Browsing by Author "Darko, Ernest"

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    Active Control of Selectivity in Organic Acid Analysis by Gas Chromatography
    (2020-03-04) Darko, Ernest; Thurbide, Kevin B.
    A new method that allows organic acid selectivity to be dynamically controlled during gas chromatography (GC) is presented. It employs dual in-series stainless steel columns, each coated with a pH-adjusted water stationary phase. The first is a 2 m column coated with a pH 11.4 phase that is connected to a second 11 m column coated with a pH 2.2 phase. In this arrangement, organic acids within sample mixtures are trapped on the first column, while the remaining non-ionizable components continue to separate and elute in the system. Later, by injecting a volatile formic acid solution, the trapped acids are released in-situ to the second column for separation and analysis as desired. The method provides good reproducibility with analyte retention times in consecutive trials yielding an average RSD of 1.9 %. Further, depending on column temperature, analytes can be readily retained for periods investigated up to about 30 minutes without significant deterioration in peak shape. This feature provides considerable control over analyte selectivity and resolution compared to conventional separations. Further, by adding a third conventional GC column in-series, both typical hydrocarbon and enhanced organic acid separations are made possible. The method is applied to the analysis of complex mixtures and matrix interference is found to be significantly minimized. Results indicate that this approach offers beneficial advantages for the selective GC analysis of such acidic analytes.
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    Capillary Gas Chromatographic Separation of Carboxylic Acids Using an Acidic Water Stationary Phase
    (The final publication is available at Springer via http://dx.doi.org/10.1007/s10337-017-3333-z, 2017-08-01) Thurbide, Kevin; Darko, Ernest
    An acidic water stationary phase is used for the analysis of carboxylic acids in capillary gas chromatography (GC). Under regular pH 7 operating conditions, these analytes are largely ionized and elute poorly, if at all, from the water phase. However, by adjusting the phase to pH 2, it is found that various acids are neutralized and can be readily eluted and separated in the system. Sulfamic acid is found to provide a stable pH for the water phase over time, whereas hydrochloric acid and other more volatile additives quickly evaporate from the column. Under optimal low pH conditions, the acidic analytes yield good peak shape and are readily observed for masses investigated down to 5 ng on-column. By comparison, on a conventional non-polar capillary GC column, the same analytes display threefold more peak tailing and are not detected for masses below 30 ng on-column. Through altering the phase pH, it is found that the selectivity between certain analytes can be potentially enhanced depending on their respective pKa values and/or ionizability. The analysis of various different samples containing carboxylic acids is demonstrated and the results indicate that this approach can possibly offer unique and beneficial selectivity in such determinations.
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    Characterization of Novel Materials as Platforms for Performing Microfluidic Gas Chromatography
    (2013-04-23) Darko, Ernest; Thurbide, Kevin
    Analytical Gas Chromatography (GC) separation techniques are constantly being developed to reduce analysis time and cost as well as improve sensitivity and accuracy. One such development is the fabrication of miniaturized microfluidic columns for use in GC. There are vast reports of miniaturized columns fabricated in silicon and even other polymeric materials. While these devices show good separation efficiency, they lack physical robustness and the thermal stability demanded by GC. Alternatively, Low Temperature Co-fired Ceramics (LTCC) and titanium are novel materials that show promise for use in microfluidic GC column fabrication. They offer advantages such as low cost, ease of fabrication, and additionally, these materials are quite strong and can withstand the high temperatures required in GC. LTCC together with titanium metals have been studied and characterized as alternative platforms for microfluidic GC in this thesis. Both LTCC and titanium tiles produced good results that compared well with commercially available GC columns. For example, a 7.5m long channel within an 11 cm x 5.5 cm LTCC tile under optimum conditions generated theoretical plates for a dodecane test analyte of about 14327 compared to 4507 for a 7.5 m long commercial capillary column. Similarly, a 15m long channel within a 15 cm x 8 cm titanium tile produced 10377 plates for the same test analyte. Both of these tiles showed high resolving power, yielding benzene – toluene resolutions of about 14.07 and 8.29 respectively for LTCC and titanium. Peak capacity was also probed using temperature programming with a simple nC8 – nC20 alkane mixture. A cumulative peak capacity of about 53 was obtained for the LTCC tile while the titanium tile gave a value of 48. Polar analyte separations on both often produced peaks with a greater degree of tailing. Additionally, packed columns fabricated on these materials were also investigated, and produced good separation efficiency with negligible flow restriction. For example, a 10 cm long channel within a 5.5 cm x 2.5 cm LTCC tile packed with 5µm C18 particles generated 2710 plates per meter. Likewise, a 10 cm long channel within a 9 cm x 5 cm titanium tile packed with 1.7µm C18 particles also produced 8430 plates per meter; a three fold increase. Neither displayed any adverse effects from operating the tiles up to 60 atm of carrier gas pressure. Results indicate that LTCC and titanium make excellent platforms for microfluidic GC. Further exploration of their properties in this area will be useful.
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    Dynamic Control of Gas Chromatographic Selectivity during the Analysis of Organic Bases
    (Scopus, 2019-05-06) Darko, Ernest; Thurbide, Kevin B.
    A novel method for controlling selectivity during the gas chromatographic (GC) analysis of organic bases is presented. The technique employs tandem stainless steel capillary columns, each coated with a pH adjusted water stationary phase. The first is a 0.5 m trap column coated with a pH 2.2 phase, while the second is an 11 m analytical column coated with a pH 11.4 phase. The first column traps basic analytes from injected samples, while the remaining components continue to elute and separate. Then, upon injection of a volatile aqueous ammonia solution, the basic analytes are released as desired to the analytical column where they are separated and analyzed. Separations are quite reproducible and demonstrate an average RSD of 1.2% for analyte retention times in consecutive trials. Using this approach, the retention of such analytes can be readily controlled and they can be held in the system for periods of up to 1 h without significant erosion of peak shape. As such, it can provide considerable control over analyte selectivity and resolution compared to conventional separations. Further, by employing a third conventional GC column to the series, both traditional hydrocarbon and enhanced organic base separations can be performed. The method is applied to the analysis of complex mixtures, such as gasoline, and much less matrix interference is observed as a result. The findings indicate that this approach could be a useful alternative for analyzing such samples
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    A Novel pH-adjusted Water Stationary Phase for Capillary Gas Chromatographic Separations
    (2019-04-11) Darko, Ernest; Thurbide, Kevin B.; Kimura-Hara, Susana; Gailer, Jürgen G.
    In this thesis, a novel pH-adjusted water stationary phase coated on a stainless steel column was investigated, characterized, and compared to other conventional columns in capillary gas chromatography (GC). The pH-adjusted water stationary phase was applied to the analysis of both organic bases and acids. The analysis of organic bases and acids on a regular pH 7 water phase is impossible to achieve due to the poor peak shapes and elution properties that result. However, these compounds readily eluted and separated well when pH 11.5 and 2.2 water stationary phase coatings are employed respectively. In the basic and acidic conditions, analytes are neutralized and are observed to produce good peak shapes even for analyte masses down to about 5ng. By comparison, analyses on a conventional non-polar capillary GC column yield more peak tailing and only analyte masses of 1 μg or higher are normally observed. Through carefully altering the pH, it is found that the selectivity between analytes can be potentially further enhanced if their respective pKa values differ sufficiently. The analysis of different pharmaceutical and petroleum samples containing organic bases and acids are demonstrated and are neatly simplified and improved using the pH-adjusted water stationary phase. Separations are also quite reproducible and provide low RSD values for the developed technique. In related work, a dynamic stationary phase switching technique was also studied and characterized for use in analytical separations involving complex mixtures. The study showed the ability to dynamically and externally control ionisable analyte retention and selectivity in GC. By first trapping these analytes, they could later be eluted on demand. Accordingly, organic bases and acids were reproducibly analyzed with much improved selectivity and resolution over other compounds. Further, by employing a triple column system, it was demonstrated that separation of hydrocarbons in complex matrices such as gasoline was achieved alongside a similar selective analysis of organic bases/acids. These results for organic acids and bases indicate that the use of a pH-adjusted water stationary phase approach can potentially offer unique and beneficial selectivity for such analytes in GC separations.
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    Sulfolane as a novel stationary phase for analytical separations by gas chromatography
    (Elsevier, 2021-11-06) Darko, Ernest; Thurbide, Kevin B.
    Sulfolane is explored as a novel stationary phase for use in analytical separations by capillary column gas chromatography with flame ionization detection (GC-FID). Stainless steel capillaries were found to provide a good substrate for coating and retaining a sulfolane phase, whereas fused silica tubing did not perform well for this. In general, the phase was found to be stable for several hours of use when using elevated carrier gas pressures (90 psi) and a small restriction (25 μm I.D. tubing) at the outlet. This normally provided good performance at temperatures up to about 200 °C with very little background interference in the FID. Given its separation properties, a short 2 m × 100 μm I.D. column was found to be preferable for most separations in this study. Measurements indicated the coating procedure yielded a sulfolane film near 4 μm thick on this column, which produced 4400 plates for benzene with a sample capacity near 30 μg. The sulfolane phase yielded good retention and peak shape for many analytes including alkanes, aromatics, alcohols, bases, sulfides, phosphites, thiols, and others. Compared to longer conventional GC columns, the relatively short sulfolane column was found to offer improved selectivity in the separation of unsaturated, aromatic, and alkane test analytes. As such the method was successfully applied to the analysis of aromatics in gasoline headspace. Results suggest that sulfolane could be a potentially useful stationary phase to further explore in GC separations.