Determination of thyreostatics in animal feed by micellar electrokinetic chromatography

The determination of the thyreostatics 2-thiouracil, its derivatives (4-methyl-2-thiouracil, 4-propyl-2-thiouracil and 4-phenyl-2-thiouracil) and methimazole in manufactured dried animal feed by micellar electrokinetic chromatography (MEKC) is described. A 99 +/- 5% extraction yield at the 20 micrograms g-1 level (n = 8) was achieved by shaking the milled fodder with methanol-1 M NaOH (80 + 20). Aliquots of the supernatant were injected in a 75 microns x 33.5 cm uncoated silica capillary using pressure; separation was performed at 23 degrees C with 15 kV (positive polarity) in a background electrolyte (BGE) containing 40 mM sodium dihydrogenphosphate, 50 mM sodium dodecyl sulfate and 15 mM Tween 20 at pH 9. When the surfactants were added to the BGE, all the thyreostatics were well resolved and the fodder extracts showed lower backgrounds. The peaks appeared within the 2.25-5.2 min range with efficiencies in the 2.5 x 10(4)-8 x 10(4) range; methimazole appeared in the vicinity of the electroosmotic migration time. Calibration curves were linear within the studied range (20-200 micrograms ml-1, r2 > 0.998). Limits of detection in the extracts of spiked fodder samples ranged from 0.25 to 0.4 microgram ml-1, which corresponded to 0.6-1.0 microgram of drug per gram of fodder. Peak area repeatabilities were about 4% at the 20 micrograms ml-1 level.     

Electrophoretic determination of thioamides, pyridine and pyrimidine derivatives

We have optimized the conditions for the separation and quantification of a mixture of heteroaromatic thioamides, potential thyreostatics: pyridine-2-thione (I), 5-trifluoromethylpyridine-2-thione (II), pyrimidine-2-thiol (III), 4-trifluoromethylpyrimidine-2-thiol (IV) by capillary zone electrophoresis using a 60 cm × 75 μm capillary (effective length of 50 cm), 15-kV voltage, and borate buffer solution (pH 9.18) as the running electrolyte. The procedure provides the estimation of analyte concentrations in the range from 6.7 × 10^?6 to 1.0 × 10^?4 M with the detection limits (by electrophoretic peak areas) 0.72 (I), 0.47 (II), 0.43 (III), and 0.76 (IV) μg/mL.     

Polymer membranes with selective gas and vapor permeation properties

Since many years synthetic membranes have been used in reverse osmosis or ultrafiltration for the separation of aqueous mixtures. More recently the separation of gases and vapors by selective membrane permeation has gained significant technical and commercial interest. The recovery of hydrogen from petrochemical purge gases and ammonia production processes or the removal of CO₂ from natural gas by selective membrane permeation are today state of the art procedures. The recovery of organic solvents from waste air streams is another very promising application of synthetic membranes. In this paper the main parameters determining the performance of a membrane in gas and vapor separation are described. The requested intrinsic properties of the polymer to be useful as a barrier for a selective gas and vapor transport are discussed. The preparation of appropriate membranes is described. Their performance in practicle applications is illustrated in selected examples.     

Trends in atmospheric trace gas measurement instruments with membrane-based gas diffusion scrubbers.

A diffusion scrubber (DS) is an excellent tool for gas analysis, and there are many types of DS devices, varying both in structure and construction. In this paper, recent work on atmospheric trace gas measurements by means of DS devices are reviewed. Theoretical considerations on representative DSs are summarized first. Then, the characteristics of the key material, a gas-diffusion membrane, are discussed, and recent improvements and novel scrubbers for highly effective collection are outlined. A chromatograph is a suitable tool for the multi-gas analysis of collected species. On the other hand, solid-state fluorescence/absorbance detectors have been developed for the continuous measurement of target gases. The methods based on DS collection and subsequent detection have high sensitivities, and the detection limits can be in the low-pptv levels. Accordingly, they are capable of measuring background levels, and detecting very low levels of contaminants in a cleanroom. Miniaturized advanced DS units, perhaps the next generation of DS devices, are introduced at the end of this review. DS systems have contributed significantly to our knowledge of the atmospheric dynamics and atmospheric chemistry.     

Qualitative and quantitative analysis of pyrolysis oil by gas chromatography with flame ionization detection and comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry

Pyrolysis oils have attracted a lot of interest, as they are liquid energy carriers and general sources of chemicals. In this work, gas chromatography with flame ionization detector (GC-FID) and two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS) techniques were used to provide both qualitative and quantitative results of the analysis of three different pyrolysis oils. The chromatographic methods and parameters were optimized and solvent choice and separation restrictions are discussed. Pyrolysis oil samples were diluted in suitable organic solvent and were analyzed by GC×GC-TOFMS. An average of 300 compounds were detected and identified in all three samples using the ChromaToF (Leco) software. The deconvoluted spectra were compared with the NIST software library for correct matching. Group type classification was performed by use of the ChromaToF software. The quantification of 11 selected compounds was performed by means of a multiple-point external calibration curve. Afterwards, the pyrolysis oils were extracted with water, and the aqueous phase was analyzed both by GC-FID and, after proper change of solvent, by GC×GC-TOFMS. As previously, the selected compounds were quantified by both techniques, by means of multiple point external calibration curves. The parameters of the calibration curves were calculated by weighted linear regression analysis. The limit of detection, limit of quantitation and linearity range for each standard compound with each method are presented. The potency of GC×GC-TOFMS for an efficient mapping of the pyrolysis oil is undisputable, and the possibility of using it for quantification as well has been demonstrated. On the other hand, the GC-FID analysis provides reliable results that allow for a rapid screening of the pyrolysis oil. To the best of our knowledge, very few papers have been reported with quantification attempts on pyrolysis oil samples using GC×GC-TOFMS most of which make use of the internal standard method. This work provides the ground for further analysis of pyrolysis oils of diverse sources for a rational design of both their production and utilization process.     

Preparative gas chromatography with glass capillary columns

The present paper describes an automated system for preparative gas chromatography with glass capillary columns, controlled by a microprocessor. The effluent from the capillary column is divided by a pneumatically controlled splitter and any desired split ratio between traps and detector can be obtained. Moreover, a second pneumatic control allows instantaneous change-over to a different split ratio, thus minimizing loss of material during collection. The effluent containing the compounds of interest is passed through a multiple manifold and collected in coiled glass capillary traps. To ensure maximum trapping efficiency even for very small amounts of material, the inner walls of the capillary traps are wetted with a suitable solvent, which gives a quantitative recovery of micro- and nanogram amounts of material. After repetitive sampling, sufficient amounts of material can be obtained for NMR spectroscopy and possibilities exist to enrich trace components with the aid of a double column system. Two examples of such applications are given, employing mixtures of both synthetic and natural origin.     

The extraction and analysis of 1,4-dioxane from water using solid-phase microextraction coupled with gas chromatography and gas chromatography-mass spectrometry

In this study, two methods are developed for the extraction of 1,4-dioxane (dioxane) from water using 80-microm carboxen-polydimethylsiloxane solid-phase microextraction fibers followed by either gas chromatography (GC)-flame ionization detection (FID) or GC-mass spectrometry (MS). With GC-FID, the lower limit of detection (LOD) for 1,4-dioxane is 2.5 microg/L (ppb) with a linear range of 5 to 10,000 microg/L, obtained by immersing the fiber in the sample for 20 min with agitation. Using GC-MS, the lower limit of quantitation is 0.5 microg/L, and the LOD is 0.25 microg/L. The upper linear range limit is 100 microg/L. Samples are extracted in 20 min using either heated headspace with agitation or direct immersion with agitation.     

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.     

Coulometric titration with gas diffusion separation step

Summary Coulometric titration is employed in combination with a gas diffusion step for the quantitative separation and determination of volatile substances. Complete separation is brought about by filling the sample into a closed circulating loop and transporting it repeatedly past a microporous membrane. The combination of membrane separation module with the coulometric cell allows rapid separation and exploitation of the precise absolute determinations obtainable by the coulometric titration method. The high precision and selectivity of the determination and quantitative separation are discussed using ammonia and sulphur dioxide analyses as examples.Part I: Fresenius Z Anal Chem (1989) 332:849–854     

Investigation of combwax of honeybees with high-temperature gas chromatography and high-temperature gas chromatography-chemical ionization mass spectrometry. II: High-temperature gas chromatography-chemical ionization mass spectrometry

Crude combwax of six various honey bee species have been analyzed by high-temperature gas chromatography (HTGC)-chemical ionization mass spectrometry after a two-step silylation procedure. An optimized chromatographic procedure, described previously, enables the separation of high-molecular mass lipid compounds resulting in a characteristic fingerprint of the combwaxes of different honeybee species. The coupling of HTGC to mass spectrometry requires appropriate instrumentation in order to achieve sufficient sensitivity at high elution temperatures and avoid loss of chromatographic resolution. Chemical ionization was carried out using methane as reagent gas in order to determine the molecular mass of the individual compounds by means of abundant quasi molecular ions. To confirm the presence of unsaturated wax esters, ammonia was used as reagent gas. More than 80 lipid constituents were separated and characterized by their mass spectra. Representative chemical ionization mass spectra of individual compounds are presented. Both, HTGC-flame ionization detection data and the results of the HTGC-mass spectrometric investigations enabled a rapid profiling of the individual classes of compounds in crude combwaxes.     

Reactivity of gas barrier membranes filled with reactive particulates

Gas barrier plastic films and structures capable of removing a permeating solute via irreversible chemical reactions in the polymer matrix are investigated in applications of active packaging for controlling contained environments. To significantly reduce permeation rates through such active barriers, high barrier reactivity is required. When the reactive species take the form of fine particulates sparingly dispersed in the matrix, the reactivity of a composite membrane is affected by a lower frequency of collisions of dissolved gas molecules with the distributed reactive particles. The mean size of the particles and their volume fraction in the matrix are identified as the critical factors affecting the overall reactivity of uniformly filled composite barriers. A two-scale coarse-grained model of reaction-diffusion in a matrix is developed by introducing a unit cell approach to estimate the reaction rate upon collision with the particles and by choosing the unit length of diffusive molecular displacement equal to the mean diameter of the reactive particles to describe diffusion across the barrier and statistics of molecule-particle collisions. The corresponding method for evaluating the barrier reactivity is described. The effects of the system parameters on the effective gas transport rates across the reactive composite barriers including polymer nanocomposite barriers with reactive additives are quantified.     

Measurements using gas chromatography with coelution and dual-isotope atomic emission detection.

Dual-isotope measurements by gas chromatography (GC)-atomic emission detection (AED) may enhance results for quantitative analyses. Adding a known amount of an isotopically labelled form of target analytes in each sample can compensate for irreproducibilities or uncertainties associated with sample pretreatments and sample loading. Similarly, fluctuations in AED temperatures, flows and interferants can be compensated via the added labelled forms if each target analyte and its isotopically labelled form coelute. Under these conditions they are subject to identical excitation environments and are measured from the same viewed volumes. Consequently, improved quantitative results may be attained by coelution in GC-AED methods which mimic isotope dilution.     

Meerwein reaction of phosphonium ions with epoxides and thioepoxides in the gas phase

Phosphonium ions are shown to undergo a gas-phase Meerwein reaction in which epoxides (or thioepoxides) undergo three-to-five-membered ring expansion to yield dioxaphospholanium (or oxathiophospholanium) ion products. When the association reaction is followed by collision-induced dissociation (CID), the oxirane (or thiirane) is eliminated, making this ion molecule reaction/CID sequence a good method of net oxygen-by-sulfur replacement in the phosphonium ions. This replacement results in a characteristic mass shift of 16 units and provides evidence for the cyclic nature of the gas-phase Meerwein product ions, while improving selectivity for phosphonium ion detection. This reaction sequence also constitutes a gas-phase route to convert phosphonium ions into their sulfur analogs. Phosphonium and related ions are important targets since they are commonly and readily formed in mass spectrometric analysis upon dissociative electron ionization of organophosphorous esters. The Meerwein reaction should provide a new and very useful method of recognizing compounds that yield these ions, which includes a number of chemical warfare agents. The Meerwein reaction proceeds by phosphonium ion addition to the sulfur or oxygen center, followed by intramolecular nucleophilic attack with ring expansion to yield the 1,3,2-dioxaphospholanium or 1,3,2-oxathiophospholanium ion. Product ion structures were investigated by CID tandem mass spectrometry (MS(2)) experiments and corroborated by DFT/HF calculations.     

Direct thermal desorption in the analysis of cheese volatiles by gas chromatography and gas chromatography-mass spectrometry: comparison with simultaneous distillation-extraction and dynamic headspace

Direct thermal desorption (DTD) has been used as a technique for extracting volatile components of cheese as a preliminary step to their gas chromatographic (GC) analysis. In this study, it is applied to different cheese varieties: Camembert, blue, Chaumes, and La Serena. Volatiles are also extracted using other techniques such as simultaneous distillation-extraction and dynamic headspace. Separation and identification of the cheese components are carried out by GC-mass spectrometry. Approximately 100 compounds are detected in the examined cheeses. The described results show that DTD is fast, simple, and easy to automate; requires only a small amount of sample (approximately 50 mg); and affords quantitative information about the main groups of compounds present in cheeses.     

Liquid chromatography fractionation with gas chromatography/mass spectrometry and preparative gas chromatography-nuclear magnetic resonance analysis of selected nonylphenol polyethoxylates

Commercial nonylphenol polyethoxylates, designated as NPnEOs, where n is the number of ethoxy groups, comprise a range of ethoxylate groups. According to the starting material nonylphenol, they may also be composed of a complex mix of isomeric nonyl substituents. In order to study more fully the heterogeneity arising from both the ethoxylate and nonyl groups, a mixture of NPnEOs is first fractionated by normal phase liquid chromatography (NPLC) into separate fractions comprising individual ethoxymers, n. Preparative collection of each early elution ethoxymer fraction allows further separation of different isomeric nonyl group components by using analytical gas chromatography/mass spectrometry (GC/MS). The nonyl isomers are not resolved in the NPLC method. The distribution of the isomeric nonyl side chain of different ethoxymers bears close resemblance with each other, and also with the original nonylphenol starting material, although separation efficiency of the nonyl isomers for each ethoxymer decreases with increasing ethoxymer number. Mass spectrometry of the separated isomers display close similarity for presumed equivalent isomers in each fraction, based on elution order of the nonyl isomers. This suggests that each corresponding peak has the same isomer structure. Mass spectra are interpreted based on branching within the nonyl side chain. Preparative GC coupled with MS and nuclear magnetic resonance spectroscopy elucidated the molecular structure of one of the resolved isomers as 4-(1,3-dimethyl-1-propyl-butyl)-phenol diethoxylate.     

Evaluation of substituted polycarbonates and a blend with polystyrene as gas separation membranes

The balance between the rate and the selectivity of transport of various gas pairs in a series of polycarbonates has been examined. Replacement of the four available hydrogens on the aromatic rings of the bisphenol-A unit with CH₃, Cl, or Br groups gives materials with a better balance of these two characteristics than the unsubstituted polycarbonate (PC). For example, using CH₃ substitution increases the permeability to O₂ by nearly a factor of four with no loss in O₂/N₂ selectivity compared with PC, while using Br substitution increases O₂/N₂ selectivity by 50% without any loss in O₂ permeability compared with PC. While these substitutions affect the permeability through both its mobility and solubility components, the remarkable selectivity effects are caused primarily by changes in relative mobility since the changes in solubility characteristics are nearly the same for all gases. These substitutions alter chain motions, cohesion, and packing as discussed. The tetramethylbisphenol-A polycarbonate forms miscible blends with polystyrene. These blends show absolute permeability coefficients which are lower than additivity while the selectivity of transport is greater. These effects are a result of the interactions between the two polymers.     

Photo-acoustic measurements of gas and aerosol absorption with diode lasers

The results of designing multipurpose high-sensitive photo-acoustic (PA) detectors and their application to high-resolution diode laser spectroscopy of molecular gases, gas analysis, and aerosol absorption measurements are summarized in this paper. The hardware and software of the diode laser spectrometer with a Helmholtz resonant PA detector providing an absorption sensitivity limit of better than 10(-7)Wm(-1)Hz(-1/2) are described. A procedure is proposed for an experiment involving the measurements of the rotational structure of hot vibrational bands of molecules. The results of the application of the nonresonant PA cell with temporal resolution of signals to measurements of weak nonresonant absorption of gases and soot aerosols are presented, and the possibility of creating a broad-band PA laser diode aerosol-meter is discussed.     

Instrumental planar chromatographic determination of benzodiazepines: comparison with liquid chromatography and gas chromatography

Diazepam, chlordiazepoxide, and midazolam are determined by high-perfromance, thin-layer chromatography (HPTLC). Results are compared with those obtained by liquid chromatography (LC) and gas chromatography (GC). Analytical conditions for all the methods are also described. Validation parameters for linearity, precision, detection, and quantitative limits are informed. Correlation study of HPTLC with LC and GC gave a correlation factor between 0.98 and 0.99. The obtained results showed that HPTLC, LC, and GC techniques are comparable for determination of such benzodiazepins in the requested working range to be analyzed in raw materials.     

The automated determination of volatile organic contaminants in ambient air and/or soil gas by gas chromatography with selective detectors

Two systems have been developed which are suitable for the determination of sub parts per billion levels of organic contaminants in ambient air and soil gas. Gas samples are passed through an adsorbent where the contaminants are trapped. Following thermal desorption the contaminants are quantitated by capillary GC employing photoionizations electrolytic conductivity, and electron capture detection. The performance of the systems is described in detail.     

On-line coupling of extraction with gas chromatography

A recent trend in the development of analytical methodologies is the integration of the sample preparation step directly with the chromatographic system. In integrated on-line systems, extraction, clean-up, separation and detection are connected with each other and the whole analytical procedure takes place in a closed, usually automated system. In this review, a critical overview of the principles and benefits of on-line coupling of extraction with gas chromatography is presented. Techniques suitable for the extraction and analysis of various types of compounds in both solid and liquid samples are presented. The potential of on-line techniques is discussed and illustrated with selected examples.