Capillary temperature sample decomposition

The decomposition point of 278° is obtained by placing the capillary in a bath already heated to 260-270°. If the sample is slowly heated, starting at room temperature, a decomposition point of 257-258° is observed. 

When selecting the desired temperature of the capillary, consideration should be given to reproducibility of sample injection and migration times, optimization of resolution, the possibility of sample decomposition, and the minimization of analysis times [3]. 

It has b n suggest (4) diat an appropriate measure of chromophore stability would be the temperature at which the NLO activity dropped to 90% of its initial value after 1/2 hour exposure. We have therefore defined a Td(l/2) as the temperature where 10% decomposition takes place in 30 minutes. The procedure followed was to thermally age the samples in sealed capillaries for 20, 40 and 60 minutes at each of several temperatures and run a DSC temperature scan of each sample. Decomposition was followed by the gradual disappearance of the melting endotherm or decomposition exotherm. The data were enter into a preprogramm spreadsheet to perform the appropriate calculations as described above. The plots as well as the data are placed on the spreadsheets, a sample of which is shown on Figure 4. 

Cool on-column Capillary Ramped temperature Track oven Low concentration or thermally labile Minimal sample discrimination and decomposition... 

In a series of papers [98-100], Bemdt described development of a new high-temperature/high-pressure flow system for continuous decomposition of biological and environmental samples. It was shown [98] that temperatures up to 260 °C and pressures up to 300 bar can be reached in a flow system when an electrically heated Teflon-lined HPLC tube is used as the digestion capillary. Digested biological... 

The capillaries are best made by drawing out carefully washed (distilled water) pyrex tubing ( 15 to 20 mm diameter). It is very important that the tubes be clean, since inorganic materials on the inside surface of the tube may cause decomposition of the sample at the relatively high temperatures often required for melting-point determinations. 

This is not a method commonly used for coordination compounds, which do not often melt without decomposition. Where applicable, it can be used by cooling from a high-temperature melt to room temperature, or by cooling a room-temperature liquid to a lower temperature. The latter is a specialized technique, usually carried out in situ on a diffractometer, with monitoring of the crystal growth by optical and X-ray methods the sample is contained in a sealed capillary tube, and selective heating may be applied by an infra-red laser to develop a single crystal. Twins and multiple crystals often result from these methods. 

The decomposition point varies with the method of determination. The values given herein were determined by placing the sample in a capillary tube, inserting the tube in the bath at a temperature 25° below the decomposition point, and heating at the rate of 2-3° per minute. 

The capillary columns are the most used columns for the separation of A-nitrosamines. These enable better separations and sensitivities than packed columns with comparatively lower temperatures, hence preventing the decomposition of thermally labile compounds. The most commonly used columns are made of siloxane polymers like DB-5 and of poliethylene glycol like DV-wax. Column lengths of 15 to 30 m and film thickness of 0.25 to 1.0 /rm are used in many applications. The high capacity of 1.0 /rm film allows the injection of large volumes of samples. This can be an important feature when TEA or MS detectors are used. 

There is a rich literature associated with studies of the breakdown or formation of specific chemical compounds and classes of chemical componnds during burning processes. A study of the thermal decomposition of pentachlorobenzene, hexachlo-robenzene, and octachlorostyrene in air contained many such citations. In this study, nearly pure 10- to 20-mg samples of the cited chemicals were decomposed in a vertical combustion furnace and the decomposition product trapped on cooled X AD-4 resin followed by charcoal tubes. The adsorbed components were desorbed with toluene and analyzed using capillary GC and GC/MS. The decomposition products formed depended upon the applied temperature, the oxygen concentration, and the residence time in the hot zone of the combustion chamber. 

In this test (Figure 3.3) the sample is held in a glass liner within a stainless steel pressure vessel, in turn inside a temperature-programmed oven. The pressure vessel has a bleed valve for venting decomposition products and a bursting disc, rated at 67 bar (l(X)0 psi). A remote pressure transducer is linked to the vessel by a steel capillary. The oven is heated at a fixed rate over the experimental temperature range, and a chart records the sample temperature and vessel internal pressure as functions of time. 

Cooling may also reduce any decomposition of the sample in the X-ray beam, which imposes some thermal loading on the crystal through absorption effects. Furthermore, it is far easier to handle air-sensitive materials in this way than for room-temperature data collection. Instead of having to provide such crystals with a stable and robust protective shield (either a coating such as epoxy resin, or a sealed thin-walled capillary tube), they can simply be handled under an inert viscous oil into which they are introduced from a Schlenk tube. The oil serves as an adhesive for mounting the crystal on the diffractometer and forms a thin protective film that vitrifies by shockcooling in the cold gas stream. Air-stable and air-sensitive samples can be handled in virtually the same way. 

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