Field fortifications

Field recovery samples are an important part of the quality control in DFR studies. Field fortifications allow the experimental data to be corrected for losses at all phases of the study from collection through sample transport and storage. Fresh laboratory fortifications monitor losses due to the analytical phase. This section details how the field recovery process was handled in the oxamyl tomato DFR study. 

Field fortification of worker exposure matrix samples (a) Purpose for having field fortification samples... 

Field fortification (commonly referred to as field spiking) is the procedure used to prepare study sample matrices to which have been added a known amount of the active ingredient of the test product. The purpose for having field fortification samples available in a worker exposure study is to provide some idea of what happens to the test chemical under the exact environmental field conditions which the worker experiences and to determine the field storage stability of the test substance on or in the field matrix materials. Field fortifications do not serve the purpose of making precise decisions about the chemical, which can better be tested in a controlled laboratory environment. The researcher should not assume that a field fortification sample by its nature provides 100% recovery of the active ingredient at all times. For example, a field fortification sample by its very nature may be prone to cross-contamination of the sample from environmental contaminants expected or not expected to be present at the field site. 

In addition, the use of field fortification samples measures the carefulness factor of the Field Scientist during the field research and allows a Study Director/Manager or distant observer to obtain a quality control estimate on the field portion of the study. For this reason, the field fortification samples are usually meant to be different from laboratory procedural fortifications and are meant to be prepared under field conditions, which are considerably more rigorous than are controlled laboratory conditions. For example, environmental factors such as heat, humidity, wind, human stress, and other human factors such as fatigue to the Field Scientist are an integral part of any field worker exposure/re-entry study. Field fortifications made to matrices under these conditions will test and readily demonstrate the ability of the Field Scientist to perform such a difficult study under trying circumstances. 

Field fortifications have also been used to measure the storage stability of the analyte in/on exposure matrices during freezer storage prior to analysis. Although use of field fortification samples for freezer storage stability is not the original purpose intended for field fortification samples, this has become an acceptable practice among scientists who work in this scientific discipline. 

After initial processing, the entire whole-body dosimeter can be used for field fortification and treated as one sample or can be cut into pieces as described above. Each... 

Another consideration when planning field fortification levels for the matrices is the lowest level for fortification. The low-level fortification samples should be set high enough above the limit of quantitation (LOQ) of the analyte so as to ensure that inadvertent field contamination does not add to and does not drive up the field recovery of the low-fortification samples. Setting the low field fortification level too low will lead to unacceptably high levels of the analyte in low field spike matrix samples if inadvertent aerial drift or pesticide transport occurs in and around where the field fortification samples are located. Such inadvertent aerial drift or transport is extremely hard to avoid since wind shifts and temperature inversions commonly occur during mixer-loader/re-entry exposure studies. 

Field fortification of urine and other types of biological fluids should be carried out as follows. 

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. 

The preparation, maintenance, and integrity of field fortification solutions during a worker exposure/re-entry field study are of the utmost importance in maintaining the integrity of such a study. 

One issue that is of some importance when considering the makeup of field fortification solutions is whether to use the technical form of the active ingredient in solvent or the formulated test product in a carrier such as water. This issue has been a point of contention for many years among scientists who perform and evaluate such studies. There are some advantages and disadvantages to either choice. 

The use of formulated material (generally suspended in water) allows the researcher to work with the form of the test material that will be the most commonly encountered under field conditions. The formulated material would be found under most circumstances on field surfaces and in the air after treatment of the field with the test product. The greatest problem with the use of formulated product in water as a field fortification suspension is the maintenance of the homogeneity of the field fortification suspension. To maintain the homogeneity of the active ingredient in the field fortification suspension, one should shake the field fortification suspension vigorously for at least one minute and immediately withdraw the aliquot for the field spike from the fortification suspension just prior to fortification of the sample. 

When fortifying air tubes or air filters, the use of the formulated active ingredient in water is not recommended since the material on the air tube or filter must dry before air is drawn over the matrix. If the field fortification is nol dry on tiie sorbenf or air filter, breakthrough of the fortification solution may occur through the air filter or air tube into the back portion of the air tube and invalidate the procedure. 

One alternative method for preparing field fortifications solutions/suspensions is to prepare each fortification sample of each matrix in a separate mini-vial in the analytical laboratory and ship the vials to the field for use. This procedure precludes the use of pipets in the field and may be useful when Field Scientists not experienced in the use of pipets are involved in the field fortification process. One disadvantage of this procedure is that the mini-vials, if not designed correctly, will be hard to handle in the field, and surface tension of the suspension or fortification solution will tend to leave unacceptable amounts of the solution/suspension in the vial or at the lip of the vial and not on the matrix in question. This procedure may lead to cross-contamination of samples as the field fortification liquid is forced from the top... 

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... 

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. 

Field fortification samples are stored under various conditions in the fleld. Generally, after the weathering period is complete, the fleld fortification samples such as dosimeter sections are wrapped in aluminum foil, placed in a pre-labeled zip-type bag, and immediately placed on dry-ice in a cooler or in a freezer. Field fortification samples such as hand washes or face wipes are prepared in labeled jars, the lids are immediately taped with electrical tape, and the jars are placed in a zip-type bag and wrapped in bubble-pack and immediately placed in frozen storage. Air tubes or air filters are collected after weathering and wrapped so as to prevent breakage. These samples are then placed in a pre-labeled zip-type bag and immediately placed in frozen storage. 

Field fortification samples may be shipped with field samples but not with controls. Controls should be kept separate from treated samples and may be placed in a separate container within the container used to ship the treated samples. Samples shipped overnight should be shipped in coolers with sufficient dry-ice to maintain the samples in a frozen state for at least 48 h in case a delay in shipment of the samples occurs. Samples should not all be shipped together in one shipment but should be split to ensure that all the samples would not be lost at the same time. A chain of custody form should accompany each separate cooler or shipping box and should list each sample that is in each box. The receiver of the shipment should fill out the chain of custody form and record the conditions of the samples upon arrival at the analytical laboratory indicating whether or not the samples were frozen, ambient, or otherwise upon arrival and if the sample integrity had been compromised during shipping. 

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