In today’s environmental landscape, PFAS (per- and polyfluoroalkyl substances) data carry more weight than ever. Regulatory actions, cleanup strategies and even public health decisions often hinge on just a few numbers in a laboratory report. Yet many practitioners rely solely on Level 2 laboratory data packages—documents that lack raw instrument data and can provide an oversimplified view of complex analytical outputs.
One may assume that lab-reported PFAS results are inherently accurate if they come from a certified lab and are reported via accepted methods like EPA 537, 533, or 1633. However, beneath the surface of these Level 2 data packages can lies a more complex story, especially when raw data are not reviewed.
At TRC, our in-depth evaluations of PFAS data have uncovered random occurrences of false positives, false negatives and methodological inconsistencies that could mislead environmental decision-making. Without access to Level 4 data that include instrument outputs and chromatograms, many critical issues can remain hidden. Scrutiny of ion confirmation, instrument variability and matrix interferences is essential. Most importantly, if the data do not make sense or results seem anomalous, reach out to the lab to discuss and make it a point to review the Level 4 data.
When a “Detection” Isn’t Real: False Positives for PFOS
Case Study #1 – The Pesticide Sample That Wasn’t What It Seemed
A pesticide sample analyzed using modified EPA Method 533 showed an alarming 2,700 ng/L detection for PFOS. The method used—while employing isotope dilution—did not require the laboratory to monitor confirmation ions. Upon reanalysis using a modified EPA 537 method (which does require monitoring of confirmation ions), the data told a very different story:
- Primary and confirmation ion ratios were outside acceptance criteria.
- No confirmation ion was detected for the major branched isomer of PFOS.
- Further investigation suggested bile acids were likely interfering with the PFOS signal.
Lesson: A reported PFOS “hit” may actually be a different compound entirely, especially in biologically complex matrices. This can impact ultimate decision-making.
Methodology Matters: One Sample, Two Instruments, Two Different Results
Case Study #2 – Monthly Monitoring Gone Wrong
A routine NPDES monitoring sample was reported with PFOS at 78 ng/L. The result was flagged by the lab noting ion ratio issues but still reported. Upon request, reanalysis was performed, this time on an instrument with improved resolution (aligned with EPA 1633); the PFOS result in the reanalysis decreased to 21 ng/L. The likely cause was again bile acid interference, which affected ion ratios on the original instrument.
Lesson: Lab instrumentation setup and method selection can alter PFAS results and impact decision-making.
False Negatives: When “Not Detected” is “Not Correct”
Case Study #3 – Field Duplicate Discrepancy in Groundwater
A PFNA detection of 290 ng/L in one sample was paired with a non-detect in the field duplicate sample. At first glance, this was perplexing. The samples were from the same location and conditions. Level 2 data offered no insight. Only by reviewing the Level 4 data (raw data and extracted ion chromatograms) was the answer found:
- The PFNA peak in the field duplicate was missed by the lab and was not integrated.
- The lab was subsequently requested to review all data associated with this project and ultimately discovered approximately 15 samples where PFNA and/or PFHpA peaks were not integrated and were incorrectly reported as not detected.
- The issue was able to be traced back to an improperly trained analyst and an inefficient secondary review within the lab.
Lesson: False negatives and subsequent regulatory exceedances can be hidden in plain sight, especially when the data reviewer is limited to summary outputs, and can therefore impact decision-making.
The Role of Confirmation Ions: A Critical Checkpoint
Confirmation ions are often the only defense against misidentification and the reporting of false positive results. EPA Method 533 does not require them but EPA Method 537.1 and 1633 do. This discrepancy can lead to inconsistent reporting, especially when labs switch methods or when historical data are included in decision-making.
Moreover, the ion ratio—a comparison of primary and confirmation ion areas—is essential to confirm the identity of PFAS compounds. If this ratio is off, your result is uncertain. Looking into the cause of ion ratio nonconformances can sometimes show potential false positive results if interferences are determined to be present.
Next Steps for Environmental Decision Makers
It’s time to raise the bar for PFAS data evaluation. In general, labs are doing a good job with PFAS analysis, but errors can happen. Here’s what every consultant, agency, and attorney should do:
- Question suspicious results — A number alone isn’t enough. If the data do not make sense to you, ask the lab.
- Request Level 4 data when results look questionable — Don’t rely solely on Level 2 summaries for high-stakes decisions. Reviewing raw data can be more costly but it will give you more assurance in the accuracy of your data.
- Understand method limitations — Not all methods are equal. Some don’t account for the interferences that matter most.
- Know your matrix — Complex samples (like tissue, biosolids, or effluent) are more prone to interference.
- Advocate for method consistency — EPA Method 1633 brings rigor, but legacy data generated under less robust methods require careful consideration.
TRC Can Help
As regulators, clients and the public demand greater clarity and accountability around PFAS, the environmental field must evolve. Data quality should no longer be judged solely by summary forms and detection limits. Instead, we must embrace a culture of deeper data review, method literacy and technical skepticism.
TRC integrates our industry-leading expertise in characterization, remediation, toxicology, forensics, regulations and strategic planning to deliver practical and creative solutions that mitigate the business and environmental risks of PFAS and other emerging contaminants. Our multidisciplinary team of experts work at the forefront of emerging contaminants with a mission to promote scientific and technological excellence and contribute to innovation in our professional, academic and client communities by creating new pathways for the world to thrive. Our tested practitioners continually expand their knowledge to deliver expert solutions for a range of emerging contaminant areas, ensuring agility, fortitude and peace of mind for the clients and industries we serve.
For help with your PFAS data issues, read more about our solutions or contact us today.
