Compounds vapor pressures

Liquid chromatography, having a resolving power generally less than that of gas phase chromatography, is often employed when the latter cannot be used, as in the case of samples containing heat-sensitive or low vapor-pressure compounds. 

PAHs are emitted mainly into the atmosphere and have been detected at long distances from their sources. Because of their low vapor pressures, compounds with 5 or more aromatic rings exist mainly adsorbed to... 

The tendency of an organic liquid or solid chemical substance to partition into the atmosphere is controlled by its vapor pressure. Compounds with high vapor pressures tend to accumulate in the gas phase. [The vapor pressure of a solid or liquid has been likened to a kind of solubility of the compound in tlie atmosphere (Mackay, 1991).] Partitioning between the gas phase and water (Section 5.5) is described by Henry s law ... 

Criteria pollutants include combustion products, participates (of various sizes), and volatile organics. Of significance, the EPA includes some slow evaporating, low-vapor pressure compounds such as butyl cellosolve as volatile organic materials. A full list of such pollutants is at the EPA site, http //epa.g0v/0ar/0aqps/peg caa/p/egcaal 1. html. 

The gas saturation method obviously suffers from errors of a different nature than the isoteniscope method. Vapor lost to surfaces within the measurement system and low vapor-trapping efficiency may lead to significant errors for low vapor pressure compounds. Spencer et al. (15) found vapor capture efficiencies as low as 90 percent, while Wasik, et al. (16) arrived at a 95-percent confidence level of +9 percent. Thomas and Sieber (17) reportedly obtained an error of +1 to 3 percent using a column-gas saturator. Spencer noted, however, that for some very low vapor pressure compounds, up to an order of magnitude variation has been reported. 

Probably the most commonly used instruments for cation impurity analysis of silicates are flame atomic absorption spectrophotometers and ion selective electrodes. In most cases, separation of silica is required to reduce interferences. The sample may also have to be diluted to bring the analyte concentration within the linear operating range. For cations, the atomic absorption spectrophotometer is more versatile than ion specific electrodes. If the analyst is concerned with the presence of heavy metals, then accessories such as a hydride system for the elements that form high vapor pressure compounds, e.g., Sb, and a mercury vapor cold trap are useful. If a large number of elements are to be determined, a substantial investment in hollow cathode and electrode discharge lamps must be made. Several gas mixtures will also be required. 

The permeation method is based on the slow penetration or permeation of molecules through a polymeric membrane. The permeation rate of the compound is determined by physical characteristics of the chemical and the membrane. Permeation is pressure and temperature dependent. Higher vapor-pressure compounds will have a higher permeation rate. Similarly, high temperature will create high internal vapor pressure that leads to higher permeation rate. 

---