Trace level enrichment of malodour analytes in water using Twister Technology

Importance of the Topic

Detecting trace-level malodour compounds in drinking water is critical for maintaining consumer confidence and meeting quality standards. Although these compounds pose no direct health risk, their extremely low olfactory thresholds can lead to strong public perception issues. Effective enrichment methods are therefore necessary to isolate and quantify these analytes via GC/MS at sub-ng/mL concentrations.

Objectives and Study Overview

This study evaluates Twister stir bar sorptive extraction techniques for trace-level enrichment of odour-causing analytes in water. Two approaches are compared: 1) ethylene glycol–silicone (EG) coated Twisters for improved recovery of polar compounds, and 2) derivatisation of phenolic analytes using acetic anhydride on standard PDMS Twisters. The performance is assessed in terms of linearity, precision, and practical applicability.

Methodology and Instrumentation

Sample preparation and extraction were conducted as follows:

• For Method 1 (EG Twisters): A 10 mL water sample (pH 3 in 5/95 MeOH/H₂O) was stirred with an EG-coated Twister for 2 hours.
• For Method 2 (PDMS Twisters + derivatisation): Samples in 5/5/90 Ac₂O/MeOH/basic water were stirred with a PDMS Twister, phenolics being derivatised in situ with acetic anhydride.

After extraction, Twisters were rinsed, dried, and thermally desorbed in a Gerstel TDU/CIS system. GC/MS analysis was performed using single ion monitoring under the following conditions:

• Column: 30 m × 0.25 mm i.d., 0.5 µm Stabilwax
• Oven program: 40 °C (2 min), 10 °C/min to 235 °C
• TDU: 40 °C (0.5 min), 720 °C/min to 220 °C (2 min)
• CIS: –100 °C to 150 °C (16 °C/s) then to 220 °C (12 °C/s)

Main Results and Discussion

Method 1 using EG Twisters achieved excellent linearity (R² ≥ 0.989) for ten representative analytes at 0.02–1 ng/mL and precision between 1.7 and 8.7 % RSD.
Method 2 with PDMS Twisters and derivatisation delivered R² ≥ 0.993 for four malodourous targets and repeatability of 4.2–8.6 % RSD. Both approaches provided robust performance without the need for solvent-intensive liquid–liquid extraction.

Benefits and Practical Applications

Twister-based enrichment offers:

• High sensitivity at sub-ng/mL levels
• Minimal solvent consumption and simplified workflow
• Adaptability to both polar and non-polar odourants
• Compatibility with standard thermal desorption–GC/MS systems

This makes the methods suitable for routine QA/QC monitoring of taste and odour issues in drinking water utilities.

Future Trends and Applications

Advancements may include novel extraction coatings targeting broader analyte classes, online coupling of stir bar extraction with GC/MS, and further automation. Derivatisation strategies could be extended to other functional groups, while miniaturised workflows may support field-deployable water quality screening.

Conclusion

Twister stir bar sorptive extraction, with either EG-coated or PDMS bars plus derivatisation, provides a sensitive, precise, and user-friendly approach for trace-level malodour analysis in water. The methods are readily implementable on existing thermal desorption–GC/MS platforms, offering a reliable solution for addressing taste and odour complaints.

Used Instrumentation

• Gerstel Multi Purpose Sampler MPS 2 XL with Maestro v1.4.8.14/3.5
• Gerstel Thermal Desorption Unit (TDU)
• Gerstel Cooled Injection System (CIS) 4
• Agilent 7890A GC
• Agilent 5975C Inert XL MSD