Application of Nutech Canister Sampling & Preconcentration System in Volatile PFAS Monitoring in Stationary Source Emission

Abstract

PFAS, due to their non-degradability and bio-toxicity, have increasingly garnered attention. While mature and reliable analytical methods exist for detecting PFAS in soil and water, measurement methods for identifying and quantifying volatile PFAS or volatile fluorinated compounds (VFCs) in stationary source emissions are limited. After upgrading the preconcentrator, Nutech developed an application method for 30 volatile PFAS in stationary source emissions—canister sampling coupled with pre-concentration and gas chromatography-mass spectrometry (GC-MS). The method’s data results show that the relative standard deviation of the average relative response factors for the 30 volatile PFAS is below 30%, the method detection limit is lower than 0.01 nmol/mol, method blanks are below the detection limit, and recoveries for spiked samples at 1.0 nmol/mol are between 70% and 130%. The relative deviation for laboratory analysis duplicates is within ±10%.

Keywords: Volatile PFAS, Pre-Concentration, Nutech, EPA OTM-50

Background

Per- and polyfluoroalkyl substances (PFAS) are a class of synthetic compounds widely used in various industrial and commercial products. They are highly fluorinated aliphatic compounds containing at least one fully fluorinated carbon atom. PFAS have a hydrophobic perfluorinated alkyl group at one end and a hydrophilic functional group at the other end. These functional groups include carboxylates, sulfonamides, phosphates, sulfates, and alcohols. PFAS possess strong carbon-fluorine (C-F) bonds, which are thermodynamically and chemically stable, making them highly resistant to biological metabolism, photolysis, hydrolysis, and other forms of chemical degradation and decomposition. Due to their unique physicochemical and biological properties—such as stability, stain resistance, water resistance, and lipid resistance—PFAS have been widely used in daily life, including in protective coatings, tableware, food packaging, firefighting foams, paints, non-stick cookware, inks, and personal care products. These widespread applications, combined with the non-degradability of PFAS chemicals, have led to new pollution issues in the environment.

Currently, there are approximately 5,000 types of PFAS on the global market, most of which are unknown (PFAS precursors), and there is no standardized analytical method to analyze these compounds, which pose significant threats to human health and the environment. In specialized laboratories, PFAS analysis in water and soil is primarily conducted using liquid chromatography-mass spectrometry (LC-MS). However, there are few reports on the use of gas chromatography-mass spectrometry (GC-MS) to analyze volatile and semi-volatile PFAS in stationary source emissions. In 2022, Nutech Instruments, commissioned by the U.S. Environmental Protection Agency (EPA), developed a pre-concentration/gas chromatography-mass spectrometry method for the pretreatment, separation, and detection of 30 volatile PFAS compounds in stationary source emissions. Based on previous research experience, we identified two major challenges in developing this analytical method:

First, among the 30 volatile PFAS compounds to be analyzed, there is carbon tetrafluoride with an extremely low boiling point, which is difficult to capture and concentrate.

Second, stationary source emissions may contain high concentrations of CO2 gas. Some waste gas samples can reach approximately 12%, and even after pressurized dilution, it remains around 4%. This affects the separation and detection of low-boiling volatile PFAS compounds, such as hexafluoroethane.

Through repeated exploration, Nutech successfully developed an analytical method for volatile PFAS in stationary source emissions, with data that has been recognized by the U.S. EPA.

1. Materials and Methods

1.1 Instruments and Reagents

Nutech 8910F Preconcentrator, Nutech Water and CO2 Removal Module, Nutech 2208 High-Precision Static Dilutor, Nutech 2104 Canister Cleaning System, Nutech Sampling Canisters (Nutech Instruments Inc., USA).

Agilent Gas Chromatograph-Mass Spectrometer (8890/5977B, Agilent Technologies, USA).

Nafion Tubing: 1.8m × 2 (Perma Pure, USA).

Chromatographic Column: GASPRO Chromatographic Column, 60m × 0.32mm × 1.4μm (Agilent Technologies, USA).

Figure 1: Volatile PFAS Monitoring and Analysis System for Stationary Source Emissions

PFAS Standard Gases: 30 types of PFAS standard gases at 200 nmol/mol, with carbon tetrafluoride (CF4) concentration at 2000 nmol/mol, provided by the U.S. EPA.

PFAS Working Standard Gases: The 200 nmol/mol standard gas is stepwise diluted to 2.5 nmol/mol (with CF4 at 25 nmol/mol) and 0.25 nmol/mol (with CF4 at 2.5 nmol/mol) working standard gases. CO2 and water are added, with the final relative humidity of the working standard gas being 50% and the CO2 concentration at 4%.

Internal Standards and Surrogates: 1.0 µmol/mol, Chlorobenzene-d5 (internal standard), Bromochloromethane, 1,4-Difluorobenzene, 4-Bromofluorobenzene (Linde SPECTRA Environmental Gases, USA).

CO2 Standard Gas: 20%.

High-Purity Nitrogen: Nitrogen from a Dewar flask, 99.999%.

High-Purity Helium: Helium from a steel cylinder, 99.999%.

Water: Distilled water free of target compounds.

1.2 Experimental Conditions

Water and CO2 Removal Module: Nafion tube temperature at 35°C, valve box temperature at 50°C, transmission line temperature at 80°C.

1.2.1 Experimental Conditions for the First 8 PFAS

Preconcentrator: Trap1 cooling temperature at -165°C, purge flow rate at 100 mL/min, purge time at 120 seconds, pre-heat temperature at 20°C, pre-heat time at 10 seconds, desorption temperature at 225°C, desorption time at 90 seconds; Focus1 cooling temperature at -196°C, cooling stabilization time at 60 seconds, flash evaporation time at 20 seconds; Injection flow rate at 80 mL/min.

GC: Initial column flow rate at 0.75 mL/min, held for 0.1 minutes, ramped to 1.5 mL/min at 5 mL/min and held until the end; Temperature program: Initial temperature at 35°C, held for 8 minutes, ramped to 70°C at 5°C/min, then ramped to 230°C at 20°C/min.

MS: EI source temperature at 230°C, MS interface temperature at 250°C, Single SIM mode, with one quantitation ion and 1–2 qualitative ions selected for each target component (see Table 1).

1.2.2 Experimental Conditions for the Remaining 22 PFAS

Preconcentrator: Trap1 cooling temperature at -53°C, purge flow rate at 120 mL/min, purge time at 1200 seconds, pre-heat temperature at 20°C, pre-heat time at 10 seconds, desorption temperature at 228°C, desorption time at 30 seconds; Focus2 cooling temperature at -170°C, cooling stabilization time at 30 seconds, flash evaporation time at 20 seconds; Injection flow rate at 80 mL/min.

GC: Constant flow rate at 1.5 mL/min; Temperature program: Initial temperature at 35°C, held for 8 minutes, ramped to 220°C at 5°C/min, then ramped to 235°C at 15°C/min and held for 4 minutes.

MS: EI source temperature at 230°C, MS interface temperature at 250°C, Single SIM mode, with one quantitation ion and 1–2 qualitative ions selected for each target component (see Table 1).

Table 1: Target Compounds of 30 Volatile PFAS

1.2.3 Qualitative and Quantitative Analysis

For each target compound, a retention time window is established by multiple injections of calibration standards. The retention time window is defined as ±3 times the standard deviation of the retention time. The retention time of the target compound in the sample should fall within this window. The quantitative ion and the auxiliary ion for each target compound must appear in the sample’s mass spectrum. At least one auxiliary ion in the sample’s mass spectrum should have a relative abundance that deviates by no more than ±30% from the relative abundance in the standard mass spectrum. Quantification is performed using the average relative response factor. For the first 8 volatile PFAS components, the external standard method (ES) is used, and the calibration curve must contain at least 7 linear points. The relative standard deviation (RSD) of the response factor (RF) should be less than or equal to 30%. For the remaining 22 volatile PFAS components, chlorobenzene-d5 is used as the internal standard, and the internal standard method (IS) is applied. The calibration curve must also contain at least 7 linear points, and the relative standard deviation (RSD) of the relative response factor (RRF) should be less than or equal to 30%.

2. Test Results

2.1 Total Ion Chromatogram

A sample is analyzed in two injections: the first injection analyzes the first eight PFAS compounds (Figure 2a), and the second injection analyzes the remaining 22 PFAS compounds (Figure 2b). The peaks for the 30 volatile PFAS compounds are well-shaped and clearly separated.

Figure 2 Chromatograms of the first 8 volatile PFAS mixed standard gas (a) and the last 22 volatile PFAS mixed standard gas (b)

2.2 Calibration Curve

The 8910F was sequentially injected with 50 mL, 100 mL, 200 mL, and 400 mL of 0.25 nmol/mol PFAS standard gas (humidified with 4% CO2) and 100 mL, 200 mL, and 400 mL of 2.5 nmol/mol PFAS standard gas (humidified with 4% CO2) to establish a calibration curve ranging from 0.0625 nmol/mol to 5 nmol/mol (with tetrafluoromethane ranging from 0.625 nmol/mol to 50.0 nmol/mol). The calibration curve was fitted using the average relative response factor.

For the first 8 PFAS compounds, quantification was performed using the external standard method, while the remaining 22 PFAS compounds were quantified using the internal standard method, with chlorobenzene-d5 as the internal standard at a concentration of 5.0 nmol/mol. The RSD of the average relative response factors for each target compound was less than 30%.

Note: The linear concentration range of tetrafluoromethane is 0.625 ppb to 50 ppb.

2.3 Method Detection Limit (MDL) and Method Blank

Seven blank spiked samples with a concentration of 0.0625 nmol/mol (tetrafluoromethane at 0.625 nmol/mol) were continuously analyzed. The standard deviation (s) was calculated, and the Method Detection Limit (MDL) was determined using the formula: MDL = t(n-1,99) × s (with a t-value of 3.143). The results for the MDL are shown in Table 2. The MDL for all target compounds is below 0.01 nmol/mol (with tetrafluoromethane’s MDL below 0.1 nmol/mol). Additionally, method blanks were tested, and the blank results for all target compounds were below the detection limits (as shown in Table 2).

2.4 Method Accuracy and Precision

The recovery rates for each target compound in the 1.0 nmol/mol blank spiked samples ranged from 70% to 130%, and the relative deviations for parallel analyses within the laboratory were within ±10%.

Table3 All Volatile PFAS Quality Control, Spiked Test, and Spiked Parallel Test Data

Note: The relative deviation is given as the absolute value.

3. Conclusion

As a participant in the collaborative experiment for the development of the U.S. EPA OTM-50 method, Nutech successfully completed the instrumentation modifications for the water and CO2 removal module and the preconcentrator, along with method development and validation experiments. The experimental content included standard curve testing, blank testing, detection limit testing, spiked accuracy testing, and repeatability testing.

The experimental results showed that:

• The relative standard deviations of the average relative response factors for all target compounds were below 30%.
• The method detection limits were below 0.01 nmol/mol (with the detection limit for tetrafluoromethane below 0.1 nmol/mol).
• The method blanks were all below the detection limit.
• The recovery rates for 1.0 nmol/mol spiked samples were within 70% to 130%.
• The relative deviation of the parallel analyses within the laboratory was within 10%.

The data results have been recognized by the U.S. EPA. Currently, the Nutech 8910F preconcentrator has been put into operation by the U.S. Environmental Protection Agency (EPA) to further improve and update the EPA OTM-50 method.