Determination of Magnesium Ascorbyl Phosphate

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Determination of Magnesium Ascorbyl Phosphate in Cosmetic Bleaching Products Using a Disposable Screen-Printed Carbon Electrode Ying Shih*a ( ) and Jyh-Myng Zen*b ( ) Department of Applied Cosmetology, Hung-Kuang Institute of Technology, Taichung 433, Taiwan, R.O.C. b Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan, R.O.C.

a

A preanodized screen-printed carbon electrode is used for the determination of magnesium ascorbyl phosphate in cosmetic bleaching products. The preanodization process exhibits a marked enhancement of the current response of magnesium ascorbyl phosphate at the screen-printed carbon electrode. The linear range is up to 240 M (correlation coefficient = 0.999) in pH 2.0 citrate buffer with a detection limit (S/N = 3) of 0.33 M. The electrode has the advantages of low cost and easy of handling and can be either disposable or reused since the renewal gives a good reproducible surface. Quantitative analysis was performed by the standard addition method for magnesium ascorbyl phosphate content in cosmetic products.

INTRODUCTION Mag ne sium ascorbyl phos phate (Mg-L-ascorbyl-2-phosphate, APM), as shown in Fig. 1, is a derivative of ascorbic acid (vitamin C) which has been recently used in cosmetic products and introduced com mercially as a skin whit ener and rad i cal scav enger. 1-3 It is known that vitamin C has important physiological effects on the skin, including the inhibition of melanic pigmentation, pro mo tion of colla gen for ma tion, and pre ven tion of free radical damage. 4-6 Un fortunately, since vitamin C can be very easily decomposed, a variety of stabilized vitamin C de riv a tives have there fore been de vel oped for the above-mentioned purpose. APM is a water-soluble, sta bilized vitamin C derivative, which can rapidly convert to vitamin C by the enzyme phosphatase abundantly present on and in the skin. 4,7 Up to now, there have been very few reports, using either spectroscopic or chromatographic methods, for the determination of APM. 4,7-9 In order to adapt dosing and to verify compliance, content monitoring is necessary. Besides, an official assay method for the analysis of APM in commercial bleaching products and skin lighteners still needs to be developed. We report here a simple elec trochemical method for the de ter mi na tion of APM us ing a preanodized screen-printed carbon electrode (SPCE). Electroanalytical tech nique in con junc tion with a dis pos able elec trode has many inherent advantages that address this kind of de tection. The merit of a screen-printed electrode is that it can be produced in large numbers at low cost. Additional advantages are rapid analytical time and the possibility of miniatur iza tion. Indeed, chem i cally mod i fied screen-printed elec trodes have been used for several determinations. 10-13

On the other hand, it is well known that the preanodization of a glassy carbon electrode has great influence on the electron transfer reaction.14-16 The preanodization followed by cathodization for a very short time introduces C=O functional group on the sur face, which can me di ate electrode transfer reactions at the interface.17-19 Recently, our group has reported an interesting observation that the selectivity and sen si tiv ity of uric acid de tec tion is dra mat i cally improved when the Nafion-coated glassy carbon electrode or bare glassy car bon elec trode is preanodized at +2.0 V vs Ag/AgCl in pH 7.0 phos phate buffer. 20 Using re flec tive mode FT-IR spectroscopy, we successfully showed that the en hance ment of sen si tiv ity on a preanodized bare glassy carbon electrode is from the direct interaction of uric acid through hy dro gen bond ing with the sur face func tional group C=O generated during preanodization. In this paper, the preanodization process is further applied to the SPCE for the detection of APM. Major factors that influence the electrode response, such as pH, preanodization potential, preanodization time, preconcentration time, and square-wave (SW) parameters were thoroughly evaluated. Practical analytical utility was demonstrated by selective

Fig. 1. Molecular structure of APM.

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measurement of APM in commercially available cosmetic bleaching products.

EXPERIMENTAL SECTION Mag ne sium ascorbyl phos phate was ob tained from Sigma (St. Louis, MO, USA). All the other com pounds (ACS-certified reagent grade) were used without further purification. Aqueous so lutions were prepared with dou bly distilled deionized water. A stock solution was prepared by dissolving 379.61 mg of APM in 100 mL water. An aliquot was diluted to the appropriate concentrations with pH 2.0 citrate-hydrochloric acid buffer before actual analysis. Elec tro chem is try was per formed on a Bioanalytical Sys tems (West Lafay ette, IN, USA) BAS-50W elec trochemical analyzer. A BAS VC-2 electrochemical cell was employed in these experiments. The three-electrode system consisted of either preanodized SPCE or a bare SPCE working elec trode, an Ag/AgCl ref er ence elec trode (Model RE-5, BAS) and a platinum wire auxiliary electrode. Since dis solved ox y gen did not in ter fere with the an odic voltammetry, no deaeration was performed. The SPCEs were fab ri cated us ing car bon inks (Acheson, Japan) and printed in a group of 16 (with a 1 mm gap be tween each) onto a poly propy lene (PP) base. The SPCEs were then cured under an UV irradiation at intensity of 1.85 mW/cm2 for 1 h. Each electrode consisted of an 8 2.5 mm working area, i.e. 20 mm2, with a 17 1.5 mm connecting strip. The SPCEs were equilibrated in the test solution con tain ing APM be fore mea sure ment. SW voltammograms were ob tained by scan ning the po ten tial from +0.7 to +1.2 V at a SW frequency of 20 Hz and SW amplitude of 60 mV. At a step height of 4 mV, the effective scan rate is 80 mV/s. The preanodization process was performed in pH 7.0 phos phate buffer as re ported previously. 20 The APM quantitation was achieved by measuring the peak current of the ox i da tion peak af ter back ground subtrac tion. The standard addition method was used to evaluate the content of APM in real sample.

Shih and Zen around +0.94 V was observed for 50 M APM. A clear increase in an odic peak was ob served when a preanodized SPCE (Fig. 2b) was used for de tec tion. Ap par ently, the preanodization process provides the SPCE an excellent accumulation force to the APM. The transport characteristic of APM at the preanodized SPCE was further investigated. The current response obtained in linear scan voltammetry at the preanodized SPCE was found linearly proportional to the scan rate, which indicated that the process was adsorption-controlled. More evidence for the adsorptive behavior of APM was dem on strated by the fact that the same voltammetric sig nal was observed when the preanodized SPCE was switched to a medium containing only supporting electrolytes after being used in measuring a APM solution. Analytical characterization Since the preanodization pro cess is es sen tial to the APM determination, the influence of the preanodization potential and the preanodization time were studied in detail. As can be seen in Fig. 3A, the peak current is constant up to +1.6 V and increases further very steeply until +2.0 V for a preanodization time of 60 s. It is because the irreversible ox i da tion of SPCE starts around +1.6 V, in tro duc ing the C=O functional groups proportionately on the surface. The

RESULTS AND DISCUSSION Voltammetric behavior Fig. 2 demonstrates the effect of preanodization on the SPCE in the determination of APM by SW voltammetry. On scan ning from +0.7 V to wards a pos i tive po ten tial at an un-preanodized SPCE (Fig. 2a), a very small anodic peak at

Fig. 2. The ef fect of preanodization on SPCE in the deter mi na tion of 50 M APM by SWV. (a) un-preanodized SPCE and (b) preanodization potential = +2.0 V, preanodization time = 60 s.

Determination of Magnesium Ascorbyl Phosphate increase in peak current can thus be explained by a substantial chem i cal inter ac tion be tween the APM and the C=O functional group present. The peak current reaches max imum around +2.0 V due to saturation of the C=O functional group on the sur face dur ing preanodization. 1 8 The preanodization time also has tremendous influence on the peak cur rent as shown in Fig. 3B. The peak cur rent increases as preanodization time increases and starts to level off at around 60 s. A preanodization potential of +2.0 V and a preanodization time of 60 s were therefore chosen for the subsequent work. Both the electrode and the detection aspects should be considered to arrive at the optimum conditions for APM determination. The effect of pH on the voltammetric response of the preanodized SPCE for 50 M APM was studied and the results are shown in Fig. 4. The current response was found to increase rapidly in a more acidic environment. A pH 2.0 citrate buffer was therefore chosen for all subsequent elec tro chem i cal mea sure ments. The ef fect of the preconcentration time at open circuit on the SW response for 100 M APM were studied next and the results obtained are shown in Fig. 5. As can be seen, the peak cur rent in-

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creases as the preconcentration time increases and starts to level off at around 10 s. It takes a longer time for the peak current to level off for a lower concentration of APM. This phenomenon is as expected and further confirms the ad sorption-controlled behavior of the preanodized SPCE. Therefore, in order to increase the sensitivity of detection, a longer time is needed for the lower concentration of APM. For convenience, a preconcentration time of 10 s was used in most of the subsequent work. The peak current obtained in SW voltammetry is dependent on various instrumental parameters such as SW amplitude, SW frequency, and step height. These parameters are interrelated and affect the response, but here only the general trends will be examined. It was found that these parameters had little effect on the peak potential. When the SW amplitude was varied in the range of 10 - 100 mV, the peak currents increased with increasing amplitude until 60 mV. However, when the amplitude was greater than 60 mV, the peak width increases at the same time. Hence, 60 mV was chosen as the SW amplitude. The step height together with the frequency defines the effective scan rate. An increase of either the frequency or the step height results in an

Fig. 3. Effect of (A) preanodization potential and (B) preanodization time on the peak current for 50 M APM ob tained at the SPCE.

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Shih and Zen

Table 1. Reproducibility of the Preanodized SPCE in the Determination of APM in Cosmetic Bleaching Solution and Standard Solutions Number of detection Bleaching solution* 5 M APM 20 M APM 100 M APM 15

10 10 10 2.33 1.54

Co efficient of variation (%) 2.16 2.48

* Dilution factor: 1/10000.

increase in the effective scan rate. The response for APM increases with SW frequency. However, above 20 Hz the peak current was unstable and obscured by a large residual current. By maintaining the frequency at 20 Hz, the effect of step height was studied. At a step height of 4 mV, the response is more accurately recorded. Overall, the optimized parameters are 20 Hz SW frequency, 60 mV SW amplitude, and 4 mV step height. The effective scan rate is 80 mV/s. Under optimal conditions, the SW voltammetric current response is linearly dependent on the concentration of APM up to 240 M in pH 2.0 ci trate buffer with slope ( A/ M), in ter cept ( A) and cor re la tion co ef fi cient of 0.0216, 0.0098 and 0.999, respectively. The detection limit (3 ) is 0.33 M. Although the main advantage of SPCE is low cost and can be disposable, the reproducibility of the

preanodized SPCE was also studied. The results obtained are summarized in Table 1. As can be seen, for 10 successive measurements a 1.54% coefficient of variation was observed in 5 M APM standard solution and 2.33% in cosmetic bleaching so lution for 15 measurements. Thus, the electrode renewal gives a good reproducibility surface. The SPCE can be easily renewed simply by repeatedly scanning in the working range in pH 2.0 citrate buffer solution after being used in measuring APM.

Fig. 4. Effect of pH on the peak current of 50 M APM ob tained at the preanodized SPCE with out preconcentration.

Fig. 5. The ef fect of preconcentration time at open cir cuit on the SWV response for 100 M APM at the preanodized SPCE.

Sample analysis The preanodized SPCE was applied to the mea sure-

Determination of Magnesium Ascorbyl Phosphate

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Table 2. Determination of APM in Cosmetic Bleaching Products with the Preanodized SPCE Original value ( M) Bleaching solution*

12.74 0.23 Bleaching cream**

11.75 0.04

10 20

Spike ( M)

Detected value after spike ( M)

30 40 50 10 20 30 40 50 21.84 0.41 32.42 1.02 41.19 0.49

51.91 61.58 23.09 31.92 41.81 50.85 64.36 100.90 103.35 98.13 100.40

0.93 0.31 0.35 0.34 0.67 0.30 0.46

Recovery (%) 99.67 103.50 95.90 96.90 95.28 103.24

* Di lution factor: 1/10000. ** Dilution factor: 2/10000.

ment of APM in commercially available cosmetic bleaching products. The accuracy of the method was determined by its recovery during spiked experiments. Two commercial cosmetic products, which contain APM, were spiked with APM standard solution at a concentration of 10, 20, 30, 40, and 50 M. Note that virtually the same recoveries were observed when the spik ing step was ap plied before or after sam ple preparation steps. As shown in Table 2, the recoveries of APM from the cosmetic matrices were satisfactory with values ranging from 95.28% to 103.50%.

CONCLUSION This study has dem on strated that the preanodized SPCE can be applied to the detection of APM in cosmetic products with excellent sensitivity and selectivity. The reliability and stability of the preanodized SPCE offers a good possibility for extending the technique in routine analysis of APM in cosmetic bleaching products. Moreover, it should be noted that the preanodized SPCE is very easy to con struct, and hence mass production is feasible.

ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Science Council of the Republic of China un der Grants NSC 88-2113-M-241-002 and NSC 88-2113-M-005-020.

Received March 25, 1999.

Key Words Magnesium ascorbyl phosphate; Screen-printed electrode; Cosmetic bleaching products.

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12. Jagner, A.; Bilitewski, U. Analyst 1994, 119, 1251. 13. Carsol, M. A.; Volpe, G.; Mascini, M. Talanta 1997, 44, 2151. 14. Zen, J.-M.; Chen, P.-J. Anal. Chem. 1997, 69, 5087. 15. Engstrom, R. C. Anal. Chem. 1982, 54, 2310. 16. Wang, J.; Pedrero, M.; Sakslund, H.; Hammerich, O.; Pingarron, J. Analyst 1996, 121, 345.

Shih and Zen 17. Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 56, 136. 18. Kamau, G. N.; Willis, W. S.; Rusling, J. F. Anal. Chem. 1985, 57, 545. 19. Randlin, J. P. in Encyclopedia of Electrochemistry of Ele ments, Bard, A. J. Ed., Mar cel Dekker, New York, 1976, Vol. 17, Ch. 1. 20. Zen, J.-M.; Jou, J.-J.; Ilangovan, G. Analyst 1998, 123, 1345.

Determination of Magnesium Ascorbyl Phosphate Fig. 1.

Molecular structure of APM. Fig. 2.

The effect of preanodization on SPCE in the determination of 50 M APM by SWV. (a) un-preanodized SPCE and (b) preanodization potential = +2.0 V, preanodization time = 60 s. Fig. 3.

Ef fect of (A) preanodization po ten tial and (B)

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preanodization time on the peak current for 50 M APM obtained at the SPCE. Fig. 4.

Effect of pH on the peak current of 50 M APM obtained at the preanodized SPCE without preconcentration. Fig. 5.

The effect of preconcentration time at open circuit on the SWV re sponse for 100 M APM at the preanodized SPCE.