Section Four

Good Laboratory Practice:Quality Assurance of Analytical

Measurements

What is Good Laboratory Practice (GLP)?

• The goal of GLP is to certify that every step of the analysis is valid.

• Standard Operating Procedures (SOP)• Quality Assurance Unit (QAU)

Validation of Analytical MethodsDefintion Example

Technique Scientific principle gives compositional information

Spectrophotometry

Method Distinct adaptation of technique selected measurement purpose

Pararosaniline method to determine SO2

Procedure Written directions to use a method

ASTM D2914Sulfur Dioxide

Protocol Set of definitive directions that must be followed, without exception

EPA Reference Method Determine SO2 Atmosphere

Fig. 4.1. General process for evaluation/validation of methodology.

Validation involves determining:

•selectivity

•linearity

•accuracy

•precision

•sensitivity

•range

•limit of detection

•limit of quantitation

•ruggedness/robustness

Standard reference materials (SRMs) best for determining accuracy.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

Fig. 4.2. Response factor plot for Figure 3.8. (p. 104)

RF = (signal – y intercept)/(concentration).

A zero slope indicates linear response over the concentration range selected.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

Fig. 4.3. Dependence of relative standard deviation on concentration.

The precision becomes poorer at low concentrations.

(Also sometimes at high concentrations, as in spectrophotometric measurements –see spectrometric error, Fig. 16.27.)

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

Fig. 4.4. Representative z-value distributions for proficiency tests with a series of laboratories.

Z = (Xi-X)/s = (mean conc. of lab – accepted conc.)/(S.D. accepted conc.)

A z-value of 2 means a lab is two S.D. away from the accepted value (95% chance of systematic error).

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)