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Unmarked glass laboratory vessels, a closed notebook, sampling spoon and empty specimen dish illustrating Zierold and colleagues 2025 source research context.

Distinct Features of Proton-Induced X-ray Emission (PIXE) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) When Used for Elemental Analysis of Nail Samples

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This study compared Proton-Induced X-ray Emission (PIXE) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for quantifying 27 elements in nail samples from 235 children living near coal-fired power plants in Kentucky. PIXE and ICP-MS showed strong concordance for calcium, copper, potassium, and zinc, but stark differences for iron, magnesium, and aluminum (PIXE-measured Fe and Mg were approximately twice ICP-MS values; PIXE aluminum (Al) was approximately seven times ICP-MS Al). Both methods are non-destructive or minimally destructive options for long-term metal exposure profiling from nails, which represent 6-12 months of cumulative exposure.

Key numbers

  • Elemental concentrations by PIXE (Q1, median, Q3; in micrograms per gram, converted from microg/m3 basis; Table 2):
    • Ca: Q1 622, median 762, Q3 920 (PIXE); Q1 622, median 752, Q3 937 (ICP-MS) — good concordance
    • Cu: Q1 3.70, median 5.04, Q3 6.59 (PIXE); Q1 4.31, median 5.18, Q3 6.95 (ICP-MS) — good concordance
    • Fe: Q1 48.35, median 67.88, Q3 107.50 (PIXE); Q1 20.41, median 31.51, Q3 46.12 (ICP-MS) — stark difference (PIXE approx 2x ICP-MS)
    • K: Q1 481, median 784, Q3 1190 (PIXE); Q1 486, median 794, Q3 1203 (ICP-MS) — good concordance
    • Zn: Q1 75.96, median 86.00, Q3 97.16 (PIXE); Q1 88.70, median 101.07, Q3 112.82 (ICP-MS) — reasonable concordance
    • Mg: Q1 212, median 255, Q3 308 (PIXE); Q1 98.41, median 115.52, Q3 140.03 (ICP-MS) — stark difference (PIXE approx 2x ICP-MS)
    • Al: Q1 139, median 205.60 (PIXE); Q1 13.17, median 19.48, Q3 29.61 (ICP-MS) — stark difference (PIXE approx 7x ICP-MS)
    • nickel (Ni): Q1 less than 0.41, median 1.45, Q3 2.60 (PIXE, 69% above LOD); Q1 0.78, median 1.40, Q3 2.84 (ICP-MS) — borderline agreement
    • lead (Pb): 1.70% of PIXE measurements above LOD (starred in table); Q1 0.27, median 0.47, Q3 0.84 (ICP-MS)
    • total arsenic (tAs): 6.81% above PIXE LOD (starred); Q1 0.07, median 0.11, Q3 0.21 (ICP-MS)
    • total mercury (tHg): 0.85% above PIXE LOD (starred); Q1 0.01, median 0.02, Q3 0.04 (ICP-MS)
    • cadmium (Cd): 0% above PIXE LOD; Q1 0.04, median 0.07 (ICP-MS)
    • tin (Sn): 8.09% above PIXE LOD; Q1 0.22, median 0.38, Q3 0.77 (ICP-MS)
  • Lin’s concordance correlation coefficient (CCC) results (Table 3): Ca, Cu, K, Zn showed strong concordance; Ni borderline; Fe, Mg, Al showed poor CCC (structural differences between methods for these elements, possibly due to surface contamination detected by PIXE but not ICP-MS, which measures after acid digestion).
  • PIXE detected 17 elements at greater than 25% of samples above LOD. ICP-MS detected 27 elements including Cd, Co, Sb, Ba, V, uranium (U) not measurable by PIXE.

Methods (brief)

Children’s nail samples collected from 235 children aged 6-14 in Jefferson and Bullitt Counties, Kentucky. PIXE analysis (Elemental Analysis Inc., Lexington KY): 3 million electron volt (MeV) proton beam, cryogenically frozen samples mixed with neutral binding agent and pressed into 5/8-inch pellets; analyzed using GUPIX software. ICP-MS analysis (Agilent 7800; children’s samples stored in plastic containers): washed with deionized water and dried; digested in 65 degrees C bath for 4 hours in 70 microL HNO3 (trace grade, Fisher Scientific A415-500) then diluted to 1 mL; additional 30 microL H2O2 added; final 50 mL volume via MILQ water. R statistical software 4.3.2; reverse Kaplan-Meier for left-censored values; CCC and Pearson correlation computed on log-transformed data.

Evidence fitness

This paper is a methods comparison study for nail biomonitoring analytical techniques. It does not measure metals in food or personal care products. It contributes to the wiki’s knowledge base on toenail and fingernail biomonitoring methodology, specifically the analytical differences between PIXE and ICP-MS for different elements. The finding of poor concordance for Fe, Mg, and Al between the two methods is relevant to any synthesis of nail biomonitoring data that pools across analytical platforms.

Limitations

PIXE cannot measure Cd, Co, Sb, Ba, V, or U reliably in nails (less than 2% above LOD for these elements). The divergence for Fe, Mg, and Al between PIXE and ICP-MS is not fully explained; surface contamination from the nail cleaning process or from the nail pellet preparation may contribute. Children living near coal-fired power plants are not a general population; results may not be representative.

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