Diviš et al. 2020 — Total mercury in fish sauces by DGT-TDA-AAS
This methods-development and application paper validates a Diffusive Gradients in Thin Films (DGT) preconcentration approach combined with thermal decomposition gold amalgamation AAS (TDA-AAS) for total dissolved mercury measurement in fish sauces — a complex, high-salt matrix that otherwise corrodes instrument components. The paper then applies the validated method to 10 commercial fish sauces from Vietnam and Thailand. Total Hg concentrations ranged from 0.8 to 42.8 µg/kg across the ten samples, well below the EU maximum of 500 µg/kg for fish products. The DGT preconcentration step achieved a ten-fold lower LOD than direct TDA-AAS analysis and was unaffected by the high NaCl content (up to 50 g/L) or the pH range (3–6) typical of fish sauce.
Mercury speciation note: the study measures total dissolved Hg only. It does not speciate methylmercury (MeHg). The cited reference Reichstädter et al. 2020 (Talanta 217, 121059) examines simultaneous Hg, Cd, and Pb determination in fish sauce by DGT and is a companion paper; this paper focuses on Hg and method validation.
Key numbers
Total Hg concentrations in undiluted fish sauce (µg/kg = ppb), n = 10 samples:
| Sample | Origin | Anchovy % | tHg (µg/kg) |
|---|---|---|---|
| FS1 | Vietnam | 95% | 4.1 |
| FS2 | Vietnam | 97% | 1.3 |
| FS3 | Vietnam | 70% | 1.8 |
| FS4 | Thailand | 70% | 1.1 |
| FS5 | Thailand | 77% | 1.5 |
| FS6 | Thailand | 55% | 0.8 |
| FS7 | Thailand | 63% | 26 |
| FS8 | Thailand | n.a. | 4.9 |
| FS9 | Thailand | 70% | 29 |
| FS10 | Thailand | 70% | 43 |
Range: 0.8–42.8 µg/kg. EU maximum for fish products: 500 µg/kg. All samples well below this limit. The three highest-Hg samples (FS7, FS9, FS10) are all Thailand-origin. Literature comparison: Mabesa et al. 1985 and Funatsu et al. 2001 reported 1–90 µg/kg in various fish sauces, consistent with these findings.
LOD of DGT-TDA-AAS: 0.071 µg/L (in solution; recalculated to ~0.6 µg/kg in undiluted sauce). LOD of direct TDA-AAS: 0.68 µg/L. Method: AMA 254 (Altec, Prague). Certified reference material ERM-BB422 (fish muscle, Hg = 0.601 ± 0.030 mg/kg); determined value 0.589 ± 0.017 mg/kg, non-significant difference (p = 0.3808).
Estimated dietary intake: assuming 4 kg/year fish sauce consumption (Asia-Pacific estimate) and mean Hg of 11.4 µg/kg, calculated EWI = 0.014 µg/kg b.w./week, representing 0.35% of the PTWI of 4 µg Hg/kg b.w./week.
Methods (brief)
Fish sauces purchased at Asian markets in Brno and Brussels. DGT pistons assembled with Purolite S924 binding gel (thiol-functional polystyrene resin, novel in this application) and 1.5% agarose diffusive gel. Samples 5-fold diluted; DGT deployed 24 h; binding gels analysed by TDA-AAS. Direct TDA-AAS also run in parallel. Results recalculated using fish sauce density 1.2 g/mL. Recovery in spiked fish sauce: 95%. RSD: 5.6%. Main limitation: 10 samples, single-purchase cross-section; no speciation of Hg species; no distinction between inorganic and organic (MeHg) mercury.
Implications
Certification: Fish sauce concentrations here are all well below the EU 500 µg/kg limit. However, the wide range (0.8–42.8 µg/kg) and the three high-outlier Thailand samples suggest origin-specific variability worth monitoring. For HMT&C, fish sauce is not currently a primary product category but is relevant to condiment and fermented-food modules. These values are total Hg; MeHg fraction in fish-fermented products is typically higher than in non-fish sources but speciation data are absent from this paper.
Courses: Methodologically important paper: illustrates DGT preconcentration for low-Hg-content matrices and the instrument corrosion challenge with high-salt condiment matrices. Good for analytical methods module.
App: Supports total Hg contamination profile for fish sauce/fermented fish condiment. Range 0.8–42.8 µg/kg; median broadly around 2–5 µg/kg for most samples, outliers at ~26–43 µg/kg. Speciation unknown. App should flag fish sauce under tHg with note that MeHg data are not available from this source.