Chapter 5

Basics of Quality Assurance

  • Quality assurance is what we do to get the right answer
    • Data quality standards:
    • get right data
    • get data right
    • keep data right
  • Raw data: measurements
  • Treated data: concentration derived from raw data by use of calibratino methods
  • Results: quanities reported after statistical analysis of treated data
  • Specifications: state how good the numbers need to be and what precautions are required in the analytical procedure
  • False positive: concentration exceeds the legal limit when, in fact, the concentration is below the limit
  • False negative: concentration is below the legal limit when it is actually above the limit
  • Selectivity: ability to distinguish analyte from other species in sample (avoid interference)
  • Sensitivity: ability to respond reliably and measurably to change in analyte concentration (slope of the calibration curve)
  • Blanks: sample not intended to contain analyte
    • account for interference by other species in the sample
    • account for trace amount of analyte found in reagents
  • Types of blanks
    • method blank
    • all components except analyte
    • subtrace response of the method blank from the response of sample before calculating the quantity of analyte
    • reagent blank
    • similar to method blank, but it has not be subjected to all sample preparation procedures
    • field blank
    • indicates if analyte is inadvertently picked up by exposure to field conditions
  • Spike Recovery
    • Matrix: everything in the unknown, other than the analyte
    • Spike (fortification): a known quantity of analyte added to a sample
  • Dealing with large numbers of samples
    • method drift can be due to changes such as room temp
    • calibration check solution should be different from one used to prepare the orginal calibration curve
    • performance test samples help to eliminate bias introduced by analyst who knows the concentration of the calibration check sample
    • standard operating procedure (SOP) outline steps the be taken for the procedure. Serve as control experiments to detect problem in the lab
  • Assessment
    • the process of:
    • collecting data to show that analytical prodecures are operating within specific limits
    • verifying that final results meet use objectives
    • if final results meet the use objectives, the method is fit for purpose

Method Validation

  • Method validation: the process of proving that an analytical method is acceptable for intended purpose
  • Selectivity: extent to which an analytical method can distinguish analyte from everything else in the sample
    • method is specific if it selects for only one analyte (no interferences)
  • Linearity: measures how well a calibration curve follows straight line
    • response is proportional to quanitiy of analyte
    • common measure linearity is the R squared
    • value of R squared > 0.995 - 0.999 is a good fit for many purposes
  • Accuracy (nearness to the truth)
    • can be demonstrated by comparing results from two or more different analytical methods
  • Precision
    • instrument precision: same quantity of one sample is repeatedly introduced into an instrument
    • intra-assay precision: analysis of aliquots of a homogeneous material several times by one person in one day with the same equipment
    • intermediate precision (ruggedness): assay is performed by different people on different intruments on different days in the same lab
    • interlaboratory precision: aliquots of the same sample analyzed by different people in different laboratories
  • Range and robustness
    • range: concentration interval over which linearity, accuracy, and precision are all acceptable
    • linear range: concentration range over which calibration curve is linear
    • dynamic range: concentration range over which there is measurable response
    • robustness: ability of an analytical method to be unaffected by small, deliberate changes in operating parameter
  • Limits of detection and quantitation
    • detection limit (lower limit of detection): smallest quantity of analyte that is significantly different from the blank
    • quantitation limit (lower limit of quantitation): smallest quantity of analyte that can be measured with reasonable accuracy
  • Reporting limit: concentration below which regulations say an analyte is “not detected”
    • does not mean analyte is not observed but that is it below the prescribed level
    • set at least 5-10 times higher than detection limit

Standard Addition

  • Standard addition: known quantities of the analyte added to the unknown
    • increase in signal indicates how much analyte was in the original unknown
    • method requires a linear response to analyte concentration
    • higher precision can be achieved when standards are added by mass instead of volume
  • Matrix effect: change in analytical sensitivity caused by something in sample other than analyte
    • sample composition affects the analytical signal

Internal Standards

  • Standards
    • standard addition: known amount of a compound-same substance as analyte- added to the unknown
    • internal standards: known amount of a compound- different from analyte- added to the unknown
    • external standards: solutions with known concentrations of analyte used to prepare a calibration curve