Ultrasound-assisted emulsification of cosmetic samples prior to elemental analysis by different atomic spectrometric techniques

Talanta 80 (2009) 109–116

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Ultrasound-assisted emulsification of cosmetic samples prior to elemental analysis by different atomic spectrometric techniques I. Lavilla, N. Cabaleiro, M. Costas, I. de la Calle, C. Bendicho ∗ Departamento de Química Analítica y Alimentaria, Área de Química Analítica, Facultad de Química, Universidad de Vigo, As Lagoas-Marcosende s/n, 36310 Vigo, Spain

a r t i c l e

i n f o

Article history: Received 13 March 2009 Received in revised form 8 June 2009 Accepted 12 June 2009 Available online 21 June 2009 Keywords: Ultrasound-assisted emulsification Cosmetics Trace elements Atomic spectrometry techniques (plasma, flame, cold vapour)

a b s t r a c t In this work, ultrasound-assisted emulsification with a probe system is proposed as a rapid and simple sample treatment for atomic spectrometric determinations (Electrothermal Atomic Absorption Spectrometry, Inductively Coupled Plasma Optical Emission Spectrometry, Flame Atomic Absorption Spectrometry and Cold Vapour Atomic Absorption Spectrometry) of trace elements (As, Cd, Cr, Cu, Hg, Mg, Mn, Ni, Sr and Zn) in cosmetic samples such as shampoos, gel (hair gel), crèmes (body milk, hair conditioner) and oil (body oil). The type of dispersion medium, the sample mass-to-dispersion medium volume ratio, as well as the parameters related to the ultrasound-assisted emulsification (sonication amplitude and treatment time) were exhaustively studied. Only 1 min of ultrasonic shaking and a dispersion medium containing 0.5% (w/v) of SDS + 3% (v/v) of HNO3 or HCl allows obtaining a stable emulsion at least for 3 months. Thermal programs, nebulization of emulsions, speed of pumps and concentration of reagents used in cold vapour generation were optimized. Calibration using aqueous standards was feasible in all cases. Calibration by the standard addition method and recovery studies was also applied for validation. Microwave-assisted digestion and Inductively Coupled Plasma Mass Spectrometry were used for comparison purposes. Relative standard deviations from analysis of five independent emulsions were less than 9% in all cases. © 2009 Elsevier B.V. All rights reserved.

1. Introduction At present, cosmetics represent an important industry worldwide, but especially in Europe. These products are subjected to restrictive regulations, such as the European Directive 76/768/EEC [1], in order to ensure their safety and usefulness [2,3]. Legislations include information requirements about: (i) substances banned (these must not be found in the finished product); (ii) substances restricted; (iii) substances subjected to positive listing (used in controlled concentration). Among these, some metals and metalloids are included, namely, Cd, Pb and Sb (prohibited all their compounds); Co, Cr and Ni (most of their compounds are prohibited); Se (only as disulphur, up to 1% w/w) or Zn (up to 1% w/w) [1]. Therefore, sensitive and selective determinations of different metals in cosmetics are necessary for the control of these products. Matrix of cosmetics is not simple; it usually contains a high number of ingredients and often requires time-consuming and tedious sample treatments [4]. Official analytical methods have been recommended in different legislations. Most of these methods use

∗ Corresponding author. Tel.: +34 986 812281; fax: +34 986 812556. E-mail address: [email protected] (C. Bendicho). 0039-9140/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2009.06.036

digestion or calcination as sample treatment and atomic spectrometry as determination technique [5,6]. It is advisable to improve cosmetic quality control using different recent developments of sample treatment but the number of papers focusing on this subject is scarce. Sample treatments, which involve the use of concentrate acids (i.e. HNO3 , HClO4 , H2 SO4 or HF) and high temperatures and pressures, similar to those recommended by the official methods, are applied [7–14]. Sometimes, even with these strong treatments, complete solubilization of a sample is not feasible. Substances such as metallic oxides and pigments form a residue that must be removed by means of filtration or centrifugation, or submitted to a fusion treatment [10,12,15]. Sample emulsification procedures have been proposed as an interesting strategy for sample solubilization when liposoluble matrices are analyzed by Atomic Spectrometry [16–23]. This allows a direct and rapid analysis of samples, avoiding the use of strong acids and large volumes of organic solvents. The use of emulsification as sample treatment for metal determination in cosmetics can minimize the above problems. Nevertheless, this strategy has been scarcely used. Vondruska [24] determined Cd by Electrothermal Atomic Absorption Spectrometry (ETAAS) in cosmetic oil forming a water emulsion. Salvador et al. [12] determined Zn and Fe by Flame Atomic Absorption Spectrometry (FAAS) in sunscreen, forming a sample emulsion by shaking with a non-ionic tensioactive and isobutyl ketone.

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In general, mechanical or manual agitation is used for emulsion formation, and in addition, agitation is required to maintain its stability during analysis [23]. The use of ultrasound for shaking allows obtaining a stable and homogeneous emulsion more quickly in comparison with mechanical processes [25]. Ultrasound-assisted emulsification is considered an interesting approach for accelerating the formation of emulsions in analytical chemistry, as well as for the determination of organic compounds and metals [26]. Two processes occurs in ultrasonic emulsification: (a) interfacial instability of the oil–water interface and (b) transient cavitation bubbles that generate micro streaming, high-pressure shock wave and high local temperature during their collapse. These phenomena accelerate the mass-transfer process between two immiscible phases by generating smaller droplet size of the dispersed phase [27,28]. However, some drawbacks related to the possible surfactant degradation can appear during ultrasonic emulsification [29]. Then, detailed studies of the process must be performed when this approach is followed. In this work, we propose a simple and fast methodology based on ultrasound-assisted emulsification with a probe as a minimalist sample treatment for the determination of metals and metalloids in different cosmetics. This sample preparation was assessed in combination with different analytical techniques: Electrothermal Atomic Absorption Spectrometry, Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Flame Atomic Absorption Spectrometry and Cold Vapour Atomic Absorption Spectrometry (CV-AAS). Different variables related to ultrasonic emulsification were studied: surfactants, sonication time, sonication amplitude, sample mass and dispersive phase volume. Microwave-assisted digestion and ICP-MS (Inductively Coupled Plasma Mass Spectrometry) were used as a comparative methodology. 2. Experimental 2.1. Instrumentation A Unicam atomic absorption spectrometer Model Solaar 939 (Cambridge, UK) equipped with a Unicam GF-90 graphite furnace and a Unicam FS-90 autosampler was used for Cd, Cr and Ni determination. A multielement hollow cathode lamp of Cr/Ni/Cu/Mn (Thermo Scientific, Cambridge, UK) and a hollow cathode lamp of Cd (Unicam) were used as radiation sources. Pyrolytically coated graphite tubes with platform were employed for Cd and without platform for Cr and Ni. ICP-OES measurements were performed using an Optima 4300 DV inductively coupled plasma optical emission spectrometer (PerkinElmer, Überlingen, Germany) equipped with an AS-90 autosampler, a concentric glass nebulizer (Meinhard Type K), a cyclonic spray chamber and nickel interface sampler and skimmer cones. Axial and radial viewings were used. ICP-OES measurement conditions for As, Cd, Cr, Cu, Mg, Mn, Ni, Sr and Zn were optimized to achieve maximum signal-to-noise ratio. A Unicam atomic absorption spectrophotometer Model Solaar 969 (Cambridge, UK) equipped with 10 cm burner head was used for Mg and Zn determination. Hollow cathode lamps Thermo Electron (Cambridge, UK) and Heraeus (Cambridge, UK) were used as radiation sources for Mg and Zn, respectively. A PerkinElmer (Überlingen, Germany) Model 4110 ZL atomic absorption spectrometer equipped with a quartz cell atomizer and a flow injection system FIAS-400 (PerkinElmer) was used for Hg determination. The hollow cathode lamp PerkinElmer Lumina was employed as radiation source for Hg. The quartz cell was maintained at room temperature during operation. ICP-MS measurements were performed using a Thermo Elemental X7 Series ICP-MS equipped with an ASX-520 autosampler (CETAC, Omaha, USA), a glass expansion concentric nebulizer, a spray chamber of quartz impact bead with a Peltier system, a quartz

vessel torch and nickel interface cones. The isotopes 111 Cd, 65 Cu, 24 Mg, 55 Mn, 60 Ni, 88 Sr and 66 Zn were measured using standard mode with guard electrode (plasma screen) and isotopes 75 As and 52 Cr were measured using cell collision technology (CCT). The operating conditions for ETAAS, ICP-OES, FAAS, CV-AAS and ICP-MS are summarised in Table 1. Thermal programs for Cd, Cr

Table 1 Operating conditions. ETAAS Element Detection wavelengths (nm) Lamp current (mA) Spectral bandpass (nm) Sample volume (␮L) Chemical modifier (␮L) Data processing Thermal program: Stage Temperature (Cd) (Cr) (Ni) Ramp (◦ C s−1 ) Hold time (s)

Unicam AAS Model Solaar 939 Cd Cr 228.8 357.9 8 10 0.5 0.2 10 20 10 – Area 1 90

2 150

5 10

10 30

Ni 232.0 15 0.5 20 –

3 800 1300 1500 50 20

4 5 1500 2800 2400 2200 0 3 3

ICP-OES

Perkin Elmer Optima 4300 DV

Elements Radiofrequency power (W) Sample flow rate (mL min−1 ) Argon flow rates (L min−1 ) Stabilization time (s) Replicate time (s)

As, Cr, Cu, Mg, Mn, Ni, Sr, Zn 1500 2 Plasma, 15.0; auxiliary, 0.2; nebulizer, 0.4 15 5

FAAS

Unicam AAS Model Solaar 969

Element Mg Detection wavelengths (nm) 285.2 Lamp current (mA) 6 Spectral bandpass (nm) 0.5 Burner height (mm) 7 ) Air/acetylene flow rate (L min−11.1 Analysis time (s) 4 CV-AAS

PerkinElmer AAS Model 4110 ZL

Element Detection wavelengths (nm) Lamp current (mA) Spectral bandpass (nm) Integration time (s) Cell temperature (◦ C) Sample volume (␮L) Data processing

Hg 253.7 6 0.7 25 100 500 Peak height

FIAS program: Step Time (s) Pump 1 (rpm) Pump 2 (rpm) Valve

Prefill 15 100 120 Fill

Zn 213.9 10 0.5 7 1.2 4

Fill Injection 10 15 1000 120120 Fill Inject

ICP-MS

Thermo Elemental X7 Series

Elements Forward power (W) Sample flow rate (L min−1 ) Argon flow rates (L min−1 ) Scanning mode Points per peak Dwell time (ms) Sweeps per peak Replicates

As, Cd, Cr, Cu, Mg, Mn, Ni, Sr, Zn 1300 0.6 Plasma, 13.0, auxiliary, 0.7, nebulizer, 0.85 Peak hop 1 10 50 3

Collision cell technology (CCT) CCT gas Flow cell gas (mL min−1 )

8% H2 and He 6

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and Ni were carefully optimized with the emulsified samples. Optimum conditions for emulsion nebulization in FAAS and ICP-OES were used. Speed of pumps in the flow injection system and concentrations of NaBH4 and HCl were also appropriately optimized. A 100 W 20 KHz Sonic and Materials (Danbury, CT, USA) high intensity ultrasonic processor, model VC 50-1, equipped with titanium microtip of 3 and 6 mm of diameter was used for ultrasound-assisted emulsification. A Multiwave 3000 Oven system (Anton Paar, Graz, Austria) equipped with a sample rotor of 8 vessels was used for microwave-assisted digestion of samples. A UVIKON XS UV/VIS spectrophotometer (Domont, France) equipped with quartz cells was used to acquire the spectrum of the emulsions. A Cannon-Fenske Serie 150 viscosimeter (Afora, Barcelona, Spain) was used for viscosity measurements. 2.2. Reagents and samples High-purity deionised water was obtained from a PETLAB ultrapure water production system (Peter Taboada, Vigo, Spain). Multielement standard stock solution of 10 mg L−1 for Cd, Cr, Cu, Mg, Mn, Ni and Sr and 100 mg L−1 for As and Zn (ICP Multi Element Standard Certipur® VI Merck, Darmstadt, Germany) were used for calibration purposes in ICP-OES and ICP-MS. Internal standardization was carried out with 89 Y (0.5 ␮g L−1 ) and 115 In (5 ␮g L−1 ) prepared from a single element standard stock solution of 1000 mg L−1 (Merck). Single element standard stock solutions containing 1000 mg L−1 were prepared dissolving the pure metal or the appropriate salts: magnesium chips (Aldrich, Steinheim, Germany), nickel metal (Aldrich), zinc fine powder (Prolabo, Fontenay-sous-Bois, France), cadmium nitrate (Panreac, Barcelona, Spain), potasium chromate (Prolabo), mercury chloride (Prolabo). Surfactants used for preparing ultrasound-assisted emulsifications were: sodium dodecyl sulphate (SDS, Fluka, Steinheim, Germany), hexadecyl trimethylammonium bromide (CTAB, Sigma–Aldrich, Milwaukee, USA), Tween 80 (Sigma–Aldrich) and triton X-100 (Merck). Concentrated HNO3 (Suprapur, Merck), H2 O2 33% (w/v) (Panreac) and HF 40% (v/v) (Merck) were used for microwave-assisted digestion procedures. Palladium nitrate (Fluka) was used as chemical modifier for Cd determination by ETAAS. Concentrated HCl of analytical grade quality (Prolabo), NaOH (Prolabo), NaBH4 (Merck) and the Antifoam A (Fluka) were used for CV-AAS measurements. Four hairs care products (conditioner, antidandruff shampoo, intensive antidandruff shampoo and hair gel) and two bath/shower preparations (body milk and body oil) were analyzed. In general, these products are emulsions and hence, a re-emulsification is necessary prior to analysis. These samples content a large number of components, such as tensioactives, fragrance agents, stabilizer agents, antimicrobial preservatives and water. Moreover, shampoos, hair gel and body milk contained a SiO2 -based compound (i.e. dimethicone). Elements like Zn (present in the anti-dandruff shampoo as pyrithione or carbonate) and Mg (present in the antidandruff shampoo as sulphate or carbonate) are active principles in shampoos. Levels of these elements were not mentioned in the commercial labels. Glassware and plasticware used throughout the work were washed with 10% (v/v) nitric acid followed by repeated rinsing with ultrapure water.

111

For the determinations by ETAAS (Cd, Cr and Ni): a portion of 15 mg of cosmetic sample was weighed into an autosampler cup and 1 mL of acid solution with surfactant (0.5% w/v SDS and 3%, v/v HNO3 solution) was added as dispersing medium. Then, the mixture was sonicated for 1 min at 20% amplitude by means of the 3 mm probe. For the determinations by ICP-OES (As, Cd, Cr, Cu, Mg, Mn, Ni, Sr and Zn) and FAAS (Mg and Zn): a portion of cosmetic sampled in the range 0.05–0.2 g was weighed into a 50 mL polyethylene tube, and then 15 mL of dispersing solution was added. This mixture was sonicated for 1 min at 20% amplitude by means of the 6 mm probe. For the determination of Hg by CV-AAS, the emulsification procedure was similar to that employed in ICP-OES and FAAS, but HCl was used instead of HNO3 in the composition of the dispersion medium. Also, addition of 200 ␮L of antifoam after sonication was needed in order to avoid foam formation in the gas–liquid separator. Blanks were treated in the same way. In all cases the emulsion obtained was directly introduced in the analytical instrument. 2.4. Procedure for microwave-assisted digestion Different digestion procedures were studied for the cosmetic samples containing a SiO2 -based compound, such as shampoos, hair gel and body milk. When digestions were carried out using only HNO3 (4 mL of concentrated nitric acid), it was necessary to filtrate in order to remove the silicate residues [8,11]. The addition of HF was also tried for the digestion of these samples (a mixture of 3 mL of HNO3 and 0.5 mL of HF was used and then, the resulting solution was heated in a sand bath nearly to dryness). The mixture of H2 O2 and HNO3 was also tried (4 mL of HNO3 and 2 mL of H2 O2 ). No significant differences were observed when the different procedures were statistically compared. Microwave-assisted digestion of the cosmetics was carried out for comparison purposes. The selected procedure was as follows: about 0.2 g of sample was weighed into the PFA vessel, and then, 4 mL of concentrated HNO3 and 2 mL of ultrapure water were added. The digestion vessels were closed and heated in a microwave oven using a preselected program: first stage of 5 min at 900 W, second stage of 2 min at 0 W and a third stage of 20 min at 900 W. After cooling, digests were quantitatively transferred into a 50-mL volumetric flask and made up to volume with ultrapure water. Blanks were treated in the same way. A dilution 1:5 was made for As, Cu, Sr and Zn measurements by ICP-MS in order to avoid matrix effects. Determinations by ICP-MS were based on a calibration graph obtained from standard aqueous solutions. Recovery studies were carried out for validation purposes. 3. Results and discussion The type of surfactant and its concentration, the acid and its concentration, the quantity of sample in relation to the volume of dispersion medium, as well as the parameters related to the ultrasound-assisted emulsification (amplitude and sonication time) were exhaustively investigated. Preliminary experiments were carried out in order to visually check the stability of emulsions and then to establish suitable conditions in which stable emulsions were formed. In this work, an oil-in-water emulsion (o/w) is formed because the aqueous medium forms the continuous phase. 3.1. Study of the dispersion medium

2.3. Procedures for ultrasound-assisted emulsification Different emulsification procedures were proposed in order to adapt this approach to some monoelemental and multielemental atomic spectrometry techniques: ETAAS, ICP-OES, FAAS and CVAAS.

Different surfactants were considered in this work: Triton X100 (non-ionic), Tween 80 (non-ionic), SDS (anionic) and CTAB (cationic). These surfactants were selected due to their high solubility in water. All them have a HLB (hydrophilic lipophilic balance) higher than 10 (dimensionless scale) and can form (o/w) emulsions.

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In general, surfactants with high HLB, as SDS (40), display high solubility in water and generally act as good solubilizing agents, detergents and stabilizers for (o/w) emulsions. The lower HLB of the others surfactants studied (15 for Tween 80, 13.5 for Triton X100, and 10 for CTAB) can predict their lowest performance in (o/w) emulsification. These surfactants have been successfully used by other workers for samples such as gasoline, crude oil or edible oil [16,22,30,31]. Surfactants play a basic role in the formation and stability of emulsions (electrostatic and/or steric repulsion between emulsion droplets mainly depend on the surfactant), but an increasing viscosity can occur. On the other hand, the cavitation pressure threshold in a liquid increases on increasing viscosity. Less viscous emulsions allow achieving cavitation more easily [32]. In order to prevent potential problems in the emulsification procedure and, especially, during the sample introduction step in the different analytical instruments, viscosity of emulsions was tested. Emulsions with different surfactant concentration were prepared: CTAB (between 0.1% and 1% w/v), Tween 80 (between 0.005% and 0.5% w/v), SDS (between 0.05% and 1% w/v) and Triton X-100 (between 0.001% and 0.5% w/v). Small quantities of surfactant were tried in order to obtain suitable blanks for trace metal determinations. The blanks obtained with the different surfactants were suitable in all cases. When ultrasound energy is used, less surfactant could be necessary, and it would represent an important advantage of ultrasound-assisted emulsification procedures. The minimum concentration of surfactant that forms a visually stable emulsion was established and then, kinematic viscosity of stable emulsions was measured. Table 2 shows the results obtained. As can be seen, emulsions formed with CTAB showed high viscosity, which was troublesome in some measurements, so this surfactant was rejected. Emulsions formed with Triton X-100 showed low stability and the breakup of the emulsion occurred at 30 min for concentrations up to 0.01% (w/v). The emulsions formed with Tween 80 showed good stability but their turbidity increased with time. SDS showed very good stability and no turbidity for concentrations up to 0.05% (w/v). Stability and turbidity studies were made by ETAAS (Ni was determined in hair conditioner emulsions prepared with different surfactants) (Fig. 1) and by spectrophotometry (absorption spectra of the formed emulsions were obtained). The results were in good agreement with those obtained visually. SDS was selected for preparing emulsions on the basis of its good stability and lack of turbidity. A SDS concentration of 0.05% (w/v) was necessary to reach stable emulsions. Then, a concentration in the range 0.1–0.5% (w/v) of SDS was used for this purpose. Stability studies were also carried out for the rest of elements. Nitric acid is usually added to the dispersion medium because it helps improving the stability of the analyte in emulsions of samples such as biodiesel [23]. HNO3 can also convert organic species, metallic solid particles and oxide species of the analytes into their inorganic forms. In addition, it has been demonstrated that the use of emulsions (gasoline and kerosene) acidified with HNO3 enables direct correlation between the signal of the analyte in the samples and the signal of it in aqueous inorganic standards [33]. When

Fig. 1. Variation of absorbance vs. time for 20 ␮g L−1 of Ni in emulsions of hair conditioner (0.015 g of sample in 1 mL of dispersion medium) prepared with different surfactants (A) , SDS 0.05%; , 0.1%; , 0.5%; , 1% (w/v); (B) Triton X-100, 0.01% (w/v) and (C) Tween 80, 0.05% (w/v).

only SDS was added to the water phase in our experiments, persistent foam and slight turbidity were observed in the formed emulsion. Therefore, acid concentration was studied in the range 0.5–4% (v/v). Foam formation was observed for acid concentrations lower than 1% (v/v), a nitric acid concentration of 3% (v/v) being selected. The use of hydrochloric acid was also considered in the application of the CV-AAS technique, although no differences were observed. 3.2. Oil/water ratio The use of a maximum amount of sample in the emulsion is desirable in order to increase the analytical sensitivity, but it also increases the viscosity, thereby reducing the cavitation and the nebulization efficiency. Then, the sample mass was varied between

Table 2 Kinematic viscosity of emulsions. Surfactant

Visually non-stable emulsions (up to % w/v)

SDS Tween 80 Triton X100 CTAB

0.05 0.01 0.005 0.5

a

Assayed range Concentration (% w/v)

K. viscosity (cSk)

0.1–1 0.02–0.5 0.01–0.5 1

0.989–1.082 1.120–1.176 1.138–1.176 43.5

K. Viscosity is kinematic viscosity. The kinematic viscosity for deionized water is 0.999 cSk.

Concentration (% w/v)

K. viscositya (cSk)

0.50 0.05 0.01 –

1.007 1.160 1.145 –

I. Lavilla et al. / Talanta 80 (2009) 109–116

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Fig. 2. Optimization of the sonication time (min) and amplitude (%) in emulsions formed with hair conditioner. (A) Absorbance of Ni obtained by ETAAS vs. ultrasound time; (B) absorbance of Ni vs. sonication amplitude; (C) emission signal of Mg by ICP-OES vs. sonication time and (D) emission signal of Mg vs. ultrasound amplitude. Experimental conditions for (A) and (B) were: 0.015 g of sample in 1 mL of dispersion medium (0.5% w/v of SDS and 3% v/v nitric acid). Experimental conditions for (C) and (D) were: 0.2 g of sample in 15 mL of dispersion medium.

0.05 and 0.2 g for 15 mL of dispersion medium and the viscosity was measured. Viscosities were found in the range 1.008–1.119 cSk. Larger sample masses caused an important increase in viscosity and besides persistent foams. Then, under these conditions, a (o/w) ratio of about 0.015 g mL−1 was established for the different analytical techniques including ETAAS, where 1 mL of dispersion medium was used. 3.3. Sonication parameters Besides the surfactant concentration and the (o/w) ratio, main parameters influencing the emulsification process included power or amplitude and sonication time. In general, it is considered that an increase in the irradiation time and power causes a decrease in droplet size and an increase in the stability of the emulsion [34]. Then, sonication time and sonication amplitude were optimized for the emulsions formed with 1 or 15 mL of dispersion medium. Different techniques and samples were used for this purpose. In all cases, the results showed that 1 min is enough to form a stable and homogeneous emulsion (Fig. 2). Higher sonication amplitude had no effect on the emulsion formation so it was maintained at 20% level. In comparison with the probe system, when an ultrasound bath was used in this work, a time of 15 min was necessary in order to obtain a transparent emulsion. When mechanical stirring was used, a turbid emulsion was obtained after 30 min. Similar times have been required by other authors with ultrasonic baths [22,35–38] and with mechanical stirring [30]. Sonication probe systems bring about some benefits for ultrasound-assisted emulsification, such as faster formation of stable emulsions and lower surfactant concentration, since cavitation is easily reached with these systems. 3.4. Studies of stability Emulsions of the different cosmetic samples, prepared as pointed out above, were visually stable at least for 3 months. For longer times, phase separation takes place.

Fig. 3. Stability study of an emulsion prepared with the antidandruff shampoo (0.2 g of sample in 15 mL of dispersion medium). Zn absorbance measured by FAAS during four consecutive days.

The stability of emulsions was investigated for a period of several days. As can be seen in Fig. 3, the analytical signal of Zn in the antidandruff shampoo emulsion indicates a very good stability. Then, no immediate determinations need to be carried out. 3.5. Calibration studies Chemical interferences caused by matrix components when emulsions are directly introduced into the analytical instruments, makes it necessary the development of different strategies for calibration. Aqueous standards have been adequate in the determination of several metals in oil-water emulsions [19,39]. de Souza et al. [18] used aqueous standards containing surfactant. Standard additions have been used with inorganic standards [38] and metalorganic standards [22]. Emulsions prepared with a single organic solvent using aqueous or organometallic stock solutions have also been used [40]. Different emulsions prepared by using purified samples and adding the metal as inorganic or as organic form have also been employed by several authors for calibration purposes [16,17,20,22,23,35].

−0.9 −0.5 +1.9 7.2832 0.5620 0.2725 −0.8 −0.7 +1.3

+2.9 – +4.0 0.3463 – 18.3887 +2.8 – +3.6

−1.5 – +2.9 +3.4

+3.3 – – +0.9 9.8919 – – 73.0435

60.7756 – 0.3936 0.6338 −2.7 – −0.2 +3.3

Change %

Body milk

In our work, potential matrix effects were studied by comparison of slopes obtained for aqueous standards and standard addition calibration methods. Calibration curves were built from three measurement replicates and, in all cases, regression coefficients were higher than 0.995. The slopes obtained and the linear ranges used are shown in Table 3. The comparison of slopes is presented in terms of percentage of change in slope (aqueous standard/standard addition). In all cases these percentages of change were less than 5%. These results indicate that there are not significant matrix effects and hence, calibration with aqueous standard can be carried out in all cases.

+1.9 – – +0.1

Change %

I. Lavilla et al. / Talanta 80 (2009) 109–116

Slope st.ad.b

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7.2708 0.5628 0.2741 −0.9 −4.3 +0.6 7.2809 0.5839 0.2758 7.1768 0.5374 0.2806 –Not determined. a Slope obtained for aqueous standards. b Slopes obtained for standard addition. c Slopes expressed in ␮g L−1 . d Slopes expressed in mg L−1 .

ICP-OES ICP-OES FAAS Srd Znd

10-100 100–1000 1–100

7.2152 0.5590 0.2776

7.2241 0.5445 0.2733

−0.1 +2.7 +1.6

+0.5 +4.0 +1.1

0.3465 – 18.4496 0.3545 – 19.2630 0.3594 – 18.9148 ICP-OES ETAAS ICP-OES Mnd Nic

10–100 1–50 10–100

0.3563 0.0084 19.1195

0.3425 0.0084 18.4681

+4.0 0.0 +3.5

−0.9 – +1.1

+0.5 – −0.7

61.5325 – 0.4059 0.6236 +0.1 – −0.4 0.0 59.7747 – 0.4064 0.6642 59.5530 – 0.4069 0.6440 −0.3 −3.4 −2.6 +3.2 60.0202 0.0088 0.4159 0.6240 59.8394 0.0085 0.4049 0.6442 ICP-OES CV-AAS ICP-OES FAAS Cuc Hgc Mgd

10–100 1–40 100–1000 100–500

ICP-OES ETAAS ETAAS ICP-OES Asc Cdc Crc

10–100 1–10 1–50 10–100

10.2203 0.1083 0.0161 73.7289

10.3741 0.1061 0.0160 73.9930

−2.1 – – −0.9 10.4393 – – 74.4422

+0.5 – −0.5 0.0

10.0303 – – 73.6395 −1.0 – – −1.6

Hair gel

−1.5 +2.1 +1.0 −0.3

10.3196 – – 74.9242

Change %

Intensive antidandruff shampoo

Slope st.ad.b Slope st.ad.b

Change %

Antidandruff shampoo

Slope st.ad.b Change %

Conditioner

Matrix Slope aqueous standa Range (␮g L−1 ) Technique

Hair care products and bath/shower preparations were analyzed following the established procedure of ultrasound-assisted emulsification and different atomic spectrometric techniques (ETAAS, ICP-OES, FAAS and CV-AAS). External calibration with aqueous standards was used throughout. As a comparative methodology, samples were analyzed following microwave-assisted digestion and ICP-MS. Analytical results from three sample preparations for both ultrasound-assisted emulsification and microwave-assisted digestion procedures and three replicates of each one are shown in Table 5. After application of a t-test, no significant differences were generally observed between the results obtained by ultrasound-assisted emulsification and those obtained following microwave-assisted digestion. In general, elements considered toxic such as Cd or Hg display concentrations below the LODs in the cosmetics under study.

Element

3.7. Analytical results for analyzed samples

Table 3 Slopes of the calibration lines obtained with external calibration (aqueous standards) and standard addition method for the analysis of different cosmetics.

Validation of the ultrasound-assisted emulsification procedure in order to demonstrate its applicability to cosmetic samples was performed. This included detection and quantification limits, linearity, precision and accuracy. Instrumental limits of detection (LOD) and quantification (LOQ) were determined following the 3 and 10 criteria, respectively. Procedural LOD and LOQ values in cosmetic samples were determined considering the instrumental LOD and LOQ values and the sample treatment (0.015 g of sample in 1 mL of solution for ETAAS and 0.2 g of sample in 15 mL of solution for ICP-OES, FAAS and CVAAS). These limits are shown in Table 4. Precision of the method, expressed as relative standard deviation (RSD), was evaluated in terms of repeatability. RSDs from five independent emulsions are shown in Table 4. Emulsions of cosmetic samples provided good precision. In all cases, repeatability from three replicates of the same emulsion was lower than 3% for all elements. Accuracy was tested by means of recovery studies because certified reference materials for determining trace elements in cosmetics were not available. These studies were carried out in triplicate upon addition of known amounts of the element under study prior to sonication. As can be seen in Table 4, quantitative results were obtained in all cases. Although mercury is used in cosmetics as a preservative in the form of phenylmercury salts and thiomersal, in this work, only inorganic mercury was determined. When ultrasound irradiation is performed in aqueous media, a series of radicals that are formed at the gas-phase interface of the cavitation bubble and to a lesser extent in the bulk solution, can reduce Hg(II) to Hg(0). However, when the medium contains HCl, this reaction does not take place [41]. Then, nitric acid was replaced by hydrochloric acid in the dispersion medium. Recovery studies confirm the maintenance of Hg(II) in the emulsion and further generation of Hg(0) from Hg(II) upon reduction with NaBH4 .

Slope st.ad.b

3.6. Validation of the proposed methodology

I. Lavilla et al. / Talanta 80 (2009) 109–116

115

Table 4 Validation of the emulsification method. Element

Technique

As Cd Cr

ICP-OES ETAAS ETAAS ICP-OES

Cu Hg Mg

ICP-OES CV-AAS ICP-OES FAAS

Mn Ni Sr Zn

LOD (␮g g−1 )

LOQ (␮g g−1 )

RSD (%)

Recoveries (%) Conditioner

a

Antidandruff shampoo

Intensive antidandruff shampoo

Hair gel

Body milk

0.11 3.2a 21a 0.23

1.7–8.7 1.2–5.6 0.6–8.3 1.0–8.7

102 95 109 99

± ± ± ±

3 3 3 5

104 96 98 101

± ± ± ±

2 2 2 1

107 97 99 99

± ± ± ±

2 2 1 2

100 102 97 99

± ± ± ±

4 2 2 2

96 100 100 97

± ± ± ±

0.3 2 2 3

0.13 5.5a 0.02 0.20

0.44 18a 0.06 0.67

1.5–7.5 0.4–4.7 1.3–8.6 0.8–9.3

94 102 91 96

± ± ± ±

5 2 2 1

90 99 91 98

± ± ± ±

3 3 3 1

99 96 97 96

± ± ± ±

2 3 3 2

95 96 107 95

± ± ± ±

4 2 2 1

94 98 92 95

± ± ± ±

2 1 2 2

ICP-OES ETAAS ICP-OES

0.02 11a 28a

0.09 35a 95a

2.0–9.1 0.8–9.2 1.7–8.3

97 ± 2 103 ± 3 94 ± 4

102 ± 2 102 ± 2 92 ± 4

101 ± 1 96 ± 3 91 ± 3

97 ± 3 93 ± 3 101 ± 3

96 ± 4 98 ± 2 95 ± 2

ICP-OES ICP-OES FAAS

3.7a 0.12 0.47

11a 0.39 1.6

0.6–9.1 0.8–5.0 0.3–5.3

91 ± 1 102 ± 4 97 ± 1

93 ± 3 94 ± 3 101 ± 1

94 ± 3 108 ± 2 98 ± 1

90 ± 3 101 ± 2 97 ± 1

90 ± 2 102 ± 2 97 ± 2

0.03 0.95a 6.2a 0.07

LODs expressed in ng g−1 .

Table 5 Concentrations (ng g−1 ) in different cosmetic samples. Element

Technique

Samples Conditioner

Antidandruff shampoo

Intensive antidandruff shampoo

Hair gel

Body milk

Body oil

As

ICP-OES ICP-MS


90 ± 6 (0.005)b 87 ± 4

234 ± 20 (1.091)b 252 ± 19




Cd

ETAAS ICP-MS







Cr

ETAAS ICP-OES ICP-MS

66 ± 4 (2.947)b
418 ± 11 373 ± 23 (0.877)b 389 ± 21 (2.065)b

159 ± 13 (1.318)b 186 ± 16 170 ± 14 (0.909)b

46 ± 4 (1.619)b
93 ± 9 (1.011)b
57 ± 6 (2.305)b
Cu

ICP-OES ICP-MS



269 ± 20 (1.349)b 292 ± 21




Hg

CV-AAS


Mg

ICP-OES FAAS ICP-MS

5.4 ± 0.1 (1.254) 5.8 ± 0.1a (2.493)b 5.5 ± 0.1a

464 ± 16 (2.121) 454 ± 13a (1.395)b 439 ± 14a

6.8 ± 0.6 (5.068) 9.5 ± 0.7a (0.364)b 9.7 ± 0.8a

38 ± 2 (0.229) 37 ± 1a (0.845)b 38 ± 2a

26 ± 2 (0.651) 24 ± 0a (2.748)b 27 ± 1a


Mn

ICP-OES ICP-MS


221 ± 20 (1.833)b 197 ± 11





Ni

ETAAS ICP-OES ICP-MS


13 ± 1a (1.008)b 10 ± 1a (2.168)b 12 ± 1a

5.4 ± 0.5a (2.295)b 4.8 ± 0.4a (0.207)b 4.8 ± 0.3a




Sr

ICP-OES ICP-MS

25 ± 1 (2.177)b 28 ± 1

149 ± 10 (3.783)b 109 ± 10

352 ± 32 (1.159)b 381 ± 30

38 ± 4 (1.241)b 42 ± 3

299 ± 26 (0.971)b 321 ± 31


Zn

ICP-OES FAAS ICP-MS


12872 ± 53a (4.306)b 12755 ± 40a (1.688)b 12694 ± 47a

2.0 ± 0.1a (2.306)b 1.9 ± 0.1a (0.097)b 1.8 ± 0.1a





b


b


b


b


b

Concentrations are expressed in ␮g g−1 . texp values are shown between parenthesis. tcrit is 2.776 (N = 3, p < 0.05). LODs for ICP-MS were: 0.01 ␮g g−1 for As, 2.0 ng g−1 for Cd, 0.04 ␮g g−1 for Cr, 0.01 ␮g g−1 for Cu, 5.0 ng g−1 for Mg, 1.8 ng g−1 for Mn, 9.3 ng g−1 for Ni, 10. ng g−1 for Sr and 0.04 ␮g g−1 for Zn. a

b

Only shampoos have a detectable content of As (87 ng g−1 in antidandruff and 271 ng g−1 in intensive antidandruff shampoos). Cr was detected at low levels (between 38 and 429 ng g−1 ). In the case of Ni, for which most of their compounds are prohibited, levels found were between the LOD and 14 ␮g g−1 . Cu concentration ranged between the LOD and 313 ng g−1 . Mg displays a large variability depending of the cosmetic sample. The antidandruff shampoo, where Mg is one of the active principles, contains around 480 ␮g g−1 . Mn concentration was between LOD and 241 ng g−1 . Sr concentration ranged between undetectable concentrations and 412 ng g−1 , with the maximum level in the intensive antidandruff shampoo. For Zn, only detectable concentrations were observed in the shampoos, with the maximum content in the antidandruff

shampoo (about 13 mg g−1 ), in which is used as an active principle. Overall, element concentrations are below the sensitizing limit proposed for consumer products and therefore, in agreement with the European legal regulations [1].

4. Conclusions The developed procedure based on ultrasound-assisted emulsification with a probe ultrasonic processor provides an accurate and precise methodology for preparation of cosmetic samples prior to elemental analysis by ETAAS, ICP-OES, FAAS and CV-AAS. It can

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be considered as an advantageous alternative in respect to digestion procedures due to its simplicity (minimal operations involved), rapidity and low consumption of reagents. These characteristics make this sample preparation suitable for routine analysis in the cosmetic industry. An emulsion with very high stability is obtained. High intensity sonication for emulsion preparation facilitates the use of a surfactant (SDS) at low concentration. Addition of acid to the dispersion medium is necessary in order to remove foaming when cosmetics containing detergents such as shampoos are analyzed. A significant reduction in treatment time used for emulsion formation is achieved with probe sonication as compared to bath sonication. Oil/water emulsions have similar viscosities in comparison with aqueous solutions and hence, no problems were found with nebulization systems in FAAS and ICP-OES instruments, with the flow injection system used in CV generation or with the autosampler used in ETAAS determinations. Also, matrix effects were not present when emulsions are used. Conventional calibration with aqueous standards can be made in all cases. Acknowledgements Financial support from the Spanish Education and Science Ministry (project CTQ2006-04111/BQU) is gratefully acknowledged. N.C. and I.C. thank the Galicia Government (Xunta de Galicia) for a grant. References [1] European Directive 76/768/EEC, Council Directive 76/768/EEC of 27 July 1976 on the approximation of the laws of the Member States relating to cosmetic products. [2] L. Gagliardi, S. Dorato, in: A. Salvador, A. Chisvert (Eds.), Analysis of Cosmetic Products, Elsevier, 2007, pp. 3–28. [3] L. Gagliardi, D. De Orsi, S. Dorato, in: A. Salvador, A. Chisvert (Eds.), Analysis of Cosmetic Products, Elsevier, 2007, pp. 45–71. [4] A. Salvador, J.G. March, M.T. Vidal, A. Chisvert, A. Balaguer, in: A. Salvador, A. Chisvert (Eds.), Analysis of Cosmetic Products, Elsevier, 2007, pp. 72–82. [5] European Directive 83/514/EEC, Third Commission Directive 83/514/EEC of September 1983 on the approximation of the laws of the Member States relating to methods of analysis necessary for checking the composition of cosmetic products.

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