Reporting Limit goals keep changing with respect to pyrethroids, with a constant push to go lower. The Reporting Limit/ desired concentration of interest can be a water quality goal, objective, EC-50, LC-50 or other regulatory limit. The LC-50 represents the Lethal Concentration at which 50% of the test organisms die, and the EC-50 represents the Effect Concentrations at which 50% of the test organisms were paralyzed or otherwise impaired. The first list of Water Quality Goals and Water Quality Objectives listed in the State of California Central Valley Regional Water Quality Control Board’s (CVRWQCB) web site were based on older data and less sensitive species with respect to pyrethroids. As work was done with a very sensitive sediment test organism, Hyalella azteca, the sediment reporting limits were targeted at 1 μg/Kg dry weight, and waters were requested “as low as possible”. From about 2000 to 2004 ‘as low as possible’ was about 0.01-0.05 μg/L, from about 2004 to 2008 was around 0.005- 0.02 μg/L. As water toxicity tests were developed using the Hyalella azteca test species, the need for lower detection limits became more urgent. The Hyalella LC-50s and EC-50s were in the low parts per trillion (ranging from 2-20 ng/L). Reporting Limits should minimally be at or below the level of effect, and preferably 10x lower than the level of interest. This is difficult to achieve in matrix. From 2008 to 2010 the lowest reported matrix MDLs were in the 0.3-2ng/L range and higher for some compounds. Recent proposed water quality goals for the Central Valley Regional Water Quality Control Board include the following:
|Proposed Acute Limit ng/L
|Proposed Chronic Limit ng/L
These proposed Chronic limits would of course require even lower reporting limits if that were possible.
Sediment reporting limits for pyrethroids should be 1μg/kg dry weight or lower to be relevant to toxicity test data.
USGS, California Dept of Food and Agriculture, and Southern Illinois University hold time studies indicate hold times as short as 3 days to 13 days depending on the analyte. Such short hold times require coordination with the lab to extract the samples in time. Some studies have examined using hexane as a ‘keeper’ to be added at time of sampling to extend hold time in water. Southern Illinois’ study concluded 20ml of hexane could extend hold time in water to seven days. Spiked sediment samples have shown reproducible recoveries after several months frozen.
Esfenvalerate and Fenvalerate co-elute and share the same mass, and are therefore not distinguishable, and are reported by some including Caltest combined. Deltamethrin thermally degrades in the GC to Tralomethrin causing the two to be indistinguishable. They are reported together.
Approach to Existing Methods
Normalization of Results
Some methods may include the practice of normalizing data to reflect extraction efficiency, and or matrix spike recoveries for a common sample group. EPA style (Clean Water Act and SW-846 manual methods) do not normalize sample results according to extraction process recovery. Extraction surrogates are reported, but data is not normalized to that recovery. It is important to know if your data is corrected for recovery or not.
Total Analyte Reporting
Caltest calibrates and quantitates samples based on the sum of the analyte’s detectable isomers. Unless otherwise noted then, results are ‘total’.
Water samples can be analyzed with or without coarse sediments and solids. Some drinking water analyses for pyrethroids have included SPE (solid phase extraction) with pre-filtering. This may suit the needs of drinking water raw water surveys but has been considered inappropriate for environmental analyses. Because pyrethroids have a strong affinity for solids, most water analyses for pyrethroids is based on ‘whole water’, that is water with all sediments/suspended solids included. Alternatively total ‘whole water’ and ‘dissolved’ fractions can be collected. When whole water samples are analyzed by liquid-liquid extraction, results will include pyrethroids bound to the solids as well as any dissolved fraction present. Pyrethroids are easily lost to surfaces they come in contact with. Sample bottles shaken, and are solvent rinsed (with lids screwed on) to try to recover all analyte possible from the sample container walls. When Solid Phase Extraction is used for a sediment laden water sample, one approach is to separate the solids by filtration, and extract those trapped sediments, and separately extract the liquid with SPE. The extracts are then combined for a ‘total’ value.
Lack of Official Approved Methods
There are no regulatory approved methods for these analytes at the concentrations of interest. The EPA method 1660 referenced in 40 CFR part 136.3 Table 1G is a HPLC method that is written with Minimum Levels of reporting in interference-free waters at 2.5 -5 μg/L. Current project reporting limits in monitoring projects sponsored by the State of California are about 1,000x lower than this EPA method.
Caltest’s approach has been to run GC/MS with Selected Ion Monitoring (SIM) to identify the correct ions, in the correct ratio that corresponds to the target analyte at the specified GC retention time. Using Mass Spec is associated with greater confidence in analyte identification when sufficient sensitivity is available. Most environmental analyses using GCMS involve the use of electron impact ionization. A more recent approach to the analyses of pyrethroids by GCMS has been the use of negative chemical ionization. This technique has been available for more than 20 years, but has not been widely reported in the environmental studies literature. Negative Chemical Ionization (NCI) allows better selectivity and sensitivity of pyrethroids. Caltest uses this version of GCMS SIM to obtain Method Detection Limits in de-ionized water of 0.14 to 0.5 ng/L.
Instrument calibrations include more than 5 points. The Reporting Limit is supported by the low calibration standard and MDL. Second Source calibration check standards are employed. Batch QC matches EPA style of Method Blank, Laboratory Control Standard, Matrix Spike and Matrix Spike duplicate for each batch of samples, with a maximum batch size of 20 samples. Extraction surrogates are included, but results of surrogates are reported as is, results of extraction surrogates are not used to normalize the results.
Caltest runs the GC/MS method per the Robinson/Syngenta sediment method, modified as needed for water analyses. Clean ups of the sample extract are used to minimize matrix affects on sensitivity and quantitation.