Meet Regulatory Standards for Impurity Analysis in Aromatic Solvents

Application Note Energy and Chemicals Author Jie Zhang Agilent Technologies (Shanghai) Co. Ltd. Abstract This study presents the dual-channel Agilent 8860B gas chromatograph (GC) system for analyzing the impurities of monocyclic aromatic solvents in accordance with American Society for Testing and Materials (ASTM) D7504. Methodological validation confirmed that column resolution, sensitivity, and quantitation accuracy meet standard requirements. Mass concentration precision (RSDs) achieved is below 1% for most of the impurities. Both channels produced consistent results within the reproducibility limits of ASTM D7504. This application note demonstrates three methods that meet ASTM D7504 requirements using helium, nitrogen, and hydrogen carrier gas. Meanwhile, the normalization quantification based on compound's effective carbon number can be easily realized using Agilent OpenLab CDS software. The 8860B GC together with the OpenLab CDS system provides a complete and verified solution for aromatic impurity analysis, covering the whole workflow from sample preparation, separation, and detection to final report generation. Analysis of Trace Impurities in Monocyclic Aromatic Solvents Following ASTM D7504 Dual simultaneous analysis using the Agilent 8860B gas chromatograph 2 Implementing ASTM D7504 on a dual-channel GC/FID platform—where two independent analytical channels configured with identical injectors, columns, and detectors operate within a single GC oven—provides significant operational advantages: 1. Enhanced laboratory throughput: By performing two injections simultaneously, the test lab can double its sample capacity per instrument, effectively reducing the cost-per-analysis and maximizing hardware ROI. 2. Method flexibility and redundancy: A dual-channel set up allows a lab to run different sample types (such as toluene on the front channel and xylene on the back) in a single oven cycle. Furthermore, it provides an immediate "back up" channel to maintain uptime during routine maintenance. For the system to be considered reliable, both channels should meet ASTM D7504 sensitivity, resolution, and precision requirements. In addition, both channels should yield consistent quantitative results for the same sample. The reproducibility limit (R) demonstrated in ASTM D7504 interlaboratory testing will be used to evaluate the result consistency between the two channels. ASTM D7504 recommends helium and hydrogen as carrier gas. In China, major Chinese petrochemical companies often align their internal quality control standards directly with ASTM D7504 when testing for export of high-purity chemical intermediates. Because nitrogen is commonly used in China, they commonly adapt the ASTM D7504 method to use nitrogen carrier gas. This application note demonstrates how dual-channel analysis on the Agilent 8860B GC, following ASTM D7504, delivers consistent and high-precision data using three types of carrier gas. Introduction The production of high-purity monocyclic aromatic hydrocarbons, such as benzene, toluene, ethylbenzene, and xylenes (BTEX), is a cornerstone of the modern petrochemical industry. These compounds serve as essential feedstocks for the synthesis of plastics, resins, and textiles. Because even trace impurities can adversely affect downstream chemical reactions and product quality, rigorous analytical testing is required to ensure these intermediates meet strict commercial specifications. ASTM D7504 is the industry-standard test method for determining the purity of, and trace impurities in, monocyclic aromatic hydrocarbons using gas chromatography (GC) with flame ionization detector (FID).1 This method is favored for its streamlined workflow; it utilizes effective carbon number (ECN) correction factors for area normalization. This eliminates the need for labor-intensive, multipoint calibrations for every individual impurity, allowing laboratories to calculate purity and quantify trace nonaromatics and aromatics with high precision. The application scope of ASTM D7504 covers a wide array of aromatic liquids, including benzene, toluene, mixed xylenes, and styrene. To comply with the method's stringent performance requirements, the GC system must demonstrate: – High sensitivity: A limit of quantitation (LOQ) of at least 0.0006% by mass. – Good resolution: Maintaining a valley-to-peak ratio of no more than 50% for critical separations (such as p-xylene and m-xylene). – Strict precision: Highly reproducible peak areas to ensure accurate purity calculations. 3 Chemicals and reagents Twenty-three chemicals were purchased from ANPEL Laboratory Technologies (Shanghai) Inc. They are common impurities found in the monocyclic aromatic solvents and dissolved in n-hexane to ~ 0.1% mass concentration for identification of each impurity. The impurity mixture was used for analytical method optimization. For column resolution and method recovery evaluation, p-xylene check standard number one is purchased from AccuStandard Inc. Lab made check standards containing common contaminants for benzene, toluene, and p-xylene were prepared according to the sample composition in previous work for precision test.2 Benzene in p-xylene (0.0006% wt) was prepared to evaluate system sensitivity. Detailed sample compositions are shown in Table 3. Compound Benzene Check Standard (Mass %) Toluene Check Standard (Mass %) p-Xylene Check Standard (Mass %) p-Xylene Standard No. 1 (Mass %) LOD Test Standard (Mass %) n-C6 0.032 – 0.003 – n-C9 – – – 0.017 Benzene 99.91 0.016 0.003 0.02 0.0006 Toluene 0.009 99.92 0.034 0.02 1,4-Dioxane 0.002 – – – Ethylbenzene – 0.027 0.245 0.1 p-Xylene 0.005 0.013 99.23 99.62 m-Xylene 0.005 0.016 0.376 0.101 Cumene 0.002 – – 0.02 o-Xylene 0.004 0.0020 0.09 0.102 Propylbenzene 0.018 0.0030 – – Butylbenzene 0.009 – 0.009 – p-Diethylbenzene – – – 0.02 Table 3. Compositions of test standards. Experimental An Agilent 8860B GC was configured with dual split/splitless inlets and dual flame ionization detectors (FIDs). The sample introduction was made by dual Agilent 7693A automatic liquid samplers (ALS) with high density turrets (16-vial). The analytical parameters and consumables are summarized in Tables 1 and 2. Data acquisition and analysis were conducted using Agilent OpenLab CDS version 2.8. ALS and Inlets Injection Volume 0.6 µL Mode Split, 100:1 Temperature 260 °C Carrier Gas He, 2.1 mL/min H2 , 1.7 mL/min N2 , 1.2 mL/min Oven Temperature He Method H2 Method N2 Method Initial Temperature 60 °C 60 °C 60 °C Initial Hold 10 min 10 min 15 min Ramp 1 Rate 5 °C/min 5 °C/min 3.5 °C/min Ramp 1 Set Point 150 °C 150 °C 150 °C Ramp 1 Hold 0 min 0 min 0 min Ramp 2 Rate 30 °C/min 30 °C/min 30 °C/min Ramp 2 Set Point 220 °C 220 °C 220 °C Ramp 2 Hold 8 min 8 min 8 min Post Run Temperature 0 °C 0 °C 0 °C Post Run Time 0 min 0 min 0 min Detector Temperature 260 °C Air 400 mL/min Hydrogen 30 mL/min Make-Up (N2 ) 25 mL/min Table 1. The system analytical parameters using three carrier gas. Configuration Column Agilent J&W HP-INNOWax, 60 m × 0.32 mm, 0.5 µm (p/n 19091N-216I) Consumables Inlet Septa Agilent nonstick advanced green (p/n 5183-4759) Inlet liner* Agilent ultra inert, low pressure drop split liner with glass wool (p/n 5190-2295) ALS Syringes Agilent Blue Line autosampler syringe, 5 µL, fixed needle, 23-26s/42/cone (p/n G4513-80206) Column Ferrules Short graphite for 0.32 mm columns, 10/pack (p/n 5080-8853) * The deactivated split liner (p/n 5183-4647) is verified and also can be used for this application. Table 2. Column and consumables. 4 Results and discussion Method development for helium, nitrogen, and hydrogen carrier gas The helium- and hydrogen-based methods utilized the oven temperature program recommended by ASTM D7504. The helium column flow rate used in previous work was reused.3 The hydrogen column flow rate was optimized with 19 of the 23 typical impurities in monocyclic aromatic solvents separated at baseline level. The nitrogen method oven temperature program was developed based on the helium method using method translator. The average linear velocity of the helium method is 29 cm/sec. Theoretically, the optimum linear velocity for nitrogen is between 10 to 15 cm/sec. To balance the analytical speed and resolution, the nitrogen average linear velocity started from 20 cm/sec, with the oven ramp rate around 70% of the helium method as shown in Figure 1. To apply the nitrogen method to the 23-impurity mixture for separation check, minor adjustments were made on the initial temperature program, extending hold up time at 60 °C from 14.3 to 15 minutes. Additionally, the ramp rate was increased to 30 °C/min from 150 °C to 220 °C to accelerate the nontarget contaminant elution. The tweaked method was then applied to 23 impurity mixture analysis for separation check. Results showed 21 compounds can be resolved at baseline level using nitrogen carrier gas. Figure 1. The nitrogen method was translated from the helium method using the method translation tool. 5 Figure 2. (A) Chromatograms of the 23-impurity mixture on the front channel using helium, nitrogen, and hydrogen carrier gases. (B) Chromatograms of 23-impurity mixture on the back channel using helium, nitrogen, and hydrogen carrier gases. 2 4 6 8 Response (pA) Response (pA) Response (pA) 2 4 6 8 12345678 910111213141516171819202122232425262728293031323334353637 2 4 6 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14/15 1617 18 19 20 21 22 23 1 2 3 4 11 12 14/15 16 18 20 21 22 23 19 17 9 10 13 8 6 7 5 1 2 3 4 5 6 7 11 14/15 16 18 20 21 22 23 8 9 10 12 13 17 19 Front channel ×101 ×101 ×101 Response (pA) Response (pA) Response (pA) ×101 ×101 ×101 Retention time (min) 123456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Retention time (min) 2 4 6 8 2 4 6 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14/15 16 17 18 19 20 21 22 23 1 2 3 4 1112 14/15 16 18 20 21 22 19 23 17 13 10 9 8 6 7 5 1 2 3 4 5 6 7 11 14/15 16 18 20 21 22 23 8 9 10 12 13 17 19 Back channel 2 4 6 8 Helium Nitrogen Hydrogen Helium Nitrogen Hydrogen B A Chromatograms of the 23-impurity mixture on the two channels using helium, nitrogen, and hydrogen carrier gases are presented in Figure 2. Their retention time (RT) can be used for identification of impurities (RT information is shown in Table 4). 6 Peak No. Compounds Name RT-He-front (min) RT-He-back (min) RT-N2 -front (min) RT-N2 -back (min) RT-H2 -front (min) RT-H2 -back (min) 1 n-Nonane 6.541 6.318 9.403 9.116 6.222 6.038 2 Benzene 8.146 7.983 11.738 11.545 7.752 7.630 3 Decane 9.634 9.173 13.768 13.161 9.142 8.764 4 Toluene 12.273 11.992 17.756 17.392 11.776 11.543 5 1,4-dioxane 13.193 13.053 19.147 18.990 12.702 12.603 6 Undecane 14.226 13.678 20.554 19.799 13.723 13.233 7 Ethylbenzene 16.179 15.846 23.496 23.060 15.676 15.390 8 p-Xylene 16.519 16.179 23.989 23.542 16.013 15.721 9 m-Xylene 16.799 16.461 24.397 23.955 16.292 16.022 10 Cumene 18.118 17.760 26.315 25.842 17.604 17.293 11 Dodecane 18.482 17.934 26.779 26.030 17.97 17.478 12 o-Xylene 18.632 18.301 27.064 26.635 18.111 17.827 13 Propylbenzene 19.544 19.179 28.38 27.898 19.107 18.698 14 p-Ethyltoluene 20.093 19.724 29.169 28.685 19.56 19.237 15 m-Ethyltoluene 20.18 19.813 29.296 28.813 19.646 19.326 16 t-Butylbenzene 20.607 20.232 29.912 29.418 20.07 19.742 17 sec-Butylbenzene 21.052 20.664 30.549 30.038 20.508 20.167 18 Styrene 21.388 21.097 31.067 30.696 20.848 20.600 19 Tridecane 22.162 21.621 32.102 31.373 21.62 21.133 20 1,3-Diethylbenzene 22.895 22.506 33.205 32.693 22.333 21.989 21 n-Butylbenzene 23.301 22.912 33.788 33.277 22.731 22.389 22 α-Methylstyrene 23.922 23.598 34.701 34.283 23.353 23.069 23 Phenylacetylene 24.861 24.625 36.074 35.783 24.291 24.010 Table 4. Retention times of the 23 impurities on front and back channels using three carrier gases. 7 Column resolution evaluation p-Xylene check standard number one was diluted to the concentration of ~ 0.03% m-xylene using highly pure p-xylene and then separated under the three methods. The separation of m-xylene and p-xylene was used to evaluate the column resolution. ASTM D7504 requires that the distance from the valley to the baseline between two peaks should not be greater than 50% of the peak height of the lower peak. For the m-xylene impurity in p-xylene solvent, the ratio of m-xylene peak height versus valley-to-baseline height should be no less than 2:1. As shown in the chromatograms (Figure 3), the selected Agilent J&W HP-INNOWax column provides H/V ratio bigger than 5:1, exceeding the resolution criterion easily. System precision System precision following ASTM D7504 was evaluated using lab-made benzene, toluene, and p-xylene check standards. Ten consecutive dual-simultaneous injections of the check standards were made under each carrier gas. Representative chromatograms using helium carrier gas on two analytical channels are shown in Figure 4. The precision of peak response and the corresponding mass concentration on both analytical channels using helium carrier gas are great with area %RSD below 1.0% for all compounds except for 1,4-dioxane and cumene due to their lower concentrations. Among all impurities, 1,4-dioxane in benzene shows the worst response precision (2.9% ~ 5.2%) on both channels. Its mass concentration was ~ 0.0022%, meaning 0.11 ng 1,4-dioxane was loaded on column for detection. The ECN of 1,4-dioxane is 3.07 indicating its absolute response factor (response/mass) is less than one‑third of hydrocarbons and monocyclic aromatic solvents. The tiny amount and low response property determined its peak was small and the area precision was not as good as other compounds. Figure 3. Resolution of p-xylene and m-xylene on dual channels using three methods. 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 15.2 15.4 15.6 15.8 16.0 16.2 16.4 16.6 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 22.6 22.8 23.0 23.2 23.4 23.6 23.8 24.0 24.2 24.4 24.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.4 0.8 1.2 1.6 2.0 Response (pA) Response (pA) Response (pA) Response (pA) 15.6 15.8 16.0 16.2 16.4 16.6 16.8 17.0 0.4 0.8 1.2 1.6 2.0 He front He back H2 front H2 back N2 front N2 back p-Xylene p-Xylene p-Xylene p-Xylene p-Xylene m-Xylene m-Xylene m-Xylene m-Xylene m-Xylene p-Xylene ×101 ×101 ×101 ×101 Response (pA) Response (pA) ×101 ×101 Retention time (min) Retention time (min) Retention time (min) 8 Figure 4. (A) Benzene, toluene, and p-xylene check standards on front channel using helium carrier gas. (B) Benzene, toluene, and p-xylene check standards on back channel using helium carrier gas 0.5 0.6 0.7 0.8 0.9 1.0 Response (pA) Response (pA) Response (pA) Response (pA) Response (pA) Response (pA) 0.5 0.6 0.7 0.8 0.9 1.0 3456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 3 456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 0.5 0.6 0.7 0.8 0.9 1.0 1 2 3 6 8 9 7 6 10 11 4 2 2 1 3 3 5 9 10 5 6 7 9 11 7 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1 2 3 6 5 6 7 8 9 10 11 4 2 2 1 3 3 9 10 5 6 7 9 11 7 Benzene check sample Toluene check sample p-Xylene check sample Benzene check sample Toluene check sample Helium method (front channel) Helium method (back channel) p-Xylene check sample ×101 ×101 ×101 ×101 ×101 ×101 Retention time (min) 1. n-Hexane 2. Benzene 3. Toluene 4. 1,4-Dioxane 5. Ethylbenzene 6. p-Xylene 7. m-Xylene 8. Cumene 9. o-Xylene 10. Propylbenzene 11. Butylbenzene A B 9 The retention time precision of three check standards running under helium method is shown in Table 6. The RT %RSD on the front channel ranged from 0.003 to 0.015% and the RT %RSD on the back channel was from 0.003 to 0.05%. Benzene Check Standard Impurity Front Channel Back Channel Difference of Two Channels Reproducibility %Mass %Mass %RSD Area Area %RSD %Mass %Mass %RSD Area Area %RSD Limits(R) n-C6 0.031794 0.408 36.974 0.402 0.031695 0.355 34.135 0.544 –9.9E-05 Benzene 99.90796 0 127,747.027 0.454 99.913267 0 117,592.951 0.652 0.005307 0.0247 Toluene 0.008925 0.267 11.288 0.546 0.009076 0.346 10.631 0.632 0.000151 0.011 1,4-Dioxane 0.002144 3.349 0.81 3.559 0.00222 5.157 0.777 5.102 7.6E-05 p-Xylene 0.004996 0.26 6.266 0.608 0.005084 0.642 5.943 0.934 8.8E-05 m-Xylene 0.004998 0.313 6.27 0.645 0.005089 0.567 5.912 0.921 9.1E-05 Cumene 0.001592 1.62 1.985 1.73 0.001615 1.391 1.864 1.691 2.3E-05 o-Xylene 0.003677 0.512 4.613 0.737 0.003735 1.01 4.339 1.223 5.8E-05 n-Propylbenzene 0.018387 0.365 22.92 0.656 0.018692 0.47 21.579 0.907 0.000305 n-Butylbenzene 0.009153 0.36 11.353 0.658 0.009249 0.503 10.624 0.951 9.6E-05 Toluene Check Standard Impurity Front Channel Back Channel Difference of Two Channels Reproducibility %Mass %Mass %RSD Area Area %RSD %Mass %Mass %RSD Area Area %RSD Limits(R) Benzene 0.015738 0.131 18.12 0.436 0.015988 0.2 18.711 0.362 0.00025 0.0022 Toluene 99.924051 0 113979.546 0.397 99.923686 0 115961.321 0.361 –0.000365 0.0131 Ethylbenzene 0.026454 0.165 29.927 0.398 0.026501 0.136 30.424 0.374 4.7E-05 0.0029 p-Xylene 0.013029 0.22 14.727 0.39 0.01305 0.217 14.982 0.435 2.1E-05 0.0029 m-Xylene 0.01564 0.157 17.679 0.432 0.01567 0.246 17.99 0.41 3E-05 0.0037 o-Xylene 0.002061 0.67 2.33 0.8 0.002074 0.878 2.381 1.037 0.000013 0.002 n-Propylbenzene 0.003027 0.618 3.401 0.745 0.00303 0.844 3.457 0.976 3E-06 0.0028 p-Xylene Check Standard Impurity Front Channel Back Channel Difference of Two Channels Reproducibility %Mass %Mass %RSD Area Area %RSD %Mass %Mass %RSD Area Area %RSD Limits(R) n-C6 0.002626 0.697 2.993 0.817 0.002631 0.971 2.818 1.043 5E-06 0.0043 Benzene 0.002712 0.744 3.398 0.725 0.002729 0.453 3.214 0.542 1.7E-05 0.0013 Toluene 0.033612 0.128 41.658 0.428 0.033803 0.181 39.379 0.416 0.000191 0.0027 Ethyl benzene 0.245836 0.025 305.307 0.484 0.246874 0.026 285.238 0.326 0.001038 0.001 p-Xylene 99.229175 0.001 121973.152 0.482 99.22707 0 114646.657 0.342 –0.002105 0.0173 m-Xylene 0.376303 0.123 462.555 0.548 0.377201 0.12 435.817 0.377 0.000898 0.0165 o-Xylene 0.087991 0.042 108.159 0.505 0.088478 0.065 102.228 0.319 0.000487 0.0024 n-Butylbenzene 0.008862 0.182 10.771 0.575 0.008878 0.224 10.143 0.297 1.6E-05 0.0017 Table 5. Impurities response and quantitation precision for benzene, toluene, and p-xylene check standards using helium carrier gas. Nitrogen and hydrogen methods showed similar precision results as the helium methods. Chromatograms obtained using nitrogen and hydrogen carrier gas and the precision summary are shown in the Appendix. 10 Limit of detection and limit of quantitation The method sensitivity was evaluated by running ten consecutive injections of 0.0006% benzene on both channels under three carrier gases. The LOD (with SNR at 3:1) and LOQ (SNR at 10:1) were calculated using the average SNR of benzene (Table 7). The LODs and LOQs achieved are better than their corresponding requirements of 0.0002 and 0.0006% in ASTM D7504. The sensitivity on two test channels is very close to each other under the same method. The quantitation limit of the helium method is the best due to the sharper peaks (compared to the nitrogen method) and cleaner baseline (compared to the hydrogen method). Carrier Gas He Method N2 Method H2 Method Channels Front Back Front Back Front Back Average SNR 43:1 40.2:1 32.0:1 28.9:1 40:1 36:1 Average Area 0.838 0.818 0.722 0.725 0.758 0.717 Area %RSD 2.336 2.623 2.464 4.779 3.973 3.895 LOD (mass %) 0.000042 0.000045 0.000056 0.000062 0.000045 0.00005 LOQ (mass %) 0.00014 0.00015 0.00019 0.00021 0.00015 0.00017 Table 7. LODs and LOQs calculated based on the average signal to noise ratio of benzene. Results consistency of dual-simultaneous injection The quantification consistency of impurities in three lab-made check standards are compared between two channels. According to ASTM D7504, the dual-channel result consistency can be confirmed with a 95% probability of being correct if their difference is below the reproducibility (R) thresholds coming from ASTM D7504 ILS results. As shown in Table 5, the dual-channel quantitation difference of each probe impurity (column 10) is below the statistical R value (column 11) in the helium method except for the ethylbenzene in p-xylene. The ethylbenzene mass concentration in the lab-made p-xylene check standard is 0.245%, which has no corresponding R value in ASTM D7504 ILS research results. D7504 shows the R value for 0.0085% ethylbenzene impurity in p-xylene is 0.001%. The higher the impurity mass concentration, the bigger the R value. Thus, the R value for 0.245% ethylbenzene is supposed to be several times of 0.001%. This assumption can be confirmed from the R values of 0.002% for 0.0265% ethylbenzene in toluene as shown in ASTM D7504. The quantitation difference for 0.245% ethylbenzene between two channels is 0.0011%, much less than the R value for 0.0265% ethylbenzene, indicating ethylbenzene quantitation between two channels is also consistent. Benzene Check Standard Compounds Front Channel Back Channel RT RT RSD% RT RT RSD% n-C6 3.916 0.008 3.884 0.006 Benzene 8.256 0.011 8.099 0.011 Toluene 12.273 0.009 12.010 0.008 1,4-Dioxane 13.198 0.014 13.073 0.010 p-Xylene 16.515 0.005 16.191 0.005 m-Xylene 16.796 0.006 16.473 0.005 Cumene 18.114 0.005 17.769 0.004 o-Xylene 18.627 0.004 18.311 0.004 n-Propylbenzene 19.539 0.004 19.187 0.004 n-Butylbenzene 23.295 0.002 22.917 0.003 Toluene Check Standard Compounds Front Channel Back Channel RT RT RSD% RT RT RSD% Benzene 8.148 0.009 7.999 0.007 Toluene 12.505 0.010 12.240 0.010 Ethylbenzene 16.177 0.005 15.859 0.005 p-Xylene 16.517 0.004 16.190 0.004 m-Xylene 16.797 0.004 16.472 0.004 o-Xylene 18.628 0.003 18.309 0.005 n-Propylbenzene 19.539 0.003 19.185 0.004 p-Xylene Check Standard Compounds Front Channel Back Channel RT RT RSD% RT RT RSD% n-C6 3.917 0.010 3.884 0.012 Benzene 8.148 0.015 7.995 0.047 Toluene 12.274 0.009 12.005 0.050 Ethylbenzene 16.212 0.014 15.891 0.037 p-Xylene 16.800 0.012 16.461 0.032 m-Xylene 16.903 0.009 16.568 0.031 o-Xylene 18.638 0.006 18.315 0.026 n-Butylbenzene 23.294 0.005 22.911 0.017 Table 6. Retention time (RT) and RT %RSD of three check standards in the helium method. 11 The results consistency between two channels were also evaluated for the nitrogen and hydrogen methods with the same conclusion as shown in the Appendix Tables A1 and A2. The excellent quantitation consistency between the front and back analytical channels is the foundation of applying dual-channel simultaneous analysis mode to high throughput sample analysis or using one channel as a back up for another channel without calibration standards. Results accuracy ASTM does not recommend a reference material for determining the bias in this test method. Thus, a recovery test was made to evaluate the quantification accuracy using the described system. Eight impurities of accurate mass were added to the high pure p-xylene solvent, in which there is zero or very low level of impurities. Then the added impurities were measured. The recovery results based on the test values, ranging from 92 to 105%, were shown in Table 8 and Figure 5. The lowest recovery in all three methods compared to other impurities was m-xylene. It is due to the nonbaseline resolution of m-xylene impurity and p-xylene solvent. Seven other compounds demonstrated excellent and comparable recovery performance in three methods. The ppm level impurities were detected with satisfactory recovery indicating the 8860B GC system is a reliable platform for accurate quality control of aromatic solvents impurity. Compound Mass % Added *Mass % Measured Results Recovery (%) He N2 H2 He N2 H2 n-C9 0.0048 0.0048 0.0046 0.0045 98.9 96.2 92.9 Benzene 0.0048 0.0050 0.0048 0.0049 103.7 100.1 102.4 Toluene 0.0041 0.0042 0.0042 0.0042 103.9 102.9 103.1 Ethylbenzene 0.0233 0.0240 0.0240 0.0239 103.3 103.1 102.8 m-Xylene 0.0288 0.027 0.027 0.027 92.7 93.8 93.9 Cumene 0.0052 0.005 0.005 0.005 99.0 100.6 98.6 o-Xylene 0.0241 0.024 0.024 0.024 100.0 100.8 100.0 p-Diethylbenzene 0.0052 0.005 0.005 0.005 98.1 100.6 98.1 * Mass % measured is the average of dual channel results under each method. Table 8. Recovery of added impurities for p-xylene. Figure 5. Recovery rate plot of added impurities for p-xylene. 75 80 85 90 95 100 105 110 n-C9 Benzene Toluene Ethylbenzene m-Xylene Cumene o-Xylene p-Diethylbenzene Recovery (%) Helium method Nitrogen method Hydrogen method Quantification and reports by OpenLab CDS ASTM D7504 quantitates the sample impurity by normalizing the adjusted response of each peak according to its ECN. OpenLab CDS software can easily implement peak area adjustment, normalization, and report generation by processing raw data using the preset method. The key settings made in data-processing method include: – Enabling manual factor mode in the calibration table to use the ECN of each impurity for response adjustment. – Enabling normalization function in calibration type setting and select Apply correction factors to normalized concentrations to implement adjusted area-based normalization. – Selecting NormAmount template or self-defined template for normalization report generation on target signal. 12 With these method settings and linking the method to acquired data, a normalization report is generated upon the date processing is completed. A quantification report for benzene check standard in the front channel is shown in Figure 6 as an example. Figure 6. The quantification report for benzene check standard (front channel). Conclusion The Agilent 8860B gas chromatography system configured with two identical analytical channels are applied for impurity analysis in monocyclic aromatic solvents using three carrier gases. Each analytical channel was evaluated according to the ASTM D7504 method in terms of resolution, sensitivity, quantitation precision, and recovery with satisfactory results. The quantitation results between the two channels are consistent with each other. – Column resolution compounds pair demonstrated good separation with H/V no less than 5:1 for 300 ppm m-xylene and p-xylene solvent. – The LODs achieved for benzene on each analytical channel are less than 0.0001%, exceeding the 0.0002% LOD required by ASTM D7504. – The response and mass concentration precisions of most impurities in check standards are below 1%, meeting or exceeding the precision requirements in D7504. – Excellent recovery results, ranging from 93 to 105%, was achieved for added impurities, indicating good quantification accuracy based on the 8860B GC system. – The quantitation difference of all probe impurities between two channels is less than the reproducibility thresholds (R), demonstrating excellent across channel consistency. Besides great instrument performance, the Agilent OpenLab CDS software can realize ECN based normalization for easy and fast quantitation and report generation. The 8860B dual channel GC system coupled with the OpenLab CDS software provides an accurate and reliable analysis of impurities in monocyclic aromatic solvents with satisfactory precision. Lab productivity can be improved significantly compared to the single channel analytical system. References 1. ASTM D7504-23. Standard Test Method for Trace Impurities in Monocyclic Aromatic Hydrocarbons by Gas Chromatography and Effective Carbon Number. ASTM International, 2023. 2. Scott, H. The Analysis of Monocyclic Aromatic Hydrocarbons by ASTM D7504 on the Agilent 8850 GC System. Agilent Technologies application note, 5994‑7409EN, 2024. 3. Zhang, Y. A Unified Method for the Analysis of Monocyclic Aromatic Solvents Using the Agilent 8860 GC System and On-Board Data Processing. Agilent Technologies application note, 5994-1586EN, 2022. 13 Appendix Figure A1. Benzene, toluene, and p-xylene check standards on front channel using nitrogen carrier gas. Figure A2. Benzene, toluene, and p-xylene check standards on front channel using hydrogen carrier gas. 4 5 6 7 8 9 Response (pA) 4 5 6 7 8 9 Response (pA) 2468 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 4 5 6 7 8 9 Response (pA) 1 2 3 6 9 10 11 4 2 2 1 3 3 5 9 10 5 6 7 9 11 7 Benzene check sample Toluene check sample p-Xylene check sample p-Xylene check sample ×101 0.6 0.8 1.0 Response (pA) 2468 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 2468 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 345678 91011121314151617181920212223242526272829303132333435363738 345678 9101112 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 Retention time (min) Retention time (min) ×101 0.6 0.8 1.0 Response (pA) 345678 91011121314151617181920212223242526272829303132333435363738 ×101 0.6 0.8 1.0 Response (pA) 1 2 3 6 8 10 11 4 2 2 1 3 3 9 10 5 67 9 11 7 Benzene check sample Toluene check sample 1. n-Hexane 2. Benzene 3. Toluene 4. 1,4-Dioxane 5. Ethylbenzene 6. p-Xylene 7. m-Xylene 8. Cumene 9. o-Xylene 10.Propylbenzene 11. Butylbenzene 1. n-Hexane 2. Benzene 3. Toluene 4. 1,4-Dioxane 5. Ethylbenzene 6. p-Xylene 7. m-Xylene 8. Cumene 9. o-Xylene 10.Propylbenzene 11. Butylbenzene 8 6 7 9 5 6 7 14 Benzene Check Standard Impurity Front Channel Back Channel Difference of Two Channels Reproducibility Mass% Mass% RSD Area Area RSD% Mass% Mass% RSD Area Area RSD% Limits (R) n-C6 0.030945 0.657 33.124 0.683 0.031025 0.599 32.662 0.684 –8E-05 Benzene 99.913267 0 117,932.271 0.652 99.913796 0 115,654.795 0.699 –0.000529 0.0247 Toluene 0.009157 0.504 10.66 0.89 0.009046 0.414 10.357 0.877 0.000111 0.011 1,4-Dioxane 0.002134 4.828 0.742 5.076 0.002093 3.639 0.716 3.771 4.1E-05 p-Xylene 0.00518 0.476 5.981 0.95 0.005123 0.741 5.818 1.17 5.7E-05 m-Xylene 0.005188 0.791 5.991 1.336 0.005129 0.737 5.824 0.858 0.000059 Cumene 0.001633 0.844 1.874 1.208 0.001639 1.403 1.85 1.441 –6E-06 o-Xylene 0.0038 0.599 4.387 1.028 0.003772 0.759 4.283 1.111 2.8E-05 Propylbenzene 0.019166 0.556 21.992 0.95 0.018928 0.48 21.36 0.938 0.000238 Butylbenzene 0.009529 0.607 10.879 1.006 0.00945 0.724 10.611 1.092 7.9E-05 Toluene Check Standard Impurity Front Channel Back Channel Difference of Two Channels Reproducibility Mass% Mass% RSD Area Area RSD% Mass% Mass% RSD Area Area RSD% Limits (R) Benzene 0.015697 0.2 18.12 0.419 0.015303 0.16 17.36 0.501 3.94E-04 0.0022 Toluene 99.923097 0 114,391.735 0.471 99.922945 0 112,292.422 0.524 1.52E-04 0.0131 Ethylbenzene 0.026886 0.108 30.357 0.532 0.027108 0.151 30.172 0.556 –0.000222 0.0029 p-Xylene 0.013231 0.164 14.94 0.561 0.013321 0.243 14.827 0.504 –9E-05 0.0029 m-Xylene 0.015919 0.215 17.974 0.562 0.016052 0.197 17.866 0.42 –0.000133 0.0037 o-Xylene 0.002093 1.467 2.364 1.624 0.002121 1.626 2.361 1.884 –2.8E-05 0.002 n-Propylbenzene 0.003077 0.58 3.453 0.697 0.00315 0.858 3.485 1.155 –7.3E-05 0.0028 p-Xylene Check Standard Impurity Front Channel Back Channel Difference of Two Channels Reproducibility Mass% Mass% RSD Area Area RSD% Mass% Mass% RSD Area Area RSD% Limits (R) n-C6 0.002571 0.879 2.718 0.794 0.002583 0.88 2.856 1.241 –1.20E-05 0.0043 Benzene 0.002684 0.51 3.121 0.74 0.00269 0.937 3.272 1.493 –6.00E-06 0.0013 Toluene 0.033585 0.218 38.62 0.851 0.033417 0.122 40.195 0.751 0.000168 0.0027 Ethylbenzene 0.247285 0.025 282.044 0.774 0.246304 0.031 293.827 0.706 0.000981 0.001 p-Xylene 99.238055 0.002 113,187.456 0.791 99.234597 0 118,381.161 0.718 0.003458 0.0173 m-Xylene 0.376102 0.543 428.981 1.243 0.377082 0.101 449.835 0.642 –0.00098 0.0165 o-Xylene 0.088837 0.075 101.324 0.801 0.088327 0.053 105.368 0.682 0.00051 0.0024 n-Butylbenzene 0.00902 0.3 10.173 0.817 0.008986 0.24 10.6 0.913 3.40E-05 0.0017 Table A1. Impurities response precision in benzene, toluene, and p-xylene check standards using nitrogen carrier gas. www.agilent.com DE-015090 This information is subject to change without notice. © Agilent Technologies, Inc. 2026 Printed in the USA, July 3, 2026 5994-9316EN Benzene Check Standard Impurity Front Channel Back Channel Difference of Two Channels Reproducibility Mass% Mass% RSD Area Area RSD% Mass% Mass% RSD Area Area RSD% Limits (R) n-C6 0.02997 0.621 31.931 1.237 0.029843 0.565 30.028 0.628 0.000127 Benzene 99.904905 0.001 117,028.727 0.736 99.907062 0 110,530 0.359 –0.002157 0.0247 Toluene 0.009356 0.394 10.841 0.639 0.009212 0.395 10.081 0.48 0.000144 0.011 1,4-Dioxane 0.002477 2.937 0.847 2.989 0.002246 3.088 0.734 2.94 0.000231 p-Xylene 0.005357 0.69 6.156 0.614 0.005266 0.508 5.716 0.653 9.1E-05 m-Xylene 0.005377 0.617 6.179 0.559 0.005278 0.633 5.728 0.722 9.9E-05 Cumene 0.001722 1.144 1.967 1.295 0.001695 0.861 1.828 0.963 2.7E-05 o-Xylene 0.003959 0.629 4.55 0.679 0.00388 0.52 4.211 0.627 7.9E-05 Propylbenzene 0.019965 0.736 22.799 0.639 0.019564 0.554 21.101 0.668 0.000401 Butylbenzene 0.00996 0.946 11.317 0.96 0.009792 0.539 10.508 0.652 0.000168 Toluene Check Standard Impurity Front Channel Back Channel Difference of Two Channels Reproducibility Mass% Mass% RSD Area Area RSD% Mass% Mass% RSD Area Area RSD% Limits (R) Benzene 0.015375 0.74 17.831 1.405 0.015102 0.752 16.678 0.823 2.73E-04 0.0022 Toluene 99.922293 0 114,619.152 0.99 99.9226 0 1,09153 0.481 –3.07E-04 0.0131 Ethylbenzene 0.027282 0.357 31.038 1 0.027268 0.348 29.542 0.644 1.4E-05 0.0029 p-Xylene 0.01345 0.465 15.302 0.961 0.013394 0.336 14.512 0.582 5.6E-05 0.0029 m-Xylene 0.016169 0.341 18.395 1.026 0.016219 0.546 17.571 0.798 –5E-05 0.0037 o-Xylene 0.002212 2.805 2.516 3.015 0.002186 2.964 2.368 2.833 0.000026 0.002 n-Propylbenzene 0.003219 1.264 3.639 1.115 0.003231 3.479 3.401 1.215 –1.2E-05 0.0028 p-Xylene Check Standard Impurity Front Channel Back Channel Difference of Two Channels Reproducibility Mass% Mass% RSD Area Area RSD% Mass% Mass% RSD Area Area RSD% Limits (R) n-C6 0.002232 1.022 2.459 1.129 0.002407 0.489 2.398 1.076 –1.75E-04 0.0043 Benzene 0.002439 0.47 2.954 1.033 0.002565 0.538 2.81 1.195 –1.26E-04 0.0013 Toluene 0.032358 0.218 38.763 0.869 0.032849 0.16 35.597 0.972 –0.000491 0.0027 Ethylbenzene 0.248691 0.016 295.476 0.928 0.247325 0.071 265.823 1.039 0.001366 0.001 p-Xylene 99.225286 0.001 117,891.859 0.933 99.215055 0.006 1,06636 1.088 0.010231 0.0173 m-Xylene 0.378024 0.197 449.142 1.032 0.390048 1.702 419.274 2.615 –0.012024 0.0165 o-Xylene 0.090252 0.075 107.23 1.04 0.08917 0.063 95.839 1.04 0.001082 0.0024 n-Butylbenzene 0.009438 0.153 11.087 0.982 0.009155 0.268 9.73 0.915 2.83E-04 0.0017 Table A2. Impurities response precision in benzene, toluene, and p-xylene check standards using hydrogen carrier gas.