Pressurized liquid extraction and GC/MS determination of model contaminants in HDPE - Application to recycling of post-consumer polyolefins using limonene
Journal of Chromatography A, Volume 1745, 2025, 465749: Fig. 1. GC/MS SIM chromatogram of a standard solution containing 1,000 µg L-1 of each surrogate. 1: chloroform. 2: deuterated toluene. 3: toluene. 4: deuterated chlorobenzene and chlorobenzene. 5: methyl salicylate. 6: phenyl cyclohexane. 7: deuterated benzophenone and benzophenone. 8: methyl stearate.
A pressurized liquid extraction and GC/MS (PLE-GC/MS) method was developed for quantifying seven model contaminants in recycled HDPE, selected based on EFSA and FDA recommendations. The method showed low detection limits, good repeatability, and high accuracy. Extraction was optimized using acetone at 100 °C for 6 min.
This method was applied in a challenge test to evaluate an innovative HDPE recycling process using limonene for polymer dissolution and precipitation. The results confirmed high cleaning efficiency, demonstrating both the effectiveness of the recycling process and the suitability of the PLE-GC/MS method for such evaluations.
The original article
Pressurized liquid extraction and GC/MS determination of model contaminants in HDPE - Application to recycling of post-consumer polyolefins using limonene
Javier Blázquez-Martín, Jorge García-Barrasa, María Teresa Tena
Journal of Chromatography A, Volume 1745, 2025, 465749
https://doi.org/10.1016/j.chroma.2025.465749
licensed under CC-BY 4.0
Selected sections from the article follow. Formats and hyperlinks were adapted from the original.
World plastic production reached 400.3 million tonnes in 2022. Of the total, the production of polyolefins from fossil sources accounted for 45.2 %. The most produced polyolefins were polypropylene (PP) and polyethylene (PE: linear low density (LLDPE), low density (LDPE), medium density (MDPE) and high density (HDPE)) [1], with main applications in the packing industry [2].
Of the 17.9 million tonnes of post-consumer packaging waste collected in the European Union in 2020, 46 % was recycled, 37 % was used for energy recovery and 17 % ended up in landfills [3]. This metric cannot be applied to polyolefins and is mainly due to the high recyclability of poly(ethylene terephthalate) (PET). HDPE recycling rates fall to 10 to 15 % in Europe [4]. As for other polymers (LDPE, PP and PS), recycling is limited as well [4].
A wide variety of extraction methods has been proposed for extracting additives and contaminants from plastics, such as total dissolution [22], solvent extraction [22], supercritical fluid extraction [23], pressurized liquid extraction [[23], [24], [25], [26], [27]] or microwave-assisted extraction [28,29]. Pressurized liquid extraction is a technique that achieves higher efficiencies and lower operating times by using organic solvents at high pressures, allowing working with temperatures higher than the selected solvent's boiling point, which increases the diffusion and desorption of compounds from the polymer, as well as increasing compound solubility.
The analysis of the PE extracts has been carried out by gas chromatography/mass spectrometry (GC/MS) [22,25,26,28], GC with a flame ionization detector (FID) [25,26] or a thermoionic selective detector (TSD) [26], headspace solid phase microextraction and GC/MS [22], liquid chromatography (LC) with UV–Vis [23,24,27,29] and MS detection [24]. GC is the preferred technique for volatile and low molecular weight compounds. Due to the volatile nature of some compounds of interest, and to allow a simultaneous detection, GC/MS was chosen for this study.
The aim of this work was to develop, optimize and validate a PLE and GC/MS-based method for the determination of seven model contaminants (proposed by the EFSA), as well as to demonstrate the cleaning efficiency of a recycling process intended for PE from different waste sources based on the dissolution-precipitation method with limonene by analysing artificially contaminated HDPE samples in a challenge test. The development of this method is of utmost importance since, to our knowledge, there are no validated procedures published for surrogates of challenge tests.
2. Materials and methods
2.4. FTIR characterization
FTIR characterization was performed to determine whether treated and untreated HDPE presented differences in its characteristic bands, as well as to determine the presence of residual limonene after the treatment. To carry out the measurements, a Perkin-Elmer FTIR spectrometer (Waltham, MA, USA) equipped with an ATR accessory was used. Resolution of 4 cm-1, 4 scans per measurement and room temperature were the selected conditions. Vibrational bands for HDPE and limonene have been assigned according to Da Silva and Wiebeck [31] and Schulz et al. [32] and can be found in Table S1 of the supplementary material.
2.5. Pressurized liquid extraction
A Dionex ASE 200 pressurized liquid extractor (Sunnyvale, CA, USA) with a solvent controller was used for the extraction process. 0.5 g of PC-HDPE flakes were placed into 11 mL extraction cells with sea sand filling up the void volume inside the cells. Extractions were carried out at 10.3 MPa (1500 psi) using acetone as solvent. Temperature varied from 60 to 100 °C, setting an upper limit below HDPE's melting point (120–130 °C) to prevent polymer melting. The extraction time varied from 2 to 10 min. The use of one, two, three and four extraction cycles was studied as well. The flush volume was 60 % and purge time was set at 60 s. A preheating step of 5 min was used. Acetone extracts containing the surrogates were transferred into volumetric flasks and made up with acetone to 50 mL after the addition of 1.25 mL of an internal standard solution. Aliquots of each extract were filtered through a 0.22 µm nylon syringe filter prior to GC analysis.
2.6. Chromatographic conditions
An Agilent 8890 gas chromatograph coupled to an Agilent 5977B single quadrupole mass spectrometer and an Agilent 7693A autosampler were used for GC analysis. The chromatographic column was an Agilent HP-5 ms (30 m x 250 µm x 0.25 µm) with a 5 % phenyl 95 % dimethylpolysiloxane phase.
The oven program started at 35 °C, holding for 0.5 min before raising the temperature to 60 °C at a rate of 10 °C min-1. Temperature was raised again to 280 °C at a rate of 70 °C min-1 and maintained for 1.8 min.
Injection was done at 380 °C with an Agilent 5190–2293 4 mm I.D. liner in splitless-split mode (0.05 min in splitless mode). The carrier gas was helium at 1.0 mL min-1. MS detection was carried out by electronic impact, with an ionization source heated at 230 °C with a voltage of 70 eV. The quadrupole temperature was 150 °C. Chromatograms were registered in SIM mode. Chromatographic parameters for each compound are listed in Table 1.
3. Results and discussion
3.1. GC/MS determination of surrogates
The analyte separation by GC was studied using surrogate standard solutions prepared in three different solvents: isopropanol, acetone and diethyl ether. Isopropanol was chosen first as it had been previously used for pressurized liquid extraction of additives in polyethylene [24,25]. In addition, it is considered as a green solvent [33], but it was discarded because it has a higher boiling point than chloroform and, therefore, interfered with its detection.
Diethyl ether and acetone proved to be better candidates as they did not interfere with the detection of any of the surrogates. However, diethyl ether's lower boiling point led to the evaporation of standard solutions even under cooling conditions, so acetone was chosen to prepare the surrogate solutions for the GC/MS analysis. Acetone is considered a green solvent as well [33].
Due to the extensive range of boiling points (from 62 °C for chloroform to 443 °C for methyl stearate), both a high injection temperature (380 °C) and a low oven start temperature (35 °C) were needed. A non-polar low-bleed column was used to carry out the separation. A chromatogram of a solution containing 1000 µg L-1 of each surrogate in acetone is shown in Fig. 1. Table 1 shows the model contaminants, their physical properties and the chromatographic parameters for its detection.
Journal of Chromatography A, Volume 1745, 2025, 465749: Fig. 1. GC/MS SIM chromatogram of a standard solution containing 1,000 µg L-1 of each surrogate. 1: chloroform. 2: deuterated toluene. 3: toluene. 4: deuterated chlorobenzene and chlorobenzene. 5: methyl salicylate. 6: phenyl cyclohexane. 7: deuterated benzophenone and benzophenone. 8: methyl stearate.
3.4. Recycling method based on dissolution-precipitation
The analytical method developed was used in the evaluation of the cleaning efficiency of a recycling method based on the dissolution-precipitation of post-consumer HDPE using limonene as the dissolution solvent which is a greener alternative to the commonly used toluene or xylene. The addition of an antisolvent to precipitate the polymer was deemed unnecessary, as the polymer precipitated by cooling down the mixture.
Both the PC-HDPE flakes and the recycled polymer were characterized by FTIR. Attention was put to find characteristic signals of the polymer and residual solvents. Of the PC-HDPE samples analysed, only HDPE characteristic bands were found. For the ultra-cleaned HDPE, the characteristic bands of the polymer were found along bands for limonene, which meant that residues of the solvent remained after the ultra-cleaning method.
To ensure the absence of any residues of limonene in the precipitated polymer, a drying step was carried out. A soft temperature of 40 °C was chosen to prevent the polymer from degrading. Aliquots of a treated sample being dried in an oven were taken at different time intervals and stored in closed glass vials at room temperature until characterization. The IR spectrum of each aliquot was registered to analyze the trends of limonene characteristic bands over time of drying.
The IR spectrums were registered six times for the aliquot taken at t = 0 h and three times for the rest of the aliquots. For each spectrum, the signal areas of limonene's characteristic bands were obtained. The variation of mean signal area over time is shown in Fig. S4 of the supplementary material. At 9 days, no signals were found. A comparison of spectrums of ultra-cleaned HDPE with 0 and 9 days of drying at 40 °C is shown in Fig. 3. No differences in HDPE bands were found between the contaminated and ultra-cleaned polymer.
Journal of Chromatography A, Volume 1745, 2025, 465749: Fig. 3. FTIR spectrums of ultra-cleaned just after cleaning (A) and after 9 days of drying at 40 °C (B). Band assignment can be found in Table S1 of the supplementary material.
4. Conclusions
A PLE-GC/MS method for the determination of seven model contaminants in post-consumer and recycled HDPE was optimized, validated and applied to the evaluation of a novel recycling method based on the dissolution and precipitation of PC-HDPE with limonene.
GC/MS allowed the quantification of the model contaminants in a single analysis, using acetone as solvent. LODs suitable for the low concentrations expected in the recycled material were achieved. The PLE conditions were optimized with a central composite design, with the compromise values allowing a fast and automated extraction process.
This method was applied to evaluate the cleaning efficiency of a novel recycling method using limonene, following EFSA recommendations to carry out a challenge test. High cleaning efficiencies were achieved for all surrogates, proving the suitability of limonene and the dissolution-precipitation process to recycle PC-HDPE.
Moreover, the proposed analytical method allowed an easy, fast analysis of HDPE samples both before and after recycling, regardless of original application of the material, proving its adequate use during the evaluation of other novel recycling methods, whether they are mechanical, chemical or alternative processes. Lastly, the use of green and bio-based solvents, lower temperatures and shorter analysis times reduced the impact on the environment.
