Hernan J. Cortes

Developments in multidimensional separation systems

Coupled-column systems in a multidimensional mode are increasingly used in order to obtain greater selectivity and sensitivity for the determination of trace components in complex matrices, and to increase the information content of an analysis in the characterization of complex samples. A review of the various strategies used in chromatography to couple orthogonal separation stages is presented, with emphasis on instrumental design and the role of miniaturization.

Determination of trace chlorinated benzenes in fuel oil by on-line multidimensional chromatography using packed-capillary liquid chromatography and capillary gas chromatography

Abstract The application of a multidimensional chromatography system with a packed-capillary liquid chromatographic (LC) column, coupled on-line with a capillary gas chromatograph, for the determination of trace chlorinated benzenes in fuel oil is described. The liquid chromatograph is utilized as a highly efficient clean-up step prior to introduction of the sample into the gas chromatograph. This significantly reduces sample preparation times and demonstrates that an on-line high-resolution LC—gas chromatography (GC) system is superior to either LC or GC alone, since the analyses described could not be accomplished by either of the two techniques used independently. The detection limits obtained ranged from 8.0 to 17 μg/g for the various chlorinated benzenes.

Multi-dimensional chromatography using on-line coupled microcolumn liquid chromatography-capillary gas chromatography for quantitative pesticide residue analysis.

Highly selective separations can be achieved by utilizing different separation modes in a multi-dimensional (on-line coupled) chromatographic system. The application of such a system utilizing microcolumn liquid chromatography coupled on-line to capillary gas chromatography for the determination of 2-(1,1-dimethylethyl)-5-pyrimidinol, a pesticide metabolite, in various corn matrices at the ng g−1 level is described. A comparative quantitative study with conventional manual sample pretreatments followed by capillary gas chromatography-mass spectrometry indicated equivalence between the two techniques. The system offers the advantages of reduced sample handling steps and analysis time, high potential for automation and adequate sensitivity.

Multidimensional gas chromatography with capillary flow technology and LTM-GC

2-D GC is a logical and cost effective extension to 1-D GC for improving the separation resolution, selectivity, and peak capacity of an analytical system. The advent of electronic pressure control systems that are accurate to the third decimal place, combined with recently innovated chromatographic devices such as capillary flow technology, has eliminated many deficiencies encountered in current conventional 2-D GC by making the technique reliable and simple to implement in both production and research analytical facilities. Low thermal mass GC (LTM-GC) was successfully integrated with capillary flow technology to further enhance overall 2-D GC chromatographic system performance by providing not only faster throughput via rapid heating and cooling, but independent temperature control for each dimension to maximize separation power. As an example, despite the enhanced peak capacity obtained from conventional 2-D GC, alkyl naphthalene isomers such as 2,3-dimethyl and 1,4-dimethyl naphthalene coeluted. These two critical compounds were well resolved (R = 5.2) using 2-D GC with LTM-GC with a similar analytical time. This paper demonstrates the benefits of combining capillary flow technology with LTM-GC to provide major enhancements to conventional 2-D GC. The synergy of these techniques is highlighted with practical industrial applications.

Practical method for the measurement of Alkyl mercaptans in natural gas by multi-dimensional gas chromatography, capillary flow technology, and flame ionization detection

Volatile sulfur compounds such as alkylmercaptans are undesired impurities in natural gas streams. As a result, natural gas treatment and purification services are essential in many industries that utilize natural gas either as a fuel or in a chemical process. While there are many analytical methods that can be employed for the measurement of mercaptans, a simple, practical, and easy-to-implement method is required for remote field deployment. An analytical method, based on multi-dimensional gas chromatography (MDGC), capillary flow technology and flame ionization has been successfully developed for the application described. Results based on the technique showed alkylmercaptans can be accurately measured with a minimum detection limit of 200 ppb (v/v) or better, a linear range of up to 100 ppm (v/v), and a relative standard deviation (n=10) of 1.2% or less were obtained by manual injection with a total sample-to-sample analysis time of less than 15 min.

Determination of chlorpyrifos in water by large-volume direct aqueous injection capillary gas chromatography

Abstract A technique is described for the injection of large-volume aqueous samples in capillary gas chromatography for the direct determination of chlorpyrifos in well water. An uncoated inlet of suitable length coupled to a thin-film methyl phenyl silicone capillary column was employed for the determination of low parts-per 10 9 (ppb) levels of the pesticide in water. Samples were injected without prior clean-up steps by using a 10-port valve, and detection was performed by electron capture. The method yielded high accuracy, precision (4.8% with 95% confidence at 0.9 ppb), and suitable linearity range (0.9–18 ppb). The method presented circumvents the extraction, filtration and centrifugation steps commonly used in the determination of trace organic components in aqueous matrices.

Instability of hexane–acetonitrile mobile phases used for the chromatographic analysis of triacylglycerides

Comprehensive 2-D LC is an emerging separation technique that has seen a rapid increase in applications in the last decade. The technique has been applied for the separation of numerous complex mixtures including triacylglycerides (TAG). Determination of TAG in food products such as rice, palm, and canola oils have been previously described and the technique of choice utilizes a silver-modified silica column with hexane-ACN as the mobile phase. Repeated retention time inconsistencies were experienced in our studies when this mobile phase was applied to the separation of natural and synthetic mixtures containing TAG. The present report summarizes a study performed to determine the relative stability of ACN, propionitrile (PCN), and butyronitrile (BCN) at concentrations ranging from 0.43 to 2.8% in hexane and heptane. The data obtained suggest that unless evaporative loss of the mobile phase is prevented, TAG retention time irreproducibility can be significant when using mobile-phase mixtures prepared with ACN or PCN. BCN should be used as the solvent modifier in cases where evaporation cannot be prevented.

Background vapor from six ionic liquids and the partition coefficients and limits of detection for 10 different analytes in those ionic liquids measured using headspace gas chromatography.

The concentration and identity of the compounds detected in the vapor above six ionic liquids by headspace gas chromatography (HS-GC) at 100 degrees C are reported. In addition, the partition coefficients for 10 different compounds in these ionic liquids and limits of detection were measured. These results provide quantitative guidance for the application of ionic liquids for HS-GC.

Dual-purpose gas chromatographic injection device for pressurized liquid and gas injection

A dual-purpose gas chromatographic injection device, capable of injecting pressurized liquid sample of up to 5000 psig and gas sample with a volume as high as 5000 microL, has been successfully developed and implemented. The injection device is synergized by the effectiveness of a classical flash vaporization of a syringe injection and the reliability of a proven rotary valve. Depending on the matrix involved, this injection device employs either a commercially available four-port internal valve for liquid sampling or a six-port external valve for gas sampling, a modified removable needle used in standard liquid syringe, and an auxiliary flow stream that can be either mechanical or electronic flow controlled for solute transfer. For pressurized liquid, the device was found suitable of up to nC(16) hydrocarbon with no observable carry-over despite the injection device was operating at ambient temperature. A relative standard deviation of less than 2% (n=20) was obtained for hydrocarbon compounds ranging from nC(8) to nC(16). For gas injection, the device performed well even under difficult chromatographic conditions such as with a low column inlet pressure of less than 1 psig. A relative standard deviation of less than 0.5% (n=10) was obtained for reactive sulfur compounds such as alkyl mercaptans. The device can be operated manually or automated with pneumatic or electrical actuator, is platform neutral, and can be moved amongst instruments without hardware modification as well as implemented for on-line or in situ applications. In this paper, the utility of the device was also demonstrated with selected GC applications of industrial significance.

The Analysis of Synthetic Organic, Neutral Polymers Using Nonaqueous Capillary Gel Electrophoresis (NACGE)

Abstract We describe, in this paper, efforts to develop an optimized, replaceable, polyethylene oxide [PEO, aka polyethylene glycol (PEG)] gel in nonaqueous CGE (NACGE) for the analysis and characterization [molecular weights (MWs)] of a series of synthetic, organic polymers, mainly polystyrenes (PS) and polymethylmethacrylates (PMMA). The approach utilizes a novel combination of organic solvent run buffers, with varying levels of water and a hydrophobic cationic monomer, to induce electrophoretic flow or forces (EPF) in the organic polymers. The gel is fixed in place, using a coated capillary to prevent electroosmotic flow (EOF), so that only the analyte polymers move through the gel with time and applied voltage. Depending on the nature of the organic gel in the capillary and the buffer additives, polymers of known MWs can be made to migrate through the capillary with time, generating a standard MW vs. migration time calibration plot, analogous to what is derived for SEC analysis of the very same polymers. Though the range of this linearity appears rather limited, perhaps because of the nature of the PEO gels available, it is still possible to determine relative MWs for a series of known, standard PS and PMMA polymers, with reasonable accuracy and precision of such measurements.