Boiling point, mobile phases

The effect of the mobile-phase composition on the operation of the different interfaces is an important consideration which will be discussed in the appropriate chapter of this book but mobile-phase parameters which affect the operation of the interface include its boiling point, surface tension and conductivity. The importance of degassing solvents to prevent the formation of bubbles within the LC-MS interface must be stressed. 

C below the boiling point of the mobile phase when a back... 

The pressurised dissolution/cooling procedure of Macko el al. [490], which uses a UV-transparent low-boiling point solvent, is fast and simple as no additional evaporation of the solvent, preconcentration or redissolution of the additive is necessary. Macko el al. [491] have given an extensive listing of HPLC analyses of aromatic antioxidants and UVAs which can be separated with n-heptane and n-hexane as the main component of the mobile phase. The method was also used for HPLC quantification of thioether antioxidants (Santonox R, Chimox 14 and Irganox PS 802) in MDPE [612],... 

In HPLC, the mobile phase is a liquid in which the sample must be soluble, and detection is most often accomplished by ultraviolet (UV) absorption. It is generally a slower process than GC however, its advantage is that the compounds to be separated are not limited by their boiling point, although low-boiling compounds are almost never separated by HPLC. Solid mixtures, as long as they are soluble in the mobile phase, can be analyzed by HPLC, whereas the solids that are typically encountered in soil analysis are not usually volatile enough to analyze via GC. 

This chapter focuses on gas-liquid chromatography, in which compounds in a sample are separated based on vapor pressures and differences in affinity for the stationary phase (a high boiling point liquid) versus the gaseous mobile phase. The time between sample injection and detection of the individual compound eluting from the column is called the retention time. Compounds that have limited solubility in the stationary phase will exit the column quickly as a large proportion will remain in the mobile phase. Compounds with polarity similar to that of the stationary phase will have longer retention times and potentially broader peaks, due to increased interaction with the stationary phase. 

Choice of Solvent. JJ-Methylpyrrolidone (NMP) was initially used as the mobile phase but proved to be unsatisfactory because of (i) high solution viscosities, (il) exceedingly small differences in refractive index between NMP and cellulose triacetate solutions, (ill) erratic base line. In view of this dichloromethane was employed. Some additional benefits derived from this mobile phase are (i) a decrease in elution volume due to low solution viscosities, (il) fast solvent recovery due to low boiling point of dichloromethane and (iii) ease of obtaining preparative GPC cuts of cellulose triacetate. 

In this technique, the stationary phase is a porous solid (such as graphite or silica gel) and the mobile phase is a gas. This type of gas chromatography demonstrates very high performance in the analysis of gas mixtures or components that have a very low boiling point. 

Reversed-phase chromatography is the term commonly applied to a system where a nonpolar liquid phase is coated on the solid support and elution carried out with an immiscible polar phase. Such systems are often necessary for separations which cannot be carried out by normal partition or adsorption chromatography. For TLC, the stationary phase is normally a liquid of high boiling point which does not readily evaporate from the adsorbent. Paraffin oil, silicone oil or n-tetradecane coated on silica gel or Kieselguhr are frequently used with water-based mobile phases such as acetone—water (3 2) or acetic acid-water (3 1). Reversed-phase chromatography is very useful for the TLC analysis of lipids and related compounds. 

Temperature has often been suggested as a useful control variable for HPLC to make a changes and to speed equilibrations leading to faster separations. The problem has been that both bonded-phase hydrolytic cleavage and solubility of silica in aqueous solvents are accelerated at elevated temperatures. Mobile phase boiling within the column can cause bubble formation and vapor locking if the critical point of the solvent is exceeded. Finally, thermal-labile compounds can suffer degradation at elevated temperatures. 

A liquid, like a gas, has no shape of its own, but it does have a definite volume. Both states of matter are referred to as fluids because of their mobility, or tendency to flow. A gas is actually a low density fluid because the molecules are much farther apart than in a liquid where molecules are in close contact with each other. For example, at room temperature and at atmospheric pressure the density of air is about 0.0012 grams (0.000042 ounces) per cubic centimeter (g/cm3), whereas the density of liquid air is approximately 0.810 g/cm3 (atits normal boiling point of—209°C, or —344°F). This corresponds to an average separation between molecules in the gas phase that is about nine times larger than that for the liquid. A liquid is thus called a condensed phase—or a high density fluid—and is roughly 1,000 times more dense than a gas. 

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