Soil Heavy Metal Determination by Atomic Fluorescence Spectrometry: Principles, Methods, and Applica

Soil heavy metal determination by atomic fluorescence spectrometry (AFS) is a highly sensitive analytical technique widely used to quantify trace-level toxic elements such as arsenic, mercury, selenium, antimony, bismuth, and tellurium in soil and sediment samples. Combining hydride generation or cold vapor sample introduction with atomic fluorescence detection, the method delivers detection limits at the µg/kg level, excellent selectivity, and compliance with internationally recognized testing standards. This article explains the technical principles, sample types, target analytes, digestion and measurement procedures, quality control requirements, and typical applications of AFS-based soil heavy metal analysis.

Technical Overview

Atomic fluorescence spectrometry (AFS) is an element-specific detection technique based on the principle that free ground-state atoms, when excited by characteristic radiation from a hollow cathode lamp, emit fluorescence light at discrete wavelengths as they return to lower energy states. In soil heavy metal analysis, the most widely implemented variant is hydride generation atomic fluorescence spectrometry (HG-AFS), in which analyte elements are chemically converted to volatile hydrides (or, in the case of mercury, to elemental vapor by cold vapor generation) before being transported by an argon stream into an argon-hydrogen flame for atomization and detection.

This hydride or cold vapor separation step physically isolates the analyte from the bulk sample matrix, which dramatically reduces spectral and chemical interferences from major soil constituents such as iron, aluminum, and calcium. As a result, AFS offers outstanding sensitivity for a specific group of hydride-forming and volatile elements — arsenic (As), mercury (Hg), selenium (Se), antimony (Sb), bismuth (Bi), tellurium (Te), germanium (Ge), and tin (Sn) — with method detection limits typically in the range of 0.001 to 0.05 mg/kg, depending on the element and the digestion procedure.

Compared with graphite furnace atomic absorption spectrometry (GFAAS) and inductively coupled plasma mass spectrometry (ICP-MS), AFS provides a cost-effective, robust, and highly selective solution for routine monitoring of these elements. Key performance characteristics include:

• Extremely low detection limits for hydride-forming elements and mercury, meeting the screening values defined in soil environmental quality standards.
• Wide linear calibration ranges, typically spanning three orders of magnitude.
• Low reagent consumption and simple instrument maintenance, making the method suitable for high-throughput laboratory workflows.
• Direct compatibility with internationally recognized standard methods such as GB/T 22105 series and the HJ 680 and HJ 702 environmental protection standards.

Testing Samples

Agricultural and Farmland Soil

Surface and profile soil samples collected from cultivated land, orchards, vegetable bases, and permanently protected farmland are analyzed to verify compliance with agricultural land risk control standards such as GB 15618. Arsenic and mercury are the primary AFS target elements in farmland screening surveys, often alongside selenium, which is monitored both as a potentially toxic element and as an essential micronutrient.

Contaminated Site Soil

Soil from industrial land, former manufacturing plants, mining and smelting areas, gasworks, pesticide plants, and electroplating facilities is tested during site investigation, risk assessment, and remediation acceptance. These samples frequently contain elevated arsenic, antimony, and mercury concentrations, and are evaluated against screening values specified in GB 36600 and site-specific assessment criteria.

Sediments and Sludge

River, lake, estuarine, and marine sediments, as well as dewatered sewage sludge intended for land application, are analyzed to characterize contamination loads and assess disposal or utilization options. Sediment digestion for AFS measurement generally follows procedures analogous to those specified in the HJ 680 standard for soils and sediments.

Background and Geological Survey Samples

Deep soil, parent material, and geochemical mapping samples collected in regional background surveys and mineral exploration programs are analyzed for arsenic, antimony, bismuth, selenium, tellurium, and mercury to establish geochemical baselines and identify mineralization-related anomalies.

Proper sample handling is critical for data quality. Samples are typically collected using stainless steel or wooden tools to avoid contamination, stored in glass or polyethylene containers, transported at low temperature, air-dried or freeze-dried, homogenized, and sieved through a 2 mm screen for moisture determination and a 0.149 mm (100 mesh) sieve for chemical analysis, in accordance with soil monitoring technical specifications such as HJ/T 166 and HJ 25.2.

Testing Items

The core testing scope of soil heavy metal determination by atomic fluorescence spectrometry covers the following elements, each reported as the total concentration on a dry-weight basis:

• Total arsenic (As): detection limit typically 0.01 mg/kg or lower; a priority controlled contaminant in both agricultural and construction land standards.
• Total mercury (Hg): detection limit typically 0.002 mg/kg; determined by cold vapor AFS due to its volatility at room temperature.
• Total selenium (Se): detection limit typically 0.01 mg/kg; monitored for both toxicity risk and nutrient status.
• Total antimony (Sb): detection limit typically 0.05 mg/kg; a key indicator near mining and flame-retardant manufacturing sites.
• Total bismuth (Bi): detection limit typically 0.05 mg/kg; relevant to geochemical exploration and industrial contamination.
• Total tellurium (Te) and germanium (Ge): specialty elements determined for geochemical and research purposes.
• Total tin (Sn): determined by hydride generation AFS, relevant near smelting and plating operations.

Depending on project requirements, AFS results are frequently combined with data for lead, cadmium, chromium, copper, zinc, and nickel obtained by ICP-MS, ICP-OES, or flame AAS to form a complete heavy metal profile of the soil.

Testing Methods

Sample Preparation and Digestion

The choice of digestion procedure depends on the target element and the governing standard method:

• For total mercury and arsenic: microwave-assisted digestion with a nitric acid–hydrochloric acid mixture followed by AFS determination, as specified in HJ 702, or water-bath digestion with aqua regia followed by cold vapor AFS, as described in GB/T 22105.1.
• For arsenic, antimony, bismuth, selenium, and tellurium: digestion with aqua regia in a water bath or temperature-controlled heating block, followed by hydride generation AFS, as specified in HJ 680 and GB/T 22105.2.
• For geological survey samples requiring total decomposition, digestion protocols based on nitric–hydrofluoric–perchloric acid mixtures followed by borate complexation of residual fluoride may be applied before hydride generation.

Throughout digestion, blank control, low-temperature operation for mercury to prevent volatilization losses, and complete dissolution of the target element's chemical forms are essential for accurate results.

Pre-Reduction and Derivatization

Arsenic and antimony in digested solutions must be pre-reduced from pentavalent to trivalent states to achieve quantitative hydride generation. This is accomplished by adding a mixed solution of thiourea and ascorbic acid and allowing sufficient reaction time, typically 30 minutes. Selenium is pre-reduced by boiling or by heating with hydrochloric acid to convert hexavalent selenium to the tetravalent form. The hydride generation reaction itself uses potassium borohydride (KBH4) dissolved in dilute sodium hydroxide as reductant, with dilute hydrochloric acid as the carrier solution, producing arsine, stibine, hydrogen selenide, and other gaseous hydrides that are separated from the liquid phase in a gas–liquid separator.

Instrumental Measurement

The separated hydrides are swept by argon carrier gas into an atomizer, where they are decomposed in an argon–hydrogen flame to free atoms. A coded hollow cathode lamp or high-intensity continuous source irradiates the atom cloud at the element-specific wavelength (for example, 193.7 nm for arsenic, 253.7 nm for mercury, and 196.0 nm for selenium), and an intensity-calibrated fluorescence signal is recorded. Quantification is performed against an external calibration curve prepared from certified standard solutions, with concentration results converted to mg/kg dry weight using the measured sample moisture content.

Quality Assurance and Quality Control

Accredited third-party laboratories operating under an ISO/IEC 17025 quality management system apply the following controls in every analytical batch:

• Method blanks and full procedural blanks to monitor contamination from reagents and glassware.
• Calibration verification using an independent second-source standard solution.
• Duplicate sample analysis, with relative deviation acceptance criteria typically within 20 percent.
• Matrix spike recoveries, generally acceptable in the range of 80 to 120 percent, or 70 to 130 percent near the detection limit.
• Analysis of certified soil reference materials to verify the accuracy of the complete digestion and measurement procedure.
• Interference checks for elevated copper, nickel, and cobalt concentrations, which at high levels can suppress hydride generation and fluorescence response.

Applications

Soil heavy metal determination by AFS supports a broad range of regulatory, industrial, and scientific applications:

• National and regional soil pollution status surveys and detailed investigation of farmland around high-risk areas.
• Contaminated site investigation, risk assessment, and post-remediation acceptance testing for land redevelopment projects.
• Environmental impact assessment and operational monitoring for mining, smelting, chemical manufacturing, and waste disposal facilities.
• Verification of compliance with soil environmental quality standards such as GB 15618 for agricultural land and GB 36600 for development land.
• Geochemical mapping and mineral exploration programs targeting arsenic, antimony, bismuth, mercury, and selenium anomalies.
• Research on soil contamination transport, bioavailability, and remediation technologies in academic and government laboratories.
• Import and export commodity inspection and due-diligence testing supporting environmental compliance declarations.

FAQ

Why is AFS preferred over ICP-MS for arsenic and mercury in soil?

AFS offers lower instrumental and operating costs, minimal spectral interferences from the soil matrix, and detection limits for arsenic and mercury that equal or exceed those of ICP-MS, while avoiding the argide and chloride-based polyatomic interferences that complicate arsenic measurement by ICP-MS. For these specific elements, AFS is often the first-choice routine method in accredited third-party laboratories.

How much soil sample is required for testing?

Typically, 20 to 50 grams of air-dried, sieved soil is sufficient for a complete AFS determination of arsenic, mercury, selenium, antimony, and bismuth, including duplicate analysis. Individual digestions generally consume 0.2 to 0.5 grams of sample per element group.

What is the typical reporting limit?

Common reporting limits are approximately 0.01 mg/kg for arsenic and selenium, 0.002 mg/kg for mercury, and 0.05 mg/kg for antimony and bismuth, all expressed on a dry-weight basis. Actual limits depend on the digestion method, sample mass, and instrument performance.

Why is thiourea–ascorbic acid added before arsenic measurement?

Arsenic in digested soil solutions exists in both trivalent and pentavalent oxidation states, but only trivalent arsenic reacts efficiently with potassium borohydride to form arsine. Thiourea and ascorbic acid reduce pentavalent arsenic to the trivalent form and also mask potential interferences from transition metals, ensuring complete and reproducible hydride generation.

What is the standard turnaround time?

For routine soil samples, accredited third-party laboratories typically report AFS results within 5 to 10 working days after sample receipt, with expedited services available for time-critical remediation and site development projects.

Can AFS measure lead and cadmium in soil?

Although hydride generation AFS methods for lead exist, they require careful pH control and are less commonly applied. Lead and cadmium in soil are more routinely determined by ICP-MS, ICP-OES, or graphite furnace atomic absorption spectrometry, and AFS analysis is usually commissioned as part of an integrated heavy metal testing package.