Analysis of Nigella sativa L. (Black Cumin) seeds for levels of heavy metals using FAAS: geospatial profiling and regional safety implications

Overview

Bekalu et al. quantified Fe, Cu, Zn, Pb, and Cd in Nigella sativa L. black cumin seeds collected from three districts in East Gojjam, Ethiopia. The study used microwave-assisted acid digestion followed by flame atomic absorption spectrometry. Lead ranged from 0.11 +/- 0.005 to 0.21 +/- 0.01 mg/kg dry weight and cadmium ranged from 0.09 +/- 0.01 to 0.14 +/- 0.007 mg/kg dry weight across the three districts. All Pb and Cd values were below the WHO herbal-material limits cited by the authors, while Fe, Cu, Zn, and Cd showed significant regional variation.

Key numbers

District mean concentrations

Table 3 reports mean +/- SD in mg/kg dry weight for black cumin seed samples.

Source-stated ranges and statistical findings

• Fe: 71.38 +/- 1.34 to 82.90 +/- 0.75 mg/kg.
• Cu: 9.18 +/- 0.22 to 22.24 +/- 2.03 mg/kg by Table 3.
• Zn: 55.27 +/- 0.33 to 71.25 +/- 2.79 mg/kg.
• Pb: 0.11 +/- 0.005 to 0.21 +/- 0.01 mg/kg.
• Cd: 0.09 +/- 0.01 to 0.14 +/- 0.007 mg/kg.
• One-way ANOVA found significant regional differences for Fe, Cu, Zn, and Cd, while Pb was not significantly different across sampling sites.
• The WHO herbal-material limits cited by the source are Pb 10 mg/kg and Cd 0.3 mg/kg; all district means are below those limits.

Methods (brief)

The study sampled Nigella sativa seeds from Dejen, Gozamen, and Sinan districts in East Gojjam, Ethiopia. Samples were subjected to microwave-assisted acid digestion and analyzed by flame atomic absorption spectrometry. Calibration standards for Fe, Cu, Zn, Pb, and Cd were prepared from 1 g/L reference solutions. Calibration R2 values were Fe 0.9991, Cu 0.9982, Zn 0.9987, Pb 0.9987, and Cd 0.9983. Spike recoveries were 94.8-108% across the measured metals. The paper reports LOD/LOQ values in Table 1, including Cd LOD 0.045 mg/kg and LOQ 0.15 mg/kg.

The matrix is black cumin seed on a dry-weight basis. The measured toxic metals are total Pb and total Cd; no arsenic, mercury, chromium, tin, aluminum, uranium, or antimony values are measured.

Implications

Certification: This source provides Ethiopia-market black cumin seed occurrence values for Pb and Cd on a dry-weight basis. It is useful as geographic-context evidence for cumin/black-cumin seed contamination, but it should not be silently pooled with US-market spice data without a market-basis decision.

Courses: Useful for teaching geospatial variance in spice and medicinal-seed contamination: Pb is highest in Dejen, Cd is highest in Dejen by Table 3, and Cu/Zn are higher in Sinan and Gozamen.

App: Adds category-level black-cumin seed values for East Gojjam, Ethiopia. The values are not brand-specific and should be treated as regional ingredient evidence.

Microbiome: Not addressed.

Wiki pages this source may touch

• Lead
• Cadmium
• Iron
• Copper
• Zinc
• Cumin
• Spices
• Herbs And Spices
• Spices
• Condiments (hot sauce, soy sauce, mustard, ketchup, mayonnaise, salsa)

Verification notes

• The PDF prints DOI 10.1007/s44371-025-00300-1, journal Discover Chemistry, and article metadata for Bekalu et al. (2025) 2:223. The PDF states open access under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
• matrices: [spices] is used as the broad routed matrix for black cumin seeds because the source describes Nigella sativa as a culinary and medicinal seed. Dry-weight basis is documented in Key numbers and Methods rather than in frontmatter matrices.
• Table 3 prints Sinan Cu as 22.24 +/- 2.03 mg/kg, and the abstract repeats the Cu range as 9.18 +/- 0.22 to 22.24 +/- 2.03. Some discussion passages later state 18.24 +/- 2.03 for Sinan Cu. This page follows Table 3 and the abstract, and treats the 18.24 value as a paper-side text inconsistency.
• The source text contains one apparent decimal typo in discussion, describing Cd as 1.2 +/- 0.04 in Gozamen. Table 3 gives 0.12 +/- 0.004 mg/kg, which is also consistent with the abstract range and with the cited WHO Cd limit comparison. This page uses Table 3.
• Table 1 gives Pb calibration R2 as 0.9987, while the discussion text describes Pb R2 as 0.9889. This page follows Table 1 for the calibration summary and records the discussion value as a source-internal inconsistency.
• Table 1 also prints Pb LOQ as 0.015 mg/kg, below the Pb LOD of 0.016 mg/kg. The Methods section therefore quotes Cd LOD/LOQ and avoids using the anomalous Pb LOQ as a quality-control anchor.
• The paper describes 81 samples overall and reports district means as mean +/- SD with n=3 in Table 3. The frontmatter sample_n captures the overall sample count; the table note captures the district-mean reporting basis.

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