The Science Behind Testing for Toxic Chemicals in Children's Car Seats
X-Ray Fluorescence (XRF)
High Definition X-ray Fluorescence (HD XRF) is an elemental analysis technique with greater sensitivity than standard XRF. Our instrument from XOS uses monochromatic excitation energies of 7, 17, and 33 keV. The spot size is one millimeter. Elements heavier than aluminum are measurable. We abbreviate HD XRF as “XRF” in this report.
Detection limits are in the low parts-per-million (ppm) or sub-ppm range for all elements of interest in this study except chlorine and phosphorus. For chlorine we consider results above 1,000 ppm to be at least semi-quantitative and for phosphorus, which is only very weakly detected by XRF, we visually inspect spectra for the presence of a peak at 2.01 keV. From the elemental composition, we learn whether heavy metals are present and can infer the likely presence of flame retardant chemicals containing bromine, chlorine, or phosphorus.
While XRF testing does not directly identify molecular structure of chemicals, detecting bromine at levels greater than 400 ppm and chlorine at levels greater than 3,500 ppm has been successfully used to infer the presence of halogenated flame retardants, depending on the sample matrix. We also have demonstrated that phosphorus detected by XRF in car seat fabrics and foams is a strong an indicator of phosphorus-based FRs. However, due to the inefficient detection of phosphorus by this method, an XRF result of "not detected" for phosphorus does not preclude the presence of chemicals containing phosphorus.
For more information on using bromine and phosphorus as proxy indicators of brominated and phosphorus-containing FRs, see the following references.
Ecology Center. Hidden Passengers - Chemical Hazards in Children’s Car Seats (2015).
Ecology Center. Traveling with Toxics - Flame Retardants & Other Chemicals in Children’s Car Seats (2016).
Allen, J. G., et al. Linking PBDEs in house dust to consumer products using X-ray fluorescence. Environ. Sci. Technol. 42, 4222–4228 (2008)
Stapleton, H. M. et al. Identification of flame retardants in polyurethane foam collected from baby products. Environ. Sci. Technol. 45, 5323–5331 (2011)
Fourier Transform Infrared Spectroscopy (FTIR)
A Nicolet iS5 FTIR spectrometer with an attenuated total reflection (ATR) accessory was used to identify the polymer type of the car seat components. Components with elevated bromine and/or phosphorus according to XRF were further analyzed after solvent extraction. First, samples were cut into small pieces and extracted in a glass vial with an appropriate solvent. After at least 24 hours, extracts were analyzed by ATR-FTIR. In most cases, we used commercial FTIR libraries to match spectra and identify extracted chemicals.
Phosphorus-based FRs present in these samples were identifiable by this method, while the bromine-based FRs were only occasionally identifiable by this method due to poor extraction efficiency. The resulting data was used in conjunction with the XRF and GC/MS or LC/MS data to help identify specific FR chemicals.
Gas chromatography and liquid chromatography coupled to mass spectrometry (GC/MS and LC/MS)
A subset of 36 fabric and soft foam (all polyurethane except one polyethylene foam) samples was chosen for a sensitive mass spectrometric analysis at Indiana University. At least one sample was taken from each seat. Scissors were wet-cleaned with isopropyl alcohol before and after each cut. Three to five grams of material for each sample were immediately placed into individual polyethylene bags to avoid cross-contamination. The samples were processed and analyzed by Indiana University. A total of 82 FR chemical analytes were targeted.
For each sample, 50 – 200 mg was subjected to a sonication-assisted extraction with 10 mL of hexane and acetone (50/50, v/v). The target organophosphate esters (OPEs) were analyzed using Ultra-Performance Liquid Chromatography (UPLC) interfaced with Triple-Quadrupole Mass Spectrometer (QQQ-MS) under Positive Electrospray Ionization (ESI+) mode. The separation was achieved on an Acquity UPLC BEH C18 column (50 mm, 2.1 mm i.d., 1.7 μm thickness), and the LC-MS was operated in Multiple Reaction Monitoring (MRM) mode.
Gas Chromatography coupled to Mass Spectrometry with Electron-Capture Negative Ionization (GC-ECNI-MS) was applied to determine the remaining analytes. The GC-MS was conducted in Selected Ion Monitoring (SIM) mode. A Rtx-1614 capillary GC column (15 m, 0.25 mm i.d., 0.1 μm thickness) was used for the chromatographic separation. Identification and quantification were based on five criteria: (1) sample peaks have approximately Gaussian shapes; (2) the retention time of sample peaks line up with those of reference standards within ±0.2 min.; (3) peak areas of quantifier and qualifier ions exhibit isotopic ratios within 15% of theoretical values; (4) the signal-to-noise (S/N) ratio of a peak is greater than 5; and (5) calculated concentrations are within the range of calibration curves; otherwise further dilution is required.
Quantifiable analytes are listed in the Data Tables (pdf).
More information on this work can be found in the following reference.
Wu Y., et al. Children’s Car Seats Contain Legacy and Novel Flame Retardants. Environ. Sci. Technol. Lett., published on the web Dec. 3, 2018.
Particle-Induced Gamma Ray Emission (PIGE) spectroscopy
Particle-Induced Gamma Ray Emission (PIGE) spectroscopy is a nondestructive technique developed to quantify total fluorine on papers and textiles. Total fluorine on textile fabrics is in most cases an indicator of per- and polyfluorinated alkyl substances (PFAS) used to impart stain resistance. The method is highly sensitive to fluorine atoms, and indicates the total fluorine present above approximately 25 ppm. This makes it a particularly appropriate analysis method for the surface concentrations of fluorine as a surrogate for PFAS. PFAS on consumer products are often intentionally applied as surface treatments because they impart water- and stain-resistant qualities.
Details of the PIGE method can be found in the following reference.
Schaider, L.A. et al. Fluorinated Compounds in U.S. Fast Food Packaging. Environ Sci Technol Lett. 4(3): 105–111 (2017).