Sampling

One of the foremost and, perhaps least appreciated of the initial problems is that of sampling. Sample size and distribution are critical because organics, which comprise the vast majority of potential samples, are invariably very poor conductors of heat.

Jones and Moyles enunciated the thin film concept in 1958. This is based on comparisons of strictly controlled pyrolytic events on the milligram, microgram and sub-microgram scales. Pyrograms of simple substances were demonstrated to be of ever-increasing complexity as sample size increased. The phenomenon was eventually shown to be due to diffusion of degradation products through partially degraded sample and longer residence times in the vicinity of the pyrolysis zone, which led to unwanted secondary reactions. For these reasons it is advisable to work on the nano- or even picogram scale whenever possible.

It should be realized that the most informative evidence leading to an unambiguous conclusion lies in the certainty that all the scissive events viewed are primary reactions (notwithstanding the fact that most primary reactions must be the formation of free radicals which are themselves stabilized by recombination). Therefore it is imperative to regularize the situation as best as possible. That is, the fastest thermal gradient with respect to time within the body of the sample, a condition that is only satisfied by presenting the sample as a very thin film, ultimately as a uni-molecular layer. Unfortunately, the very nature of most samples submitted for pyrolysis prior to gas chromatography is that they are reluctant to form the uni-molecular layers that are theoretically essential. So a compromise has to be made and as thin a layer is distributed on the effective surface of the chosen pyrolyser as is practically possible.

When a sample is soluble in a volatile solvent the deposition problem is ameliorated but immediately poses further problems. What if the solvent lacks wetting power for metallic surfaces? Addition of an amount (e.g. up to 5% v/v) of tetrahydrofuran or dioxan as a wetting agent rarely affects solubilization and overcomes the beading that characterizes a nonwetting system.

After deposition of the sample, residual solvent is best removed in a vacuum oven at a temperature little above ambient. However should the sample be insoluble it is reduced to fine powder by slow freeze-grind-ing in order to minimize the risk of thermal degradation arising from localized heating due to mechanical friction. It is sonically mixed with a blend of a polar solvent and water containing 1% w/w of a refined natural gum or a purified poly(vinyl alcohol), and then stuck on the probe and dried in the manner already suggested. Conflicting pyrolytic fragments arising from breakdown of the adhesive used can be subtracted from the total pyrogram if deemed necessary; experience, however, has shown that there is little significance in any contribution from the presence of a relatively small amount of an alien material provided that the same routine is used in comparative exercises.

It cannot be emphasized enough that much depends on maintenance of strictly sterile conditions as always.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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