US EPA Other Test Method (OTM) 50

OTM-50 is an analytical method from the US EPA for monitoring volatile fluorinated compounds (VFCs) in stationary sources. Air samples are collected in canisters and analysed using thermal desorption coupled with gas chromatography–mass spectrometry (TD–GC–MS).

The destruction of perfluoroalkylated and polyfluoroalkylated substances (PFAS) is a vital step in global efforts to reduce their ongoing impact on human health and environment. However, if the destruction process used doesn’t completely mineralise the PFAS, lower-molecular-weight volatile fluorinated compounds (VFCs) may be released into the environment.

OTM-50 is one of two “Other Test Methods” published by the US EPA for monitoring fluorinated substances in air, and focuses on VFCs (the other method is OTM-45, and applies to PFAS themselves). OTM-50 was written to enable monitoring of PFAS destruction and mitigate risks from the release of VFCs. It can also be applied to testing of industrial emissions and air monitoring applications more broadly.

What are the key challenges for VFC analysis?

The instrumentation and consumables used for VFC analysis need to be considered carefully. Particular analytical challenges are:

• Incredibly volatile compounds, which are not catered for by standard analytical system configurations and methods
• High humidity, which can negatively impact repeatability and the long-term functioning of the analytical system
• Varying CO₂ levels, which can be very high (up to 15% has been reported), leading to bias and preventing quantitative reporting of some compounds.

Markes’ approach to tackling these challenges are outlined below.

Can the most volatile species in OTM-50 be reliably monitored on a single system?

OTM-50 lists 30 VFCs that are “products of incomplete destruction” of larger PFAS species. They are all fully fluorinated, with a C₁–C₈ framework, and include known degradation products such as Fluoroether E-1 (which results from degradation of GenX).

Tetrafluoromethane (CF₄, Freon 14) is the most difficult compound to monitor from the list and is also one of the most important. CF₄ is a potent greenhouse gas with a global warming potential that is 50,000 times greater than CO₂, and is produced when mineralisation of other PFAS is not complete.

CF₄ is difficult to trap even with the strongest sorbents because of its high volatility (b.p. –128°C). However, the backflush design of the focusing trap in Markes’ UNITY-xr instrument means that it can be captured at –30°C in sample volumes up to 25 mL without breakthrough.

The other 29 compounds have retention volumes of over 1 L, giving the analyst the flexibility to meet the limits of detection needed. For example, running the same sample twice but with different volumes ensures that all 30 components can be quantified under the conditions that generate the most reliable results

How can water management for OTM-50 help to improve results?

The presence of humidity in a sample can reduce retention time stability, speed up degradation of the GC column phase, cause split discrimination upon injection, and result in a rapid decline in MS sensitivity. When trapping at sub-zero temperatures (which is required for OTM-50), humid samples can also result in ice formation within the focusing trap, resulting in poor reproducibility and potentially causing damage to the analytical system.

In a typical OTM-50 protocol, some water will be removed within the sample train prior to transfer into the canister, but the relative humidity can still remain very high. However, with Markes’ analytical system for OTM-50, the residual water in the sample is removed using the Kori-xr water management module, avoiding any negative impact on results.

How can high-CO₂ samples be successfully managed within OTM-50?

The high levels of CO₂ that can be present in stationary sources can result in severe chromatographic interference (especially for C₂F₆, with which CO₂ co-elutes), decreasing the sensitivity of the analysis.

Samples can be diluted prior to analysis to manage the concentration of CO₂, but on its own this is insufficient to resolve the above issues. Therefore, in Markes’ analytical system for OTM-50, the sample, once collected on the focusing trap, is purged at a slightly elevated temperature, prior to the main desorption step. This significantly reduces the levels of CO₂, negating chromatographic interference of key species (C₂F₆) and enabling quantitation.

What compounds are included in OTM-50?

The 30 compounds specified in OTM-50 are listed below, in order of decreasing volatility.