Emsure

Ultrapure water (UPW) with a resistivity of 18.2 MΩ cm was used for the preparation of solutions and dilution analytical standards using mercury standard solution (traceable to SRM from NIST, Hg(NO₃)₂ in HNO₃, 2 mol·L⁻¹, Merck, Darmstadt, Germany). For atomic fluorescence spectrometry (AFS) measurements, argon gas (99.996%, MTI IndustrieGase AG, Ulm, Germany) was used as a carrier gas. Reference measurements according to the EPA method 1631 required potassium bromide (VWR International GmbH, Darmstadt, Germany), potassium bromate (VWR International GmbH), hydroxylammonium chloride (VWR International GmbH), hydrochloric acid, 37% (p.a. EMSURE^®, Merck, Darmstadt, Germany), and tin(II) chloride (≤0.000001% Hg, p.a. EMSURE^®, Merck, Darmstadt, Germany). DOC-containing model solutions were prepared from dilution of Suwannee River Humic Acid Standard III (3S101H, International Humic Substance Society) and Suwannee River Fulvic Acid Standard III (3S101F, International Humic Substance Society). For synthesis of the thin films, ethanol (Merck, absolute for analysis), Pluronic^® F-127 (Sigma-Aldrich, Darmstadt, Germany), and titanium tetrachloride (Merck) were applied. Moreover, demineralized water and synthetic air gas mixture (20.5 Vol-% oxygen, rest nitrogen) were used.

Throughout this study, two ICP-MS multi-elemental solutions were used for the preparation of calibration standards: Standard stock A contained 100 mg/L (Ag, Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, Pb, Sb, Se, Si, Ti, Tl, V and Zn) (SPC Science, Quebec, Canada) and standard stock B contained 10 mg/L (Au, Ir, Pd, Pt, Os, Rh, and Ru) (CPI International, Santa Rosa, CA, USA). A freshly prepared acid mix of 0.1% HCl (37% fuming, EMSURE, Merck KGaA, Darmstadt, Germany) and 0.65% HNO₃ (EMSURE, Merck KGaA, Darmstadt, Germany) prepared in ultrapure water was used for dilution of the calibration standards. Calibration standards were prepared daily in the concentration ranges of 0–20.00 μg/L for standard A and 0–2.00 μg/L for standard B. To all calibration standards, 50.00 μL 1000 μg/L Y and In was added, and the calibration standards were diluted with the acid mixture to a final volume of 10.00 mL (5.00 μg/L internal standard). A 7-point standard curve was applied for both calibration standards.

Zinc 1000 mg/L stock solution (Centipur ®), analytical grade 65% HNO₃ (EMSURE ®), and analytical grade absolute ethanol (EMSURE ®) were purchased from Merck (Darmstadt, Germany). ZnO raw material (UV-Cut-ZnO-61-DM™) was obtained from Grant Industries (New Jersey, USA). Lipsticks, liquid foundations, and cream emulsions were obtained from Belcorp (Lima, Perú). Deionized water (resistivity ≥ 18 MΩ ∗ cm) was prepared with a Barnstead™ Easy Pure™ II water purification system.

Antibiotic standards, Disodium EDTA (OmniPur®), Oxalic acid (ReagentPlus®), Sodium sulphate (≥99%), n-hexane (Emsure®), Dichloromethane (LiChrosolv®), Methanol (LiChrosolv®) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Acetonitrile (≥99.9%) was sourced from Fisher Scientific, Loughborough, Leicestershire, while the SPE columns (200 mg x8 mL tubes) were purchased from Supelco™, Bellefonte, USA. The 0.45 μm GHP ACRODISC 13 mm disposable syringe filter unit was purchased from Agilent, whereas the deionised water (Ultrapure water) used in this study was produced using an Integral 10 Elix Milli-Q system with an LC (Biopak) polisher (Massachusetts, USA). Other apparatuses used include the Waring laboratory blender – Z272205 (Sigma-Aldrich, St. Louis, MO, USA), and the Vortex mixer VM18 (Schiltern Scientific, Beds, UK).

Menthol and capsaicin were selected to evaluate sensitivity to cooling and pungent sensations, respectively. The menthol and capsaicin solutions were made from single stock solutions. L-menthol (≥ 99%, Sigma-Aldrich, Steimheim, Germany) and natural capsaicin (#360376, Sigma-Aldrich, Steimheim, Germany) were first dissolved in 96% ethanol (EMSURE®, Sigma-Aldrich, Steimheim, Germany). These stocks were diluted with water to reach the final concentrations (Table 1) and supplemented with ethanol to standardise all stimuli to equal ethanol concentration of 0.5% (v/v) for menthol and 0.1% (v/v) for capsaicin, as ethanol may also elicit chemesthetic stimulation. The preparation procedure including the concentrations referred to a previous study [26 (link)] followed by a series of pilot tests.Table 1

Sample series for chemesthetic sensitivity test with menthol and capsaicin solutions

Chemesthetic modality	Chemical compound	Concentrations (ppm)
		Low	Medium	High
Cooling	Menthol	7.8	31.3	125.0
Pungency/spiciness	Capsaicin	0.1	1.0	10.0

Blood was drawn using 21G × ¾″ ×  7″ BD Safety-Lok^tm vacutainers (Becton, Dickinson and Company, USA) in 8.5 ml BD Vacutainer® SST™ II Advance Tubes (BD, USA). Blood was allowed to cloth for 30 min at RT, whereafter tubes were centrifuged for 10 min at RT at 1000×g.
To activate serum components (performed according to [7 (link)]), 30 μL serum was added to 200 μL DMEM (Gibco) with additional 0.1% w/v Bovine Serum Albumin (BSA) (Sigma-Aldrich). Subsequently, 10 μL 4 M HCl (Merck) was added, and samples were incubated on a roller for 1 h at 4 °C. Hereafter, acid was neutralized using 10 μL 4 M NaOH (Emsure®, Sigma-Aldrich). Non-acidified sera were treated by simultaneously adding 10 μL 4 M HCl and 10 μL 4 M NaOH. Acidified sera were used the very same day.