Tomasz Gierczak

Prevention of water vapour adsorption by carbon molecular sieves in sampling humid gases.

The water uptake by the solid sorbents Carbosieve S-III, Carboxen 569, 1000 and 1001, all of which are used for sampling of volatile organic compounds from the atmosphere. was examined using a direct experimental approach. The content of retained water is affected both by the trap temperature and the initial water vapour concentration in the sampled gas. Two different adsorption mechanism are operative. At low relative humidities (RH) only active polar centres are involved. This adsorption is so weak that negative water interferences can easily be managed. Another mechanism, the micropore volume filling, involves substantial amounts of water, becomes operative once the threshold value for relative humidity (RHth) is surpassed. RHth is 45+/-3% for Carboxen 1000 decreasing to 35+/-3% for the three other sorbents studied. A novel but simple strategy was tested for water management: moderate heating of the trap during the sampling (a warm trap method). The temperature elevation required depends on the RHth characteristic for the specific sorbent, and RH and the temperature of the sampled gas. Usually the 5-15 degrees C elevation is sufficient; only under extreme RH conditions is an elevation of 20 degrees C necessary. The diagrams are given to determine this elevation. Since the sample RH is significantly decreased at an elevated temperature the negative effect of water uptake on the safe sampling volume is alleviated. Consequently the sampled gas volume can be as large as desired which decreases detection limits.

Adsorption of water vapour in the solid sorbents used for the sampling of volatile organic compounds

The adsorption of water vapour in the sorbents used to sample volatile organic compounds (VOCs) from the atmosphere was investigated by frontal gas chromatography. Air of 95% relative humidity (RH) was passed through the sorbent bed and the uptake of water was monitored at the outlet of the trap. Graphitized carbons and non-polar polymeric sorbents, such as Tenax and Chromosorb 106, show poor water trapping of generally less than 5 mg of water per gram of sorbent. Polar polymeric sorbents, e.g. Chromosorb 108, Porapak T and Porapak N, sorb more water which is, however, weakly bound and easily removed by purging the sorbent bed with a dry gas. Carbon molecular sieves, e.g. Carbosieve S-III, Carboxen 569, Carboxen 1000 and Carboxen 1001, adsorb substantial amounts of water, corresponding to the volume of micropores. An important feature is a lack of adsorption at low RHs. Measures to prevent water adsorption in sampling even very humid gases are advanced. The RH of sampled gas should be decreased below a threshold value (RHthr). The RHthr is 50% for Carboxen 1000 and less than that for the other sorbents studied. Practical implementation of the suggested method is discussed.

Dry purge for the removal of water from the solid sorbents used to sample volatile organic compounds from the atmospheric air

The dry purging technique used to remove water from the air nand water sampling adsorbents in volatile organic compounds (VOC) analysis nwas investigated. As a sampling simulation step, a fixed volume of humid nair was passed through the tube filled with the sorbent bed. Desorption by nthe dry gas purging followed. The concentration of water vapour in the gas nat the outlet of the trap was directly measured in the course of all nexperiments. No more than 300 ml of dry gas is enough for complete removal nof water from Tenax, Chromosorb 106, and Carbotraps B and C, even if large nvolumes of air at relative humidity as high as 95% are sampled. Adsorbed nwater can also be purged effectively from the carbon molecular sieves: nCarbosieve S-III and Carboxen 569, 1000 and 1001. Carboxen 1000 is the neasiest and Carbosieve S-III the most difficult case that requires the npurging gas volume larger by about 60–100%. Carboxen 569 and 1001 noccupy an intermediary position. For carbon molecular sieves the dependence nof water vapour concentration at the outlet of the sampling tube on the dry ngas volume is very characteristic: a long segment that corresponds to the nconstant concentration is followed by a sharp decrease until the water is nremoved completely. The volume of dry gas necessary to achieve this task ndepends on the sample magnitude and relative humidity and on the desorption ntemperature. The adsorbent mass exerts a very small effect. The latter nphenomenon is unexpected but very important for analytical practice. nIncrease in the adsorbent mass prevents the losses of weakly adsorbed nanalytes without the need to resort to increasing the purging gas volume. nThe water desorption process can easily be monitored and automated by nplacing a humidity sensor in the outlet channel of the purging gas.

Mutual isomerization of cyclopentyl and 1-Penten-5-y1 radicals

Unimolecular reactions of mutual isomerization of cyclopentyl and 1-penten-5-yl radicals have been investigated by chemical activation. The radicals were generated by adding energized hydrogen atoms (EH about 23 kcal mol−1) to the double bond of either cyclopentane or 1,4-pentadiene. Based on the extensive steady-state RRKM calculations employing the experimental data from this work as well as from the literature, the threshold energies for the cyclopentyl ring opening and closure are 32 ± 0.3 and 16.2 ± 0.3 kcal mol−1, respectively. The entropy of activation for the ring opening is close to zero.

Analysis of α-acyloxyhydroperoxy aldehydes with electrospray ionization–tandem mass spectrometry (ESI-MSn)

A series of α-acyloxyhydroperoxy aldehydes was analyzed with direct infusion electrospray ionization tandem mass spectrometry (ESI/MS(n)) as well as liquid chromatography coupled with the mass spectrometry (LC/MS). Standards of α-acyloxyhydroperoxy aldehydes were prepared by liquid-phase ozonolysis of cyclohexene in the presence of carboxylic acids. Stabilized Criegee intermediate (SCI), a by-product of the ozone attack on the cyclohexene double bond, reacted with the selected carboxylic acids (SCI scavengers) leading to the formation of α-acyloxyhydroperoxy aldehydes. Ionization conditions were optimized. [Mu2009+u2009H](+) ions were not formed in ESI; consequently, α-acyloxyhydroperoxy aldehydes were identified as their ammonia adducts for the first time. On the other hand, atmospheric-pressure chemical ionization has led to decomposition of the compounds of interest. Analysis of the mass spectra (MS(2) and MS(3)) of the [Mu2009+u2009NH(4)](+) ions allowed recognizing the fragmentation pathways, common for all of the compounds under study. In order to get detailed insights into the fragmentation mechanism, a number of isotopically labeled analogs were also studied. To confirm that the fragmentation mechanism allows predicting the mass spectrum of different α-acyloxyhydroperoxy aldehydes, ozonolysis of α-pinene, a very important secondary organic aerosol precursor, was carried out. Spectra of the two ammonium cationized α-acyloxyhydroperoxy aldehydes prepared with α-pinene, cis-pinonic acid as well as pinic acid were predicted very accurately. Possible applications of the method developed for the analysis of α-acyloxyhydroperoxy aldehydes in SOA samples, as well as other compounds containing hydroperoxide moiety are discussed.

Multistep derivatization method for the determination of multifunctional oxidation products from the reaction of α-pinene with ozone

A novel three-step analytical method was developed which enables the simultaneous detection and identification of multifunctional oxygenated products resulting from the reaction of α-pinene with ozone. The method consists of the following steps: conversion of carbonyl groups to methyloximes using methyloxyamine, conversion of carboxylic acids to methyl esters using trimethylsilyldiazomethane (TMSD), and conversion of alcohols to trimethylsilyl ethers using N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA). The derivatization procedure at each stage was optimized yielding the appropriate amount of derivatization reagent, reaction temperature and time. The newly developed analytical procedure manages without processes of extraction and evaporation to dryness at any stage. Total time for sample analysis is short ca. 3h. The characteristic ions of derivatives and common pattern for ion fragmentation in capillary gas chromatography electron impact mass spectrometry (GC-EI-MS) analysis were elucidated and discussed.

Isomerization of chemically activated secondary butyl radical

Photolytic decomposition of hydrogen sulfide followed by the addition of energized hydrogen atoms to cisbut-2-ene gives excited sec-butyl radicals. The radicals undergo a 1,3 hydrogen migration as is evidenced by the good agreement between the RRKM calculations and experiment. The contribution of a 1,2 hydrogen shift cannot be excluded. The threshold energies are 153.8 and 176.8 kJ mol−1 for 1,3 and 1,2 isomerization, respectively.AbstractФотолитическое разложение сульфида водорода с последующим присоединением знергичных атомов водорода к цисбутену-2 приводит к возбужденным втор.-бутильным радикалам. В радикалах происходит 1,3-миграция водорода, о чем свидетельствует хорошее совпадение расчетных результатов с экспериментальными. Однако, нельзя пренебрегать 1,2-смещением водорода. Энергии порога равны 153,8 и 176,8 кДж/моль для 1,3-и 1,2-изомеризаций, соответственно.

Reaction of photochemically generated hot hydrogen atoms with 1-butene

Abstract The reaction of hot hydrogen atoms with 1-butene was studied in the gas phase at pressures in the range 0.4–400 Torr (0.5–533 hPa). Hydrogen atoms were generated by exposing HI to the action of UV light (λ = 334 and 313 nm). Some of the hydrogen atoms add to the double bond in a first collision yielding highly excited sec-butyl and n-butyl radicals; the remainder undergo thermalization according to a step-ladder model. The evidence that hot hydrogen atom addition occurs is based on kinetic considerations—both experimental and calculated RRKM rate constants for decomposition of the excited butyl radicals are given—and is supported by the observation that the contribution of non-terminal addition of hot hydrogen atoms reaches a level of about 30%, whereas thermal hydrogen atoms add mainly (about 94%) to the terminal carbon atom. The hot hydrogen atom + olefin chemical activation technique provides an interesting tool for the investigation of highly excited radicals.

Dissociation of isomerization of excited C3H5 radicals in the gas phase

Abstract Highly excited C3H5 radicals were formed in the gas phase by the addition of hot hydrogen atoms (E = 22.9 kcal mol−1) to either allene or propyne. The hydrogen atoms were generated by the action of UV radiation (253.7 nm) on H2S. 2-propenyl radicals undergo mainly cleavage of the Cue5f8H bond yielding allene or propyne; the latter decomposition channel predominates. The main reaction of 1-propenyl radicals is cleavage of the Cue5f8C bond yielding methyl radicals and acetylene. Rice—Ramsperger—Kassel—Marcus calculations indicate that isomerization of the propenyl radicals into the allylic structure may be of importance. The calculations involving allyl radicals with an excess energy as high as about 81 kcal mol−1 demonstrate that dissociation yielding allene and a hydrogen atom is not effective; isomerization can occur prior to dissociation but the rate constants for both processes are markedly lower than that of the propenyl radical. The implications of these results with respect to the determination of the photolysis mechanism for gaseous olefins are briefly discussed.

Qualitative and Quantitative Analyses of the Halogenated Volatile Organic Compounds Emitted from the Office Equipment Items

Indoor sources of the halogenated volatile organic compounds (halogenated VOCs) emissions into the office air were identified in this study. Mixtures of the organic pollutants emitted from 16 plastics samples or from finishing materials were analysed. The VOCs emitted from the samples were acquired in a small environmental test chamber with the clean air circulation system, at 296u2009±u20092u2009K. The VOCs emitted were collected on three-layer adsorption tubes. Identification of the emitted VOCs was based on thermal desorption (TD) combined with the capillary gas chromatography – mass spectrometry technique (GC/MS). The tests identified several organic pollutants and most of these VOCs were chlorinated organic compounds: tetrachloroethene (identified in 68.7% samples), bromo-dichloromethane (in 56.2% samples), chlorobenzene and 1,4-dichlorobenzene (both in 43.7% samples). Concentration levels were compared for halogenated compounds and for some other VOCs (e.g., benzene, toluene, ethylbenzene, xylenes). The largest emission level was found for toluene (≤181.6u2009µgu2009m−3) and styrene (≤24.5u2009µgu2009m−3). The electric wall switch was found to be the most important source of halogenated VOCs and a 16.4u2009µgu2009m−3 (about 10.3% of TVOC emitted). The felt-backed carpet was the second most abundant (11.1u2009µgu2009m−3), about 11.9% of TVOC.