Rapid Determination of Eight Related Aromatic Acids in the P-Phthalic

Rapid Determination of Eight Related Aromatic Acids in the p-Phthalic Acid Mother Liquid Using an Agilent 1260 Inﬁ nity LC System and an Agilent Poroshell 120 SB-C18 Column

Application Note

Author Abstract Xinlei Yang This Application Note describes a method to determine eight aromatic acids Agilent Technologies Co. Ltd simultaneously in p-phthalic acid mother liquid using an Agilent 1260 Infi nity Shanghai, China Quaternary LC system with a diode array detector (DAD). Good resolution was achieved for all eight target analytes on an Agilent Poroshell 120 SB-C18 column without interference from other impurities of the mother liquid. The analysis was completed in 13 minutes. Due to the excellent performance of the 1260 Infi nity LC System, good precision was obtained using six sequential injections. The results showed an RSD % ≤ 0.18 % (peak area) and RSD % ≤ 0.05 % (retention time) for eight aromatic acids. This study showed that this method is applicable for the analysis of related aromatic acids in the p-phthalic acid mother liquid. Introduction Experimental Mixed working solution Purifi ed p-phthalic acid (PTA) is Instruments A 2.5 mL amount of each stock an important raw material in the solution was pipetted into a 100-mL production of synthetic resin, polyester This method was developed on an volumetric fl ask and diluted to 100 mL Agilent 1260 Infi nity Quaternary LC fi ber, and other chemical products. with MeOH/H2O (10/90, V/V). The The content of p-toluic acid (TA) and System, consisting of a quaternary concentration of each standard in the 4-carboxybenzaldehyde (4-CBA) is often pump (G1311B, built-in four-channel mixed working solution was 25 µg/mL. used as a critical indicator to control vacuum degasser), an autosampler the quality of PTA. This is because both (G1329B), a thermostatted column Sample preparation compartment (G1316A), a diode array compounds can induce a side chain Mother liquid was provided by a PTA reaction to terminate the polymerization detector (G4212B), and an OpenLab CDS ChemStation Edition (C.01.03). manufacturer and fi ltered through a process, and decrease the molecular 0.45-µm PTFE membrane before use. A weight of the product. However, due to Standard solution 1 mL solution was pipetted into a 100-mL the impurities from p-xylene and from volumetric fl ask, followed by the addition the manufacturing process, there can Stock solution of 2 mL of 1 % (V %) ammonia water. It be many other by-products in PTA and was diluted to 100 mL with MeOH/H O Eight aromatic acids: trimellitic acid 2 its mother liquid. The majority of the by- (10/90, V/V) and fi ltered through a products are the related aromatic acids, (TMLA), hydroxymethyl-benzoic acid (HMBA), orthophthalic acid (OA), 0.22-µm PTFE membrane before UHPLC such as TA and 4-CBA. The detection of analysis. these aromatic acids in p-phthalic acid terephthalic acid (TA), mother liquid helps determine the real- 4-carboxybenzaldehyde (4-CBA), time oxidation process, and provides data isophthalic acid (IA), benozic acid for further purifi cation. (BA), and p-toluic acid (p-TOL) were purchased from Sigma-Aldrich. A 50.0 mg With the growing global demand for PTA, amount of each standard was weighed manufacturers are increasingly concerned and put into a 50-mL volumetric fl ask about the analysis of aromatic acids in separately. A 1 mL amount of 1 % (V %) mother liquid as it pertains to the quality ammonia water was added for solvation control of the fi nal product. Monitoring and the solution was diluted to 50 mL the mother liquid could help to optimize with MeOH/H2O (10/90, V/V). The the oxidation process and reduce the concentration of each standard was manufacturing cost. HPLC is an analytical 1.0 mg/mL. tool routinely used in modern laboratories to analyze aromatic acids in fi nal PTA, UHPLC conditions residues from purifi cation, and waste water2–4. This Application Note develops Table 1. UHPLC conditions. a new method for the determination of eight related aromatic acids in p-phthalic Parameter Value acid mother liquid by reversed-phase Column Agilent Poroshell 120 SB-C18, 3.0 × 100 mm, 2.7 µm UHPLC with an Agilent Poroshell Column temperature 40 °C SB-C18 column. The method has been Wavelength 240 nm (4 nm)/Ref 360 nm (40 nm) successfully used for sample analysis by Mobile phase A 0.1 % TFA in H2O a PTA manufacturer. Mobile phase B MeOH Flow 0.8 mL/min Injection volume 2 µL Gradient Time (min) B % 0 10 4 30 7 60 10 60 Post time 3 minutes

2 Results and Discussion Selection of mobile phase TMLA OA 4-CBA

An acidic mobile phase was considered 0.1 % H3PO4 to beneﬁ t the retention and resolution for these compounds due to their acidic property. Three common acid additives OA 0.1 % FA TMLA 4-CBA (formic acid, H3PO4, and TFA) were tested at 0.1 % (V %) concentration in mAU the aqueous phase. The pH of 0.1 %

H3PO4, 0.1 % FA, and 0.1 % TFA was 2.3, 2.7, and 2.0 respectively, as measured OA by a pH meter. Figure 1 shows that the TMLA 4-CBA elution order of eight analytes did not 0.1 % TFA change with the three mobile phases, however, the peak shape and resolution 0 1 2 3 4 5 6 7 8 min were different from each other. When 0.1 % FA was added in the aqueous Figure 1. Chromatograms of eight aromatic acids under different acidic mobile phases phase, TMLA eluted as a shoulder peak, (Concentration 25 µg/mL of each analyte). possibly due to the low capability of the mobile phase to suppress the ionization Conditions for Figure 1 of TMLA. TMLA is a triprotic acid, whose Column Agilent Poroshell SB-C18 (3.0 × 100 mm, 2.7 µm) pKa1 is approximately 2.5. The OA peak Flow rate 1.0 mL/min tailed under 0.1 % H PO and 0.1 % FA, 3 4 Column temperature 50 °C while the 4-CBA peak fronted under 0.1 % FA. Additionally, 0.1 % TFA and 0.1 % Wavelength 230 nm Injection volume 1 µL H3PO4 lead to better resolution for OA and HMBA. Therefore, 0.1 % TFA in aqueous Mobile phase A aqueous phase with different additives (see chromatogram) phase was selected as the ﬁ nal mobile Mobile phase B MeOH phase based on the resolution and peak Gradient Time (min) % B shape. 0 10 4 15 7 60 9 60 Post time 3 minutes

3 Selection of detection wavelength Due to their chemical structures, the eight aromatic acids have different maximum adsorption wavelengths (Figure 2). To simplify the method, 240 nm was selected as measurement wavelength. The ﬁ nal method parameters are listed in Table 1.

mAU TMLAmAU HMBA mAU OAmAU TA

70 140 70 100 60 120 60 80 50 100 50 40 80 40 60 30 60 30 40 20 20 40 20 10 10 20 0 0 0 0 220 260 300 340 380 nm 220 260 300 340 380 nm 220 260 300 340 380 nm 220 260 300 340 380 nm mAU4-CBA mAUIA mAUBA mAU P-TOL

175 60 250 100 150 50 200 80 125 40 150 60 100 30 75 100 40 20 50 50 10 20 25 0 0 0 0 220 260 300 340 380 nm 220 260 300 340 380 nm 220 260 300 340 380 nm 220 260 300 340 380 nm

Figure 2. UV spectra of eight aromatic acids.

4 Reproducibility Sample analysis Six sequential 25 μg/mL injections of The sample was prepared according to standard solution were used to validate the procedure previously outlined, and injection reproducibility under the analyzed under the optimized conditions previously discussed optimized method (Table 1). Figure 3 shows that the (Table 1). Table 2 shows the retention impurities from the sample (small peaks time and peak RSDs of eight aromatic eluting between 7 and 10 minutes) do not acids separately. All RSD values were interfere with the p-TOL (8.05 minutes). less than 0.05 % for retention time and 0.18 % for peak area, indicating excellent reproducibility.

Table 2. RSD values of eight aromatic acids by six sequential injections.

Compounds TMLA HMBA OA TA 4-CBA IA BA p-TOL Retention time (RSD, %) 0.05 0.04 0.04 0.04 0.03 0.02 0.02 0.01 Peak area (RSD, %) 0.18 0.16 0.11 0.12 0.13 0.13 0.12 0.09 mAU Blank 150 100 50 0 Peak id 0 2 4 6 8 min 1. TMLA 8 2. HMBA 4 mAU 8.053 3. OA STDs 25 µg/mL 2 4.950 150 3.553 4. TA 6 7 5. 4-CBA 100 1 3 5 5.791 6.552 6. IA 2.842 4.000 5.240 50 7. BA 0 8. p-TOL 0 2 4 6 8 min 6.551 mAU 8.054 Sample 5.794 150 2.852 100 4.006 4.957 50 3.563 5.241 0 0 2 4 6 8 min

Figure 3. Chromatograms of blank, standard mix, and sample solution.

5 Conclusion Refereneces Aromatic acids are the major components 1. Purifi ed Terephthalic Acid for Industrial in PTA mother liquid, whose contents Use - Determination of p-Toluic Acid, could refl ect the oxidation process of 4-carboxybenzaldehyde – HPLC p-xylene directly. This Application Note Method(draft), GB/T xxxx 2013. shows a method to rapidly determine eight aromatic acids using an Agilent 2. Bang Li, Fang Han, “Simultaneous Poroshell SB-C18 column without Determination of Eight Compounds any sample interference. Additionally, in Residue from Purifi cation of aromatic acids, which are diffi cult to Terephthalic Acid by High Performance degrade, exist in the wastewater of PTA Liquid Chromatography”, Speciality manufacturers extensively. The level Petrochemicals, (5): 67-69, 2005. of these compounds can provide some 3. Chuang-Ji Liu, Zhen-Xia Du, useful data for wastewater treatment. “Determination of Aromatic Acids This method may be used for wastewater in Terephthalic Acid Wastewater monitoring in the PTA industry. by Ultra-performance Liquid Chromatography/ultraviolet Detection”, Journal of Bejing University of Chemical Technology (Natural Science), (39): 29-32, 2009.

4. Na Yuan, Jun-Qin Qiao, and Hong-Zhen Lian, “Simultaneous Determination of Nine Related Substances in p-Phthalic Acid Residue by RP-HPLC”, Journal of Chromatographic Science, (50): 410-413, 2012.

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