Analyzing halogenated organic compounds with CIC according to DIN 38409-59
What are PFASs?
Per- and polyfluorinated alkyl substances (PFASs) are a classification of thousands of organic molecules in which all of the hydrogen atoms on at least one methyl or methylene carbon atom are replaced by fluorine . Due to this characteristic, PFASs have unique chemical and physical properties including their water and oil repelling quality, making them especially interesting for industrial usage . These substances are highly stable due to the strong C-F bond which causes them to strongly resist degradation, earning them the nickname «forever chemicals». PFASs are therefore known to be extremely persistent and accumulate in humans, animals, and the environment . Research into the adverse health effects of some of these substances is increasing, causing restrictions to be put on their usage and growing public interest in monitoring these compounds and their degradation products.
After the invention of PFASs in the 1930s, the first commercialized production for their end products began in the following decade . The first companies to launch products containing PFASs were DuPont (under their Teflon™ brand) in 1946  and 3M (with Scotchgard™) in the 1950s .
Aside from commercial usage in consumer products, PFASs were also widely used in aqueous film-forming foams (AFFF). These foams were created to extinguish hydrocarbon fuel-based fires, and as such were deployed to military bases, airports, oil rigs, and municipal fire departments. These locations are now potential sources of PFASs leaching into the surrounding environment . Possible contamination and distribution pathways of PFASs are illustrated in Figure 1.
Restriction and elimination of first generation PFASs
PFOA and PFOS have simply been replaced with more novel substitutes like the ammonium salt of hexafluoropropylene oxide dimer acid (HFPO-DA, commercially known as «GenX»), 9-chlorohexadecafluoro-3-oxanonane-1-sulfonate (F-53B), and sodium p-perfluorous nonenoxybenzene sulfonate (OBS) . These chemicals are not affected by the ban on PFOA, PFOS, and related substances, but that does not imply that they are any less toxic or less persistent.
What about other halogenated substances?
In addition to the fluorinated organic compounds presented earlier in this article, chlorinated, brominated, and iodinated organic compounds are also used for numerous industrial and commercial applications and are thus released into the environment . These substances are also formed as byproducts during industrial processes or water treatment and then released extensively into the environment .
As an example, organochlorine substances have been of public interest since the 1980s due to rising awareness about the adverse health effects of dioxins and polychlorinated biphenyls (PCBs) . Back then, public authorities had to take action, which is reflected by a general ban on those substances in most countries today. Brominated organic compounds are still widely used as flame retardants, and iodinated organic compounds are commonly used in healthcare as x-ray contrast media .
Monitoring and analyzing halogenated organic compounds
Analysis of halogenated organic compounds in environmental samples has already been mandatory for decades in many countries. To fulfill this mandate, the sum parameter of AOX (adsorbable organically bound halogens) is used to describe the amount of adsorbable organically bound chlorine, bromine, and iodine in water samples. The existing norms DIN ISO 9562 and EPA Method 1650 describe the determination of AOX by adsorption followed by combustion and microcoulometric titration. However, using titration as detection technique only allows the determination of AOX expressed as chlorine, and not the determination of the individual fractions AOCl, AOBr, and AOI, nor the determination of AOF (adsorbable organically bound fluorine) at all.
Advanced analysis of halogenated organic compounds according to DIN 38409-59
The new DIN 38409-59 (Determination of adsorbable organically bound fluorine, chlorine, bromine, and iodine (AOF, AOCl, AOBr, AOI) by means of combustion and subsequent ion chromatography measurement) fills the gaps left by DIN ISO 9562 and EPA Method 1650 by enabling the determination of AOF, AOCl, AOBr, and AOI as individual fractions and as the sum parameter CIC-AOX(Cl).
Each step of the new method is illustrated in Figure 3, starting with adsorption of the sample, followed by sample transfer to the sample boat(s), combustion of the activated carbon, and adsorption of the halogens followed by analysis of the analytes using ion chromatography (IC).
The key element of this method is the adsorption of the organohalogens on activated carbon. It facilitates their preconcentration while inorganic halogens are eliminated as they do not adsorb to the carbon material. Because the concentration of inorganic halogens in environmental samples exceeds the concentration of organohalogens by several magnitudes, it is crucial to only adsorb the organohalogens and to properly remove the inorganic halogens. The required quantification limits according to DIN 38409-59 of 2 µg/L AOF, 10 µg/L AOCl, 1 µg/L AOBr, and 1 µg/L AOI can easily be reached this way, and interferences by inorganic components are therefore minimized.
Figure 3. Complete setup for analysis of AOX according to DIN 38409-59. In the first step, adsorption of the sample is performed on the APU sim (Analytik Jena). The activated carbon is transferred into combustion boats (step 2) which are then introduced to the combustion module (Analytik Jena) consisting of a combustion oven with Auto Boat Drive (ABD) and an autosampler (MMS5000) in step 3. Next, the volatilized halogens are transported into the absorber module (920 Absorber Module) for absorption. Finally, the halogens are analyzed by IC (930 Compact IC Flex) and the results are evaluated using the MagIC Net software from Metrohm (step 5) .
For AOCl, AOBr, and AOI analysis, the sample pH value needs to be adjusted to pH <2, similar to the methods presented in DIN ISO 9562 and EPA Method 1650. Contrary to this, AOF analysis is only performed with neutralized samples. This differentiation is critical because inorganic fluorine tends to adsorb on to the activated carbon when under acidic conditions and therefore falsify the results.
- Automated adsorption of the samples is performed using a sample preparation system (e.g., the APU sim from Analytik Jena). A 100 mL portion of the neutralized (or acidified) sample is passed through two columns arranged in series – both filled with activated carbon – to which any organohalogens in the sample are adsorbed. In a second step, inorganic halogens are removed from the columns by rinsing them off with 25 mL of a dedicated solution.
- After the adsorption step, the activated carbon is removed from the column and directly transferred into a ceramic boat for analysis with combustion ion chromatography (CIC). It is up to the user whether the content of the two columns is analyzed individually in two separate ceramic boats (e.g., to determine the efficiency of each column) or with a single analysis together in one boat.
- Combustion of the activated carbon takes place at temperatures above 950 °C in the presence of argon and oxygen. For pyrohydrolytic combustion purposes, ultrapure water is continuously added to improve the combustion process. Under these conditions, the organohalogens are volatilized and then converted into their ionic forms by absorption in ultrapure water.
- The absorption of these analytes takes place in the 920 Absorber Module. As no further oxidation of the halogens is necessary, ultrapure water is an adequate absorber solution. The absorber solution is automatically transferred to the IC with a Dosino (and Dosing unit) from Metrohm—a precise dosing apparatus that enables variable injection volumes (4–200 µL) for injection into the IC. This technique, also referred to as MiPT (Metrohm intelligent Partial Loop Injection Technique), is also used to automatically calibrate the IC from a single standard solution by using variable injection volumes. This results in better calibrations and less time spent preparing separate standards manually.
- After injection into the IC, the halogens are separated on an anion exchange column. Sequential suppression is used to decrease the background conductivity and increase the analyte sensitivity before conductivity detection. The chromatogram of a wastewater sample analyzed according to DIN 38409-59 is shown in Figure 4.
Figure 4. Chromatograms of a wastewater sample where the activated carbon from the two adsorption columns was analyzed individually. An AOF concentration of 7.85 µg/L was found on the first carbon column and 1.46 µg/L on the second carbon column, equal to a total AOF concentration of 9.31 µg/L for the sample. This is the result after blank subtraction. The respective AOF blanks are also shown in grey .
An innovative solution for screening PFASs
Thousands of chemicals are classified as PFASs, but targeted analysis using LC-MS/MS is restricted to the determination of a small number of predefined substances from this group. Therefore this approach provides researchers with limited information regarding actual contamination levels and usually does not provide any information on precursors or newly developed per- and polyfluorinated alkyl substances.
On the other hand, sum parameters (e.g., AOF) provide more extensive information about the total amount of PFASs contaminating a sample. The new DIN 38409-59 provides a standardized approach including sample preparation for reliable and reproducible results. Thus, AOF is the ideal parameter for screening PFASs in water samples prior to any further targeted analysis. Additionally, DIN 38409-59 can also be used to report AOCl, AOBr, and AOI values, and therefore provides the full information regarding the organohalogen content in the respective sample.