Multi-element method for determination of trace elements in sunscreens by ICP-AES

Abstract

An inductively coupled plasma atomic emission spectrometric (ICP-AES) method was developed for multi-element analysis of sunscreen creams and lotions. The objective was the simultaneous determination of Ti (TiO₂ being is the only authorized inorganic UV filter in the European Union) and several minor, trace or toxic elements (Al, Zn, Mg, Fe, Mn, Cu, Cr, Pb and B) in the final products. Two alternative pretreatment procedures were examined: (i) total acid digestion in closed pressurized vessels prior to sample introduction into the plasma and (ii) direct introduction of sample in the form of emulsified slurry. The latter was proved inefficient for several types of creamy samples due to their high viscosity and insolubility. Several acid mixtures were examined for wet digestion because of the complex and fatty matrix of creams and lotions. Plasma parameters like nebulizer argon gas flow rate and radiofrequency incident power were optimized in order to improve the atomization. The recovery of the proposed acid digestion method was evaluated using spiked samples. The calculated recoveries were 95.0% for Ti, 98.2% for Zn and 101.3% for Fe, and the detection limits were 0.2 μg g⁻¹ for Ti, 0.2 μg g⁻¹ for Zn and 0.5 μg g⁻¹ for Fe, respectively. Possible interference from the presence of Ti on the sensitivity of each analyte was examined. Finally the method was applied successfully to several commercial sun protection products and the results were compared with those obtained by atomic absorption spectrometry as reference method.

Introduction

Sunscreen products are widely used to prevent sunburn, reduce premature photo-ageing and skin cancer risk and allow tanning and longer exposure to sunlight [1], [2]. The active sunscreen agents are classified in two categories (i) inorganic UV filters which reflect scatter or absorb broadband the UV radiation and (ii) organic UV filters which attenuate solar UV rays by absorbing the radiation and are classified in two groups as UVA and UVB filters [3], [4], [5]. Various formulations containing combinations of inorganic and organic UV filters were developed aiming to improve the protective effect and sunscreen efficacy against both UV-B (290–320 nm) and UV-A (320–400 nm) radiation.

The regulatory authorities in the European Union and USA [6], [7], [8] have established lists of the authorized UV filters and the corresponding maximum allowed concentrations in the final products [9]. In these lists, only titanium dioxide is an inorganic UV filter, sometimes called physical UV filter, and it is applied in the form of micronised pigment of TiO₂ which is easily incorporated in emulsions.

The analysis of the sunscreen cosmetics is required since sun protection factor (SPF) is related to the content of UV filters in the commercial products [10]. There is also necessity to ensure that the concentration levels of metals and oxides are lower than the established limits, since there are several known undesirable dermatological side-effects mainly from organic UV filters [1], [5].

For inorganic UV filters like titanium dioxide, Schulz et al. [11] found that there is no evidence of skin absorption. Although titanium and zinc oxides are considered as inert substances without ability to penetrate stratum corneum [2], [12] several other reports are contradictory [5]. There is a concern about the photocatalytic activity of titanium or zinc oxides, which facilitates the generation of reactive oxygen and hydroxide radicals. The research in the field of inorganic UV filters is continuous towards deactivation of titanium oxide by coating with Al₂O₃ or SiO₂ [5], while Yabe and Sato [13] investigated cerium oxide as an alternative to titanium and zinc oxides inorganic UV filter.

Many products containing titanium dioxide and zinc oxide have been commercialised, and several sunscreens can also include iron oxide in order to improve the skin color appearance. As it is mentioned above, it is generally assumed that the sun protection factor (SPF) of a sunscreen is dependent on the amount of product applied and the percentage of active ingredient(s) [10]. Analytical control of the final composition is a matter of great interest because the raw materials employed in sunscreen formulations may sometimes not be carefully purified [4]. Consequently, reliable and fast methods are required to check whether they conform to existing legislation and also for quality control purposes. There are no official methods to determine these inorganic constituents in sunscreen cosmetics. In addition to Ti and Zn, other metals like Cr, Cu, Mn, Pb, etc. are also of importance and must be determined in such skin products due to their allergic or toxic properties. It is interesting that not more than 80 papers have been published for all types of suncare products analysis in the period 1976–2004 as described in a critical review by Salvador and Chisvert [1]. However, the majority of the reports refer to organic UV absorbers by liquid chromatographic (LC) methods [1], [8], [9], [14], [15], [16], [17] and very few to inorganic UV filters [4], [18], [19], [20].

Despite of the fact that sunscreen products contain usually more than one metal oxides either as inorganic filters or other constituents, few analytical methods have been reported in the literature for multi-element analysis of commercial sunscreens. Inductively coupled plasma techniques (ICP-AES and ICP-MS) are very convenient multi-element tools for routine analysis applicable to a wide variety of applications including cosmetic and pharmaceutical, because they offer the significant advantage of improved sensitivity and selectivity as compared to other techniques [20], [21], [22], [23], [24].

Titanium was determined in sunscreen creams for first time by Mason [18]. Atomic absorption spectrometry (AAS) was applied and the samples were decomposed with a mixture of sulphuric acid, ammonium sulphate and hydrogen peroxide. Based on ICP-AES determination, several acidic mixtures including nitric, phosphoric or hydrofluoric acids were comparatively tested for their efficacy in dissolving titanium dioxide [19]. Titanium could be also determined by ICP-AES after acid digestion in a microwave oven, followed by fusion of the titanium dioxide with KHSO₄, and finally dissolving the residue in concentrated sulphuric acid [4]. Zinc and iron were determined independently by flame AAS after emulsion formation of the samples with isobutyl methyl ketone (IBMK) and Nemol as surfactant [4]. XRF compared to ICP-AES was also used for titanium in cosmetic products [20].

Direct introduction of solid or oily matrices in the form of a slurry or an emulsion is an alternative technique to the acid digestion of the sample, and sometimes offers significant advantages [21], [22]. Time-consuming digestion steps and possible losses or contamination are avoided by slurry introduction. The slurry introduction technique requires a suitable nebulization system and careful adjustment of plasma parameters in order to achieve robust conditions. However this is not always possible for emulsion-type slurries, like those obtained from sunscreen creams.

The objectives of this work were to develop and optimize methods of simultaneous determination of the most common inorganic UV sunscreen agents like titanium, zinc and iron as well as several other elements likely to be found in sunscreen cosmetics. Also there is a constant interest to monitor any trace of toxic elements in commercial products, and for this reason, the developed method included the determination of toxic heavy metals, e.g. lead and chromium, which must not be present in cosmetics [25]. The performance of three digestion procedures was examined for quantitative multi-element analysis of sunscreen creams. Alternatively direct introduction of the sample as emulsified slurry into the nebulization system of inductively coupled plasma was studied. The performance characteristics of the proposed method were evaluated and compared to the results obtained after conventional acid digestion of the sunscreen creams.