Song et al. 2024 — HPLC-ICP-MS method for Cr(VI) in foods and reduction mechanism
A fast HPLC-ICP-MS method was developed for Cr(VI) speciation in food, achieving separation in 1.5 minutes using a weak anion-exchange column (Sepax Proteomix WAX-NP5) with 70 mM NH4NO3 mobile phase at pH 7.0. The LOD was 0.1 µg/kg, substantially better than most published HPLC-ICP-MS methods (range 0.2–50 µg/kg). When applied to seven food products (milk powder, rice flour, whole wheat bread, yoghurt, white vinegar, orange juice, green tea), Cr(VI) was not detected in any sample. Spiking experiments revealed that Cr(VI) rapidly converts to Cr(III) on contact with food matrices, with conversion accelerated by vitamin C, tea polyphenols, acidic conditions (pH 3.0), and elevated temperatures (80°C). These findings support the conclusion that natural reducing agents in food convert Cr(VI) to Cr(III), making Cr(VI) detection in most foods unlikely with conventional protocols that allow contact time before extraction.
Key numbers
LOD: 0.1 µg/kg Cr(VI) by HPLC-ICP-MS (this study); prior published methods ranged 0.2–50 µg/kg.
Analysis time: 1.5 min per sample (compared to 5–11 min for existing methods).
Cr(VI) in 7 food samples: not detected (< 0.1 µg/kg) in milk powder, rice flour, whole wheat bread, yoghurt, white vinegar, orange juice, green tea.
Recovery spiking (Cr(VI) added directly to extraction solution): 91.70–111.85% across 5, 25, 50 µg/kg spike levels.
Recovery spiking (Cr(VI) mixed with food then extracted after 3 h): 0–102.38%; Cr(VI) completely undetected in orange juice and green tea at all spike levels after 3 h contact, demonstrating rapid reduction by vitamin C and tea polyphenols.
Cr(VI) reduction time in food components:
- Vitamin C (0.01 g/L): complete conversion within 20 min
- Tea polyphenols (1 g/L): complete conversion within 24 min
- Whey proteins (1 g/L): complete conversion after 24 h
- Gelatin (1 g/L): complete conversion after 24 h
- Fructose (1 g/L): complete conversion after 60 h
- Cellulose (1 g/L): complete conversion after 60 h
High temperature (80°C vs. 25°C) accelerated reduction rates by 1.04–10.11 fold; acidic conditions (pH 3.0 vs. 7.0) accelerated reduction 4.47–6.81 fold for proteins, sugars, and cellulose.
Methods (brief)
HPLC-ICP-MS using Agilent 1260 HPLC + Agilent 7700 ICP-MS. Weak anion-exchange column (Sepax Proteomix WAX-NP5, 50 × 4.6 mm, 5 µm). Mobile phase: 70 mM NH4NO3 at pH 7.0, 0.8 mL/min. Injection volume: 5 µL. Monitored isotope: m/z = 52 (Cr). Alkaline extraction modified from US EPA method 3060A (0.5 M NaOH + 0.28 M Na2CO3 + MgCl2, ultrasonic 20 min). LOD = 3σ/signal-noise. Linear range 0–50 µg/kg (R² = 0.9996). RSD = 1.20% for 5 µg/kg standard (n=5). Published in MDPI Toxics under CC BY 4.0.
Implications
Certification: This paper’s key finding for certification is that Cr(VI) is not detectable in common processed foods including dairy, grain products, juices, and tea, because natural food components reduce Cr(VI) to Cr(III) within minutes to hours. For HMT&C purposes where Cr-VI is a target analyte, this paper supports the view that standard methods with prolonged contact time before extraction will systematically underestimate Cr(VI) if it were present; the fast extraction protocol used here provides the most reliable upper bound. The result of ND in all 7 tested foods is consistent with the broader literature finding that Cr(VI) is generally absent from plant-based foods.
Courses: Excellent teaching case for the distinction between total Cr and Cr-VI: foods can contain detectable total Cr while having effectively zero Cr-VI because the food matrix itself converts Cr(VI) to Cr(III). Vitamin C and tea polyphenols are quantitatively the most potent reducing agents for Cr(VI) in food.
App: Supports treating Cr-VI as not detectable in most processed foods and beverages, while total Cr may still be present. The wiki and app should maintain the iAs/tAs and Cr/Cr-VI distinctions scrupulously; this paper provides mechanistic evidence for why.
Microbiome: Relevant to Cr speciation in gut: Cr(III) is the predominant form reaching the gut even if Cr(VI) is consumed, given rapid reduction in acidic food matrices.
Wiki pages updated on ingest
- chromium
- chromium-hexavalent
- arsenic-speciation (analogous speciation challenge documentation)
- tea
- rice-flour