A Novel Extraction Procedure for Stir Bar Sorptive Extraction (SBSE): Sequential SBSE for Uniform Enrichment of Organic Pollutants in Water Samples

Significance of the topic

Stir bar sorptive extraction (SBSE) is a powerful solvent-free technique for concentrating trace organic pollutants in water samples.
Achieving uniform enrichment across compounds with widely varying polarity and volatility is critical for reliable multi-residue monitoring in environmental analysis.

Objectives and Study Overview

The study introduces a sequential SBSE protocol designed to improve extraction recovery and uniformity for 80 model organic pollutants in water.
The workflow consists of two consecutive extractions on a single 5 mL sample: an unmodified first step targeting hydrophobic analytes (log Ko/w > 4.0) and a salted second step (30 % NaCl) for medium and low‐polarity compounds (log Ko/w < 4.0).
Comparisons were made with conventional SBSE performed with and without salt addition.

Methodology and Instrumentation

The sequential SBSE procedure was carried out as follows:

• Sample: 5 mL aqueous aliquot in a 10 mL headspace vial.
• First extraction: one PDMS‐coated stir bar (24 µL) for 1 h at 1500 rpm without additives.
• Second extraction: same vial with added 30 % NaCl and a fresh stir bar for 1 h at 1500 rpm.
• After each step, stir bars were rinsed, dried, and placed together in a single glass desorption liner.

Thermal desorption and GC–MS analysis:

• Thermal Desorption Unit (TDU) with MPS2 autosampler and CIS4 PTV inlet.
• Agilent 6890N GC coupled to a 5973 inert mass selective detector.
• Column: HP-5ms, 30 m × 0.25 mm, 0.25 µm film.
• Desorption: 40 °C to 280 °C at 720 °C/min, cryo-trap at –100 °C, then inject to GC.
• MS in EI scan mode (m/z 58–510), quantifier and qualifier ions selected per analyte.

Main Results and Discussion

Sequential SBSE achieved overall recoveries between 82 % and 113 % for 75 compounds (log Ko/w > 2.5), with only five analytes below 80 % recovery.
Conventional SBSE with or without salt addition provided less than 80 % recovery for 23 and 41 compounds, respectively.
Linearity was excellent (r2 > 0.9900) across 20–1000 ng/L, and limits of detection were below 10 ng/L for most analytes even in scan mode.
A pilot screening of river water samples detected 11 pesticides at 7.2–52 ng/L, demonstrating practical applicability.

Benefits and Practical Applications of the Method

The sequential SBSE approach delivers:

• Uniform enrichment across a broad polarity range without compromising hydrophilic or hydrophobic analyte recovery.
• Low detection limits (< 10 ng/L) using only 5 mL of sample.
• High throughput by simultaneous desorption of two stir bars.
• Reduced glass adsorption artefacts and enhanced reproducibility (RSD < 10 %).
• Suitability for routine environmental monitoring of trace organic pollutants.

Future Trends and Potential Applications

Further developments may include:

• Applying sequential SBSE with alternative modifiers (organic solvents, pH adjusters) or sorbent coatings (RAM, PDMS/DVB).
• Automation and scaling for high-throughput analysis in regulatory and QA/QC laboratories.
• Extension to other matrices such as beverages, food extracts, and biological fluids.
• Coupling with high-resolution mass spectrometry for non-target screening of emerging contaminants.

Conclusion

The novel sequential SBSE procedure combines two targeted extraction steps on the same small sample volume to achieve exhaustive and uniform enrichment of organic pollutants over a wide polarity range.
This technique significantly improves recovery, sensitivity, and reproducibility compared to conventional SBSE approaches, making it a valuable tool for environmental trace analysis.

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