Determination of residual solvents in pharmaceuticals by thermal desorption-GC/MS.

A novel method for the determination of residual solvents in pharmaceuticals by thermal desorption (TD)-GC/MS has been established. A programmed temperature pyrolyzer (double shot pyrolyzer) is applied for the TD. This method does not require any sample pretreatment and allows very small amounts of the sample. Directly desorbed solvents from intact pharmaceuticals (ca. 1 mg) in the desorption cup (5 mm x 3.8 mm i.d.) were cryofocused at the head of a capillary column prior to a GC/MS analysis. The desorption temperature was set at a point about 20 degrees C higher than the melting point of each sample individually, and held for 3 min. The analytical results using 7 different pharmaceuticals were in agreement with those obtained by direct injection (DI) of the solution, followed by USP XXIII. This proposed TD-GC/MS method was demonstrated to be very useful for the identification and quantification of residual solvents. Furthermore, this method was simple, allowed rapid analysis and gave good repeatability.     

铁皮石斛的裂解气相色谱指纹图谱及其系统聚类分析

采用裂解气相色谱/质谱法(Py-GC/MS)测定了10种不同产地的铁皮石斛并结合系统聚类分析法比较了这些铁皮石斛的指纹图谱,采用释放气体分析法考察了裂解温度对指纹图谱的影响。结果表明,0.4 mg样品在450 ℃下可瞬间裂解,10种样品的指纹图谱具有相似性,且重现性好;采用系统聚类分析能区别不同产地的样品。本法快速、简便、准确,不失为药材质量控制的良好方法。     

Thermoanalytical characterization of polymers: A comparative study between thermogravimetry and evolved gas analysis using a temperature-programmable pyrolyzer

Evolved gas analysis (EGA) was carried out on 15 synthetic polymer samples using a temperature programmable pyrolyzer as a heating unit which was on-line coupled with a MS detector. A deactivated stainless steel tube and a vent free adapter were used to couple the pyrolyzer with the MS detector, and they were placed in a GC oven at 300°C to avoid condensation of evolved gases with high boiling point. Thermograms of polystyrene measured by this system (Py-EGA-MS) showed shifts of the peak temperature to the higher temperature region as the sample mass increased. It was found that the S/N ratios of EGA thermograms were 420-fold superior to those of differential thermogravimetry (DTG) thermograms for the same sample mass of 0.20 mg. Since a good linear relationship was obtained between peak temperatures obtained by Py-EGA-MS and DTG, it can be concluded that Py-EGA-MS can be used to obtain reliable data on thermal properties of samples with high sensitivity and using less sample.     

Influence of the sample temperature on the desorption of matrix molecules and ions in matrix-assisted laser desorption ionization

The effect of the (initial) sample temperature on the threshold laser fluences and the increase of signal intensities with laser fluence has been investigated for UV-laser desorbed 2,5-dihydroxybenzoic acid (DHB) ions and (photoionized) neutral DHB molecules using a ‘flat-top’ laser profile for desorption. A linear increase in threshold fluence with decreasing temperature was observed for neutrally desorbed molecules as well as ions in the investigated temperature range of approximately − 100°C to + 20°C. The results are discussed and interpreted in the framework of a quasi-thermal desorption model (IJMSIP 141 (1995) 127–148).     

Sensitive determination of styrene and related compounds in human body fluids by headspace capillary gas chromatography with cryogenic oven trapping

Summary A simple and sensitive method is presented for determination of styrene, toluene, ethylbenzene, isopropylbenzene andn-propylbenzene in human body fluids by capillary gas chromatography (GC) with cryogenic oven trapping. After heating a blood or urine sample containing each compound andp-diethylbenzene (internal standard, IS) in a 7.0-mL vial at 60°C for 20 min, 5 mL of headspace vapor was drawn into a glass syringe and injected into a GC. All vapor was introduced into an Rtx-Volatile middle bore capillary column in splitless mode at oven temperature of 20°C to trap entire analytes, and the oven temperature then programmed to 280°C for GC measurements by flame ionization detection. The present conditions gave sharp peaks of each compound and IS, and low background noises for whole blood or urine samples.     

Pyrolysis of liquefied petroleum gas assisted by radicals desorbed from mesh catalyst surface

The purpose of this study is to understand the reactions on the catalyst surface and in the gas phase during the catalytic pyrolysis of light hydrocarbons. To avoid the complexity of internal pore diffusion and heat transfer limitation, nickel mesh without pore structure was used as a catalyst for the catalytic pyrolysis of a commercial liquefied petroleum gas (LPG) sample in a quartz tube reactor and in a wire‐mesh reactor over a temperature range of 600–850°C. With a Ni mesh catalyst, no catalyst deactivation associated with coke formation was observed at high gas flow rate. Our experimental results indicate that the desorption of radicals from the catalyst surface is an important process in the catalytic pyrolysis of LPG using the Ni mesh catalyst. The desorption of radicals across the gas–catalyst interface is greatly facilitated by increasing gas flow rate passing through the mesh. The desorbed radicals would initiate and/or enhance the gas‐phase radical chain reactions and lead to improved reaction rates for the pyrolysis of LPG although the product selectivities remained unchanged. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 637–646, 2003     

Pyrolysis field ionization mass spectrometry of carbohydrates: Part b: Polysaccharides

The application of pyrolysis in combination with field ionization (FI) mass spectrometry for the characterization and identification of polysaccharides is reported. Polysaccharides such as xylan, agarose and alginic acid, which contain monomer subunits of different elemental composition, can be differentiated in a straightforward manner by the field ionization spectra of their Curie-point pyrolysates. Polysaccharides with hexosyl subunits, such as cellulose, galactan, laminaran and mannan, were pyrolysed by Curie-point pyrolysis and show photographically recorded FI spectra which differ in the relative heights of their pyrolysis peaks. Characteristic pyrolysis products are formed, which can be identified or assigned structures on the basis of accurate mass measurements, direct isotopic determination and by analogy with established chemical procedures and mechanisms.Oven pyrolysis of polysaccharides combined with electrical detection of the FI spectra at low mass resolution gives a higher sensitivity and better reproducibility for all peaks over the whole mass range. From sample amounts of about 40 μg, spectra are obtained by raising the oven temperature by 0.4°C/s. Utilizing repetitive magnetic scanning, registration and signal processing by the data system, the standard deviation of the peak heights for five repeated measurements is about 10%. Accumulation of about 30 spectra in a limited mass range on a multi-channel analyser gives results which vary by about 2–3% on average, despite a lower sample consumption (20–30 μg). Oven pyrolysis between 250 and 400°C yields significant differences in the spectra of differently linked mannans and allows an unequivocal differentiation of these isomers. Following FI, field desorption (FD) spectra were obtained from pyrolysis products condensed on the emitter surface by heating of the emitter wire between 10 and 30 mA. The cations of alkali metals, such as Na*, K* and Cs*, can be registered in this way. Most interesting is the detection of the molecular ions of monomer and oligomer subunits of the polysaccharides as complementary analytical information in the FD mode. Obviously, condensation of these neutral, thermal products on the emitter surface occurs without field ionization and desorption is initiated by supply of thermal energy to the adsorbed sample layer.     

Transport behavior of carbon disulfide into poly aryl ether ether ketone

The sorption/desorption of carbon disulfide into/from PEEK as a function of crystallinity and temperature was investigated. The sorption curves of carbon disulfide into PEEK show only two major regions: (a) an increase of penetrant weight with time, and (b) a limiting equilibrium value (solubility). This is in contrast to the sorption of toluene into PEEK which shows three regions. The solubility of carbon disulfide decreases with increasing crystallinity, but temperature has little effect on the solubility in the temperature range of 25–40°C. An acceleration in penetration rate at the later stage of diffusion is observed for PEEK films whose crystallinity is greater than 13.4%, suggesting Supercase II diffusion. Carbon disulfide can be desorbed completely from PEEK in contrast to other fluids, such as toluene or methylene chloride, which are difficult to desorb. The normalized weight loss of carbon disulfide during desorption is an exponential function of square-root time. Solvent-induced crystallization was observed. Crystallinity was estimated from both the measured density and microhardness of the desorbed polymer and polymer which had undergone a sorption/desorption/resorption/desorption cycle. © 1996 John Wiley & Sons, Inc.     

High-temperature pyrolysis of poly(vinyl chloride): Gas chromatographic-mass spectrometric analysis of the pyrolysis products from PVC resin and plastisols

A pyrolysis–gas chromatographic–mass spectrometric technique for analyzing the pyrolysis products from polymers in an inert atmosphere is described. Initial studies encompassing the pyrolysis of poly(vinyl chloride) homopolymer and a series of PVC plastisols (based on o-phthalate esters) have provided a complete qualitative and semi-quantitative analysis of the pyrolysis products from these materials. PVC resin yields a series of aliphatic and aromatic hydrocarbons when pyrolyzed at 600°C; the amount of aromatic products is greater than the amount of aliphatic products. Benzene is the major organic degradation product. A typical PVC plastisol [PVC/o-dioctyl phthalate (100/60)] yields, upon pyrolysis, products that are characteristic of both the PVC matrix and the phthalate plasticizer. The pyrolysis products from the plasticizer dilute those from the PVC portion of the plastisol and are, in turn, the major degradation products. There are no degradation products resulting from an interaction of the PVC with the plastisol. The pyrograms resulting from pyrolysis of the various plastisols of PVC can be used for purposes of “fingerprinting.” Identification of the major peaks in a typical plastisol pyrogram provides information leading to a precise identification of the plasticizer. The pyrolysis data from this study were related to a special case of flammability and toxicity.     

Optical studies of pyrolyzed polyacrylonitrile

The results of measurements of the optical absorption spectra, IR absorption spectra, thermogravimetric analysis, and elementary analysis of pyrolyzed polyacrilonitrile are presented as a function of the pyrolysis temperature T_p and of the duration of pyrolysis. At T_p ～ 200°C, an intermediate phase was discovered, containing conjugated CN sequences and a completely unreacted carbon backbone. The optical absorption data imply that the resulting polymer is a semiconductor with a delocalized π-electron system and an energy gap E_g ? 2.5 eV. For T_p > 260°C, the weight loss rapidly increases, and the absorption edge gradually broadens and shifts to lower energies. The resulting polymer (after higher-temperature pyrolysis) contains CN and C?C? C conjugation sequences, but appears to be a complex structure consisting of a mixture of different chemical species.     

Crystallization during polymerization of poly-p-xylylene. III. Crystal structure and molecular orientation as a function of temperature

Polymerization of p-xylylene was carried out from the gas phase with monomer produced by the pyrolysis of [2,2]-p-cyclophane. The crystalline form and preferred orientation of as-polymerized polymer deposited at various temperatures (?196 to 80°C) were investigated by x-ray diffraction methods. The melting behavior and other thermal transitions were studied by DSC. At 80°C the polymer film deposit is a mixture of the α and β forms, while between 60 and 0°C the deposit is of the α form. At lower temperature the polymer deposit is mainly of the β form, which shows diffuse reflections. At liquid nitrogen temperature it is of the β form with sharp reflections, contaminated with a small amount of oligomer. It was also found that at low temperatures, fibrillar crystals grow from the substrate in a direction 45° against the gas flow, and at even lower temperature, well-oriented filmlike crystals grow perpendicular to the substrate surface.     

Pyrolysis – capillary gas chromatography combined with on-line alkylation for the compositional analysis of liquid crystalline aromatic polyesters

Pyrolysis–capillary gas chromatography combined with on-line alkylation has been employed for compositional analysis of liquid crystalline aromatic polyesters (LCPs) based on p- hydroxybenzoic acid. The fundamental conditions were examined using an LCP prepared from p-hydroxybenzoic acid, terephthalic acid, and 4,4′-biphenol and every constituent of a sample weighing ca. 50 μg was almost quantitatively recovered as its dimethyl derivative in the pyrogram obtained following pyrolysis at 400°C in the presence of 1 μl of 25% tetramethyl-ammonium hydroxide in methanol. The compositions of a variety of terpolyesters and LCPs containing isophthalic acid or 2-hydroxy-6-naphthoic acid units have been precisely deter-mined by pyrolysis-methylation GC.     

Thermochemical Properties of the Lattice Oxygen in W,Mn-Containing Mixed Oxide Catalysts for the Oxidative Coupling of Methane

Mixed NaWMn/SiO₂ oxide, samples containing individual components (Na, W, Mn) and their double combinations (Na–W, Na–Mn, W–Mn) supported on silica were studied by temperature programmed reduction (TPR) and desorption (TPD), and heat flow calorimetry during their reoxidation with molecular oxygen in pulse mode. The NaWMn/SiO₂ mixed oxide was shown to contain two different types of reactive lattice oxygen. The weakly-bonded oxygen can be reversibly released from the oxide in a flow of inert gas in the temperature range of 575?900°C, while the strongly-bonded oxygen can be removed during the reduction of the sample with hydrogen at 700–900°C. The measured thermal effect of oxygen consumption for these two oxygen forms are 185 and 350 kJ/mol, respectively. The amount of oxygen removed at reduction (~443 μmol/g) considerably exceeded the amount desorbed in an inert gas flow (~56 μmol/g). The obtained results suggest that the reversible oxygen desorption is due to the redox process in which manganese ions are involved, while during the temperature programmed reduction, mainly oxygen bonded with tungsten is removed.     

Low temperature pyrolysis of carboxymethylcellulose

Unlike the common high temperature pyrolysis of carboxymethylcellulose (CMC) targeting activated carbon, this study investigates the pyrolytic behaviour of plain CMC at low temperatures ranging between 260 and 300 °C. Preliminary experiments were conducted using differential scanning calorimetry to define the temperature range necessary for the process. Low-temperature pyrolysis was then simulated using thermogravimetric analysis under inert atmosphere. Investigations reveal that a minimum holding temperature of 260 °C is required for an isothermal process, at which pyrolysis is terminated after around 26 min. Increasing exposure temperature reduces pyrolysis time. Within the range of the investigated sample and CMC particle size, no significant effects could be measured regarding the decomposition behaviour. The resulting char was further analysed using X-ray diffraction and Fourier transform infrared spectroscopy. Visual inspection was conducted using scanning electron microscopy. Upon pyrolysis, originally longitudinally shaped CMC was found to be converted into spherical particles of functionalised amorphous carbon with an average particle size of 41 µm.     

Acid Properties of SiMCM-41 Mesoporous Molecular Sieve

The SiMCM-41 mesoporous molecular sieve was synthesised by the hydrothermal method. The physicochemical characterisation by infrared spectroscopy, X-ray diffraction and thermogravimetry, showed that the material presents a well-defined structure. In this study, the determination of the total acidity and relative strength of the acid sites of the SiMCM-41,was performed by desorption of adsorbed n-butylamine combined with thermogravimetric measurements. The total acidity, determined by means of the amount of amine desorbed normalised by mass of solid, was equivalent to 0.927 mmol g^–1, in the temperature range from 96 to 235°C. By using the Flynn and Wall integral kinetic model, at 5, 10 and 20°C min^–1 heating rates, it was determined that the activation energy to desorb n-butylamine was 90.7 kJ mol^–1, in the same temperature range, evidencing that SiMCM-41 presents only weak acid sites on its surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Multisimplex Optimisation of the Purge-and-Trap Preconcentration of Volatile Fatty Acids, Phenols and Indoles in Cow Slurries

In this work the simultaneous purge-and-trap (P&;T) preconcentration of volatile fatty acids (acetic acid, propanoic acid, butanoic acid, iso-butanoic acid, pentanoic acid, iso-pentanoic acid, hexanoic acid and heptanoic acid), phenols (phenol, 4-methylphenol and 4-ethylphenol) and indoles (indole and 3-methylindole) from cow slurries was carried out in order to quantify them by gas chromatography flame ionisation detection (GC–FID). The optimisation of the preconcentration was performed using spiked cow slurries and the variables studied were the nature of the trap, the purge and heating time and the amount of inert salt (NaCl) added. Other parameters such as purge flow, sample volume, desorption time and temperature and baking time and temperature were kept constant. The first variable studied was the nature of the preconcentration trap. Four different commercially available traps (Vocarb 4000, Vocarb 3000, BTEXTRAP and Tenax/silica gel/carbon) were studied and Vocarb 3000 gave best results. Once the optimum trap was chosen, appearance of possible memory peaks were studied and significant signals were observed for 4-methylphenol, 4-ethylphenol, indole and 3-methylindole. In order to improve the blanks after each analysis the following actions were taken: (1) the needle sparger was located in the headspace of the sample and (2) all the replaceable parts of the P&;T (sample tube, needle sparger, Teflon ferrules, nuts, etc.) were washed, sonicated in acetone for 15 min and dried in an oven at 350 °C. Once the blank problems were minimised, the remaining variables (purge and heating time and the amount of NaCl) were optimised using the MultiSimplex^® program, which enabled the study of several variables and responses simultaneously. Optimum conditions for the simultaneous preconcentration were obtained after 24 experiments and the final optimised preconcentration conditions were as follows: 0.4 g of NaCl were added to 10 mL sample which was heated at 80 °C for 10 min and purged for 20 min while the Vocarb 3000 trap remained at room temperature. Once the purge step was over, the trap was desorbed at 250 °C for 6 min and baked at 260 °C for 10 min. After the optimisation of the P&;T based preconcentration, the precision (within and among days), accuracy and detection limits of the method were studied.     

Studies of grass forages and their fibrous fractions by pyrolysis gas chromatography – ion trap detection

Grass forage samples with different fiber content and composition, and their fibrous fractions, have been analyzed by pyrolysis – gas chromatography with ion trap detection. The identities of the pyrolysis fragments indicated their origin to be carbohydrates, lignin, and nitrogen compounds. Comparison of the resulting quantitative data by factorial variance revealed pyrogram peaks attributable to neutral detergent fiber, cellular contents, and lignin. Quantitative pyrolysis data from these components showed the same trends as those recorded by the method of Goering and Van Soest [1].     

Selenium determination in tissue samples of Nile tilapia using ultrasound-assisted extraction

This paper proposes a method to determine selenium in samples of fish muscle and liver tissue using ultrasound assisted extraction process, and analysed by graphite furnace atomic absorption spectrometry (GFAAS). The selenium content was extracted by 0.10 M HCl at the optimal extraction conditions which were established as follows: sample mass of 100 mg; granulometry of the sample <60 μm; sonication time of five 40 s cycles; and sonication power of 136 W. The selenium determinations were performed by GFAAS, at a drying temperature of 120°C/250°C, pyrolysis temperature of 1300°C, atomization temperature of 2300°C, and cleaning temperature of 2800°C. Palladium nitrate was used as a chemical modifier coinjected with the samples, and tungsten as a permanent modifier. The concentration of selenium determined in the pool of fish muscle and liver tissue were 280.4±4.2 e 592.3±6.7 μg kg⁻¹, respectively. The accuracy and precision of the proposed extraction method were evaluated using certified standard Bovine Muscle — NIST 8414. The results obtained by the ultrasonic extraction method were equivalent to those obtained by the method of acid mineralization of samples in a microwave oven     

Fullerenes from kerogen by laser ablation fourier transform ion cyclotron resonance mass spectrometry

Raw oil shale, kerogen (demineralized shale) and carbonaceous residues from kerogen pyrolysis in the range 350–700°C (at 50°C intervals) were studied by laser ablation Fourier transform ion cyclotron resonance mass spectrometry using the fundamental frequency of Nd: YAG laser (1064 nm). Normally, pyrolysis of the raw materials produces oil and the resulting residues have decreased hydrogen to carbon ratios and exhibit relative increases in aromatic carbons. Raw shale and kerogen give positive-ion spectra with mainly protonated species of m/z 100–400. Laser ablation positive-ion mass spectra of the pyrolysis products of the kerogen show the presence of C₆₀, C₇₀ and other fullerene ions with a distribution of higher mass fullerene ions up to m/z 4000. Using high laser powers (100–3000 MW cm^?2), the residue from pyrolysis at 350°C initially did not produce any fullerene ions (apart from traces of C₆₀ and C₇₀), but after continued ablation a cavity was formed in the target and a wide distribution of fullerene ions was obtained with subsequent laser pulses. Residues obtained from the pyrolysis of kerogen at 400–500°C produced fullerene ions at both low (4–200 kW cm^?2) and high laser powers. The 550°C pyrolysis residue gave only small amounts of C₆₀ and C₇₀ positive ions at low laser power whereas residues from the pyrolysis of kerogen above 550°C did not give fullerene ions over a wide range of laser powers. It is proposed from the above results that the changes in the aromatic nature of the kerogen residues with increasing pyrolysis temperature are directly related to the ease of fullerene formation. This is possibly due to the formation of large polycyclic aromatic systems at pyrolysis temperatures above 400°C, formed in the residues. It should be noted that the shale samples (raw or pyrolysed) did not generate fullerene ions under any of the conditions employed in these experiments.     

Determination of the temperature—time profile of the sample in pyrolysis—gas chromatography

The mean temperature-time profile in a sample has been determined in order to establish whether it is possible to equate the temperature of the pyrolyzer with that of the sample being pyrolyzed. This was done by studying the degradation of cis-1,4-polybutadiene be sequential pyrolysis. The rate of degradation was determined for different amounts (0.5–20 μg) of sample. The temperature dependence of the degradation rate was determined from an Arrhenius plot. The experimental results were combined with theoretically derived expressions for the heating of samples subjected to pyrolysis. The temperature–time profiles of different amounts of sample could then be calculated.It was found that samples thinner than 1 μm were heated to a mean temperature of 95% of the temperature of the pyrolyzer in 40 ms or faster. Too thin samples can give rise to noticeable catalytic effects.By choosing the appropriate pyrolysis conditions of sample thickness, time and temperature it is possible to assume that the temperature of the sample is nearly the same as that of the pyrolyzer.