Analysis of 28 volatile organic compounds in tap water by gas chromatography-mass spectrometry

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Materials and Methods

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Instruments and Reagents

Agilent Technologies 7890A - 5975C Gas Chromatography-Mass Spectrometry (United States, Agilent); Teledyne Tekmar Velocity XPT Purge and Trap (United States, Tekmar); 5 ml Purge Tube (United States, Tekmar); Electronic Balance (Switzerland, METTLER, accurate to 0.0001 g); Milli-Q Ultra-pure Water System (United States, Millipore); Vials, 40 ml, baked at 180°C for at least 2 hours before use; Microsyringe, 10, 100 μl; Chromatographic-grade methanol; Milli-Q ultra-pure water; Standard Material: 502/524 VOC Mix, containing 54 VOCs, concentration 2000 μg/ml (United States, Supelco); Internal Standard: Fluorobenzene (Germany, Dr. Ehrenstorfer, 99.5%); Carrier gas: High-purity helium; Purge gas: High-purity nitrogen.

1.1.2 Measuring Conditions

1.2.1 Chromatographic Conditions: Column DB-WAX (20 m × 0.18 mm × 1.0 μm), pressure: 20 psi; column flow: 1.10 ml/min; column temperature: 40°C for 3 min, 10°C/min to 100°C, hold 0 min; 25°C/min to 225°C, hold 3 min; inlet temperature: 150°C; split ratio: 50:1.

1.2.2 Mass Spectrometry Interface Temperature: 280°C; Ion Source Temperature: 230°C; Quadrupole Temperature: 150°C; EM Voltage: 1400 V; Solvent Delay: 0 min.

1.2.3 Purge and Trap Conditions: Purge sample volume: 5–10 ml; purge time: 11 min; purge flow: 40 ml/min; desorption temperature: 250°C; desorption flow: 300 ml/min; desorption time: 2 min.

1.1.3 Standard Preparation and Analysis

Accurately weigh fluorobenzene internal standard, prepare a stock solution, dilute with methanol to 20–100 μg/ml. Use a 2000 μg/ml VOC mixed standard, dilute to 10–100 μg/ml. Prepare standard series at 0, 0.15, 1.0, 2.0, 5.0, 10.0, 20.0, 50.0, 100.0 μg/L. Add 5–10 μl of internal standard solution to each sample vial, analyze using purge and trap conditions. Standards should be freshly prepared daily.

1.1.4 Sample Collection and Analysis

Bake 40 ml sample bottles, fill completely with water, add 25 mg ascorbic acid, seal at 4°C, and analyze as soon as possible. Before analysis, add 5–10 μl of internal standard solution, then perform purge and trap, followed by GC-MS analysis.

2 Results and Discussion

2.1 Experimental Parameter Determination

To minimize interference, the internal standard method was used. A calibration curve was established by plotting the peak area ratio of the standard ion to the internal standard against concentration. As shown in Table 1, 28 VOCs exhibited good linearity in the range of 0–50 μg/L, with correlation coefficients above 0.996. Six replicate samples at 20–100 μg/L showed RSD between 2.12% and 13.0%. The detection limit for each component ranged from 0.001 to 0.01 μg/L.

2.1.2 Experimental Condition Optimization

Chromatographic columns like HP-VOC, HP-5, and DB-624 can be used for VOC analysis, but they result in longer analysis times. This study used a DB-WAX column (20 m × 0.18 mm × 1.0 μm), with an initial temperature of 40°C for 3 min, ramping at 10°C/min to 100°C, holding for 0 min, then 25°C/min to 225°C, holding for 3 min. With a split ratio of 50:1, this setup provided excellent separation of 28 VOCs in 14 minutes, offering high efficiency. Although the retention times of xylene and bromoform were similar, differences in their quantification ions ensured accurate identification.

2.1.2.2 Purge and Trap Conditions

The choice of trap material depends on its capture and release efficiency. Commonly used traps include Tenax, silica gel, molecular sieves, Carbopack B, Carboxen 1000, and VOCARB 3000. This study used VOCARB 3000, which demonstrated good performance. Based on references, a purge time of 7–11 min, purge flow of 40 ml/min, desorption temperature above 220°C, and desorption time of 2 min ensures complete VOC removal. Using these parameters, tap water samples achieved recovery rates of 85.5% to 108.6%, meeting analytical requirements. After desorption, the trap is baked at 300°C for 3 min to remove residual impurities and prevent interference.

2.1.3 Determination of Actual Samples

According to the urban drinking water hygiene monitoring network, 42 samples were collected from a city's tap water supply. Each sample received 5–10 μl of internal standard, followed by purge and trap-GC-MS analysis. VOCs such as chloroform, bromoform, dichloromethane, styrene, and others were detected at concentrations between 0.003 and 0.040 mg/L. Blank samples spiked with standards showed recovery rates of 85.5% to 108.6%, confirming the method’s reliability.

3 Conclusion

This paper establishes a rapid method for determining 28 VOCs in tap water using purge and trap coupled with gas chromatography-mass spectrometry. The method is simple, fast, and suitable for routine analysis. A preliminary study on VOC content in tap water revealed that increasing industrial and urban activity has worsened water pollution. Trace contaminants, if not properly managed, can react with disinfectants to form harmful byproducts. To ensure safe drinking water, it is crucial for relevant departments to strengthen regulation, control environmental risks, and improve water disinfection processes to protect public health.