Detecting Trace-Level Pyrethroid Insecticides in Sewage and Ocean Sediments With GC-MS/MS

As the use of pyrethroid insecticides continues to increase in the United States, it is becoming increasingly important to monitor levels of the compounds in environmental waters, river and ocean sediment and wastewater effluent. Pyrethroids are lethal to mayflies, gadflies and other invertebrates at extremely low levels, and are toxic to fish and other aquatic organisms. The insecticides have been found to be unaffected by secondary treatment systems at municipal wastewater treatment facilities, appearing in the effluent. Effective pyrethroid detection techniques are vital for environmental monitoring and for understanding the impact of the compounds in the environment, even at lower nonlethal levels.

Any method for pyrethroid detection must be able to discriminate effectively against complex matrices and provide trace-level detection. This article details the use of GC-MS/ MS to screen a group of pyrethroid insecticides at low concentrations in complex matrices of sewage outfall and ocean sediments. Electron and negative chemical ionization techniques are also compared to examine which delivers the best results in these complex matrices.

Introduction

Pyrethroids are synthetic insecticides based on the structure of pyrethrins, which are derived from chrysanthemum flowers. The compounds are used in a range of household insecticides and repellents because they are stable, toxic and effective against a wide range of pests. Pyrethroids work by altering nerve function, which causes paralysis in target insect pests, eventually resulting in death.1 Pyrethroids that have entered the environment through agricultural and domestic use have proved to be lethal to many invertebrates and toxic to fish and other aquatic organisms.

The stability and highly hydrophobic nature of pyrethroids means they remain in sediment and have been identified in influent, effluent and biosolids from sewage treatment plants in Europe and the United States. Pyrethroids have also been found in secondary-treated municipal wastewater in California at concentration levels above the LC50 (lethal concentration) for the test system organism, Hyalella azteca.2 To address this, the California Department of Food and Agriculture has implemented monitoring programs requiring low to sub-parts-per-billion reporting limits in soil and water samples.

Since the use of pyrethroids continues to increase in both urban and agricultural settings, it is important to have robust, sensitive methods that are capable of measuring these compounds at the limits required in both water and sediment. Such methods will also help scientists understand pyrethroid behavior in the environment.3

In pesticide monitoring, discrimination against matrix components arising from wastewater outfall (sewage) and sediment extracts is critical to the success of any method. Gas chromatography-coupled triple quadrupole tandem mass spectrometry (GC-MS/MS) is an ideal technique to analyze pyrethroids because it can discriminate effectively against matrices and provide trace-level detection. Recent advances in GC-MS/MS systems, such as the axial ion source and lens-free design of the Bruker SCION TQ (Bruker Daltonics, Billerica, Mass.), result in robust operation and high sensitivity for the pyrethroid insecticides.

Case study

To determine the best method of analysis, a group of pyrethroids was analyzed using GC-MS/MS. Electron ionization (EI) and negative chemical ionization (NCI) techniques were optimized and compared in terms of calibration range, method detection limits and precision. The SCION TQ triple quadrupole mass spectrometer coupled to a Bruker 456 GC and CP-8400 liquid autosampler (Agilent Technologies, Santa Clara, Calif.) was used with a standard hot splitless injection, which reduced instrument matrix load compared to often-used large-volume programmed temperature vaporization (PTV). The instrument parameters are shown in Table 1.

Table 1 – Instrument parameters for pesticide quantification using SCION TQ GC-MS/MS coupled to a 456 gas chromatograph

Calibration and instrument detection limits were determined for both NCI and EI modes. The calibration standards were prepared in dichloromethane with a concentration range of 0.1–50 ppb. For electron ionization, the average relative standard deviation (%RSD) and r2 values for all compounds were 8.7% and 0.9995, respectively. For negative chemical ionization, the average %RSD was 9.8% and r2 was 0.9990. The added sensitivity observed for NCI allowed several compounds to have an extended lower calibration range, from 1 ppt to 5000 ppt. Instrument detection limits were performed by analyzing seven replicates at 0.1 ppb. Analysis with NCI demonstrated excellent sensitivity.

Figure 1 – Ocean sediment extract (a) and sewage extract (b) used for matrix studies.

For matrix injections, sewage outfall and ocean sediments were prepared. A microwave digestion technique was used on the samples and resulted in highly colored extracts (Figure 1). Severe matrix interference was noted for several compounds in the EI method and resulted in very highly biased results.

As shown in Figure 2b, there is interference in the EI/MS/MS analysis of the ocean sediment samples for allethrin/pallethrin, cypermethrin isomer and deltamethrin at the transitions studied. Elevated baselines and distorted peaks hinder quantitation, even though the ion ratios are in range. The NCI/MS/MS analyses for the same sediment extract shown in Figure 2a demonstrate comparable results to the pure standard and give acceptable percent recoveries.

Figure 2 – Comparison of NCI (a) and EI (b) extracted ion chromatograms for (top to bottom) allethrin/pallethrin, cypermethrin isomers and deltamethrin.

The sewage samples exhibited similar interferences in EI/MS/MS but to a lesser extent. For example, deltamethrin-spiked sewage showed some elevation in baseline but gave a similar response to the calibration standard.

Conclusion

The increasing use of pyrethroids has highlighted the importance of environmental monitoring, particularly since these compounds have been found in even secondary-treated municipal wastewater and biosolids. Due to the toxic nature of pyrethroids and resulting environmental impact, low reporting limits in water and soil samples is required. However, this demands robust and sensitive instruments and methods. Advanced GC-MS/MS techniques with multiple reaction monitoring provide a platform for low-level pesticide analysis by delivering good specificity and selectivity in complex matrices. It has also proved important to evaluate the ionization technique alongside the matrix to deliver enhanced performance. In this case, NCI combined with MS/MS clearly provides the best results for most pyrethroids in the matrices studied.

References

  1. http://epa.gov/oppsrrd1/reevaluation/pyrethroids-pyrethrins.htmlhttp://www.waterboards.ca.gov/water_issues/programs/swamp/ docs/cwt/guidance/3722.pdf (use in 2004 double that of in 1992).
  2. Markle, J.C.; van Buuren, B.H. et al. Pyrethroid pesticides in municipal wastewater: a baseline survey of publicly owned treatment works facilities in California in 2013; http://www.tritac.org/documents/Pyrethroid-pesticides-in-municipal-wastewater_012214.pdf.
  3. Hladkik, M.L.; Smalling, K.L. et al. Methods of analysis—determination of pyrethroid insecticides in water and sediment using gas chromatography/mass spectrometry; http://pubs.usgs.gov/tm/tm5c2/ tm5c2.pdf.

Joe Anacleto is vice president, Applied Markets, Bruker Daltonics, 40 Manning Rd., Manning Park, Billerica, Mass. 01821, U.S.A.; tel.: 905-691-2724; e-mail: [email protected]; www.bruker.com

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