introduction to pharmaceutical analysis. what is analytical chemistry analytical chemistry is the...

Introduction to Pharmaceutical Analysis

What is analytical Chemistry

• Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials

• Qualitative analysis reveals the identity of the elements and compounds in a sample.

• Quantitative analysis indicates the amount or concentration of each substance in a sample.

Pharmaceutical Analysis

• Questions pharmaceutical analysis methods are used to answer:• Is the identity of the drug in the formulated product correct?• Does this formulation contain solely the active ingredient or are

additional impurities present?• What is the stability of the drug?• What is the rate of drug release from its formulation?• Do the identity and purity of pure drug meet specification?• What are the concentrations of specified impurities?• What are the concentration of drug in plasma or biological fluids?• What are the pKa values, partition coefficients, solubilities, and

stability of drug under development?

Chemical Purity

• Chemical purity can be described as complete freedom from foreign matter. A state of absolute purity (100%) is virtually unattainable.

• Criteria that determine the acceptable level of purity:– The cost-effectiveness of the process.– The purification methods.– Partly on the stability of the product.– Toxicity of the impurities – The dose of the active ingredient.

The source of impurities in pharmaceutical chemicals

1) Raw materials• Pharmaceutical chemicals may be isolated from

– Biological sources: plant, animal, microbiological fermentation– Chemically synthesized

2) Manufacturing Process:(a)The starting material and its impurities(b) The intermediates (c)Reagents, solvents and catalysis used in the process(d)Reaction vessel, traces of metallic ions

The source of impurities in pharmaceutical chemicals

3) Chemical and physical instabilityChemical instability: Pharmaceutical substances can undergo

chemical decomposition when stored under non-ideal conditions: light, traces of alkali and acids, air oxidation, water vapor, carbon dioxide

Ex. The acid-catalyzed degradation of β-lactam antibiotics (amoxicillin) is accelerated by heavy metals

Physical instability: Change in crystal size and form, change in particle size

4) Manufacturing hazards: Particulate contamination, Cross contamination, microbial contamination

Some important terms to describe analytical results

• Precision describes the reproducibility of measurements, in other words the closeness of results that have been obtained in exactly the same way. Determined by simply repeating the measurement on replicate samples.

• Described by standard deviation, variance

• Accuracy indicates the closeness of the measurement to the true or accepted value and is expressed by the error.


• Error types (quantitatively) • The absolute error E of a measurement is the difference

between the measured value and the true value. The sign of the absolute error tells you whether the value in question is high or low. If the measurement result is low, the sign is negative; if the measurement result is high, the sign is positive.

• E = Xi - Xt , where Xi is the measured value, Xt is the true

• Relative Error Er is a more useful quantity than the absolute error. The percent relative error is given by the expression

•

t


• Quantitative determination of iron from aqueous solution, where is the mean of six samples


• Error types (qualitatively) • 1) Systematic errors: Those which can be avoided and they

include: • a) Instrumental and reagent errors: those arise from faulty

balance for example or reagents that are contaminated with other material

• B) Errors of method

• 2) Random errors: those errors can not be avoided. They appear as slight variations in successive measurements due to causes that the analyst can’t control. They are mainly due to personal factors.


• Selectivity: The selectivity of a method is a measure of how capable it is of measuring the analyte alone in the presence of other compounds contained in the sample.

• Sensitivity: is a measure of the ability of the method to determine low concentrations of certain analyte.

• Robustness: refer to how resistant the precision and accuracy of an assay to small variations in the method, e.g. changes of instrumentations, slight variations in extract procedures

Pharmacopoeial Standards

• Pharmacopoeias are official documents that provide guidelines, specifications, and analytical procedures for various drug substances.

• British Pharmacopoeia (BP)• United states pharmacopoeia (USP)• European Pharmacopoeia• Japanese pharmacopoeia

• Drug manufacturers must comply with these pharmacopoeias according to the regulations of different countries.

C16H18N3NaO4S 371.4 69-52-3 Action and use Antibacterial. Preparation Ampicillin Injection DEFINITIONAmpicillin sodium contains not less than 91.0 per cent and not more than the equivalent of 100.5 per cent of sodium ampicillinCHARACTERS A white or almost white powder, hygroscopic, freely soluble in water, sparingly soluble in acetone, practically insoluble in fatty oils and in liquid paraffin. IDENTIFICATIONUsing TLC (described in details in the monograph)TESTS Appearance of solution Specific optical rotationASSAY Examine by liquid chromatography

Ampicillin monograph as described in BP

Ampicillin monograph as described in BP (continued)

STORAGEStore in an airtight container . If the substance is sterile, store in a sterile, airtight, tamper-proof container . LABELLINGThe label states, where applicable, that the substance is free from bacterial endotoxins. IMPURITIESDescribes all potential impurities and their accepted limit generated from synthesis, intermediates, raw material or chemical degradation of ampicillinExample: heavy metals not more than 20 ppm

Some basic concepts

• Concentration• Is a measure of how much of a material (solute) is dissolved in

certain solution and can be expressed in different way, which are known as the units of concentrations.

• There are physical and chemical basis of expressing concentration.

• Chemical units interested in knowing how many moles of the material present in each unit of volume (L) of the solution

• Physical units interested in knowing how many grams (mg, μg,..) present in unit volume (L, ml, μl,…)

Some basic concepts

• Most common physical units used in pharmaceutical analysis: mg/ml, μg/ml, part per million (ppm) which means number of mass units (mg, g, …) in one million of that mass units (according to BP definition)

• The term % is another way of expressing concentrations physically. According to BP these units can be used in four different ways:

• 1) (%w/w) to indicate the number of grams of solute in 100 g of solution

• 2) (%w/v) to indicate the number of grams of solute in 100 ml of solution

• 3) (%v/v) to indicate the number of milliliters of solute in 100 ml of solution

• 4) (%v/w) to indicate the number of milliliters of solute in 100 g of solution

Some basic concepts

• The USP states that if the unit percentage (%) was used without specifying if it was (w/w, w/v, v/v) then it should understood as follow:

• Solids in semisolid (creams and ointments), then it’s w/w• Solution or suspension of a solid substance in a liquid, then

it’s w/v• Liquid in liquid, then it’s v/v.

• Chemical Units, molarity (M) and normality (N)• Molarity: number of moles in each liter of solution• Normality: number of equivalents in one liter of solution

Some basic concepts• Analytical molarity: total number of moles of a solute in one

liter of the solution.• Equilibrium, or species molarity: the molar concentration of a

particular species in a solution at equilibrium. • Equilibrium molar concentrations are symbolized by placing

square brackets around the chemical formula of the species. [H2SO4]

Example

• Example. Calculate the analytical and equilibrium molar concentrations of the solute species in an aqueous solution that contains 285 mg of trichloroacetic acid, Cl3CCOOH (163.4 g/mol)in 10 ml (the acid is 73% ionized in water)

• Converting Between Concentration Units• A concentrated solution of aqueous ammonia is 28.0% w/w

NH3 and has a density of 0.899 g/ml. What is the molar concentration of NH3 (17.04g/mol) in this solution?

• The maximum allowed concentration of chloride in a municipal drinking water supply is 2.50 × 102 ppm Cl–. When the supply of water exceeds this limit, it often has a distinctive salty taste. What is this concentration in moles Cl–/liter?

Examples

• Q1. Describe the preparation of 500 ml of 0.0740 M Cl- solution from solid BaCl2.2H2O (244.3 g/mol).

• 4.52 g of BaCl2.2H2Oin 500 ml water

• Example. What is the molarity of K+ in aqueouse solution that contains 63.3 ppm of K3Fe(CN)6 (329.2 g/mol).

• M = 5.77 X 10-4 mol/L

Calculation the pH value of aq. Solutions of strong and weak acids and bases

• The pH of a solution is defined as –log[H+].• In pure water the conc. of hydrogen ions is governed by the

equilibrium:


• Strong acids and bases• They are completely ionized in water.• What is the pH of 0.054M solution of HCl?• What is the pH of 0.01M KOH?

• Weak acids and bases

• What is the pH of 0.01 M acetic acid? Pka = 4.76


• Salts of weak acids and weak bases.• Salt of weak acid is conjugate base and the salt of weak base is

conjugate acid. • What is the pH of a solution (0.025 M) of sodium benzoate? (Pka of

benzoic acid 4.2)• Solutions of mixtures of a weak acid and a conjugate base.• Henderson–Hasselbalch equation

• Calculate the pH of a solution prepared by dissolving 242.2 mg of a primary amine (base) in 10 ml of 0.17 M HCl and dilution to 100 ml with water. Pka = 8.08 (Mwt of amine 121.1 g/mol)

Chemistry of acidic and basic compounds

• Some basic principles to predict the relevant strength of an acid or base:

• 1) Dissociation of positively charged acid is easier than neutral or negatively charged acid.

• Dissociation of diprotoic acid is always easier (lower pKa) than the second dissociation.

• 2) Protonation of base is easier when it’s more negative. • For 1 and 2, as the distance between the charge carrying

atom and the acidic or basic group increases, the above effect decrease

Chemistry of acidic and basic compounds

• pKa of carboxylic acid 2.31, 2.88. 3.51• 3) Electron-withdrawing groups increase acidity and electron-

donating groups increase basicity.• 4) All aromatic acids are stronger than their comparable

aliphatic acids.

H3N COOH

H3NCOOH

H3N COOH

I

II

III

• Describe the preparation of 100 ml of 6.0 M HCl from a concentrated solution that has density of 1.18 and is 37% (w/w) HCl (36.5 g/mol). Mconc. X Vconc. = Mdil. X Vdil.

Buffer solutions

• By definition, a buffer solution resists changes in pH with dilution or with addition of acids or bases.

• Generally, buffer solutions are prepared from a conjugate acid/base pair, such as acetic acid/sodium acetate or ammonium chloride/ammonia.

• pH of buffer solution is calculated using Henderson–Hasselbalch equation

• Buffer capacity: defined as the number of moles of a strong acid or a strong base that causes 1.00 L of the buffer to undergo a I.00 unit change in pH.

• Buffer capacity depends on the concentration ratio of acid and its conjugate base or base and its conjugate acid

Buffer solutions

• The pKa of the acid chosen for a given application should lie within ±I unit of the desired pH for the buffer to have a reasonable capacity.

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