Determination of total lead in lipstick: Development and validation of a microwave-assisted digestion, inductively coupled plasma&mdash;mass spectrometric method

Nancy M. Hepp; William R. Mindak; John Cheng

DETERMINATION OF TOTAL LEAD IN LIPSTICK 409 lipstick samples and no variation in ICP–MS values for lipstick portion sizes ranging from 0.1 to 0.4 g (see Figure 1). The fi nal method parameters are outlined in Table II. For each lipstick sample, duplicate portions and portions fortifi ed at 0.5 and 1.0 μg Pb/g were digested. A Pb solution in 1% HNO3 was used for fortifi cation. Each digestion batch of 12 vessels also included a blank, a blank + 0.02 μg Pb/g, an organic Pb RM (0.100 μg Pb/g in base oil 20), and an RM with mineral content (estuarine stediment, SRM 1646a). RESULTS METHOD VALIDATION Accuracy of the method was demonstrated by measuring recoveries of Pb from RMs and from fortifi ed lipstick samples. Since no lipstick-type RM was available, several RMs were analyzed representing varying matrix types. NIST 1635 trace elements in coal (complex matrix, organics) NIST 8435 whole milk powder (high fat matrix) NIST 1084a wear metals in oil and SPEX ORG-PB8-2Y/Z lead in base oil 20 (oily matrices Figure 1. Portion size effect before and after adding excess boric acid. Table II Final Method Parameters ● 0.3-g lipstick portion ● 2 ml HF + 7 ml HNO3 ● CEM XP-1500+ vessels, MARS Microwave Digestion System ● Heat in two steps to 200°C, hold for 30 min: Stage Power (watts) Ramp (min) Pressure (psi) Temperature (°C) Hold (min) 1 1200 15:00 0800 130 3:00 2 1200 15:00 0800 200 30:00 ● Cool to 50°C, vent, add 30 ml 4% boric acid ● Heat to 180°C, hold for 10 min ● Dilute to 200 ml

JOURNAL OF COSMETIC SCIENCE 410 containing organically complexed Pb) and NIST 1646a estuarine sediment (refractory mineral matrix). Pb recoveries from RMs with and without HF are shown in Table III. Complete recovery of Pb from the RMs was obtained by simple HNO3 digestion, with the exception of estuarine sediment, for which complete recovery was obtained only when HF was used in the digestion. Each lipstick sample was fortifi ed at two levels and analyzed following HNO3/HF digestion, with recoveries averaging 98.1%. Absence of matrix infl u- ence was shown by sequentially diluting several analytical solutions, with no signifi cant differences. Analytical solution stability was demonstrated by analysis of three representative analytical solutions over time. Two analytical solutions containing approximately 0.25 and 1.0 μg Pb/l were analyzed on days 1, 3, 7, and 14 using freshly prepared standard solutions on each day. There was 5% variation over the time period. A 10-μg Pb/l standard solution prepared on day 1 and analyzed with the analytical solutions on subsequent days behaved similarly. Method precision was demonstrated by between-day and within-day repeatability experiments. A 3% relative percent difference (RPD) was observed from analyzing 22 portions of lipstick composite over three days, and 2% RPD was obtained from analyzing 12 portions of one lipstick brand over three days. The precision of the instrument was tested by analyzing an analytical solution seven times on one day, yielding 2% RPD. The ruggedness of the method was demonstrated by varying the analytical parameters. There were no signfi cant differences in Pb results with portion size variations of 0.1 to 0.4 g. The volume of HF was varied from 0 to 4 ml (0, 0.5, 1.0, 2.0, 3.0, and 4.0 ml). Pb recov- ery became constant when ≥1 ml was used. The 4% boric acid solution amount was var- ied from 6 ml to 60 ml (6, 20, 30, 40, and 60 ml), with Pb recovery becoming constant when ≥20 ml was used. Solutions also became clear, eliminating the need for fi ltration or centrifugation before ICP–MS analysis. There was no signifi cant change in Pb recovery when the maximum disgestion temperature was lowered from 200°C to 180°C. As a measure of quality control, each digestion batch included two RMs: lead in base oil 20 (representing organic Pb in an oily matrix) and estuarine sediment (representing a mineral matrix). Recovery of Pb from the RMs is shown in Figure 2. Method blanks and method blanks spiked near the detection level were also included in each digestion batch. The average and standard deviation for blanks from fi fteen batches Table III Lead Recoveries from Reference Materials and Lead Values from a Composited Lipstick With and Without HF* Certifi ed value (μg Pb/g) ±95% C.I. HNO3 only (μg Pb/g) HF + HNO3 (μg Pg/g) NIST 1635 (trace elements in coal) 1.9 0.2 1.8 (95%) 1.8 (95%) NIST 1084a (wear metals in oil) 101.1 1.3 103.7 (103%) SPEX ORG-PB8-2Y/Z (lead in base oil 20) 1000 1000 (100%) NIST 8435 (whole milk powder) 0.11 0.05 0.10 (91%) 0.11 (100%) NIST 1646a (estuarine sediment) 11.7 1.2 8.2 (70%) 10.8 (93%) Composited lipstick — — 0.29 2.91 *Values are the average of 3 to 15 samples. Recoveries are indicated in parentheses.

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JOURNAL OF COSMETIC SCIENCE 408 melting and mixing together eleven tubes of lipstick (same brand and shade, but several lot numbers). These lipsticks were placed in a beaker submerged in a water bath at 85°C and stirred with a propeller-type mixer. Initial attempts to completely dissolve a lipstick sample using typical microwave- assisted HNO3 digestion were unsuccessful. A cloudy, white suspension remained after the treatment. Also, replicate results for a single lot of lipstick showed variations in Pb recovery for different lipstick portion sizes and maximum digestion temperatures (see Table I). The portion size and temperature effects on Pb recovery suggested that microwave-as- sisted digestion with HNO3 was incomplete for Pb and that levels of Pb recovered would vary depending on digestion parameters. Therefore, other preparation procedures were investigated that might achieve total recovery of Pb. A dry ash procedure was tried in which a lipstick sample was heated gradually to 540°C and held at that temperature for an hour. The resulting residue was treated with HNO3 and HCl but would not com- pletely dissolve. The acid leachate was diluted with water and analyzed by ICP–MS. Results were variable and lower compared to values obtained with microwave-assisted acid diges- tion. Similarly, sodium carbonate fusion at 1075°C resulted in a residue that would not completely dissolve in HNO3 or HCl and produced variable values for Pb. The results indicated that some Pb must be associated with the undissolved refractory mineral matter in the lipsticks and suggested that HF would be necessary to break down the minerals. Based on work of other investigators (10), the initial HF dissolution procedure used 0.3-g portions of lipstick, 7 ml HNO3 + 2 ml HF, and sealed Tefl on microwave digestion ves- sels. A two-step procedure was used for digestion. The vessels were heated to 130°C over 15 minutes and held at that temperature for three minutes before ramping to 200°C over 15 minutes and holding for an additional 30 minutes. The vessels were allowed to cool to 50°C and then were vented. Initially 6 ml of 4% boric acid was then added to each vessel and the solutions were heated to 170°C over 15 minutes and held for ten minutes to complex the HF. Boron forms a strong complex with fl uoride according to the reaction shown in equation 1. After cooling and venting, the solutions were diluted to a fi nal vol- ume of 200 ml for ICP–MS analysis. However, the digests were still cloudy, with a ge- latinous precipitate appearing upon centrifugation. Also, results varied depending on the analytical portion. Equation 1 is as follows: 4HF + B(OH)3 → HBF4 + 3H2O (1) Therefore, an excess of boric acid was used to dissolve any insoluble fl uorides. The revised HF digestion procedure used 30 ml of 4% boric acid and resulted in clear solutions for all Table I Analytical Portion and Temperature Effects on Pb Determined After Digestion with Nitric Acid Portion size (g) Maximum digestion temperature (°C) μg Pb/g 0.3 240 1.4 0.1 240 2.1 0.3 200 0.5

DETERMINATION OF TOTAL LEAD IN LIPSTICK 409 lipstick samples and no variation in ICP–MS values for lipstick portion sizes ranging from 0.1 to 0.4 g (see Figure 1). The fi nal method parameters are outlined in Table II. For each lipstick sample, duplicate portions and portions fortifi ed at 0.5 and 1.0 μg Pb/g were digested. A Pb solution in 1% HNO3 was used for fortifi cation. Each digestion batch of 12 vessels also included a blank, a blank + 0.02 μg Pb/g, an organic Pb RM (0.100 μg Pb/g in base oil 20), and an RM with mineral content (estuarine stediment, SRM 1646a). RESULTS METHOD VALIDATION Accuracy of the method was demonstrated by measuring recoveries of Pb from RMs and from fortifi ed lipstick samples. Since no lipstick-type RM was available, several RMs were analyzed representing varying matrix types. NIST 1635 trace elements in coal (complex matrix, organics) NIST 8435 whole milk powder (high fat matrix) NIST 1084a wear metals in oil and SPEX ORG-PB8-2Y/Z lead in base oil 20 (oily matrices Figure 1. Portion size effect before and after adding excess boric acid. Table II Final Method Parameters ● 0.3-g lipstick portion ● 2 ml HF + 7 ml HNO3 ● CEM XP-1500+ vessels, MARS Microwave Digestion System ● Heat in two steps to 200°C, hold for 30 min: Stage Power (watts) Ramp (min) Pressure (psi) Temperature (°C) Hold (min) 1 1200 15:00 0800 130 3:00 2 1200 15:00 0800 200 30:00 ● Cool to 50°C, vent, add 30 ml 4% boric acid ● Heat to 180°C, hold for 10 min ● Dilute to 200 ml

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DETERMINATION OF TOTAL LEAD IN LIPSTICK 407 equipment and is subject to matrix absorption errors. AAS with electrothermal atomiza- tion analysis was also considered. However, since sample digestion would be necessary, inductively coupled plasma–mass spectrometry (ICP–MS) was chosen because of its po- tential for better sensitivity and speed. Thus, the effort focused on developing a method using microwave-assisted acid digestion for sample preparation and determination by ICP–MS. EXPERIMENTAL CHEMICALS, REAGENTS, AND REFERENCE MATERIALS Twenty shades of lipstick sold in the United States under ten brand names were pur- chased from retail stores or provided by manufacturers. Multiple samples with the same lot number were obtained for several shades in order to compare analytical techniques. Six RMs were used for comparing different preparation techniques and for quality assurance: wear-metals in lubricating oil, SRM 1084a (NIST, Gaithersburg, MD) whole milk pow- der, SRM 8435 (NIST) estuarine sediment, SRM 1646a (NIST) lead in base oil 20 standard, ORG-PB8-2Y/Z (SPEX CertiPrep, Inc., Metuchen, NJ) base oil 20 standard (SPEX CertiPrep, Inc.) and trace elements in coal, SRM 1632c (NIST). American Society for Testing and Materials (ASTM) type 1 grade water was used to pre- pare reagents, standards, and analytical solutions. Pb standards (0, 0.1, 0.5, 1.0, and 10 ng Pb/ml), Pb stock (0.1 and 1.0 μg Pb/ml), and thallium internal standard (0.1 μg Tl/ml) solutions were prepared from commercial ICP–MS grade single-element analyte solu- tions (High-Purity Standards, Charleston, SC). Trace metals grade (TMG) HNO3 (Fisher Scientifi c, Pittsburgh, PA) was used for cleaning laboratory ware and digestion vessel lin- ers. Optima grade HNO3 and HF (Fisher Scientifi c) were used for sample, stock, and standard solutions. Boric acid (Puratronic grade, Alfa Aesar, Ward Hill, MA) was used to prepare 4% boric acid solution, which was conveniently dispensed with a bottle-top dis- penser. A 0.100-μg Pb/g in base oil 20 stock solution was prepared from 1000 μg Pb/g (SPEX ORG-PB8-2Y/Z organo-metallic standard solution) serially diluted to 10.00 and then to 0.100 μg Pb/g with SPEX base oil 20. Lipsticks were digested using XP-1500 Plus vessels in a MARS microwave digestion oven (CEM Corp., Matthews, NC). Pb determinations were performed on an Agilent 7500c ICP–MS (Agilent Technologies, Inc., Santa Clara, CA) equipped with a Peltier cooled Scott double-pass spray chamber and a MicroMist nebulizer (Glass Expansion, West Melbourne, Victoria, Australia). The built-in peristaltic pump was used to deliver the analytical and thallium internal standard solutions to the nebulizer at 0.17 ml/min and at 0.01 ml/min, respectively. The analytical and internal standard solutions were merged with a Tee fi tting. METHOD DEVELOPMENT Lipstick is a challenging matrix of many ingredients including waxes, oils, dyes, and pigments (11). The pigments may include refractory minerals such as alumina, silica, titanium dioxide, and mica. Preliminary experiments with one lot of lipstick revealed an easily detectable amount of Pb, but quantitative results varied depending on preparation technique. In order to evaluate preparation techniques, a composite was prepared by

JOURNAL OF COSMETIC SCIENCE 408 melting and mixing together eleven tubes of lipstick (same brand and shade, but several lot numbers). These lipsticks were placed in a beaker submerged in a water bath at 85°C and stirred with a propeller-type mixer. Initial attempts to completely dissolve a lipstick sample using typical microwave- assisted HNO3 digestion were unsuccessful. A cloudy, white suspension remained after the treatment. Also, replicate results for a single lot of lipstick showed variations in Pb recovery for different lipstick portion sizes and maximum digestion temperatures (see Table I). The portion size and temperature effects on Pb recovery suggested that microwave-as- sisted digestion with HNO3 was incomplete for Pb and that levels of Pb recovered would vary depending on digestion parameters. Therefore, other preparation procedures were investigated that might achieve total recovery of Pb. A dry ash procedure was tried in which a lipstick sample was heated gradually to 540°C and held at that temperature for an hour. The resulting residue was treated with HNO3 and HCl but would not com- pletely dissolve. The acid leachate was diluted with water and analyzed by ICP–MS. Results were variable and lower compared to values obtained with microwave-assisted acid diges- tion. Similarly, sodium carbonate fusion at 1075°C resulted in a residue that would not completely dissolve in HNO3 or HCl and produced variable values for Pb. The results indicated that some Pb must be associated with the undissolved refractory mineral matter in the lipsticks and suggested that HF would be necessary to break down the minerals. Based on work of other investigators (10), the initial HF dissolution procedure used 0.3-g portions of lipstick, 7 ml HNO3 + 2 ml HF, and sealed Tefl on microwave digestion ves- sels. A two-step procedure was used for digestion. The vessels were heated to 130°C over 15 minutes and held at that temperature for three minutes before ramping to 200°C over 15 minutes and holding for an additional 30 minutes. The vessels were allowed to cool to 50°C and then were vented. Initially 6 ml of 4% boric acid was then added to each vessel and the solutions were heated to 170°C over 15 minutes and held for ten minutes to complex the HF. Boron forms a strong complex with fl uoride according to the reaction shown in equation 1. After cooling and venting, the solutions were diluted to a fi nal vol- ume of 200 ml for ICP–MS analysis. However, the digests were still cloudy, with a ge- latinous precipitate appearing upon centrifugation. Also, results varied depending on the analytical portion. Equation 1 is as follows: 4HF + B(OH)3 → HBF4 + 3H2O (1) Therefore, an excess of boric acid was used to dissolve any insoluble fl uorides. The revised HF digestion procedure used 30 ml of 4% boric acid and resulted in clear solutions for all Table I Analytical Portion and Temperature Effects on Pb Determined After Digestion with Nitric Acid Portion size (g) Maximum digestion temperature (°C) μg Pb/g 0.3 240 1.4 0.1 240 2.1 0.3 200 0.5

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DETERMINATION OF TOTAL LEAD IN LIPSTICK 411 are shown in Table IV, along with the aggregate RM results. Method blank results were used to estimate the detection limit of 0.04 μg Pb/g using equation 2 (12): Detection limit = (2 · t · σ · √(1 + 1/N)) (2) LIPSTICK SURVEY AND COMPARISON WITH VALUES BY OTHER METHODS Twenty-two lipstick samples (not including the composite), identifi ed by brand, shade, and lot number, were analyzed for Pb by the validated method. The results are summa- rized in Table V. All of the lipsticks contained detectable amounts of Pb, with values ranging from 0.09 to 3.06 μg/g and an average amount of 1.07 μg/g. Despite the limited size and color range of the survey samples (all were red shades), samples from a few manufacturers (A – C) appeared to contain the highest levels of lead. As stated above, recoveries from lipsticks fortifi ed with lead nitrate, Pb(NO3)2, and ana- lyzed by the validated method (using HNO3 and HF) averaged 98.1%. However, recov- eries from some lipsticks were equally good using digestion with HNO3 alone (see Table III). Good recoveries were observed for all but one RM (the mineral-containing estuarine sediment) using either technique. This suggests that Pb-containing minerals were pres- ent in some but not all lipsticks. Figure 2. Lead recoveries from reference materials. Table IV Mean Values and Standard Deviations for Reference Materials, Blank, and Composited Lipsticks Analyzed Repeatedly NIST 1646a [estuarine sediment (μg/g)] SPEX ORG-PB8-2Y/Z [lead in base oil 20 (μg/g)] Method blank (μg/g) Blank + 0.02 μg Pb/g (μg/g) Composite lipstick Value (n = 13) Ref. value Value (n = 15) Ref. value Value (n = 15) Value (n = 22) Average 10.84 11.7 ± 1.2 0.100 0.100 0.019 0.037 2.91 S.D. 0.36 — 0.007 — 0.008 0.009 0.09

JOURNAL OF COSMETIC SCIENCE 412 The method developed here is for total Pb. Our studies showed that variable amounts of Pb can be extracted depending upon experimental conditions such as analytical portion, acids used, temperature, decomposition procedure, etc. Consistent results could usually be obtained only by including HF in the digestion procedure. To illustrate this, fi ve lipstick lots and the composite were analyzed by digestion with HNO3 alone or with HNO3 + HF. Substantially higher values were obtained for three of the lots and the composite using HNO3 + HF compared to HNO3 alone. Pb values for two different lots of the same lipstick were equivalent by digestion with HNO3 alone or with HNO3 + HF (see C-4, lots a and b). Results reported by the Campaign for Safe Cosmetics (CSC), in which several of the same brands and shades of lipsticks were analyzed (2), are also listed in Table V. The CSC method used a 0.5-g portion, extraction with sulfuric and nitric acids, and ICP–MS analysis. Lot numbers were not reported by the CSC. Differences among values obtained by the three digestion techniques can be explained by the presence of mineral content in some formulations, as well as by the fact that Pb is easily precipitated as a sulfate after sulfuric acid is used in the CSC extraction. Mica, a mineral permitted as a color additive, which frequently contains small amounts of Pb (13), would require HF for complete dissolution. The use of sulfuric acid in the CSC ex- Table V Lead Content in Lipsticks by Validated Method and by Two Other Techniques Product Shade Lot FDA: HF + HNO3 (μg Pb/g) FDA: HNO3 (μg Pb/g) CSC (μg Pb/g) (lot #s unknown) A 1 a 1.40 b 1.20, 1.22 0.12, 0.56 c 3.06 1.90 d 3.05 B 1 Composite 2.91 0.29 0.02 a 2.38 A 2 a 2.24 0.03, 0.03 C 1 a 1.79 0.02, 0.02, 0.06 A 3 a 1.76 0.28 C 2 a 1.53 b 0.62, 0.68 0.50, 0.65 c 1.47 1.20 B 2 a 1.37 0.91 0.02 b 0.83, 0.81 C 3 a 1.21 0.19 B 3 a 1.04 0.02, 0.03 C 4 a 0.67 0.60 0.58 b 0.79 0.74, 0.80 D 1 a 0.55 0.02 D 3 a 0.48 0.03 D 2 a 0.43 0.02 E 1 a 0.33 0.09 C 5 a 0.23 0.12 F 1 a 0.17 0.12 G 1 a 0.15 0.02, 0.04 H 1 a 0.12 0.21 I 1 a 0.10 0.02, 0.03 J 1 a 0.09 0.02

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