Drug analysis (4)

Question 1

Atomic spectroscopy

Question 2

Atomic Emission Spectroscopy

Answer 2

• Atomic Emission Spectroscopy=> AES
• Atomic Absorption Spectroscopy=> AAS

Question 3

Atomic Emission Spectrometry (Flame photometry)

Answer 3

• The test solution is aspirated into the excitation region where metal ions are
  
  • Desolvated
  • Vapourised
  • Atomized by a flame, discharge or plasma

Question 4

Atomic Emission Spectroscopy

Answer 4

• Sufficient energy to promote a small % of the atom into high energy levels
• The atoms decay back to lower levels by emitting light
• Simultaneous excitation of all atoms in sample => simultaneous detection using a polychromator with multiple detectors

Question 5

Atomic emission spectroscopy

Temperature instability problems

Answer 5

• fuel/Oxidant determines the temperature and hence the extent of atomisation and excitation
• Temperature instability
  
  • Affects the extent of itemisation and ionisation
  • The large effect in AE- Exponential T sensitivity of excited state population
  • A 10K temp rise changes the excited state population by as much as 4%
• Alkali metal have very strong emission and are usually measured by AES in the clinical analysis using relatively simple equipment

Question 6

Atomic Absorption spectroscopy

Answer 6

• Specific light source
• Hollow cathode- filled with Fe
• Emits light at frequencies characteristic of Fe
• One element at a time cf AES
• Compare I_o and I (Beer-Lambert law)

Question 7

Atomic absorption spectroscopy

Answer 7

• Compare I_o and I (Beer-Lambert law)
• The majority of the test species are in the ground electronic state
• Varying the temperature by 10K hardly affects the ground-state population and does no noticeably affect the signal in AAS

Question 8

AAS and AES

Answer 8

• High sensitivity (ppm, sometimes ppt)
• Sharp lines
• Little overlap between different elements in the same sample- SELECTIVE
• Not as accurate as some wet methods- precision rarely better than 1-2%
• Equipment expensive but widely available
• Need calibration curves- standard solutions

Question 9

AES vs AAS differences

Answer 9

• High throughput of samples
  
  • Slowe throughput of samples
• Some instruments can measure over 60 elements in a sample simultaneously
  
  • Single element determinations
• Simpler equipment
  
  • Needs light source
• Errors from flame temperature instability
  
  • Far less prone to flame temperature instability errors

Question 10

Ion selective electrodes

Answer 10

• Commonly used to determine e.g. Cl^-, SO₄^-, NO₃^-, Na⁺, K⁺, NH₄⁺
• The ion-selective electrode (ISE) is placed in a stirred solution of the sample along with the reference electrode
• The potential between the two is measured

Question 11

Ion-selective electrodes

Answer 11

• The tip of the ISE comprises a membrane which is able to bind selectively the desired ion e.g. for a cation
• A⁺ + membrane => A-membrane
• Adding A⁺ to the sample moves the equilibrium to the right
• Inside the electrode, A⁺, already bound to the electrode, is released to restore the equilibrium
• This changes the potential inside the electrode

Question 12

Ion-selective Electrodes

Answer 12

• c= Concentration*of the ion
• *Electrodes respond to the activity of uncomplexed analyte ions
• Ligands must be absent or masked
• If the ionic strength is held constant, concentration is proportional to activity and the electrode measures concentration
• Thus an inert salt is added to raise all the standards and samples to the same high and constant ionic strength

Question 13

Ion-selective electrode

Advantages vs Disadvantages

Answer 13

• Advantages
  
  • Short response time
  • Can be selective
  • A wide range of linear response
  • Non-destructive
  • Non-contaminating
  • Unaffected by colour or turbidity
  • Lower capital costs cf AAS/AES
• Disadvantages
  
  • Precision (>2%) lower than for titrimetric methods
  • Less sensitive than AAS/AES