Chromatography


Trace level analysis for Extractable Organic Fluorine (EOF) screening in water matrices, by Combustion Ion-Chromatography


Robbert van Wessel, TE Instruments


Extractable Organic Fluorines (EOF) are a group of compounds that contain fl uorine atoms and are classifi ed as persistent organic pollutants. EOF concentration values are the result of measuring a certain group of components which are also categorised as PFAS components. In most cases, PFAS (per- and polyfl uoroalkyl substances) are human-made products that originally didn’t occur in our environment. As a result, they are resistant to degradation and can accumulate in our environment. Due to this PFAS is also known as the forever chemicals. PFAS is used in a wide variety of industrial and commercial applications, including the production of non-stick cookware, fi refi ghting foams, and textiles.


PFAS compounds can be harmful to human health and the environment and may cause a variety of health problems. On an environmental scale, these compounds can interfere with the normal functioning of ecosystems. As such, the presence of PFAS in the environment can be a sign of environmental pollution which makes monitoring an important factor for the assessment of environmental quality.


Several examples of PFAS compounds include: • Perfl uorooctanoic acid (PFOA) • Perfl uorooctane sulfonate (PFOS) • Perfl uorononanoic acid (PFNA) • Perfl uorohexane sulfonate (PFHxS)


The origin of PFAS comes from several sources: industrial manufacturing processes and/ or emissions, agricultural runoff, municipal waste, or from consumer products. Currently, thousands of different PFAS components have been produced and identifi ed, which makes the analyses for these components more and more complex.


PFAS can be found in a variety of environmental matrices, including soil, sediment, and water. PFAS can be analysed as EOF (Extractable Organic Fluoride) or as AOF (Adsorbable Organic Fluoride). Depending on the sample preparation, results are either classifi ed as EOF or AOF.


AOF is typically measured using a column adsorption method on which PFAS is adsorbed on activated carbon, by feeding liquid sample material through these columns. The PFAS-loaded carbon is then introduced into a combustion analyser.


EOF is the result of a pre-concentration technique called SPE (Solid Phase Extraction) in which PFAS components are retained on a selective stationary phase and fl ushed off with a suitable solvent/ liquid or a combination of both. This fl ushed-off liquid is then introduced into a combustion analyser.


In both cases, EOF or AOF is a total bulk analysis and gives the total sum of all Organic Fluorine containing compounds.


New standardised methods for screening purposes to measure total fl uorine in water matrices have been introduced or are in the process of development. These methods all focus on pre-treating the samples using AOF:


• DRAFT EPA Method 1621: Screening method for the determination of Adsorbable Organic Fluorine (AOF) in aqueous Matrices by Combustion Ion Chromatography.


• WK 68866 - New Test Method for Determination of Adsorbable Organic Fluorine in Waters and Waste Waters by Adsorption on Activated Carbon followed by Combustion Ion Chromatography


• DIN 38409-59 German standard method for the examination of adsorbable organically bound fl uorine, chlorine, bromine, and iodine (AOF, AOCl, AOBr, AOI) by means of combustion and subsequent ion chromatography in water, wastewater, and sludge.


• DRAFT ISO 18127 Determination of adsorbable organically bound fl uorine, chlorine, bromine, and iodine (AOF, AOCl, AOBr, AOI) Method using combustion and subsequent ion chromatographic measurement.


In this paper, we will discuss a possible alternative sample pretreatment technique using solid phase extraction (SPE) for measuring EOF instead of AOF.


Sample preparation


Solid-phase extraction (SPE) is a sample preparation technique that is used to isolate and enrich analytes of interest from a complex sample matrix. It is based on the principle of chromatography, in which the analytes are selectively retained on a solid phase while the matrix components are washed away.


When using SPE, it is important to select the correct sorbent phase and the proper wash/ fl ush procedures. The most used cartridges for PFAS currently are based on weak anionic exchange material (microporous polystyrene-divinylbenzene with amine functional groups), but other new stationary phases are to be in development.


In essence, SPE consists of the following procedural steps: 1. Preparation of the SPE columns (conditioning/washing) 2. Loading of sample material onto the SPE columns 3. Washing away undesired matrix and/or other components 4. Flushing of the desired measuring components 5. Preconcentration of fl ushed sample material by evaporation 6. Accurate addition of solvent applicable for combustion analysis 7. Injection in a combustion analyser coupled with an ion chromatograph (C-IC)


Depending on the preparative SPE equipment, liquid handling for all these steps is carried out by using a vacuum and/or positive displacement pressure (accommodated by syringe drives/pumps).


Advantages of SPE (EOF) over Activated Carbon material (AOF)


Matrix interference:


Real-world sample matrices often contain complex challenges, such as impurities, contaminants, or co-existing compounds, that can interfere with the accurate measurement of the target analyte. SPE allows for the selective removal of these interfering compounds, ensuring that only the analyte of interest is measured.


Selectivity:


SPE can be used to selectively isolate and enrich analytes of interest from a complex sample matrix by using specifi c types of sorbents. This can improve the sensitivity and accuracy of subsequent analytical methods.


Reproducibility:


SPE is a reproducible method for sample preparation. This is important for ensuring that the results of analytical methods are accurate and reliable.


Blank levels: The blank levels of EOF analysis are much lower than those of activated carbon (AOF) analysis. This is because EOF analysis uses a more selective method for the removal of interfering compounds.


Automation:


Several SPE setups can be automated, which can save time and improve the effi ciency of sample preparation. This is especially important for large sample batches.


Sample introduction: Liquid sample introduction is always preferred over solid sample introduction in a combustion analyser. This is because liquid samples are easier to handle and larger volumes can be introduced.


INTERNATIONAL LABMATE - APRIL 2024


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