Neutron activation analysis of titanium in Suncare (or Make-up) products

The EU regulation 2016/1143 of 13 July 2016 amends Annex VI to Cosmetics Regulation 1223/2009, to introduce Titanium Dioxide in its nano-form (TiO2NPs) and make it an authorized UV-filter, except in spray products. In this context, the availability of suitable analytical methods for the determination of titanium (Ti) and its physicochemical form in in unmodified commercial sunscreens has become of utmost importance. For this, we have developed Instrumental Neutron Activation Analysis (INAA), a method of elemental analysis using techniques of nuclear transmutation, for the determination of Ti in cosmetic products. The analysis involves three steps: (i) preparation of samples and standards. 5-20 mg of powdered or liquid cosmetic samples were introduced into plastic vials (0.6x 3cm, suitable for food contact, Kartell). As standard, we used 50 μL of a Standard Reference Material Titanium Standard Solution intended for use as a primary calibration standard for the quantitative determination of Ti (NIST, SRM 3162a, certified value of titanium of 9.879 mg/g ± 0.019 mg/g in HNO3 10% + HF 2%); (ii) neutron activation. The vials containing samples and standards were introduced into cylindrical polythene containers which were transferred in the pneumatic irradiation system, named Rabbit, in the outer core of the Triga Mark II nuclear reactor of Pavia. Neutron irradiations were carried out for 30 s at thermal neutron flux of 7.4 x1012 neutrons cm-2 s-1. The nuclear reaction used for Ti determination was 50Ti (n,γ)51Ti. The Rabbit system, directly connected to the LENA Radiochemistry Laboratory by means of a high-speed pneumatic transfer system, allowed the immediate counting of the induced 51Ti radionuclide in irradiated samples; (iii) radioactive counting. After neutron irradiation samples and standards were decayed for 3 min in order to decrease the background radiation and the gamma-ray emissions counted for 5 min by computer-based high resolution gamma-ray spectrometry using a Ge detector (EG & G Ortec Int, GA, USA) coupled to a multichannel analyzer (Laben, USA). The software used for the acquisition and processing of data and analysis of the gamma ray spectra was GammaVision for Windows Model A66-B32, Version 6.07 (EG&G-ORTEC). For the determination of the Ti the 321 keV line photon emission of the induced 51Ti radionuclide (T1/2 = 5.76 min) was evaluated. INAA here developed: (i) doesn’t need correction for reagent blank and is free of matrix interference (ii) it is simple (no sample pretreatment such as digestion or dissolution, is necessary) and fast (approximately 10min / sample) reaching a good sensitivity (20 ng Ti) and accuracy (better than 10%). However, INAA is not able to distinguish in solution between nanoparticulate and dissolved Ti. In spite of this some applications we have carried out showed an important and complementary analytical role of the nuclear technique in relation to problems concerning TiO2NPs and cosmetic products. Applications concern: (i) the analysis of Ti in commercial sunscreen and make-up products, proving INAA an excellent analytical tool for screening work in relation to analytical quality controls of cosmetic formulations; (ii) the development of novel TiO2NPs in coating of mesoporous silica (SiO2) nanosphere for research purpose. Interestingly, in this case INAA was able to measure simultaneously in the same sample both titanium and silicon; (iii) the use as reference method to validate the development of single particle ICP-MS (spICPMS), the emerging analytical method to characterize and quantify simultaneously TiO2NPs concentration, size, size distribution and dissolved Ti concentration in consumer products, including commercial sunscreens.