Journal of Chromatography A, 773 (1997) 163–168 Determination of aspartame and phenylalanine in diet soft drinks by high-performance liquid chromatography with direct spectrofluorimetric detection * ´ ´ Kazimierz Wrobel , Katarzyna Wrobel ´ Instituto de Investigaciones Cientıficas, Universidad de Guanajuato, L. de Retana No. 5, 36000 Guanajuato, Mexico Received 10 December 1996; revised 21 February 1997; accepted 24 February 1997 Abstract Application of spectrofluorimetric detection is proposed in this work for the determination of aspartame and its main hydrolysis product phenylalanine in diet soft drinks by high-performance liquid chromatography. Separation was achieved on a LiChrosorb RP18 column with the mobile phase methanol–acetonitrile–phosphate buffer (2:17:81), pH 4.3. Native fluorescence of the two analytes ( l 5205 nm, l 5284 nm) was used for detection. The detection limits for aspartame and ex em 21 21 21 phenylalanine were 0.06 mg l and 0.01 mg l and calibration graphs were rectilinear in the range 0.5–40 mg l and 21 0.1–10 mg l , respectively. The proposed method was applied to determination of aspartame and phenylalanine in soft drinks (Diet Pepsi, Diet Coke and Diet Sprite) and the results obtained using two different detectors (spectrofluorimetric and spectrophotometric) were compared and discussed. Keywords: Soft drinks; Detection, LC; Food analysis; Aspartame; Phenylalanine; Artificial sweeteners 1. Introduction together with other food additives [9–11] and also with some of its decomposition products, as aspartic Over the last few decades, the demand for dietary acid, diketopiperinaze, and aspartylphenylalanine food has significantly increased. Aspartame ( N-L-a- [12–14]. Several authors have used reversed-phase aspartyl-L-phenylalanine methyl ester) is a low- high-performance liquid chromatography (HPLC) calorie sweetener commonly used in carbonated soft with spectrophotometric detection (205, 214, 215 drinks and beverages [1]. Several analytical methods nm) and with a mobile phase containing phosphate have been proposed for its determination in food buffer and acetonitrile and / or methanol [12–16]. products, titration with lithium methoxide being the However, quantitation of phenylalanine in soft drinks official method [2]. More recently, spectrophoto- has not been reported. It is well known that spectro- metric [3,4], enzymatic [5–7] and spectrofluorimetric fluorimetric detection allows not only better sen- [8] procedures have been reported. sitivity but also is more specific than spectrophoto- On the other hand, aspartame was determined metric detection. Hayakawa et al. [17] used pre- column derivatization of aspartame and its hydrolysis * products to form the fluorescent 1-cyanobenz- Corresponding author. Fax: (152) 473 26252; e-mail ,katarzyn quijote.ugto.mx.. [ f ]isoindole derivatives, which were separated by 0021-9673 / 97 / $17.00 Copyright  1997 Elsevier Science B.V. All rights reserved PII S0021-9673(97)00224-0