Jan Niedzielski

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

How much do the ion-dipole capture rate constants obtained by transition state theory differ from those given by trajectory calculations?

The ion-dipole capture rate constant is calculated by using the macrovariational version of transition state theory for the conservation of resultant orbital-rotational momentum. The results are obtained in the nearly analytical form and show practically no differences with the results obtained by much more time consuming trajectory calculations.

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.

Double Stark effect in the theory of ion-dipole capture processes

Equations have been derived to describe the energy of a linear dipole in the weak external rotating electric field. This formalism is used to estimate the effect of the principle of resultant momentum conservation on the rate constant for the ion–dipole capture calculated according to the statistical adiabatic channel model.

Gas phase photolysis of 1-butene at 147 nm (8.4 eV)

Abstract The photolysis of gaseous 1-pentene was carried out in a static system using the xenon resonance line at 147 nm (8.4 eV) at pressures in the range 0.5 – 400 Torr (0.7 – 533 hPa). Only decomposition processes were studied and no attempt was made to establish the pattern of free radical reactions. The major dissociation products observed were ethylene, allene, propylene, 1,3-butadiene, acetylene and propyne. The minor products included methane, ethane, propane, some C 5 H 8 and C 4 H 6 hydrocarbons, and 1-butene. The radical species were identified using scavengers such as oxygen, H 2 S and HI. The pressure dependence of the yields of the major radicals (C 3 H 5 , CH 3 , C 2 H 5 , C 2 H 3 and C 3 H 7 ) was established. The C 2 H, C 4 H 5 , C 4 H 7 , CH 2 and C 3 H 3 radicals were found to be unimportant. The primary decomposition channels are established. The main processes are the cleavage of a CH bond with a yield φ of 0.45 – 0.48 and the cleavage of a CC bond with a yield φ of 0.47. The allylic CC bond appears to be the only CC bond which undergoes primary rupture. All four primary intermediates, i.e. C 5 H 9 , C 3 H 5 , C 2 H 5 and H, are energized. The radicals either decompose (isomerization prior to decomposition is possible in some cases) or undergo collisional stabilization; some hydrogen atoms add to the double bond prior to thermalization. Some details of the secondary processes are established but the overall mechanism is too complex to be fully interpreted.

Adsorption of water vapour from humid air in carbon molecular sieves: Carbosieve S-III and Carboxens 569, 1000 and 1001

The adsorption of water vapour in carbon molecular sieves (CMS) used to determine volatile organic compounds (VOCs) in air was investigated. The CMS mass in the trap was found not to affect the mass of retained water under conditions of incomplete saturation of the adsorbent bed with water. Thus, the restrictions commonly imposed on the CMS mass are not necessary. The usefulness of four different CMSs to sample large volumes of humid air was estimated. Carboxen 1000 exhibited the best performance. To assess the magnitude of CMS mass in the trap in dependence on the volume, the relative humidity and the temperature of the sample, the use of a novel parameter, called the water vapour interference factor, was suggested.

Effect of quantization of rotational energy on the ion-molecule capture

Abstract Failure to account for the quantized character of the energy of the molecule is one of the main reasons for the differences between measured and classically calculated rate constants for the ion-molecule charge transfer reactions. The quantization is especially important at low temperatures even for molecules that feature small dipole momenta.

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.