Field fortified sample

Another important quality measure is the analysis of the field-fortified samples. The field-fortified samples have been handled, shipped and stored in the same manner as the treated samples. Any loss of analyte in the field-fortified samples will be used to correct residue levels of the field treated samples. It is best practice to keep the storage interval as short as possible to minimize losses. The acceptable storage time will vary according to the stability of each compound in aqueous solution. A good guideline is to analyze the samples within 30 days of sampling. 

A set of quality control parameters includes field blank, field fortified blank, md field fortified sample. 

Normally, the analysis of the field fortified samples should be performed even more frequently (e.g., 5%-10% of the samples). However, when the analysis is performed on biological samples, the volume available is generally limited especially in the case of blood, where from an ethical point of view, the use of a large amount must also be avoided. Therefore, the analysis of one field fortified sample every 10 samples can represent a good compromise. 

On the other hand, if this protocol is applied to the determination of water contamination and the volume of the sample available is not limited, it is more reliable to perform an analysis of field fortified samples on 5%-10% of the samples. 

After every 10 field samples, analyze a field fortified sample by adding an amount of target analyte equal to or higher than the real concentration of the sample selected for fortification. 

Field Fortified Sample The held sample with the addition of a known concentration of the compound(s) under investigation (in this protocol DCM, TCE, and PCE). Analysis of a fortihed sample assists in identifying chemical interferences inherent in the matrix and in assessing the accuracy of the method. 

Untreated (control) soil is collected to determine the presence of substances that may interfere with the measurement of target analytes. Control soil is also necessary for analytical recovery determinations made using laboratory-fortified samples. Thus, basic field study design divides the test area into one or more treated plots and an untreated control plot. Unlike the treated plots, the untreated control is typically not replicated but must be sufficiently large to provide soil for characterization, analytical method validation, and quality control. To prevent spray drift on to the control area and other potential forms of contamination, the control area is positioned > 15 m away and upwind of the treated plot, relative to prevailing wind patterns. 

To fortify a sample, the label from a fortification sampling vial was removed and secured to the pre-labeled sample jar. Spike vials were individually shaken before use. The cap was discarded, the contents of the vial were poured into the sample jar and then the vial was dropped into the sample. The sample jar was capped with a Tefion-lined lid, hand shaken to mix, placed in a Kapak bag and sealed. Jars were placed immediately in storage freezers. In all cases, quality control samples were transported and stored with their corresponding field plot samples throughout sample handling and shipment to the analytical facility. 

Such field-fortified matrix samples are absolutely necessary as a part of any worker exposure or re-entry study in order for the behavior of the active ingredient to be... 

Field-fortified air tubes or filters attached to air sampling pumps should not be placed on tables having other spiked matrices since volatilization of the active ingredient from the other matrices may lead to contamination of the spiked air samples. 

Control urine should be collected from individuals who have no apparent past history of exposure to the active ingredient. This control urine must be stored frozen until used for field fortification purposes. The urine is then thawed, shaken well, and a certain amount should be aliquoted into a small jar/bottle to use for field fortification. The active ingredient is then added to the urine using a 1-mL volumetric pipet, the solution is shaken well, and the sample is immediately frozen. Occasionally, the fortified sample can be left at room temperature or at some lower temperature in a liquid state to simulate field storage during collection of the urine sample. After leaving the sample at such temperatures for the prescribed length of time, the sample is immediately stored frozen. 

Some more recent field techniques have focused on the location of the preparation of field fortification samples and have taken some of the responsibility for the preparation of the field fortification samples from the field personnel and placed them with the analytical laboratory. For example, it is becoming more common for the analytical laboratory to prepare air sample field fortifications in the analytical laboratory, freeze them, and ship them to the field for use in a frozen state. Whereas there may be some advantage to this technique in that the air tube fortification samples may possibly be fortified more accurately in the laboratory under controlled conditions than if done in the field, there are some inherent scientific problems with this method. First, one reason for the field fortification is to test the ruggedness of the field techniques of the researcher under extreme field conditions. Second, the act of freezing and thawing the sorbent matrix within the air mbe itself may have an impact on the recovery of the analyte from the air tube after exposing the sorbent to field conditions... 

Field forms can be used to document the fortification of the matrices during the field-fortifying phase of the study. The matrix fortified, the sample number, the identification and the amount of fortification solution used, the time fortified, and the time that the matrices were removed from the field and stored should be... 

Field control and field-fortified urine samples were prepared with urine from individuals with no known exposure to chlorpyrifos. Some control urine samples were left unfortified (controls), and some control urine samples were spiked with a "low" amount of 3,5,6-TCP, shaken, and stored on dry ice until analysis. Some control samples were also fortified with a "high" amount of 3,5,6-TCP. 

Grant et al. [30] found that nitramine and nitroaromatic explosive residues in real field soil samples were stable under refrigeration, but nitroaromatics used to fortify samples degraded rapidly, even when samples were refrigerated. Therefore, fortified soils can lead to significant errors. 

Data are assigned to one of five classes, A through E, based on the results of experiments to determine the recovery of the analyte from fortified samples. Data from the three kinds of recovery experiments mentioned previously (laboratory recovery, storage stability and field recovery) are used to classify the exposure data. This is described in Table 5.1 (PHED, 1992). 

Field recovery Data generated to determine the loss of analyte from sample-collection devices fortified in the field, when subjected to the same environmental conditions (e.g. temperature, light, relative humidity, wind, etc.) and duration as field-exposure samples (OECD, 1997 Norman, Ch. 10). 

JC-radiolabclled crop samples as well as fortified samples of each commodity prior to its use in determining field residue levels. 

The actual mass chromatograms and spectra are received seven days later at which time they are also reviewed. Any discrepancies between the two sets of data are immediately rectified. Data are qualified based on adherence to the method criteria plus the results of the field blank, fortified sample, and performance evaluation sample. Data are qualified as either valid or invalid for remedial site work. 

If the fortified sample samples continue to fall outside the control range, despite the positive results obtained after the field fortified blank duplicate analysis, then the results related to the unfortified sample under investigation should be labeled as suspect due to matrix effect. 

As with any residue method, a method used in an LSMBS method should include analysis of control commodities to demonstrate adequate selectivity and analysis of fortified control samples to demonstrate recovery. These aspects present a particular challenge in every food-based market basket survey, because, unlike field residue studies, control samples of known provenance are not available. 

Fresh oxamyl standards were prepared for each fortification event. Concentrations of 50 and 400 qg mL analytical-grade oxamyl were prepared in a 20% acetonitrile-80% FIPLC-grade water solution. The solutions were tranferred in 1-mL aliquots into uniquely identified vials so that each vial contained the correct volume of oxamyl standard to fortify one quality control sample. The vials were shipped as needed during the course of the study to each field site. 

A reference substance can be either the formulated test substance suspended in water or the technical or analytical grade active ingredient of the test substance dissolved in a solvent. The reference substance is normally used to fortify field matrices to develop information on the field storage stability of the active ingredient. Reference substances should be prepared at the analytical facility where the matrix samples are to be analyzed. Methods to prepare reference substances for field use will be discussed later. 

Travel fortification samples are a type of field fortification that is usually prepared in the fleld to allow the investigator to determine the stability of the active ingredient on matrices without weathering. Such matrices are fortified and placed immediately in frozen storage. Usually, one set of travel fortification samples for each matrix is prepared for each five sets of weathered fleld fortification samples. The samples are then stored and shipped using the same procedures as all other samples prepared in the fleld. 

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