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Scientific Investigations Report 2009–5189

National Water-Quality Assessment Program

Adjustment of Pesticide Concentrations for Temporal Changes in Analytical Recovery, 1992–2006

By Jeffrey D. Martin, Wesley W. Stone, Duane S. Wydoski, and Mark W. Sandstrom

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ABSTRACT

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Recovery is the proportion of a target analyte that is quantified by an analytical method and is a primary indicator of the analytical bias of a measurement. Recovery is measured by analysis of quality-control (QC) water samples that have known amounts of target analytes added (“spiked” QC samples). For pesticides, recovery is the measured amount of pesticide in the spiked QC sample expressed as percentage of the amount spiked, ideally 100 percent. Temporal changes in recovery have the potential to adversely affect time-trend analysis of pesticide concentrations by introducing trends in environmental concentrations that are caused by trends in performance of the analytical method rather than by trends in pesticide use or other environmental conditions.

This report examines temporal changes in the recovery of 44 pesticides and 8 pesticide degradates (hereafter referred to as “pesticides”) that were selected for a national analysis of time trends in pesticide concentrations in streams. Water samples were analyzed for these pesticides from 1992 to 2006 by gas chromatography/mass spectrometry. Recovery was measured by analysis of pesticide-spiked QC water samples. Temporal changes in pesticide recovery were investigated by calculating robust, locally weighted scatterplot smooths (lowess smooths) for the time series of pesticide recoveries in 5,132 laboratory reagent spikes; 1,234 stream-water matrix spikes; and 863 groundwater matrix spikes. A 10-percent smoothing window was selected to show broad, 6- to 12-month time scale changes in recovery for most of the 52 pesticides.

Temporal patterns in recovery were similar (in phase) for laboratory reagent spikes and for matrix spikes for most pesticides. In-phase temporal changes among spike types support the hypothesis that temporal change in method performance is the primary cause of temporal change in recovery. Although temporal patterns of recovery were in phase for most pesticides, recovery in matrix spikes was greater than recovery in reagent spikes for nearly every pesticide. Models of recovery based on matrix spikes are deemed more appropriate for adjusting concentrations of pesticides measured in groundwater and stream-water samples than models based on laboratory reagent spikes because (1) matrix spikes are expected to more closely match the matrix of environmental water samples than are reagent spikes and (2) method performance is often matrix dependent, as was shown by higher recovery in matrix spikes for most of the pesticides.

Models of recovery, based on lowess smooths of matrix spikes, were developed separately for groundwater and stream-water samples. The models of recovery can be used to adjust concentrations of pesticides measured in groundwater or stream-water samples to 100 percent recovery to compensate for temporal changes in the performance (bias) of the analytical method.


Suggested citation:

Martin, J.D., Stone, W.W, Wydoski, D.S., and Sandstrom, M.W., 2009, Adjustment of pesticide concentrations for temporal changes in analytical recovery, 1992–2006: U.S. Geological Survey Scientific Investigations Report 2009–5189, 23 p. plus appendixes.


Contents

Foreword

Abstract

Introduction

Purpose and Scope

Analytical Method for Pesticides

Measurement of Recovery

Spike Solutions

Laboratory Reagent Spikes

Matrix Spikes

Sources, Preparation, and Review of Recovery Data

Temporal Changes in Recovery

Robust, Locally Weighted Scatterplot Smooths (lowess smooths)

Comparison of Reagent Spikes and Matrix Spikes

Magnitude of Temporal Changes in Recovery in Matrix Spikes

Comparison of Groundwater and Stream-Water Matrix Spikes

Model Performance

Limitations of Modeled Recovery

Future Investigations

Summary

Acknowledgments

References Cited

Appendixes

First posted September 28, 2009

For additional information:
Jeff Martin
Pesticide National Synthesis Project
National Water-Quality Assessment Program
U.S. Geological Survey
jdmartin@usgs.gov

See also the Pesticide National Synthesis Web page:

http://water.usgs.gov/nawqa/pnsp/

This report is updated and supplemented by USGS Data Series 630: http://pubs.usgs.gov/ds/630/

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.



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