28 Environmental Laboratory
IMPROVED SENSITIVITY FOR THE DETECTION OF PER- AND POLYFLUORINATED ALKYL SUBSTANCES (PFAS) USING DIRECT INJECTION METHOD ON THE WATERS XEVO™ TQ ABSOLUTE
INTRODUCTION The determination of per- and polyfl uorinated alkyl substances (PFAS) is an essential part of water analysis today. The requirements for sensitivity for the detection of PFAS are extremely demanding since the regulations in Europe for the quality of drinking water ground water and surface waters include mandatory parametric values and environmental quality standards for a selection of PFAS at extremely low concentrations (pg/L to ng/L). In the following article, we describe how highly sensitive LC-MS/MS technologies and mitigation of background contamination help to check for compliance with these challenging limits using a direct injection approach.
Regulatory framework for PFAS in water in Europe
In Directive (EU) 2020/2184 of 16 December 2020 on the quality of water intended for human consumption (the recast Drinking Waters Directive - DWD), new parameters were specifi ed for the fi rst time that defi ned limits for PFAS compounds in drinking water in Europe.1
A parametric value of 0.1 µg/L was applied
to the “sum of PFAS”, a group of PFAS considered a concern as regards water intended for human consumption, namely 20 selected carboxylic and sulfonic acids with chain lengths from C4 to C13 listed in point 3 of Part B of Annex III. Preserving and improving the quality of surface water is a declared goal of the European Directive 2000/60/EC (the Water Framework Directive - WFD), which established a framework for Community action in the fi eld of water policy.2
Directive 2013/39/EU subsequently
amended earlier legislation regarding priority substances and defi ned various types of Environmental Quality Standards (EQS) for PFOS and its derivatives.3
The term “derivatives” is used to
refer to any or all of the substances which contain the PFOS moiety and may break down in the environment to give PFOS. The annual average (AA) EQS in freshwater is extremely low at 0.65 ng/L whereas the maximum allowable concentration (MAC) for PFOS for inland surface waters is much higher (36 µg/L). To check compliance with the limits and standards, sensitive, accurate and reliable methods are required.
Two analytical approaches have been used to reach detection limits low enough to check compliance with regulatory requirements: with and without a sample clean-up/analyte enrichment step. With the former, water samples can be subjected to solid-phase extraction (SPE), typically using a weak anion exchange sorbent (eg OasisTM
WAX). However, reliance
on SPE for enhanced sensitivity has some disadvantages. It adds to the cost of the analysis in terms of the need for extra consumables and additional time. SPE can also be a source of PFAS contamination, concentrated by evaporation of the solvents and leaching from the sorbent, which has a negative impact on the performance of the method. The alternative to SPE is direct injection of the water sample on to the LC-MS/MS system, which avoids one source of contamination and results in a quicker overall method, and which could also lead to increased sample throughput as well as less solvent waste. However, the sensitivity of the LC-MS/MS system selected needs to be suffi cient for the detection of PFAS at the extremely low levels without any enrichment or clean-up steps.
A direct injection method was developed for the practical implementation of this requirement, which achieves a limit of quantifi cation of 0.001 µg/L for the individual substances. Various factors played a role in the development of the method in order to reach ultimate sensitivity in routine sample analysis.
Action to mitigate background
contamination issues PFAS from fl uoropolymers and coating are ubiquitous in common
IET MARCH / APRIL 2023
Figure 1: delay mechanism with the PFAS kit
Figure 2: PFOS in wastewater infl uent at various spiked concentrations
sampling and analytical equipment and can easily contaminate solvents and reagents. Hence, laboratory contamination is common and near impossible to eliminate. Background issues with perfl uorobutane sulfonic acid and its potassium salt (PFBS) are extremely common. Such contamination can lead to reporting of false positives and/or higher reporting limits due to elevated background. It is essential to take steps to try to mitigate this contamination:
• Avoid all products likely to contain fl uoropolymers – eg vials with Tefl on seals
• Minimise risks from contaminated dust and air
• Screen all analytical consumables, solvents and reagents including water purifying systems
• Carry out extensive use of procedural blanks from sample storage all the way through the LC-MS/MS step
• Implement traceability systems for the consumables used in the laboratory
To exclude contaminations from the LC system, it can be modifi ed with a PFAS Installation Kit. This establishes a complete fl ow path for analysis of PFAS-containing samples while minimizing interference from background contamination. It involves removing and replacing components such as some
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