Determining factors affecting the accuracy of SEM-EDX data-based quantitative chemical analysis for identifying naturally occurring individual carcinogenic erionite fibers

Our plain-English summary of Determining factors affecting the accuracy of SEM-EDX data-based quantitative chemical analysis for identifying naturally occurring individual carcinogenic erionite fibers, by W.W. Fan, A.F. Gualtieri, A. Hamilton, et al (Scientific Reports, 2025).

The study

• Authors: W.W. Fan, A.F. Gualtieri, A. Hamilton, J.P. Patel, J.A. Salmond
• Journal: Scientific Reports
• Published: 2025;15(1):25316
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Erionite is a fibrous zeolite classified as a Group 1 carcinogen and may pose a significant public health hazard when aerosolized into individual respirable-sized fibers. SEM-EDXA has been widely used to identify individual erionite fibers in environmental samples as it combines morphological imaging with elemental analysis. However, the reliability of SEM-EDXA data when applied to the quantitative chemical analysis of individual erionite fibers rather than bulk samples remains uncertain. This study analyzed 325 individual erionite fibers (obtained from a bulk sample with high-purity) across a range of fiber widths and four commonly used sample preparation methods, using two different SEM-EDS systems. SEM-EDXA results were compared with previously acquired EPMA reference data from the bulk sample to assess analytical accuracy. Systematic overestimations of Si and Mg and underestimations of Al, K, and Ca were observed. Framework elements (Si and Al) exhibited relatively stable detection in fibers > 0.5 μm. However, preparation methods-deionized water dispersion and hydrogen peroxide digestion-introduced greater variability, likely due to ion exchange and cation mobilization. Despite the purity of the erionite bulk sample, none of the analyzed individual fibers fully met the established quantitative chemical criteria for erionite identification. These results highlight the need for pre-calibration with erionite standards and the application of correction index to improve SEM-EDX accuracy, and subsequent confirmation of mineralogy using TEM-SAED, when determining whether individual fibers found in airborne samples are erionite.

© 2025. The Author(s).

In plain English

This laboratory study tested how accurately scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX) measures the chemistry of single erionite fibers. Erionite is a fibrous zeolite mineral, and SEM-EDX is used to identify fibers in air and other environmental samples. The authors ground a high-purity erionite sample that had already been analyzed by electron probe microanalysis (EPMA). They prepared fibers four ways: dry spreading, deionized water dispersion, and two hydrogen peroxide digestions. They then analyzed 325 fibers on two SEM-EDX instruments. Compared with the EPMA values, silicon was overestimated (about 13% on the older instrument, 2% on the newer) and aluminum underestimated (32% and 4%). Potassium and calcium were detected at lower levels. Among the 94 fibers narrower than 0.5 μm, none showed sodium. Water dispersion and peroxide digestion added variability. None of the 325 fibers met all published chemical criteria for identifying erionite. The authors concluded that SEM-EDX remains viable for preliminary identification, especially for fibers wider than 0.5 μm. They added that it needs calibration with erionite standards, correction factors, and confirmation by TEM-SAED.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory analytical-methods study comparing SEM-EDX measurements of individual fibers with previously obtained EPMA reference data for the same bulk erionite sample
• Who was studied: 325 individual erionite fibers ground from one high-purity volcanic erionite bulk sample (about 2 wt% impurities, smectite and illite; suggested to be erionite-K). 173 fibers (width 0.96 plus or minus 0.83 μm) were analyzed on SEM-EDS.1 and 152 fibers (width 0.75 plus or minus 0.71 μm) on SEM-EDS.2. No people were studied.
• Exposure: Not a human exposure study. The factors tested were fiber width, four sample preparation methods (dry and air dispersed on carbon tape; deionized water dispersion and filtration; hydrogen peroxide digestion of bulk sample; hydrogen peroxide digestion of bulk sample plus roadside dust from shrub twigs), and two SEM-EDS instruments (an older Si(Li) detector and a newer silicon drift detector), both run at 15 kV with 60 s acquisition.
• Main results:
  • Compared with the EPMA reference (Si 28.43 wt%, Al 7.56 wt%), silicon was overestimated by about 13% on SEM-EDS.1 and 2% on SEM-EDS.2. Aluminum was underestimated by 32% on SEM-EDS.1 and 4% on SEM-EDS.2.
  • Potassium and calcium were consistently detected below their EPMA values (K 2.63 wt%, Ca 1.27 wt%). Magnesium and sodium appeared at higher concentrations than EPMA.
  • Potassium measured 51.8% to 73.1% of the EPMA reference on SEM-EDS.1 and 53% to 70% on SEM-EDS.2. Underestimation was more pronounced in hydrogen peroxide-digested fibers.
  • Among the 94 fibers narrower than 0.5 μm (both systems), only 54% (n = 51) showed potassium, 40% (n = 38) calcium, 41% (n = 39) magnesium, and none sodium.
  • The authors reported that results from both systems were more homogeneous for fibers wider than 0.5 μm, for all elements except sodium.
  • Dry and air-dispersed fibers gave the most stable results. Fibers prepared by deionized water dispersion or hydrogen peroxide digestion showed more scatter and lower detection of extra-framework cations, particularly potassium and magnesium.
  • None of the 325 fibers met all the published chemical criteria for identifying erionite.
  • Results for individual criteria:
• Si + Al approximately 36: 87% of fibers (282 of 325).
• Tsi ratio in the erionite range: 13% (43 fibers), rising to 56% after correction by the instrument intercept values.
• Mg below 0.8: 29% (93 fibers).
• Mg/(Ca + Na) below 0.15: no fibers.
  • For the charge-balance criterion, 9% of fibers (n = 28) fell within plus or minus 10%, and 24% (n = 78) within plus or minus 20%. No fiber met both the Mg below 0.8 and plus or minus 20% charge-balance criteria.
  • The newer SEM-EDS.2 was closer to the EPMA values for silicon and aluminum. The older SEM-EDS.1 was closer for potassium, calcium and magnesium in terms of regression intercepts.
• The authors' conclusion: The authors concluded that variation from small fiber size, instrument settings and sample preparation limits SEM-EDX for reliable quantitative chemical analysis of single erionite fibers. They judged that relatively stable silicon and aluminum detection makes it a viable tool for preliminary identification, particularly for fibers wider than 0.5 μm. They called for pre-calibration with erionite standards from the same locality and systematic correction factors, and the abstract adds confirmation by TEM-SAED.
• Limitations the authors state:
  • Natural chemical variation within the erionite sample may have affected the results.
  • The uneven surface geometry of natural fibers may explain some variation.
  • The effect of the sample coating was not assessed separately but was grouped with instrument effects.
  • Residual impurities (2%) in the sample may have introduced minor deviations.
  • The proposed correction index applies only to the specific sample and instrument.

Topics

Medical terms in this study, explained in our Lexicon: Carcinogenic, erionite, Zeolite, Elemental Analysis, Cation, Mineralogy, Selected Area Electron Diffraction (SAED).

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