ISO 27108:2010 — Water Quality — Determination of Pesticides by SPME and GC-MS

1. Scope and Principle of SPME-GC-MS Method for Water Analysis

ISO 27108:2010 specifies a method for determining selected plant treatment agents and biocide products in drinking water, groundwater, and surface water using solid-phase microextraction (SPME) followed by gas chromatography-mass spectrometry (GC-MS). The method achieves determination limits of at least 0.05 micrograms per liter for most target compounds, making it suitable for compliance monitoring against stringent drinking water regulations including the EU Drinking Water Directive and US EPA maximum contaminant levels. The standard was developed by ISO/TC 147/SC 2 to address the growing concern about pesticide contamination of water resources worldwide.

SPME is a solvent-free extraction technique that integrates sampling, extraction, and concentration into a single step. A fused-silica fibre coated with a polymeric adsorbent is immersed directly in the water sample, and analytes partition into the fibre coating based on their equilibrium distribution coefficients. After extraction equilibrium is reached, the fibre is transferred to the GC injector where analytes are thermally desorbed and transferred to the chromatographic column. The solvent-free nature of SPME eliminates solvent disposal costs and exposure risks, and the technique requires only small sample volumes of 8 to 16 milliliters, making it ideal for laboratory and field applications.

2. Analytical Procedure and Method Validation

The SPME procedure is performed by placing 8 to 16 mL of water sample in a glass vial with a magnetic stir bar, adding sodium chloride to control ionic strength and enhance extraction efficiency through the salting-out effect, and immersing the SPME fibre for a defined extraction time typically ranging from 30 to 60 minutes with constant stirring. Extraction time must be optimized for the target compounds — shorter times may not achieve equilibrium while longer times reduce sample throughput. After extraction, the fibre is withdrawn into the protective needle, transferred to the GC injector, and thermally desorbed at 250 to 280 degrees C. The fibre is then conditioned in a clean injector port to prepare for the next extraction.

GC-MS analysis uses electron impact ionization in selected ion monitoring mode for maximum sensitivity and selectivity. Quantification is performed using internal standard calibration with deuterated analogues of target analytes where available, which corrects for variations in extraction efficiency, injection volume, and mass spectrometer response. The standard provides comprehensive data including retention times, mass spectra, and quantification and confirmation ions for all target compounds in informative annexes. The method must be validated for each laboratory’s specific instrument configuration, with acceptable performance criteria defined for linearity, precision, accuracy, and detection limits.

Calibration is performed using standard solutions prepared in reagent water, with the calibration range covering the expected concentration range in real samples. The standard recommends a minimum of five calibration levels plus a blank. Linearity is assessed through the correlation coefficient of the calibration curve, which should be at least 0.99. If any compound shows significant matrix effects in real samples, standard addition calibration or matrix-matched calibration should be used instead of external standard calibration.

3. Engineering Insights for Environmental Analysis Laboratories

The SPME-GC-MS method offers significant advantages over traditional liquid-liquid extraction and solid-phase extraction methods. Solvent consumption is virtually eliminated, reducing both cost and environmental impact. Sample volumes are much smaller — 8 to 16 mL compared to 200 to 1000 mL for SPE methods. Sample preparation time per analysis is comparable for automated systems, but SPME eliminates solvent evaporation and reconstitution steps required in SPE. However, SPME has some limitations: extraction is equilibrium-based rather than exhaustive, so extraction efficiency depends on fibre type, extraction time, temperature, and matrix composition. Quantification must be performed carefully accounting for these factors.

Matrix effects are more pronounced in surface water and wastewater analysis due to dissolved organic matter and suspended particles that can compete for fibre binding sites or alter distribution coefficients. The standard recommends matrix-matched calibration or standard addition for complex samples. Salinity effects must be controlled through consistent addition of sodium chloride to all samples, standards, and blanks. The addition of salt not only controls ionic strength variation between samples but also enhances extraction efficiency for many analytes through the salting-out effect. Fibre selection is critical — the standard specifies 100 micrometer PDMS for non-polar compounds and 65 micrometer PDMS/DVB for more polar analytes.

The standard includes informative annexes with gas chromatograms, mass spectra, and precision data from interlaboratory studies. These resources are valuable for method development and troubleshooting. The mass spectra in particular help analysts confirm compound identity when retention time shifts occur due to column aging or instrument maintenance. The standard notes that applicability can be extended to other compounds not explicitly listed, but full validation is required including linearity, precision, accuracy, and detection limit studies for each additional compound. For compounds not amenable to GC-MS, such as highly polar or thermally labile pesticides, alternative methods including LC-MS/MS should be considered.

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