Vapor-liquid chromatography

An interesting feature of the hydrolysis of dichlorosilanes is the formation of many stable ring structures. In addition to the cyclic tetramer, D4, simple mixing of (CH3)2SiCl2 with alkaline water leads to appreciable quantities of D3, Dj, D , and so on. Vapor-liquid chromatography is able to confirm the presence in hydrolyzate of rings up to D40. Production of many silicone compounds starts with redistilled hydrolyzate, which is predominantly D4. 

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. 

In many applications in mass spectrometry (MS), the sample to be analyzed is present as a solution in a solvent, such as methanol or acetonitrile, or an aqueous one, as with body fluids. The solution may be an effluent from a liquid chromatography (LC) column. In any case, a solution flows into the front end of a mass spectrometer, but before it can provide a mass spectrum, the bulk of the solvent must be removed without losing the sample (solute). If the solvent is not removed, then its vaporization as it enters the ion source would produce a large increase in pressure and stop the spectrometer from working. At the same time that the solvent is removed, the dissolved sample must be retained so that its mass spectrum can be measured. There are several means of effecting this differentiation between carrier solvent and the solute of interest, and thermospray is just one of them. Plasmaspray is a variant of thermospray in which the basic method of solvent removal is the same, but the number of ions obtained is enhanced (see below). 

These factors make it necessary to reduce the amount of solvent vapor entering the flame to as low a level as possible and to make any droplets or particulates entering the flame as small and of as uniform a droplet size as possible. Desolvation chambers are designed to optimize these factors so as to maintain a near-constant efficiency of ionization and to flatten out fluctuations in droplet size from the nebulizer. Droplets of less than 10 pm in diameter are preferred. For flow rates of less than about 10 pl/min issuing from micro- or nanobore liquid chromatography columns, a desolvation chamber is unlikely to be needed. 

Gas chromatography (GC) deals with volatile substances that can be vaporized into a gas stream. Liquid chromatography (LC) concerns mostly nonvolatile substances dissolved in a liquid stream. 

The modern electronic industry has played a very important role in the development of instrumentation based on physical-analytical methods As a result, a rapid boom in the fields of infrared, nuclear magnetic resonance (NMR), Raman, and mass spectroscopy and vapor-phase (or gas-liquid) chromatography has been observed. Instruments for these methods have become indispensable tools in the analytical treatment of fluonnated mixtures, complexes, and compounds The detailed applications of the instrumentation are covered later in this chapter. 

To measure gas and water vapor permeability, a film sample is mounted between two chambers of a permeability cell. One chamber holds the gas or vapor to be used as the permeant. The permeant then diffuses through the film into a second chamber, where a detection method such as infrared spectroscopy, a manometric, gravimetric, or coulometric method isotopic counting or gas-liquid chromatography provides a quantitative measurement (2). Die measurement depends on the specific permeant and the sensitivity required. 

Liquid Chromatography-Mass Spectrometry Vaporizer Seated capillary Skimmer... 

For 2,3,4,6-tetrachlorophenol hardly any discoloration was detected above 200°C instead the sample vaporized rapidly. Its sodium salt reacted similarly to sodium pentachlorophenate except that its exothermic decomposition (Figure 4) is less clearly defined no crystallization occurred on cooling, and the yield of hexachlorodioxins was much lower, the remaining products being higher molecular weight materials. The hexachlorodioxins were identified by gas-liquid chromatography as two isomers in a ratio of 35 65. 

As there often exists a substantial difference in volatility between the solutes typically analyzed by liquid chromatography and the solvents used for their separation, it should be possible to use selecti solvent vaporization as an enrichment technique in LC/MS. ThiS inclple is employed on-line in the moving belt... 

Hussam, A., Carr, P. W. (1985) A study of a rapid and precise methodology for the measurement of vapor-liquid equilibria by headspace gas chromatography. Anal. Chem. 57, 793-801. 

Techniques for analysis of different mercury species in biological samples and abiotic materials include atomic absorption, cold vapor atomic fluorescence spectrometry, gas-liquid chromatography with electron capture detection, and inductively coupled plasma mass spectrometry (Lansens etal. 1991 Schintu etal. 1992 Porcella etal. 1995). Methylmercury concentrations in marine biological tissues are detected at concentrations as low as 10 pg Hg/kg tissue using graphite furnace sample preparation techniques and atomic absorption spectrometry (Schintu et al. 1992). 

Gas chromatography (GC) can also be referred to as vapor-phase chromatography (VPC) and even gas—liquid chromatography (GLC). Usually the technique, the instrument, and the chart recording of the data are all called GC ... 

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