Total organic arsenic determination in surface water is essential for evaluating water quality, ecological risk, and compliance with environmental standards such as GB 3838 and WHO guidelines. Modern speciation analysis—typically combining high-performance liquid chromatography (HPLC) with inductively coupled plasma mass spectrometry (ICP-MS) or hydride generation atomic fluorescence spectrometry (HG-AFS)—allows laboratories to separate and quantify individual arsenic species with high sensitivity and selectivity. This technical article details the complete detection procedure, from sample collection and preservation through chromatographic separation, quantification, and quality control.
Technical Overview
Arsenic in surface water exists in two principal chemical categories: inorganic species, mainly trivalent arsenite As(III) and pentavalent arsenate As(V), and organic species, including monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), trimethylarsine oxide (TMAO), and arsenobetaine (AsB). Inorganic arsenic is generally recognized as the more toxic fraction, but organic arsenic compounds are increasingly monitored because they serve as indicators of biological methylation processes, industrial contamination, and agricultural runoff, and because their environmental mobility differs from that of inorganic forms.
Total organic arsenic is not measured as a single direct parameter. Instead, it is obtained either by chromatographically separating and summing all organic arsenic species, or by the difference method, in which the concentration of inorganic arsenic species is subtracted from the concentration of total arsenic. Both approaches require careful, speciation-preserving sampling because arsenic species can interconvert or degrade during storage.
The regulatory context underscores the importance of this analysis. GB 3838 sets a limit of 50 μg/L for total arsenic in Class III surface waters in China, while the WHO guideline value for arsenic in drinking water is 10 μg/L. Accredited third-party laboratories and environmental monitoring agencies therefore apply internationally recognized procedures based on standards such as HJ 700, HJ 694, ISO 17294-2, and ASTM D5673 to generate defensible speciation data.
Testing Samples
Rivers, Streams, and Estuarine Waters
Flowing water bodies are sampled at representative cross-sections, typically at mid-depth and away from banks, stirrer effects, or visible discharge points. Rivers downstream of mining, smelting, glass manufacturing, or agricultural regions receive priority because methylated arsenic species such as MMA and DMA are frequently associated with these activities.
Lakes, Reservoirs, and Ponds
Standing water bodies are sampled at multiple depths and locations to account for thermal stratification and seasonal algal activity, both of which influence arsenic methylation. Samples collected near drinking water intakes are of particular concern for risk assessment purposes.
Sample Collection and Preservation Requirements
Proper handling is the most critical factor in obtaining valid organic arsenic results. The following requirements apply:
- Collect samples in precleaned HDPE or PTFE bottles that have been acid-washed and rinsed with ultrapure water.
- Filter samples in the field through a 0.45 μm membrane to remove particulate-bound arsenic and halt biological transformation.
- For speciation aliquots, do not acidify; acidification can alter species distribution. Store at 4 °C in the dark and analyze within 48 hours, optionally adding EDTA to stabilize arsenic species.
- For the total arsenic aliquot, acidify to pH below 2 with high-purity nitric acid immediately after filtration.
- Record field parameters—pH, conductivity, temperature, redox potential, and chloride concentration—since high chloride affects ICP-MS measurement of arsenic.
Testing Items
A complete organic arsenic evaluation typically reports the following parameters, all expressed in μg/L as arsenic:
- Total arsenic, determined after acid digestion.
- Arsenite, As(III).
- Arsenate, As(V).
- Monomethylarsonic acid, MMA.
- Dimethylarsinic acid, DMA.
- Trimethylarsine oxide, TMAO, and arsenobetaine, AsB, where required.
- Total inorganic arsenic, calculated as the sum of As(III) and As(V).
- Total organic arsenic, calculated as the sum of all organic species or as total arsenic minus total inorganic arsenic.
Testing Methods
Method Selection and Principles
The reference technique is HPLC-ICP-MS, in which an anion-exchange column separates the arsenic species before they are introduced into the mass spectrometer, aligned with the principles of ISO 17294-2, ASTM D5673, and HJ 700. A cost-effective alternative is HPLC coupled with hydride generation atomic fluorescence spectrometry (HG-AFS), consistent with HJ 694, which offers excellent sensitivity for arsenic at low capital cost. Total arsenic is confirmed independently by ICP-MS after microwave-assisted nitric acid digestion.
Step 1: Field Sampling and Filtration
- Rinse the sample bottle two to three times with filtered site water before final collection.
- Draw the sample through a 0.45 μm membrane syringe or in-line filter directly into the bottle, avoiding aeration.
- Prepare parallel aliquots: one unacidified for speciation, one acidified with nitric acid for total arsenic.
Step 2: Preservation and Transport
- Place speciation aliquots in a cooler at 4 °C and transport to the laboratory within 24 hours.
- Keep samples in the dark to prevent photochemical interconversion of species.
- Verify holding times; speciation samples exceeding 48 hours before analysis should be flagged or rejected.
Step 3: Sample Pretreatment
- For total arsenic: digest the acidified aliquot with high-purity nitric acid using microwave heating or a hot block until the solution is clear, then dilute to volume with ultrapure water.
- For speciation: dilute the unacidified aliquot with mobile phase, filter again through a 0.22 μm membrane, and, where validated, add EDTA to complex cations that could shift species equilibrium.
- Analyze samples with high dissolved solids after appropriate dilution to protect the chromatographic column and plasma.
Step 4: Chromatographic Separation
- Use a strong anion-exchange column maintained near 25 °C.
- Employ a buffered mobile phase, commonly 10–20 mmol/L ammonium phosphate or ammonium carbonate adjusted to pH 5.8–6.0, at a flow rate of approximately 1.0 mL/min.
- Inject 20–100 μL of prepared sample; typical elution order resolves AsB, As(III), DMA, MMA, and As(V) within 10–15 minutes.
- Confirm baseline separation of all target peaks using a mixed species standard before processing samples.
Step 5: Instrumental Detection
- ICP-MS detection monitors mass-to-charge ratio 75 for arsenic; apply a collision-reaction cell with helium or hydrogen to suppress the polyatomic interference from argon chloride at m/z 75, which is significant in chloride-rich surface waters.
- HG-AFS detection generates arsines from the chromatographic effluent using potassium borohydride in acidic medium, followed by atomization and fluorescence measurement.
- Verify instrument tuning, oxide ratios, and sensitivity daily with a performance check solution.
Step 6: Calibration and Quantification
- Prepare species-specific multi-point calibration curves, for example 0.1, 0.5, 1, 5, 10, and 20 μg/L, in the same matrix as the mobile phase.
- Require a linear regression coefficient of at least 0.995 for each species.
- Quantify each species from its chromatographic peak area against the corresponding calibration curve.
- Report method detection limits, which typically range from 0.05 to 0.5 μg/L per species depending on technique and matrix.
Step 7: Quality Control and Assurance
- Analyze a method blank and a laboratory reagent blank with each batch to confirm absence of contamination.
- Duplicate analyses should agree within a relative percent difference of 20% or less.
- Matrix spike recoveries for MMA and DMA must fall within 80–120%.
- Verify accuracy with certified reference materials or proficiency-test samples where available.
- Run continuing calibration verification standards every ten samples and recalibrate if recovery drifts beyond 90–110%.
- Check chromatographic recovery by comparing the sum of species with independently measured total arsenic; agreement within 80–120% validates the speciation run.
Step 8: Data Processing and Reporting
Total organic arsenic is calculated using one of two equivalent approaches:
- Summation method: add the individually quantified concentrations of MMA, DMA, TMAO, AsB, and any other resolved organic species.
- Difference method: subtract the sum of As(III) and As(V) from the total arsenic concentration.
The final report should present each species concentration, the calculated total organic arsenic value, uncertainty, detection limits, holding-time compliance, and complete quality control results so that reviewers can trace every reported figure to validated measurements.
Applications
Accurate determination of total organic arsenic in surface water supports a wide range of regulatory, industrial, and scientific activities:
- Compliance monitoring of rivers, lakes, and reservoirs against environmental quality standards such as GB 3838.
- Source water safety assessment for municipal drinking water intakes.
- Supervision of wastewater discharge from mining, smelting, semiconductor, glass, and agrochemical operations.
- Ecological and human health risk assessment in contaminated watersheds.
- Remediation performance evaluation at arsenic-impacted sites.
- Academic research on arsenic biogeochemical cycling, methylation pathways, and trophic transfer.
FAQ
What is the difference between total arsenic and total organic arsenic?
Total arsenic encompasses every arsenic species in the dissolved sample, both inorganic and organic. Total organic arsenic refers only to the carbon-containing arsenic compounds, principally MMA, DMA, TMAO, and AsB, obtained either by direct chromatographic summation or by subtracting inorganic arsenic from total arsenic.
Why is speciation analysis necessary when total arsenic is easier to measure?
Different arsenic species differ markedly in toxicity, mobility, and treatment behavior. Regulatory limits and health risk evaluations increasingly demand species-level information, so measuring only total arsenic cannot distinguish highly toxic inorganic forms from generally less toxic organic species.
What detection limits can be achieved?
With HPLC-ICP-MS, species-level detection limits of 0.05–0.2 μg/L are routinely achievable, while HPLC-HG-AFS typically delivers detection limits near 0.1–0.5 μg/L, both of which are far below the GB 3838 Class III limit of 50 μg/L for total arsenic.
How quickly must samples be analyzed after collection?
Unacidified speciation samples should be analyzed within 48 hours of collection when stored at 4 °C in the dark, because microbial activity and photochemical reactions can interconvert arsenic species. Acidified samples for total arsenic may be held longer, commonly up to one month.
Which standards govern this testing?
Key references include GB 3838 for surface water quality classification, HJ 700 and HJ 694 for elemental and atomic fluorescence methods, ISO 17294-2 and ASTM D5673 for ICP-MS determination of elements in water, and ISO 11969 for atomic absorption determination of arsenic. Accredited third-party laboratories typically build validated procedures on this combined framework.
Can the difference method replace chromatographic speciation?
The difference method is acceptable for screening when chromatographic equipment is unavailable, but chromatographic speciation is strongly preferred because it positively identifies each organic species, avoids error accumulation from subtracting two measurements, and reveals unexpected species that the difference method cannot detect.