Determination of Monomethylarsenic (MMA) in Soil: Pretreatment Protocols and HPLC-ICP-MS Analysis

Monomethylarsenic (MMA, monomethylarsonic acid) is a toxic organoarsenic species found in soils affected by arsenical pesticide use, microbial methylation, and industrial contamination. Accurate MMA determination depends critically on a gentle, speciation-preserving pretreatment and extraction step, followed by anion-exchange HPLC separation and ICP-MS detection. This technical guide explains the complete analytical workflow—from sample drying and sieving to extraction, chromatography, quantification, and quality control—for laboratories and environmental consultants performing soil arsenic speciation.

Technical Overview

Monomethylarsonic acid (MMA, CH3AsO(OH)2) is a pentavalent, methylated arsenic species that enters soil through three principal pathways: biotic and abiotic methylation of inorganic arsenic, degradation of historical organoarsenical herbicides such as monosodium and disodium methyl arsenate, and deposition from industrial sources including smelting and wood-preservative handling. Although regulatory frameworks such as GB 36600-2018 and GB 15618-2018 control arsenic in soil on a total-concentration basis, toxicity and mobility differ substantially among arsenic species, so speciation data provide essential additional insight for risk assessment.

MMA cannot be measured by conventional acid digestion, because aggressive digestion (aqua regia, nitric acid microwave digestion) destroys the carbon–arsenic bond and converts all species to inorganic arsenic. Instead, determination of MMA requires a mild aqueous or phosphoric acid extraction that leaches the species intact from the soil matrix, followed by chromatographic separation and element-specific detection. The standard analytical platform couples anion-exchange high-performance liquid chromatography (HPLC) with inductively coupled plasma mass spectrometry (ICP-MS), consistent with the detection requirements of ISO 17294-2 and EPA Method 6020B and the speciated-isotope-dilution concepts outlined in EPA Method 6800.

The overall workflow therefore consists of four linked stages: (1) gentle sample pretreatment (freezing, freeze-drying, grinding, sieving); (2) species-preserving extraction; (3) HPLC separation of MMA from inorganic arsenic and dimethylarsenic (DMA); and (4) ICP-MS quantification at mass-to-charge ratio 75 with interference control. Because every subsequent step depends on the quality of the pretreatment and extraction, this stage is the decisive factor in data reliability.

Testing Samples

Agricultural, Orchard, and Golf Course Soils

Soils with a documented or suspected history of methylarsenical herbicide application are the most common sample type. Typical MMA concentrations range from below 0.01 mg/kg in background soils to several mg/kg at heavily treated sites, usually accompanied by elevated inorganic arsenic.

Industrial and Contaminated Site Soils

Soils from former pesticide manufacturing plants, wood-treatment facilities, tailings areas, and smelter-affected land often contain complex arsenic species mixtures. These matrices typically contain high iron and manganese oxide contents that bind arsenic strongly, which must be considered when selecting the extraction reagent.

Sediments and Ditch Deposits

Anoxic sediments favor microbial methylation and may accumulate MMA and DMA. Redox changes during sampling can alter species distribution, so sediment samples should be collected with minimal air exposure and processed rapidly.

Sample Submission and Preservation Requirements

• Minimum sample quantity: 100–500 g of homogenized soil per determination.
• Containers: pre-cleaned amber glass jars or polyethylene bottles; avoid metal containers.
• Preservation: transport at 4 °C in the dark; where possible, freeze subsamples immediately upon receipt.
• Holding time: extraction within 7–14 days when refrigerated; shorter holding times (24–72 hours) are strongly recommended when inorganic arsenic redox species are also targeted.
• Documentation: full chain-of-custody records, sampling depth, GPS coordinates, and site history.

Testing Items

The core testing item is the concentration of monomethylarsenic in soil, reported as MMA (or as arsenic on a dry-weight basis) in mg/kg. The HPLC separation is inherently multi-species, so the same run normally quantifies the full arsenic speciation profile:

• Monomethylarsenic (MMA / MMAA-V) — the target analyte.
• Arsenite As(III) and arsenate As(V) — inorganic species.
• Dimethylarsenic (DMA / DMAA-V) — co-methylated species.
• Optional: trimethylarsine oxide and other minor species where site history indicates them.
• Supporting determinations: total arsenic by aqua regia extraction (HJ 803-2016) or microwave digestion (HJ 680-2013) to evaluate extraction efficiency, and dry-matter content for result correction.

Results are reported in mg/kg on a dry-weight basis, together with the extraction efficiency (sum of species versus total arsenic) and full quality-control data.

Testing Methods

Principle

A weighed portion of prepared soil is extracted with a reagent that desorbs arsenic species from mineral surfaces without breaking the C–As bond. After centrifugation and membrane filtration, the extract is injected onto an anion-exchange HPLC column. At the mobile-phase pH (typically 5.5–6.2), MMA is monovalent anionic and is separated from neutral As(III), weakly anionic DMA, and divalent As(V). The ICP-MS detects arsenic eluting from the column in time-resolved mode at m/z 75, and MMA is quantified by external calibration against species-specific standards.

Sample Pretreatment Before Extraction

Pretreatment follows the general logic of ISO 11464 but is modified to protect arsenic speciation:

• Freeze-drying (lyophilization) at low temperature is the preferred drying method; oven drying above 40 °C and prolonged air-drying can shift species distribution and are avoided.
• Dried samples are homogenized and ground in an agate mortar or cryogenic mill; metal grinders are avoided to prevent contamination.
• Sieving: pass through 2 mm to remove debris, then grind and sieve to 0.25 mm (60 mesh) or finer for extraction, and to 0.149 mm for total-arsenic determination.
• Prepared material is stored in the dark at 4 °C in sealed containers and analyzed promptly.

Extraction (The Critical Pretreatment Step)

Two extractant systems are widely validated for soil arsenic speciation:

• Phosphoric acid system: 0.5–1.0 mol/L H3PO4, frequently with 0.1–0.5 mol/L ascorbic acid added to keep the medium reducing and suppress oxidation of As(III). Phosphate displaces arsenate and methylarsenate strongly sorbed on Fe/Mn oxides, giving high extraction efficiency in contaminated mineral soils.
• Phosphate buffer system: 10–50 mmol/L ammonium dihydrogen phosphate adjusted to pH 5–6, a gentler extractant well matched to the chromatographic mobile phase and suited to organic-rich or moderately contaminated soils.

Typical extraction conditions:

• Weigh 0.5–1.0 g (to 0.0001 g) of prepared soil into a polypropylene centrifuge tube.
• Add extractant at a solid-to-liquid ratio of 1:10 to 1:20 (w/v).
• Extract by ultrasonication (30–60 min, bath kept below 40 °C), temperature-controlled water bath (55–70 °C, 2–6 h), or microwave-assisted extraction (55–80 °C, 10–30 min). Temperatures above approximately 80 °C are avoided to prevent species transformation.
• Centrifuge at 4000–10000 rpm for 10–15 min; optionally repeat extraction of the residue and combine extracts to improve recovery.
• Filter the supernatant through a 0.22–0.45 μm membrane filter and analyze within 24–48 h, storing extracts at 4 °C in the dark.

Chromatographic Separation

• Column: strong anion-exchange HPLC column, 250 mm × 4.1 mm, 10 μm particle size (or equivalent guard plus analytical column).
• Mobile phase: 10–25 mmol/L ammonium phosphate buffer (NH4H2PO4 or NH4H2PO4/(NH4)2HPO4), pH 5.5–6.2, optionally with 0.5–1 % methanol to sharpen peaks.
• Flow rate: 1.0 mL/min; column temperature: 25–30 °C; injection volume: 10–50 μL.
• Typical elution order: As(III) (near void), DMA, MMA, As(V); MMA retention time is confirmed daily with a pure standard.

ICP-MS Detection

• Monitor isotope: 75As (sole arsenic isotope, m/z 75).
• Interference control: 40Ar35Cl+ from chloride is suppressed using helium collision mode with kinetic energy discrimination, or eliminated by oxygen reaction mass-shift to 75As16O+ at m/z 91; phosphoric acid extracts are preferred partly because they introduce no chloride.
• Internal standard: germanium (72Ge or 74Ge) added post-column via a mixing tee.
• Acquisition: time-resolved analysis with dwell times of 100–300 ms per point to define chromatographic peaks precisely.

Calibration and Quantification

• External calibration with species-specific MMA standards covering 0.1–100 μg/L as arsenic, with correlation coefficients ≥0.995.
• Peak-area (or peak-height) integration; species identified by retention-time matching within ±2 % of the standard.
• Calculations correct for extraction volume, sample mass, dilution factors, and dry-matter content to yield mg/kg dry weight.
• Method detection limits for MMA in soil are typically ≤0.01 mg/kg, with quantification limits of 0.03–0.05 mg/kg.

Quality Assurance and Quality Control

• Reagent blanks and extraction blanks carried through the complete procedure each batch.
• Duplicate analyses with relative deviation ≤20 % for MMA concentrations above the quantification limit.
• Matrix spike recoveries for MMA in the range 70–130 %.
• Continuing calibration verification every 10 samples; recalibration if drift exceeds ±10 %.
• Extraction-efficiency check: sum of extracted species compared with total arsenic; significantly low recoveries are reported and interpreted with caution.
• Use of certified reference materials or interlaboratory-verified soils where available; all procedures performed under an ISO/IEC 17025-compliant management system.

Applications

Determination of monomethylarsenic in soil supports a wide range of environmental and scientific objectives:

• Contaminated site investigation: distinguishing pesticide-derived organoarsenic from geogenic inorganic arsenic at brownfield, orchard, golf course, and former agrochemical plant sites.
• Risk assessment and regulatory support: supplementing total-arsenic screening under GB 36600-2018 and GB 15618-2018 with toxicity-relevant speciation data and supporting human-health and ecological risk models.
• Remediation verification: tracking species-specific concentration changes before and after soil washing, stabilization, or bioremediation to confirm genuine arsenic removal rather than species interconversion.
• Food-chain safety studies: evaluating MMA availability for crop uptake in agricultural soils and its transfer into rice, vegetables, and groundwater.
• Biogeochemical research: quantifying microbial methylation and demethylation rates, redox-driven species cycling in flooded or anoxic soils, and arsenic mobility in wetlands and sediments.
• Due diligence and litigation support: establishing whether methylarsenic signatures are attributable to specific historical herbicide or industrial sources.

FAQ

Why can't total-arsenic digestion methods be used to determine MMA?

Aqua regia, nitric acid microwave digestion (e.g., EPA Methods 3050B/3051A, HJ 680-2013, HJ 803-2016) completely oxidizes the methyl group and the C–As bond, converting MMA into inorganic arsenic. Only total arsenic can be measured this way. MMA determination requires a mild extraction that leaches the molecule intact, followed by chromatographic separation before detection.

Does interconversion between species occur during extraction?

MMA is one of the more stable arsenic species under the recommended extraction conditions. The principal transformation risk concerns As(III)/As(V) redox cycling, which is minimized by adding ascorbic acid to phosphoric acid extractants, controlling extraction temperature below 80 °C, and analyzing extracts within 24–48 h. Spike-recovery experiments verify that no significant MMA degradation occurs in each matrix.

What extraction efficiency should be expected?

Phosphate-based extractants typically recover 40–100 % of total arsenic depending on mineralogy, with strongly bound arsenic in iron-rich contaminated soils extracted less completely than weakly bound arsenic. The extraction efficiency is always reported alongside species concentrations so results can be interpreted correctly.

What are typical detection limits and reporting units?

With a 1.0 g sample and 20 mL extractant, HPLC-ICP-MS typically achieves method detection limits of about 0.002–0.01 mg/kg and practical quantification limits of 0.03–0.05 mg/kg for MMA, reported in mg/kg on a dry-weight basis.

How should samples be collected and shipped for speciation analysis?

Collect with clean stainless-steel or plastic tools into amber glass containers, minimize headspace and air exposure, keep samples at 4 °C in the dark, and ship with frozen gel packs under a documented chain of custody. Freeze or freeze-dry upon receipt; never oven-dry samples intended for speciation.

Is the method standardized?

Soil arsenic speciation is performed by accredited third-party laboratories using validated in-house procedures, since the HPLC-ICP-MS measurement platform is anchored in ISO 17294-2 and EPA Method 6020B, with speciation quality-control concepts drawn from EPA Method 6800 and sample pretreatment aligned with ISO 11464. Method validation data, including precision, recovery, and detection limits, should be requested with any report.