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People have developed a keen interest in and studied the effects of introducing carbon-fluorine bonds into organic compounds. This process can significantly affect the physical and chemical properties of organic compounds, especially when compared to non-fluorinated analogs. 1
Image source: Analytik Jena, USA
Most key products in the fields of chemistry and life sciences rely on the functions and beneficial properties of organic fluorine chemistry, including drugs, polymers, and fine and specialty chemicals.
The increasing production of fluorinated organic substances has led to their increasing release into the environment. The Stockholm Convention (Annex B restriction) and the European Water Framework Directive (WFD) have listed these substances as persistent organic pollutants. .
Perfluorinated and polyfluoroalkyl substances (PFAS) represent a wide variety of synthetic chemicals, and they present a series of analytical challenges, including their widespread presence in environmental samples. 2
PFAS has the potential to contaminate the public drinking water system that serves approximately 19 million people in the United States, prompting the U.S. Environmental Protection Agency (EPA) to implement a strong and urgent PFAS action plan—the most detailed and far-reaching The institutional plan aims to address an emerging chemical of concern. 2,3
The EPA has established an unenforceable health recommendation level of 70 parts per trillion (ppt) for the total amount of pentafluorooctanoic acid (PFOA) and heptafluorooctane sulfonic acid (PFOS) in drinking water.
Throughout 2021, EPA has been working to develop a rapid screening tool that can identify the presence and absence of total PFAS. The upcoming standard operating procedure (SOP) will be applied to quantify total organic fluorine (TOF)4.
This requires the development of a sensitive, rapid and direct TOF detection method suitable for monitoring and managing environmental pollution.
This method utilizes solid phase extraction (SPE) and high resolution-continuous source graphite furnace molecular absorption spectroscopy (HR-CS GF MAS).
The solid phase extraction (SPE) procedure has been thoroughly discussed in other studies, detailing its potential to extract fluorinated compounds. These studies indicate that HR-CS GF MAS detects total fluorine by forming gallium fluoride (I) (GaF) molecules in situ.
The research presented here aims to improve the performance of total fluorine (TF) analysis in wastewater using a combination of a new calibration strategy and optimization of the HR-CS GF MAS method.
It is necessary to develop and optimize species non-specific responses for HR-CS GF MAS in order to quantify fluorinated organic compounds as a sum parameter.
Previous studies have emphasized the applicability of gallium for the most sensitive detection of fluorine through the formation of diatomic GaF. The characteristic molecular absorption of GaF can be detected by HR-CS GF MAS.
Before injection, adjust the furnace tube with molecular forming agent (Ga) and modifier (Pd/Mg/Zr/Ba) to obtain the best signal.
The existing literature confirms that the melting point and vapor pressure of fluorinated compounds have a considerable influence on the recovery rate of different compounds. With this in mind, the drying and pyrolysis procedures of the furnace program have been optimized to help minimize the potential loss of fluorinated compounds.
In order to effectively evaluate the optimization method, a total of 24 different fluorine compounds were mixed as QC samples. These 24 compounds were selected to contain different numbers of fluorine atoms and boiling points.
Using this QC sample, a calibration strategy can be developed to obtain the best recovery rate.
The contrAA® 800 G graphite furnace atomic absorption spectrometer and AS-GF autosampler were used throughout the research process. This is controlled using ASpect CS software, which enables the determination of fluorine.
Before use, the graphite tube is coated with zirconium six times (35 µL stock solution). The fluorine content can then be determined by measuring the intensity of the gallium fluoride absorption band.
Throughout the analysis process, in the presence of gallium, fluorine is stoichiometrically converted to GaF. The furnace program contains an adjustment program to increase sensitivity, and each sample has been analyzed in three replicates.
Table 2 and Table 3 provide detailed information on method settings and furnace programs.
Table 1. Instrument specifications. Source: Analytik Jena, USA
Table 2. Method settings and evaluation parameters. Source: Analytik Jena, USA
Note: *- used in the conditioning step
Table 3. GaF molecular detection furnace program. Source: Analytik Jena, USA
Note: *-modifier injection is used for pretreatment, ^-sample injection
The melting point and vapor pressure of fluorine-containing compounds play an important role in the analysis of total fluorine.
If the vapor pressure of the corresponding substance at a given temperature is high, the analyte may be transformed into a gas phase and transported through the argon flow of the furnace program before it can be detected and analyzed.
In order to reduce this risk, it is important to choose calibration standards to ensure that these standards exhibit similar behavior to the sample itself. Therefore, three different calibration groups were prepared to find the best calibration strategy for wastewater (Table 4).
Table 4. Concentration groups. Source: Analytik Jena, USA
The 1000 ppm NaF stock solution is used to prepare calibration standards for inorganic fluoride calibration (inorganic F).
There are three frequently encountered fluorinated compounds-PFOS, PFOA and HFPO-DA-used to generate organic fluorine (organic F) standards, while inorganic and organic mixture standards use NaF, PFOS, PFOA and HFPO-DA Generated.
Table 5 shows the concentration, absorbance, and overall linearity of the calibration. The calibration curve and the measured blank value show that the lowest LOD (4.00 µg/L) can be obtained using inorganic and organic mixture calibration.
Inorganic fluoride calibration standards have the highest signal, while organic calibration standards have the lowest signal (Table 5). It is determined that the thermal stability of organic and inorganic substances affects their different signal responses.
Table 5. Calibration type and quality. Source: Analytik Jena, USA
During the drying and pyrolysis steps of the entire furnace process, many volatile perfluoro and polyfluoroalkyl substances are lost to a certain extent, but the inorganic fluoride remains thermally stable during this process.
Table 6 shows the clear and interference-free GaF absorption spectra (blue) of the three calibration groups at 211.248 nm.
Table 6. The characteristic signal shape and spectral vicinity of the analyte spectrum. Source: Analytik Jena, USA
*-Blue: GaF analyte signal, red: background signal
The QC standards of 24 fluorinated organic compounds showed a recovery rate of 38% in the entire inorganic F calibration strategy (Table 7).
When NaF solution is used as the calibration standard, the change in signal response will result in low PFAS recovery. When spiked with 100 µg/LF (as NaF), the QC sample showed a good spike recovery of 114%.
Table 7. Inorganic F calibration measurement results. Source: Analytik Jena, USA
The recovery rate of QC samples was 190%, and the organic F calibration strategy was adopted. This overestimation of PFAS is the result of the different thermal stability of different organic fluorinated compounds.
Due to the greater stability and improved signal response of NaF, adding 100 µg/L NaF to QC can produce a spike recovery of 275%.
Both QC and wastewater samples were added with a 125 µg/LF TOF mixture; a mixture of the same compounds as the organic calibration standard. The QC and waste water recovery rates are both 74%.
Table 8. Measurement results of organic F calibration. Source: Analytik Jena, USA
*TOF = PFOS, PFOA, HFPO-DA (ratio 1:1:1)
The calibration strategy for organic and inorganic F mixtures was determined to provide the best approximation of the sample analyte characteristics, as shown in Table 9.
The QC sample recovery rate is determined to be 100%. Both QC samples and wastewater samples were added with a 125 µg/LF TF mixture.
The compounds in the TF mixture are the same as those in the calibration standards for inorganic and organic mixtures.
The spike recovery rates for QC samples and wastewater samples were determined to be 85% and 86%, respectively.
QC samples also added inorganic F. Due to the increased thermal stability of inorganic NaF, the recovery rate was overestimated by 167%.
The three calibration strategies are compared. This highlights the significant influence of the calibration components on the analysis results, and further emphasizes that the inorganic and organic F mixture calibration strategy is the best choice for this particular QC sample.
Table 9. Measurement results for calibration of inorganic and organic mixtures. Source: Analytik Jena, USA
*TF = NaF, PFOS, PFOA, HFPO-DA (ratio 1:1:1:1)
To evaluate the long-term stability of the method, we tested QC samples on three different days (every other week).
These three tests used the calibration curve of test 1 without recalibration. During this period, the RSD of these three tests was 5.7% (Table 10). This confidently establishes the long-term stability of the method.
Table 10. Long-term stability test. Source: Analytik Jena, USA
A relatively high blank value was observed during method optimization. This is mainly due to the carryover effect of the sample introduced into the system.
The addition of HNO3 and TritonX-100 to the rinse solution resulted in lower blank values. It is important to use fluorine-free modifiers throughout the process, and it is recommended to use high-purity salts to prepare these, such as gallium(III) nitrate hydrate.
The research presented here demonstrates a fast, direct and sensitive replenishment method for the analysis of total fluorine in wastewater.
This study illustrates the successful application of the MAS method for measurement using the contrAA 800 G instrument. It was discovered that optimized furnace procedures and calibration strategies can provide high sample throughput, sensitivity, and accuracy.
Each sample requires 3.5 minutes for each repetition, and a calibration strategy of inorganic and organic mixtures is used to achieve 100% QC sample recovery. It is also possible to use inorganic and organic mixture calibration strategies to achieve the lowest LOD (4 ppb).
The results of fluorine determination by HR-CS GF MAS have good reproducibility and long-term stability. This method does not require any additional cleaning steps, and proves that the use of the AS-GF autosampler can easily achieve sample dilution and addition.
This information is derived from materials provided by Analytik Jena US and has been reviewed and adapted.
For more information on this source, please visit Analytik Jena US.
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