Abstract:

A method for the determination of 25 organochlorine pesticides (OCPs) in the atmosphere using isotope dilution high-resolution gas chromatography/high-resolution mass spectrometry (ID-HRGC/HRMS) was developed. Sample extraction was performed using an accelerated solvent extractor (ASE). The extraction parameters were as follows: the extraction solvent was 50% (v/v) hexane in dichloromethane, the extraction temperature was 100 ℃, the static time was 8 min, the cell was rinsed with 60% cell volume using the aforementioned extraction solvent, the purging time was 180 s with N₂ gas, and the extraction proceeded through three cycles. The eluting solutions of common cartridges such as florisil, graphitized carbon black, alumina, and silica were determined via cartridge elution tests. Use of the aforementioned cartridges alone cannot remove the pigments in the air sample solution. Subsequently, all possible pairwise combinations of the four cartridges were used for sample cleaning, and only the combination of florisil and graphitized carbon black was found to completely remove the pigments. Thus, the combination of florisil and graphitized carbon black cartridges using 10 mL toluene for elution was determined as the final cleaning method in this study. A high-resolution mass spectrometer equipped with a gas chromatograph was used for quantification. A fused-silica capillary column (Rtx-CL Pesticides2, 30 m×0.25 mm×0.2 μm) was used to separate the target compounds. Injection was performed in the splitless mode at 250 ℃. The flow rate of nitrogen gas was maintained constant at 1 mL/min. The oven temperature was 110 ℃ (1 min), 20 ℃/min up to 210 ℃, 1.5 ℃/min up to 218 ℃ (1 min), and 2 ℃/min up to 260 ℃ (1 min). HRMS was conducted at >8000 resolution, the source temperature was 280 ℃ in the electron impact mode using ionization energy of 35 eV, and measurements were performed in the selective ion monitoring (SIM) mode. Twenty-five OCPs were identified by comparing their GC retention times with those of the corresponding labeled compounds, and the actual ion abundance ratios of two exact m/z values with the corresponding theoretical values. The 25 OCPs were quantified by average relative response factors (RRFs), and the relative standard deviations (RSDs) of the RRFs with six calibration solutions were no more than 20%. The linear range of this method was 0.4 to 800 μg/L, and the correlation coefficients (R²) were higher than 0.992. To validate the method, clean materials (one quartz fiber filter (QFF) and two polyurethane foam (PUF) plugs) were spiked with 100 pg, 400 pg, and 15 ng native OCP standards, respectively; the RSDs of the 25 OCPs for each spiked level ranged from 0.64% to 16%. The spiking recoveries of the native OCPs ranged from 67.2% to 135%. Penetration experiments were conducted by sampling various volumes of air (15-1000 m³) using a filter-PUF/PUF high-volume active sampler. The breakthrough volume was sampled when the amount of OCPs collected in the PUF of the non-sampling end reached 5% of the total amount collected by both PUFs. When a high-volume active sampler with filter-PUF/PUF was used as an adsorbent for sampling atmospheric OCPs, a serious breakthrough of pentachlorobenzene (PeCB) occurred. The effective sampling volume of hexachlorobenzene (HCB) was very low, and was no more than 30 m³ under the standard conditions (101.325 kPa, 273 K). The effective sampling volumes of other OCP compounds should be no more than 1200 m³. This will necessitate the use of high-adsorption-capacity adsorbents such as the PUF-XAD (a styrene-divinylbenzene copolymer) sandwich used for sampling air PeCB and HCB. Calculation with the effective sampling volumes from the penetration experiment revealed that the limits of detection of the 25 OCPs were in the range of 0.002 to 0.7 pg/m³. Thus, the detection levels of OCPs in this study were reduced to at least 2% of the current monitoring standards. Analysis of air samples in Beijing showed that all the target compounds except for trans-heptachlor epoxide, endrin, cis-nonachlor and 4,4'-DDD were 100% detected in the air samples. The concentrations of HCB (in volumes of 15-30 m³) ranged from 514 to 563 pg/m³, while those of the other OCPs (in a volume of 600 m³) ranged from 0.01 to 18.9 pg/m³. The recoveries of surrogate standards in this sample analysis were in the range of 33.9% to 155%, which satisfied the requirements of EPA Method 1699. Because of the very high detection limits, the current related monitoring standards cannot meet the requirements of atmospheric OCP analysis, especially at the ultra-trace level. In addition, highly sensitive monitoring standard methods are urgently needed. This method is suitable for analyzing most atmospheric OCPs, even at the ultra-trace level. It also lays the foundation for a new standard method formulation and provides strong support for the implementation of relevant international conventions.

Key words: isotope dilution (ID), high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS), organochlorine pesticides, ambient air