Methodology and Quality Assurance/Quality Control of Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 120

precision. Details are described as follows for several commonly used laboratory QC
samples.
Reagent blanks are analyte-free water analyzed with the samples. Such
blanks will detect any introduction of contaminants and artifacts into
samples. Preparation blanks/method blanks are analyte-free matrix (reagent
water) that are carried throughout the entire preparation and analysis and
contain all of the reagents in the same volumes as used in the processing of
the samples. They are used to detect any contamination (false positive
results) during the sample preparation and analytical process. Instrument
blanks are the analyte-free reagents (water or solvent) inserted in the
analytical run, normally injected between high concentration and low
concentration samples. Instrument blanks are used to flush the system in
trace analysis and used to determine whether memory effects (carryover) are
present in the analytical system.
Reagent water spikes are analyte-free water spiked with the analyte of
interest and are prepared in the same way as the samples. Such spikes
monitor the effectiveness of the method. The level of the spike addition is
recommended to be five times the method quantification limit.
Performance evaluation (PE)/laboratory control samples (LCS) contain
known identities and concentrations of target analytes in certified clean water.
PE samples are usually prepared by a third party and submitted ''blind'' to
analytical laboratories, while the LCS samples are prepared by the laboratory
so that the laboratory knows the contents of the sample. Neither PE nor LC
samples are affected by matrix interference, and thus they can provide a clear
measure of laboratory error. The EPA has a guideline regarding the minimal
number of analytes to be used in spikes (Popek, 2003).
Matrix spikes and matrix spike duplicates (MS/MSDs) are field samples
spiked in the laboratory using the same spiking chemicals described for LCS.
Same as PE and LCS, predetermined quantities of stock solutions of certain
analytes are added to a sample matrix prior to sample extraction/digestion
and analysis. Samples are split into duplicates, each of the analytes detected.
The percentage recovery is calculated to assess the accuracy, and the relative
percent difference (RPD, see Eq. 2.9) is calculated and used to assess
analytical precision. Unlike PE and LCS sample, MS/MSDs also indicates
the degree to which matrix interference will affect the analysis.
Laboratory duplicates are aliquot of the same sample that are prepared and
analyzed at the same time, but submitted and analyzed as separate samples
(see also field duplicates). The analyst does not know that they are
duplicates. Discrepancies in duplicate samples indicate poor analytical
reproducibility (the analytical precision) and/or the poor homogenization
in the field.
Reference materials (blind QC check samples) are obtained from an
independent source with known analytical level(s). The analyst is not told