Determination of Total Organic Arsenic in Surface Water: Analytical Methods and Technical Guide

Total organic arsenic in surface water is a critical indicator of water quality and ecological risk, because organoarsenic species differ from inorganic arsenic in toxicity, mobility, and environmental fate. This article presents professional analytical approaches for determining total organic arsenic in surface water, including HPLC–ICP-MS speciation, hydride generation atomic fluorescence spectrometry, and difference-based calculation, together with sampling protocols, quality control requirements, and relevant international and national standards. It is intended for environmental chemists, water quality managers, and accredited third-party laboratories engaged in surface water monitoring and risk assessment.

Technical Overview

Arsenic enters surface water through geological weathering, mining drainage, industrial effluents, and agricultural runoff, and it exists in both inorganic and organic chemical forms. In unpolluted natural waters, inorganic species, namely arsenite As(III) and arsenate As(V), usually predominate, while organoarsenic compounds such as monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), trimethylarsine oxide (TMAO), and arsenobetaine (AsB) appear as products of microbial methylation or as residues of anthropogenic chemicals, including legacy arsenical herbicides and organoarsenic feed additives.

Total organic arsenic refers to the sum of all organically bound arsenic species in a water sample. Although most water quality standards regulate total arsenic, GB 3838-2002, the surface water environmental quality standard, sets a limit of 50 µg/L for Class III waters, and the WHO drinking water guideline value is 10 µg/L as total arsenic, the organic fraction is indispensable for refined risk assessment because the toxicological profiles of MMA, DMA, and AsB differ substantially from those of As(III) and As(V). Since no single instrumental channel responds equally to all arsenic species, total organic arsenic is determined either by fully speciating the sample or by subtracting inorganic arsenic from independently measured total arsenic.

• Inorganic arsenic: arsenite As(III) and arsenate As(V), the most toxic and typically dominant forms in surface water.
• Methylated arsenic: MMA and DMA, produced by microbial methylation and discharged from industrial and agricultural sources.
• Higher organoarsenicals: TMAO, arsenobetaine, arsenocholine, and substituted phenylarsonic acids from specific industrial origins.

Testing Samples

River and Stream Water

Representative samples are collected at midstream sections away from bank effects, bridge scour zones, and direct discharge points, using grab or depth-integrated sampling depending on the monitoring objective. Flow conditions and recent rainfall events should be recorded because storm runoff can transiently elevate organoarsenic loads from watershed sources.

Lake and Reservoir Water

Thermal stratification and seasonal turnover influence arsenic speciation through redox changes in the hypolimnion. Samples are taken at defined depths along a vertical profile, with the epilimnion, thermocline, and bottom water sampled separately when transformation processes are under investigation.

Estuarine and Brackish Surface Water

Salinity gradients in estuaries impose strong matrix effects, particularly the polyatomic interference on arsenic determination caused by chloride. These samples require collision or reaction cell operation, matrix-matched calibration, and dilution verification to guarantee accurate species quantification.

Sample Collection and Preservation

Samples are collected into pre-cleaned amber borosilicate glass or PTFE bottles and filtered through 0.45 µm membranes when the dissolved fraction is required. For speciation analysis, strong acidification must be avoided because it can alter species distribution; accepted practice involves EDTA addition to buffer pH and complex cations, storage at 4 °C in the dark, and analysis within 48 hours, or within a validated holding period not exceeding seven days. Field blanks, trip blanks, and field duplicates accompany every sampling campaign.

Testing Items

The analytical program for total organic arsenic comprises the following determinands, all reported as arsenic mass concentration in µg/L as As:

• Total arsenic, determined after complete digestion or by full-speciation summation.
• Arsenite, As(III).
• Arsenate, As(V).
• Monomethylarsonic acid, MMA.
• Dimethylarsinic acid, DMA.
• Trimethylarsine oxide, TMAO.
• Arsenobetaine, AsB, and other organoarsenic compounds where present.
• Total inorganic arsenic, calculated as As(III) plus As(V).
• Total organic arsenic, calculated by species summation or by the difference method.

Testing Methods

Speciation Analysis by HPLC–ICP-MS

High performance liquid chromatography coupled with inductively coupled plasma mass spectrometry is the reference technique for arsenic speciation. An anion-exchange column with an ammonium carbonate or ammonium phosphate mobile phase at pH 8.5 to 9.0 separates As(III), As(V), MMA, and DMA within approximately 10 to 15 minutes, while cationic species such as arsenobetaine are resolved on a complementary cation-exchange column when required. The ICP-MS monitors mass 75 under helium collision mode or oxygen reaction mode with mass shift to AsO+ at mass 91, which removes the ArCl+ polyatomic interference and is essential for chloride-bearing samples. Species detection limits of 0.01 to 0.1 µg/L as As are routinely achieved. Instrument qualification, calibration, and reporting follow ISO 17294-2 and ASTM D5673, adapted for chromatographic coupling.

LC–HG-AFS and IC–ICP-MS Alternatives

Liquid chromatography coupled with hydride generation atomic fluorescence spectrometry provides a cost-effective speciation platform consistent with the principles of HJ 694-2014. Because MMA and DMA generate hydrides less efficiently than inorganic arsenic, an online UV photo-oxidation or persulfate oxidation step is inserted between the column and the hydride generator to equalize the response across species. Ion chromatography coupled with ICP-MS offers higher chromatographic resolving power and is advantageous for complex estuarine matrices where overlapping peaks and high ionic strength challenge conventional HPLC separations.

Total Arsenic Determination

Total arsenic is quantified independently by ICP-MS in accordance with ISO 17294-2, ASTM D5673, and HJ 700-2014, or by hydride generation AFS after acidification and prereduction following HJ 694-2014. When non-hydride-active organoarsenicals are suspected, a mild UV-persulfate or microwave digestion must precede the total measurement to ensure complete conversion to As(V); omitting this step underestimates total arsenic and distorts the calculated organic fraction.

Calculation and Reporting of Total Organic Arsenic

Two equivalent approaches are accepted by the technical community:

• Direct summation: total organic arsenic equals the sum of MMA, DMA, TMAO, AsB, and all other resolved organoarsenic species.
• Difference method: total organic arsenic equals total arsenic minus the sum of As(III) and As(V).

The two values should agree within the combined measurement uncertainty, and a mass balance between the sum of species and the independently measured total arsenic, closing within 80% to 120%, validates the dataset. Results are reported as µg/L as As with individual species concentrations, detection limits, and the calculation route stated explicitly.

Quality Assurance and Quality Control

• Multi-point calibration with species-specific standards, correlation coefficient of at least 0.999, and continuing calibration verification every ten samples.
• Internal standard recovery, using germanium or rhodium, maintained within 70% to 130% for all ICP-MS determinations.
• Matrix spike recoveries between 75% and 125% and duplicate relative percent difference no greater than 20%.
• Analysis of certified reference materials of natural water and participation in interlaboratory proficiency testing schemes.
• Field blanks, transport blanks, chromatographic retention-time confirmation, and, where uncertainty exists, verification on a second column chemistry.
• Method detection limits established according to recognized regulatory protocols and reverified at defined intervals.

Applications

Speciation-based determination of total organic arsenic supports a broad range of environmental, industrial, and regulatory activities:

• Routine surface water quality monitoring against GB 3838-2002 and basin-specific management objectives.
• Pollution source tracing that distinguishes geogenic arsenic from industrial or agricultural organoarsenic discharges.
• Human health and ecological risk assessment for drinking water sources, fishery waters, and recreational waters.
• Verification of remediation performance at contaminated sites and evaluation of downstream transport impacts.
• Biogeochemical research on arsenic methylation, redox transformation, and cycling in lakes, reservoirs, and estuaries.
• Technical support for environmental impact assessments and discharge permit compliance demonstrations.

FAQ

Why is total organic arsenic measured separately from total arsenic?

Toxicity, mobility, and bioavailability differ sharply among arsenic species. Inorganic As(III) and As(V) are classified carcinogens, methylated species such as MMA and DMA are of intermediate concern, and arsenobetaine is practically non-toxic. Total arsenic alone can therefore either overstate or understate actual risk, whereas the organic fraction enables a scientifically defensible assessment.

Can total organic arsenic be read directly from the instrument?

No. It is obtained either by summing individually separated organoarsenic species or by subtracting inorganic arsenic from independently determined total arsenic. For this reason, paired speciation and total arsenic analyses are always commissioned together in a professional monitoring program.

What detection limits are achievable?

HPLC–ICP-MS typically delivers species detection limits of 0.01 to 0.1 µg/L as As, and LC–HG-AFS achieves approximately 0.1 to 0.5 µg/L. Both performance levels are adequate for the sub-µg/L to low µg/L concentrations at which organoarsenic species occur in surface water.

How must samples be preserved to keep arsenic species stable?

Strong acidification must be avoided because it can convert species. Accepted practice combines 0.45 µm filtration, EDTA addition, storage in amber bottles at 4 °C in the dark, and analysis within 48 hours for speciation, or within a validated holding period not exceeding seven days.

Which standards govern this analysis?

Total arsenic measurement by ICP-MS follows ISO 17294-2, ASTM D5673, and HJ 700-2014; hydride generation AFS follows HJ 694-2014; and surface water quality evaluation references GB 3838-2002. Speciation procedures are additionally supported by internationally recognized hydride generation and chromatographic-coupling methodologies adopted by accredited third-party laboratories worldwide.