Comprehensive Two-dimensional Gas Chromatographic Separations Research Articles

Ionic Liquid Stationary Phase for Improving Comprehensive Two-dimensional Gas Chromatographic Separation of Polychlorinated Naphthalenes

The performance of comprehensive two-dimensional gas chromatography coupled to mass spectrometry (GC×GC-MS) using a column combination of a non-polar stationary phase (DB-5MS) and an ionic liquid stationary phase (SLB-IL60) in the first- and the second dimension has been evaluated for the suitable separation of polychlorinated naphthalenes (PCNs). The optimization of the GC×GC-MS method was carried out using different oven temperature programs and modulation conditions, achieving the best results using a ramp temperature rate of 0.75 °C min−1 and a modulation time of 12 s. Under these conditions, efficient separation of all PCN congeners present in Halowax formulations was achieved in 140 min, resolving some critical closed eluting isomers, such as CN-33/34/37, highly toxic CN-66/67 or CN-71/72 pairs, among others. These findings represent a significant improvement in the congener-specific separation of PCNs over the 1D-GC and GC×GC methodologies already published and the DB-5MS × SLB-IL60 column combination offered the orthogonality required for the congener-specific determination with a high peak capacity. The GC×GC-MS method was applied to the characterisation of Halowax formulations, obtaining similar compositional profiles than those previously reported.

Comprehensive two-dimensional gas chromatographic separations with a temperature programmed microfabricated thermal modulator

Comprehensive two-dimensional gas chromatography (GC×GC) with a temperature-programmed microfabricated thermal modulator (μTM) is demonstrated. The 0.78cm2, 2-stage μTM chip with integrated heaters and a PDMS coated microchannel was placed in thermal contact with a solid-state thermoelectric cooler and mounted on top of a bench scale GC. It was fluidically coupled through heated interconnects to an upstream first–dimension (1D) PDMS-coated capillary column and a downstream uncoated capillary or second-dimension (2D) PEG-coated capillary. A mixture of n-alkanes C6–C10 was separated isothermally and the full-width-at-half-maximum (fwhm) values of the modulated peaks were assessed as a function of the computer-controlled minimum and maximum stage temperatures of μTM, Tmin and Tmax, respectively. With Tmin and Tmax fixed at −25 and 100°C, respectively, modulated peaks of C6 and C7 had fwhm values<53ms while the modulated peaks of C10 had a fwhm value of 1.3s, due to inefficient re-mobilization. With Tmin and Tmax fixed at 0 and 210°C, respectively, the fwhm value for the modulated C10 peaks decreased to 67ms, but C6 and C7 exhibited massive breakthrough. By programming Tmin from −25 to 0°C and Tmax from 100 to 220°C, the C6 and C7 peaks had fwhm values≤50ms, and the fwhm for C10 peaks remained<95ms. Using the latter conditions for the GC×GC separation of a sample of unleaded gasoline yielded resolution similar to that reported with a commercial thermal modulator. Replacing the PDMS phase in the μTM with a trigonal-tricationic room temperature ionic liquid eliminated the bleed observed with the PDMS, but also reduced the capacity for several test compounds. Regardless, the demonstrated capability to independently temperature program this low resource μTM enhances its versatility and its promise for use in bench-scale GC×GC systems.

μGC × μGC: Comprehensive Two-Dimensional Gas Chromatographic Separations with Microfabricated Components

The development and characterization of a microanalytical subsystem comprising silicon-micromachined first- and second-dimension separation columns and a silicon-micromachined thermal modulator (μTM) for comprehensive two-dimensional (i.e., μGC × μGC) separations are described. The first dimension consists of two series-coupled 3.1 cm × 3.1 cm μcolumn chips with etched channels 3 m long and 250 μm × 140 μm in cross section, wall-coated with a poly(dimethylsiloxane) (PDMS) stationary phase. The second dimension consists of a 1.2 cm × 1.2 cm μcolumn chip with an etched channel 0.5 m long and 46 μm × 150 μm in cross section wall-coated with either a trigonal tricationic room-temperature ionic liquid (RTIL) or a commercial poly(trifluoropropylmethyl siloxane) (OV-215) stationary phase. The two-stage, cryogen-free μTM consists of a Si chip containing two series-coupled, square spiral channels 4.2 cm and 2.8 cm long and 250 μm × 140 μm in cross section wall-coated with PDMS. Conventional injection methods and flame ionization detection were used. Temperature-ramped separations of a simple alkane mixture using the RTIL-coated second-dimension ((2)D) μcolumn produced reasonably good peak shapes and modulation numbers; however, strong retention of polar compounds on the RTIL-coated (2)D μcolumn led to excessively broad peaks with low (2)D resolution. Substituting OV-215 as the (2)D μcolumn stationary phase markedly improved the performance, and a structured 22 min chromatogram of a 36-component mixture spanning a vapor pressure range of 0.027 to 13 kPa was generated with modulated peak fwhm (full width at half-maximum) values ranging from 90 to 643 ms and modulation numbers of 1-6.

Comprehensive Two-Dimensional Gas Chromatographic Separations with a Microfabricated Thermal Modulator

Rapid, comprehensive two-dimensional gas chromatographic (GC × GC) separations by use of a microfabricated midpoint thermal modulator (μTM) are demonstrated, and the effects of various μTM design and operating parameters on performance are characterized. The two-stage μTM chip consists of two interconnected spiral etched-Si microchannels (4.2 and 2.8 cm long) with a cross section of 250 × 140 μm(2), an anodically bonded Pyrex cap, and a cross-linked wall coating of poly(dimethylsiloxane) (PDMS). Integrated heaters provide rapid, sequential heating of each μTM stage, while a proximate, underlying thermoelectric cooler provides continual cooling. The first-dimension column used for GC × GC separations was a 6 m long, 250 μm i.d. capillary with a PDMS stationary phase, and the second-dimension column was a 0.5 m long, 100 μm i.d. capillary with a poly(ethylene glycol) phase. Using sets of five to seven volatile test compounds (boiling point ≤174 °C), the effects of the minimum (T(min)) and maximum (T(max)) modulation temperature, stage heating lag/offset (O(s)), modulation period (P(M)), and volumetric flow rate (F) on the quality of the separations were evaluated with respect to several performance metrics. Best results were obtained with a T(min) = -20 °C, T(max) = 210 °C, O(s) = 600 ms, P(M) = 6 s, and F = 0.9 mL/min. Replicate modulated peak areas and retention times were reproducible to <5%. A structured nine-component GC × GC chromatogram was produced, and a 21 component separation was achieved in <3 min. The potential for creating portable μGC × μGC systems is discussed.

Methodology for the derivatization and quantification of dialkyl phosphate esters in petroleum samples

Dialkyl phosphate esters are used as gellants in some oil well fracturing processes for conventional oil production. Residual amounts of these compounds that remain in the well represent a source of potential contamination of the crude oil produced from the well. This contamination results in fouling of refinery equipment. Current methodologies for the analysis of alkyl phosphates have a relatively poor detection limit (0.5 ± 1.0 µg mL−1) and provide no speciation information: only a total phosphorus concentration. Herein, we present an approach that permits speciation of individual alkyl phosphates and quantification of individual compounds with limits of detection and precision that represent an improvement on the existing methodology. The method that we present relies upon derivatization of dialkyl phosphates to their trimethylsilane (TMS) esters followed by a comprehensive two-dimensional gas chromatographic separation. Two dialkyl phosphate esters, dibutyl and bis(2-ethylhexyl) phosphate were used as model compounds. Recovery of spiked samples in pure solvent and a model petroleum mixture ranged from 92% to 120%. This approach represents a significant first step in the development of an analytical solution to the challenge of phosphorus contamination facing the oil industry.

Profiling analysis of volatile compounds from fruits using comprehensive two-dimensional gas chromatography and image processing techniques

An image processing approach originating from the proteomics field has been transferred successfully to the processing of data obtained with comprehensive two-dimensional gas chromatographic separations data. The approach described here has proven to be a useful analytical tool for unbiased pattern comparison or profiling analyses, as demonstrated with the differentiation of volatile patterns (“aroma”) from fruits such as apples, pears, and quince fruit. These volatile patterns were generated by headspace solid phase microextraction coupled to comprehensive two-dimensional gas chromatography (HS-SPME-GC × GC). The data obtained from GC × GC chromatograms were used as contour plots which were then converted to gray-scale images and analyzed utilizing a workflow derived from 2D gel-based proteomics. Run-to-run variations between GC × GC chromatograms, respectively their contour plots, have been compensated by image warping. The GC × GC images were then merged into a fusion image yielding a defined and project-wide spot (peak) consensus pattern. Within detected spot boundaries of this consensus pattern, relative quantities of the volatiles from each GC × GC image have been calculated, resulting in more than 700 gap free volatile profiles over all samples. These profiles have been used for multivariate statistical analysis and allowed clustering of comparable sample origins and prediction of unknown samples. At present state of development, the advantage of using mass spectrometric detection can only be realized by data processing off-line from the identified software packages. However, such information provides a substantial basis for identification of statistically relevant compounds or for a targeted analysis.

Modulation-induced error in comprehensive two-dimensional gas chromatographic separations

There is a fundamental difference between data collected in comprehensive two-dimensional gas chromatographic (GC × GC) separations and data collected by one-dimensional GC techniques (or heart-cut GC techniques). This difference can be ascribed to the fact that GC × GC generates multiple sub-peaks for each analyte, as opposed to other GC techniques that generate only a single chromatographic peak for each analyte. In order to calculate the total signal for the analyte, the most commonly used approach is to consider the cumulative area that results from the integration of each sub-peak. Alternately, the data may be considered using higher order techniques such as the generalized rank annihilation method (GRAM). Regardless of the approach, the potential errors are expected to be greater for trace analytes where the sub-peaks are close to the limit of detection (LOD). This error is also expected to be compounded with phase-induced error, a phenomenon foreign to the measurement of single peaks. Here these sources of error are investigated for the first time using both the traditional integration-based approach and GRAM analysis. The use of simulated data permits the sources of error to be controlled and independently evaluated in a manner not possible with real data. The results of this study show that the error introduced by the modulation process is at worst 1% for analyte signals with a base peak height of 10 × LOD and either approach to quantitation is used. Errors due to phase shifting are shown to be of greater concern, especially for trace analytes with only one or two visible sub-peaks. In this case, the error could be as great as 6.4% for symmetrical peaks when a conventional integration approach is used. This is contrasted by GRAM which provides a much more precise result, at worst 1.8% and 0.6% when the modulation ratio ( M R) is 1.5 or 3.0, respectively for symmetrical peaks. The data show that for analyses demanding high precision, a M R of 3 should be targeted as a minimum, especially if multivariate techniques are to be used so as to maintain data density in the primary dimension. For rapid screening techniques where precision is not as critical lower M R values can be tolerated. When integration is used, if there are 4–5 visible sub-peaks included for a symmetrical peak at M R = 3.0, the data will be reasonably free from phase-shift-induced errors or a negative bias. At M R = 1.5, at least 3 sub-peaks must be included for a symmetrical peak. The proposed guidelines should be equally relevant to LC × LC and other similar techniques.

Effect of first-dimension column film thickness on comprehensive two-dimensional gas chromatographic separation

In comprehensive two-dimensional gas chromatography (GC × GC), samples experience two-dimensional separation implemented by a modulator which helps preserve the first-dimension separation and facilitates the second-dimension separation by periodically collecting, focusing and launching the material from the primary column onto the secondary column with a different stationary phase. Column overloading in GC × GC is a considerable problem, aggravated by the fact that two columns are involved. Broad first-dimension peaks of an analyte help produce smaller fractions of the analyte in the second-dimension, reducing the chance of secondary column overloading. One of the means to generate broad peaks in the first-dimension is to use thick film primary columns. A series of primary columns of various film thickness were tested in the study, and the results indicate that when other conditions are kept constant, 1 μm film columns often provide better resolution in both first and second-dimension but at the expense of a much longer separation time; 0.1 μm is clearly inadequate for GC × GC separation; 0.5 and 0.25 μm film columns seem to be the best compromises.

Flow model for coupled-column gas chromatography systems

One of the challenges in performing comprehensive two-dimensional gas chromatographic separations is being able to predict the average linear velocities of the carrier gas in the two columns, especially when they have different diameters. The problem is compounded for loop-type modulators, where two thermal trapping zones that switch from hot to cold and back simultaneously are separated by a delay loop. If the linear velocity in the loop is not tuned to the length of the loop and the modulation period, the dual-stage modulation may cease to work properly. A model has been developed that calculates the flow rates in the columns and predicts appropriate delay loop dimensions for a given set-up. Additionally, the model determines the pressure ramp that needs to be used in order to maintain constant average linear velocity within the modulator loop throughout the course of the separation.

Prediction of comprehensive two-dimensional gas chromatographic separations: A theoretical and practical exercise

Two-dimensional gas chromatography offers an unsurpassed and ordered separation combined with a very high peak capacity. Especially when applied to the separation and quantitative characterisation of complex mixtures it constitutes a leap forward with respect to state-of-the-art capillary GC. This is realised by the two orthogonal separations to which an entire sample is subjected. However, the selection of the proper combination of stationary phases and the temperature program is rather complicated and time consuming. A model is developed to predict which combination of columns is the most appropriate for a specified separation problem. The model is based on calculating retention times and peak widths of the compounds to be separated, on both columns, thus predicting the eventual chromatogram. It starts from estimating the retention factors ( k) of the compounds at their elution temperatures. This is performed with the help of the (calculated) vapour pressures and the enthalpic contribution to the activity coefficient as obtained from the Kováts retention indices. Some examples illustrate the usefulness of the model.