In pesticide monitoring, discrimination against matrix components arising from wastewater outfall (sewage) and sediment extracts is critical to the success of any method.
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Calibration and instrument detection limits were determined for both NCI and EI modes. The calibration standards were prepared in dichlo- romethane 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.
For matrix injections, sewage outfall and ocean sediments were pre- pared. 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 iso- mer 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.
a b
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Figure 1 – Ocean sediment extract (a) and sewage extract (b) used for matrix studies.
AMERICAN LABORATORY • 25 • MARCH 2015
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