Determination of heavy metals on the non-woven in wet wipes using ICP-MS

Choi, Song, and Kim digested the nonwoven sheet from 24 wet wipes and measured arsenic, cadmium, mercury, nickel, lead, antimony, and thallium. Antimony was the frequent detect. Cadmium, mercury, and thallium were not detected in any wet wipe. Results are means of three preparations, in micrograms per liter of the digest solution. The paper also reports eight mask packs. Those are a different product.

Key numbers

The unit on Tables 4, 5, and 6 is µg/L. A dash is not detected. The mass taken was approximately 0.2 g. The digest was diluted with 25 mL of water. The paper does not say the solution was made up to 25 mL, and it does not state a final volume after the 8 mL of acid. The printed result stays in µg/L. A reconstruction in mg/kg is shown below and is not a number from the paper.

Limits of detection and quantification, Table 4, ICP-MS unless noted: arsenic 0.02 and 0.07 µg/L; cadmium, lead, antimony, and thallium 0.01 and 0.03 µg/L; nickel 0.11 and 0.33 µg/L. Mercury, on the mercury analyzer, is 0.025 and 0.80 µg/L in the table. The methods paragraph prints the mercury quantification limit as 0.08 µg/L and the arsenic quantification limit as 007. The table is the figure used here.

Wet-wipe detects, Table 5. Codes are the paper’s. Uses and fibers are the paper’s labels.

Counts on the 24 wet-wipe sheets: antimony in 12, lead in 6, arsenic in 3 (all three United States sheets), nickel in 2. Cadmium, mercury, and thallium: none detected. The other sheets in Table 5 are dashes for every element.

Mask packs are Table 6, not baby wipes. Combined sheet-plus-liquid nickel was 103.52 ± 2.02 µg/L (code GA) and 53.69 ± 1.45 µg/L (code IT). For code GA, antimony was 8,895.28 ± 42.86 µg/L in the nonwoven and 38.95 ± 1.86 µg/L in the liquid. The authors did not publish that split for the wet wipes.

Methods (brief)

Sheets were bought as 21 domestic wet wipes and 3 from the United States, plus 4 domestic and 4 international mask packs. Microwave digestion used a Milestone Start D: about 0.2 g, 7 mL of 70 percent nitric acid, 1 mL of 50 percent hydrofluoric acid, then dilution with 25 mL of ASTM type 1 water. Antimony, arsenic, cadmium, lead, nickel, and thallium were measured on a PerkinElmer NexION 300X ICP-MS. Mercury was a separate Milestone DMA-80 analysis with no digestion. Calibration standards were 0, 10, 50, and 100 µg/L for the ICP-MS elements and 0, 2.5, 5, and 10 µg/L for mercury. Reported R-squared values were 0.9992 to 0.9999. Each reported value is a mean of three preparations.

The panel does not include tin, aluminum, or chromium. Arsenic is not speciated. Mercury is total mercury from the analyzer, not methylmercury. Antimony is not speciated.

Reconstruction, not a printed result

Section 2.2 says: “Approximately 0.2 g of sample was added into a PTFE vessel containing 7 mL of 70% HNO3 and 1 mL of 50% HF.” After microwave digestion, “The samples were then transferred and diluted with 25 mL of ASTM type 1 water.” That is dilution with 25 mL. It is not a statement that the flask was brought to 25 mL. The mass is the word “approximately,” not a weight recorded for each code. Mercury was measured on a DMA-80 with no digestion, so this arithmetic does not apply to mercury, or to the cadmium and thallium non-detects.

If a reader treats the final volume as exactly 25 mL (0.025 L) and the mass as exactly 0.200 g (0.0002 kg), the content estimate is:

mg/kg = C(µg/L) × 0.025 / 0.200 = C × 0.125.

Worked example, code ML, the rayon-only infant sheet: 19,558.35 × 0.125 = 2,444.79375, which rounds to 2,444.79 mg/kg. If the 8 mL of acid is assumed to remain and the 25 mL of water is added to it, the volume is 33 mL and the factor is 0.033 / 0.200 = 0.165. The same sheet is then 19,558.35 × 0.165 = 3,227.12775, or 3,227.13 mg/kg. Neither figure is printed. The table keeps the fiber label on the row. Detects only are shown. A dash stays a dash.

Washington Ecology measured 40.6 to 70.6 ppm antimony in three wipe components. Under the 25 mL assumption, the rayon-only reconstruction (2,444.79 mg/kg) is about 35 times the highest of those components and about 60 times the lowest. The lowest antimony reconstruction, code MA at 23.48 mg/kg, sits below that Washington range. The two bases are still not the same measurement. The reconstruction is not used as the paper’s result, and it is not placed next to a cosmetic limit as if the paper had reported milligrams per kilogram.

Evidence fitness

The table supports a statement about these 24 sheets in digest-solution units, with the fiber words the authors printed. It supports the authors’ hypothesis that antimony trioxide can be a polyester catalyst, as a hypothesis. It does not make that hypothesis a rule: the highest antimony result is the rayon-only sheet, and several polyester-labeled sheets had none detected.

It does not support a milligrams-per-kilogram content as a printed result. The reconstruction above is labeled as a volume assumption. It does not support a comparison with a cosmetic impurity limit in ppm or µg/g, a dermal dose, or a share of the market. Mask-pack rows stay out of baby-wipe tallies.

Limitations

The set is not a random sample of wipes on sale. Brand names are not in the table. The United States rows are three sheets. A printed dash is not detected relative to the Table 4 limits; it is not a formal statement that the element is absent from the product class. The mercury quantification limit disagrees between the methods paragraph and Table 4. Mass is approximate, and the final volume is not explicit. The milligrams-per-kilogram columns are a reconstruction under two stated volume assumptions. They are not the result the paper printed.

Related evidence

• Baby wipes
• Heavy metals in baby wipes
• Antimony
• Nickel
• Lead
• Metals in Children's and Consumer Products and Packaging

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