20 Environmental Laboratory - Spotlight on PFAS OVERCOMING THE CHALLENGES OF REDUCING
BACKGROUND INTERFERENCE FOR LC/MS/MS TRACE PFAS ANALYSIS
Per- and polyfl uoroalkyl substances (PFASs) represent a large group of thousands of anthropogenic compounds that have been produced and widely used in many sectors including automotive, food processing and packaging, textiles, construction and household products, electronics, fi refi ghting, and medical articles.
T
hese compounds have unique physical and chemical characteristics: they all contain carbon-fl uorine bonds
(among the strongest chemical bonds in organic chemistry), that means they are highly stable and resistant to degradation, and they are known to persist in the environment longer than any other man-made substance. This, along with their ubiquitous use, have led to the accumulation of PFAS in the environment, with growing concern of human exposure to these chemicals.1-6
The optimization of analytical methods for identifi cation and quantifi cation of PFASs is essential for risk assessment. Because of its high sensitivity, selectivity and robustness, the most widely used analytical method for PFAS detection is based on Liquid Chromatography coupled with Mass Spectrometry technique (LC/MS/MS). Coupling SPE with LC/MS/MS has been one of the most popular approaches to PFAS analysis in aqueous samples, and has been employed in EPA Method 537.1, as well as ISO 25101.7,8
But the key challenge of measuring ppt levels of PFAS is that these compounds are ubiquitous throughout the environment and accumulate everywhere, including the laboratory equipment and accessories. In fact, many of the components used in liquid chromatographs, mass spectrometers, and solid phase extraction systems are made of polytetrafl uoroethylene (PTFE) or PTFE copolymers, which leach PFAS compounds and cause background interference during a sample measurement. Even the use of glass sample containers can generate additional challenges, the glass in fact adsorbs PFAS compounds.
Reducing PFAS background
To reduce background contamination and reach accurate ultra- trace levels, every step of the analytical protocol must be free of PFAS materials: from sample collection to sample preparation and analysis.
To start, purchase high quality mobile phases (LCMS grade solvents). Additionally, instead of utilizing conventional glass vials with PTFE-lined septa, polyethylene vials and caps are necessary
Figure 1. Reducing background from pump and mobile phases.
to reduce the possibility of contamination. The HPLC pump, autosampler, and SPE system all contain PFAS components that require mitigation as well.
To combat interference from these sources, a delay column may be installed in the fl ow path between the pump and the autosampler, as shown in Figure 1.
The delay column captures PFAS contaminants coming from the mobile phase, the solvent lines, or the pump before they reach the autosampler. As a result, the captured compounds elute via the gradient later than the analyte peak in the sample (see chromatograms in Figure 1) allowing clear separation of PFAS contaminants from the analytes of interest, enabling more authentic measurements of PFAS in the sample.
In many cases, the HPLC autosampler contains fl uoropolymer tubing which will introduce PFAS contamination upon sample injection. It is recommended to replace all tubing with high performance polyether ether-ketone (PEEK).
SPE extraction confi gurations normally include an abundance of fl uoropolymers. The tubing connecting sample bottles to the SPE cartridges can be a signifi cant source of PFAS contamination. Replacement of all transfer tubing with linear low-density polyethylene (LLDPE) or PEEK tubing is necessary to avoid PFAS leaching. In addition, some of the valving on the manifold
Figure 2. PerkinElmer QSight®
220 LC/MS/MS Triple Quadrupole system.
IET JANUARY / FEBRUARY 2023
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