Methodology and Quality Assurance/Quality Control of Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 121

the true concentration of the sample. These QC samples measure and
validate the analytical system and the performance of the analyst.
Calibration standards are the standard solutions used to obtain calibration
curves, including a calibration blank and a series of several concentrations.
For example, one calibration blank and four concentration levels are
normally required for inorganic standard calibration curves (Section 2.1.5).
If an internal calibration procedure is used, internal standards are added to
all standards, QC sample, and all other samples. Internal standards should be
the chemicals not normally present in the sample and interfere with the
compounds of interest.
QC check standards (or calibration verification standard, CVS) are standard
solutions with known concentrations. They should be certified (purchased
from the EPA or from other sources), or independently prepared standard from
a source other than the calibration standard. They are used to verify that the
standards and the calibrations are accurate and also to confirm the calibration
curve. The value is accepted within10% deviation from the 100% recovery.
Number of Analytical (Laboratory) QA/QC Samples
The required number of laboratory QC samples is better discussed with a concept
called batch, which is the basic unit of analytical QC. It can be defined as ''a group of
samples that behave similarly with respect to the sampling or testing procedure being
employed and that are processed as a unit.'' A batch, by default, is considered to be a
group of 20 samples that can be logically grouped together (same lots of reagents
used in sample preparation, analyzed together with the same instrument within a
sequence, or the same time period or in continuous sequential time periods); thus
various types of batches exist, such as extraction or digestion batches, matrix spike
batches, clean-up batches,oranalysis batches. Keep in mind that the one per twenty
(5%) frequency is a default that has been used in many EPA programs for many
years. This means that for each batch or at least one per twenty samples there should
have a QC sample including blanks, spikes, and duplicates of various types. Adding
all these QC samples together, the total number of required QC samples should be
around 25% of the samples sent out for laboratory analysis. Keep also in mind that
internal and surrogate standard should be added to every blank, field, and QC
sample.
Practical tips
Having a proper number of QC samples during sample preparation and
analysis will help to diagnose problems if errors are suspected. For most part,
the analyst may not have to do the analysis over again from the beginning if
proper QA/QC protocols are followed, thus saving time at the end.
When loading a batch of samples in an auto-sampler tray, always keep
calibration standard, external standard, blank in certain sequence in one