Distillation analysis, advantages

The most frequently used methods of analyte isolation and concentration for organic compounds involve distillation, extraction auid adsorption techniques. Some typical applications of these techniques and their attendant -advantages and disadvantages for the analysis of trace organic solutes in water are summarized in Table 8.1 [4,26]. These methods will be elaborated on below and in subsequent sections of this chapter. 

Advantages High analysis rate 3-4 elements per hour Applicable to many more metals than voltammetric methods Superior to voltammetry for mercury and arsenic particularly in ultratrace range Disadvantages Nonspecific absorption Spectral interferences Element losses by molecular distillation before atomisation Limited dynamic range Contamination sensitivity Element specific (or one element per run) Not suitable for speciation studies in seawater Prior separation of sea salts from metals required Suspended particulates need prior digestion About three times as expensive as voltammetric equipment Inferior to voltammetry for cobalt and nickel... 

Extractive distillation is evaluated as an alternative to ordinary distillation for the separation of propylene-propane mixtures. Particular attention is given to the necessary compromises between different design factors solvent concentration within the primary column, solvent selectivity, solvent loss, etc. A major expense is associated with the sensible heat requirements of the circulating solvent process modifications so as to minimize this expense are discussed. The process analysis explores combinations of solvent selectivity and other solvent properties which might make extractive distillation attractive. It appears that in almost all cases extractive distillation offers no advantage compared with ordinary distillation. Only in special cases may circumstances warrant extractive distillation. External factors favoring the use of extractive distillation are identified. 

A procedure was developed by Hiatt (1983) and Dreisch and Munson (1983) to identify and quantify 1,1-dichloroethane in fish tissue samples by GC/MS, employing a fused-silica capillary column (FSCC) and vacuum distillation (extraction). An advantage of the vacuum extraction is that the system does not require elevated temperatures or the addition of reagents, which could produce unwanted degradation products (Hiatt 1981). The FSCC provides a more attractive approach than packed column for chromatographic analysis of volatile organic compounds, because FSCC can be heated to a higher-temperature (350°C) than that recommended for packed column thereby... 

Without access to the ion resin, Bronk and Ward (1999) switched to vacuum distillation to remove NH4+, followed by peroxide oxidation under UV radiation to convert DON to NOs". The nitrate was then reduced to nitrite and isolated by solvent extraction using tricholoroethanol (Olson, 1981) (Section 3.3). As noted above, Miyajima et al. (2005) have recently also analyzed DON by conversion to nitrate but employed GC-MS analysis of the PFB derivative. Their approach has the advantage of requiring small sample sizes, which also permits simply drying the sample to remove NH4+ during the DIN removal step. Clearly, any of the methods for NOs analysis (Section 3.3) would be suitable for this purpose too. 

The researches have dealt with each phase of the pentosan determination. Many modifications have been introduced in the distillation procedure, in attempts to obtain ma.ximal yields of furfural. For e.xample, some workers prefer to distil with 23 % hydrobromic acid, rather than with the conventional 12% hydrochloric acid. Others distil in the presence of added sodium chloride, to avoid changes in acid concentration. Steam distillation has been used by a number of workers, who claim theoretical yields of furfural from pentoses, but Launer and Wilson found no advantage either in salts or in steam in the analysis of pulps and papers. Interfering substances are of two types materials other than pentosans which form furfural in the pentosan analysis, and substances which yield products which may be determined as furfural. TJronic acids and polyuronides yield furfural, although not quantitatively, and, in the case of materials containing appreciable quantities of these substances, it is usual to make a correction. The value of the correction to be applied has been determined experimentally by several workers, with somewhat differing results. 

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