Liquid chromatographic and spectrofluorimetric determination of aspartame and glutamate in foodstuffs following fluorescamine fluorigenic labelling

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1 Analytica Chimica Acta, 270 (1992) Elsevier Science Publishers B.V., Amsterdam Liquid chromatographic and spectrofluorimetric determination of aspartame and glutamate in foodstuffs following fluorescamine fluorigenic labelling F. Garcla Sfinchez and A. Aguilar Gallardo Department of Analytical Chemistry, Faculty of Sciences, University of Mdlaga, Mdlaga (Spain) (Received 13th April 1992) Abstract A spectrofluorimetric method for determining aspartame (AS) and sodium glutamate (GL) based on labelling with fluorescamine (FC) was developed. Mixtures of AS and GL were analysed by isodifferential synchronous derivative spectrofluorimetry. R.S.D.s of 6.4% and 3.0% were obtained by the individual methods. Detection limits of 0.03 and 0.1 p.g ml I were obtained for AS and GL, respectively. On the basis of the FC derivatization procedure, a liquid chromatographic method to determine AS and GL was developed. R.S.D.s of 0.62 and 0.34% were obtained with fluorimetric detection and 2.26 and 3.14% with photometric detection at 395 nm. Detection limits were 0.04 and 0.02 /xg ml-x (spectrofluorimetric detection) and 0.2 and 0.1 /xg ml 1 (spectrophotometric detection) for AS and GL, respectively. These methods were applied to the determination of AS in carbonated soft drinks and GL analyses in commercial dried soups. Keywords: Fluorimetry; Liquid chromatography; Aspartame; Foods; Glutamate Aspartame, a dipeptide (N-L-a-aspartyl-Lphenylalanine 1-methyl ester), has a sweetening power 180 times that of sucrose [1]. It is often used as substitute for saccharin in foods and as a sweetener in carbonated drinks. It is synthesized from L-aspartic acid and L-phenylalanine [2]. Although aspartame is not toxic, high concentrations may cause illness and so the aspartame content in foods is controlled by the health authorities in many countries. Methods for the determination of aspartame in foods are generally based on gas chromatography [3] and liquid chromatography (LC) [4-7]. L-Glutamic acid enhances flavours and the Correspondence to: F. Garcla SAnchez, Department of Analytical Chemistry, Faculty of Sciences, University of MAlaga, MAlaga (Spain). sodium salt is widely used as a food additive. Although about metric tons of glutamate are consumed annually, high doses may be toxic and the World Health Organization sets a limit of 120 mg kg -1 body weight as the maximum daily dose [8]. The Association of Official Analytical Chemists recommends potentiometric titration of the previously ion-exchanged monosodium glutamate [9]. Spectrophotometric [10], spectrofluorimetric [11] and chromatographic methods [12-14] have also been described. In this work, the optimum experimental conditions were investigated for the spectrofluorimetric determination of aspartame and glutamate based on fluorophore generation by derivatization with fluorescamine (FC). Reversed-phase LC determination of AS and GL with precolumn derivatization was also carried out and applied to real samples /92/$ Elsevier Science Publishers B.V. All rights reserved

2 46 F. Garc[a Sdnchez and A. Aguilar Gallardo /Anal. Chim. Acta 270 (1992) EXPERIMENTAL Apparatus The spectra (range nm) were obtained at room temperature with a Perkin-Elmer LS-50 luminescence spectrometer, fitted with a xenon lamp (9.9 W) pulsed at line frequency, 1 x 1 cm fused-silica cells and a R928 Hamamatsu photomultiplier. Slit widths were set at 5/5 nm. The emission spectra were not corrected for non-linear instrumental response. The LC equipment included a Merck-Hitachi L-6200 pump. Injections were made with a Merck-Hitachi AS-4000 autosampler. A Merck- Hitachi L-4250 UV-visible detector was also used. The analogue signals were converted into digital signals by a Merck-Hitachi D-6000 interface. Integration was made with a PC/AT computer and the instrumental parameters were controlled by Merck-Hitachi HM software. A Spherisorb $5 ODS-2 reversed-phase column (200 x 4.6 mm i.d. 5 /zm particle size) and a Spherisorb $5 ODS-2 precolumn (60 x 4.6 mm i.d.; 5/zm particle size) was used; the mobile phase was buffer (ph 9)- acetonitrile-methanol at a flow-rate of 0.5 ml min - 1 Sequential spectra of eluted peaks were recovered with a Model ABI-1000S diode-array detector (Applied Biosystems). The data obtained with this system were processed and analysed by means of Labcal software (Galactic, Salem, NH). The LS-50 spectrofluorimeter was used as a detector on-line with the UV-visible or diodearray spectrophotometer. Reagents Aqueous stock standard solutions of the L- aspartyl-l-phenylalanine methyl ester (Aldrich) (5.91 x 10-3 M) and monosodium L-glutamate (Aldrich) (3.59 X 10-3 M) were prepared weekly. Working standard solutions were prepared from these by appropriate dilution. Analytical-reagent-grade fluorescamine {4- phenylspiro[furan-2(3h),l'-phthalan]-3,3'-dione} was obtained from Aldrich and was dissolved in acetone (1 mg ml-1). Phosphate buffer solution (0.2 M) and sodium borate-hc1 buffer solution (0.2 M) were pre- pared from potassium and sodium salts (Merck), respectively. All solvents were of analytical-reagent grade (Merck) and demineralized water was used throughout. LC mobile phases were of Lichrosolv gradient grade (Merck). Spectrofluorimetric method In 10-ml volumetric flasks were placed the appropriate volumes containing mixtures of AS and GL with concentrations between 0.1 and 1 /zg m1-1, 1 ml of FC (3.4 x 10-3 M) and 2 ml of buffer solution (ph 9). The solution was then diluted with water. The synchronous first-derivative spectra were recorded at AA = 90 nm for AS and AA = 85 nm for GL. The relative fluorescence intensity (RFI) and the derivative values (expressed in cm) were converted into units of concentration by applying the corresponding regression equations or calibration graphs. LC method Aliquots of /zl of aqueous standard solutions of AS and GL (10/xg ml-1) were introduced in a 1.5-ml flask and 150/~1 of a 3.4 x 10-3 M acetone solution of FC and 300 /zl of ph 9 buffer solution were added. The mixture was diluted to 1.5 ml and shaken for 10 s. Volumes of 10/zl of this solution were injected into the chromatograph and eluted with ph 9 buffer-acetonitrile-methanol ( , v/ v/v) at a flow-rate of 0.5 ml min -~. Two detectors on-line (UV-visible and spectrofluorimetric) set at 395 and 395/482 nm, respectively, gave the peaks corresponding to GL and AS at t R = 3.12 and 4.14 min, respectively, for UV-visible detection and at 3.5 and 4.5 min, respectively, for fluorimetric detection. RESULTS AND DISCUSSION Because of their amine character, AS and GL react with FC to form two fluorophores whose spectra are very similar. Figure 1 shows the excitation and emission spectra of the FC derivatives of AS and GL under the final experimental con-

3 F. Garc(a Sdnchez and A. Aguilar Gallardo /Anal. Chim. Acta 270 (1992) ditions. As expected, the spectral parameters for both compounds are similar. Each compound was characterized by its well resolved excitation maximum and its single emission peak, at 393/483 and 397/482 nm for AS and GL, respectively. The operating parameters for the individual compounds can be optimized to give one analytical method for each. However, as this work shows, the spectra of both compounds overlap considerably and this precludes the use of normal spectrofluorimetry to determine the individual compounds in mixtures. Consequently, after fixing the individual optimum conditions for determining AS and GL, a new set of conditions was selected to obtain good emission signals for each compound before carrying out the analysis of mixtures of AS and GL. As a fluorigenic reagent for amino compounds, FC lacks selectivity, which emphasizes the need for more detailed information about the effect of the main reaction conditions so that the fluorescence yield might be improved to permit the 200 R.F.I l ph Fig. 2. Influence of ph on the relative fluorescence intensity of (1) AS and (2) GL. [AS]=[GL] = 5 /zg ml-1; [FC]= M. 120 R.F.I O Wavelength (nm) Fig. 1. Excitation and emission spectra of FC derivatives of (1, 2) AS and (3,4) GL and the respective blank solutions (1'-4'). [AS]= 2.5 p~g ml -~, ph 7.5; [GL]= 2.5/xg ml -~, ph 9.0. selective analysis of mixtures of fluorophores ~ith FC. Influence of reaction variables The effect of ph on fuorescence intensity was explored by carrying out several assays of solutions that contained 5/.Lg ml-1 AS or GL and 2 ml of different buffer solutions that covered the ph range , together with 1 ml of FC standard solution. The results in Fig. 2 show that the maximum fluorescence for the AS fluorophore occurred at ph 7.5 and that of GL at ph 9. In both instances, the narrow range in which the fluorescence intensity was maximum suggests that careful control of the solution ph is required. On the other hand, to obtain good yields in the labelling reactions of mixtures of both compounds, the ph setting must be a compromise and in this work ph 9 appeared to be the optimum. FC reacts very quickly with primary amines (tl/2 = ms), but frequently a great excess of FC is needed to produce good thermodynamic

4 48 F. Garc{a Sdnchez and A. Aguilar Gallardo ~Anal. Chim. Acta 270 (1992) F, [ " I i I I I I FC/Analyte molar ratio Fig. 3. Influence of the FC/analyte molar ratio on the relative fluorescence intensity of (1) AS (ph 7.5) and (2) GL (ph 9.0). equilibrium conditions, as described previously []5]. The effect of FC concentration on fluorophore formation was observed by measuring the fluorescence intensity for each compound at different FC/analyte molar ratios, while all other experimental conditions were kept constant at the optimum values. Figure 3 shows that maximum response was obtained when the FC/analyte molar ratio was within the range 12 : 1-20 : 1; in subsequent work a ratio of 15 : 1 was employed for the individual determinations of AS and GL. For the simultaneous determination of the two compounds a molar ratio of 25 : 1 was selected. Influence of instrumental parameters Spectrofluorimetric method. The synchronous scanning first-derivative fluorimetric approach takes advantage of the band-narrowing effect of synchronous scanning to maximize the first-derivative amplitudes. In this way, better sensitivity may be obtained, especially if the normal spectra show broad profiles and, on the other hand, improved selectivity in multi-component analyses was obtained when the graphical method of plotting isodifferential amplitude measurements was used. The wavelength scanning interval (Aem - Aex = AA), a critical parameter for spectral shape, was optimized by using the sequential recovery of synchronous spectral data with newly modified software. The best results were obtained at AA = 90 and AA = 85 nm for AS and GL, respectively. As can be seen in Fig. 1, the excitation/emission spectra of AS and GL overlap. However, as shown in Fig. 4, the first derivatives of the synchronous spectra corresponding to the same component at different concentrations have values of zero on the ordinate scale (differential fluorescence). This so-called isodifferential point indicates that the contribution of this component to the overall derivative signal is zero, as discussed earlier, and consequently the amplitudes of the derivative spectra from this wavelength to the break with the experimental derivative curve of the mixture are independent of this component. Figure 4 also shows that the isodifferential points corresponding to the AS and GL series are located at 393 and 397 nm, respectively. LC Method. Optimization of LC parameters was performed by seeking the separation of the peaks corresponding to GL and AS derivatives, allowing separate peak integration. Figure 5 shows the contour plot of 500 spectra collected during dl/dx o t /5~" \,o t - ot -40 J ~ / / -60 x "-. /- / / -80 I,,, Wavelenglh (nrn) Fig. 4. Synchronous first-derivative spectra of (solid lines) AS and (dashed lines) GL at several different concentrations. [AS]=[GL]= 0.1, 0.3, 0.5, 0.8 and 1.0 ~g ml i.

5 F. Garc[a Sdnchez and A. Aguilar Gallardo lanai. Chim. Acta 270 (1992) chromatographic elution using a time interval in the events programme of the diode-array detector of 0.03 spectra per minute. Three peaks are detected at t R = 3.20, 4.00 and 4.25 min, arising from GL, hydrolysed FC and AS, respectively, with the UV-visible detector set at 395 nm. Peak purity analysis [16] shows that both 3.20 and 4.25 min are retention times at.01 r,.) e~ s_. o M 380 4~ Nanometers i~- ~ m oq ~o 4~,o '.~o o NanomeLcrs Fig. 5. Three-dimensional and contour plot of the chromatographic elution of 1/xg ml- ~ AS and 1 p.g ml- ~ GL after derivatization with FC.

6 50 F. Garc{a Sdnchez and A. Aguilar Gallardo /Anal Chim. Acta 270 (1992) which eluted AS and GL virtually pure (purity = for both). In comparison with these values, near to unity, the purity index of the peak at t R = 4.00 min is It should be pointed out that because the detectors are connected in series, some delay in t R from one to other detector signal is produced. Spectrofluorimetric detection at Aex c = 395 and,hem = 482 nm shows only two peaks at 3.5 and 4.5 min corresponding to GL and AS derivatives of FC, respectively. Quantitative analysis Spectrofluorimetric method. Calibration graphs were constructed by analysing a series of samples of known AS and GL concentrations. Studies with solutions of AS and GL alone established that the concentrations of AS and GL correlated well with the synchronous derivative signals. The detector response was linear over the concentration range /xg m1-1. The equations obtained by least-squares treatment were DS 1= 58.3[AS] and r=0.990(n=5) DS ~ = 49.7[GL] r = (n = 5) for AS and GL, respectively, where DS 1 is the first-derivative signal, in arbitrary units. To evaluate the reproducibility of the individual methods, a series of six solutions with concentrations of 0.5 /zg ml-i were prepared. The results are given in Table 1. LC method. The detector reponse was linear over the concentration range /zg m1-1 for spectrofluorimetric and 1-4/xg ml- ~ spectropho- tometric detection. The equations obtained by least-squares treatment were as follows: spectrofluorimetric detection: A = 150.2[GL] r = (n = 5) A=121.9[AS] spectrophotometric detection: A=2625[GL]-1656 r=0.999(n=5) r=0.991(n=4) A = 1391[AS] r = (n = 4) where A is area under the peak, in arbitrary units, and concentrations are in/zg ml-'. The standard deviations of the LC methods were calculated from three determinations at each of six different concentrations covering the calibration range. The six standard deviations obtained were averaged; this value is taken as the standard deviation of the method, and is used to calculate limiting values. Table 1 also gives other analytical figures of merit of the methods. The detection limit, CL(k = 3), and determination limit, C O (k = 10), are reported as recommended by IUPAC [17]. Comparison of the methods The spectrofluorimetric methods for the determination of AS and GL were compared with the corresponding methods using LC (y) and spectrofluorimetric (x) detection, by regression analysis [18]. The values of slope (a), intercept (b) and correlation coefficient (r) (n = 8) for GL are a = , b = and r = and for AS are a = , b = and r = , showing excellent agreement and the absence of any systematic error. TABLE 1 Analytical parameters of the proposed methods Method C e (p~g ml-1) a CQ (/Lg ml-l) a t R (min) a Error (%) b R.S.D. (%) c AS GL AS GL AS GL AS GL AS GL Synchronous first-derivative spectrofluorimetry LC- spectrofluorimetric detection LC- spectrophotometric detection a For definitions, see text. b Error = 100t S.D./,7. c For six determinations of 0.5 ~g m1-1.

7 F. Garda Sdnchez and A. Aguilar Gallardo /Anal Chim. Acta 270 (1992) TABLE 2 Spectrofluorimetric interference study at 0.5/zg ml-i AS and at 0.5/xg ml 1 GL Interferent AS : interferent Recovery (%) GL: interferent Recovery (%) wt. ratio (first deriv.) wt. ratio (first-deriv.) Dulcin 1 : : : Sodium cyclamate 1 : : : Saccharin 1 : : : Ascorbic acid 1 : : : Methyl paraben 1 : : : : Benzoic acid 1 : : : : Nicotinamide 1 : : : : TABLE 3 Spectrofluorimetric analysis of binary mixtures of AS and GL AS : GL wt. ratio AS GL Taken Found Error (%) Taken Found Error (%) (/xg ml -I) (~gm1-1) (Izg ml 1) (/.Lg ml-1) 1 : : : : : : Interference study The effects of other food additives on the determination of 0.5 tzg ml -~ AS and 0.5/zg ml -~ GL by the first-derivative synchronous method were studied. The results, summarized in T~ble 2, show that good recoveries of AS and GL were obtained at interferent to analyte ratios below 1:20. In general, the errors were negative and only methyl paraben and benzoic acid in the determination of AS and sodium cyclamate and dulcin in the determination of GL interfered positively, at interferent to analyte ratios higher than 10:1. Because the amounts of interferent assayed were fairly high, the interferences are probably spectral (inner filter effect). The results obtained in the determination of each food additive in binary mixtures for differ- TABLE 4 Spectrofluorimetric determination of AS in carbonated soft drinks Sample No. Synchronous first derivative Added Found Mean recovery (~g ml-i) (/xg ml-l) a (%)a _ _ _ _ _ _ ± ± ± ± _ _ _ ± _3 a Mean+S.D. (n=3).

8 F. Garda Sdnchez and A. Aguilar Gallardo /Anal Chim. Acta 270 (1992) TABLE 5 LC analysis of spiked binary mixtures of GL and AS in dried soups GL Added Mean recovery ~ R.S.D. ~ Added Mean recovery a R.S.D. a (/~g ml- 1) (%) (%) (/zg ml- l) (%) (%) AS a n=3. TABLE 6 LC analysis of spiked binary mixtures of GL and AS in carbonated soft drinks GL Added Mean recovery a R.S.D. a Added Mean recovery a (p.g ml- i) (%) (%) (~g ml- 1) (%) AS R.S.D. a (%) an=3. TABLE 7 Determination of monosodium glutamate in dried soup Method Volume (/zl) a Found (/xg ml- x) b Glutamate (#g ml 1 soup) c Glutamate (g kg 1 soup) d Spectrofluorimetric LC _ Aliquot of final solution taken for analysis (see text), b Concentration of GL in final solution (mean + S.D., n = 3). c Concentration of GL in original soup solution (mean _+ S.D., n = 3). d Concentration of GL in original dried soup (mean _+ S.D., n = 3). ent AS/GL ratios are given in Table 3. It can be seen that the reported method gave satisfactory results for the binary mixtures tested. Determination of AS in carbonated beverages and of GL in dried soups The usefulness of the methods developed in this work was evaluated by applying the methods to the determination of AS in carbonated drinks and to that of GL samples of commercial powdered soups. Sample preparation of carbonated drinks was simple; they were degassed by immersion in an ultrasonic bath before analysis. The samples of commercial dehydrated and homogenized soup were mixed with demineralized water (3 g in 500 ml) and agitated vigorously. The solutions were filtered through Whatman No. 1 filter-paper, then 25 ml were dialysed for 24 h at room temperature against 1000 ml of deionized water. The dialysed solution was concentrated to 25 ml in a rotary vacuum evaporator. Aliquots of the remaining solution were taken for analysis. Table 4 gives the results obtained by applying the spectrofluorimetric procedure to the determination of AS in carbonated soft drinks, and Tables 5 and 6 those for the LC determination of

9 F. Garc{a Sdnchez and A. Aguilar Gallardo /Anal. Chim. Acta 270 (1992) GL and AS in dried soups and soft drinks by LC. The results obtained demonstrate the effectiveness of the proposed methods in determining the analytes assayed in these type of samples. Table 7 presents the results obtained in the determination of endogenous GL in dried soups by applying the spectrofluorimetric and LC methods. The results show that the spectrofluorimetric procedure reports a higher concentration than the LC method, probably because of the more pronounced effect of the concomitants in the matrix on the overall fluorimetric signal. However, both procedures give comparable results. The authors thank the Comisi6n Interministerial de Ciencia y Tecnologla (Project PB ) for supporting this study. REFERENCES 1 M.R. Cloninger and R.E. Baldwin, J. Food Sci., 39 (1974) G.E. Inglett, Food Technol., 35(3) (1981) I. Furda, P.D. Malizia, M.G. Kolor and P.J. Vernieri, J. Agric. Food Chem., 32 (1975) B.B. Woodward, G.P. Heffelfinger and D.I. Ruggles, J. Assoc. Off. Anal. Chem., 62 (1979) T.A. Tyler, J. Assoc. Off. Anal. Chem., 67 (1984) N.G. Webb and D.D. Beckman, J. Assoc. Anal. Chem., 67 (1984) M. Nishigima, M. Kanmuri, Y. Waturi and Y. Kimura, Shokuhin Eiseigaku, 17 (1976) Toxicological Evaluation of Certain Food Additives and Contaminants, 29th Meeting of the Joint FAO/WHO Expert Committee Food Additives, World Health Organization, Geneva, E. Fernandez-Flores, A.R. Johnson and V.H. Blomquist, J. Assoc. Off. Anal. Chem., 52 (1969) V.D. Yanchuk and V.V. Petrenko, Farmatsiya, 37(1)(1988) E.D. Coppola, S.N. Christie and J.G. Hanna, J. Assoc. Off. Anal. Chem., 58 (1975) P. Sporns, J. Assoc. Off. Anal. Chem., 65 (1982) A.T. Rhys Williams and S.A. Winfield, Analyst 107 (1982) H.J. Keller, K.Q. Do, M. Zollinger, K.M. Winterhalter and M. Cuenod, Anal. Biochem., 166 (1987) J.A.F. de Silva and N. Stronjny, Anal. Chem., 47 (1975) G. Webster, Int. Lab. News, December (1991). 17 IUPAC, Spectrochim. Acta, Part B, 33 (1978) J.C. Miller and J.N. Miller, Statistics for Analytical Chemistry, Horwood, Chichester, 2nd edn., 1988.