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┬ęDeployed POCIS

Chemical Analysis

Bioassay/Toxicity testing Fig. 8.4. General POCIS processing scheme.

Enrichment and Fractionation

Chemical Analysis

Bioassay/Toxicity testing Fig. 8.4. General POCIS processing scheme.

POCIS extract is ready for analysis using common analytical instrumentation or for bioassay or toxicity testing.

Any instrumental technique suitable for quantification of the targeted analyte can be used by the analyst. Gas chromatography (GC) with an electron capture, nitrogen-phosphorus, or flame ionization detectors, GC-mass spectrometry, liquid chromatography (LC) with diode array detection, and LC coupled with mass spectrometry have been used for the analysis of POCIS extracts. Also, POCIS extracts have been subjected to various bioassay techniques including Microtox®, the yeast estrogen screen (YES), and the yeast androgen screen (YAS) to determine the toxicological significance of sequestered chemicals. Blank POCIS used for quality control purposes have not shown any matrix effects interfering with these techniques.

8.4.2 Data quality consideration

The POCIS approach is well suited as a screening tool for determining the presence/absence, sources, and relative amounts of chemicals (ranking) at the study sites. For projects requiring estimates of ambient HpOC concentrations, calibration data are needed for targeted chemicals. At the current state of the technology, sampling rates are available for only a limited number of HpOCs and site conditions.

However, in cases where Rs data are available, the agreement between estimated water concentrations and measurements by traditional water sampling methods has been quite good. For example, in the River Thames (United Kingdom) concentrations of the herbicides diuron and isoproturon derived from POCIS data and measured with traditional methods varied by a maximum of 1.7-fold, with only one exception [23].

The application of appropriate quality control (QC) procedures or criteria is a mandatory consideration in the use of any passive sampler. QC samples must address issues of analyte recovery, background in sampler components, and any contamination incurred during transport, deployment, retrieval, storage, processing, enrichment/fractionation, and analysis. These issues are typically addressed by the following types of QC samples: fabrication blanks, process blanks, reagent blanks, field blanks, matrix spikes, and procedural spikes. Fabrication blanks are POCIS that are constructed concurrently with the deployed POCIS, but are stored frozen in an inert atmosphere until processing of the entire sample set begins. Fabrication blanks account for interferences and/or contamination incurred from the POCIS components, laboratory storage, processing, and analysis. Process blanks are POCIS prepared just prior to initiation of the processing of exposed samplers and these, along with fabrication blanks, account for the analytical background. Reagent blanks consist of identical portions of all solvents used in the extraction of the POCIS. These blank sample extracts are carried along with the POCIS samples extracts through the entire analytical procedure and provide information on background due to laboratory reagents and procedures. Field blanks are POCIS samples that account for contamination during transport to and from study sites, and during deployment and retrieval of exposed POCIS. Field blanks are stored frozen in vapor-tight containers between the deployment and retrieval periods. Matrix spikes are used to determine the recoveries of the target compounds from the POCIS sorbents and to establish ''control limits'' for the analytical process. Procedural spikes can be used to determine the recovery of target compounds for an individual procedural step, multiple steps, or for the whole analytical process. Often, radiolabeled compounds (14C- or 3H-labeled) are used as procedural spikes to provide a rapid indication of the performance of an individual step. Because as many as seven types of spikes and blanks are used for POCIS sampling and analysis, the amount of QC samples required often ranges between 20 and 50% of the total number of samples. The exact percentage of QC used is dependent on project goals, which should be determined during the developmental phase of a project.

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