This article includes some methods for 1 identification, 2 limit tests for impurities and 3 analysis of selected examples of drugs used medically according to pharmacopoeias.
The analysis of drugs enables us to take an idea about the following :
- The exact percentage of the active drug in formulation in comparison with the stated one.
- The presence of impurities and other active ingredients in the formulation.
- The stability of the drugs in formulation.
- Measuring the rate of release of the active drug from the formulation.
- The concentration of specified impurities in the pure drug substance.
- The concentration of the drug in a sample of tissue or biological fluids.
- The physicochemical parameters of drug substances under development. Sources of Errors in Drug Analysis
- Incorrect weighing and transfer of analytes and standards.
- Inefficient extraction of the analyte from a matrix (e.g. tablets).
- Incorrect use and the wrong calibration of pipettes, burettes or volumetric flasks for volume measurements.
- Failure to use an analytical blank.
- Assay conditions that cause degradation of the analyte.
- Interference of excipients in the measurement of the analyte
The precision of Assay :
It is the measure of its ability to produce close agreement between the results for several determinations of the same sample.
Analytical Blank :
This consists of all the reagents or solvents used in the analysis without any of the analyte being present.
A true analytical blank should reflect all the operations to which the analyte in real sample is subjected. It is used in checking that reagent or indicators do not contribute to the volume of titrant required for a titration. Limit of detection
It is the smallest amount of the analyte, which can be detected by a particular method.
Sensitivity :
The sensitivity of a method indicates how responsive it to small changes in the concentration of any analyte.
Selectivity :
The selectivity of a method is how capable it measures the analyte alone in presence of other compounds contained in the sample.
Expressions of concentration :
Molarity :
Molarity is designated by the symbol M preceded by a number that is the number of moles of the designated solute contained in an amount of the designated solvent that is sufficient to prepare one liter of solution.
Normality :
Normality is designated by the symbol N preceded by a number that is the number of equivalents of the designated solute contained in an amount of the designated solvent that is sufficient to prepare one liter of solution.
Molality :
Molality is designed by the symbol m preceded by a number that is the number of moles of the designated solute contained in one kilogram of the designated solvent.
Percent weight in volume (%w/v)
Expresses the number of grams of a constituent in 100 ml of solution or liquid preparation.
Percent volume in volume (%v/v)
Expresses the number of millilitres of a constituent in 100 ml of solution or liquid preparation.
Percent weight in weight (%w/w)
Expresses the number of grams of a constituent in 100 gram of solution or preparation.
Milligram Percent (mg %)
This term is used to express the number of milligrams of a substance in 100 ml of liquid.
Parts per million (ppm)
It is used for very dilute solutions, where the concentration of the ingredient is negligible. ppm Means 1/1000,000.
Example: 5ppm of iron in water means 5/1000,000 =1/200,000 = 0.0005 %.
Simple conversion of concentration to mg /ml.
To convert product percentage strength to mg/ml, multiply the percentage strength expressed as a whole number, by 10.
Example:
Convert 5% (w/v) to mg/ml.
5 X 10 = 50 mg /ml
5 % means 5g /100ml = 5000 mg/100ml = 50 mg /ml.
Solubility
Pharmacopeial substances are indicated by the descriptive terms in the accompanying table.
Description
Parts of Solvent required for 1 part of solute
Very soluble
Less than 1
Freely soluble
From 1-10
Soluble
From 10-30
Sparingly soluble
From 30-100
Slightly soluble
From 100-1000
Very slightly soluble
1000-10,000
Practically insoluble or insoluble
10,000 and over