APPLICATION OF ATOMIC ABSORPTION SPECTROSCOPY IN PHARMA-CEUTICAL ANALYSIS
The elements present in a host of pharmaceutical substances are determined quantitatively by atomic absorption spectroscopy, for example : Pd in carbenicillin sodium ; Cu, Pb and Zn in activated charcoal ; Fe in ascorbic acid ; Ag in cisplatin ; Ph and Zn in copper sulphate ; Zn in glucogen ; Zn in insulin ; Pb in oxprenolol hydrochloride ; Ni in prazosin hydrochloride ; Zn in sodium sulphite heptahydrate, and Cd and Pb in zinc oxide.
Insulin zinc suspension is nothing but a neutral suspension of insulin in the form of water insoluble complex with ZnCl2. Determination of both total zinc and zinc in solution is performed on a sample of the supernatant liquid obtained by centrifuging the suspension. The percentage of total zinc and of zinc in solution varies according to the strength of the preparation viz., 40, 80 or 100 units ml–1.
Stock solution of Zn (5000 mcg ml–1) : Dissolve Zn metal (Anala-R-Grade) 2.5 g in 5 M HCl (20 ml) and dilute to 500 ml with DW ; HCl (0.1 M) : 10 ml ;
To the 2 ml of well-shaken suspension add HCl (0.1 M ; 1 ml) and dilute with water to 200 ml. Spray the solution by adopting the standard procedure and read off the concentration of zinc from a calibration curve prepared with solution containing 0.5, 1, 2, and 3 mcg ml–1 of Zn.
Carbenicillin sodium : 1.0 g ; sulphuric acid (36 N or 18 M) : 2.0 ml ; mixture of nitric acid (70% w/v) and hydrochloric acid (35% w/w or 11.5 M) [3 : 4] : 5.0 ml ; hydrochloric acid (11.5 M) : 3.0 ml ; palladiun solution (standard) [Dissolve 1.670 g of Palladium (II) chloride in 200 ml of a 50% v/v solution of hydrochloric acid (11.5 M) with the aid of heat, cool and add sufficient water to produce 1 litre] : This standard palladium solution contains 1 mg of Pd in 1 ml ;
Moisten 1 g of carbenicillin sodium in a silica crucible with 2 ml of sulphuric acid. Heat, gently at first, then more strongly until all carbon is removed and a white ash is obtained. Allow to cool and add 5 ml of a mixture of nitric acid and hydrochloric acid and evaporate to dryness on a water-bath. Add 3 ml of hydrochloric acid, warm to dissolve and add sufficient water to produce 25 ml.
Place in each of three similar graduated flasks equal volumes of the solution of the substance pre-pared as above. Add to all but one of these flasks a measured quantity of the specified standard solution of palladium to produce a series of solutions containing increases amounts of Pd. Dilute the contents of each flask to the required volume with DW.
After calibrating the instrument as stated above, introduce each solution into the generator 3 times and record the steady reading at 248 nm. If the generator is a flame, wash the apparatus thoroughly with DW after each introduction ; if a furnace is used, fire it after each introduction. Plot the mean of the readings against concentration on a graph the axes of which intersect at zero added Pd and zero reading. Extrapolate the straight line joining the points until it meets the extrapolated concentration axis. The distance between this point and the intersection of the axes represents the concentration of Pd present in the prepared solution of carbenicillin sodium.
A number of pharmaceutical substances official in BP (1993) can be assayed by adopting the above procedures of AAS as detailed in the following Table 26.1.