Drug Analysis

Question 1

What is a raw material specification?

Answer 1

A list of tests and corresponding limits which the results of the tests must be within. This is often based on the pharmacopeial monographs. Manufacturers may also make their own specifications for the bulk drugs made or bought.

Question 2

By what three means may impurities enter the produced drug?

Answer 2

Synthesis, storage, related substances.

Question 3

How may impurities from synthesis enter the produced drug?

Answer 3

From starting materials, reagents, catalysts, solvents, or by-products. They may also enter from the processing plant; heavy metals from soldered joints and iron from steel vessels.

Question 4

How may impurities enter the produced drug during storage?

Answer 4

From degradation products generated through hydrolysis or photolysis. As well as moisture.

Question 5

In terms of impurities, what are related substances?

Answer 5

These are usually specific synthetic precursors, by-products and degradation products chemically related to the drug. They may have biological activity similar to that of the drug.

Question 6

What do the limits of impurities depend on?

Answer 6

The toxicity of the products (how dangerous), the feasibility (can it be done). Pharmacopeias may explain the limits.

Question 7

Describe UV analysis.

Answer 7

Looks at chromophores, areas of conjugation (alternating double and single bonds). Identifies functional groups and can be used to identify acids or bases.
If the chromophores of the drug and the impurities are similar then UV analysis is ruled out.

Question 8

Put UV, IR, and NMR in order of greatest sensitivity to least sensitivity.

Answer 8

UV, IR, NMR.

Question 9

Put UV, IR, and NMR into the order of greatest information to the least information.

Answer 9

UV, IR, NMR.

Question 10

Why can UV analysis sometimes be inaccurate for organic compounds?

Answer 10

UV energy is similar to the bonding energy for organic compounds so can photolyse the compound.

Question 11

What does a greater UV wavelength suggest?

Answer 11

Greater conjugation and a larger chromophore.

Question 12

What characteristic of chromophores leads to it being affected by pH change?

Answer 12

Acid or base properties.

Question 13

How does an increase in electron conjugation change the lambda max?

Answer 13

AN increase in conjugation leads to an increase in lambda max.

Question 14

How does a decrease in electron conjugation change the lambda max?

Answer 14

A decrease in electron conjugation decreases lambda max.

Question 15

What is Beer’s Law?

Answer 15

Light absorption is proportional to the concentration of absorbing species.

Question 16

What is Lambert’s Law?

Answer 16

Light absorption is proportional to path length.

Question 17

Give some sample related causes for deviation from the Beer-Lambert Law.

Answer 17

Overlapping contaminant, Degradation (photolysis), Fluorescence, Tautomerisation (pH effects, temperature).

Question 18

Why can tautomerisation cause deviations from the Beer-Lambert Law?

Answer 18

Different tautomers can have different absorption. This can be pH or temperature dependent.

Question 19

Give some other causes for deviation from the Beer-Lambert Law.

Answer 19

Stray light, non-monochromatic light sauce, mismatched cells, sensitivity, solvent absorption.

Question 20

Why are calibration curves used when carrying out spectroscopy?

Answer 20

To establish the validity of the Beer-Lambert Law for each drug under the measurement conditions to be used under an appropriate concentration range.

Question 21

Describe the instrumentation of single-beam spectrophotometers.

Answer 21

An intense source of UV light (deuterium or hydrogen lamp - depending on the spectrum), Prism or diffraction grating monochromator, capable of high precision. Used for absorbance determination at a fixed wavelength, not to obtain a spectrum.

Question 22

Describe the instrumentation of double-beam spectrophotometers.

Answer 22

Similar to single-beam instrumentation. Radiation split into two beams by a rotating mirror. One beam passes through the sample. The other beam passes through the blank reference cell. The two beams are compared to give the absorbance. Suitable for fixed wavelength readings and whole spectra.

Question 23

What is a photodiode array?

Answer 23

A series of lamps at different wavelengths which is crude but gives more info. It is used more in HPLC.

Question 24

Give some pharmacopeial applications of spectrophotometry.

Answer 24

Assay of single drugs, assay of mixtures of drugs, colourimetric methods, tablet dissolution, limit test for impurities, assays for bulk drugs or extracts.

Question 25

Give some other applications for spectrophotometry.

Answer 25

Physiochemical methods, pKa, velocity constants in enzymatic reactions.

Question 26

Assaying mixtures of drugs can be hard if they have similar chromophores. How can this be worked around?

Answer 26

One can derivatise one of the drugs so it moves away from the other in the spectra.

Question 27

Explain the instrumentation of dispersive spectrophotometers (infra-red).

Answer 27

• Scanned absorbance.
• Single beam.
• Double beam (uses a reference beam to compensate for solvent absorbance).
• Usually a real-time chart paper readout.
• Computational subtraction (sometimes).
• Low intensity at high absorption.

Question 28

EXplain the instrumentation of Fourier transform infra-red (FTIR).

Answer 28

• An interferometric method.
• Light from the whole range is split into two beams (photons out of phase).
• One beam travels along a longer path to the other.
• One beam passes through the sample.
• Beams recombined.
• Interference pattern (interferogram) - the sum of all interference patterns from every wavelength.
• Converted via a Fourier transformation to an absorbance spectrum.

Question 29

What are the advantages of Fourier-transform infra-red (FTIR)?

Answer 29

Fast (all measured at once), reproducible, accurate, high resolution, low sample concs, low stray light levels, digital data.

Question 30

Describe atomic emission spectroscopy (AES) (flame photometry).

Answer 30

The test solution is aspirated into the excitation region where metal ions are desolvated, vapourised, and atomised by a flame, discharge, or plasma.
When the atoms are excited, photons are released and this goes through a monochromator, detector, amplifier, and then shown on a readout.

Question 31

By how much does a 10K temperature change the excited population? How does this affect the results?

Answer 31

4%. This can introduce error to the results.

Question 32

Where is AES found and for what use?

Answer 32

It is commonly found in hospitals for detecting electrolyte concentrations.

Question 33

Describe atomic absorption spectroscopy (AAS).

Answer 33

The test solution is aspirated into the excitation region where metal ions are desolvated, vapourised, and atomised by a flame, discharge, or plasma.
Light from a lamp specific for an atom is shone through the sample and detected. Providing a readout.
The emitted ions can be equated to the cuvette in UV analysis.

Question 34

Why is AAS sometime preferred to AES?

Answer 34

There is less flame temperature fluctuation in AAS, leading to less error in the results.

Question 35

What are the similarities of AAS and AES

Answer 35

High sensitivity, sharp lines with little overlap between different elements, not as accurate as some wet methods, expensive equipment, need calibration curves.

Question 36

How are ion-selective electrodes used? How do they work?

Answer 36

The ion selective electrode is placed in a stirred solution of the sample along with a reference electrode. The potential difference between the two is measured. The ion selective electrode has a membrane permeable to the ions allowing them to affect the potential inside the electrode. Adding the ion the sample moves the equilibrium, increasing the ions bound to the membrane.

Question 37

What are the advantages of using an ion-selective electrode?

Answer 37

Short response time, can be selective, wide range of linear response, non-destructive, non-contaminating, unaffected by colour or turbidity, lower costs.

Question 38

What are the disadvantages of ion-selective electrodes?

Answer 38

Precision is lower than for titrimetric methods, less sensitive.

Question 39

Why are some compounds to be assayed by GLC converted to their TMS derivatives?

Answer 39

To increase their volatility, making them easier to assay by way of GLC.