LECTURE 1B

Formulating the Question

• know the type of information that is to be obtained

• translate general questions into specific questions

Selecting Analytical Procedure

• number of components always influences the choice of method

• analytical methods that have been defined and validated and currently used in regulatory laboratories

AOAC - Association of Official Analytical Chemists

ASTM - American Society for Testing in Materials

USP - United States Pharmacopoeia

BP - British Pharmacopoeia

Sampling

• sample must faithfully represent the top of the bulk

• sample must have the same properties that are representative of the block of the material

• use mortar and pestle to turn heterogenous solids into homogenous powders

• methods that are destructive in nature → dissolving/volatilize the sample

• destructive means it is difficult to recover the sample at the end of the analysis

Sample Preparation

• dissolution → done on solids

• concentration → the analyte in trace amounts

• interferents - species in an analyte that can affect measurements

• selective precipitation - done on mixtures, uses titration/centrifugation to separate particles

Analysis Proper

• replicates of sample - portions of the material of approximately the same size, taken at the same time, and in the same manner

• blank sample - used to confirm that the analytical system is free of analyte contaminants/interferents (potential interference may be present in the sample itself or in the reagent used during the analysis)

• note: blanks do not contain the sample

• method blank - can be prepared using the sample itself or in the reagent used during the analysis

Reporting and Interpretation

• results are valid only if these were obtained from replicate measurements and if the data were completed statistically

Drawing conclusions'

• all analytical methods have inherent strengths and weakness