Field duplicates

Sample numbers have been randomised to minimise regional bias, help separate false from true anomalies and obtain meaningful estimates of the variance of duplicates. Field duplicates, analytical duplicates, in-house standards and certified reference materials are introduced at regular intervals in the analytical streams. 

A total of 72 samples were collected from the three soil orientation traverses, including three field duplicate samples. Samples were collected every 50 m along each traverse. A single pit was dug at each site to between 50 and 60 cm in depth. Soil was collected from the bottom of the hole, and sieved to 100% passing 8 mesh on site. 

In each case, initial sampling should be combined with field duplicates for quality analysis and control. 

The RPD calculated for pairs of identical environmental samples (field duplicates) is the measure of total sampling and analysis precision, which combines the precision of sampling, sample handling, and the precision of sample preparation and analysis. Precision of field duplicates may be significantly affected by matrix interferences and by inherent sample variability. That is why the SAP should make a distinction between analytical precision determined from LCS/LCSD pairs and total sampling and analysis precision determined from field duplicate pairs and adopt separate acceptance criteria for each. 

A common practice to collect one duplicate sample for every 10 field samples may be excessive for some projects. The DQOs should justify the need and the frequency of field duplicate collection. 

Field duplicates increase the sampling and analysis costs by at least 10 percent. However, field duplicate precision data are rarely used for project decisionmaking or in evaluation of data usability. 

Field duplicates are samples collected at the same time and from the same sampling point as the corresponding primary samples. As a rule, field duplicates represent 10 percent of the total number of the field samples for each matrix (one field duplicate for every 10 field samples). Field duplicate precision calculated as the RPD (Equation 1, Table 2.2) enables us to evaluate the total sampling and analysis precision for water samples and the inherent sample variability for soil samples. For soil, the extent of inherent sample variability at the sampling point is a qualitative measure of sample representativeness. 

To prevent the laboratory from intentionally making field duplicate data look comparable to the primary sample data, fictitious sample numbers are assigned to field duplicates to conceal their identity. Once analytical results have been obtained from the laboratory, the project team, who has the information on the field duplicate s real identity, will compare the field duplicate and the corresponding primary sample results. 

Field duplicates for water samples are collected by sequentially filling sample containers with water from a sampling device. Water sample field duplicate precision summarily measures the analytical precision and the reproducibility of the sampling procedure. Unless gross contamination in the form of a separate phase is present, field duplicate precision for properly collected and analyzed duplicate water samples is typically below 30 percent. 

Unhomogenized duplicates are soil sample aliquots sequentially collected from the same sampling point into separate sample containers. Unhomogenized duplicate sample data provide information on soil variability with respect to contaminant distribution at the sampling point. Vastly different contaminant concentrations in unhomogenized field duplicate results indicate high contaminant variability. 

In each of these soil field duplicate collection techniques, matrix variability is a decisive factor that cannot be entirely controlled. Consequently, field duplicate RPD cannot be controlled and it should not be compared to a numeric standard, such as an acceptance criterion. Soil field duplicates are best assessed qualitatively by drawing conclusions from the comparison of the identified contaminants and their concentrations. 

Substantial sample variability affecting field duplicate precision and, consequently, the total sampling and analysis precision is to be expected for many types of soil samples and even some types of water samples as shown in Example 2.5. High sample variability has been documented for any sample collection technique for VOC analysis (Vitale, 1999 Popek, 1999). Depending on the project DQO, the... 

Example 2.5 Samples with expected low field duplicate precision... 

Low field duplicate precision may be expected for the following types of samples ... 

Quality assurance samples are replicates of field samples that are analyzed at two different laboratories, a primary laboratory that provides analytical services to the project and a referee or QA laboratory. The purpose of QA samples is to establish data comparability. QA samples are collected in the same manner as field duplicates and usually from the same sampling points. Three replicate samples from a single sampling point will represent a field sample, a field duplicate (QC sample), and a QA sample. 

Sample type (field sample, field duplicate, equipment blank)... 

As required by the SAP, collect a field duplicate sample into three En Core samplers as unhomogenized split samples obtained as close as possible to the location of the primary sample. 

For this type of sampling, the collection of additional sample volumes for MS/ MSD analysis is not possible. The liners, however, usually contain enough soil for the analysis of the sample itself and for MS/MSD analysis. In this case, we designate one of the field samples as MS/MSD on the COC form. Collocated field duplicates are usually collected as the middle and the lower liners from the same depth interval or occasionally by placing another borehole next to the location of the primary sampling point. 

Results of the field QC samples (trip blanks, field duplicates, equipment blanks, etc.) are part of the data packages and, together with field samples, they are evaluated to establish whether they are valid. But what is the significance of these data Like the field samples, these data were collected for a well-defined need, use, and purpose. They provide part of the answer to the question Are the data representative of the sampled matrix ... 

The chemist also evaluates field duplicate data to assess the total sampling and analysis precision of water and the inherent sample variability of soil as a measure of the sampling point representativeness. From the Sample Tracking Log or other field records the chemist identifies field duplicate samples and evaluates the correlation between the results of the primary sample and the duplicate. 

For water samples, the RPD of field duplicate results (Equation 1, Table 2.2) is a measure of total sampling and analysis precision. The RPD values are compared to the acceptance criterion specified in the SAP. A common field duplicate precision acceptance criterion for water samples is 30 percent. Field duplicate results that do not agree within 30 percent may indicate several problems as follows ... 

Insufficient analytical accuracy may be a cause of poor field duplicate precision. For example, low analytical accuracy that is typical for samples with low contaminant concentrations reduces analytical precision. 

The chemist identifies the probable cause of inadequate field duplicate precision and forms an opinion on a corrective action that will allow avoiding the same... 

The evaluation of soil field duplicates is somewhat complicated because numeric acceptance criteria are not appropriate for soil. Affected mainly by sample variability, the RPD values for soil sample duplicates may vary vastly. The sampling method also affects soil field duplicate precision, as shown in Chapter 2.6.2. Because soil duplicate RPD cannot be controlled, the application of a standard, such as an acceptance criterion, for their evaluation is not reasonable or practical. 

While evaluating soil field duplicate data, the chemist may calculate the RPD as a relative measure of sample variability the values, however, should be carefully interpreted in qualitative terms. If the same contaminants are identified and their concentrations are of the same order of magnitude, the variability is deemed negligible. Great disparities in the types and concentrations of the detected contaminants indicate significant variability. Example 5.9 illustrates this point. 

The sample data are never qualified based on field duplicate precision, and the information gained from the evaluation of soil duplicates usually does not have any effect on project decision-making. The only situation, when these data may... 

Example 5.9 Interpretation of field duplicate soil sample results... 

This example presents the typical results of field duplicate sample analysis that can be expected for VOCs and SVOCs in soil. The wide range of RPD values emphasizes the point that the use of numeric acceptance criteria for assessment of field duplicate precision is not practical. 

Contaminant PQL Sample Result Field Duplicate Result RPD (percent) Sample type... 

Poor correlation between field duplicate results is expected for certain types of samples and contaminants, as shown in Example 2.5, Chapter 2.6.2. For such samples, to reduce the cost of sampling and analysis, field duplicates should not be collected at all, as the data will not provide any meaningful information. 

Comparability is a DQI that comprises accuracy, precision, and representativeness. It may be assessed as interlaboratory precision and calculated as the RPD between results obtained at two laboratories. But more often, comparability, similar to field duplicate precision, is evaluated qualitatively. Data are never qualified based on comparability evaluation. 

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