CHMB16 Chapter 5

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 calibration method. • 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 under.  False Negative: says the result is below but it is actually above the limit. o Selectivity: being able to distinguish analyte from other species in the sample. 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 the sample.  Method Blank: a sample containing all components except analyte and it is taken through all steps of the analytical procedure.  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 are used. • 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 • 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 electrophoretic separation. 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 blanks. 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% standard. • 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