2.1. Chemicals and reagents

1,4-Dioxane, 1,4-dioxane-d8, sodium chloride, acetone and methanol were purchased from Sigma- Aldrich (St. Louis, MO, USA). Water was purified by milli-Q equipment (Millipore Corp., Milford, MA, USA).

2.2. Water sampling and extraction procedure

Surface water samples were collected from 16 basins in the Gum River without headspace in 40 mL VOC bottles containing 1 drop of 2 M HCl. The sampling sites were selected to uniformly represent all streams of the River. All samples were collected in duplicate, and 5 mL water were placed in a 10 mL headspace vial containing 2 g NaCl. 50 μL of 1,4- dioxane-d8 solution (1.0 mg/L in milli-Q water) as an internal standard was added in the sample as soon as possible, and then the vial was sealed with a cap. The sample vial was placed in a heating block for 30 min at 90 ℃.

2.3. Gas chromatography-mass spectrometry

An Agilent 6890/5973 N instrument and a Combipal Autosampler (Agilent, USA), which allows automated HS injections, were used. All mass spectra were obtained with an Agilent 6891/5973N instrument. The ion source was operated in the electron ionization mode (EI; 70 eV, 230 ℃) and full-scan mass spectra (m/z 40-800) were recorded for analyte identification. An HP-InnoWax capillary column (40 m × 0.25 mm I.D. × 0.25 μm film thickness) was used. Samples were injected in the splitless mode. The flow rate of helium as a carrier gas was 1.0 mL/min. The injector temperature was set at 210 ℃. The oven temperature programs were as follows: an initial temperature of 35 ℃ (held for 3 min) increased to the final temperature of 220 ℃ at 8 ℃/min. Head space conditions of 1,4-dioxane are as described in Table 1. The ions selected by SIM were m/z 88, 58 and 57 for 1,4-dioxane and m/z 96, 64 and 62 for 1,4-dioxane-d8 (internal standard).

Table1.

Head space conditions of 1,4-dioxane

2.4. Calibration and quantification

Calibration curves for 1,4-dioxane were established by extraction after adding 0.5, 2.0, 5.0, 20, 50, 100 and 250 ng of the standard and 50 ng of the internal standard (1,4-dioxane-d8) to 5.0 mL of water. The ratios of the peak area of the standard to that of the internal standard were used in the quantification of the compounds.

3.1. Method validation

For the GC separation of 1,4-dioxane, the use of the semi-polar stationary phase was found to be efficient. Chromatograms are shown in Fig. 1. As can be seen from the figure, the peaks of 1,4-dioxane and the internal standard are symmetrical and separation of the analytes from the background compounds in the water samples was good. The retention time of 1,4-dioxane was 8.19 min. There were no extraneous peaks observed in a chromatogram of spiked water in the near of the retention time of 1,4-dioxane.

Fig.1.

GC-MS chromatogram from (a) a spiked ultra-pure water sample at 10 μg/L and a river water sample (1,4- dioxane was detected in concentration of 0.1 μg/L (b) and 1,4-dioxane-d8 was spiked at 10.0 μg/L (c)).

The method was validated for linearity, the limit of detection (LOD), the limit of quantification (LOQ), precision and accuracy.

Calibration curves of the 1,4-dioxane were constructed by extraction of the spiked water samples. Examination of the typical standard curve by computing a regression line of peak area ratios for the 1,4- dioxane to the internal standard on concentrations using a least-squares fit demonstrated a linear relationship with correlation coefficients of 0.9997. The line of best fit for 1,4-dioxane is y = 0.0952x + 0.0134 over a range of 0.1-50 μg/L, where x is the analyte concentration (μg/L) and y is the peak area ratio of the analyte to the internal standard.

We have defined and calculated as following: LOD and LOQ were defined by 3.14 and 10 times the standard deviation for replicate determinations (n=7) from samples spiked at the concentration of 0.01 μg/L in stream water, in which the analyte was not detected. The LOD in this study was 0.02 μg/L and the LOQ was 0.07 μg/L using a 5.0 mL sample.

The accuracy can be assessed by determining the recovery in spiked samples: Intra-day accuracy was evaluated using five spiked samples at concentrations of 10.0 and 50.0 μg/L for 1,4-dioxane. The inter-day accuracy was determined using sample recovery on three different days. The accuracy was in the range of approximately 96 to 99%, and the precision of the assay was less than 6%, as shown in Table 2.

Table2.

Intraday and interday laboratory precision and accuracy results for the analysis of 1,4-dioxane in water (n=5)

3.2. Water analysis

We analyzed the target compound in 44 surface water samples. 1,4-Dioxane was detected in a concentration range of 0.49-43.0 μg/L in 13 samples of the 44 surface water samples collected from Gum-River, Korea. In 1^st survey (in June, 2012), 1,4- dioxane was detected in a concentration range of 0.61-43.0 μg/L (mean=2.2 μg/L) in 3 samples of the 22 surface water samples. Otherwise, in 2^nd survey (in September, 2012), 1,4-dioxane was detected in a concentration range of 0.49-7.8 μg/L (mean=1.7 μg/ L) in 10 samples of the 22 surface water samples.