAMEs within fractions of reversed-phase high-performance liquid chromatography of transesterified fish extracts were derivatized to pentafluorobenzyl esters, which were subjected to GC/mass spectrometry (MS) with negative ion chemical ionization (NICI). Pentafluorobenzyl esters displayed reasonably good GC characteristics, a very high ionization efficiency and a low degree of fragmentation. Chloride ion chromatograms extracted at m/z 35 and 37 from full scans were utilized for locating traces of chlorinated unknowns in the GC elution profile so that their mass spectra could be readily displayed. Significant ions displayed in the mass spectrum scanned in a narrow range of retention time where a chlorinated unknown was located were evaluated using ion chromatograms extracted at the m/z of these ions. The chromatographic peaks of those ions derived from the analyte were expected to center at that specific retention time, whereas those originating from matrix compounds were not. The isotopic patterns of chlorinated ions were also examined against their theoretical relative abundances. Using this approach, three metabolism-related dichloro fatty acids previously identified by GC/XSD in filet extracts of white sucker sampled downstream from a bleached kraft pulp mill were confirmed: dichlorooctadecanoic, dichlorohexadecanoic and dichlorotetradecanoic acids. In addition, an isomer of dichlorotetradecanoic acid was found in a reference fish sample. As sample preparation is critical in this application, improved conditions for hydrolysis and pentafluorobenzyl esterification are also discussed.     

Open tubular column supercritical fluid chromatography/mass spectrometry on a benchtop mass spectrometer

A benchtop SFC/MS system is described which utilizes supercritical carbon dioxide in a 50 micron diameter open tubular column interfaced directly to an unchanged commercially available benchtop capillary GC/MS system equipped with a chemical ionization (CI) source. A small amount of methane reagent gas was admitted co-axially to a capillary restrictor at the exit of the capillary chromatographic column. This make-up gas served as the CI reagent gas and appeared to optimize the sensitivity of the system while providing abundant (M+1) ions for the analytes investigated in this study. Good chromatographic intergrity was obtained for the GC/MS test compound, decafluorotriphenylphosphine (DFTPP), but the capillary restrictor appeared to cause some tailing of the ion current profiles resulting from low nanogram levels of caffeine and some fatty acid esters. Improvements in the SFC/MS capillary restrictor interface and the pumping system of the benchtop GC/MS system should increase the capability of this system for future applications.     

Determination of impurities in illicit methamphetamine seized in Korea and Japan

Impurities produced during synthesis of methamphetamine (MA) show different patterns under various synthetic conditions. Valuable information on the origins and smuggling routes can be obtained by using impurities as chemical fingerprints. We have detected more than 100 compounds from 436 MA samples seized in Korea by gas chromatography-flame ionization detector and gas chromatography-mass spectrometer, among which 31 impurities and three additives were identified. Twenty-six impurity peaks including unknowns were selected as the indicators of similarity, and were used as variables for cluster analysis. Cluster analysis result showed that part of the MA samples seized in Japan might have the same origin as those seized in Korea. It means that broad-based cooperation is necessary for efficient regulation of MA. Synthetic trends of the MA seizures of Korea were monitored by cluster analysis with 16 MA samples synthesized by three different methods in the previous work. We could find comparable changes of synthetic trends, which might have been influenced by domestic regulations and international situations.     

Trace level detection and identification of chemicals related to the chemical weapons convention from complex organic samples

A solid-phase extraction (SPE) method involving selective usage of solvents has been developed for trace level identification of chemical warfare agents (CWAs) present in a complex organic background. The total ion chromatograms obtained from direct gas chromatography-electron ionization mass spectrometric analysis of samples spiked with CWAs in the presence of diesel are very complex and dominated by hydrocarbon peaks and the same after treatment with SPE show distinct peaks corresponding to spiked chemicals. The recovery of samples from SPE is found to be 70-85% at the 10 ppm level.     

A rapid HPLC method for the determination of carboxylic acids in human urine using a monolithic column

A rapid HPLC method for the determination of carboxylic acids in urine samples using a Chromolith Performance RP/18e 100/4.6 with Chromolith Guard Cartridge RP/18e 10/4.6 (Merck KgaA, Darmstadt, Germany) was developed. The method facilitates the simultaneous determination of aromatic hydrocarbon metabolites mandelic acid (MA) and phenylglyoxylic acid (PGA) from styrene and ethylbenzene, hippuric acid (HA) from toluene and 2-, 3-, 4-methylhippuric acids (MHA) from xylene. 3-Hydroxybenzoic acid (3-HBA) was used as internal standard. A chromatographic run is completed within less than 5 min for styrene, ethylbenzene and toluene metabolites, and within 10 min for xylene metabolites. The detection limits are 9 mg L^–1 urine for MA, 1.25 mg L^–1 urine for PGA, 4.9 mg L^–1 urine for HA, 22 mg L^–1 urine for 2-MHA, and 18.5 mg L^–1 urine for 3-MHA.No significant differences of the MA, PGA and HA concentrations in human urine samples obtained by HPLC chromatography on LiChrosorb RP 18 and on Chromolith RP/18e columns were found. The results were evaluated by using ANOVA.Abbreviations MA mandelic acid - PGA phenylglyoxylic acid - HA hippuric acid - MHA methylhippuric acid - 3-HBA 3-hydroxybenzoic acid - ANOVA analyses of variance - GC gas chromatography - HPLC high-performance liquid chromatography     

Quantification of naphthalenes in jet fuel with GC x GC/Tri-PLS and windowed rank minimization retention time alignment

Comprehensive, two-dimensional gas chromatography (GC x GC) is used in conjunction with trilinear partial least squares (Tri-PLS) to quantify the percent weight of naphthalenes (two-ring aromatic compounds) in jet fuel samples. The increased peak capacity and selectivity of GC x GC makes the technique attractive for the rapid, and possibly less tedious analysis of jet fuel. The analysis of complex mixtures by GC x GC is further enhanced through the use of chemometric techniques, including those designed for use on 2-D data such as Tri-PLS. Unfortunately, retention time variation, unless corrected, can be an impediment to chemometric analysis. Previous work has demonstrated that the effects of retention time variation can be mitigated in sub-regions of GC x GC chromatograms through the application of an objective retention time alignment algorithm based on rank minimization. Building upon this previous work, it is demonstrated here that the effects of retention time variation can be mitigated throughout an entire GC x GC chromatogram with an objective retention time alignment algorithm based on windowed rank minimization alignment. A significant decrease in calibration error is observed when the algorithm is applied to chromatograms prior to construction of Tri-PLS models. Fourteen jet fuel samples with known weight percentages of naphthalenes (ASTM D1840) were obtained. Each sample was subjected to five replicate five-minute GC x GC separations over a period of two days. A subset of nine samples spanning the range of weight percentages of naphthalenes was chosen as a calibration set and Tri-PLS calibration models were subsequently developed in order to predict the naphthalene content of the samples from the GC x GC chromatograms of the remaining five samples. Calibration models constructed from GC x GC chromatograms that were retention time corrected are shown to exhibit a root mean square error of prediction of roughly half that of calibration models constructed from uncorrected chromatograms. The error of prediction is lowered further to a value that nearly matches the uncertainty in the standard percent weight values (ca. 1% of the median percent volume value) when the aligned chromatograms are truncated to include only regions of the chromatogram populated by naphthalenes and compounds of similar polarity and boiling point.     

Application of Comprehensive Two‐Dimensional Gas Chromatography (GC×GC) to the Qualitative Analysis of Essential Oils

This paper investigates the separation of moderately complex samples by comprehensive two‐dimensional gas chromatography (GC×GC). The analysis of peppermint (Mentha piperita) and spearmint (Mentha spicata) essential oil components, including acetates, alcohols, furans, ketones, sesquiterpenes, and terpenes, was achieved by one‐dimensional gas chromatography with quadrupole mass spectrometry detection (GC/MSD) and GC×GC with flame ionization detection. Peppermint essential oil was found to contain 89 identifiable peaks by GC×GC compared to 30 peaks in the GC/MSD chromatogram. Likewise, 68 peaks were found in the GC×GC chromatogram of spearmint (compared to 28 in GC/MSD). Plots of the first dimension versus second dimension retention times provided a fingerprint of the two essential oils, which revealed 52 similar compounds between the two essential oils as opposed to 18 matches by 1D GC.     

Determination of a polymeric hindered amine light stabilizer in polypropylene formulated with magnesium hydroxide flame retardant by reactive thermal desorption-gas chromatography.

A polymeric hindered amine light stabilizer (HALS), Tinuvin 622 (MW [symbol: see text] 4000), in PP materials formulated with a magnesium hydroxide flame retardant was determined by reactive thermal desorption (RTD) gas chromatography (GC). Two kinds of the HALS components that were formed through the RTD in the presence of tetramethylammonium hydroxide [(CH3)4NOH, TMAH] were clearly observed in the chromatograms of the PP samples, with negligible interference from the other additives and the PP substrate. Here, the coexisting flame retardant was proved to affect significantly the RTD process of the occluded HALS. As a result, the recovery of the HALS components in the RTD-GC chromatograms of the PP samples increased with increase in the content of the flame retardant. This enhancement of the HALS recovery is attributed mainly to the preferential exposure of the HALS on the surface of the ground PP sample through the interaction between the polymeric HALS and the flame retardant in the molten PP during kneading. In spite of such a considerable action of the flame retardant, the observed intensities of the characteristic peaks of HALS by RTD-GC showed a good linear relationship with the HALS content in the PP samples with constant content of the flame retardant (50 phr); this relationship could be used as the calibration line for the determination of the polymeric HALS in the PP materials containing the flame retardant.     

Shifting of gas chromatographic retention times due to solvent effects — a study using sulfur chemiluminescence detection

Gas chromatrography (GC) with sulfur chemiluminescence detection (SCD) is an outstanding combination for selectively determining trace concentrations of sulfur compounds in hydrocarbon samples. GC peaks can be identified by retention times when reproducible, automated injection techniques are used. However, as described in this work, analysts should be on the look out for retention shifting due to solvent effects from sample components. Three examples of retention shifting are presented: (1) thiophene by benzene; (2) methylthiophenes by toluene; and (3) dibenzothiophene by gas oil. Depending on samples and analysis conditions, retention shifts from a few hundredths to 1.5 min have been observed. Such retention shifts are likely to cause errors in peak identifications. Therefore, when using SCD, simultaneous FID monitoring is recommended as an aid in evaluating chromatograms for possible retention shifting due to solvent effects.     

Supercritical fluid chromatography with redox chemiluminescence detection

Summary The new selective and sensitive redox chemiluminescence detector (RCD), originally developed for gas chromatography, has been successfully interfaced with a capillary column supercritical fluid chromatograph. Interfacing the SFC-RCD was accomplished simply through utilization of the redox reaction chamber presently used for both capillary and packed column GC. The feasibility of this new separation-detection scheme was demonstrated with SFC chromatograms of antioxidant isomers and some compounds of biological relevance. Mobile phase compatibility and signal quenching are also discussed.

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Chromatographie mit überkritischen fluiden phasen unter Verwendung der Redox-Chemiluminescenz-Detektion