Effect of Temperature on Analyte Ionization

The s pT a of five weak electrolytes of different chemical nature (butylamine, A,A-dimethylaniline, phenol, and benzoic acid) in 50% methanol/water at 20-50°C were determined by Castells et al. [108], and the values are shown in Table 4-15. The effect of temperature was the greatest for the basic compound butylamine, and a lesser effect was observed for the weaker bases pyridine and A,/V-dimethylaniline and the weakly acidic phenol. 

Therefore the temperature of the separation should also be taken into consideration when performing method development, especially for basic compounds. Basic compounds that have pA values 6 usually experience the greatest changes in retention with increase in temperature. The pA values of these basic compounds decrease with an increase in temperature, thereby making them more neutral when analyzed at higher temperatures. 

Mobile phase pH 7.8, acetonitrile-phosphate buffer (40 60, v/v), with ionic strength (I) maintained at 0.05 M. Buffer concentration adjusted to maintain I constant at different temperatures. (Reprinted from reference 111, with permission.) 

TABLE 4-15. Values for Acidic and Basic Compounds as a Function of Temperature 

50v/v% Methanol-Water Temperatnre (K) Phenol IpK, Butylamine Pyridine IpK, A,A-Dimethylaniline IpK, 

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