Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 432–435 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy j ourna l homepage: www.e lsev ie r .com/ locate /saa A paper platform for colorimetric determination of aluminum hydrochloride in antiperspirant samples Amanda Letícia Polli Silvestre, Maria Izabel Milani, Eduardo Luiz Rossini, Leonardo Pezza, Helena Redigolo Pezza ⁎ Instituto de Química, Universidade Estadual Paulista “Julio de Mesquita Filho”, UNESP, R. Prof. Francisco Degni 55, P.O. Box 355, 14800-900 Araraquara, SP, Brazil ⁎ Corresponding author. E-mail address: hr.pezza@unesp.br (H.R. Pezza). https://doi.org/10.1016/j.saa.2018.06.049 1386-1425/© 2018 Elsevier B.V. All rights reserved. a b s t r a c t a r t i c l e i n f o Article history: Received 28 March 2018 Received in revised form 11 June 2018 Accepted 13 June 2018 Available online 15 June 2018 A simple, fast, low-cost, portable, and eco-friendly method using a spot test on a paper platform, together with diffuse reflectance spectroscopy, was developed and validated for the quantification of aluminum hydrochloride, a potential neurotoxic agent, in antiperspirant samples. The determination of aluminum hydrochloride was per- formed at a wavelength of 615 nm, bymeasuring consumption of the purple colorimetric reagent Alizarin S, due to reaction with aluminum. The linear rangewas from 10.0 to 125.0 mg L−1 and could be described by the equa- tion: AR= 0.4479− 0.002543 CAl (R= 0.999). The limits of detection (LOD) and quantification (LOQ)were 3.06 and 10.2 mg L−1, respectively. The method was specific, accurate, and repeatable, with relative standard devia- tion (RSD) b5.0%. The recoverywas between 92.2 and 103.4%. Themethodwas successfully used for the determi- nation of aluminum hydrochloride in commercial antiperspirant samples, revealing concentrations below the maximum permitted by current legislation. © 2018 Elsevier B.V. All rights reserved. Keywords: Diffuse reflectance Aluminum Paper platform Antiperspirant 1. Introduction Personal hygiene has become a part of the daily routine worldwide, reflecting the concern of individuals about their welfare. Personal hy- giene products such as antiperspirants and deodorants are considered indispensable, because they are no longer merely a way to maintain a sensation of cleanliness, but also contain components that treat and beautify the skin, such as vitamin E [1, 2]. Perspiration is a fundamental physiological process whose function is to control and maintain the body temperature at around 37 °C [3]. There are three different ways to reduce or control underarm odor: minimization or elimination of the secretions from both types of sweat glands; prevention of bacterial growth; and absorption of body odors. For these purposes, two different product categories are avail- able, namely deodorants and antiperspirants [2]. Deodorants only con- trol odor, while antiperspirants are formulated to reduce local perspiration and usually also contain components to combat bad odor. Different to deodorants, antiperspirants reduce the amount of sweat produced locally by the human body and are therefore considered drugs, since they alter the normal physiology of the body [1]. Aluminum and its derivates have a broad range of applications in many different fields, including in the cosmetics and pharmaceutical in- dustries [4]. Many of the antiperspirant products currently used contain aluminum compounds as the active agents [5–7]. Aluminum hydrochloride (AHC) is themain active compound in an- tiperspirants. Its mechanism of action consists of the precipitation of AHC in the interior of the sweat glands, producing insoluble aluminum hydroxide that binds in the gland and blocks sweat secretion [1, 8]. Some antiperspirants, especially metal salt solutions, also exhibit bacte- ricidal action and deodorant effects [7]. The skin is the main route for entry of aluminum contained in antiperspirant products, and this trans- dermal absorption can lead to aluminum overload, which may result in anemia, bone disease, or even dementia, notably in individuals with poor renal function [9]. The metal is potentially neurotoxic [10], so its accumulation can cause diseases such as Parkinson's, Alzheimer's, en- cephalopathy, and osteomalacia [11, 12]. Furthermore, the daily use of antiperspirants can irritate the skin, causing rashes and burning sensa- tions, while more severe skin irritation can lead to desquamation or even necrosis [13]. In order to regulate the sale and use of this type of product, it is fun- damental to ensure adequate supervision by governmental agencies. In Brazil, the entity responsible is the National Health Surveillance Agency (ANVISA), which stipulates a maximum of 25% of aluminum salts in the composition of antiperspirants and deodorants [14]. In Brazil, these products are classified as risk grade 2, with potential danger to health [15–17]. There are several methods that can be used to determine aluminum in antiperspirant samples, including pre-column derivatization and HPLC-UV/Vis detection [18], flow injection analysis with spectrophoto- metric detection [19], and atomic absorption spectroscopy [20], among others. Most of these methods require the use of organic solvents or http://crossmark.crossref.org/dialog/?doi=10.1016/j.saa.2018.06.049&domain=pdf https://doi.org/10.1016/j.saa.2018.06.049 hr.pezza@unesp.br https://doi.org/10.1016/j.saa.2018.06.049 http://www.sciencedirect.com/science/journal/13861425 www.elsevier.com/locate/saa Fig. 1. Diffuse reflectance spectra of Alizarin S in alkaline medium (blue line) and the Al- Alizarin S complex (red line). Examples of spot tests for the blank and an aluminum solution. 433A.L.P. Silvestre et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 432–435 derivatization steps, whichmay cause harm to the operator and the en- vironment. Additional considerations are that the analyses can be very expensive and require specialized operators. An alternative to these methods is to use a spot test together with diffuse reflectance spectroscopy, which is a simpler, faster, and less ex- pensiveway to determine an analyte. In this type of analysis, aliquots of the reagent and sample are placed on a solid support, with quantifica- tion by measurement of a change in the analyte, such as a color alter- ation or even gas evolution [21]. Paper provides a good solid platform for diffuse reflectance measurements, due its high contrast and bright white background [22]. Paper platforms can be obtained from renew- able sources and require the use of minimal volumes of reagent and sample, generating a negligible waste that is easily incinerated [23]. In order to improve the analytical response and color uniformity, the paper platforms used for spot tests can be impregnatedwithwax, creat- ing hydrophobic barriers so that the solutions applied are restricted to thedelimited area [24]. Several studies have described low cost andpor- table methodologies that use hydrophobic barriers in paper [25–27]. The present work describes a method for the determination of alumi- num in antiperspirant samples, using a colorimetric reaction in a spot test delimited with hydrophobic barriers in a paper platform. The reddish re- action product wasmeasured using diffuse reflectance spectroscopy. This method is fast, cheap, eco-friendly, and very simple to use. 2. Materials and Methods 2.1. Materials and Solutions All reagents used were analytical grade. Aluminum nitrate nonahydrate and Alizarin S were purchased from Vetec. Sodium hy- droxide was purchased from Sigma-Aldrich. Solutions and dilutions Fig. 2. Reaction between Alizarin S an employed deionized water (18.2 MΩ cm) obtained from a Milli-Q sys- tem (Millipore). A 0.1% (m/m) solution of Alizarin S in deionized water was employed for the aluminum determination. A stock solution of 1000mg L−1 of aluminumwas prepared in deionizedwater.Working solutions of aluminumwere freshly prepared by appropriate dilution of the stock solution in 1.00 mol L−1 NaOH. 2.2. Sample Preparation Seven different samples of antiperspirant were purchased locally in the city of Araraquara (São Paulo State, Brazil). A mass of 0.125 g of each sample was transferred to a beaker, followed by addition of 25 mL of 2.00 mol L−1 NaOH solution. The mixture was heated at ebul- lition for 5 min, under constant stirring. After cooling, the content was filtered using a quantitative paper. The beaker and the filter were washedwith 50mL of 1.00mol L−1 NaOH solution, collecting thewash- ings in a 100 mL volumetric flask that was completed to the meniscus with deionized water. 2.3. Preparation of the Paper Platform Hydrophobic wax barriers (15 mm diameter, 0.75 mm thickness) were designed as described byMilani and co-workers [28], using graph- ical software (CorelDRAW X5). The printing was performed onto Whatman No. 1 filter paper, using a wax printer (Xerox Phaser 8560) andwax toner (Genuine Xerox Solid Ink Black), as described by Carrilho and co-workers [24]. Thepaperwasheated for 120 s at 120 °C for forma- tion of the hydrophobic barriers. 3. Methodology In the spot test, a 15 μL aliquot of aluminum working solution or sample was applied to the center of the delimited area. After drying at room temperature, 15 μL of Alizarin S solution was added, followed by further drying for about 10 min. Diffuse reflectance measurements were performed using a portable spectrophotometer (USB2000, Ocean Optics) operated with OOIBase32 software (Ocean Optics). 3.1. Study of Matrix Interferences Matrix interferences were evaluated using recovery tests. The sam- ple matrix was fortified with standard solutions at levels from 50% to 200%, followed by diffuse reflectance spectroscopy measurements. 4. Results and Discussion 4.1. Preliminary Tests TheAlizarin S chromogenic reagentwas selected for the colorimetric determination of aluminum in antiperspirant. In an alkaline medium, the reagent has a purple color, while reaction with Al3+ (Fig. 1) results in the formation of a red complex (Fig. 2) [29–31]. A strongly alkaline d aluminum in alkaline medium. Table 1 Figures of merit of the proposed method. Parameter Value Linear range 10.0–125.0 mg L−1 Wavelength 615 nm Calibration curve AR = 0.4479 − 0.002543 CAl Coefficient of correlation (R) 0.999 LOD 3.06 mg L−1 LOQ 10.2 mg L−1 Fig. 3. Influence of pH on the reaction between aluminum and Alizarin S, and on the development of color. 434 A.L.P. Silvestre et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 432–435 medium (NaOH at 1 mol L−1) was selected for use in the development of themethod, due to the color difference in the presence and in the ab- sence of aluminum (Fig. 3), which enabled the quantification of alumi- num by diffuse reflectance spectroscopy. In an acid medium (pH 4), there was no development of a different color in the presence of AHC. In a neutral medium, a gelatinous precipitate of Al(OH)3 was formed, while in an alkalinemedium(pH9), therewas amodest color difference in the presence of aluminum, compared to the blank. Furthermore, the samples were prepared in 1 mol L−1 NaOH medium, eliminating a pH adjustment step for AHCdetermination, and due to the error in pHmea- surements under alkaline conditions, with a decrease in the pH value in the presence of high concentrations of Na+, it is preferable toworkwith OH– concentrations, rather than the pH scale. Absorption by the reagent and the analyte occurs in the samewave- length range, but the reagent absorption presents a bathochromic shift. Therefore, the quantification of aluminum was achieved by measure- ment at 615 nm of the consumption of the purple reagent, avoiding spectral interference between the product and the reagent. The best analytical responsewas achieved by first adding the aliquot of the sample or aluminum standard solution, followed by addition of the reagent solution. The use of the hydrophobic barriers provided a more homogeneous and intense coloration, decreasing the standard de- viation and improving the analytical performance. The colored product was stable for at least 70 min after the reaction. Fig. 4. Analytical curve for analytical response (AR) vs. aluminum concentration, and the change of the absorption spectra with increase of the aluminum concentration. 4.2. Figures of Merit An analytical curve was constructed using the aluminum working standard solutions at concentrations from 10 to 125mg L−1. A linear re- lationship was found between the analytical response (AR, at 615 nm) and the aluminum concentration (CAl), described by AR = 0.4479 − 0.002543 CAl (Fig. 4), with R= 0.999 indicating good linear correlation. The repeatability of the proposedmethodwas evaluated using intra- day and inter-day relative standard deviations (%RSD), at two concen- trations. For a 25 mg L−1 aluminum solution, the %RSD values were 2.7% (intra-day) and 3.0% (inter-day). The corresponding values for a 50 mg L−1 solution were 1.3% and 1.9%, respectively. These results showed that the new method was repeatable and could be used for the determination of aluminum in antiperspirant samples. The limits of detection (LOD) and quantification (LOQ) were calcu- lated according to the IUPAC recommendations [32], using the following expressions: LOD= 3 ∗ σ / S and LOQ= 10 ∗ σ / S, where σ is the stan- dard deviation ofmeasurements of the blank (n=10) and S is the slope of the linear range. The values obtained were 3.06 mg L−1 (LOD) and 10.2 mg L−1 (LOQ). The technique was sufficiently sensitive to be able to determine aluminum in antiperspirant samples. Table 1 presents the figures of merits of the proposed method. 4.3. Sample Analysis Themethod developedwas used to determine the concentrations of AHC in seven different antiperspirant samples (Table 2). In addition to the quantitative analysis, a semi-quantitative visual determination could be performed by comparison of the colors obtained with a color palette (Fig. 5), whichwould be useful in situationswhere no diffuse re- flectance spectrometer is available. All the samples analyzed presented aluminum hydrochloride con- centrations below the limit established by legislation [14], indicating compliance of the cosmetics industries with product quality criteria intended to protect the population,with no cases of excessive quantities of aluminum that might lead to health problems in consumers. 4.4. Study of Matrix Interferences Antiperspirants contain various other substances besides aluminum hydrochloride, such as alcohols, EDTA, glycerin, paraffin, oil, and per- fume. The presence of these substances could lead to increases or Table 2 Quantification of AHC in commercial samples using the proposed method. Sample AHC concentrationa (%) A 23.0 ± 1.8 B 19.9 ± 0.9 C 15.6 ± 0.3 D 20.2 ± 0.6 E 20.2 ± 0.6 F 11.1 ± 0.5 G 12.2 ± 0.3 a The maximum level allowed by Brazilian legislation is 25%. Fig. 5. Color palette for semi-quantitative visual determination of aluminum. 435A.L.P. Silvestre et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 432–435 decreases of the analyte response, due tomatrix effects. The possible ex- istence of such effects was evaluated using recovery tests. The recovery values obtained for AHC were between 92.2% and 103.4%, indicating an absence of interference from the compounds normally found in antiper- spirants. Therefore, the sample matrix had no significant influence on the AHC determination [33]. Furthermore, aluminum is the only metal present at high concentrations in these types of formulations. Conse- quently, if the matrix happened to be contaminated with other metals that could bind to Alizarin S, the aluminum concentration would be much higher, hence minimizing any possible interference. 5. Conclusions A newmethodwas developed for the determination of aluminumhy- drochloride in antiperspirant samples. 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Introduction 2. Materials and Methods 2.1. Materials and Solutions 2.2. Sample Preparation 2.3. Preparation of the Paper Platform 3. Methodology 3.1. Study of Matrix Interferences 4. Results and Discussion 4.1. Preliminary Tests 4.2. Figures of Merit 4.3. Sample Analysis 4.4. Study of Matrix Interferences 5. Conclusions Acknowledgements References