CALTEST CONTRIBUTION TO NOR-CAL SETAC
Written by Peter W. Halpin for the Summer Newsletter, 2005
TOXICITY CONCERNS
Pyrethroid pesticides are not new, but are being used more frequently as replacements for the organophosphorus pesticides Diazinon and Chlorpyrifos. Pyrethroids were expected to bring less inadvertent run-off toxicity than the pesticides they replaced, but newer evidence indicates that they cause toxicity in the sediment portion of the stream, and that dissolved levels may cause sub-lethal stresses to fish.
Pyrethroid pesticides are also gaining more attention as recent toxicity studies indicate much lower thresholds of toxicity than indicated in existing Water Quality Goals. Generally speaking the analysis of pyrethroid pesticides has been limited to reporting levels higher than the concentrations of interest for many analytes. More recently at least two non-commercial labs have provided reporting limits in the 0.01 to 0.05 ug/L range. Environmentally relevant reporting limits would ideally be in the single ng/L range, and 0.1 ng/L for permethrin and cypermethrin. Currently there are no regulatory approved methods.
The analytes of interest at this point seem to be represented by the following list:
Bifenthrin, Cyfluthrin, Cyhalothrin, Cypermethrin, Esfenvalerate/Fenvalerate, Fluvalinate, Fenpropathrin, Permethrin, Resmethrin, Tralomethrin/Deltamethrin.
ANALYTICAL TECHNIQUES
Three existing analytical techniques are in use, and discussed in the literature. They are HPLC (High Pressure Liquid Chromatography) by EPA Draft Method 1660, GC/ECD (Gas Chromatography with Electron Capture Detector), and GC/MS-SIM (Gas Chromatography with Mass Spectrometer operated in the Single Ion Monitoring mode of narrow range scanning).
EPA’s Draft Method 1660 lists reporting limits in the 2.5-5 ug/L range, with MDLs in the 1-2 ug/L range. This HPLC method is solely reliant on retention time for analyte identification. Confirmation is by retention time match on a dissimilar column. Many existing Water Quality Goals are in the 50 to 180 ug/L range making the HPLC method appear to be an appropriate choice for some pyrethroid monitoring . However, the Water Quality Goals are 0.002ug/L for Cypermethrin and 0.03 ug/L in freshwater and 0.001 ug/L in saltwater for Permethrin. For Water Quality Goals at these levels the reporting limits provided in the HPLC method are 1000 to 5000x too high. The most recent toxicology data indicates a need for reliable quantitation of pyrethroids in the low ng/L range in water, and sub ug/Kg for sediments.
Gas Chromatography with an Electron Capture Detector (GC/ECD) is the most commonly used method of analysis of pyrethroids because of the extreme sensitivity of the detector. Analyte identification is limited to retention time match and identification of multiple isomers for some analytes. Reporting limits for this method are 5-80 ng/L. This method has been used in government and university labs. GC/ECD confirmation of analyte identity is based on retention time match for the analyte on a dissimilar column. Where possible, results have been confirmed with mass spectrum analyses, but GCMS sensitivity has not been low enough to confirm at the same reporting levels as the ECD.
The GC/MS-SIM method used by Caltest is not single ion monitoring, but a narrow-range scan for three masses of each analyte. The analyte of concern is qualitatively identified by relative retention time and the ratio of the quantitation ion abundance to the qualifying ion abundance. The third ion is used for monitoring purposes, and only needs to be present. This is considered three-dimensional data while GC provides two-dimensional data. The three dimensional data provides more confidence in identification.
Reporting limits in water are 5-10 ng/L, and 0.33ug/Kg wet wt. in sediment. Known problems include the inability to separately identify or quantitate Esfenvalerate/Fenvalerate, and Deltamethrin/Tralomethrin. These compounds should be reported together as they are indistinguishable by the GC and GCMS methods. Caltest’s reporting levels are based on a 1-Liter sample with methylene chloride extraction concentrated to 1mL final volume in water, and 30g sediment extracted by cold-sonication using methylene chloride and acetone with a final volume extract at 2ml. Cold sonication is used due to the analytes being thermally unstable.
The importance of the GC/MS-narrow range scan method is reporting limits low enough to be relevant to toxicity ranges demonstrated in recent studies, and the ability to identify the analyte with confidence even in complex matrixes. Because the mass spectral analyses provides an additional level of confidence in analyte identification, GC/MS is preferred over GC/ECD when the reporting limits by GC/MS are adequate. Additionally, it can be used as a filter to in effect ‘remove’ interferences and identify a peak that is partially obscured by interferences, which can’t be done with conventional GC/ECD. Where the sensitivity of the mass spec meets data quality objectives, or at least meets the sensitivity of an alternative method, the mass spec is usually the better choice.
CLOSING THOUGHTS
Pyrethroids are of interest in many matrixes, clean water, dirty run-off, industrial wastewater, and sediments. The robust mass-filtering capability of the mass spec makes it ideal to handle the demands of multiple matrix effects.
Caltest’s Richard Heines contributed to this article. He directed the GCMS narrow range scan method development at Caltest. The approach Caltest has employed is based on the work of the USGS, Denver Water Quality Lab, and the California Dept. of Food and Agriculture.
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