Standard Operating Procedure for the Determination of Total Organic Arsenic in Surface Water

The determination of total organic arsenic in surface water is a key requirement for environmental compliance monitoring, ecological risk assessment, and water quality management under national surface water standards such as GB 3838-2002. This standard operating procedure describes validated analytical workflows, principally anion-exchange HPLC coupled with ICP-MS or hydride generation atomic fluorescence spectrometry, covering sampling, pretreatment, calibration, instrumental operation, quality control, and data reporting. It serves as a practical technical reference for environmental monitoring agencies, research institutions, and accredited third-party laboratories.

Technical Overview

Organic arsenic refers to arsenic species in which the arsenic atom is covalently bonded to one or more carbon-containing groups. In surface water, the principal organoarsenicals include monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), trimethylarsine oxide (TMAO), and arsenobetaine (AsB), along with phenylarsonic compounds in waters impacted by historic agrochemical or animal-feed additive use. These species enter aquatic systems through microbial methylation of inorganic arsenic, agricultural runoff, industrial discharge, and geothermal inputs.

Because toxicity, mobility, and bioaccumulation behavior differ markedly among arsenic species, regulatory and scientific practice increasingly requires speciation information rather than total arsenic alone. The national surface water quality standard GB 3838-2002 specifies a maximum of 50 μg/L total arsenic for Class I–III waters, and international drinking water guidelines are typically set at 10 μg/L; the organic fraction is usually a minor but analytically significant component of the total burden.

Two complementary approaches are recognized for determining total organic arsenic. Direct speciation separates and quantifies individual organoarsenic species by anion-exchange HPLC coupled to inductively coupled plasma mass spectrometry (ICP-MS) or hydride generation atomic fluorescence spectrometry (HG-AFS), with the organic species summed. Alternatively, a difference method calculates total organic arsenic as total arsenic, determined after oxidative digestion in accordance with ISO 17294-2 or ASTM D5673, minus the sum of the inorganic species As(III) and As(V). Typical method detection limits are 0.02–0.1 μg/L per species by HPLC-ICP-MS, with a linear working range extending to 50–100 μg/L.

Testing Samples

Rivers and Streams

Flowing waters are sampled at predetermined cross-sections using grab or depth-integrated techniques. Moderate to high suspended solids are common, so both filtered (dissolved, 0.45 μm) and unfiltered aliquots should be collected because arsenic species partition between the dissolved and particulate phases.

Lakes and Reservoirs

Thermal stratification, algal activity, and seasonal turnover strongly influence arsenic methylation. Depth-profile sampling across the epilimnion, thermocline, and hypolimnion is recommended wherever redox-driven speciation changes are expected.

Estuarine and Brackish Surface Waters

Elevated chloride introduces the 40Ar35Cl+ spectral interference at m/z 75 and challenges chromatographic retention. Samples with salinity above 2‰ should be analyzed with collision or reaction cell technology and, where necessary, matrix-matched calibration.

Industrial and Agricultural Impact Zones

Waters downstream of mining, smelting, geothermal discharge, wood preservative production, or historic organoarsenical pesticide application warrant full speciation. Concentrations may exceed the linear range and require dilution with mobile phase rather than ultrapure water to preserve chromatographic behavior.

Testing Items

The core deliverable of this procedure is total organic arsenic in surface water, reported in μg/L as arsenic (As). The following items are determined to support this result:

• Total organic arsenic, the sum of all quantified organoarsenic species
• Monomethylarsonic acid (MMA)
• Dimethylarsinic acid (DMA)
• Trimethylarsine oxide (TMAO)
• Arsenobetaine (AsB) and other minor organoarsenicals present above the reporting limit
• Inorganic arsenic species: arsenite As(III) and arsenate As(V)
• Total arsenic, for verification of mass balance and the difference method
• Supporting parameters: pH, conductivity, dissolved organic carbon, and suspended solids, which govern species stability

Testing Methods

Method A: HPLC-ICP-MS Speciation (Primary Method)

Species are separated on a strong anion-exchange column (250 mm × 4.1 mm, 10 μm quaternary ammonium polymer packing or equivalent) using a volatile ammonium carbonate or ammonium phosphate buffer (10–25 mmol/L, pH 8.5–9.0) at 1.0 mL/min and 25–30°C, with a 50–100 μL injection volume. Baseline separation of As(III), As(V), MMA, and DMA is achieved within 10–15 minutes. The column effluent is routed directly to the ICP-MS nebulizer, and arsenic is monitored at m/z 75 under helium collision mode (kinetic energy discrimination) to suppress the ArCl+ polyatomic interference. Germanium (m/z 72) or rhodium (m/z 103) is recommended as an internal standard added post-column. Total organic arsenic equals the sum of the chromatographically resolved organoarsenic peaks. This approach extends the instrumental principles of ISO 17294-2 and ASTM D5673 to chromatographic coupling.

Method B: HPLC-HG-AFS Speciation

This is a cost-effective alternative for laboratories equipped with atomic fluorescence instrumentation. The same anion-exchange separation feeds a post-column reaction with hydrochloric acid (5–10%, v/v) and potassium borohydride (0.5–2%, m/v, in 0.5% KOH) to generate arsines that are swept into an argon-hydrogen flame for atomic fluorescence detection. Because the hydride yields of MMA and DMA are incomplete, an online UV/persulfate oxidation module is installed between the column and the hydride generation unit to convert organoarsenicals quantitatively. Detection limits of 0.1–0.5 μg/L per species are routinely achieved.

Method C: Difference (Subtraction) Method

Total arsenic is determined on an acidified aliquot by ICP-MS or AFS after appropriate digestion, and inorganic arsenic (As(III) + As(V)) is determined by speciation on the preserved unacidified aliquot. Total organic arsenic is then calculated as total arsenic minus inorganic arsenic. This method is acceptable for screening purposes but carries the combined uncertainty of two determinations and should not replace direct speciation when species-specific concentrations are required.

Interferences and Matrix Control

• 40Ar35Cl+ and 40Ca35Cl+ interferences at m/z 75, controlled by collision cell operation and by the early chromatographic elution of chloride near the void volume
• High dissolved organic carbon, which may shorten retention times and broaden peaks; guard columns and periodic replacement of the analytical column inlet frit are recommended
• Salinity above 2‰, which requires matrix-matched calibration and dilution to protect the ICP-MS interface cones
• Cross-species conversion during storage, minimized by immediate filtration, dark storage at 4°C, and analysis within 48 hours

Reagents and Instrumentation

• Ultrapure water (18.2 MΩ·cm at 25°C) for the preparation of all solutions
• Mobile phase of ammonium carbonate or ammonium phosphate buffer, prepared fresh, filtered through a 0.22 μm membrane, and degassed
• Sub-boiling distilled nitric acid for total arsenic preservation and digestion
• Potassium borohydride and hydrochloric acid of guaranteed reagent grade for hydride generation
• Certified single-species stock solutions of As(III), As(V), MMA, and DMA (1000 mg/L as As), traceable to a national metrology institute
• HPLC system with an anion-exchange analytical column and matching guard column
• ICP-MS equipped with a collision or reaction cell, or an atomic fluorescence spectrometer with online hydride generation
• Pre-cleaned HDPE sample bottles, 0.45 μm low-arsenic syringe filters, pH meter, and analytical balance with 0.1 mg readability

Step-by-Step Operating Procedure

Sample Collection and Preservation

• Collect samples in pre-cleaned HDPE bottles while wearing powder-free nitrile gloves; rinse bottles two to three times with sample water before filling.
• Prepare field blanks with ultrapure water and transport blanks with a known low-level standard.
• Filter one aliquot through a 0.45 μm membrane within 12 hours of collection for dissolved speciation; retain an unfiltered aliquot for whole-water total arsenic.
• Do not acidify the speciation aliquot; acidification shifts As(III)/As(V) equilibria and may cause demethylation during storage.
• Store samples at 4°C in the dark and analyze speciation aliquots within 48 hours (72 hours maximum); acidify the total arsenic aliquot to pH below 2 with nitric acid and analyze within 14 days.

Calibration and Standards Preparation

• Prepare a mixed working standard of As(III), As(V), MMA, and DMA in mobile phase at six levels spanning 0.1–20 μg/L.
• Verify calibration linearity with a correlation coefficient of at least 0.995 for each species; apply weighted regression (1/x) where low-level curvature is present.
• Confirm calibration with an independent mid-range check standard; the acceptance criterion is ±10% of the nominal concentration.
• Inject a mobile phase blank after the highest standard; carryover must be below 1% of the preceding response.

Instrument Setup and System Suitability

• Equilibrate the chromatographic column with at least 10 column volumes of mobile phase until a stable baseline is obtained.
• Tune the ICP-MS daily: verify sensitivity at mass 75, an oxide ratio CeO+/Ce+ below 3%, and a doubly charged ratio Ce++/Ce+ below 3%.
• Demonstrate chromatographic resolution of at least 1.5 between adjacent arsenic peaks and retention time repeatability (RSD ≤ 2%, n = 5) using a system suitability solution before sample analysis.

Sample Analysis Sequence

• Run the sequence as follows: calibration standards, independent verification, method blank, laboratory control sample, then samples with a continuing calibration verification, duplicate, and matrix spike every ten injections.
• Dilute samples exceeding the calibration range with mobile phase, reanalyze, and apply the dilution factor.
• Review each chromatogram for integration integrity, retention time drift, and unassigned peaks; reinject if any criterion fails.

Calculation and Reporting

• Calculate species concentrations from the calibration curve in μg/L as arsenic; total organic arsenic equals the sum of MMA, DMA, TMAO, AsB, and any other quantified organoarsenicals.
• For the difference method, compute total organic arsenic as total arsenic minus the sum of As(III) and As(V) and verify that the mass balance closes within ±15%.
• Report results to two significant figures together with the method detection limit, reporting limit, and dilution factors; flag any result below the reporting limit as non-detect.

Quality Control and Assurance

Each analytical batch must include a method blank, a laboratory control sample, a duplicate analysis, and a matrix spike. Typical acceptance criteria are blanks below one-half of the reporting limit, a duplicate relative percent difference of 20% or less, and matrix spike recoveries of 70–130% (80–120% for laboratory control samples). Method detection limits are established from at least seven replicate low-level fortified samples using the t-statistic at the 99% confidence level and are verified at least annually. Participation in interlaboratory proficiency testing and the routine use of certified reference materials are strongly recommended, and all quality control data should be maintained on control charts with documented corrective actions for any breach of limits, consistent with accreditation to ISO/IEC 17025.

Applications

This operating procedure supports a wide range of environmental and regulatory applications:

• Compliance monitoring of rivers, lakes, and reservoirs against surface water quality standards such as GB 3838-2002
• Source apportionment, where elevated methylated species indicate active microbial transformation and phenylarsonic compounds point to specific industrial or agrochemical inputs
• Ecological and human health risk assessment within drinking water source protection zones
• Performance verification of arsenic removal and site remediation systems
• Biogeochemical research on arsenic cycling, methylation, and trophic transfer in aquatic ecosystems
• Technical support provided by accredited third-party laboratories for environmental consulting, permitting, and dispute resolution

FAQ

What is the difference between total arsenic and total organic arsenic?

Total arsenic is the sum of all arsenic species after oxidative digestion, whereas total organic arsenic comprises only the organoarsenicals such as MMA, DMA, TMAO, and arsenobetaine. Determining the organic fraction requires chromatographic separation, because a total arsenic measurement alone cannot distinguish between inorganic and organic forms.

Why must samples for speciation not be acidified?

Nitric acid preservation can oxidize As(III) to As(V), destabilize organoarsenicals, and promote demethylation during storage, distorting the species distribution. Speciation aliquots are therefore filtered, refrigerated at 4°C in the dark, and analyzed promptly, while only the total arsenic aliquot is acidified.

What detection limits can be achieved?

With HPLC-ICP-MS, method detection limits of 0.02–0.1 μg/L per species are routinely attainable. HPLC-HG-AFS typically achieves 0.1–0.5 μg/L per species, which remains adequate for most surface water monitoring programs.

How is the chloride interference on mass 75 controlled?

The 40Ar35Cl+ polyatomic ion overlaps with arsenic at m/z 75. It is controlled by operating the ICP-MS in helium collision mode, optionally using hydrogen or oxygen reaction cell mass shift, and by exploiting the chromatographic separation under which chloride elutes close to the void volume ahead of the arsenic species of interest.

Is the difference method acceptable for reporting total organic arsenic?

The difference method is acceptable for screening and quality verification, but it combines the uncertainties of two separate determinations. Direct chromatographic speciation is preferred whenever species-specific data or defensible low-level results are required.

Which standards govern this determination?

Evaluation criteria derive from GB 3838-2002 for surface water quality classification. Instrumental determination of total arsenic follows ISO 17294-2 and ASTM D5673, with ASTM D2972 covering classical arsenic methods, and sector-specific environmental standards provide detailed procedures for AFS- and ICP-MS-based arsenic measurement in individual jurisdictions.

How long can samples be stored before analysis?

Speciation aliquots should be analyzed within 48 hours of collection and never beyond 72 hours, even at 4°C in the dark. Acidified total arsenic aliquots remain valid for up to 14 days when stored as prescribed.