Noble gases temperature effect

A final note must be made about a common problem that has plagued many kinetic treatments of reactive intermediate chemistry at low temperatures. Most observations of QMT in reactive intermediates have been in solid matrices at cryogenic temperatures. Routinely, reactive intermediates are prepared for spectroscopy by photolyses of precursors imbedded in glassy organic or noble gas (or N2) solids. The low temperatures and inert surroundings generally inhibit inter- and intramolecular reactions sufficiently to allow spectroscopic measurements on conventional and convenient timescales. It is under such conditions, where overbarrier reactions are diminished, that QMT effects become most pronounced. 

For many elements, the atomization efficiency (the ratio of the number of atoms to the total number of analyte species, atoms, ions and molecules in the flame) is 1, but for others it is less than 1, even for the nitrous oxide-acetylene flame (for example, it is very low for the lanthanides). Even when atoms have been formed they may be lost by compound formation and ionization. The latter is a particular problem for elements on the left of the Periodic Table (e.g. Na Na + e the ion has a noble gas configuration, is difficult to excite and so is lost analytically). Ionization increases exponentially with increase in temperature, such that it must be considered a problem for the alkali, alkaline earth, and rare earth elements and also some others (e g. Al, Ga, In, Sc, Ti, Tl) in the nitrous oxide-acetylene flame. Thus, we observe some self-suppression of ionization at higher concentrations. For trace analysis, an ionization suppressor or buffer consisting of a large excess of an easily ionizable element (e g. caesium or potassium) is added. The excess caesium ionizes in the flame, suppressing ionization (e g. of sodium) by a simple, mass action effect ... 

To successfully use high-resolution molecular spectroscopy to study tunneling, two conditions have to be met suppression of hot bands and removal of inhomogeneous broadening. In the traditional technique of equilibrium sample preparation these conditions are mutually exclusive To decrease the hot band intensity one needs to lower the temperature, which entails the condensation of a sample and, consequently, appearance of inhomogeneous spectral effects which are due to intermolecular interactions in the solid. To some extent, a compromise is achieved in the matrix isolation method, where the intermolecular interactions between the guest and host molecules are minimized by using the noble gas matrix. However, even in this case the asymmetry of the potential is... 

As with other compounds, solution effects can elevate the condensation temperatures of clathrate guest species. Sill and Wilkening calculated that in a gas of solar composition the major clathrate, and the first to form, will be ice-methane, and that noble gases can substitute for the methane at temperatures higher than decomposition temperatures for noble gas clathrates. They calculate, for example, that in a total nebular pressure of 2 x 10 atm (high in comparison with most model pressures currently considered of about 10 4 atm ), ice-methane clathrate at 80 K will have dissolved 99% of the available Xe (and substantially smaller amounts of the other noble gases). 

Unlike elemental concentrations, isotopic compositions are only affected a little by chemical differentiation processes. Mass-dependent isotopic fractionations can arise in chemical partitioning (cf. Section 2.9), of course, but on the scale of interest in the present context, plausible fractionation effects are small, especially at the high temperatures prevalent in the mantle. We can thus be much more confident that a noble gas isotopic composition measured in a mantle-derived sample is indeed characteristic of its mantle source. Representative mantle ranges for selected isotopic ratios are presented in Table 6.2. 

Vibrational spectroscopy is successfully employed to quantitative analysis of gases, especially if real time and on-line analyses are needed. In order to compensate the effects of pressure broadening, it is worthwhile to carry out all measurements at the same total pressure. To this end, the sample is placed in an inert gas, such as nitrogen or a noble gas, and the pressure raised to a defined value. The partial pressure instead of the concentration is used in the Lambert-Beer law. The calibration curve is valid only at the calibration temperature. If the temperature of the sample deviates from this temperature, the partial pressure has to be corrected by the Gay-Lussac law. 

The temperature dependences of the Henry s Law coefficients of the different gases listed in Table 3.6 are quite variable (Fig. 3.11). Helium, the least soluble noble gas, has very little solubility temperature dependence between 0 and 30 °C. On the other hand, Kr, the second most soluble of the non-radioactive noble gases, is much less soluble at higher temperatures. More details about gas solubilities are presented in the chapter on air-sea gas exchange (Chapter 10). Another notable aspect of the temperature dependence of the gas solubilities is that they are not linear. Thus, mixing between parcels of water of different temperatures at saturation equilibrium with the atmosphere results in a mixture that is supersaturated. This effect has been observed for noble gases in the ocean and may ultimately have a utility as a tracer of mixing across density horizons. 

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