141 |
Negative Temperature Programming using Microwave Gas ChromatographyBao, Jianxiong 14 July 2005 (has links)
The microwave region of the electromagnetic spectrum lies between the infrared radiation and radio frequency region and corresponds to wavelengths of 1 cm to 1 m (frequencies of 30 GHz to 300 MHz respectively). Gaisford and Walters developed the microwave GC oven in 2000, which operated at 919 MHz. The microwave GC oven has been engineered to generate a uniform microwave field around an open tubular capillary column with elimination of cold spots which are common in a domestic microwave oven. Only the column is heated in a microwave GC oven. In microwave GC, ramp rates in excess of 6 degree/sec, isotherms as high as 450 degree, and 1 min cool-down times from 300 to 50 degree can be achieved. In conventional GC, only positive temperature programming is used because the oven cools down slowly due to the great thermal mass of the oven. Short cool-down time in microwave-heating GC makes it possible to employ negative temperature programming for the separation of compounds during the process.
The theory and feasibility of negative temperature programming in microwave GC were investigated in this research. Negative temperature programming was employed to analyze and quantitate four different critical pairs of non-volatile and volatile compounds. The influence of cooling rate, holding time, and reheating rate in negative temperature programs for enhanced resolution were investigated. The results obtained from negative temperature programming were compared to those from positive temperature programming. Negative temperature programming afforded greater resolution of critical pairs of analytes. / Master of Science
|
142 |
Determination of Solute Descriptors for Illicit Drugs Using Gas Chromatographic Retention Data and Abraham Solvation ModelMitheo, Yannick K. 08 1900 (has links)
In this experiment, more than one hundred volatile organic compounds were analyzed with the gas chromatograph. Six capillary columns ZB wax plus, ZB 35, TR1MS, TR5, TG5MS and TG1301MS with different polarities have been used for separation of compounds and illicit drugs. The Abraham solvation model has five solute descriptors. The solute descriptors are E, S, A, B, L (or V). Based on the six stationary phases, six equations were constructed as a training set for each of the six columns. The six equations served to calculate the solute descriptors for a set of illicit drugs. Drugs studied are nicotine (S= 0.870, A= 0.000, B= 1.073), oxycodone(S= 2.564. A= 0.286, B= 1.706), methamphetamine (S= 0.297, A= 1.570, B= 1.009), heroin (S=2.224, A= 0.000, B= 2.136) and ketamine (S= 1.005, A= 0.000, B= 1.126). The solute property of Abraham solvation model is represented as a logarithm of retention time, thus the logarithm of experimental and calculated retention times is compared.
|
143 |
A study of the carbohydrates of cocoa beans by gas chromatographyAndersen, David A. January 1968 (has links) (PDF)
Thesis (Ph. D.)--Pennsylvania State University, 1968. / Includes bibliographical references.
|
144 |
The identification and quantitation of complex polycyclic aromatic hydrocarbon mixtures in environmental samples using comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometryManzano, Carlos A. (Carlos Andres) 27 June 2013 (has links)
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants and are mostly products of the incomplete combustion of organic material. PAHs are often found in environmental samples as a complex mixture of isomers. In addition, the same sources that produce complex PAH mixtures also produce other poorly characterized mixtures of organic compounds, commonly referred to as an unresolved complex mixture (UCM), that act as matrix interferences in the chromatographic analysis of samples.
Conventional one-dimensional chromatographic techniques, such as gas chromatography coupled to mass spectrometry (GC/MS), are not sufficient for the analysis and quantitation of complex PAH mixtures present in environmental samples due to the high degree of overlap in compound vapor pressures, boiling points, and mass spectral fragmentation patterns. Therefore, the separation and quantitation of complex mixtures of individual PAH compounds in environmental samples requires high chromatographic resolution.
Comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC��GC/ToF-MS) was used for this study. GC��GC/ToF-MS uses two different gas chromatographic columns, with different separation mechanisms, for the analysis of complex environmental samples. In theory, the peak capacity in GC��GC/ToF-MS is equivalent to the product of the individual peak capacities of each column used. However, in practice, this is rarely obtained because of the existing correlation between the two GC columns used. This dissertation is a compilation of three studies related to analytical method development for the identification and quantitation of complex PAH mixtures (including parent-PAHs, alkyl-PAHs, oxy-PAHs, nitro-PAHs, thio-PAHs, chloro-PAHs, bromo-PAHs and PAHs with molecular weight higher than 300 Da) that may be present in environmental samples using novel column combinations in GC��GC/ToF-MS.
The use of a liquid crystal column (LC-50) in the first dimension, followed by a nano-stationary phase column (NSP-35) in the second dimension, was evaluated for the separation of a standard PAH mixture containing 97 different PAHs. Two standard reference materials purchased from NIST (NIST SRM1650b ��� Diesel Particulate Matter and NIST SRM1975 ��� Diesel Extract) were used, after extraction and cleanup, for method validation and comparison between the commonly used non-polar �� polar column combination and the LC-50 �� NSP-35 column combination with high orthogonality. As part of the method validation, an aliquot of NIST SRM1975 (Diesel extract), without sample cleanup was also analyzed for PAHs, showing that the LC-50 �� NSP-35 column combination was accurate (with an average absolute percent difference of approximately 30%) for the identification and quantitation of complex PAH mixtures in environmental samples, with reduced sample preparation prior to analysis. In addition, the LC-50 �� NSP-35 column combination was used for the analysis of PAHs sorbed to polystyrene pellets deployed in an urban bay area as passive water samplers because one-dimensional GC/MS was ineffective due to the presence of a strong unresolved complex mixture (UCM) and matrix interferences. / Graduation date: 2013 / Access restricted to the OSU Community at author's request from Dec. 27, 2012 - June 27, 2013
|
145 |
Parylene Microcolumn for Miniature Gas ChromatographNoh, Hongseok "Moses" 14 May 2004 (has links)
This research contributes to worldwide efforts to miniaturize one of the most powerful and versatile analytical tools, gas chromatography (GC). If a rapid, sensitive and selective hand-held GC system is realized, it would have a wide range of applications in many industries and research areas. As a part of developing a hand-held GC system, this research focuses on the separation column, which is the most important component of a GC system. This thesis describes the development of a miniature separation column that has low thermal mass and an embedded heating element for rapid thermal cycling. The worlds first thin polymer film (parylene) GC column has been successfully developed. This thesis includes: first, a study of theoretical column performance of rectangular GC column; second, the design optimization of parylene column and embedded heating element; third, the development of new processes such as parylene micromolding and stationary phase coating technique for parylene column; fourth, the fabrication of parylene GC column with an embedded heating element; and lastly, the testing and evaluation of parylene GC column through GC analysis.
|
146 |
The Analysis of Volatile Impurities in Air by Gas Chromatography/Mass SpectrometryTalasek, Robert Thomas 05 1900 (has links)
The determination of carbon monoxide is also possible by trapping CO on preconditioned molecular sieve and thermal desorption. Analysis in this case is performed by gas chromatography/mass spectroscopy, although the trapping technique is applicable to other suitable GC techniques.
|
147 |
Activity coefficients at infinite dilution for ILs : methyltrioctylammonium cation and bis(trifluoromethylsulfonyl)imide or thiosalicylate anions using glcGwala, Nobuhle V. January 2009 (has links)
Dissertation submitted in fulfilment of the requirements for the Masters Degree in Technology: Chemistry, Durban University of Technology, 2009. / The activity coefficients at infinite dilution ( γ13 ) were calculated for alkanes, alkenes,
cycloalkanes, alkynes, ketones, alcohols and aromatic compounds from gas liquid
chromatography (glc) measurements at three temperatures (303.15 and 313.15 and 323.15) K.
The γ13 values were calculated from the retention data for two ionic liquids (ILs) with the
same cation but different anions. The ionic liquids: [methyltrioctylammonium
bis(trifluoromethylsulfonyl)imide and methyltrioctylammonium thiosalicylate] were used as
the stationary phase. The γ13
data for methyltrioctylammonium thiosalicylate were higher
than for methyltrioctylammonium bis(trifluoromethylsulfonyl)imide.
For each temperature and each ionic liquid, γ13
values were determined for two columns and
the average γ13
values were used for the calculation of the partial molar excess enthalpies at
infinite dilution, H E
1 Δ , using the Gibbs-Helmholtz equation. Graphs of In γ13
vs 1/T were
plotted to determine H E
1 Δ at T = 303.15 K. The selectivity, S12 , value for the
hexane/benzene separation was calculated from the ratio of the avarage γ13
values to
determine the suitability of the ILs as an entrainer for extractive distillation in the separation
of aromatic and aliphatic compounds.The S12
values were also compared to the literature
values for other ionic liquids. It was found that the longer chain alkyl group attached to the
cation or anion gave lower S12 values. Both ILs have S12 values greater than one and in
theory can be used as an entrainer for the hexane/ benzene separation. The capacities at
infinite dilutions, k1
, were also calculated from the inverse of the avarage γ13
values for each
IL at each temperature. / National Research Fund; Postgraduate Development and Support Directorate.
|
148 |
Stir bar sorptive extraction and gas chromatography : mass spectrometry for the analysis of biological matricesStopforth, A. 12 1900 (has links)
Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2007. / This study describes the development of simplified analytical methods for the analysis of trace quantities of selected naturally occurring target compounds in complex biological matrices by stir bar sorptive extraction (SBSE) and gas chromatography/mass spectrometry (GC/MS). SBSE facilitates the direct extraction of organic compounds from aqueous samples by allowing the solutes to partition between the aqueous phase and a glass stir bar that is coated with a layer of polydimethylsiloxane (PDMS). The partitioning of polar compounds into the PDMS coating was enhanced by using different derivatization techniques in combination with SBSE. The derivatization of polar functional groups was performed with ethyl chloroformate, acetic acid anhydride, and O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine directly in the aqueous samples. Headspace derivatization of compounds containing a secondary alcohol group was performed directly on the stir bar coating in the presence of acetic acid anhydride vapors. The derivatized compounds were thermally desorbed (TD) and analyzed on-line by GC/MS. A number of experimental parameters, including salt addition, temperature and time were optimized to improve the recovery of the derivatized compounds by SBSE. The optimized methods were validated in terms of linearity, precision, and detection and quantitation limits prior to performing the quantification. Trace levels of tuberculostearic acid, a marker of tuberculosis, was detected in sputum samples that were decontaminated and concentrated before being analyzed by SBSE-TD-GC/MS. The method is sufficiently sensitive to detect the marker without the need to culture the organisms, namely M. Tuberculosis. The analysis of 4-hydroxynonenal has also been demonstrated by detecting trace levels of this oxidative stress marker in urine samples obtained from healthy volunteers. Furthermore, abnormally low testosterone/epitestosterone ratios were detected in a group of HIV positive patients by means of SBSE-TD-GC/MS. Further research is required to determine the clinical significance of this finding in the context of HIV infection. Finally, the excessive urinary excretion of estrone and 17β-estradiol following the administration of a high dose of the conjugated equine estrogens to a female volunteer has also been demonstrated.
|
149 |
PHYSICAL AND CHEMICAL CHARACTERIZATION OF TAILORED CHROMATOGRAPHIC ADSORBENTSDell'Ova, Vincent Edward January 1980 (has links)
The reaction between nitrogen-containing organic compounds (pyridines and amines) and alumina was studied through the use of chromatographic titrations and elemental analysis. The aluminum-nitrogen bond was the basis for the preparation of a series of new chromatographic adsorbents. A series of amines and pyridine homologs were bound to alumina and evaluated as chromatographic stationary phases. The surface coverage was determined using elemental analysis. The adsorption properties of the stationary phases were investigated by determining differential enthalpies, entropies, and free energies of adsorption for a selected group of organic moieties. Rate theory was used to examine further the interaction between the molecular probes and the tailored supports. Relative peak broadening was measured both as a function of flow rate and temperature to provide a chromatographic evaluation of molecular probe-adsorbent interaction occurring during the chromatographic process. Results indicated that the single-molecule moieties used as tailoring agents served as excellent deactivating agents but produced no significant changes in the selectivity of the adsorbents. Pellicular beads were synthesized by using 4-vinylpyridine as a linking agent between the alumina substrate and a series of polymers. The polymers used in this study were polystyrene, polymethylmethacrylate, and polyacrylonitrile. Each type of polymer-coated bead was prepared at different loading levels. Scanning electron microscopy was used to examine the gross change in the surface and elemental analysis used to determine the polymer loading. The adsorption properties of the polymeric pellicular supports were studied by determining the aforementioned thermodynamic quantities and by rate theory. The relative peak broadening was mentioned as a function of polymer type, loading, flow rate, and temperature. The chromatographic behavior of the selected molecular probes on the pellicular supports was sensitive to polymer type and loading. It was established that alumina can be modified with amines and pyridines and that a molecule possessing a nitrogen atom and a polymerization site can be used as an adhesive interface to chemically link polymeric phases to alumina. Currently, there are no commercially available modified aluminas. It has been demonstrated here that the preparation of such supports is feasible and can lead to chromatographically useful products.
|
150 |
Gas chromatography/olfactometry and descriptive analysis of Valencia orange juiceShah, Rohan 08 June 1998 (has links)
Heat treated orange juice, both pasteurized and concentrate, are being increasingly
consumed in the U.S. Orange juice is primarily heat treated to increase its shelf life, by
curbing the growth of microorganisms; and to inactivate pectin methylesterase, which
demethylates pectin and leads to cloud loss in the juice. However, because of heat
processing, orange juice undergoes undesirable flavor changes that decrease its
acceptability to consumers.
The objectives of this study were to differentiate between fresh frozen and heat
treated orange juice employing descriptive analysis, and to determine by Osme, a gas
chromatography-olfactometry (GCO) method, odor active volatiles that were either
lacking or created in the heat treated juice. The second objective was to determine how
changes in the odor-active volatile profile of heat treated orange juice, relates to changes
in the aroma and flavor intensities of the samples as assessed by descriptive analysis.
Through descriptive analysis, the panel was successful in significantly (p<0.05)
separating the fresh, pasteurized, and concentrate samples. Orange, orange peel, sweet,
and grassy descriptors were found to be important for fresh aroma and flavor, while
cooked, yam, metallic, tamarind, green bean and artificial orange descriptors were higher
in heat treated samples.
Using Osme, it was possible to separate fresh frozen from heat treated orange
juice, on the basis of their aroma profiles. Fresh frozen samples show a higher
concentration of peaks tentatively identified as gamma-butyrolactone, citral, nonanal,
carvone, perillaldehyde, carvyl propinate, valencene, and other unidentified peaks
possessing descriptors such as floral, lime, citrus, pine, bamboo leaf, metallic, and vinyl.
Pasteurized samples show a larger concentration of peaks tentatively identified as
hexanol, octanol, nerol / carveol, myrcene, 2-octanone, p-cymene, terpenen-4-ol, betacitronellol,
and other unidentified peaks with descriptors such as cilantro, vinyl, melon,
mushroom, and metallic.
Descriptors such as orange, orange peel, sweet, grapefruit, and grassy are more
pronounced in the fresh samples and are similar to the odor descriptors of Osme peaks
higher in the fresh samples. Descriptors such as cooked, artificial orange, yam, metallic,
tamarind, and green bean are higher in the pasteurized samples, and are similar to the
odor descriptors of peaks higher in these samples. / Graduation date: 1999
|
Page generated in 0.0847 seconds