Heat-sensitive compounds, development

A particle beam interface (PBI) better serves the purpose of LC-EI-MS [55-57]. The PBI removes the solvent by nebulization into an evacuated desolvation chamber from where the evolving microscopic sample particles are transferred into an El ion source via a jet separator (Fig. 5.14). Designs different from PBI have also been developed [57]. The PBI is comparatively robust and attained popularity in particular for low- to medium-polarity analytes, but has some drawbacks such as poor sensitivity especially with water-rich mobile phases, moderate linearity with polar compounds, and low tolerance for heat-sensitive compounds [58,59]. The most recent addition to LC-EI interfaces, also the simplest and most elegant solution, makes use of the very low liquid flow rates of nano-LC equipment [60,61]. The flow from a 30 pm i.d. fused silica capillary column is passed... 

Although elegant, this approach also has particular limitations. First, because the test insects (Spodoptera litura) refused to consume any of the silica gel beneath the diet, the method can only identify antifeedants sufficiently polar to diffuse into the artificial diet in biologically effective concentrations. Second, heat-sensitive compounds may be broken down as the agar is poured over the plate. Thus, while positive results may provide dramatic and useful information, negative results are ambiguous. Finally, this technique is useful only for species for which an artificial diet has been developed (see following section). 

The increased use of IV-methyl carbamate insecticides in agriculture demands the development of selective and sensitive analytical procedures to determine trace level residues of these compounds in crops and other food products. HPLC is the technique most widely used to circumvent heat sensitivity of these pesticides. However, HPLC with UV detection lacks the selectivity and sensitivity needed for their analysis. In the late 1970s and early 1980s, HPLC using post-column hydrolysis and derivatization was developed and refined with fluorescence detection to overcome these problems. The technique relies on the post-column hydrolysis of the carbamate moiety to methylamine with subsequent derivatization to a fluorescent isoindole product. This technique is currently the most widely used HPLC method for the determination of carbamates in water" and in fruits and vegetables." " ... 

It becomes a problem in semantics to set a time limit for "development within which a process can be considered "spontaneous or "instantaneous . These two words seem to apply well to such extremely sensitive compounds as Nitrogen Triodide and Cupric Azide, which explode at the slightest touch when dry and, in addition, explode at a fairly low temperature. Attempts to correlate initiation in such cases with the attainment of a certain temperature seem unrealistic, especially in view of differences between relative sensitivity of different compounds to mechanical and thermal influences. For example, Mercuric Azide is so sensitive to impact that it explodes even under water, hut its heat sensitiveness is about the same as that for Cadmium Azide, which has been reported not to explode by percussion (Ref 5) Information about susceptibility of different explosives to spontaneous detonation is highly important from the viewpoint of safety. In Refs which follow are listed examples of spontaneous detonations of substances, some of them previously considered safe in this respect... 

Also propellant charges for rockets and guns have also been developed by compounding solid explosives such as nitramines (e.g. -> Cy-clonite) with plastics. Plastic explosives and plastic propellants are of interest, if low thermal and impact sensitivity is needed (- LOVA -> Armor Plate Impact Test -> Friction Sensitivity -> Heat Sensitivity - Impact Sensitivity -> Projectile Impact Sensitifity - Susan Test). 

Elemental sulfur in either its ore or its refined state can generaUy be recognized by its characteristic yeUow color or by the generation of sulfur dioxide when it is burned in air. Its presence in an elemental state or in a compound can be detected by heating the material with sodium carbonate and mbbing the fused product on a wet piece of silver metal. A black discoloration of the silver indicates the presence of sulfur. The test is quite sensitive. Several other methods for detecting smaU amounts of elemental sulfur have also been developed (34). 

As the vapor leaves the tube, the compounds in the sample are detected by a device such as a thermal conductivity detector. This instrument continuously measures the thermal conductivity (the ability to conduct heat) of the carrier gas, which changes when a solute is present. The detection techniques are very sensitive, allowing tiny amounts of solutes to be detected. Many environmental monitoring and forensic applications have been developed. 

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