Headspace Gas Chromatography for the Determination of 12 Volatile Aromatic Hydrocarbons in Soil

Volatile aromatic hydrocarbons in soil (such as 12 typical pollutants including benzene, toluene, xylene, and chlorobenzene) primarily originate from human activities such as industrial emissions, petroleum spills, and the use of organic solvents. These compounds exhibit notable toxicity, persistence, and bioaccumulation characteristics. They can be transmitted through the food chain, posing a direct threat to agricultural product safety and human health, with certain risks of carcinogenicity and teratogenicity.

This article references the standard method "Soil and Sediment—Determination of Volatile Aromatic Hydrocarbons—Headspace/Gas Chromatography" (HJ 742-2015) and utilizes the Wayeal’s gas chromatograph GC6100, equipped with an FID detector and a headspace autosampler, for the detection of volatile aromatic hydrocarbons in soil.

Keywords: Volatile Aromatic Hydrocarbons; Headspace; Gas Chromatography; FID Detector; Soil

1. Experiment Method

1.1 Instrument Configuration

Table 1 Configuration List of Gas Chromatography System

1.2 Experimental Materials and Auxiliary Equipment

Standard stock solution (1000μg/mL) of 12 volatile aromatic hydrocarbons: Certified reference material purchased externally, stored under light-proof, airtight conditions at temperatures below 4°C for refrigeration.

Methanol: Chromatographic grade.

Sodium chloride: Guaranteed reagent grade (baked at 400°C in a muffle furnace for 4 hours before use, cooled to room temperature in a desiccator, and then stored in a ground-glass bottle for later use).

Phosphoric acid: GR grade

Quartz sand: Analytical reagent grade, 20–50 mesh

Reciprocating shaker: Oscillation frequency 150 strokes/min;

Analytical balance.

Carrier gas: High-purity nitrogen;

Hydrogen generator;

Air generator;

Fully automated headspace sampler: Temperature control accuracy of ±1°C;

Headspace vials: Glass headspace vials (20mL).

1.3 Test Conditions

1.3.1 Reference Conditions for Headspace Sampler

Heating and equilibrium temperature: 85°C

Heating and equilibrium time: 50 min

Injection valve temperature: 100°C

Transfer line temperature: 110°C

Injection volume: 1.0 mL (sample loop).

1.3.2 Reference Conditions for Gas Chromatograph

Chromatographic column: Wax capillary column, 30 m × 0.32 mm × 0.5 μm.

Temperature program: Initial column temperature of 35°C held for 6 minutes; heated to 150°C at a rate of 5°C/min and held for 5 minutes; then heated to 200°C at 20°C/min and held for 5 minutes.

Column flow rate: 1 mL/min

Injection port temperature: 250°C

Detector temperature: 250°C

Air flow rate: 300mL/min

Hydrogen flow rate: 40 mL/min

Makeup flow rate: 10mL/min

Split injection: Split ratio 5:1

1.4 Solution Preparation

1.4.1 Saturated Sodium Chloride Solution

Measure 500mL of ultrapure water, adjust the pH to ≤2 using phosphoric acid, add 180g of sodium chloride, dissolve and mix thoroughly. Store in a light-protected environment at 4°C.

1.4.2 Linear Standard Working Solutions

Measure 25μL, 50μL, 100μL, 250μL, and 500μL of the volatile aromatic hydrocarbon standard stock solution (1000μg/mL) separately into 5mL volumetric flasks pre-filled with a small amount of methanol. Then dilute to volume with methanol to obtain standard solutions with concentrations of 5μg/mL, 10μg/mL, 20μg/mL, 50μg/mL, and 100μg/mL, respectively. Add 2g of quartz sand, 10mL of saturated sodium chloride solution, and 10μL of each of the aforementioned linear standard working solutions sequentially into five headspace vials. This preparation yields a five-point calibration curve series with target compound masses of 50ng, 100ng, 200ng, 500ng, and 1000ng, respectively.

2. Result and Discussion

2.1 Qualitative Analysis of Reference Standards

Fig 1 Blank Chromatogram

Fig 2 Chromatogram of Volatile Aromatic Hydrocarbons Standard Solution (500ng)

Table 2 Chromatographic Parameters of Volatile Aromatic Hydrocarbons Standard Solution (500ng)

Note: As shown in the chromatogram above, the resolution between the chromatographic peaks of all volatile aromatic hydrocarbon compounds exceeds 1.5, meeting the requirements for experimental analysis.

2.2 Linear

Fig 3 Standard Curves and Correlation Coefficients of Volatile Aromatic Hydrocarbons

Note: The standard working curve for the volatile aromatic hydrocarbons tested in this analysis was constructed at mass levels of 50ng, 100ng, 200ng, 500ng, and 1000ng. All components showed excellent linearity with correlation coefficients exceeding 0.999, meeting the requirements for experimental analysis.

2.3 Precision

Fig 4 Chromatogram of Volatile Aromatic Hydrocarbons Standard Solution (50ng)

Fig 5 Chromatogram of Volatile Aromatic Hydrocarbons Standard Solution (200ng)

Fig 6 Chromatogram of Volatile Aromatic Hydrocarbons Standard Solution (1000ng)

Table 3 Precision Chromatography Parameters

Note: Six replicate analyses were performed for volatile aromatic hydrocarbon samples at mass levels of 50ng, 200ng, and 1000ng. The relative standard deviations (RSDs) were in the ranges of 0.6–3.2%, 1.6–3.1%, and 1.4–3.2%, respectively. The relative deviations of the chromatographic peaks for all compounds complied with the standard requirements.

2.4 Limit Of Detection

Fig 7 Chromatogram of Limit of Detection Solution (50ng)

Table 4 Method LOD and Lower Limit of Determination for Each Component of Volatile Aromatic Hydrocarbons.

The volatile aromatic hydrocarbon standard solution (50ng) was repeatedly injected eight times. Based on the calculations, when the soil sample amount is 2g, the method detection limits for the 12 volatile aromatic hydrocarbons range from 0.7μg/kg to 3.0μg/kg, and the lower limits of determination range from 2.8μg/kg to 12.0μg/kg, all of which meet the standard requirements.

2.5 Sample Testing

Fig 8 Chromatogram of Soil Sample

Weigh 2g of the sample into a headspace vial, promptly add 10.0 mL of saturated sodium chloride solution, and seal immediately. Shake the vial on a reciprocating shaker at 150 strokes/min for 10 minutes. Analysis indicated that no benzene series compounds were detected in the soil sample.

2.6 Spiked Recovery Test

Fig 9 Chromatogram of Spiked Soil Sample (Spiking level: 100μg/kg).

Table 5-1 Soil Sample Spiked Recovery (Spiking level: 25μg/kg).

Table 5-2 Soil Sample Spiked Recovery (Spiking level: 100μg/kg)

Table 5-3 Soil Sample Spiked Recovery (Spiking level: 500μg/kg)

Analyses were conducted on soil samples spiked at levels of 25μg/kg, 100μg/kg, and 500μg/kg. The spiked recovery ranges for the 12 target compounds were 92.4%–99.8%, 87.7%–103.7%, and 94.9%–101.6%, respectively.

3. Conclusion

This method employed the Wayeal’s headspace gas chromatography GC6100, equipped with an FID detector and a headspace autosampler, for the detection of volatile aromatic hydrocarbons in soil. The experimental results indicate that the resolution between chromatography peaks of all volatile aromatic hydrocarbon compounds is greater than 1.5, meeting the requirements for experimental analysis. When the standard working curve mass ranged from 50 to 1000ng, all components in the standard solution exhibited excellent linearity with correlation coefficients exceeding 0.999, meeting the requirements for experimental analysis. The results of precision testing, method detection limits, lower limits of quantification, and spiked recovery experiments all comply with standard requirements. This method can be employed for the detection of volatile aromatic hydrocarbons in soil.