PFAS, or perfluorinated alkylated substances, are a family of fluorosurfactants. These substances are used in food packaging, particularly for fried fast food, because oil leaks through packaging are inhibited by their lipophobicity.
However, PFAS has been known to bioaccumulate and have been connected to adverse health outcomes for humans. C8 compounds, which are long-chain PFAS with 8 or more carbon atoms per chain, have longer biological half-lives and, in the USA, have been voluntarily discontinued in food packaging.
Other fluorinated hydrocarbons and shorter chain PFAS continue to be used. However, some countries and individual states in the USA have been considering limits on total organofluorine compounds allowed to be used in food packaging.
The potential applications for in situ elemental analysis have been expanded through the use of field-portable handheld LIBS analyzers. These analyzers allow for rapid quantification and screening of any element in nearly any matrix type.
LIBS has the ability to test light elements like fluorine that make it particularly well suited to applications where other field-portable elemental analysis techniques (for example, X-ray fluorescence spectroscopy (XRF)) are insensitive to the lightest elements.
An illustration of the LIBS process is shown in Figure 1.
Figure 1. Illustration of the LIBS process. Image Credit: SciAps, Inc.
For measurements, a SciAps Z-300 handheld LIBS analyzer was used (Figure 2 shows the Z-901 Flourine, the instrument that superseded the Z-300).
Figure 2. New for 2021: SciAps Z-901 handheld LIBS analyzer. Internal purge gas cartridge is shown on left. Image Credit: SciAps, Inc.
LIBS analyzers from SciAps have several advanced features:
- Sample positioning enabled by the integrated camera
- X/Y rastering of laser across the sample
- Ability to enhance F signal via helium purge gas
- Capability to minimize bremsstrahlung background with time gating of CCD detector
- Wide spectrometer range covering 185-190nm
LIBS will not speciate the fluorine-containing compound, although detecting fluorine is taken as a marker for the potential presence of PFAS.
Several samples of food packaging were obtained from fast-food restaurants and stores in the Boston area. Samples were not specially prepared in any way and were presented to the analyzer and tested as-is.
A 250 ns gate delay was used for the time gating of the CCD.
LIBS spectra were acquired for 150 consecutive pulses, with each sample manually rastered across the laser focus. All of the samples were tested in replicates of 10, with spectra averaged together.
The samples for the experiment included:
- Microwave popcorn bag
- Compressed cardboard French fry box
- Corrugated cardboard sandwich box
- Paper bags used for oily foods
- Paper wrappers for baked goods and sandwiches
Image Credit: SciAps, Inc.
Results and Discussion
Spectra were inspected for the F (l) peak at 685.60nm - the most prominent overlap-free peak in most matrix types.
There is a secondary F (l) peak at 690.25nm, but it is weaker and also overlaps with another unidentified peak at 690.40nm that was present in many blank samples.
Spectra plotted in the chart below have been corrected to remove any baseline offsets and have had Savitzky-Golay smoothing applied to reduce noise levels.
There were varying levels of F detected in paper wrappers (Fig. 3) for sandwiches.
Figure 3. F 685.60nm signal from various paper wrappers. Image Credit: SciAps, Inc.
F appeared only on the food side for some samples like cardboard hamburger boxes (Fig. 4). Tests were performed on French fry cardboard boxes from three separate fast-food chains, but none showed detectable levels of F.
Figure 4. Hamburger clamshell cardboard box. F was present on the food facing side (solid) but not outside of the box (dashed). Image Credit: SciAps, Inc.
There was a weak LIBS signal overall for a tortilla chip bag while still showing some F signal on the food side of the bag. There was no F was detected on the outside of the bag (Fig. 5).
Figure 5. F signal on food side (solid) of the tortilla chip bag, but not outside (dashed). Image Credit: SciAps, Inc.
Store-brand microwave popcorn bags showed the highest levels of F. Both sides had detectable levels of F, although with higher levels on the food side (Fig. 6).
Figure 6. Microwave popcorn bag. F is present on both sides, but at higher levels on the food side. Image Credit: SciAps, Inc.
To detect levels of fluorine, the F 685.60nm peak in each of the 10 replicates was integrated and then the average and standard deviation of the signal was calculated. Positive detection was defined as samples with an average signal greater than twice the standard deviation (2σ, or 95% confidence).
Table 1. Samples highlighted in blue meet 95% confidence detection threshold. Source: SciAps, Inc.
LIBS cannot determine whether the F was intentionally added now or whether the detected F is from PFAS or other F-based compounds because it is an elemental analysis technique.
There was no determination of a quantitative detection limit for F, meaning some samples marked as non-detect may have poor laser-sample coupling, producing weak LIBS plasma, they may have F inhomogeneously distributed on the surface, or they may even contain lower levels of F.
Using portable handheld LIBS analyzers has been shown to effectively screen food packaging for the presence of fluorine, which may be indicative of the presence of organofluorine compounds or PFAS.
Should elemental action levels be established, further work could quantitatively calibrate a LIBS analyzer for F levels in packaging.
A concern for any further quantitative work is the lack of matrix-matched certified reference materials. Further work could also include correlating quantitative LIBS F concentrations with the content of PFAS determined using techniques like LC/MS.
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This information has been sourced, reviewed and adapted from materials provided by SciAps, Inc.
For more information on this source, please visit SciAps, Inc.