Frants R. Lauritsen

Catabolism of leucine to branched‐chain fatty acids in Staphylococcus xylosus

Aims:  Staphylococcus xylosus is an important starter culture in the production of flavours from the branched‐chain amino acids leucine, valine and isoleucine in fermented meat products. The sensorially most important flavour compounds are the branched‐chain aldehydes and acids derived from the corresponding amino acids and this paper intends to perspectivate these flavour compounds in the context of leucine metabolism.

A new membrane inlet for on-line monitoring of dissolved, volatile organic compounds with mass spectrometry

Abstract A conventional membrane inlet for the mass spectrometric monitoring of gases has a considerable length of connection tube between the membrane and the ion source. This design is unsuitable for the measurement of organic compounds as they absorb strongly to the walls of the connection tube, giving rise to long response times. In this paper, we describe a new inlet design, where the membrane is mounted close to the ion source. The initial transients in the response to various alcohols were measured and compared with the transients found with a conventional inlet. Faster response times were achieved with the new inlet. The transients were found to be composed of two components, one from the membrane and one from adsorption/desorption processes on the vacuum walls in the connection tube. Good agreement was found between the measured data and a simple model of adsorption/desorption processes in a vacuum system.

Linking Soil O2, CO2, and CH4 Concentrations in a Wetland Soil: Implications for CO2 and CH4 Fluxes

Oxygen (O(2)) availability and diffusivity in wetlands are controlling factors for the production and consumption of both carbon dioxide (CO(2)) and methane (CH(4)) in the subsoil and thereby potential emission of these greenhouse gases to the atmosphere. To examine the linkage between high-resolution spatiotemporal trends in O(2) availability and CH(4)/CO(2) dynamics in situ, we compare high-resolution subsurface O(2) concentrations, weekly measurements of subsurface CH(4)/CO(2) concentrations and near continuous flux measurements of CO(2) and CH(4). Detailed 2-D distributions of O(2) concentrations and depth-profiles of CO(2) and CH(4) were measured in the laboratory during flooding of soil columns using a combination of planar O(2) optodes and membrane inlet mass spectrometry. Microsensors were used to assess apparent diffusivity under both field and laboratory conditions. Gas concentration profiles were analyzed with a diffusion-reaction model for quantifying production/consumption profiles of O(2), CO(2), and CH(4). In drained conditions, O(2) consumption exceeded CO(2) production, indicating CO(2) dissolution in the remaining water-filled pockets. CH(4) emissions were negligible when the oxic zone was >40 cm and CH(4) was presumably consumed below the depth of detectable O(2). In flooded conditions, O(2) was transported by other mechanisms than simple diffusion in the aqueous phase. This work demonstrates the importance of changes in near-surface apparent diffusivity, microscale O(2) dynamics, as well as gas transport via aerenchymous plants tissue on soil gas dynamics and greenhouse gas emissions following marked changes in water level.

Release of VOCs and Particles During Use of Nanofilm Spray Products

Here, we present emission data on VOCs and particles emitted during simulated use of four commercial nanofilm spray products (NFPs) used for making easy-to-clean or self-cleaning surfaces on floors, ceramic tiles, and windows. The aim was to characterize the emitted VOCs and to provide specific source strength data for VOCs and particles released to the airduring use of the products. Containers with NFP were mounted on a spray-stand inside a closed stainless steel chamber with no air exchange. NFPs were sprayed in amounts corresponding to 1 m2 surface toward a target plate at a distance of 35 cm. Released VOCs were measured by a combination of air sampling on Tenax TA adsorbent followed by thermal desorption GC/MS and GC/FID analysis and real time measurements using a miniature membrane inlet mass spectrometer. Particles were measured using a fast mobility particle sizer and an aerosol particle sizer. A number of VOCs were identified, including small alcohols, ketones and ethers, chlorinated acetones, a perfluorinated silane, limonene, and cyclic siloxanes. The number of generated particles was on the order of 3 x 10(8) to 2 x 10(10) particles/m3 per g sprayed NFP and were dominated by nanosize particles.

Novel pyrazine metabolites found in polymyxin biosynthesis by Paenibacillus polymyxa

A complex mixture of methyl-branched alkyl-substituted pyrazines was found in the growth medium of the polymyxin-producing bacterium Paenibacillus polymyxa, and of these, seven are new natural compounds. A total of 19 pyrazine metabolites were identified. The dominant metabolite was 2,5-diisopropylpyrazine as identified using a combination of high-resolution mass spectrometry, (1)H- and (13)C-nuclear magnetic resonance, gas chromatography-mass spectrometry as well as co-elution with an authentic standard. Its biosynthesis was correlated with growth and production was strongly stimulated by valine supplementation. The other pyrazine metabolites, all related pyrazines with either one, two or three alkyl substituents, were identified by means of their mass spectral data and/or co-elution with authentic standards.

Quantitative determination of semivolatile organic compounds in solution using trap-and-release membrane inlet mass spectrometry

This paper discusses the use of trap-and-release membrane inlet mass spectrometry (T&R-MIMS) for the quantitative determination of semivolatile organic compounds in real samples. We found that the T&R-MIMS technique is particular sensitive to relatively polar, semivolatile organic compounds. For example, the detection limits for the acids acetylsalicylic acid and phenoxyacetic acid were lowered by a factor of 100 as compared with those possible with standard MIMS, and caffeine was detectable only with the T&R-MIMS method. The detection limits were in the parts-per-billion range, and the dynamic range was 3 orders of magnitude. As a practical example of the application of the T&R-MIMS technique, we used it for the quantitative analysis of caffeine in ground coffee and tea leaves. Good agreement between T&R-MIMS and HPLC determinations was found, and the reproducibility of the whole analytical system for caffeine determination (extraction procedure and T&R-MIMS determination) was within 10% as relative standard deviation. However, for coffee, a large background from the essential oils prevented low-level work, such as the determination of residual caffeine in decaffeinated coffee. Obviously, the analysis of many complex matrixes will require the use of tandem mass spectrometry.

ADVANCES IN MEMBRANE INLET MASS SPECTROMETRY (MIMS)

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Membrane inlet ion mobility spectrometry for on-line measurement of ethanol in beer and in yeast fermentation

Abstract A membrane inlet ion mobility spectrometric method was developed for on-line measurement of ethanol concentration in beer and in yeast fermentation. Main parts of the measurement system are an ion mobility spectrometer, M90, and a custom built membrane inlet. The M90 ion mobility spectrometer is unique in the sense that positive and negative ions are simultaneously measured at 6 different detection channels, 3 for positive ions and 3 for negative ions. The custom built membrane inlet utilized a microporous polypropylene membrane for sample introduction into the M90 instrument. The developed measurement system allowed analysis of ethanol at 0.05% ( v v ) levels, clearly sufficient for the applications reported here. Good signal linearity was observed at positive ion channels 1 and 3 in ethanol concentration range 0.2−10% ( v v ), calculated correlation coefficients were 0.989 and 0.999, respectively. Agreement between the ethanol concentration measured by membrane inlet ion mobility spectrometry and the values declared on the beer bottle labels was good. Reproducibility of signals of the detection channels which gave the best response for ethanol was excellent in ethanol concentration measurement of beer samples, coefficient of variation was 1% for all these channels. Membrane inlet ion mobility spectrometry was also shown to be capable of on-line ethanol measurement, confirmed by membrane inlet mass spectrometry, during yeast fermentations.

Chloroform in a pristine aquifer system: Toward an evidence of biogenic origin

Ambient air, soil air, and groundwater were monitored for volatile halogenated organic compounds in a pristine spruce forest at Klosterhede, Denmark. Although this location is remote from industrial areas and free of any point sources of either soil or groundwater contamination, several volatile chlorinated hydrocarbons were detected. The concentration profiles of chloroform indicated a formation of this compound in the soil. Low concentrations were found in ambient air (0.02 ng L−1), higher concentrations in the upper soil layers (9.6 ng L−1 at 0.75 m below the surface), and a decrease with increasing depth (down to 1.5 ng L−1 just above the groundwater table at 7.5 m below the surface). For the other identified chlorinated compounds the concentration profiles were different from those of chloroform and exhibited less systematic variation between ambient air and soil air and with increasing depth. In the phreatic groundwater, high concentrations (up to 1.6 μg L−1) of chloroform and low concentrations (<0.01 μg L−1) of the other chlorinated compounds identified in air were found. Of the seven volatile brominated and iodinated compounds analyzed in ambient air, soil air, and groundwater, only bromoform was found and only in the upper soil layer. For chloroform a formation in the spruce forest soil is suggested, leading to concentrations in the groundwater in the micrograms per liter range.

Microporous membrane introduction mass spectrometry with solvent chemical ionization and glow discharge for the direct detection of volatile organic compounds in aqueous solution

Abstract The use of a microporous polypropylene membrane for the introduction of aqueous samples into a mass spectrometer in amounts sufficient for the water vapor to be used as reagent gas in chemical ionization (CI) is demonstrated. Solute molecules pass the membrane together with the water and can be analyzed using CI-MS and tandem mass spectrometry (MS-MS). Glow discharge, instead of a filament, is used to ionize the reagent gas, since the high water pressure was found to shorten the filament lifetime to a few hours. The glow discharge was efficient only within a very narrow pressure range, which was controlled simply by adjusting the water flow into the ion source by changing the membrane probe temperature. Using solvent CI with glow discharge, it is possible to measure a broad range of organic compounds with detection limits below 100 ppb and with an accuracy of ±5%. No enrichment of analytes as compared to the water solvent is achieved with the porous membrane and most volatile compounds show similar detection limits. Detection limits and response times were also measured with a silicone membrane introduction system using methane CI. The detection limits using microporous membrane introduction and solvent CI with glow discharge were at least an order of magnitude lower for small polar compounds like alcohols, ketones, aldehydes and acids than with the silicone membrane introduction system, whereas hydrophobic compounds like aromatics and chlorocarbons gave similar results for the two techniques. The response times of the flow injection membrane introduction system were found to be similar for the microporous and the silicone membranes used.