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

Volatile Organic Compounds (VOCs) are chemical substances that evaporate easily at room temperature, with boiling points below 200°C. These compounds often result from environmental pollution or water purification processes, such as disinfection. Due to their wide variety, high volatility, and low concentrations in water (often in the range of μg/L), direct analysis is challenging. Common methods for detecting VOCs in water include headspace gas chromatography, solid-phase microextraction, and purge and trap techniques combined with gas chromatography or mass spectrometry. Purge and trap is a widely used pre-treatment method because it requires small sample volumes, allows for high concentration, avoids solvent contamination, and is relatively simple. This technique can be used with advanced gas chromatography-mass spectrometry to perform both qualitative and quantitative analysis of up to 28 different VOCs.

With the rapid development of the economy, industrial wastewater, urban sewage, and agricultural activities have increasingly polluted natural water sources. Trace pollutants in water are difficult to remove during treatment and may react with disinfectants to form harmful by-products. To ensure the safety of drinking water, it is recommended that authorities strengthen monitoring, control environmental risks, improve disinfection technologies, and reduce health threats associated with contaminated water.

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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 Trap (United States, Tekmar), 5 ml Purge Tube (United States, Tekmar); Electronic Balance (Switzerland, METTLER, accurate to 0.1 mg); Milli-Q Ultra-pure Water System (United States, Millipore); Vials, 40 ml, baked at 180°C for at least 2 hours before use; Micro-syringe, 10 and 100 µl; Chromatographic-grade methanol; Milli-Q ultra-pure water; Standard material: 502/524 VOC Mix, containing 54 VOCs, concentration 2000 µg/ml (Supelco, United States); Internal standard: Fluorobenzene (Dr. Ehrenstorfer, Germany, 99.5%); Carrier gas: High-purity helium; Purge gas: High-purity nitrogen.

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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, 0 min hold, 25°C/min to 225°C, 3 min hold; 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.

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Standard Preparation and Analysis

Accurately weigh fluorobenzene internal standard, prepare a stock solution, dilute with methanol to 20–100 µg/ml. Take 2000 µg/ml VOC mixed standard, dilute to 10–100 µg/ml. Prepare standard series at 0, 0.150, 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 40 ml vial filled with ultrapure water, analyze using purge and trap-GC-MS. Standards should be freshly prepared daily.

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Sample Collection and Analysis

Bake 40 ml sample bottles, fill with water without air gaps, 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 to each sample, purge, adsorb, and analyze using GC-MS.

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Results and Discussion

2.1 Experimental Parameter Determination: Using the internal standard method, plot the peak area ratio of the standard quantification ion to the internal standard against concentration to create a calibration curve. As shown in Table 1, 28 VOCs showed good linearity (R² > 0.996) in the range of 0–50 µg/L. Six replicate samples at 20–100 µg/L had RSD between 2.12% and 13.0%. Detection limits were between 0.001 and 0.01 µg/L.

2.1.2 Optimization of Experimental Conditions: Compared to other columns like HP-VOC or DB-624, the DB-WAX column (20 m × 0.18 mm × 1.0 µm) provided better separation of 28 VOCs in 14 minutes. Although xylene and bromoform had similar retention times, their quantification ions differ, so no interference occurred. The mixed standard total ion chromatogram is shown in Figure 1.

2.1.2.2 Purge and Trap Conditions: The VOCARB®3000 trap was chosen for its high capture and release efficiency. Optimal conditions included a purge time of 11 min, purge flow of 40 ml/min, desorption temperature of 250°C, desorption flow of 300 ml/min, and desorption time of 2 min. These settings resulted in a recovery rate of 85.5%–108.6% for tap water samples.

2.1.3 Determination of Actual Samples: A city’s tap water was tested, and VOCs like chloroform, bromoform, and styrene were detected at concentrations between 0.003 and 0.040 mg/L. Spiked samples showed recoveries of 85.5%–108.6%, confirming the method’s reliability.

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Conclusion

This study established a rapid method for determining 28 VOCs in tap water using purge and trap-GC-MS. The method is simple, fast, and effective. A preliminary investigation into VOC levels in a city's tap water revealed increasing contamination due to industrial waste, urban sewage, and agricultural runoff. These contaminants not only persist during water treatment but also react with disinfectants to form harmful by-products. To ensure safe drinking water, stricter regulations, improved disinfection technologies, and enhanced environmental controls are essential to reduce health risks.