Multipurpose cold injector for high resolution gas chromatography

The multipurpose cold injector described in this paper represents a solution for an universal sampling system for high resolution gas chromatography. The system is modular and is built around the Carlo Erba cold on-column injector provided with secondary cooling. An auxiliary module, easily fixable on the bottom of the on-column injector, creates a temperature programmable vaporizing chamber making the system also suitable for cold split-splitless injections or solvent venting prior to the sample transfer into the capillary. The system can be operated manually or in a fully automatic mode using the auto-sampler for cold on-column injections. The experimental data illustrate its benefits and limitations.     

Microfabricated planar glass gas chromatography with photoionization detection

We report the development of a microfabricated gas chromatography system suitable for the separation of volatile organic compounds (VOCs) and compatible with use as a portable measurement device. Hydrofluoric acid etching of 95 × 95 mm Schott B270 wafers has been used to give symmetrical hemi-spherical channels within a glass substrate. Two matching glass plates were subsequently cold bonded with the channels aligned; the flatness of the glass surfaces resulted in strong bonding through van der Waals forces. The device comprised gas fluidic interconnections, injection zone and 7.5 and 1.4 m long, 320 μm internal diameter capillaries. Optical microscopy confirmed the capillaries to have fully circular channel profiles. Direct column heating and cooling could be achieved using a combination of resistive heaters and Peltier devices. The low thermal conductivity of glass allowed for multiple uniform temperature zones to be achieved within a single glass chip. Temperature control over the range 10–200 °C was achieved with peak power demand of approximately 25 W. The 7.5 m capillary column was static coated with a 2 μm film of non-polar dimethylpolysiloxane stationary phase. A standard FID and a modified lightweight 100 mW photoionization detector (PID) were coupled to the column and performance tested with gas mixtures of monoaromatic and monoterpene species at the parts per million concentration level. The low power GC-PID device showed good performance for a small set of VOCs and sub ng detection sensitivity to monoaromatics.     

Annular denuder tube preconcentrator for nitrobenzene determination by gas chromatography with electron-capture detection

A platinum/lead alloy-coated annular denuder tube was employed to concentrate nitrobenzene from nitrogen atmospheres of relative humidity < 5% at ambient temperatures. The limit of detection achieved was 1.7 ng of nitrobenzene, which could be obtained from a concentration of 0.15 μg m^?3 of nitrobenzene in nitrogen. Quantification was carried out at concentrations above 0.12 μg m^?3 of nitrobenzene in nitrogen over a 4-month period, although ageing and deterioration of the denuder system were noted, necessitating regular recalibration. The results support claims made elsewhere regarding the suitability of denuder tubes for organic vapour determination, in this instance with an analyte-selective collection surface.     

A double plasma gas chromatography injector and detector

A direct-current, chip-based plasma has been used for gas sample injection in gas chromatography. A second identical plasma chip has been used as the excitation source for an optical emission detector. The first plasma is normally continually sustained during operation, causing continuous ionisation/fragmentation of the sample, whilst the second plasma records the optical emission downstream. For injection, the first plasma is briefly interrupted, introducing a "plug" of unmodified sample into the system. Injection plug sizes of between 5 and 50 [micro sign]l have been reproducibly obtained, although significantly smaller volumes may be possible with the use of smaller cross-section columns, lower flow rates and/or shorter plasma interruption times.     

Multi-stage preconcentrator/focuser module designed to enable trace level determinations of trichloroethylene in indoor air with a microfabricated gas chromatograph

This article describes the development and characterization of a multi-stage preconcentrator/focuser (PCF) module designed to be integrated with a microfabricated gas chromatograph (μGC) for autonomous, in situ determinations of volatile organic compounds. The PCF module has been optimized specifically for the determination of trichloroethylene (TCE) vapors at low- or sub-parts-per-billion concentrations in the presence of common indoor air co-contaminants in residences at risk of vapor intrusion (VI) from surrounding TCE-contaminated soil. It consists of three adsorbent-packed devices arranged in series: a pre-trap of conventional (tubular metal) design for capturing interferences with vapor pressures <3 torr; a high-volume sampler, also of conventional design, for capturing (and transferring) TCE and other compounds with vapor pressures within the range of ~3 to 95 torr; and a microfocuser (μF) consisting of a micromachined Si chamber with an integrated microheater for focusing and injecting samples into the separation module. The adsorbent masses, sampling and desorption flow rates, and heating profiles required for selective, quantitative capture and transfer/injection of TCE are determined for each of the devices, and the assembled PCF module is used to analyze a test atmosphere containing 200 parts-per-trillion of TCE and 27 relevant co-contaminants with a conventional downstream capillary column and electron-capture detector. An average TCE transfer efficiency of 107% is achieved for a 20 L air sample, with a preconcentration factor of ~800,000.     

Determination of vapor pressures using gas chromatography

The determination of vapor pressures, p0, of compounds with low vapor pressures (10(-8) Pa < p0 < 10(3) Pa) is becoming increasingly important as a result of the need to measure p0 for environmentally sensitive compounds such as organophosphorus pesticides, biphenyls, dioxins and alkylbenzenes. Under strict conditions, the components of gas-liquid chromatography (GLC) (a volatile solute, an involatile solvent and a mobile carrier gas) are in equilibrium and as a result it is possible to use the technique to measure equilibrium properties such as vapor pressure. The technique is rapid, reliable and reproducible. These advantages have tempted many workers to measure physiochemical properties, including vapor pressures, under conditions for which the basic theories do not hold. In this review, the GLC techniques used to measure vapor pressures from GLC data together with the basic theory, limitations of the techniques and some recent measurements are discussed.     

Water vapor as a carrier gas in capillary gas chromatography

The use of water vapor as carrier gas in capillary chromatography is analyzed. Under equivalent conditions, a comparison of water vapor and helium as a mobile phase was performed during the separation of diesel fuel components. The advantage of water vapor was demonstrated. The resolutions of peaks were calculated for a pair of substances, C₁₈-iso and C₂₀ (4.4 for water vapor and 3.9 for helium). The Van Deemter dependences were measured, and it was demonstrated that the substantially lower viscosity of the water vapor compared to helium makes it possible to perform the separation of substances in a broader range of linear velocities, comparable to hydrogen used as mobile phase. The prospects for using water vapor was demonstrated in chromatographic analysis of hydrocarbons.     

The palladium/silver membrane focusing injector — A new inlet system for thermal desorption capillary gas chromatography

Summary A new sample focusing technique for capillary gas chromatography is described. The system is designed as a focusing inlet for thermally desorbed gas samples. A solid-sorbent trapped sample is thermally released from the sample tube, transferred to a palladium/silver membrane chamber by a hydrogen carrier gas stream and retained there by the gas separation membrane, which is highly permeable to the carrier gas. After focusing in the membrane chamber the sample is injected onto the separation column. This technique allows focusing and injection of highly volatile compounds in capillary gas chromatography without using any coolant. The injection performance for n-alkanes is shown to be comparable to the cryofocusing technique.     

Liquid membranes for gas/vapor separations

A review on developments of liquid membranes (LMs) in the field of gas and vapor separation of the last 16 years is presented. Liquid membrane configurations employing supports, i.e. immobilized, supported and contained liquid membranes are focussed and detailed information on the respective materials, i.e. supports (supplier, type, thickness, pore width, porosity, tortuosity), liquids and carriers, are presented together with their specific separation tasks. Performance of different LMs in terms of permeability and selectivity as well as stability (duration of testing, applied differential pressures) are compared and discussed. Finally, different preparation methods of LMs are illustrated.     

Development of an ultra-micro sample injector for gas chromatography using an ink-jet microchip.

An ultra-micro sample injector for gas chromatography (GC) was developed. An ink-jet microchip, originally used for industrial recorder, was modified at the edge near to an orifice, and fixed into the GC. In order to evaluate the characteristics of this injector, a sample injector and a thermal conductive detector (TCD) were connected directly, while water was used as the test sample. The volume of the droplet, the interval time and the back-pressure to the ink-jet microchip were investigated. Within the range of 1 - 5 nL volume injected sample, the TCD response according to the amount of the sample volume (the volume of one droplet from the ink-jet microchip was about 1 nL) was obtained. A good reproducibility of the peak area was obtained to be about 1.0% of the RSD value. In order to compare the injection method of the ink-jet chip with that using a micro-syringe, the method using the ink-jet chip could introduce 1/1000 of the amount of the sample and gave reproducible results.     

Integrated explosive preconcentrator and electrochemical detection system for 2,4,6-trinitrotoluene (TNT) vapor

This article reports on an integrated explosive-preconcentration/electrochemical detection system for 2,4,6-trinitrotoluene (TNT) vapor. The challenges involved in such system integration are discussed. A hydrogel-coated screen-printed electrode is used for the detection of the thermally desorbed TNT from a preconcentration device using rapid square wave voltammetry. Optimization of the preconcentration system for desorption of TNT and subsequent electrochemical detection was conducted yielding a desorption temperature of 120 °C under a flow rate of 500 mL min⁻¹. Such conditions resulted in a characteristic electrochemical signal for TNT representing the multi-step reduction process. Quantitative measurements produced a linear signal dependence on TNT quantity exposed to the preconcentrator from 0.25 to 10 μg. Finally, the integrated device was successfully demonstrated using a sample of solid TNT located upstream of the preconcentrator.     

Boronate functionalised polymer monoliths for microscale affinity chromatography

Novel macroporous monolithic stationary phase materials suitable for microscale boronate affinity chromatography were developed.     

Characterization of polymers by multi-step thermal desorption/programmed pyrolysis gas chromatography using a high temperature PTV injector

Thermal treatment hyphenated with gas chromatography is a versatile and powerful tool in the study of polymer characterization. An inexpensive system where thermal treatment at different temperatures occurs inside a Programmable Temperature Vaporization injector (PTV) is described. The samples investigated, commercial plastics, are complex mixtures that contain several polymers and additives. These plastics as well as their pure constituents are subjected to multi-step thermal treatment. The individual chromatograms of the various constituents of the polymeric sample are correlated with those of the final material in order to identify additives (thermal desorption) and degradation products (pyrolysis). Results obtained with the new method indicate the interesting potentials of the technique for the characterization of polymer compositions. Reproducibility of absolute and relative peak areas has been considered and found to be acceptable. The absence of a heated transfer line and switching valves, which are always present in conventional set-ups, eliminates the risk of losses of high molecular weight components. Further advantages of the technique proposed are simplicity, versatility, and its inexpensive nature.     

气相色谱法测定及预测有机磷阻燃剂的蒸气压

以p,p′-二氯二苯三氯乙烷(p,p′-DDT)为参考化合物,采用气相色谱法(GC)测定了14种有机磷阻燃剂(OPFRs)在不同温度(T)条件下的蒸气压(P_(GC))及其蒸发热。根据获得的蒸气压值,发现14种OPFRs的log P_(GC)随着温度的增加而减小。初步探讨了分子结构与log P_(GC)的相关性,结果发现卤代烷基磷酸酯的log P_(GC)低于烷基和芳基磷酸酯,且随着分子摩尔体积的增大,相应的log P_(GC)呈下降趋势。采用逐步回归的方法,建立了OPFRs的相对保留时间(RRT)与log P_(GC)之间的定量构效关系(QSPR)模型,模型的累计交叉有效判别系数Q2cum为0.946。模型对OPFRs的log P_(GC)预测值与实验值有良好的相关性(r=0.980),结果表明所建模型具有良好的稳健性和预测能力。利用建立的方法,通过测定的保留时间参数以及定量构效关系模型,可有效预测目前没有标准品的OPFRs的log P_(GC)值。     

Retention-index based vapor pressure estimation for polychlorobiphenyl (PCB) by gas chromatography

Summary Vapor pressures for 133 individual polychlorobiphenyl congeners as subcooled liquids were determined by two different approaches on the basis of gas-chromatographic retention indices obtained with two different non-polar stationary phases. The approach which is based on the retention indices obtained on a methyl-50% octyl polysiloxane phase (SB Octyl 50) and on reference vapor pressures of PCB congeners, and thus contains less approximations, should yield more accurate results than the method which uses retention indices obtained on a methyl polysiloxane phase (OV 101) and reference vapor pressure data taken from the n-alkanes of the retention index system. A systematic deviation is observed between the values obtained by the two different methods. The first method gives constantly slightly higher values for the vapor pressure. This will be caused by the different separation characteristics of the two non-polar stationary phases used, as well as by uncertainties in the reference data for the vapor pressure calculations.     

Special cooling system for the on-column injector in capillary gas chromatography eliminating discrimination of sample compounds

On-column injection into a hot gas chromatograph oven involves the danger that the bottom part of the needle is warmed up above the boiling point of the solvent in the sample. Consequently parts of the sample are evaporated out of the needle instead of being introduced as a liquid into the capillary column. This causes losses and hence discrimination of the less volatile parts of the sample which remain in the needle. A technical solution is proposed using a system for cooling the capillary during the injection, down to the injection point in the oven.     

A micro gas preconcentrator with improved performance for pollution monitoring and explosives detection

This paper presents the optimization of a micro gas preconcentrator based on a micro-channel in porous and non-porous silicon filled with an adequate adsorbent. This micro gas preconcentrator is both applicable in the fields of atmospheric pollution monitoring (Volatil organic compounds—VOCs) and explosives detection (nitroaromatic compounds). Different designs of micro-devices and adsorbent materials have been investigated since these two parameters are of importance in the performances of the micro-device. The optimization of the device and its operation were driven by its future application in outdoor environments. Parameters such as the preconcentration factor, cycle time and the influence of the humidity were considered along the optimization process. As a result of this study, a preconcentrator with a total cycle time of 10 min and the use of single wall carbon nanotubes (SWCNTs) as adsorbent exhibits a good preconcentration factor for VOCs with a limited influence of the humidity. The benefits of using porous silicon to modify the gas desorption kinetics are also investigated.