Cryogenic Focusing Research Articles

Cryogenic focus measurement system for a wide-field infrared space telescope

We describe a technique for measuring focus errors in a cryogenic, wide-field, near-infrared space telescope. The measurements are made with a collimator looking through a large vacuum window, with a reflective cold filter to reduce the background thermal infrared loading on the detectors and optics. The vacuum window and cold filter introduce a wavefront error that we characterize using an autocollimating microscope. For the 200 mm diameter aperture f/3 space telescope SPHEREx, we achieve a focus position measurement with a ∼15µm systematic and a ∼5µm statistical error.

Comparison of a Molecular and an Immobilized Gadolinium(III)‐tris[(1R,4S)‐3‐heptafluorobutanoyl‐camphor] as Catalyst in the Asymmetric Danishefsky‐Hetero‐Diels‐Alder‐Reaction

On-column reaction gas chromatography combines the power of separation and rapid analysis of reactants and reaction products with screening of reactions in a single step. Not only conversions but the reaction rates at various temperatures can be obtained from single measurements, making this approach superior to the time-consuming measurements typically performed in reaction progress analysis. However, this approach has only been used in the investigation of interconversion processes, rearrangement reactions, and only a few examples of higher-order reactions are known. Here we present the screening of immobilized gadolinium(III)-tris[(1R,4S)-3-heptafluorobutanoyl-camphor] in the Danishefsky-hetero-Diels-Alder-reaction by enantioselective on-column reaction gas chromatography utilizing cryogenic focusing to achieve catalytic conversions in this higher-order reaction and subsequent separation of the enantiomeric product mixture to determine the enantiomeric ratio. The results obtained by this approach could be transferred to the conventional batch reaction at a larger scale, demonstrating that on-column reaction chromatography provides reliable results in the screening of enantioselective reactions.

Needle-type extraction device for the purge and trap analysis of 23 volatile organic compounds in tap water

We developed a rapid determination technique for trace volatile organic compounds (VOCs) in tap water by introducing a novel needle-type extraction device coupled to a purge-and-trap method. To extract a wide range of VOCs, a new extraction needle containing particles of divinylbenzene and activated carbon was developed in this study. During the active sampling of the headspace gas in a glass vial by the extraction needle, pure N2 gas was used for purging the aqueous sample. After the optimization of several experimental parameters, such as the addition of the salt and conditions of dry purging and desorption, the extraction performance of the device and method was evaluated for 23 VOCs that are typically found in tap water samples. The quantification limits of the method were 0.6μg/L for 1,1-dichloroethylene and less than 0.5μg/L for other VOCs, with good repeatability being confirmed for all the target compounds. Taking advantage of the excellent recovery of VOCs, the determination of VOCs in real tap water samples was carried out successfully. Because the developed method does not require sample heating and/or cryogenic focusing, simple and rapid analyses can be performed along with satisfactory sensitivity for typical tap water samples.

Targeted multidimensional gas chromatography using a heart-cutting device and cryogenic focusing for the determination of benzophenone derivatives in foodstuffs

Photoinitiators are used to promote the polymerization process during the curing of varnishes or inks on cartonboard packaging. Depending on storage conditions and shelf life, these substances are able to migrate through the packaging layer into the foodstuff. This type of contamination phenomenon is therefore becoming a critical issue in terms of food safety. In order to tackle this problem, a fast and selective method was developed for the determination of benzophenone and three methylbenzophenone isomers in cereal-based foodstuffs and their cardboard packaging. Food samples or packages were efficiently extracted by pressurized liquid extraction using acetonitrile, and the extracts were directly injected onto the analytical system. The analysis was performed by multidimensional gas chromatography-mass spectrometry using a heart-cutting approach to reduce the background noise from complex matrices. The strategy employed two distinct cuts each containing its proper deuterated internal standard leading to accurate quantification. By integrating a cryofocusing effect, an enhancement in signal/noise ratio was achieved by a factor >10, which markedly decreased the sensitivity threshold. Moreover, baseline separation of the critical isomers allowed their unequivocal determination. The method was fully validated on cereal-based foodstuffs based upon an analysis of variance, and excellent performances were obtained at the decision limit making this method well suited for official food controls.

High throughput analysis of atmospheric volatile organic compounds by thermal injection – isothermal gas chromatography – time-of-flight mass spectrometry

Sixty one volatile organic compounds (VOCs) from a standard gas mixture were separated via isothermal gas chromatography coupled with time-of-flight mass spectrometry (GC–TOFMS) in a ∼35s separation time window (∼45s separation). The VOCs in the standard gas mixture were selected based on the EPA TO-15 methodology. The high throughput separation was achieved with a relatively high total peak capacity (nc∼114), by simultaneously minimizing both on-column and off-column peak width broadening. The on-column contributions to peak width broadening were minimized by taking into account and applying GC separation theory for the selection of column dimensions and carrier gas flow rate conditions. Both fast cryogenic focusing and re-injection of compounds (implemented via a commercially available thermal modulator and referred to herein as thermal injection (TI) and fast TOFMS detection (100scans/s)) were applied to reduce off-column sources of peak width band broadening (sometimes referred to as off-column band broadening). Cryogenic focusing during TI and minimal band broadening-based dilution during separation resulted in preconcentration factors for the detected peaks ranging from 78 (1,4-dichlorobenzene) to 420 (propylene). Since the injected volume for preconcentration was 500μl, and based on the detected noise levels at selected m/z for each analyte compound, the concentration limit of detection (LOD) ranged from 67ppbv (parts per billion by volume) for propylene, to 4ppbv for freon-12. While application of standard VOC analysis conditions leads to separation times typically ranging from ∼30 to 50 min, the isothermal GC–TOFMS method reported herein represents a 40-fold improvement in analysis time while maintaining peak capacity and detection sensitivity that is comparable to traditional GC–MS VOC analysis.

The Spitzer Space Telescope

The Spitzer Space Telescope, which has operated very successfully since 2003 in its unique Earth-trailing solar orbit, is NASA's Great Observatory for infrared astronomy. We provide a quick overview of the optical characteristics of Spitzer and review the observatory design. The main emphasis is on two unique on-orbit activities used to optimize the scientific return from Spitzer: 1. an unusual approach to focusing the telescope that minimized the use of the cryogenic focus mechanism, and 2. a methodology for extending the cryogenic lifetime of Spitzer by actively controlling the telescope temperature.

Supercritical fluid chromatography hyphenated with twin comprehensive two-dimensional gas chromatography for ultimate analysis of middle distillates

This paper reports the conditions of online hyphenation of supercritical fluid chromatography (SFC) with twin comprehensive two-dimensional gas chromatography (twin-GCxGC) for detailed characterization of middle distillates; this is essential for a better understanding of reactions involved in refining processes. In this configuration, saturated and unsaturated compounds that have been fractionated by SFC are transferred on two different GC x GC columns sets (twin-GCxGC) placed in the same GC oven. Cryogenic focusing is used for transfer of fractions into the first dimension columns before simultaneous GCxGC analysis of both saturated and unsaturated fractions. The benefits of SFC-twin-GC x GC are demonstrated for the extended alkane, iso-alkane, alkene, naphthenes and aromatics analysis (so-called PIONA analysis) of diesel samples which can be achieved in one single injection. For that purpose, saturated and unsaturated compounds have been separated by SFC using a silver loaded silica column prior to GC x GC analysis. Alkenes and naphthenes are quantitatively recovered in the unsaturated and saturated fractions, respectively, allowing their identification in various diesel samples. Thus, resolution between each class of compounds is significantly improved compared to a single GCxGC run, and for the first time, an extended PIONA analysis of diesel samples is presented.

Rapid determination of trans-resveratrol in red wine by solid-phase microextraction with on-fiber derivatization and multidimensional gas chromatography–mass spectrometry

There has been considerable public interest and a growing number of scientific studies linking certain phenolic compounds in grapes and wines, particularly trans-resveratrol ( trans-3,5,4′-trihydroxystilbene, TRA), to human health benefits. Typical TRA concentrations in wine are very low. It is a polar compound with very low volatility, which makes it difficult to extract and to separate on a gas chromatography (GC) column without derivatization. In this study, a new method for trace analysis of TRA was developed using solid-phase microextraction (SPME) with on-fiber silylation derivatization. Multidimensional GC equipped with a heartcut valve and cryogenic focusing was coupled with a mass-selective detector and used for improved separations and analysis. The effects of SPME fiber selection, extraction time, temperature, and desorption time were investigated. The derivatization conditions, time/temperature and the volume of derivatization reagent were also optimized. The calibration curve was linear over the concentration range from 10 ng L −1 to 5 mg L −1, with a correlation coefficient of 0.9996. The average recovery of TRA in red wine was 83.6 ± 5.6%. The method detection limit (MDL) for TRA in ethanol:water (12.5:87.5, v/v) solution in this study was 7.08 ng L −1 whereas the MDL for TRA in pure water was 2.85 ng L −1. The new method was used to test the TRA content in six selected Iowa red wine samples. Measured concentrations varied from 12.72 to 851.9 μg L −1.

Nitric oxide isotope tracing at biological trace levels

The many roles of nitric oxide (NO) in biology and its tremendous therapeutic applications have drawn much attention to this molecule; yet little is still known on the chemical mechanisms that enable NO to achieve these roles in living systems. Here we describe new methodology, based on isotope labeling, for identifying and quantifying biochemical sources and targets of NO, in tissues and in biological fluids at biological trace levels. To detect 14NO and the stable isotope 15NO we have adapted a chemical assay, designed originally for chemiluminescence detection, to take advantage of mass spectrometric detection. Using a standard Sievers reaction vessel, coupled to an HP GCD1800A gas chromatograph / mass spectrometer via cryogenic focusing in a 1 ml loop, we were able to reach detection limits of 10 pmol and 50 pmol for 14NO and 15NO isotopes, respectively. The difference in detectability currently arises from oxygen contamination contributing to the background at 15NO, due to the limited mass selectivity of our instrument. A mass spectrometer upgrade, now under way, is expected to improve our sensitivity and selectivity manifold to where the limit of detection for both 14NO and 15NO isotopes should reach the single pmol range. At these levels our mass spectrometric approach should greatly expand the analytical capabilities available to investigators in the field of NO Chemical Biology. Made possible by NIH Grant Number P20 RR16456 from the INBRE Program of the National Center for Research Resources.

Static headspace--gas chromatography: theory and practice

Static headspace--gas chromatography: theory and practice

Mercury Speciation in Sediments of a Tropical Coastal Environment

The speciation of mercury in tropical coastal areas, although scarcely studied, has presented an odd behaviour when compared with the results obtained in temperate environments. In this work, we measured the concentrations of mercury species (mercuric mercury, methyl-mercury and dimethyl-mercury) in the sediments of Sepetiba Bay (Brazil) and compared these with geochemical parameters. Twenty-eight sediment samples were extracted in an open microwave system and mercury speciation was carried out by ethylation, cryogenic focusing, gas chromatography and quartz furnace atomic absorption spectrometry. Fraction < 63 µm, organic carbon and total sulphur contents and redox potential were measured in order to outline the geochemical characteristics of the sediments. While mercuric mercury presented concentrations ranging between 22.65 and 134.61 ng g−1, methyl-mercury concentrations ranged between < 2.0 and 4.4 ng g−1. Only a few dimethyl-mercury concentrations were relatively high (up to 14.6 ng g−1). The results of mercuric mercury presented an unexpected positive correlation with sulphur contents showing that mercury is not forming stable sulphide complexes, even under very reducing conditions. This would render mercury available for the formation of methyl- or dimethyl-mercury that would be both fixed in the sediments and volatilised.

Statistical-overlap theory of column switching in gas chromatography: applications to flavor and fragrance compounds.

First-column gas chromatograms (GCs) of hundreds of flavor and fragrance compounds, and second-column GCs of specific regions of these GCs, are predicted using thermodynamic databases in commercial software. A statistical-overlap theory of column switching with cryogenic focusing then is developed by mimicking the predicted GCs by two kinds of Monte Carlo simulations. In the first kind, a probability distribution is calculated for the number of compounds in a region of the first-column GC, based on the number of observed peaks in the region, the number of observed peaks in the second-column GC, and the retention-time distributions and breadths of single-component peaks in both GCs. In the second kind, criteria are established for the theory's application. The theory is applied to 12 regions of first-column GCs. The theory predicts the number of compounds in all of them and shows that separation rarely is complete in second-column GCs, when 10 or more compounds are transferred between columns. The theory also rationalizes the tedious search required to find good separation conditions by showing that column-switching gas chromatography with cryogenic focusing is inherently statistical. The number of peaks in the second-column GC can be greater than, less than, or equal to the number of peaks in the relevant region of the first-column GC, and the good conditions sought by researchers to substantially improve separation correspond to favorable "rolls of the dice" found only by trial and error.

Speciation of volatile antimony compounds in culture headspace gases of Cryptococcus humicolus using solid phase microextraction and gas chromatography–mass spectrometry

AbstractDirect analysis of the volatile antimony compounds stibine (SbH3), monomethylantimony, dimethylantimony (Me2Sb) and trimethylantimony (Me3Sb) using solid phase microextraction (SPME) with polydimethylsiloxane fibres and gas chromatography–mass spectrometry (GC–MS) is described. The best analyte to background signal ratio was achieved using a 20 min extraction time. Antimony species were separated using a 3% phenylmethylsilicone capillary column operated at a column pressure of 70 kPa, a flow rate of 1.4 ml min−1 and temperature ramping from 30 to 36 °C at 0.1 °C min−1. Cryogenic focusing of desorbed species was required to achieve resolution of antimony species. The optimized SPME–GC–MS method was applied to the analysis of headspace gases from cultures of Cryptococcus humicolus incubated with inorganic antimony(III) and (V) substrates. The headspace gases from biphasic (aerobic–anaerobic) biomass‐concentrated culture incubations revealed the presence of SbH3, Me2Sb and Me3Sb. Stibine was the major antimony species detected in cultures amended with inorganic antimony(V). Me3Sb was the sole volatile antimony species detected when cultures were amended with antimony(III). Copyright © 2002 John Wiley &amp; Sons, Ltd.

Aroma-active components in fermented bamboo shoots.

Bamboo shoots (Phyllostachys pubescens) were fermented and prepared in a traditional Taiwanese manner. Static and dynamic headspace extractions of volatile compounds were conducted by solid phase microextraction (SPME) and by cryogenic focusing purge and trap, respectively. Volatile analysis was conducted with gas chromatography and mass spectrometry. Gas chromatography-olfactometry (GCO) was conducted utilizing the Osme time-intensity method. Of 70 volatile compounds detected, 29 possessed aroma activity, and the most odor active included p-cresol (barn-like), 2-heptanol (mushroom), acetic acid (vinegar), and 1-octen-3-ol (mushroom). SPME extracted 66 compounds, purge and trap extracted 14 compounds, and 12 compounds were common to both methods. The Osme GCO technique coupled with SPME is an effective tool for the extraction and evaluation of aroma-active headspace volatiles.

A Source of Dimethyl Disulfide and Dimethyl Trisulfide in Grain Spirit Produced with a Coffey Still

The regulation of dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS) concentrations in new-make grain spirit in the Scotch whisky industry are important because these polysulfides are significant flavor congeners. Methional could be a precursor in the pathway of DMDS and DMTS formation; therefore, the relationship between wash methional concentrations at the end of fermentation and DMDS and DMTS in spirit from two stills operating in parallel was investigated. Methional from wash and DMDS and DMTS from spirit were measured by purging them onto Tenax adsorbent and then injecting the volatiles onto a GC column using thermal desorption and cryogenic focusing. Sulfur compounds eluting from the column were quantified using a sulfur-specific Sievers chemiluminescence detector. Methional added as a “spike” to wash and DMDS and DMTS added as spikes to spirit had recoveries of 104.5, 103.9, and 98.3% respectively. This established the validity of the analytical technique. Concentrations of methional varied in wash from ≈50 to 710 μg/L. There was a linear positive correlation between wash methional and DMDS during two periods of distillery production (P < 0.01 for period 1 and P < 0.001 for period 2) investigated. Linear correlations between wash methional and DMTS were also found during period 2 (P < 0.001); however, correlations were only significant during period 1 (P < 0.1, still 1 and P < 0.05, still 2). In conclusion, DMDS and DMTS concentrations in new-make spirit are related to the concentration of methional in wash, although operation of stills can in particular modulate the final DMTS concentrations.

Method for measurement of volatile oxygenated hydrocarbons in ambient air

An automated gas chromatographic method for the quantitative determination of oxygenated (C 2C 5 carbonyls and C 1C 2 alcohols) and some non-oxygenated (C 5C 8) hydrocarbons in ambient air has been developed. The analytical system consists of a gas chromatograph with a cryogenic sampling trap, a precolumn for the separation of water and other interfering compounds, a cryogenic focusing trap and two analytical columns connected in series. Substances are detected either by flame ionization or by a mass spectrometer. Ozone is removed by a potassium iodide scrubber placed upstream the sampling trap. External gas standards generated by a permeation device are used for calibration. The detection limits range between 0.03 and 0.08 ng (depending on the compound), equivalent to 5 to 56 ppt in 1 l of sampled air. The method was tested by an intercomparison with a different gas chromatographic technique for the determination of NMHC. The system has been applied since 1994 for measurements in ambient air. Data obtained during an intensive campaign in summer 1995 at the field station Wank (1778 m a.s.l.) near Garmisch-Partenkirchen, Germany, are reported and compared with NMHC mixing ratios measured simultaneously in the same air masses.

Determination of volatile organic compounds in air using a dehumidified and ventilated diffusive sampler, thermal desorption and gas chromatography with flame ionization detection

A portable dehumidified and ventilated diffusive sampler (DVDS) incorporating three diffusive tubes and a domestic battery-driven fan dryer was developed to solve the problems of high humidity and stagnant air for environmental monitoring of volatile organic compounds (VOCs). Using Tenax GR as adsorbent, thermal desorption and cryogenic on-column focusing prior to capillary GC analysis with flame ionization detection, the 10 most common aromatic and halogenated VOCs were determined with the DVDS, giving linear working ranges 3.2 µg m–3–770 mg m–3 for 1 h sampling, sampling rates of 0.64–0.98 ml h–1 and RSD of 4–6%. The good repeatability is due to the use of a small-diameter diffusive tube (0.32 mm id) and reproducible ventilation in the DVDS. The optimized analytical parameters are temperature 10–40 °C, relative humidity 0–100%, sampling time 15–480 min and storage for up to 21 days. Quality assurance procedures were developed for the calibration and preparation of the DVDS. The reliability of the method was established by parallel determinations at a given field station using both the DVDS and the active sampling method.

Determination of trace quantities of volatile sulfur compounds in aqueous solutions by gas chromatography after purge and trap isolation and cryogenic focusing

AbstractA method for the determination of volatile organic sulfur compounds, such as dimethylsulfide (DMS), at the nanogram level in aqueous samples is described. A purge and trap technique was used to remove and preconcentrate DMS and other sulfur compounds from aqueous solutions. Trapping of the isolated compound was performed on Tenax TA and Carbosieve III S. The water vapor from the purging stream was removed by a commercially available Nafion tube inserted in a plastic container packed with molecular sieve 5A. Analytes, after thermal desorption, were recollected in a short fused silica capillary tube cooled with the vapors of liquid nitrogen to a temperature ca. −150°C. Next, sulfur compounds were revolatilized and directly transferred to the analytical capillary column for analysis. Detection was performed with a flame photometric detector. The whole procedure was optimized and checked using a homemade valving device easily coupled to any gas chromatograph. The method has been used to measure DMS and other volatile sulfur compounds in seawater of the Baltic Sea and in samples of water from three rivers in the Gdańsk, Poland, region.

Solid-Phase Microextraction for Flavor Analysis

The application of solid-phase microeffraction (SPME) for flavor analysis has been studied. SPME headspace and liquid sampling were tested for 25 common flavor components in dilute aqueous solution. The addition of salt generally enhances SPME adsorption. Larger sample volume and a smaller volume of headspace over the liquid sample also increase the sensitivity of SPME-based analysis. For thermal desorption following SPME, a GC injector liner with small diameter, e.g., 1 mm i.d., improves resolution and obviates the need for cryogenic focusing. The SPME technique was applied to authentic samples: ground coffee, a fruit juice beverage, and a butter flavor in vegetable oil. The advantages and limitations of SPME are discussed

Measurements of VOCs from the TAMS Network

Target volatile organic compounds (VOCs) were measured at a network of urban air monitoring locations in Boston, Chicago, Houston, and the Seattle/Tacoma area. Following a pilot-scale field evaluation of available techniques for determining concentrations of VOCs in ambient air, a technique based on evacuated stainless steel canisters was selected to collect whole air samples. Twenty-four-hour integrated samples were collected every twelfth day at ten sites over a 2-year study period. Battelle Columbus Laboratory (BCL) analyzed the samples for 25 target VOCs using cryogenic focusing, gas chromatographic separation and mass selective detection with flame ionization detection as backup. Duplicate canister samplers were operated each sampling period at one of the ten sites in the Toxic Air Monitoring System (TAMS) network to estimate overall method precision. In addition, every 10th analysis was repeated by BCL to obtain a measure of analytical precision. Finally, each sampling period a clean evacuated canis...