Chapter 5: Quality Assurance and Calibration Methods
Basics of Quality Assurance
• Quality Assurance: what we do to get the right answer for our purpose.
• Raw Data: individual values of a measured quantity.
• Treated Data: concentrations/amounts derived from raw data by use of
• Results: quantities reported after statistical analysis of treated data.
• Use Objectives: states purpose for which results will be used (drinking water is
usually disinfected with chlorine which kills microorganisms).
• Specifications: stating how good the numbers need to be and what
precautions are required in the analytical procedure.
o Sampling Requirements: must collect representative samples and
analyte must be preserved after sample is collected.
o Accuracy and Precision
o Rate of False Results
False Positive: says that the result is over but it is actually
False Negative: says the result is below but it is actually above
o Selectivity: being able to distinguish analyte from other species in the
o Sensitivity: the capability of responding reliably and measurably to
changes in analyte concentration (sensitivity = slope of calibration
curve = change in signal / change in analyte concentration).
o Acceptable Blank Values: account for interference by other species in
Method Blank: a sample containing all components except
analyte and it is taken through all steps of the analytical
Reagent Blanks: similar to a method blank, but it has not been
subjected to all sample preparation procedures.
Field Blank: similar to method blank but it has been exposed to
the site of sampling.
o Recovery of Fortification or Spike: a known quantity of analyte added
to a sample to test whether response to a sample is the same as that
expected from a calibration curve (% recovery = Cspiked sample –
Cunspiked sample / Cadded x 100).
o Calibration Checks: we analyze solutions formulated to contain known
concentrations of analyte.
o Quality Control Samples: help eliminate bias introduced by an analyst
who known the concentration of the calibration check sample.
• Standard Operating Procedures: stating what steps will be taken and how they
will be carried out are the bulwark of quality assurance.
• To gauge accuracy, calibration checks, fortification recoveries, quality control
samples and blanks are used.
• To gauge precision, replicate samples and replicate portions of same sample
• Chain of Custody: the trail followed by a sample from the time it is collected to
the time it is analyzed and archived.
• Assessment: the process of collecting data to show that analytical procedures
are operating within specified limits and verifying that final results meet use
objectives. o Control Charts: used to monitor performance on blanks, calibration
checks and spiked samples to see if results are stable over time or to
compare the work of different employees.
o Standard Protocols: provide directions for what must be documented
and how the documentation is to be done.
• Method Validation: the process of proving that an analytical method is
acceptable for its intended purpose.
• Specificity: the ability of an analytical method to distinguish analyte from
everything else that might be in the sample.
o Electrophoresis: an analytical method in which substances are
separated from one another by their differing rates of migration in a
strong electric field.
o Electropherogram: a graph of detector response versus time in an
o Baseline Separation: the detector signal returns to its baseline before
the next compound reaches the detector.
• Linearity: measures how well a calibration curve follows a straight line,
showing that response is proportional to the quantity of analyte.
o Each standard should be prepared and analyzed three times with
o Square of the Correlation Coefficient (R^2): R^2 = [∑(x – xaverage)(y –
yaverage)^2 / ∑(x – xaverage)^2 • ∑(y – yaverage)^2
o An acceptable value of R^2 might be ≥ 0.98 for the range 0.1 to 2 wt%
and the y-intercept should be < 10% of the response for the 2 wt%
• Accuracy: nearness to the truth.
o Analyze a certified reference material in a matrix (everything in the
sample except the analyte) similar to that of your unknown. Your
method should find the certified value for analyte in the reference
material, within the precision of your method.
o Compare results from two or more different analytical methods. They