Multipass acoustically open photoacoustic detector for trace gas measurements

What we believe to be a novel multipass, acoustically open photoacoustic detector designed for fast-response, high-sensitivity detection of trace gases and pollutants in the atmosphere is demonstrated. The acoustic pulses generated by the absorption of the light pulses of a tunable optical parametric oscillator by target molecules are detected by an ultrasonic sensor at 40 kHz. The photoacoustic signal is enhanced by an optical multipass arrangement and by concentration of the acoustic energy to the surface of the ultrasonic sensor. The detection sensitivity, estimated from CO2 measurements around a 2 microm wavelength, is approximately 3.3 x 10(-9) W cm(-1).     

Performance evaluation of an in-injection port thermal desorption/gas chromatographic/negative ion chemical ionization mass spectrometric method for trace explosive vapor analysis

A gas chromatographic method utilizing thermal desorption of Tenax TA and sol-gel sorbent traps has been developed and validated for the analysis of trace explosive vapor with negative ion chemical ionization mass spectrometric detection. Sorbent tubes were packed with Tenax TA and sorbent particles prepared in-house by the sol-gel process. Thermal desorption was performed within a split/splitless injection port with minimal instrument modification. Performance was characterized by relative thermal desorption recovery, precision (reproducibility), linearity of the calibration, and method detection limits. Method validation was performed with a series of dinitrotoluenes, dinitrobenzene, trinitrotoluene, trinitrobenzene, two aminodinitrotoluenes, three nitroesters, and two nitramines. The performance of Tenax TA and the sol-gel sorbents is evaluated based on the method validation data. The method was applied to the analysis of trace explosive vapor collected and concentrated with sol-gel solid sorbent traps from the headspace of a smokeless gunpowder sample.     

Intercomparison of infrared cavity leak-out spectroscopy and gas chromatography-flame ionization for trace analysis of ethane

Comparison of two different methods for the measurement of ethane at the parts-per-billion (ppb) level is reported. We used cavity leak-out spectroscopy (CALOS) in the 3 microm wavelength region and gas chromatography-flame ionization detection (GC-FID) for the analysis of various gas samples containing ethane fractions in synthetic air. Intraday and interday reproducibilities were studied. Intercomparing the results of two series involving seven samples with ethane mixing ratios ranging from 0.5 to 100 ppb, we found a reasonable agreement between both methods. The scatter plot of GC-FID data versus CALOS data yields a linear regression slope of 1.07 +/- 0.03. Furthermore, some of the ethane mixtures were checked over the course of 1 year, which proved the long-term stability of the ethane mixing ratio. We conclude that CALOS shows equivalent ethane analysis precision compared to GC-FID, with the significant advantage of a much higher time resolution (<1 s) since there is no requirement for sample preconcentration. This opens new analytical possibilities, e.g., for real-time monitoring of ethane traces in exhaled human breath.     

Nanofibrous PANI-based conductive polymers for trace gas analysis

Electrospun nanofibres have been confirmed to be very good candidates for ultra-sensitive gas sensors since they greatly improve surface area to volume ratios of coatings, which in turn affect two additional and crucial features for sensors: high sensitivity and fast response time. Electrospinning is a simple method for the deposition of long (up to several centimetres) nanofibres, aligned or non-woven, directly onto suitable transducers. Such a structured layer may have better properties than a compact film, providing faster adsorption and minimising some bulk effects (i.e. long diffusion-desorption time, analyte entrapment, etc.). Electrospun conductive polymers (CPs) have been specifically investigated for developing smart sensors whose electrical properties change upon interactions with the analytes. Polyaniline is one of the most interesting CPs for gas sensing, because of its conductive features, when doped, as well as its thermal stability and sensing performance. The sensing mechanisms are different, depending on the nature of both PANi and the targeting analytes. Thus, various blends of polyaniline and insulating host polymers have been planned, prepared, deposited and studied to optimise the properties of sensors consequent to the combination of the electrical conductivity of CP and of the physical properties of the host polymer. Host polymer carriers cause great modifications to the topology of the interacting surface (diameter and length of the fibres, roughness, porosity, presence of beads and grains, non-woven framework and branched junctions, adhesion, etc.), in addition to the different affinity to the analytes tested. However, they enable electrodes to function over a wider dynamic range of gas or vapour concentrations. The polymer features have been also characterised over a range of water vapour concentrations and temperatures.     

In situ surface sampling of biological objects and preconcentration of their volatiles for chromatographic analysis

This report describes a rolling stir bar sampling procedure for volatile organic compounds (VOCs) present on various biological surfaces. In combination with thermal desorption/gas chromatography/mass spectrometry, this analytical technique was initially tested for quantitative profiling of human skin VOCs. It is also applicable to additional hydrophobic surfaces such as agricultural products, plant materials, and bird feathers. Use of embedded internal standards provides highly reproducible and quantitative results for a wide variety of sampled trace components. The samples of collected human skin VOCs and standards were found stable under cool storage conditions for at least 14 days, making this approach suitable for field biological and agricultural studies. Additionally, this methodology appears to have potential for forensic and toxicological investigations, as suggested through the analyses of VOC profiles of the human thumb prints recovered from a nonbiological smooth surface.     

浅谈纯化器在气相色谱分析中的应用

介绍载气纯化器对高灵敏度检测器的作用,通过试验和建立数学模型来阐明配置载气纯化器有利于提高气体中杂质含量测定的准确性和仪器的稳定性及灵敏度。     

A three dimensional beam profile monitor based on residual and trace gas ionization

A three dimensional beam profile monitor based on tracking the ionization of the residual gas molecules in the evacuated beam pipe is described. Tracking in position and time of the ions and electrons produced in the ionization enables simultaneous position sampling in three dimensions. Special features which make it possible to sample very low beam currents were employed. The characteristics of this detector make it particularly suitable for sampling beams produced at radioactive beam facilities, provided an auxiliary gas feed can be utilized.     

A mobile mass spectrometer for comprehensive on-line analysis of trace and bulk components of complex gas mixtures: parallel application of the laser-based ionization methods VUV single-photon ionization, resonant multiphoton ionization, and laser-induced electron impact ionization

A newly developed compact and mobile time-of-flight mass spectrometer (TOFMS) for on-line analysis and monitoring of complex gas mixtures is presented. The instrument is designed for a (quasi-)simultaneous application of three ionization techniques that exhibit different ionization selectivities. The highly selective resonance-enhanced multiphoton ionization (REMPI) technique, using 266-nm UV laser pulses, is applied for selective and fragmentationless ionization of aromatic compounds at trace levels (parts-per-billion volume range). Mass spectra obtained using this technique show the chemical signature solely of monocyclic (benzene, phenols, etc.) and polycyclic (naphthalene, phenathrene, indol, etc.) aromatic species. Furthermore, the less selective but still fragmentationless single photon ionization (SPI) technique with 118-nm VUV laser pulses allows the ionization of compounds with an ionization potential below 10.5 eV. Mass spectra obtained using this technique show the profile of most organic compounds (aliphatic and aromatic species, like nonane, acetaldehyde, or pyrrol) and some inorganic compounds (e.g., ammonia, nitrogen monoxide). Finally, the nonselective ionization technique laser-induced electron-impact ionization (LEI) is applied. However, the sensitivity of the LEI technique is adjusted to be fairly low. Thus, the LEI signal in the mass spectra gives information on the inorganic bulk constituents of the sample (i.e., compounds such as water, oxygen, nitrogen, and carbon dioxide). Because the three ionization methods (REMPI, SPI, LEI) exhibit largely different ionization selectivities, the isolated application of each method alone solely provides specific mass spectrometric information about the sample composition. Special techniques have been developed and applied which allow the quasi-parallel use of all three ionization techniques for on-line monitoring purposes. Thus, a comprehensive characterization of complex samples is feasible jointly using the characteristic advantages of the three ionization techniques. Laboratory applications show results on rapid overview characterization of mineral oil-based fuels and coffee headspace. The first reported field applications include timely resolved on-line monitoring results on automobile exhausts and of waste incineration flue gas.     

Ultrafast gas chromatographic separation of organophosphor and organosulfur compounds utilizing a microcountercurrent flame photometric detector

A microcountercurrent flame photometric detector (microcc-FPD) was adapted and optimized for ultrafast gas chromatographic (GC) separation and detection of organophosphor (OP) and organosulfur (OS) compounds on short chromatographic columns. Air and hydrogen are introduced to the microcc-FPD from opposite directions, creating a hydrogen-rich flame. In this microcc-FPD, combustion takes place between the burner tips without touching them. The separation between the tips and the flame reduces heat loss from the flame to the surrounding environment, resulting in low hydrogen consumption and a compact flame. The microcc-FPD is capable of detecting very narrow (13 ms) chromatographic peaks. An ultrafast GC separation of a group of six OP and OS compounds is achieved within less than 5 s using fast temperature programming of a 0.5-m-long microbore column. Very fast separations are also demonstrated on a 1-m-long microfabricated column consisting of 150-microm-wide, 240-microm-deep channels, etched in a 1.9-cm square silicon chip, covered with a Pyrex wafer, and statically coated with dimethyl polysiloxane. With a hydrogen flow rate of 10 mL/min, the detection limit for OP is 12 pg of P/s and 3 ng of S/s for OS compounds at a signal-to-noise ratio of 2. The coupling of a microfabricated column and a miniature FPD is an important step toward the development of a miniaturized GC-FPD capable of ultrafast detection of low levels of OP and OS compounds.     

Acoustic flame detector for gas chromatography

A novel gas chromatography detector is described that uses acoustic signals from a partly premixed hydrogen-air flame burning on top of a capillary. The device, referred to as the acoustic flame detector (AFD), is based on the measurement of the frequency of acoustic transients generated at the burner under a range of operating conditions. The presence of trace amounts of analyte in the flame was found to increase the frequency of these sonic bursts from the baseline level of ～100 Hz. The response of the AFD for n-dodecane, measured as the shift in frequency, was determined to be linear over ～3 orders of magnitude, with a minimum detectable level of about 1-5 ng C/s using the current system. The sensitivity correlates roughly with carbon content, except for certain organometallics (Sn, Mn), which gave substantially enhanced signals. Some tailing was observed but became serious only for particular types of organometallics. The noise of the system was predominantly of the 1/f type. The effects of flow conditions, burner geometry, and flame gas constituents were investigated. The oscillations could be followed by acoustic, visual, electrical, and optical means. The AFD mechanism is shown to involve oscillatory chemical kinetics, in which the flame front (the inner cone) temporarily enters a few millimeters into the capillary during each cycle, thereby creating the acoustic signal.     

Study of metastable superconducting detector response to irradiation using SQUID-readout

We describe the results of experiments on the³⁵S irradiation of a metastable superconducting detector between 2.0 and 3.4 K using a second-derivative rf SQUID readout operated in an integral mode. The detector consisted of 3×10⁶ tin grains, distributed in diameter from 10–25 micron. Comparison with computer simulation based on a hot border model consistent with the measurement protocols yields reasonable agreement despite the large timing uncertainties associated with the measuring systematics of the SQUID system.     

Selective detection of volatile aromatic compounds using a compact laser ionization detector with fast gas chromatography

We report results for a new gas chromatography detector that is comparatively sensitive and far more selective for aromatic compounds than the traditional photoionization detector. The detection means is multiphoton ionization at atmospheric pressure. The ionization source in these experiments is a diode-pumped passively Q-switched microchip laser operating at 266 nm. Experiments were conducted with the detector interfaced to a fast gas chromatograph. For <20 s elution time, limits of detection were <1 pg for toluene, ethylbenzene, xylenes, and isopropylbenzene; the limit of detection for benzene is approximately 10 pg. Detector response was linear over 5 orders of magnitude, including these low levels. Negligible signals were observed for nonaromatic ketones, aldehydes, ethers, and cycloalkanes at levels as high as 0.1 microg (10 mg/L concentration). Detector efficiency after fast GC separation was 0.002% when using a detector cell with a radius of 1.1 cm and a purge gas flow of 500 mL/min. The advantages of this detector are further illustrated by the fast GC analysis of fuel samples.     

Metrological aspects of natural gas analysis by the chromatographic method

We consider the metrological characteristics of the chromatographic method for determination of the component content of natural gas according to GOST 23781- 87. We note the disadvantages inherent in the calibration method recommended by the GOST standard. We present the results of measurements of the composition of natural gas obtained on the high-precision chromatographic system at the All-Union Scientific-Research Institute of Metrology, which makes it possible to solve problems of certification of natural gas samples at the level of primary standards and to transfer the concentration scale for the components in natural gas from the standard chromatographs to working chromatographs. We establish that the figures of merit for the precision of the Institute's apparatus are not inferior to those for the apparatus of the US National Institute of Standards and Technology.Translated from Izmeritel'naya Tekhnika, Vol. 38, No. 4, pp. 62–65, April, 1995.     

Extraction chromatographic methods in the sample preparation sequence for thermal ionization mass spectrometric analysis of plutonium isotopes

A sample preparation sequence for actinide isotopic analysis by thermal ionization mass spectrometry (TIMS) is described that includes column-based extraction chromatography as the first separation step, followed by anion-exchange column separations. The sequence is designed to include a wet ashing step after the extraction chromatography to prevent any leached extractant or oxalic acid eluent reagents from interfering with subsequent separations, source preparation, or TIMS ionization. TEVA resin and DGA resin materials, containing extractants that consist only of C, N, O, and H atoms, were investigated for isolation of plutonium. Radiotracer level studies confirmed expected high yields from column-based separation procedures. Femtogram-level studies were carried out with TIMS detection, using multiple monoisotopic spikes applied sequentially throughout the separation sequence. Pu recoveries were 87% and 86% for TEVA and DGA resin separations, respectively. The Pu recoveries from 400 μL anion-exchange column separation sequences were 89% and 93% for trial sequences incorporating TEVA and DGA resin. Thus, a prior extraction chromatography step in the sequence did not interfere with the subsequent anion-exchange separation when a simple wet ash step was carried out in between these column separations. The average measurement efficiency for Pu, encompassing the chemical separation recoveries and the TIMS ionization efficiency, was 2.73% ± 0.77% (2σ) for the DGA resin trials and 2.67% ± 0.54% for the TEVA resin trials, compared to 3.41% and 2.37% (average 2.89%) for two control trials. These compare with an average measurement efficiency of 2.78% ± 1.70%, n = 33 from process benchmark analyses using Pu spikes processed through a sequence of oxalate precipitation, wet ash, iron hydroxide precipitation, and anion-exchange column separations. We conclude that extraction chromatography can be a viable separation procedure as part of a multistep sequence for TIMS sample preparation.