SOP: Heavy Metals Sample Digestion
Instruments like ICP-MS are only as good as the digestion that feeds them. Undigested fat or carbon residue suppresses signals and clogs nebulizers, while overly aggressive conditions lose volatile elements. This procedure gives a clean, complete digest for lead, cadmium, mercury, and arsenic determination.
Chemical analysis converts invisible composition into numbers that specifications, labels, and safety decisions depend on. Consistent technique matters as much as good instruments: small variations in timing, temperature, or reagent preparation shift results enough to fail a batch that should pass, or pass one that should fail.
Purpose
To digest food samples completely for heavy metal determination by atomic spectroscopy, producing a clear solution free of organic matrix.
Scope
Applies to fish, rice, vegetables, canned foods, and spices for lead, cadmium, total arsenic, and mercury (with appropriate modifiers). Use closed-vessel microwave digestion for volatile elements.
Equipment and Materials
- Microwave digestion system with PTFE vessels, or hot block with digestion tubes
- Concentrated nitric acid, trace-metal grade
- Hydrogen peroxide, 30 percent, trace-metal grade
- Deionized water, 18 megaohm
- Analytical balance
- Volumetric flasks, Class A
- Certified reference material for verification
Procedure
- Homogenize the sample. Weigh 0.5 g of dry sample (or 2 g of wet sample) into a clean PTFE digestion vessel. Record the exact weight.
- Add 6 mL of concentrated nitric acid and 2 mL of hydrogen peroxide. Let predigest uncapped in the fume hood for 15 minutes to control foaming.
- Seal the vessels and run the microwave program: ramp to 180 C over 15 minutes, hold 20 minutes, then cool.
- Vent carefully in the fume hood after cooling below 50 C.
- Transfer quantitatively to a 50 mL volumetric flask and make to volume with deionized water. The digest must be clear and colorless to pale yellow.
- Prepare a method blank and a certified reference material with every batch.
- If particles remain, filter through an acid-washed filter, noting the filtration on the record.
Recording Results
This SOP yields digests for instrumental analysis. Record digestion weights, acid lots, program used, and final volumes. The reference material recovery must be 80 to 120 percent before sample results are accepted.
Quality Control
Run a reagent blank with every batch and subtract it from sample results. Analyze every sample in duplicate; the relative percent difference between duplicates must not exceed 10 percent for concentrations well above the detection limit. Include a certified reference material or a validated in-house control sample with each batch and plot the result on a Shewhart control chart. Investigate any point outside two standard deviations and reject the batch if a point falls outside three standard deviations. Verify the calibration of volumetric glassware annually against Class A tolerances, and check the analytical balance daily with certified weights. Standardize titrants monthly and label with the expiry date. Document all quality control results in the batch record; sample results must not be reported when associated QC fails.
Troubleshooting
- Dark or cloudy digest: incomplete oxidation. Increase peroxide or extend the hold time; reduce sample weight for fatty matrices.
- High blanks: contaminated acids or vessels. Use trace-metal grade reagents and acid-wash vessels.
- Vessel venting during run: too much sample or too fast a ramp. Reduce weight and extend the ramp.
- Low mercury recovery: mercury needs gold stabilizer or closed-vessel conditions; never open-digest for mercury.
Safety Notes
- Concentrated nitric acid causes severe burns and toxic fumes. Full face protection, acid gloves, and fume hood are mandatory.
- Hydrogen peroxide is a strong oxidizer; never mix with organics outside the vessel.
- Microwave vessels are pressurized when hot. Cool fully before opening.