Qualitative supercritical fluid extraction coupled to capillary gas chromatography

The usefulness and ease of utilizing supercritical fluid extraction (SFE) directly coupled to capillary gas chromatography (GC) as quantitative or qualitative analytical problem-solving tools will be demonstrated. As an alternative to conventional liquid solvent extractions, SFE presents itself as a means to achieve high extraction efficiencies of different compounds in complex solid matrices in very rapid tims frames. Moreover, SFE has an additional advantage of being able to achieve distinct extraction selectivities as a function of the solubilizing power of the supercritical fluid extracting phase. For on-line SFE/GC, the extraction effluent is directly transferred to the analytical chromatograph. On-line SFE/GC involves the decompression of pressurized extraction effluent directly into a heated, unmodified capillary split injection port of the GC. In this respect, SFE introduction into GC can be used as an alternative means of GC injection, comparable to such modes of injection as pyrolysis and thermal desorption. This paper will show applications of SFE/GC where mass spectrometric detection together with flame ionization detection was used for component identification from environmental, tobacco, and petroleum matrices.     

Measurement of retention in comprehensive two-dimensional gas chromatography using flow modulation with methane dopant

Flow modulation of methane-doped carrier gas is used to visualize the second dimension hold-up time in GC × GC continuously throughout the run. This provides an internal reference of hold-up time and presents a straightforward means of examining retention in each dimension of GC × GC. Retention factors on similar and dissimilar column pairs are examined. Stationary phase bleed is shown to be retained by the second dimension column.     

Comprehensive two-dimensional separations based on capillary high-performance liquid chromatography and microchip electrophoresis

A novel comprehensive two-dimensional (2-D) separation system coupling capillary high-performance liquid chromatography (cHPLC) with microchip electrophoresis (chip CE) is demonstrated. Reversed-phase cHPLC was used as the first dimension, and chip CE acted as the second dimension to perform fast sample transfers and separations. A valve-free gating interface was devised simply by inserting the outlet-end of LC column into the cross-channel on a specially designed chip. A home-made confocal laser-induced fluorescence detector was used to perform on-chip high-sensitive detection. The cHPLC effluents were continuously delivered to the chip and pinched injections of the effluents every 20 seconds were employed for chip CE separation. Gradient elution of cHPLC was carried out to obtain the high-efficiency separation. Free-zone electrophoresis was performed with triethylamine buffer to achieve high-speed separation and prevent sample adsorption. Such a simple-made comprehensive system was proved to be effective. The relative standard deviations for migration time and peak height of rhodamine B in 150 sample transfers were 3.2% and 9.8%, respectively. Peptides of the fluorescein isothiocyanate (FITC)-labeled tryptic digests of bovine serum albumin were fairly resolved and detected with this comprehensive 2-D system.     

Comprehensive two-dimensional capillary supercritical fluid chromatography in stop-flow mode with synchronized pressure programming

A comprehensive two-dimensional capillary supercritical fluid chromatography method was developed. The interface consisted of a ten-port valve, a capillary trap and two fused silica restrictors. The primary column was operated in stop-flow mode: the flow in the primary column was stopped during the separation of the second dimension. The pressure of the system was controlled with a single pump. The pressure program was synchronized with the sampling: the pressure was only ramped up during the sampling time, when the primary column effluent was transferred from the first dimension to the trap, and was maintained constant during the second-dimension separation. All of the operations were automated using in-house software. The separation characteristics of the present system can be readily regulated by changing the size of the restrictors and/or the programmed pressure rate. The use of synchronized pressure programming allowed the sampling duration and/or the second-dimension separation time (and therefore, the total analysis time) to be changed without affecting the separation pattern. Widely different selectivities were attained depending on the combination of the three columns with different polarities (such as the nonpolar DB-1, the medium-polarity DB-17 and the polar DB-WAX columns) used. The present system afforded improved separation and identification capabilities for analytes in complex mixtures.     

Computer-assisted optimization of temperature programming and flow rate in capillary gas chromatography

A computer-assisted method is presented for simultaneous optimization of three-factor of temperature programming (initial temperature, temperature rising rate and carrier gas flow rate) for the separation of eleven compounds in capillary gas chromatography. The optimization of the separation over the experimental region is based on a special polynomial from fifteen preliminary experiments using the resolution as the selection criterion. Computer scanning technique was used for optimum selection on three dimensions. Excellent agreement was found between the predicted data and the experimental results.     

Electroosmotic flow modulation in capillary electrophoresis by organic cations from ionic liquids

This paper describes the ability of several ionic liquids cations for electroosmotic flow modulation in capillary electrophoresis. Organic salts based on phosphonium, sulfonium, cysteinium, ammonium, and guanidinium cations were selected to study this property. In addition, the synergistic effect of these compounds in cyclodextrin chiral separation was also evaluated. In comparison with most studied imidazolium-based ionic liquids, several of the cations studied, are stronger modifiers in terms of electroosmotic flow (EOF) modulation. Phosphonium-based compounds and tri-octyl methylammonium chloride ([Aliquat]Cl) had the strongest ability to reverse EOF both in acidic and in basic conditions and had the lowest EOF reversal concentrations in the presence of hydroxypropyl-β-cyclodextrin. EOF modulation ability of phosphonium cations also contributed to the improvement of chiral separation of DL-propranolol by hydroxypropyl-β-cyclodextrin at lower concentrations in comparison with most commonly used EOF modulators such as tetrabutylammonium phosphate.     

Application of capillary gas chromatography to environmental analysis

Routine analytical methods for benzo[a]pyrene and nitroarenes in airborne particulates are presented. Benzo[a]pyrene in airborne particulates was easily analyzed by a capillary GC/NCIMS. Analysis time was about 10 min and analytical values obtained by this method agreed well with those by HPLC/spectrofluorometry. Nineteen nitroarenes were analyzed by capillary GC/FTD following a simple clean-up procedure. Fifteen out of 19 nitroarenes analyzed were mutagenic for Salmonella typhimurium TA100 and TA98, and 4 nitroarenes were carcinogenic to experimental animals.     

Factors affecting peak shape in comprehensive two-dimensional gas chromatography with non-focusing modulation

In the case of a non-focusing modulator for comprehensive two-dimensional gas chromatography (GC × GC), the systematic distortions introduced when the modulator loads the second-dimension column give rise to a characteristic peak shape. Depending on the operating conditions this systematic distortion can be the dominant component of the second-dimension elution profiles in the GC × GC peak. The present investigation involved a systematic investigation of peak shape in pulsed-flow modulation (PFM)–GC × GC. It is shown that low flow ratio can lead to significant peak skewing and increasing the flow ratio reduces the magnitude of peak skewing. Validation of the peak shape model is made by comparison with experimental data. The residuals from the fitting process (normalised to the maximum detector response) vary between –1.5% and +2.6% for an isothermal model and between –1.0% and +3.0% for a temperature-programmed model.     

Chemometric analysis of comprehensive two-dimensional gas chromatography data using cryogenic modulation

The two-dimensional (2D) data structure generated under a high resolution GC×GC system with a small number of samplings taken across the first dimension is evaluated for the purpose of the application of chemometric deconvolution methods. Chemometric techniques such as generalized rank annihilation method (GRAM) place high demands on the reproducibility of chromatographic experiments. For GRAM to be employed for GC×GC data interpretation, it is critical that the separation method provides data with a bilinear structure; the peak-shape and retention times on both columns must be reproducible. With a limited number of samplings across a ¹D (first dimension) peak (e.g. four to six samplings) repeatability of the pattern of the modulated peaks (controlled by the modulation phase) becomes important in producing a bilinear data structure. Reproducibility of modulation phase can be affected by both reliability of the modulation period and reproducibility of the retention time of the peak on the first column (which arises from oven temperature and carrier flow rate stability). Evaluation of within-run and run-to-run retention time reproducibility (retention time uncertainty) on both columns, and modulation phase reproducibility using a modulated cryogenic system for a pair of overlapping components (fatty acid methyl esters) was undertaken. An investigation of the quality of data to permit quantification of each component by using GRAM deconvolution, was also conducted. Less than 4% run-to-run retention time uncertainty was obtained on column 1 and less than 9% run-to-run and within-run retention time uncertainty was obtained on column 2, where these R.S.D. measures are reported normalised to peak widths on each respective dimension. The R.S.D. of duplicate quantification results by GRAM ranged from 2 to 26% although the average quantification error using GRAM was less than 5%.     

The use of alternative fluids in on-line supercritical fluid extraction – capillary gas chromatography

A key feature differentiating analytical supercritical fluid extraction (SFE) from conventional liquid extraction is the possibility of varying the solvent strength of a supercritical fluid to achieve selective extractions of specific target compounds, or functional classes of compound, from complex matrices. This can be accomplished by using supercritical fluids other than carbon dioxide, for example, sulfur hexafluoride, nitrous oxide, or sulfur hexafluoride-modified carbon dioxide. The use of these fluids will be demonstrated by the characterization of complex environmental and petroleum matrices by directly coupled SFE – capillary GC. On-line SFE-GC involves the decompression of pressurized extraction fluid directly into the heated, unmodified capillary split injection port of the chromatograph. This paper will also show how, by adjustment of the extraction temperature and pressure, SFE selectivity may be further enhanced.     

Automated optimization of split ratio in capillary gas chromatography

An electronic gas-flow controller system called Advanced Flow Control (AFC), which controls not only carrier gas pressure at the column inlet but also the total gas flow including the split flow, was designed and evaluated. BASIC programming of repetitive analyses of standard mixtures under varied split ratios and pressure programs allowed automated optimization of those conditions for the desired column loading and resolution between adjacent peaks.     

Flow of the carrier gas in open-tubular capillary columns in temperature-programmed gas chromatography

A second-order non-linear partial different equation was derived to describe the dependence of carrier gas pressure in the column on the column distance and the time under temperature programmed conditions. This equation was solved numerically by the modified finite difference method for various column parameters. Constant inlet and outlet pressures were used as boundary conditions. The retention times calculated on assumption of a constant pressure profile along the column. Significant differences between retention times of corresponding solutes calculated by the two methods were found, especially when relatively long columns(L>50m) with small internal diameter(d<0.3mm)and high temperature program rates (r>5°/min)are used.     

A flexible loop-type flow modulator for comprehensive two-dimensional gas chromatography

The present investigation is focused on a simple flow modulator (FM), for comprehensive two-dimensional gas chromatography (GC×GC). The interface is stable at high temperatures, and consists of a metallic disc (located inside the GC oven) with seven ports, which are connected to an auxiliary pressure source via two branches, to the first and second dimension, to a waste branch (linked to a needle valve) and to an exchangeable modulation loop (2 ports). The ports are connected via micro-channels, etched on one of the inner surfaces of the disc. Modulation is achieved using a two-way electrovalve, connected on one side to the additional pressure source, and to the two metal branches, on the other. An FM enantio-GC×polar-GC method (using a flame ionization detector) was optimized (a 40-μL loop was employed), for the analysis of essential oils. As an example, an application on spearmint oil is shown; the method herein proposed was subjected to validation. Finally, an FM GC×GC diesel experiment was carried out, using an apolar-polar column combination, to demonstrate the effectiveness of the modulator in the analysis of a totally different sample-type.     

Element-selective detection using capillary gas chromatography with atomic emission detection

Capillary GC coupled to an atomic emission detector (AED) provides a powerful new hyphenated technique for the separation and characterization of complex mixtures and compounds. The AED provides simultaneous and truly specific multi-element detection. The specificity of detection reduces the need for the complex sample pretreatment procedures which are necessary to reduce the interference from co-eluted substances which is experienced with detectors such as the FID and the ECD. A range of environmentally significant problems has been studied, including PCB analysis, the characterization of the reaction products of a novel waste treatment process, and the profiling of sulfur-containing species formed by the pyrolysis of various types of coal.     

Direct coupling of capillary supercritical fluid chromatography with supercritical fluid extraction using modified carbon dioxide

Capillary supercritical fluid chromatography has been directly coupled with supercritical fluid extraction using modified carbon dioxide. The mixed fluids were prepared with a single pump on-line mixing system. The most important step in the SFE-SFC interface was the elimination of the modifier solvent. This was achieved by use of a coupled trap, 0.1 mm i.d. and 0.53 mm i.d. capillary tubing connected in series, with the collected solutes refocused on the second (0.53 mm i.d.) trap before transfer into the separation column. This enabled complete elimination of various modifier solvents and high efficiency collection of the solutes. The effect of the modifier on trapping efficiency was investigated using methanol, ethanol, dichloromethane, hexane, and toluene at a variety of concentrations. n-Eicosane was, for example, trapped quantitatively by modified carbon dioxide containing up to 13 % (w/w) methanol. The use of the technique has been demonstrated by selective extraction of n-paraffins, fatty acid methyl esters, and alcohols from a silica matrix; the effect of different modifiers on the extraction of a mixture of pesticides from soil has also been investigated.     

Residue analysis of organophosphorus pesticides in food with two-dimensional gas chromatography using capillary columns and flame photometric detection

Two-dimensional gas chromatography with two capillary columns of different polarity (SP 2100 and OV 225) is used for pesticide residue analysis in food samples. By means of “live chromatography” applying pneumatic switching technique according to Deans, unequivocal identification of 57 organophosphorus pesticides in food samples is achieved at trace concentrations. The instrument is equipped with only one injection port and one flame photometric detector. On-line data processing is very helpful, especially in calibration and checking the system's reliability with the multitude of test compounds. The complete pesticide residue analysis including clean-up of about six food samples can be completed by one person in 8 hours.     

Analysis of antiepileptic drugs by capillary gas chromatography with on-column injection

A simple and quick gas chromatographic method has been developed for the determination of up to six commonly used antiepileptic drugs in human serum. The antiepileptics are isolated from serum by solid phase extraction on to a reversed phase sorbent and recovered with ethyl acetate as eluent. The ethyl acetate eluate is suitable for direct on-column injection on to a phenyl methyl siloxane capillary column; hydrogen is used as carrier gas and the compounds are separated with a two-ramp temperature program. Detection is by FID. The reproducibility of the method varies between 2 and 6% RSD, depending on the drug and the level analyzed; limits of detection were found to be 14–51 pg and minimum assayed concentrations in serum were between 14 and 51 ng/ml.