Total organic arsenic is a key indicator for evaluating arsenic speciation, bioavailability, and ecological risk in surface water environments. This article presents a professional comparison of mainstream determination techniques, including HPLC-ICP-MS, HPLC-HG-AFS, and the total-minus-inorganic arsenic difference method, with respect to analytical principle, detection limit, precision, accuracy, and applicability. It serves as practical technical guidance for environmental monitoring laboratories, researchers, and water quality managers.
Technical Overview
Arsenic in surface water occurs in multiple chemical forms, primarily the inorganic species arsenite As(III) and arsenate As(V), together with organoarsenicals such as monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), trimethylarsine oxide (TMAO), and occasionally arsenobetaine (AsB). Methylated species are generated through biological methylation by algae, microorganisms, and sediments, and their distribution reflects both natural geochemical processes and anthropogenic inputs such as pesticide runoff, mine drainage, and industrial discharge. Because toxicity, mobility, and treatment behavior differ markedly among species, total arsenic alone is insufficient for a complete risk evaluation.
Total organic arsenic is generally defined either as the arithmetic sum of all quantified organoarsenical species (MMA + DMA + TMAO + AsB, etc.) or operationally as the difference between total arsenic and the sum of inorganic arsenic species (As(III) + As(V)). In China, GB 3838-2002 specifies a Class III surface water limit of 0.05 mg/L for total arsenic, while GB 5749-2022 and the WHO guideline value for drinking water are both 0.01 mg/L. Although current regulations target total arsenic, speciation data and total organic arsenic results are increasingly demanded in environmental impact assessments, pollution source apportionment, and ecological research.
Several analytical approaches are available for determining total organic arsenic, and the choice among them involves trade-offs in sensitivity, selectivity, throughput, equipment cost, and susceptibility to matrix interference. This article compares the principal methods under the following dimensions:
- Analytical principle and scope of species coverage
- Method detection limit (MDL) and linear range
- Precision (repeatability RSD) and accuracy (spike recovery)
- Resistance to matrix interference (chloride, transition metals, organic matter)
- Instrumentation cost, operational complexity, and applicability
Testing Samples
Rivers and Streams
River water affected by mining, smelting, geothermal sources, or historical organoarsenical pesticide use is the most common matrix. Samples are typically collected at background, discharge-influenced, and downstream control sections in accordance with HJ 91.1-2019 and the ISO 5667 series. Arsenic concentrations usually range from below 0.1 μg/L to several tens of μg/L, with organic species frequently constituting 5–30% of total arsenic in biologically active reaches.
Lakes and Reservoirs
Stratified lakes and drinking water reservoirs exhibit strong seasonal speciation shifts. In epilimnetic waters with active algal methylation, DMA can become a dominant species during summer blooms, while hypolimnetic anoxic waters favor As(III). Depth-profiled sampling is often required to capture vertical gradients in total organic arsenic.
Estuarine and Brackish Surface Water
Estuarine samples contain elevated chloride, typically above 1000 mg/L, which creates the polyatomic interference 40Ar35Cl+ at mass 75 in ICP-MS and suppresses hydride generation efficiency in AFS. Dilution, collision/reaction cell technology, or chromatographic separation must be applied, making these samples a critical test of method robustness in comparative studies.
Sample Collection and Preservation
For speciation analysis, samples are filtered on site through 0.45 μm membranes into pre-cleaned amber glass or HDPE bottles, optionally buffered with a small amount of EDTA to complex transition metals, and transported at 4 °C in the dark. Strong acidification must be avoided because pH below 2 can interconvert MMA, DMA, and inorganic species over storage. Analysis should be completed within 48–72 hours for speciation, while samples for total arsenic may be acidified with nitric acid to pH below 2 and held longer in accordance with HJ 493-2009 and ISO 5667-3.
Testing Items
The core deliverable is the concentration of total organic arsenic in μg/L, supported by a full speciation profile and total arsenic for mass balance verification. Typical testing items include:
- Total organic arsenic (sum of MMA, DMA, TMAO, AsB and other organoarsenicals, or total As minus inorganic As)
- Individual species: As(III), As(V), MMA(V), DMA(V), TMAO, arsenobetaine
- Total arsenic after persulfate or nitric acid digestion
- Auxiliary matrix parameters: pH, dissolved organic carbon, suspended solids, chloride, sulfate, iron, and manganese
Testing Methods
Method A: HPLC-ICP-MS Coupled Technique
High performance liquid chromatography is coupled on-line to inductively coupled plasma mass spectrometry. Species are separated on a polymeric strong anion-exchange column using an ammonium carbonate or phosphate buffer eluent at pH 6–9, with injection volumes of 20–100 μL and run times of 10–20 minutes. The ICP-MS monitors m/z 75 in helium collision mode (kinetic energy discrimination) to suppress the 40Ar35Cl+ interference. Species results are summed to obtain total organic arsenic directly. Reference frameworks include US EPA Method 6020B, ISO 17294-2, ASTM D5673, and HJ 700-2014 for the ICP-MS measurement component.
- Method detection limit: approximately 0.005–0.05 μg/L per species
- Linear range: typically 0.05–200 μg/L, with correlation coefficients r ≥ 0.999
- Repeatability: RSD generally below 5% at 1–5 μg/L
- Spike recovery: 85–110% for MMA and DMA in surface water
- Advantages: highest sensitivity and selectivity, simultaneous multi-speciation, direct sum result
- Limitations: high capital and operating cost, requires skilled operators and stable chromatographic conditions
Method B: HPLC-HG-AFS Coupled Technique
Liquid chromatography is coupled to hydride generation atomic fluorescence spectrometry. Because methylated arsenicals respond inefficiently to direct hydride generation, a post-column online oxidation step using potassium persulfate under UV or thermal catalysis converts MMA and DMA to hydride-active forms before AFS detection. This approach is widely adopted in routine monitoring laboratories because AFS hardware is economical and well established under HJ 694-2014 and US EPA Method 1632 concepts.
- Method detection limit: approximately 0.05–0.3 μg/L per species
- Repeatability: RSD generally below 10% at 1–5 μg/L
- Spike recovery: 80–120%, dependent on oxidation efficiency
- Advantages: low cost, single-element sensitivity for arsenic, well suited to China regulatory practice
- Limitations: lower sensitivity than ICP-MS, requires careful control of oxidation and hydride generation conditions; susceptible to suppression by transition metals, antimony, and selenium
Method C: Difference Method (Total Arsenic Minus Inorganic Arsenic)
The simplest approach quantifies total arsenic after alkaline persulfate digestion by ICP-MS or AFS, then determines inorganic arsenic separately, either by selective hydride generation at controlled pH for As(III) plus prereduced As(V), or by a standalone chromatographic run. Total organic arsenic is calculated by subtraction. This method requires only standard laboratory equipment.
- Method detection limit: approximately 0.1–0.5 μg/L for the calculated result, governed by the poorer of the two component MDLs
- Advantages: minimal instrumentation, fast, suitable for screening large sample batches
- Limitations: uncertainty accumulates from two independent determinations; incomplete persulfate oxidation of AsB or arsenosugars causes negative bias; non-target species are invisible to the mass balance; results should be treated as operational rather than species-specific
Comparative Summary
For trace-level work in pristine surface water, HPLC-ICP-MS provides the best sensitivity, precision, and species coverage and is the recommended reference method. HPLC-HG-AFS offers the best cost-to-performance ratio for routine monitoring at low μg/L levels and delivers directly species-resolved data. The difference method is acceptable for screening or when organic arsenic constitutes a substantial fraction of total arsenic, but it should be cross-validated against a chromatographic method whenever regulatory or research conclusions depend on the result. In high-chloride estuarine matrices, chromatographic separation ahead of the detector is strongly preferred to control spectral interference.
Quality Assurance and Method Validation
Method validation should follow HJ 168-2020 or US EPA 40 CFR Part 136 Appendix B protocols. Key QC elements include method blanks, laboratory control samples, matrix spikes at 1–5 μg/L with acceptance of 70–130% recovery, duplicate analysis with RSD below 20%, seven-replicate MDL determination, calibration verification every 10 samples, and participation in interlaboratory comparison or analysis of certified reference materials. Species mass balance, defined as the sum of species divided by independently measured total arsenic, should fall within 80–120% to confirm quantitative recovery of the chromatographic method.
Applications
Determination of total organic arsenic in surface water supports a wide range of regulatory, scientific, and engineering objectives:
- Routine surface water quality monitoring and compliance screening against GB 3838-2002 and drinking water source protection requirements
- Ecological and human health risk assessment, where organic arsenic fractions modify toxicity-weighted exposure estimates
- Pollution source tracing and apportionment, since MMA/DMA dominance indicates biological methylation while inorganic dominance suggests industrial or geothermal inputs
- Biogeochemical research on arsenic cycling, algal methylation, and sediment–water exchange in lakes and reservoirs
- Performance evaluation of water treatment processes, including coagulation, adsorption, and oxidation–filtration trains
- Environmental impact assessment and remediation verification at contaminated sites, mining areas, and former pesticide manufacturing zones
FAQ
What is included in total organic arsenic in surface water?
Total organic arsenic comprises all arsenic species containing arsenic–carbon bonds, predominantly MMA, DMA, and TMAO, and occasionally AsB near biologically productive waters. Analytically it is reported either as the direct sum of chromatographically resolved organic species or as total arsenic minus the sum of As(III) and As(V).
Which method offers the lowest detection limit?
HPLC-ICP-MS typically achieves per-species detection limits of 0.005–0.05 μg/L, roughly one order of magnitude better than HPLC-HG-AFS at 0.05–0.3 μg/L and clearly superior to the difference method, which is limited to about 0.1–0.5 μg/L for the calculated organic fraction.
Why should speciation samples not be strongly acidified during preservation?
Strong acidification can catalyze species interconversion, including degradation of methylated arsenicals and oxidation of As(III) to As(V), distorting the organic-to-inorganic distribution. Speciation samples are therefore filtered, kept chilled and dark, and analyzed promptly, whereas only total arsenic samples are acidified to pH below 2 with nitric acid.
When is the difference method acceptable?
The difference method is appropriate for screening surveys, high-concentration samples, and matrices where organic species represent a significant fraction of total arsenic. For low-level regulatory decisions or research-grade speciation data, a chromatographic method such as HPLC-ICP-MS or HPLC-HG-AFS should be used, or at minimum used to verify the difference result.
What are the main interferences in these methods?
In ICP-MS, chloride generates the 40Ar35Cl+ interference at m/z 75, controlled by collision cell helium mode or chromatographic separation. In hydride generation AFS, high concentrations of nickel, cobalt, copper, antimony, bismuth, and selenium suppress signal; masking with thiourea–ascorbic acid or dilution is applied. In the difference method, incomplete persulfate oxidation of refractory organoarsenicals is the principal bias source.
Is there a regulatory limit specifically for organic arsenic in surface water?
Current standards including GB 3838-2002 and international drinking water guidelines regulate total arsenic rather than organic arsenic specifically. However, speciation data are increasingly requested in risk assessment and scientific evaluation because inorganic arsenic is substantially more toxic than methylated species, and accredited third-party laboratories routinely provide species-resolved reporting alongside total arsenic.