Trace Impurities in Solvents

The reason for stressing the importance of working with relatively pure reagents and solvents is that the rates of many reactions are extremely sensitive to the presence of trace impurities in the reaction system. If there is reason to suspect the presence of these effects, a series of systematic experinlents may be carried out to explore the question by seeing how the reaction rate is affected by the intentional addition of impurities. In many cases, lack of reproducibility between experiments may be an indication that trace impurity effects are present. 

Criteria for solvent purity include careful measurement of physical constants such as melting point, refractive index, or conductance, but even these techniques may not detect some trace impurities. In many instances gas-liquid or other sensitive chromatographic methods may be used. 

Gopalkrishnan, M. Radhakrishnan, K. Dhami, P.S. Kulkami, V.T. Joshi, M.V. Patwardhan, A.B. Mathur, J.N. Determination of trace impurities in uranium, thorium and plutonium matrices by solvent extraction and inductively coupled plasma atomic emission spectrometry, Talanta 44 (1997) 169-176. 

A signal also appears at about 8 7.30 ppm and this can be identified as an impurity since its integrated intensity corresponds to only about 0.2 protons. It is due to the proton of chloroform which is present as a trace impurity in the solvent, deuterochloro-form (CDcy. 

There are two main sources of drift, both due to non equilibrium conditions in the column and the detector. If the detector, column and mobile phase are not in thermal equilibrium, then serious drift will occur. This can be eliminated by careful temperature control of column and detector. Another and more common source of drift arises when the stationary phase and mobile phase have not been given sufficient time to come into equilibrium. This type of drift often occurs when changing the mobile phase composition and mobile phase should be pumped through the chromatographic system until a stable baseline is achieved. Trace impurities in the mobile phase can cause prolonged drift and longterm noise and so very pure solvents must be used for the mobile phase. Distilled in glass solvents may not necessarily be sufficiently pure to ensure drift-free detector operation. 

The mechanism of the reaction between nitric oxide and the amines is not known at present. In a reported 14) kinetic study low concentrations of NO were utilized. Reaction of NO with trace impurities in the solvent gave rise to a species whose spectrum was mistakenly assumed to be that of nitric oxide. The three-step scheme presented above is favored, with the first step probably being rate-controlling. 

Impurities are described in many texts on solvents [4, 5]. Many organic impurities can be identified by GC or GC/MS techniques for instance, MIBK can be found to contain mesityl oxide, the precursor to the manufacture of MIBK (Figure 4.6) [20]. Common impurities in solvents include H20, DMF [21], and AcOFl (in EtOAc and other solvents). Some solvents are inhibited with trace amounts of antioxidants, e.g., CH2C12 (stabilized with amylene or cyclohexane) andTFIF and 2-Me-THF (stabilized with BHT, butylated hydroxytoluene ). These impurities can influence the desired chemistry positively [22] and negatively, so and they should be considered. 

Table 1.11 H NMR data of the most common laboratory solvents as trace impurities in CDCI3 (in ppm)...

Particularly in the analysis of trace amounts of alkaloids, e.g., in studies of metabolism, such minor impurities in solvents may have great influence. 

Some typical industrial applications of HS-SPME are analysis of trace impurities in polymers and solid samples, the determination of solvent residues in raw materials, ppt odour analysis, organics in water, etc. SPME can also be used for quaUtative headspace sampling of fruit volatiles [1000]. Since equilibrium rather than exhaustive extraction occurs in the micro-extraction methods, SPME is suitable for field monitoring. 

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