Optimizing PFAS sample preparation with WAX SPE: Faster load using Biotage® PrepXpert-8

Per- and polyfluoroalkyl substances (PFAS) have become a global focus in environmental analysis. With this attention comes the need for reliable, efficient sample preparation methods. The U.S. EPA has established several methods for PFAS analysis, EPA 537.1, EPA 533, and the more recent performance-based EPA 1633A, all of which rely on solid phase extraction (SPE) for sample cleanup and preconcentration.

Solid phase extraction in EPA PFAS methods

SPE plays a critical role in isolating PFAS analytes from complex aqueous matrices. EPA Method 537.1 uses styrene-divinylbenzene (SDVB) sorbents, while EPA Methods 533 and 1633A employ weak anion exchange (WAX) sorbents.

•      SDVB (EPA 537.1): A hydrophobic polymer sorbent that captures PFAS primarily through reversed-phase interactions.
•      WAX (EPA 533 and 1633A): Combines reversed-phase retention with anion exchange functionality, making it better suited for a broader range of PFAS compounds, especially short-chain variants.

The throughput bottleneck

The chemistry of the sorbent influences recommended loading rates. SDVB methods (EPA 537.1) allow for faster flow rates of 10–15 mL/min, whereas WAX methods (EPA 533 and 1633A) recommend slower loading rates around 5 mL/min. While these conservative recommendations help minimize breakthrough risk, they also create a significant bottleneck in sample throughput.

Table 1 summarizes the recommended sample volumes, SPE media, bed mass, and flow rates for each method.

When processing large sample volumes, these slower flow rates can drastically extend total run times. And since this doesn’t include additional SPE steps (conditioning, washing, elution), the impact on laboratory efficiency is even greater.

Visualizing the bottleneck

As shown in Figure 1, load times increase significantly at lower flow rates, especially for larger sample volumes. This highlights the throughput limitations of conservative WAX SPE methods.

Figure 1: Sample load times increase with volume and decrease with flow rate. This chart highlights how slower flow rates significantly extend processing time, especially for larger volumes.

Optimizing WAX SPE with the Biotage^® PrepXpert-8

Recognizing this challenge, we explored whether faster load rates could be validated for WAX SPE. To remove human variability and ensure consistent processing, we used the Biotage^® PrepXpert-8 automated SPE system.

Biotage^® PrepXpert-8 provides several advantages:

•      Eight-channel processing: Extracts eight samples simultaneously.
•      Split-mode operation: Runs two different methods at once (four channels per side) which streamlines method development.
•  

This dual-mode capability is illustrated in Figure 2.

Figure 2: Split method mode on Biotage^® PrepXpert-8 allows two methods to run simultaneously.

Method optimization: Faster load rates for EPA 1633A

Since EPA 1633A is performance-based, it served as the ideal platform for optimization. We focused on 500 mL QC samples extracted using EVOLUTE^® PFAS 533 (150 mg WAX SPE cartridges). Our target was clear: enable 24 extractions of 500 mL samples in under 5 hours on a single Biotage^® PrepXpert-8. Achieving this required load rates greater than 10 mL/min.

Figure 3 shows the relationship between flow rate and total extraction time for 24 samples, demonstrating the time savings achieved at higher flow rates.

Figure 3: Total extraction time for 24 EPA 1633A samples decreases as sample load rate increases. The highlighted range (15–25 mL/min) shows the optimal balance between speed and performance on the Biotage^® PrepXpert-8 system.

Evaluation results

Samples were spiked with 40 native PFAS analytes (22–625 ng/L) and 24 isotopically labeled surrogates, known as EIS analogues (Extraction Internal Standards). These are stable isotope-labeled versions of PFAS compounds used to correct for variability during extraction and analysis.

Two load rates were tested: 15 mL/min and 20 mL/min, with all other method steps kept constant. Using split-mode operation, both methods ran simultaneously, making evaluation efficient.

The results demonstrated that both 15 mL/min and 20 mL/min provided acceptable recoveries for both native PFAS and EIS surrogates. This demonstrates that throughput can be significantly improved without compromising data quality.

As shown in Figures 4 and 5, average recoveries for both native PFAS compounds and isotopically labeled surrogates (EIS) fall within the EPA 1633A method’s acceptance limits. This confirms that increasing the flow rate to 15 or 20 mL/min does not compromise method performance.

Figure 4: Average recoveries of native PFAS compounds at 15 mL/min and 20 mL/min (n=4), compared to EPA 1633A method acceptance limits. Both flow rates show consistent recoveries within the defined low and high thresholds.

Figure 5: Average recoveries of isotopically labeled surrogates (EIS) at 20 mL/min. Most compounds fall within the EPA 1633A acceptance range, supporting the method’s robustness at higher flow rates.

Conclusion: speed without compromise

PFAS analysis requires precision, but efficiency cannot be overlooked. Traditional WAX methods limit laboratories with slow loading rates, which unnecessarily extend processing times.

Our evaluation shows that with automation on the Biotage^® PrepXpert-8, WAX SPE load rates can be increased three to four times beyond EPA’s conservative recommendations while maintaining method integrity.

Key outcomes:

     Faster throughput: 24 × 500 mL samples in under 5 hours
     Reliable performance under EPA 1633A
     Streamlined method development with split-mode automation

By pushing beyond traditional limits, laboratories can achieve both accuracy and efficiency in PFAS testing. This turns one of the biggest throughput bottlenecks into an opportunity for increased productivity.