Distillation analysis, disadvantages

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... 

Wils et al. (25,26) previously reported an entirely different approach to TDG analysis. TDG in urine was converted back to sulfur mustard by treatment with concentrated HC1. The sample treatment is less straightforward than the methods described above, but analysis as sulfur mustard is facile. Urine, plus 2H8-TDG as internal standard, was cleaned up by elution through two C18 cartridges. Concentrated HC1 was added and the sample stirred and heated at 120 °C. Nitrogen was blown over the solution and sulfur mustard isolated from the headspace by adsorption onto Tenax-TA. The method was used to detect TDG in urine from casualties of CW attacks (see below). A disadvantage of this method is that it may convert metabolites other than TDG to sulfur mustard. This is supported by the detection of relatively high levels of analytes in urine from control subjects. Vycudilik (27) used a similar procedure, but recovered the mustard by steam distillation and extraction. 

There are a wide variety of methods available for the analysis of alkaloids in tobacco. Each method is associated with certain advantages and disadvantages, but fills a certain niche for tobacco scientists. If the percent total alkaloids determined as nicotine suffices, then steam distillation or autoanalyzer are appropriate methods. When secondary alkaloids need to be quantitatively determined, then one of the chromatographic methods is more appropriate. 

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