3.3 Chemical Analysis
91
the expected concentration range; (2) provide the data with the accuracy and
precision to successfully achieve the DQOs specified in the QA plan; and (3) not
exceed logistical limitations with regard to sample collection, sample preparation, or laboratory capabilities. For example, a method for determining NH 4
+
concentrations might provide data over the necessary concentration range
with a sufficiently high level of accuracy and precision but may require that
the analysis be done within 12 h of collection. Such a short holding time might
not be feasible for samples collected from remote sites or might be beyond the
capabilities of the laboratory.
Selection of a method that is capable of meeting the required data accuracy
and precision specified in the DQOs does not ensure that this level of data quality will be achieved. Rigorous QC and QA procedures must be followed as part
of the method implementation. QC refers to procedures that identify results
during chemical analysis that do not meet DQOs, thereby triggering immediate corrective action that usually involves reanalysis of that sample. DQOs are
generally based on (1) the precision and accuracy levels required by the project
and the laboratory and (2) the analytical limits of the methods used. A key component of QC is the introduction of artificial samples of known concentration,
which are associated with a specific set of project samples. QC procedures are
generally focused on instrument performance.
Additional procedures, referred to as QA, which are also evaluated by DQOs,
are used to document laboratory performance through the introduction of
artificial and natural samples that are not associated with a specific set of project samples, but reflect the accuracy and precision of sample preparation and
analysis, including instrument performance. Recommended protocols for QA
and QC are described in Chapter 4 of this book.
Each method used to determine a chemical concentration involves a complex set of procedures, reagents, and instrumentation. Any variation in these
factors can potentially change the result, yielding a different concentration
value. Therefore, each method requires an SOP that is strictly adhered to by the
analyst each time that the method is implemented. All details of the method
must be documented in the SOP, which must be available to any potential user
of the data. Each SOP must be dated, authored, and approved. Any change to an
SOP requires that the date of preparation also be changed on the SOP document.
We have not attempted here to recommend specific SOPs for implementation of analysis methods. SOPs are specific to individual laboratories and
instrumentation and can be influenced by the expected concentrations in a
particular study. Analytical methods, and thus SOPs, will inevitably change as
instrumentation and technology improve. Moreover, as indicated, the specific
analyses, methods, and details of SOPs must complement the project objectives and DQOs. Rather than recommending specific SOPs for general use, we
recommend that each project, in consultation with the analytical laboratory,
91
the expected concentration range; (2) provide the data with the accuracy and
precision to successfully achieve the DQOs specified in the QA plan; and (3) not
exceed logistical limitations with regard to sample collection, sample preparation, or laboratory capabilities. For example, a method for determining NH 4
+
concentrations might provide data over the necessary concentration range
with a sufficiently high level of accuracy and precision but may require that
the analysis be done within 12 h of collection. Such a short holding time might
not be feasible for samples collected from remote sites or might be beyond the
capabilities of the laboratory.
Selection of a method that is capable of meeting the required data accuracy
and precision specified in the DQOs does not ensure that this level of data quality will be achieved. Rigorous QC and QA procedures must be followed as part
of the method implementation. QC refers to procedures that identify results
during chemical analysis that do not meet DQOs, thereby triggering immediate corrective action that usually involves reanalysis of that sample. DQOs are
generally based on (1) the precision and accuracy levels required by the project
and the laboratory and (2) the analytical limits of the methods used. A key component of QC is the introduction of artificial samples of known concentration,
which are associated with a specific set of project samples. QC procedures are
generally focused on instrument performance.
Additional procedures, referred to as QA, which are also evaluated by DQOs,
are used to document laboratory performance through the introduction of
artificial and natural samples that are not associated with a specific set of project samples, but reflect the accuracy and precision of sample preparation and
analysis, including instrument performance. Recommended protocols for QA
and QC are described in Chapter 4 of this book.
Each method used to determine a chemical concentration involves a complex set of procedures, reagents, and instrumentation. Any variation in these
factors can potentially change the result, yielding a different concentration
value. Therefore, each method requires an SOP that is strictly adhered to by the
analyst each time that the method is implemented. All details of the method
must be documented in the SOP, which must be available to any potential user
of the data. Each SOP must be dated, authored, and approved. Any change to an
SOP requires that the date of preparation also be changed on the SOP document.
We have not attempted here to recommend specific SOPs for implementation of analysis methods. SOPs are specific to individual laboratories and
instrumentation and can be influenced by the expected concentrations in a
particular study. Analytical methods, and thus SOPs, will inevitably change as
instrumentation and technology improve. Moreover, as indicated, the specific
analyses, methods, and details of SOPs must complement the project objectives and DQOs. Rather than recommending specific SOPs for general use, we
recommend that each project, in consultation with the analytical laboratory,