TOP Assay analysis for Perfluorinated Compounds - Technical Update
Apr 11, 2018
TOP assay is an oxidative sample pre-treatment method used for sample extracts to convert PFAS (Perfluoroalkyl Substances) to perfluoroalkyl acids that are then quantified by the chosen analytical technique.
As a result of the growth of the synthetic chemical industry, over the last 50 years, thousands of new fluorinated molecules have become prevalent in the environment. A multitude of poly- and perfluoroalkyl substances (PFAS), valued for their ability to repel both water and oils, have been used for decades for diverse applications such as water repellent "outdoor" fabrics, stain resistant carpets, non-stick frying pans and firefighting foams.
PFOS (perflourooctane sulphonate) and PFOA (perfluorooctanoic acid) are the best known examples of PFAS. Their potential impact on human health has been recognised globally as they are extremely persistent, bioaccumulative and toxic.
As a result, PFOS is restricted under the Stockholm convention and classed as a persistent organic pollutant (POP), with PFOA being actively considered for inclusion.
However, more recently, concern has expanded to a much wider number of the PFAS now present in the environment such as perfluorinated or polyfluorinated alkyl substances compounds (PFAS) found in aqueous film forming foam (AFFF) and mulitple other products. These perfluorinated or polyfluorinated compounds are sometimes termed "precursors" as they can biotransform to form more simple perfluorooctanoic acids such as PFOA.
As a result, there are significant analytical challenges to overcome when considering how to assess soil and groundwater contaminated with PFAS as there are mulitple analytes to consider, not just PFOS and PFOA.
Routine analysis of these compunds is undertaken by Liquid Chromatography - Triple Quadrupole Mass Spectrometry (LC-QQQ). This analytical technique is very selective and sensitive, allowing for compounds which have been calibrated for to be detected at low concentrations, typically around 1 ng/l or less in water and 1 ug/kg in soil.
The following table shows the compounds that can be analysed by this technique.
*including branched and linear