Determination of Monomethylarsenic (MMA) in Soil: Testing Methods, Standards, and Technical Guidelin

Monomethylarsenic (MMA) is a methylated organoarsenical species whose toxicity, mobility, and bioavailability in soil differ fundamentally from those of inorganic arsenic, making species-specific measurement essential for accurate environmental risk assessment. This technical article explains how accredited third-party laboratories determine monomethylarsenic in soil, covering sample preservation, extraction procedures, HPLC-ICP-MS and HPLC-HG-AFS analytical techniques, quality control requirements, and the relevant national and international standards.

Technical Overview

Monomethylarsenic, present in soil predominantly as monomethylarsonic acid (MMA, CH3AsO(OH)2), is a methylated organoarsenical compound in which arsenic exists in the pentavalent oxidation state. MMA enters and persists in soil through several pathways: microbial methylation of inorganic arsenic under flooded or anaerobic conditions, historical application of organoarsenical herbicides such as monosodium methanearsonate (MSMA) on cotton fields, turf, and golf courses, and degradation of arsenic-bearing feed additives in manure and poultry-litter amended soils.

Because the toxicity, solubility, plant uptake, and leaching behavior of arsenic depend strongly on its chemical form, total arsenic determination alone cannot support a reliable risk assessment. Speciation analysis, which quantifies MMA alongside arsenite As(III), arsenate As(V), and dimethylarsinic acid (DMA), provides the decisive information for contaminated site characterization, remediation design, and regulatory compliance. The analytical difficulty lies in preserving the original species distribution during sampling, drying, and extraction, since arsenic species readily interconvert through oxidation, reduction, and methylation reactions. Modern practice therefore couples mild, speciation-preserving extraction with chromatographic separation and element-specific detection, typically high performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) or with hydride generation atomic fluorescence spectrometry (HPLC-HG-AFS).

Testing Samples

Agricultural and Paddy Soils

Rice paddies and other periodically flooded agricultural soils are priority sample types, because anaerobic microbial communities actively methylate inorganic arsenic and MMA can accumulate in pore water and be taken up by crops. Upland soils treated with MSMA-based herbicides or arsenical pesticides are also routinely submitted.

Contaminated Site Soils

Soils from former pesticide manufacturing plants, wood preservation facilities using chromated copper arsenate (CCA), mining and smelting areas, and tailings-affected land frequently contain elevated methylated arsenic fractions and require speciation-based screening against risk control values such as those in GB 36600-2018.

Sediments and Dredged Materials

River, lake, estuarine, and marine sediments, as well as dredged sediments destined for land application or disposal, are analyzed to evaluate arsenic methylation status and release potential under changing redox conditions.

Amended Soils, Sludge, and Compost

Soils receiving biosolids, composts, and animal manure derived from feed operations may carry organoarsenical residues, and speciation testing verifies whether MMA or DMA is accumulating in the receiving soil.

Testing Items

Testing programs for methylated arsenic in soil typically include the following deliverables, all reported on a dry-weight basis:

• MMA concentration in soil, expressed in mg/kg dry weight, quantified by species-specific calibration.
• Complete arsenic speciation suite covering As(III), As(V), MMA, and DMA, allowing calculation of the inorganic-to-organic arsenic ratio.
• Total arsenic determined after complete microwave digestion with nitric acid or aqua regia, used to check the mass balance of extractable species, typically expected to recover a defined proportion of total arsenic depending on soil mineralogy.
• Method performance parameters, including method detection limit (MDL), reporting limit, spike recoveries, and duplicate precision for the analytical batch.

Testing Methods

Sample Collection and Preservation

Samples are collected following technical specifications for soil environmental monitoring such as HJ/T 166-2004. Because arsenic speciation is unstable, samples should be transported cold and dark, stored at 4 degrees Celsius, and analyzed promptly. Freeze-drying followed by gentle grinding and sieving, typically to below 0.25 mm, is the preferred preparation route; oven drying at elevated temperature is avoided because it promotes oxidation of As(III) and degradation of methylated species.

Extraction of Arsenic Species

A weighed portion of soil, commonly 0.2 to 1.0 g, is extracted with 10 to 25 mL of a mild extractant under conditions that release bound species without transforming them. Widely used extractants include 0.5 to 1.0 mol/L phosphoric acid, often stabilized with a small concentration of ascorbic acid or hydroxylamine hydrochloride, and dilute ammonium carbonate solutions for water-soluble fractions. Extraction is performed by microwave assistance at 50 to 60 degrees Celsius for 30 to 60 minutes, ultrasonic extraction, or end-over-end shaking for up to 16 hours. The extract is centrifuged, filtered through a 0.22 or 0.45 micrometer membrane, and analyzed within 24 to 48 hours, with refrigerated storage of extracts if delay is unavoidable.

Chromatographic Separation

Separation is achieved on a strong anion-exchange HPLC column, typically 250 mm by 4.1 mm with 10 micrometer particles. A common isocratic mobile phase is 10 to 15 mmol/L ammonium phosphate buffer adjusted to pH 5.8 to 6.0 at approximately 1.0 mL/min, with injection volumes of 10 to 50 microliters. Under these conditions the typical elution order is As(III), DMA, MMA, and As(V), with MMA eluting in the middle of a 10 to 15 minute run. Alternative methods employ ammonium carbonate or ammonium sulfate mobile phases under gradient elution for complex soil matrices.

Detection by HPLC-ICP-MS

ICP-MS detection offers the highest sensitivity and is the technique of choice for low-level determination. The column effluent is routed directly into the nebulizer of the ICP-MS, and arsenic is monitored at mass-to-charge ratio 75. Collision cell technology with helium, or reaction mode with oxygen measuring AsO+ at mass-to-charge ratio 91, is used to suppress the polyatomic interference from argon chloride on arsenic. Species-specific external calibration is established over the range of approximately 0.05 to 20 micrograms per liter, with correlation coefficients of 0.999 or better. The approach aligns with the general measurement principles of EPA Method 6020B, EPA Method 200.8, and ISO 17294-2, yielding soil-based method detection limits commonly in the range of 0.001 to 0.01 mg/kg.

Detection by HPLC-HG-AFS

For laboratories equipped with atomic fluorescence spectrometers, the post-column effluent is mixed with potassium persulfate under heating to oxidize organoarsenicals uniformly to As(V), followed by hydride generation with alkaline potassium borohydride in hydrochloric acid. The arsine hydrides are swept by argon into the atomizer and quantified at the arsenic resonance line near 193.7 nm. This adaptation of the hydride generation principles in EPA Method 1632A is cost-effective, free of argon chloride interference, and achieves soil detection limits typically in the range of 0.005 to 0.05 mg/kg.

Total Arsenic Support Determination

Total arsenic is measured independently after microwave digestion following HJ 680-2013 for soil and sediment using atomic fluorescence detection, or after aqua regia extraction with ICP-MS detection consistent with HJ 803-2016, or by the atomic fluorescence procedure of GB/T 22105.2-2008. The comparison of speciated extractable arsenic against total arsenic confirms extraction efficiency and data reliability.

Quality Assurance and Quality Control

Each analytical batch incorporates method blanks, continuing calibration verification standards within plus or minus 10 percent of the true value, laboratory duplicates with relative percent deviation generally below 20 percent, and matrix spike samples with MMA recoveries typically acceptable in the 70 to 130 percent range. Retention time identification windows are set within plus or minus 5 percent of calibration standards, and certified reference soils such as GBW series materials are digested to verify total arsenic accuracy. Spike-recovery experiments on real soil matrices are essential because speciation-oriented certified reference materials remain limited.

Applications

Determination of MMA in soil supports a wide range of environmental management and research objectives:

• Contaminated site investigation and human health risk assessment where inorganic and organic arsenic fractions carry different toxicological weights.
• Screening and compliance testing of development land against risk control values under GB 36600-2018.
• Verification of remediation performance for soil washing, solidification and stabilization, and bioremediation strategies targeting arsenic speciation change.
• Rice and crop safety management, including evaluation of methylated arsenic uptake from flooded paddies.
• Environmental impact assessments for former MSMA herbicide storage areas, CCA wood treatment sites, and mining or smelting legacies.
• Scientific research on arsenic biogeochemical cycling, microbial methylation and demethylation, and redox-driven species transformation.

FAQ

Why must MMA be measured separately instead of relying on total arsenic?

Total arsenic does not reveal the chemical form. MMA and other methylated species differ from inorganic arsenic in toxicity, soil mobility, plant uptake, and remediation behavior. Regulatory frameworks and risk models increasingly demand species-specific data, so measuring MMA directly provides information that total arsenic analysis cannot supply.

What detection limits can be expected?

With HPLC-ICP-MS, soil-based method detection limits of approximately 0.001 to 0.01 mg/kg are routinely achieved. HPLC-HG-AFS typically delivers detection limits of 0.005 to 0.05 mg/kg, which remains sufficient for most contaminated site investigations.

How do laboratories prevent species transformation during sample handling?

Laboratories minimize transformation by cold chain transport, storage at 4 degrees Celsius, freeze-drying instead of oven drying, mild acidic extraction below 60 degrees Celsius, and rapid analysis of filtered extracts, typically within 24 to 48 hours. Spike-recovery tests verify that conversion has not occurred during the procedure.

Which technique is preferable, HPLC-ICP-MS or HPLC-HG-AFS?

HPLC-ICP-MS offers superior sensitivity, multi-element capability, and robustness in complex matrices, making it the international benchmark. HPLC-HG-AFS provides an economical alternative with excellent arsenic selectivity and is widely used where atomic fluorescence instrumentation is standard. Both, when properly validated, produce equivalent qualitative conclusions.

Which standards govern this analysis?

There is no single universal standard dedicated solely to soil MMA. In practice, laboratories combine sampling guidance such as HJ/T 166-2004, total arsenic standards such as HJ 680-2013, HJ 803-2016, and GB/T 22105.2-2008, ICP-MS measurement principles from EPA Method 6020B, EPA Method 200.8, and ISO 17294-2, and hydride generation concepts from EPA Method 1632A, all within a validated laboratory method.

Can MMA be artificially created or destroyed during the test itself?

Genuine risk exists if aggressive digestion or high-temperature extraction is applied. Validated methods therefore use phosphoric acid or dilute salt extractants at controlled temperatures, confirm identity by retention time matching, and demonstrate species stability through matrix spike recoveries and hold-time studies before data are reported.