Approximately 5% of the total number of samples analyzed consist of laboratory duplicates. For dissolved analytes, after a sample is analyzed, the same sample container is placed farther along in the automatic sampler and re-analyzed. The mean of the two values is reported as the concentration for that sample. If a difference of >10% is observed between replicates, then all of the replicates for that particular analytical run are carefully reviewed. If only one of the duplicate pairs is in question, then only that sample is re-analyzed. If all show a similar trend, then instrumentation/reagent problems are suspected and the analytical run is halted until such time as the problem is resolved. This procedure is practiced for all dissolved analytes that are not consumed completely in the analytical procedure. For those that are completely consumed and for particulate analytes, duplicate samples constitute actual duplicate samples collected in the field and analyzed in the same analytical run.
Values for each duplicate analyzed are recorded in a separate QA/QC data file along with the sample number, sample collection date and analysis date. The mean concentration and standard deviation of the replicates are calculated in this data file.
In the case of particulate carbon and nitrogen, total suspended solids and chlorophyll a, 10% of the total number of samples are analyzed as duplicates. This generates sufficient quality assurance data to compensate for the omission of laboratory spikes for these non-aqueous samples.
Laboratory duplicates serve as an indicator of instrument stability, consistency in laboratory sample preparation and analysis, as well as an estimate of field proficiency.