W3.1_Pharmaceutical Analysis

Question 1

What is the purpose of pharmaceutical analysis and sample preparation?

Answer 1

• Applied in drug discovery and development, formulation stability testing, drug metabolism, pharmacokinetics and pharmacodynamics, quality control and quality assurance
• Sample preparation: to isolate/purify drugs, to concentrate/derivatise/stabilise analyte

Question 2

What is the purpose of extraction/purification? State the different methods to extract solid and liquid phases (3/4).

Answer 2

• To isolate pure drug from crude form, remove interfering constituents for analysis
• Solid-phase (stationary + mobile phases): SPE/SPME, filtration, chromatography
• Liquid-phase (solvent-solvent mobile phase): separating funnel, countercurrent distribution for immiscible liquids, crystallisation, fractional distillation for miscible liquids

Question 3

Explain the purpose of solid phase extraction (SPE). Describe the process of SPE and state its advantages over LLE.

Answer 3

• Used for sample preparation to remove interferences from sample, give more reliable results, concentrate analytes to improve sensitivity
• Capture: sample is filtered through sorbent particles -> analyte captured from liquid matrix
• Elute: concentrated analytes eluted with solvent -> eluted sample collected
• Caused by different strength of interactions with stationary phase
• Advantages over liquid-liquid extraction (LLE): quick, less labour, less solvent, better recovery, easier to automate, more selective in solvents/adsorbents

Question 4

Referring to the diagrams, explain the process of immiscible liquid-liquid extraction (LLE) by relating to lipophilicity and pH and giving examples of typical pH of different functional groups. What are the common problems that could occur in separating funnel (4)?

Answer 4

• Usually in water and organic solvent by separating funnel/countercurrent distribution apparatus
• D value extremely useful in extraction -> acid-base extraction to give (un)ionised compounds
• pH heavily utilised in selective extraction (ionised -> in form of salt soluble in water, neutral -> soluble in organic solvents)
• ex. typical pKa of amines (>9), phenols (8-10), carboxylic acids (<6)
• Common problems in separating funnel: emulsions from vigorous shaking, unclear boundary, mixed layers, unsafe venting procedures

Question 5

State the four different types of miscible LLE. What is the purpose of using a vacuum and drawing away water vapour in LLE?

Answer 5

• Rotary evaporation (vacuum and heat)
• Fractional distillation (different boiling points)
• Fractional crystallisation (different solubility)
• Sublimation (freeze drying from solid to gas)
  -> use of vacuum and drawing water vapour away, good for heat labile compounds/biologicals (ex. vaccines, proteins, viruses, bacteria, plasma)

Question 6

Explain the differences between qualitative and quantitative analysis and give examples of them. What is the purpose of back titration?

Answer 6

• Qualitative analysis: to report characteristics of product
• ex. melting/boiling points, appearance, solubility, refractive index, viscosity, optical rotation, IR peaks
• Quantitative analysis: to report the actual concentration
• ex. volumetric/titrimetric (chemical change such as acid-base/complexation/redox/precipitation), spectroscopic (interaction of energy with matter), chromatographic (stationary and mobile phases)
• Back titration may be used when direct titration reaction is slow or end point is hard to obtain