Field spikes

Spike recoveries for samples are used to detect systematic errors due to the sample matrix or the stability of the sample after its collection. Ideally, samples should be spiked in the field at a concentration between 1 and 10 times the expected concentration of the analyte or 5 to 50 times the method s detection limit, whichever is larger. If the recovery for a field spike is unacceptable, then a sample is spiked in the laboratory and analyzed immediately. If the recovery for the laboratory spike is acceptable, then the poor recovery for the field spike may be due to the sample s deterioration during storage. When the recovery for the laboratory spike also is unacceptable, the most probable cause is a matrix-dependent relationship between the analytical signal and the concentration of the analyte. In this case the samples should be analyzed by the method of standard additions. Typical limits for acceptable spike recoveries for the analysis of waters and wastewaters are shown in Table 15.1. ... 

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. 

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. 

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. 

Field fortifications were prepared to check the field/storage stability of the dermal dosimeters, handwashes, and air filters. The field fortifications were prepared using the formulated product undiluted for "high" level spikes and diluted with water (-1 pg/mL chlorpyrifos) for the "low" level field spikes. Field fortification solutions for urine were prepared from a 3,5,6-TCP standard in acetonitrile utilizing an 1.2-pg/mL solution for the "high" field fortifications and an -0.01-ug/mL solution for the "low" level fortifications. 

Table 4 Field Controls and Field Spikes (Worker Exposure)...

Table 5 presents data on field spike recoveries. Recoveries of chlorpyrifos from "low" spiked substrates in the California studies ranged from 62.5 to 126%, and recoveries of chlorpyrifos from "high" spiked substrates ranged from 93.7 to 133%. In the Florida, Arizona, and Michigan studies, all recoveries ("low" and "high") ranged from 61 to 158%. The field recoveries cited above were found to be reasonable and within the range of field recoveries seen in many worker exposure studies. 

Table 5 Chlorpyrifos Field Spike Recoveries, Chlorpyrifos Worker Exposure Study...

Laboratory analytical recoveries and field spike recoveries were acceptable for all substrates encountered in this series of studies. Calculated penetration factors were similar to penetration factors reported in the literature. 

Field spikes with laboratory standards as well as spraying solutions were taken at a wide range of concentrations to determine the stability of the samples during both sampling and storage and possible contamination during the sampling procedure. Field blanks were also taken. 

The primary sample types used for field spiking were freshly prepared soapy distilled water (soapy water), air filter cassettes set up with 2.0 L/min. of air flow, and foil-backed patches of underwear cloth with a cover flap of coveralls cloth. The spiking solution was applied to the underwear material and the coveralls patch was then folded down to cover the spiked area. The patch was then exposed to air and sunlight for the duration of the trial in an area upwind from the trial site. The washwater samples for spiking consisted of 50-mL samples of soapy water prepared by putting on latex examination gloves and washing with Ivory soap in deionized water prior to the trial in the same way the operator would wash his hands. 

Field-spiked and exposed for duration of trial (five replicates/trial)... 

Coupons have also been used to estimate dissipation rates. For such use, it is best if the coupons are made of materials representative of honsehold furnishings, flooring or other residential surfaces (i.e. fabrics, carpet, wood and Formica), and they shonld be placed on or near the surfaces they represent. Replicate coupons (e.g. three) shonld be placed at each location for best characterization, and field spikes are particnlarly important. Coupons shonld be exposed pre-application and at several time intervals post-application to provide snfficient measnrements to project residne half-lives . The same measnres to prevent or limit the evaporative losses given above shonld also be exercised in order to assnre that the coupons reflect the residnes remaining on the snrfaces being evalnated at the time of collection. 

Field validation by comparison to accepted method or by field spiking. 

Coadsorption. Collection of other compounds can also affect recovery. The chemistry of all the compounds collected must be considered and the relative effects tested in the laboratory or in field spiking experiments. 

The field data QA/QC samples consisted of duplicate samples, field spikes with NIST (National Institute of Standards and Technology) traceable spike standards, field blanks, and breakthrough sections. Frontier Global Sciences (FGS), the project laboratory, provided all analytical services as well as all sampling media including spiked sorbent trap. FGS is a NELAP (National Environmental Laboratory Accreditation Program) accredited and ISO 17025 accredited laboratory specializing in the analysis of trace and heavy metals. 

Many sponsors request that field spike samples be taken. Field spikes of each sample matrix should bracket the expected range of analytical results generated from the assay of the samples. Spikes just above the screening level are necessary to validate recoveries and insure an acceptable analytical method. The number of field spikes should be completely planned and described in the protocol. These samples can be used for quality control and as samples for storage stability during shipment and laboratory storage. 

Spike Recoveries One of the most important quality assessment tools is the recovery of a known addition, or spike, of analyte to a method blank, field blank, or sample. To determine a spike recovery, the blank or sample is split into two portions, and a known amount of a standard solution of the analyte is added to one portion. The concentration of the analyte is determined for both the spiked, F, and unspiked portions, I, and the percent recovery, %R, is calculated as... 

Spike recoveries on method blanks and field blanks are used to evaluate the general performance of an analytical procedure. The concentration of analyte added to the blank should be between 5 and 50 times the method s detection limit. Systematic errors occurring during sampling and transport will result in an unacceptable recovery for the field blank, but not for the method blank. Systematic errors occurring in the laboratory, however, will affect the recoveries for both the field and method blanks. 

A good example of a prescriptive approach to quality assessment is the protocol outlined in Figure 15.2, published by the Environmental Protection Agency (EPA) for laboratories involved in monitoring studies of water and wastewater. Independent samples A and B are collected simultaneously at the sample site. Sample A is split into two equal-volume samples, and labeled Ai and A2. Sample B is also split into two equal-volume samples, one of which, Bsf, is spiked with a known amount of analyte. A field blank. Dp, also is spiked with the same amount of analyte. All five samples (Ai, A2, B, Bsf, and Dp) are preserved if necessary and transported to the laboratory for analysis. 

The first sample to be analyzed is the field blank. If its spike recovery is unacceptable, indicating that a systematic error is present, then a laboratory method blank. Dp, is prepared and analyzed. If the spike recovery for the method blank is also unsatisfactory, then the systematic error originated in the laboratory. An acceptable spike recovery for the method blank, however, indicates that the systematic error occurred in the field or during transport to the laboratory. Systematic errors in the laboratory can be corrected, and the analysis continued. Any systematic errors occurring in the field, however, cast uncertainty on the quality of the samples, making it necessary to collect new samples. 

If the field blank is satisfactory, then sample B is analyzed. If the result for B is above the method s detection limit, or if it is within the range of 0.1 to 10 times the amount of analyte spiked into Bsf, then a spike recovery for Bsf is determined. An... 

The following data were obtained for the repetitive spike recoveries of field samples. ... 

Figure 11-14 shows the calculated hole density (upper panel) and the electric field (lower panel) as a function of position for the three structures. For the devices with a hole barrier there is a large accumulation of holes at the interface. The spike in the hole density at the interface causes a rapid change in the electric field at the interface. The field in the hole barrier layer is significantly larger than in the hole injection layer. For the 0.5 eV hole barrier structure, almost all of the... 

Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

A seizure is accompanied by a burst of spikes in the EEG. Between these so-called iclal phases are solitary EEG interictal spikes. Each of them represents the field potential associated with a burst of action potentials in a group of neurons within the epileptic focus (Fig. 16.2). 

Figure 6 shows the sequence of events in a laser desorption FTMS experiment. First, a focused laser beam traverses the analyzer cell and strikes the crystal normal to the surface. Molecules desorbed by the thermal spike rapidly move away from the crystal and are ionized by an electron beam which passes through the cell parallel to the magnetic field and 3 cm in front of the crystal. 

Storage stability studies for carfentrazone-ethyl compounds on crop matrices have shown a pattern of stability for at least 7-24 months, depending on the study program or the maximum sample storage interval for the study. Carfentrazone-ethyl was not stable in field corn starch, potato tuber and bovine kidney. The residue results indicated that a significant portion of carfentrazone-ethyl was converted to C-Cl-PAc in these matrices however, the total amount of carfentrazone-ethyl and C-Cl-PAc accounted for the original spiking level. Since both carfentrazone-ethyl and C-Cl-PAc were determined in these stability studies, the instability of carfentrazone-ethyl was not of any concern. 

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. 

---