Sediment heavy metal analysis by atomic fluorescence spectrometry (AFS) is a highly sensitive analytical technique used to quantify trace-level toxic elements—such as arsenic, mercury, selenium, antimony and bismuth—in marine, estuarine, river and lake sediments. Accredited third-party laboratories routinely apply hydride generation atomic fluorescence spectrometry (HG-AFS) and cold vapor AFS in accordance with standards such as HJ 702-2014, HJ 680-2013 and GB 17378.5-2007 to support environmental impact assessments, dredging projects and ecological risk evaluations. This article explains the analytical principle, sample types, testing scope, digestion and measurement procedures, and typical applications of AFS-based sediment testing.
Technical Overview
Sediments act as the ultimate sink for heavy metals released from industrial discharge, mining, agriculture and urban runoff. Because contaminants accumulated in sediment can be re-mobilized into the water column and aquatic food web, quantitative determination of toxic metals in sediment is a core requirement of marine monitoring programs, port engineering projects and contaminated-site investigations. Atomic fluorescence spectrometry (AFS), particularly in its hydride generation (HG-AFS) and cold vapor (CV-AFS) configurations, is one of the most effective techniques for this purpose, offering exceptional sensitivity for elements that are difficult to determine accurately at trace levels by conventional flame atomic absorption spectrometry.
Principle of Hydride Generation Atomic Fluorescence Spectrometry
In HG-AFS, the acidified sample solution is reacted with an alkaline sodium or potassium borohydride solution, which chemically reduces analytes such as arsenic, antimony, bismuth, selenium and tellurium into volatile gaseous hydrides. These hydrides are separated from the liquid matrix and swept by an argon carrier gas into a heated quartz tube atomizer, where they are dissociated into free atoms. The atoms are excited by element-specific radiation from a hollow cathode lamp or boosted-discharge hollow cathode lamp, and the characteristic fluorescence emitted as they return to the ground state is measured by a photomultiplier detector. Fluorescence intensity is directly proportional to analyte concentration over the calibrated range.
For mercury, no hydride is required: mercuric ions are directly reduced to elemental mercury vapor, which is transported to the atomizer and measured by cold vapor AFS at 253.7 nm. Because the hydride generation and cold vapor steps separate the analytes from the bulk matrix, spectral and chemical interferences are greatly reduced compared with direct solution nebulization techniques.
Why AFS Is Well Suited to Sediment Analysis
- Extremely low detection limits, typically in the range of 0.001 to 0.05 mg/kg (dry weight), meeting the strict thresholds used in sediment quality classification.
- High selectivity for hydride-forming elements and mercury, with minimal spectral overlap.
- Matrix separation during hydride generation suppresses interference from the major constituents of sediment digests.
- Lower instrument and operating costs compared with ICP-MS, with excellent day-to-day reproducibility.
- Direct alignment with national sediment monitoring standards, including HJ 702-2014, HJ 680-2013 and GB 17378.5-2007.
Testing Samples
Marine and Estuarine Sediments
Surface and subsurface sediments collected from open sea areas, coastal zones and estuaries are the most common sample type. Analysis typically supports compliance evaluation against marine sediment quality standards such as GB 18668-2002, under which, for example, Class I sediments are limited to 20 mg/kg for arsenic and 0.2 mg/kg for mercury—thresholds that demand the low detection limits provided by AFS.
River, Lake and Reservoir Sediments
Freshwater sediments from rivers, lakes, impoundments and urban waterways are analyzed to trace upstream pollution sources, assess ecological risk and inform dredging or capping decisions. Where such sediments are managed under soil contamination frameworks, results may also be compared with screening values in standards such as GB 36600-2018.
Harbor, Channel and Dredged Material
Pre-dredging characterization and post-dredging disposal assessments require reliable data on arsenic and mercury, which are among the most critical contaminants controlling the classification of dredged spoil for ocean dumping, beneficial reuse or upland disposal. AFS methods provide the sensitivity needed for these regulatory decisions.
Sediment Cores and Archived Samples
Dated sediment cores are analyzed layer by layer to reconstruct the historical record of contaminant inputs, distinguish natural geochemical background from anthropogenic enrichment, and establish baselines for remediation projects.
General sample requirements include a minimum of 100 g of wet sediment per element group, collection and storage in accordance with the marine monitoring specification GB 17378.3-2007, preservation in the dark at low temperature or by freezing, and subsequent freeze-drying (preferred for mercury, to prevent volatilization losses), grinding in non-contaminating agate ware, and sieving to pass a 100-mesh (150 μm) or finer sieve before digestion.
Testing Items
Atomic fluorescence spectrometry is the method of choice for the hydride-forming elements and mercury in sediments. Typical testing items and approximate method detection limits (which vary with sample mass, digestion route and instrument conditions) are listed below:
- Arsenic (As) — total arsenic, typical detection limit approximately 0.01 mg/kg.
- Mercury (Hg) — total mercury by cold vapor AFS, typical detection limit approximately 0.002 mg/kg.
- Selenium (Se) — total selenium, typical detection limit approximately 0.01 mg/kg.
- Antimony (Sb) — total antimony, typical detection limit approximately 0.01 mg/kg.
- Bismuth (Bi) — total bismuth, typical detection limit approximately 0.02 mg/kg.
- Tellurium (Te) and germanium (Ge) — determinable on request where instrument conditions permit.
Elements such as copper, lead, zinc, cadmium, chromium and nickel are not determined by AFS; they are quantified in parallel by atomic absorption spectrometry, ICP-OES or ICP-MS, allowing a complete heavy metal profile of the sediment from a single homogenized sample.
Testing Methods
Sample Pretreatment
Field samples are homogenized, freeze-dried or oven-dried at low temperature, disaggregated and ground in an agate mill, and sieved to the required particle size. All subsampling is performed by coning and quartering or riffling to ensure representativeness, and moisture content is determined on a separate subsample so that results can be reported on a dry-weight basis.
Digestion
Two principal digestion approaches are prescribed by the national standards:
- Water-bath aqua regia digestion (HJ 702-2014): a weighed portion of sediment is digested with aqua regia in a boiling water bath at approximately 95 °C, a simple and robust procedure suited to routine high-throughput monitoring.
- Microwave-assisted acid digestion (HJ 680-2013): the sample is digested with a hydrochloric–nitric–hydrofluoric acid mixture in a closed-vessel microwave system, achieving more complete decomposition of silicate lattices; boric acid is subsequently added to complex residual fluoride before measurement.
- For marine sediments, the determination of arsenic, mercury and selenium is also specified in GB 17378.5-2007, ensuring consistency across national marine monitoring programs.
Pre-reduction and Hydride Generation
Speciation control is essential for quantitative hydride generation. Arsenic(V) and antimony(V) are pre-reduced to their trivalent states using a thiourea–ascorbic acid solution with a standing time of at least 30 minutes. Selenium(VI) is pre-reduced to selenium(IV) by heating the digest in a concentrated hydrochloric acid medium. Mercury is measured directly by cold vapor generation. The digests are then mixed online with hydrochloric acid carrier solution and an alkaline sodium borohydride reductant to generate hydrides or mercury vapor, which are stripped by argon gas into the quartz tube atomizer shielded by a secondary argon–hydrogen flame.
Instrumentation, Calibration and Quality Control
Measurements are performed with multi-point external calibration curves prepared from certified standard solutions, with correlation coefficients typically required to be no lower than 0.995. Quality assurance follows ISO/IEC 17025 principles and includes:
- Full procedural blanks and reagent blanks in every batch.
- Analysis of certified reference materials, such as GBW-series marine and stream sediment reference materials, with recoveries within the certified uncertainty ranges.
- Matrix spikes and duplicates with spike recoveries typically between 80% and 120% and relative deviations within method-specified limits.
- Continuing calibration verification at defined intervals to confirm instrument stability.
- Interference control for transition metals such as nickel, cobalt and copper, using masking agents, dilution or standard addition where matrix effects are significant.
Applications
- Marine sediment quality assessment and classification against GB 18668-2002 for ocean dumping permits and coastal zone management.
- Environmental impact assessments and environmental acceptance monitoring for ports, docks, submarine pipelines, bridges and offshore wind projects.
- Pre- and post-dredging evaluation of harbor and navigation channel sediments, and classification of dredged material for disposal or reuse.
- Ecological risk assessment using geochemical indices, such as the geo-accumulation index and the potential ecological risk index, which require accurate low-level arsenic and mercury data.
- Pollution source apportionment and baseline surveys in rivers, lakes and estuaries.
- Historical contamination reconstruction through analysis of dated sediment cores.
- Verification of sediment remediation projects, including capping, in-situ stabilization and monitored natural recovery.
- Academic research in geochemistry, hydrology and environmental science.
FAQ
Which heavy metals in sediment can be determined by atomic fluorescence spectrometry?
AFS is optimized for arsenic, mercury, selenium, antimony, bismuth, tellurium and germanium. For conventional heavy metals such as copper, lead, zinc, cadmium and chromium, laboratories combine AFS with atomic absorption spectrometry, ICP-OES or ICP-MS within the same testing program.
What detection limits can be achieved?
With standard sample masses, method detection limits are typically around 0.002 mg/kg for mercury and approximately 0.01 mg/kg for arsenic, selenium and antimony, comfortably below the strictest marine sediment quality thresholds and suitable for background-level studies.
Which standards govern AFS testing of sediments?
The principal standards are HJ 702-2014 (water-bath digestion), HJ 680-2013 (microwave-assisted digestion) and GB 17378.5-2007 (marine sediment analysis), supplemented by GB 18668-2002 for quality classification and ISO/IEC 17025 for laboratory competence. Testing should be commissioned from accredited third-party laboratories with the relevant method scope.
Why is pre-reduction necessary before hydride generation?
Borohydride efficiently generates hydrides only from the lower oxidation states of the analytes. Without pre-reduction of arsenic(V), antimony(V) and selenium(VI), hydride generation is incomplete and results would be significantly biased low.
How should sediment samples be collected and stored?
Samples should be collected with non-metallic sampling tools, stored in clean glass or PTFE containers, kept in the dark and chilled or frozen as soon as possible, and freeze-dried in the laboratory. Mercury is prone to loss and contamination, so prompt preservation and dedicated clean handling are essential.
Are matrix interferences a problem in sediment digests?
The hydride generation step removes most matrix constituents before atomization, and thiourea-based masking further suppresses interference from transition metals. For unusual matrices, standard addition calibration and certified reference material verification are used to confirm accuracy.