Determination of arsenic and mercury by atomic fluorescence spectrometry (AFS) is a highly sensitive analytical technique capable of detecting these toxic elements at trace and ultra-trace levels in water, food, soil, cosmetics, and pharmaceuticals. Combining hydride generation or cold vapor generation with atomic fluorescence detection, the method offers detection limits at the ng/L level, excellent selectivity, and full compliance with international and national standards such as GB 5009.11, GB 5009.17, HJ 694, and ISO-based testing protocols. This article provides a comprehensive technical overview of the principles, sample types, testing workflow, quality control measures, and typical applications of AFS-based arsenic and mercury determination.
Technical Overview
Atomic fluorescence spectrometry (AFS) is a trace-element analytical technique that measures the characteristic fluorescence emitted by free atoms in the gaseous state after excitation by an element-specific radiation source. For arsenic and mercury, the technique is typically coupled with vapor generation chemistry: hydride generation atomic fluorescence spectrometry (HG-AFS) for arsenic, and cold vapor atomic fluorescence spectrometry (CV-AFS) for mercury. This combination dramatically improves transport efficiency into the atomizer and separates the analyte from the sample matrix, resulting in exceptionally low detection limits and minimal spectral interference.
In the AFS workflow, the acidified sample is merged with a potassium borohydride (KBH4) solution. Arsenic is converted to arsine (AsH3) gas, while mercury ions are reduced directly to elemental mercury vapor. An argon stream carries the generated vapors into a quartz atomizer heated to approximately 200 °C, where the hydride is thermally decomposed into free atoms. A high-intensity hollow cathode lamp emits radiation at the resonance wavelengths of arsenic (193.7 nm) or mercury (253.7 nm), exciting the atoms, and the resulting fluorescence intensity is measured by a photomultiplier tube and is directly proportional to the analyte concentration over a wide linear range.
Key performance characteristics of AFS for arsenic and mercury include:
- Detection limits typically in the range of 0.01–0.10 µg/L for arsenic and 0.002–0.05 µg/L for mercury, far superior to flame atomic absorption spectroscopy.
- Linear dynamic range spanning three to four orders of magnitude.
- Low reagent consumption and high sample throughput, with a single determination completed in approximately 30–60 seconds.
- Mature standardization, including GB 5009.11, GB 5009.17, GB/T 5750.6, HJ 694, HJ 702, and GB/T 21191 covering food, drinking water, environmental water, soil, and instrument specifications.
- Compatibility with hyphenated techniques such as HPLC-AFS for mercury speciation (methylmercury) and inorganic arsenic speciation.
Testing Samples
Water and Wastewater
Drinking water, groundwater, surface water, seawater, and industrial effluents are among the most common sample matrices. Water samples are generally acidified with nitric acid to pH below 2 at the time of collection and analyzed directly or after simple oxidation pretreatment, making AFS the reference technique for verifying compliance with drinking water quality standards such as GB 5749 and environmental discharge limits under HJ 694.
Food and Agricultural Products
Rice and other cereals, aquatic products and seaweed, edible fungi, vegetables, meat, eggs, dairy products, beverages, edible oils, condiments, and health foods are routinely tested. Arsenic and mercury are priority contaminants in the food chain due to bioaccumulation in marine organisms and the uptake of arsenic by paddy rice, and testing supports compliance with contaminant limits specified in GB 2762.
Soil, Sediment, and Solid Waste
Agricultural soil, contaminated site soil, river and marine sediments, sewage sludge, composts, and fertilizers are digested with acid mixtures before AFS determination, following methods such as HJ 702 and related environmental standards. These data support soil pollution surveys, land-use risk assessment, and remediation verification.
Cosmetics and Personal Care Products
Skin-lightening creams and other cosmetic formulations carry a recognized risk of mercury adulteration, while arsenic may enter products through mineral-derived ingredients. AFS determination after acid digestion verifies conformity with cosmetic safety standards that impose strict impurity limits.
Pharmaceuticals and Traditional Chinese Medicine
Herbal materials, herbal extracts, finished traditional medicine preparations, and chemical pharmaceuticals are monitored for heavy metal impurities in accordance with pharmacopoeial heavy metal limit tests, where the low detection capability of AFS is particularly valuable for plant-derived matrices with complex backgrounds.
Biological and Occupational Health Samples
Urine, blood, and hair specimens are analyzed for biological monitoring of populations with occupational or dietary exposure to arsenic and mercury, leveraging the ultra-trace sensitivity of CV-AFS and HG-AFS.
Testing Items
Typical testing items offered through AFS-based analysis include:
- Total arsenic in water, food, soil, cosmetics, and pharmaceutical matrices.
- Inorganic arsenic (arsenite As(III) and arsenate As(V)) in rice, seaweed, and aquatic products by extraction coupled with HG-AFS.
- Total mercury in all of the above matrices by CV-AFS.
- Methylmercury speciation in fish, seafood, and sediment by HPLC-AFS coupling.
- Arsenic and mercury leachability testing for solid waste classification under hazardous waste identification standards such as GB 5085.3.
Method detection limits depend on the matrix and digestion procedure, but typical achievable values are 0.01–0.05 µg/L for arsenic and 0.002–0.01 µg/L for mercury in aqueous solution, corresponding to 0.005–0.02 mg/kg for arsenic and 0.002–0.01 mg/kg for mercury in solid samples digested under closed-vessel conditions. These limits are well below the maximum residue and contaminant levels established in food safety, drinking water, and environmental regulations, providing substantial regulatory margin.
Testing Methods
Sample Preparation and Digestion
Solid samples are homogenized and digested using one of several established approaches: closed-vessel microwave digestion with nitric acid (optionally with hydrogen peroxide), high-pressure vessel digestion, or hot-plate wet digestion with nitric-perchloric or nitric-sulfuric acid mixtures. Water samples for total mercury may undergo oxidative pretreatment with bromate-bromide reagent followed by reduction of excess oxidant with hydroxylamine hydrochloride. For mercury determination, digestion temperatures are kept moderate to prevent volatilization losses, and all digestion vessels are rigorously acid-cleaned to minimize blank contamination.
Pre-Reduction of Arsenic
Because potassium borohydride efficiently generates arsine only from As(III), pentavalent arsenic in the digest must first be pre-reduced. This is achieved by adding a mixed solution of 5% thiourea and 5% ascorbic acid in hydrochloric acid medium and allowing the reaction to stand for at least 30 minutes. Mercury requires no pre-reduction step, as Hg(II) is quantitatively reduced to elemental vapor by borohydride under the same acidic conditions.
Vapor Generation and Instrumental Determination
The pretreated sample is introduced into the vapor generation system, where it reacts with a reducing solution of 0.5–2% potassium borohydride stabilized in 0.2–0.5% sodium hydroxide, with 2–10% hydrochloric acid as the carrier. The generated arsine or mercury vapor is separated from the liquid phase in a gas-liquid separator and swept by argon into the quartz atomizer. Instrumental parameters—lamp current, photomultiplier negative high voltage, carrier gas and shield gas flow rates, and atomizer height—are optimized according to the instrument operating manual and validated during method development. Quantification is performed against an external calibration curve prepared from certified arsenic and mercury standard solutions, with calibration correlation coefficients typically required to be 0.995 or better.
Quality Control and Data Validation
Accredited third-party laboratories apply a comprehensive quality assurance framework to every analytical batch:
- Method blanks and reagent blanks to monitor contamination from acids, vessels, and the vapor generation system.
- Calibration verification with an independent check standard at regular intervals.
- Duplicate sample analysis with relative deviation acceptance criteria, typically within 10%.
- Matrix spike recoveries in the range of 80–120% to verify accuracy in the presence of sample matrix effects.
- Certified reference materials (CRMs) of matching matrix, such as rice flour, soil, or sediment CRMs, analyzed with each batch.
- Detection limit verification and periodic instrument performance checks such as lamp energy and blank stability.
Reported results are corrected for blanks and dilution factors, expressed on a wet-weight or dry-weight basis for solids as required by the applicable standard, and accompanied by measurement uncertainty where specified by the client or regulation.
Applications
AFS-based arsenic and mercury determination serves a wide spectrum of regulatory, commercial, and scientific purposes:
- Food safety surveillance and product release testing against contaminant maximum limits for rice, aquatic products, seaweed, and beverages.
- Drinking water quality verification for municipal suppliers, bottled water producers, and water treatment facilities.
- Environmental impact assessments, pollution surveys, and remediation verification for soil, sediment, and groundwater projects.
- Import and export commodity inspection, where certificate-of-analysis data must meet the destination market's regulatory limits.
- Cosmetic and pharmaceutical raw material and finished product quality control under GMP quality systems.
- Occupational health monitoring programs and epidemiological studies of exposed populations.
- Geological and geochemical exploration, where arsenic and mercury serve as pathfinder elements.
The method's combination of ultra-trace sensitivity, robustness, low operating cost, and alignment with internationally recognized GB, ISO, and ASTM testing frameworks makes it the preferred technique for laboratories that must routinely quantify arsenic and mercury across diverse and demanding sample matrices.
FAQ
What detection limits can be achieved for arsenic and mercury by AFS?
Under optimized conditions, method detection limits of approximately 0.01 µg/L for arsenic and 0.002 µg/L for mercury in aqueous samples are routinely achievable, which corresponds to low µg/kg levels in digested solid matrices. Actual limits depend on the digestion procedure, reagent purity, and blank control, and each laboratory establishes its own statistically validated detection limits for each matrix.
Why is thiourea-ascorbic acid added before arsenic determination?
Potassium borohydride generates arsine efficiently only from trivalent arsenic, while arsenic in digested samples is predominantly present as pentavalent arsenate. Thiourea reduces As(V) to As(III), and ascorbic acid accelerates the reduction and stabilizes the system. Without this pre-reduction step, arsenic recovery would be incomplete and biased low.
What is the difference between total arsenic and inorganic arsenic testing?
Total arsenic measures all chemical forms of the element after complete digestion, whereas inorganic arsenic testing quantifies only the toxicologically more significant As(III) and As(V) species, usually after extraction with dilute acid under heating. Food safety standards for rice and aquatic products set limits specifically on inorganic arsenic, so speciation-capable extraction methods coupled with HG-AFS are required for compliance testing.
Can arsenic and mercury be determined in the same run?
Yes. Modern atomic fluorescence spectrometers are commonly configured with dual channels and dual hollow cathode lamps, allowing simultaneous determination of arsenic and mercury from a single digest, which improves throughput and reduces sample consumption.
How should samples be preserved before arsenic and mercury analysis?
Water samples should be collected in acid-cleaned glass or polyethylene containers and acidified with nitric acid to pH below 2 immediately after collection; samples for mercury preservation may additionally require potassium dichromate. Solid food and soil samples should be stored cool and analyzed promptly, with homogenization before subsampling to ensure representativeness.
What quality control evidence should a test report include?
A compliant report from an accredited third-party laboratory should identify the applied standard method, digestion and determination conditions, method detection limits, blank-corrected results with units, and supporting QC data such as spike recoveries, duplicate precision, and certified reference material results, together with the laboratory's accreditation scope covering the specific method and matrix.
