Aliphatic amines and amino acids photo-oxidation

Aliphatic amines and amino acids photo-oxidation 

Aliphatic amines, such as diethylamine, and amino acids, such as glycine and glutamic acid, do not show any specific absorption (Fig. 70). In this case, a photo-oxidation (using a low-pressure mercury lamp and a potassium peroxodisulphate solution as oxidant) could lead to nitrate formation (Fig. 71), easily determined by UV spectrophotometry (see Chapter 4). 

In contrast with the previous case, this procedure is not specific and only show that the photo-oxidised substance contains nitrogen. 

Carbohydrates do not absorb in the UV-visible range, but a photo-degradation step is able to reveal their presence in an aqueous solution [20], Contrary to the previous case where the association of UV irradiation with a chemical oxidant (peroxodisulphate) has demonstrated its efficiency, this procedure is not adapted for carbohydrate determination. The oxidation rate of this method is too fast, and absorbing intermediates (carbonyl compounds) are only weakly observed, because they are quickly oxidised into carboxylic acids. Thus, milder oxidation conditions (such as a simple UV lamp) should be used in order to provide an appropriate oxidation reaction that allows the formation of an intermediate compound with a stable absorption peak (Fig. 72). 

The pH value affects the UV photodegradation efficiency. From the same concentration of initial sugar solution, the absorbance obtained is greater for alkaline pH than for neutral or acidic pH. Thus, an isosbestic point is found in the pH series of the absorption spectra (Fig. 73). Alkaline conditions are needed for solutions containing low sugar concentrations. However, for solutions with high sugar concentrations, it is still better to use alkaline conditions because this reduces the analysis time. 

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