Dual-purpose gas chromatographic injection device for pressurized liquid and gas injection

A dual-purpose gas chromatographic injection device, capable of injecting pressurized liquid sample of up to 5000 psig and gas sample with a volume as high as 5000 μL, has been successfully developed and implemented. The injection device is synergized by the effectiveness of a classical flash vaporization of a syringe injection and the reliability of a proven rotary valve. Depending on the matrix involved, this injection device employs either a commercially available four-port internal valve for liquid sampling or a six-port external valve for gas sampling, a modified removable needle used in standard liquid syringe, and an auxiliary flow stream that can be either mechanical or electronic flow controlled for solute transfer. For pressurized liquid, the device was found suitable of up to nC₁₆ hydrocarbon with no observable carry-over despite the injection device was operating at ambient temperature. A relative standard deviation of less than 2% (n = 20) was obtained for hydrocarbon compounds ranging from nC₈ to nC₁₆. For gas injection, the device performed well even under difficult chromatographic conditions such as with a low column inlet pressure of less than 1 psig. A relative standard deviation of less than 0.5% (n = 10) was obtained for reactive sulfur compounds such as alkyl mercaptans. The device can be operated manually or automated with pneumatic or electrical actuator, is platform neutral, and can be moved amongst instruments without hardware modification as well as implemented for on-line or in situ applications. In this paper, the utility of the device was also demonstrated with selected GC applications of industrial significance.     

Gas-chromatographic headspace analysis of phenol and cresols in soils by direct acetylation

A quick, simple and selective procedure has been developed for the determination of phenol, o-cresol, m-cresol, and p-cresol in soil samples. The method is based on the gas chromatographic headspace analysis of phenols as acetate derivatives. These acetates were prepared directly in the wetted soil samples by acetic anhydride in the presence of KHCO_3˙ With this procedure sample handling is reduced to a minimum as desorption and derivatization of the analytes are developed from the wetted samples inside the vial of the head space sampler. The acetate esters were analyzed using a 60 m × 0.56 mm DIIDP capillary column (di-isodecyl-phthalate), using 2-chlorophenol as an internal standard. The detection limit was between 0.03–0.08 μg g⁻¹ for the different species considered. Soil samples having carbon contents lower than 5 % can be analyzed by the proposed procedure.     

Nitrogen isotopic analyses by isotope-ratio-monitoring gas chromatography / mass spectrometry

Amino acids containing natural-abundance levels of ¹⁵N were derivatized and analyzed isotopically using a technique in which individual compounds are separated by gas chromatography, combusted on-line, and the product stream sent directly to an isotope-ratio mass spectrometer. For samples of N₂ gas, standard deviations of ratio measurement were better than 0.1‰ (Units for δ are parts per thousand or per million (‰).) for samples larger than 400 pmol and better than 0.5‰ for samples larger than 25 pmol (0.1‰ ¹⁵N is equivalent to 0.00004 atom % ¹⁵N). Results duplicated those of conventional, batchwise analyses to within 0.05‰. For combustion of organic compounds yielding CO₂/N₂ ratios between 14 and 28, in particular for N-acetyl n-propyl derivatives of amino acids, δ values were within 0.25‰ of results obtained using conventional techniques and standard deviations were better than 0.35‰. Pooled data for measurements of all amino acids produced an accuracy and precision of 0.04 and 0.23‰, respectively, when 2 mnol of each amino acid was injected on column and 20% of the stream of combustion products was delivered to the mass spectrometer.     

The α-AlB12 structure

The crystal structure of α-AlB₁₂, reported recently by Higashi, Sakurai, and Atoda is confirmed by an independent investigation of a different crystal. The space group is P4₁2₁2 (or P4₃2₁2) and our lattice parameters are a = 10.161(7)Å, c = 14.283(8)Å. The structure was partially solved by Patterson methods, when the full structure was communicated to us by Higashi et al. Utilizing 2393 reflections, our structure refinement yields an R value of 2.6% with very low standard deviations for structural parameters (0.0001 for atomic coordinates of boron atoms). Within the standard deviations there is excellent agreement with all parameters determined by Higashi et al. It is found, nevertheless, that the structural parameters of α-AlB₁₂ do not serve to account for the reported Debye-Scherrer patterns of “BeB₆,” “LiB₆,” or β-tetragonal boron, all of which have the same cell dimensions and space group as α-AlB₁₂.     

Detection of Benzo[a]pyrene in Fried Food by Ultrasound-Assisted Matrix Solid-Phase Dispersion and Isotope Dilution GC–MS

A simple, sensitive and reliable analytical method was developed for the detection of benzo[a]pyrene in fried food using gas chromatography-mass spectrometry with an isotope-labelled internal standard. Samples were directly extracted and purified by the ultrasound-assisted matrix solid-phase dispersion (MSPD) procedure. The simple pretreatment procedures were tested with different absorbents (C18, NH₂, Oasis, and SiO₂). The optimal ultrasonication-assisted MSPD was achieved by MSPD-SiO₂ and sonication for 10 min in an ultrasonic bath. The samples were quantified using benzo[a]pyrene-d₁₂ as the internal standard. An analysis of the samples spiked with different levels of benzo[a]pyrene showed recoveries ranging from 84.6 to 103.2 %, with an RSD of 3.21–8.32 %, depending on the spiking level. This method was thus shown to be suitable for the detection of benzo[a]pyrene in fried food.     

Radio gas chromatography of14C-labelled compounds by measuring the radioactivity of the FID combustion products after GLC analysis

Summary A simple method of radio gas chromatography, which avoids the necessity for an effluent gas stream splitter and a special reactor after the GLC column, has been described. The system uses the FID as a combined mass detector and combustion furnace for the conversion of¹⁴C-labelled compounds into¹⁴CO₂ and operates the FID in series with the¹⁴CO₂ detection system. Specific activity values of weakly¹⁴C-labelled compounds such as organic methyl esters and TMS sugars can be determined precisely with the standard error of the mean less than 3%.     

Gas transport in TiO2 nanoparticle-filled poly(1-trimethylsilyl-1-propyne)

Titanium dioxide (TiO₂) nanoparticles were dispersed via solution processing in poly(1-trimethylsilyl-1-propyne) (PTMSP) to form nanocomposite films. Nanoparticle dispersion was investigated using atomic force microscopy and transmission electron microscopy. At low-particle loadings, nanoparticles were dispersed individually and in nanoscale aggregates. At high-particle loadings, some nanoparticles formed micron-sized aggregates. The gas transport and density exhibited a strong dependence on nanoparticle loading. At low-TiO₂ loadings, the composite density was similar to or slightly higher than that predicted by a two-phase additive model. However, at particle loadings exceeding approximately 7 nominal vol.%, the density was markedly lower than predicted, suggesting that the particles induced the creation of void space within the nanocomposite. For example, when the TiO₂ nominal volume fraction was 0.35, the polymer/particle composite density was 40% lower than expected based on a two-phase additive model for density. At low-nanoparticle loading, light gas permeability was lower than that of the unfilled polymer. At higher nanoparticle loadings, light gas permeability (i.e., CO₂, N₂, and CH₄) increased to more than four times higher than in unfilled PTMSP. At most, selectivity changed only slightly with particle loading.     

Fast response speed of mechanically exfoliated MoS2 modified by PbS in detecting NO2

MoS₂, acting as a promising gas sensing material, has shown huge potential in monitoring of toxic and harmful gases at room temperature. However, MoS₂-based gas sensors still suffer from poor gas sensing performance such as poor sensitivity, long response time. Constructing the heterostructure is an effective approach to improve gas-sensing performance of MoS₂. Herein, PbS@MoS₂ composites synthesized by mechanical exfoliation combining with wet-chemical precipitation are used to investigate its performance in detecting NO₂ at room temperature. The response value of PbS@MoS₂ gas sensor against NO₂ is significantly improved compared with the pure MoS₂ gas sensor. At the same time, the modification with PbS also accelerates the response speed of MoS₂, and the response time is almost reduced by two orders of magnitude, from hundreds of seconds to less than ten seconds. The enhanced response value and fast response time are mainly benefited from the modulation effect of NO₂ to PbS@MoS₂ heterostructure and the mechanically exfoliated MoS₂ surface with few defects. This work can be expected to provide useful guidance for designing composite materials with excellent gas sensing properties.     

Asensitive,Selective, and Rapid Colorimetric Method to Determine Cyanide Contamination in Containers,Capsules, and Liquids

Abstract

A new formulation of the Konig colorimetric method for cyanide is selective, has increased stability, and is sensitive to a few ppb. The formulation can be placed as a strip on the safety seals of bottles to indicate tampering, placed in a syringe and used to sample head space gas in commercial products or liquids to test for tampering, or placed in the desiccant container in bottles of capsules or tablets. Reaction with head space gas requires just a few seconds, and less than an hour is required if the cyanide must first permeate through capsules. Liquids also can be either drawn directly into the syringe, or the cyanide evolved by adding an Alka-seltzer tablet to purge the gas. Reactions are immediate. Hydrogen sulfide and ammonia which interfere with several other cyanide tests, do not interfere with this formulation. A mixture of N-chlorosuccinimide and succinimide oxidizes CN^- to CN⁺. The cyanogen ion selectively reacts with 4-phenylpyridine to form a dialdehyde, which is then coupled with either barbituric acid (red rapidly turning to blue) or 3-methyl-l-phenyl-2-pyrazolin-5-one (purple slowly turning to green black).     

Ultrasonic nebulization extraction coupled with on-line gas chromatography for determination of trans-anethole in spices

Ultrasonic nebulization extraction (UNE) coupled with on-line gas chromatography (GC) was proposed for the determination of trans-anethole in fruits of Illicium verum Hook. f. and Foeniculum vulgare Mill. The extraction was performed in a common self-made extraction system. In the UNE the analyte was transferred and enriched from the solid sample to gas phase. The sample gas containing analyte obtained by UNE was introduced into the sampling loop with the purging gas (N₂). And then the sample gas in the sampling loop was introduced into the GC column. Several experimental parameters of on-line UNE-GC, including sampling time, flow rate of purging gas, standstill time and temperature of tubing, were optimized. The calibration curve ranging from 0.05 to 1.5 mg g⁻¹ for determining the trans-anethole was obtained. The recoveries for determining trans-anethole are between 99.2% and 111.2% and RSDs are less than 8.3% when the UNE was applied. The analytes can rapidly be extracted and transferred from the solid sample to gas phase. The analytes in the gas phase are easier to be introduced into GC system than those in the solid and liquid phase. Compared with off-line systems, the proposed on-line system is more suitable to detect volatile compounds.     

Headspace Liquid-Phase Microextraction of Short-Chain Fatty Acids in Plasma, and Gas Chromatography with Flame Ionization Detection

A novel method for analysis of short-chain fatty acids (SCFA) in blood plasma has been developed by combining headspace liquid-phase microextraction (HS-LPME) and gas chromatography with flame ionization detection. HS-LPME conditions, for example working solution volume, salt concentration, and extraction temperature and time, were optimized by means of an L₉ (3⁴) orthogonal array design. The effects on extraction efficiency of the type of solvent and the volume of the microdrop, the pH of the sample matrix, and the stirring rate were examined. Under optimum conditions the linear calibration ranges for SCFA were from 0.08 to 80 µg mL^?1 (r ²>0.9921) with detection limits of 0.02 to 0.08 µg mL ^?1 and relative standard deviation less than 9.81%. Recovery from spiked blood plasma samples was more than 70.5%. The method was satisfactorily applied to analysis of SCFA in rat blood plasma samples from the blind-loop model.     

Determination of Biacetyl in Beer by Liquid Chromatography with Sensitized Phosphorescence Detection

Abstract

A new determination method for biacetyl in beer is developed. After a simple sample preparation method and separation by HPLC, biacetyl is detected by sensitized room temperature phosphorescence. A detection limit of 0.5ppb biacetyl in beer can be obtained. The linearity of response is 0.5–150ppb; the relative standard deviation is 4.2% at the 16ppb level. Prepared samples are stable for at least 20 hours when kept in closed vials. The separation of biacetyl and 2,3-pentanedione, an α-diketone also present in beer, is described. It is found that sensitized phsophorescence detection is about a factor of 30 less sensitive for 2,3-pentanedione. The reliability of the method is demonstrated by comparison with a routine head space gas chromatography method.     

Mode-filtered light methane gas sensor based on cryptophane A

A mode-filtered light sensor has been developed for methane (CH₄) gas determination at ambient conditions. The proposed chemosensor consisted of an annular column which was constructed by inserting an optical fiber coated with a thin silicone cladding of cryptophane A into a fused-silica capillary. When CH₄ was introduced to the sensor, selective inclusion of CH₄ into the silicone layer would cause a change in the local refractive index of the cladding, resulting in the change of mode-filtered light that emanated from the fiber. Three detection windows were set alongside the capillary to propagate the light to a charge-coupled device (CCD). The changes of mode-filtered light on exposure to various concentrations of CH₄ were thus simultaneously monitored. The mode-filtered light intensity decreased with the increase in concentration of CH₄. The dynamic concentration range of the sensor for CH₄ was 0.0-16.0% v/v with a detection limit of 0.15% v/v. The highest sensitivity was found at the channel furthest away from the excitation light source. The response time (t_95%) was about 5 min. The reproducibility was good with a relative standard deviation (RSD) of less than 7% from evaluating six cryptophane A-coated fibers. Oxygen, hydrogen and carbon dioxide showed very little interference on detection but interferences from dichloromethane and carbon tetrachloride were observed. The proposed mode-filtered light sensor has been successfully applied to determine CH₄ samples and the accuracy was good. Our work offers a promising approach for CH₄ detection.     

Gas permeabilities of polyurethane films for fresh produce packaging: Response of O2 permeability to temperature and relative humidity

Thermoplastic shape memory polyurethane (SMPU) polymers were synthesized, cast to films, and their gas barrier properties were characterized. In addition, performance of an optical method was assessed by measuring oxygen permeability (P_O2) of the films. P_O2 of the SMPU film was at least two times higher than that of low density polyethylene (LDPE and increased at higher relative humidity. Permselectivity (P_CO2/P_O2) of the SMPU film was 15, which is approximately three times higher than for LDPE. The film absorbed circa 18% water vapor at 98% relative humidity. The optical method agreed very well (maximum 20% deviation) with a standard carrier gas method in P_O2 measurement. Overall our results show that SMPU is an attractive polymer for fresh produce packaging.     

In situ sulfur isotopes (δS and δS) analyses in sulfides and elemental sulfur using high sensitivity cones combined with the addition of nitrogen by laser ablation MC-ICP-MS

The sulfur isotope is an important geochemical tracer in diverse fields of geosciences. In this study, the effects of three different cone combinations with the addition of N₂ on the performance of in situ S isotope analyses were investigated in detail. The signal intensities of S isotopes were improved by a factor of 2.3 and 3.6 using the X skimmer cone combined with the standard sample cone or the Jet sample cone, respectively, compared with the standard arrangement (H skimmer cone combined with the standard sample cone). This signal enhancement is important for the improvement of the precision and accuracy of in situ S isotope analysis at high spatial resolution. Different cone combinations have a significant effect on the mass bias and mass bias stability for S isotopes. Poor precisions of S isotope ratios were obtained using the Jet and X cones combination at their corresponding optimum makeup gas flow when using Ar plasma only. The addition of 4–8 ml min⁻¹ nitrogen to the central gas flow in laser ablation MC-ICP-MS was found to significantly enlarge the mass bias stability zone at their corresponding optimum makeup gas flow in these three different cone combinations. The polyatomic interferences of OO, SH, OOH were also significantly reduced, and the interference free plateaus of sulfur isotopes became broader and flatter in the nitrogen mode (N₂ = 4 ml min⁻¹). However, the signal intensity of S was not increased by the addition of nitrogen in this study. The laser fluence and ablation mode had significant effects on sulfur isotope fractionation during the analysis of sulfides and elemental sulfur by laser ablation MC-ICP-MS. The matrix effect among different sulfides and elemental sulfur was observed, but could be significantly reduced by line scan ablation in preference to single spot ablation under the optimized fluence. It is recommended that the d₉₀ values of the particles in pressed powder pellets for accurate and precise S isotope analysis should be less than 10 μm. Under the selected optimized analytical conditions, excellent agreements between the determined values and the reference values were achieved for the IAEA-S series standard reference materials and a set of six well-characterized, isotopic homogeneous sulfide standards (PPP-1, MoS₂, MASS-1, P-GBW07267, P-GBW07268, P-GBW07270), validating the capability of the developed method for providing high-quality in situ S isotope data in sulfides and elemental sulfur.     

The estimation of phase modifications in polymers by the method of “molecular probes” in gas chromatography—III. Glass transition behaviour and compatibility in mixtures of poly(methyl methacrylate) with poly(vinyl chloride)

Using the method of “molecular probes” in gas chromatography, glass-transitions in mixtures of poly(methyl methacrylate) with poly(vinyl chloride) are found. The strong variation of T_g for poly (methyl methacrylate) in mixtures with more than 20% of that polymer suggests interactions although T_g for poly(vinyl chloride) remains constant. The existence of a single T_g in mixtures with less than 20% poly(methyl methacrylate), on the other hand, suggests possible compatibility in these mixtures.     

Alleviation of polyatomic ion interferences for determination of chlorine isotope ratios by inductively coupled plasma mass spectrometry

A simple variation in sample preparation and introduction allows the measurement of chlorine isotope ratios by inductively coupled plasma mass spectrometry (ICP/MS). Dissolution of the sample in D₂O rather than H₂O attenuates the major polyatomic ion ³⁶ArH⁺ and frees m/z 37 for determination of ³⁷Cl⁺. The isotope ratio ³⁵Cl/³⁷Cl in a 50 mg/L solution of Cl as LiCl is determined with a relative standard deviation of 0.21%. Sample memory is low, as the ³⁵Cl signal decays to less than 1% of its original value after ～2 min of cleanout with D_2O . The detection limit for Cl using this procedure is approximately 20 μg/L.     

Use of calibration gases in the U.S. acid rain program

The United States Acid Rain Program continuous emission monitors (CEMs) have been successful in producing quality-assured data 95% of the time, and in meeting a relative accuracy standard of less than or equal to 10.0% at over 99% of the CEMs in the program. One key reason for this high accuracy is the required use of high quality calibration gases in certification and quality assurance/quality control (QA/QC) tests. An annual QA audit helps ensure high quality calibration gases. A third party purchases gases from gas vendors. An Environmental Protection Agency (EPA) laboratory analyzes the gases and compares the results with the tag value on the cylinder. The results are posted on an EPA website. This allows purchasers of calibration gases to buy gases from vendors producing the most accurate gases. Over time, we believe it also results in better accuracy from all gas vendors. Because of a change in SO₂ quantification methodology, SO₂ emissions were underreported by approximately 2% between 1989 and 1996. EPA, the National Institute for Standards and Technology and calibration gas vendors collaborated to produce a correction policy and a standard correction form to be used by affected electric utility plants. Calibration gas cylinder tag values were required to be corrected by 1 January, 1997. In the future, it is possible that cleaner, more varied sources will be regulated for greenhouse effect, ozone and toxic emissions control. This will probably require more accurate CEMs, lower calibration gas concentrations, and a broader menu of gas mixtures. Received: 23 December 1999 Accepted: 12 December 2000     

催化氧化处理工业废气中高浓度甲醛

制备了用于温和条件下催化氧化去除工业废气中高浓度甲醛(HCHO)的1%Pt-4%CeO₂/AC催化剂. 将高浓度甲醛的催化氧化过程与双甘膦氧化制备草甘膦的反应过程集成在一起,使草甘膦合成过程中产生和排放出来的甲醛(100-300 mg/m³)在通过催化剂床层时被完全除去. 系统研究了温度、空速和甲醛含量对甲醛去除率的影响. 在气体空速(GHSV)低于20000 h^-1时废气中几乎所有的甲醛都被氧化,处理后的废气中的甲醛含量低于0.1 mg/m³,甲醛的转化率为99.1%-100%. 当GHSV为30000- h^-1,催化剂床层温度为12℃时,生产废气通过催化剂床层后的甲醛含量小于1.5 mg/m³,甲醛的转化率为97.56%-99.99%. 1%Pt-4%CeO₂/AC催化剂的中试试验结果表明,处理后最终尾气中甲醛含量小于10 mg/m³,有效地防止了甲醛对人们健康的危害,具有良好的产业化前景.     

The ageing phenomenon of polyethersulphone hollow fibre membranes for gas separation and their characteristics

We have studied the ageing behaviour of PES/NMP (polyethersulphone/N-methyl pyrrolidone) hollow fibres for gas separation that were prepared from 35% and 37% dope. The effect of ageing on hollow fibres spun from low and high shear rate (103 vs. 862 s⁻¹) has also been investigated, in terms of their transport properties (permeation flux and separation performance), thermal, mechanical and tensile properties. Hollow fibres in this study were aged for around four months in ambient air at room temperature prior to testing.In general, the gas permeation flux drops steeply during the 40 days following fabrication and levels off thereafter. The O₂/N₂ selectivity decreases slightly over time. Hollow fibres spun with high shear rate seem to age faster than those spun with low shear rate. The gas fluxes of both membranes were found to follow a log–log relationship with ageing time. For almost all the gases used in this study, the gas flux decay rate, calculated from the slope of the log–log plot of gas flux vs. ageing, is higher for membranes spun with high shear rate. The effect of shear rate on ageing is less significant for smaller gas molecules that travel faster such as He and H₂. No significant effect of ageing on gas selectivity was observed. Experimental results also indicate that the storage modulus and loss modulus of the hollow fibres increase with ageing. Hollow fibres spun with high shear rates give a slightly higher increase in these moduli than those spun at low shear rates. Surprisingly, tangent δ (energy dissipation) and glass transitional temperature are not sensitive to ageing. We also found that the tensile yield strength and Young's modulus of the hollow fibres increase slightly with ageing. The hollow fibre membranes spun at high shear rates also show a higher increment in tensile yield stress. However, the change in Young's modulus due to ageing was similar for fibres spun with high and low shear rates.