TITLE: Polyethylene terephthalic acid deconstruction product analysis by UHPLC-DAD
AUTHORS: Hannah M. Alt, Kelsey J. Ramirez, Stefan J. Haugen, William E. Michener, Gregg T. Beckham

[DESCRIPTION]
This analytical procedure outlines a rapid and accurate method for the quantification of three key PET deconstruction products, namely terephthalic acid (TPA), 2-hydroxyethyl terephthalic acid (MHET), and bis(2-hydroxyethyl) terephthalic acid (BHET), using an ultra high performance liquid chromatography with diode array detection (UHPLC-DAD) system. This method combines the advantages of UHPLC for improved resolution and separation efficiency with the sensitivity of DAD for accurate and reliable quantification.



[BEFORE_START]
All solvents and chemicals used are listed in the ‘Materials’ section. These are excluded from in-line references to maintain clarity and keep the steps concise.  




[GUIDELINES]
This protocol utilizes an ultra high performance liquid chromatography diode array detector (HPLC-DAD) system manufactured by Agilent Technologies as referenced in 'Materials'. A similar UHPLC-DAD system can be utilized however, some parameter nomenclature may deviate depending on the manufacturer. 

[STEPS]
SECTION: Mobile Phase Preparation
5. Mobile Phase A:
a solution of 20 mM phosphoric acid in UPW using the ratio of 1.34 mL of 85% phosphoric acid per 1 L of UPW. Ensure sufficient volume for analysis of all samples and standards.

Mobile Phase B:
Methanol
SECTION: Sample Preparation
4. Samples must be filtered through a 0.2 µm or smaller filter prior to injection on the UHPLC     
Samples expected to be over the linear range of the instrument should be diluted to ensure accurate analysis and avoid carryover.
SECTION: UHPLC-DAD Parameters
6.  
 

 

SECTION: Preparation of Standards
1. Terephthalic acid (TPA) preparation

Prepare a buffer solution for the TPA standard
Prepare a 1M potassium phosphate dibasic (K2HPO4) buffer solution by weighing 87 g of K2HPO4 into 500 mL of 18.2MΩ⋅cm ultrapure water (UPW).
Prepare a 1M potassium phosphate monobasic (KH2PO4) buffer solution by weighing 68 g of KH2PO4 into 500 mL of UPW.
Prepare a 1M phosphate buffer solution by combining 94 mL of K2HPO4 (dibasic) buffer and 6 mL of KH2PO4 (monobasic) buffer.
Check that the pH of the buffer solution is 8.0 ± 0.1 pH units (7.9-8.1). Adjust pH with 10N NaOH or concentrated phosphoric acid if necessary.

Prepare the TPA standard
Weigh out dry TPA standard and create 1000 µg/mL TPA stock standard using the phosphate buffer solution created in the previous step as the stock standard diluent. Example: Weighed 9,800 µg (9.8 mg) of powdered TPA and add 9.8 mL of phosphate buffer solution.
Create a 250 µg/mL TPA working standard using the 1000 µg/mL TPA stock standard and UPW as the diluent. Example: Pipetted 2.5 mL of 1000 µg/mL TPA stock standard and add 7.5 mL of UPW.
Create the following calibration standards using the 250 µg/mL TPA standard working solution and UPW as the diluent.

 

SECTION: Preparation of Standards
2. 2-Hydroxyethyl terephthalic acid (MHET) preparation

Prepare a 1:1 solution of UPW and tetrahydrofuran (THF)
Weigh out neat MHET standard and create a 1000 µg/mL MHET stock standard using the 1:1 solution of ultrapure water (18.2MΩ⋅cm) and THF created in the step 2.1. Example: weigh 8,700 µg (8.7 mg) of powdered MHET and added 8.7 mL of 1:1 UPW and THF.
Create a 250 µg/mL MHET working standard using the 1000 µg/mL MHET stock standard and the 1:1 solution of UPW and THF. Example: Pipetted 2.5 mL of 1000 µg/mL MHET stock standard and 7.5 mL of 1:1 UPW and THF solution.
Create the following calibration standards using the 250 µg/mL MHET standard working solution and UPW as the diluent.

 

SECTION: Preparation of Standards
3. Bis(2-hydroxyethyl) terephthalic acid (BHET) preparation

Weigh out neat BHET standard and create a 1000 µg/mL BHET stock standard using tetrahydrofuran (THF). Example: Weighed 9,500 µg (9.5 mg) of powdered BHET and added 9.5 mL of THF.
Create a 250 µg/mL BHET working standard using the 1000 µg/mL stock standard and THF. Example: Pipetted 2.5 mL of 1000  µg/mL stock standard and 7.5 mL of THF.
Create the following calibration standards using the 250 µg/mL BHET standard working solution and THF as the diluent.


 


SECTION: Analytical Quality Control
7. Calibrations
All compounds must have a correlation coefficient (r2) of greater than or equal to 0.995 using a linear fit. It is recommended to weight the calibration with a 1/x weighting.

Calibration verification standards
A calibration verification standard (CVS) is a standard from the calibration that is re-analyzed every 20 or fewer samples to ensure instrument drift remains within the determined acceptable criteria. Acceptable recoveries for this analysis are ±10% of the expected value. All reported data was bracketed by acceptable CVS recoveries. Acceptance criteria may differ between instruments and should be determined experimentally.



SECTION: Example Chromatogram
8.