Atomic fluorescence spectrometry (AFS) is a highly sensitive and cost-effective technique for determining trace heavy metals and metalloids in soil, with particular strength for arsenic, mercury, selenium, antimony, bismuth, and tellurium. This technical article explains the complete hydride generation and cold vapor AFS workflow—from sample digestion and pre-reduction to instrumental measurement and quality control—in alignment with standards such as HJ 680, HJ 702, GB/T 22105, GB 15618, and GB 36600. It also outlines typical sample types, measurable elements, detection limits, laboratory applications, and answers to frequently asked questions for environmental professionals.
Technical Overview
Atomic fluorescence spectrometry (AFS) is an element-specific optical technique in which free ground-state atoms, produced in an atomizer, absorb characteristic radiation from an element line source—typically a hollow cathode lamp (HCL) or boosted-discharge hollow cathode lamp—and subsequently re-emit the absorbed energy as fluorescence. Under low-concentration conditions, the fluorescence signal is directly proportional to the number of atoms in the optical path, enabling quantitative measurement at trace and ultra-trace levels.
In soil analysis, AFS is almost always implemented as hydride generation AFS (HG-AFS) or cold vapor AFS (CV-AFS). In hydride generation, acidic sample solutions react with an alkaline sodium or potassium borohydride (NaBH4 or KBH4) reductant to convert dissolved arsenic, selenium, antimony, bismuth, tellurium, tin, germanium, and lead into gaseous hydrides (for example, AsH3, H2Se, SbH3, BiH3, TeH2, SnH4, GeH4, PbH4). These volatile species are separated from the liquid matrix in a gas-liquid separator, swept by argon into an argon-hydrogen flame, atomized, and excited by the lamp radiation. Mercury is determined by cold vapor generation: dissolved mercury is chemically reduced to elemental mercury vapor, which is transported directly to the atomizer without the need for a flame.
The chemical vapor separation step is the key advantage of AFS for soil work. Because the analyte is physically separated from the bulk digestion matrix—iron, aluminum, calcium, magnesium, alkali metals, and dissolved salts—spectral and matrix interferences are dramatically reduced compared with direct solution nebulization techniques. Combined with high-intensity line sources and low-noise photomultiplier detection, HG-AFS and CV-AFS routinely achieve solution detection limits in the low nanogram-per-liter range, translating to solid-phase method detection limits on the order of 0.002 to 0.05 mg/kg for most target elements. Instruments are relatively economical to purchase and operate, consume modest amounts of argon, and permit sequential multi-element determination in a single digested aliquot.
From a regulatory standpoint, AFS is the reference method of choice in Chinese national and industry standards for mercury, arsenic, selenium, antimony, and bismuth in soil and sediment, and it is routinely deployed by accredited third-party laboratories supporting soil pollution surveys, agricultural land classification under GB 15618-2018, and development-land risk screening under GB 36600-2018. It complements ICP-OES, ICP-MS, and graphite furnace atomic absorption spectrometry, often outperforming them in sensitivity and selectivity for the hydride-forming elements and mercury.
Testing Samples
Agricultural and Farmland Soil
Paddy soil, upland field soil, vegetable plots, orchard soil, and greenhouse soil are the most frequently submitted sample types. The primary concerns are arsenic, mercury, and selenium accumulation from irrigation water, phosphate fertilizers, manure, pesticides, and atmospheric deposition, because these elements transfer readily into the food chain. AFS supports risk screening against GB 15618-2018 and the evaluation of soil-to-crop uptake risk for food safety programs.
Development Land and Redevelopment Site Soil
Soil from former industrial plants, fuel stations, landfills, and brownfield parcels destined for residential, commercial, or public-use redevelopment is tested to determine whether contaminant concentrations fall below the screening values of GB 36600-2018. Mercury and arsenic are among the mandatory inorganic indicators for such risk assessments, and AFS provides the sensitivity required to demonstrate compliance at low screening thresholds.
Mining, Smelting, and Industrial Site Soil
Soil surrounding nonferrous metal mines, smelters, coal-fired power plants, chemical works, and battery or electronics manufacturing sites often contains elevated antimony, arsenic, bismuth, tin, tellurium, and mercury. AFS is well suited to characterizing these multi-element signatures, delineating plumes, and supporting remedial investigation and feasibility studies.
Sediment, Sludge, and Organic-Rich Matrices
River, lake, estuary, and marine sediments, as well as sewage sludge, compost, and biogas residues, can be analyzed by the same AFS procedures, since the applicable standards (HJ 680-2013 and HJ 702-2014) cover both soil and sediment. These matrices typically require careful attention to organic matter digestion and mercury loss control.
Background and Geochemical Survey Samples
Regional background soil, soil profile samples, and geochemical mapping samples benefit from the low detection limits of AFS, which allow reliable quantification near natural background concentrations of arsenic and selenium that would challenge less sensitive techniques.
Testing Items
Atomic fluorescence spectrometry is the preferred or reference technique for the following analytes in soil and sediment, with typical solid-phase method detection limits depending on the digestion route and instrument configuration:
- Total mercury (CV-AFS), typically 0.002 to 0.01 mg/kg, a mandatory screening item in both agricultural and development-land standards.
- Total arsenic (HG-AFS), typically 0.01 to 0.05 mg/kg, a high-toxicity metalloid and key regulatory indicator.
- Total selenium (HG-AFS), an essential trace element at low levels but a contaminant of concern at elevated concentrations.
- Total antimony (HG-AFS), relevant to mining and smelting impact assessment.
- Total bismuth (HG-AFS), a geochemical tracer often determined together with antimony.
- Total tellurium (HG-AFS), associated with nonferrous metallurgy and electronics manufacturing.
- Total germanium and total tin (HG-AFS), determined with specialized hydride generation chemistry.
- Total lead (HG-AFS), determined as lead hydride with an oxidant-enhanced borohydride system; commonly cross-checked against ICP or AAS methods.
- Chemical speciation, including inorganic As(III) and As(V), monomethylarsonic and dimethylarsinic species, and inorganic mercury versus methylmercury, by coupling high-performance liquid chromatography (HPLC-AFS).
- Aqua regia extractable versus total content determinations, clearly distinguished in test reports because Chinese soil standards specify aqua regia digestion as the basis for "total" mercury and arsenic screening values.
Testing Methods
Sample Collection and Preparation
Samples are collected following HJ/T 166-2004, the technical specification for soil environmental monitoring, using grid- or judgment-based layouts appropriate to the assessment objective. For mercury determinations, samples intended for speciation or volatile-sensitive analysis should be chilled or frozen immediately; for total metal analysis, samples are air-dried or freeze-dried at low temperature, debris and stones are removed, and the material is crushed and sieved. Total metal digestion uses the fraction passing a 0.149 mm (100-mesh) nylon sieve, while the less than 2 mm fraction is reserved for moisture, pH, and supporting parameters.
Digestion Procedures
Two digestion routes are standard. The first, specified in HJ 680-2013, employs aqua regia digestion in a boiling water bath: weighed soil portions are reacted with hydrochloric-nitric acid mixture, and the extract is diluted to volume for direct AFS measurement of mercury, arsenic, selenium, bismuth, and antimony. The second, specified in HJ 702-2014, uses closed-vessel microwave digestion with nitric-hydrochloric acid, offering shorter processing times, lower blank contribution, minimal mercury volatilization loss, and better recovery from organic-rich or sulfide-rich matrices. Hot-plate digestion with acid mixtures is also accepted in some laboratories when validated. Because mercury is volatile, digestion temperature control, closed systems, and recovery verification with certified reference materials are essential.
Pre-Reduction and Valence State Adjustment
Hydride generation efficiency depends strongly on analyte oxidation state. Pentavalent arsenic and antimony are pre-reduced to their trivalent forms by adding a mixed thiourea–ascorbic acid solution and standing for at least 30 minutes, which simultaneously masks transition-metal interferences. Hexavalent selenium is reduced to tetravalent selenium by heating with concentrated hydrochloric acid. Proper valence control is a prerequisite for quantitative and reproducible hydride formation.
Hydride Generation, Cold Vapor Generation, and Measurement
The prepared acidic sample solution is pumped continuously and merged with hydrochloric acid carrier and alkaline potassium borohydride reductant (commonly 0.5 to 2 percent KBH4 in dilute KOH). Generated hydrides or mercury vapor are stripped by argon flow through a gas-liquid separator and carried to an argon-hydrogen flame atomizer. Element-specific hollow cathode lamps excite the atoms, and fluorescence intensity is detected at right angles to the excitation beam, minimizing scattered-light background. Quantification uses external calibration with matrix-matched standards prepared in the same acid and pre-reductant system as the samples, and mercury determinations benefit from periodic purging to control carryover and memory effects.
Interferences and Their Control
Principal interferences include suppression of hydride generation by elevated concentrations of copper, nickel, and cobalt, chloride-related matrix effects, residual nitric acid (which depresses selenium and arsenic signals), and spectral overlap from co-generated hydrides at extreme concentration ratios. Mitigation measures include thiourea–ascorbic acid masking, dilution of the digest, hydrochloric acid matrix matching, sample spiking and standard-addition verification for difficult matrices, and reanalysis of high-concentration samples after dilution to prevent cross-contamination of subsequent runs.
Quality Assurance and Quality Control
A defensible AFS dataset requires a full quality system: method blanks to monitor reagent and digestion contamination; calibration verification standards with correlation coefficients typically at or above 0.995; continuing calibration checks within acceptance windows; duplicate analyses with relative percent differences generally below 20 to 30 percent; matrix spike recoveries typically within 80 to 120 percent; and analysis of certified soil reference materials to confirm digestion efficiency and accuracy. Method detection limits are established statistically in accordance with HJ 168 and are re-verified whenever the digestion route or instrument configuration changes.
Applicable Standards
- HJ 680-2013, Soil and sediment — determination of mercury, arsenic, selenium, bismuth and antimony — atomic fluorescence spectrometry (water-bath aqua regia digestion).
- HJ 702-2014, Soil and sediment — determination of mercury, arsenic, selenium, bismuth and antimony — microwave digestion / atomic fluorescence spectrometry.
- GB/T 22105.1-2008, GB/T 22105.2-2008 and GB/T 22105.3-2008, Soil quality — determination of total mercury, total arsenic and total lead by atomic fluorescence spectrometry, in three parts.
- GB 15618-2018, Soil environmental quality — risk control standard for soil contamination of agricultural land.
- GB 36600-2018, Soil environmental quality — risk control standard for soil contamination of development land.
- HJ/T 166-2004, Technical specification for soil environmental monitoring.
- HJ 168, Guidelines for the formulation and revision of environmental monitoring analytical method standards, applied to detection limit determination and validation.
Applications
Atomic fluorescence analysis of soil heavy metals underpins a wide range of environmental and regulatory activities. In detailed site investigations, AFS mercury and arsenic data drive risk screening decisions, delineation of contamination boundaries, and the design of remediation programs for industrial and mining land. In agricultural land management, AFS results support soil pollution status surveys, farmland classification and safe-utilization planning, and source apportionment for irrigation and fertilizer inputs. Post-remediation validation relies on AFS to demonstrate that arsenic, mercury, and selenium concentrations meet risk control values before land reuse.
Beyond compliance work, AFS supports environmental impact assessments for new industrial and mining projects, emergency response investigations after accidental spills, long-term monitoring around landfills and smelters, and geochemical baseline mapping. Research applications include soil-plant transfer studies of arsenic and selenium, bioavailability research, and HPLC-AFS speciation studies that distinguish toxic inorganic species from less harmful organic forms. Accredited third-party laboratories routinely combine AFS with ICP-MS, ICP-OES, and AAS in integrated inorganic contaminant packages, ensuring that every regulated metal and metalloid is covered by the most sensitive and selective technique available.
FAQ
Q: Which elements can be determined in soil by atomic fluorescence spectrometry?
A: AFS is strongest for the vapor-forming elements: mercury (cold vapor), and arsenic, selenium, antimony, bismuth, tellurium, tin, germanium, and lead (hydride generation). Selected instrument configurations can also extend to cadmium and zinc. Cadmium, chromium, copper, nickel, and other regulated metals are typically determined by complementary ICP or AAS techniques within the same test package.
Q: Why choose AFS instead of ICP-MS or atomic absorption for arsenic and mercury?
A: For arsenic, antimony, selenium, and mercury, hydride generation or cold vapor AFS offers lower effective detection limits than routine ICP-OES or flame AAS, freedom from argon-chloride and matrix interferences that affect ICP-MS, and lower instrument and operating costs. It is also the reference method cited in the relevant national and industry standards.
Q: What is the difference between "total" content and aqua regia extractable content?
A: In Chinese soil environmental standards, the screening values for mercury and arsenic are based on aqua regia digestion, so aqua regia data are directly comparable with the regulatory limits. Total content obtained with hydrofluoric acid digestion dissolves silicates as well and yields higher values for lithogenic elements. Reports should always state which basis was used.
Q: How are soil samples preserved for mercury analysis?
A: For total mercury, samples should be dried at low temperature or freeze-dried, since oven drying at high temperature can cause mercury loss. For mercury speciation, samples must be transported cold, kept frozen, and analyzed promptly. Sample containers should be glass or certified polyethylene, and cross-contamination from metallic tools must be avoided by using non-metallic or corrosion-resistant sampling equipment.
Q: Can AFS perform arsenic speciation in soil?
A: Yes. Coupling liquid chromatography with AFS (HPLC-AFS) separates and quantifies As(III), As(V), monomethylarsonic acid, and dimethylarsinic acid in soil pore water, extracts, or digests. Speciation data are valuable for risk assessment because inorganic arsenic species are substantially more toxic than the organic species.
Q: What is a typical analytical workflow and turnaround?
A: A typical workflow comprises sample receipt and drying, grinding and sieving, digestion (water bath or microwave), pre-reduction, hydride or cold vapor generation, AFS measurement against calibration standards, and full quality control review. Standard orders are commonly completed within five to ten working days, while expedited services can be arranged through accredited third-party laboratories when project schedules demand faster reporting.