Being a manager, you must be vigilant. While working in a laboratory, analysts come across various activities ranging from analysis, reporting and calculation. Under the guidance of manager, analyst takes utmost care during each and every step but more often lose focus during calculation. Managers must clear doubts about calculations and shall teach analyst how to think logically about different factors used during calculation. In today’s article I am going to walk you through few familiar equations used in laboratory along with precautions to be taken while selecting values of few common factors.
Standard potency: The standard solution is used for calculation of assay, residual solvents and impurities. The potency of the standard needs to be considered during calculation. The potency of standard ‘on as such’ or ‘as is basis’ must be considered in case if standard is not dried before use. The potency on ‘dried basis’ must be used in case if standard is dried before use. If the reference standard is used and if container label does not specify potency value- then use 100% as the potency.
The calculation formula used for % assay calculation in case of external standard method is as below
Test response Standard Conc.
% Assay = ---------------------------------- x ---------------------------- x100
Standard response Test Conc.
LOD/Water content value: The %assay of drug substance is more often reported on either dried basis or anhydrous basis. The LOD or water content is used to convert ‘as such assay’ to dried or anhydrous basis. Below formula is used for the calculation.
Test response Standard Conc. 100
% Assay (on dried or = ------------------------ x--------------------- x------------------------x100
(anhydrous basis) Standard response Test Conc. (100 – LOD/Water)
Note: All available digits of the analytical determination of LOD/Water content must be preserved when calculating the assay result.
Average weight: The test preparation for assay of finished product such as tablets is either done by ‘crush method’ or by ‘drop method’. In case of ‘crush method’, analyst have to take suitable number of tablets, crush them to fine powder and then weigh equivalent amount of crushed powder to prepare test solution. During ‘crush method’ technique, analyst have to use average weight during assay calculation. Refer below calculation formula for more details.
Test response Standard Conc. Avg. Wt.
% Assay = ---------------------------------- x ---------------------------- x---------------------x100
Standard response Test Conc. LC
Note: All available digits of the analytical determination of average weight must be preserved when calculating the assay result.
As mentioned in above equation, average weight is used for calculation of assay. The average weight is also one of the test parameter defined in specification. Analyst must not use the average weight evaluated in another experiment for calculation of assay. Analyst must evaluate average weight based on the tablets weighed during assay experiment and use the same during assay calculation.
Density of liquid products: For liquid products, label claim is often expressed in terms of ‘weight per volume’. For example, in liquid syrup of paracetamol, Paracetamol content is 500mg per 5ml. In case if product is not viscous, analyst can take suitable ‘volume’ of sample during test solution preparation. However, in case if product is viscous, analyst must take ‘weight’ of sample during test solution preparation. The density will be applicable during calculation in case latter technique is used. Refer below assay calculation formula when ‘weight’ is taken during test preparation.
Test response Standard Conc. Designated Volume
% Assay = ---------------------- x -------------------- x-----------------------------x Density x 100
Standard response Test Conc. LC
Note: All available digits of the analytical determination of density must be preserved when calculating assay result.
Designated volume, labelled claim (LC) and Density must be in same unit. For example, if designated volume is in “ml” and LC is in mg; then Density must be expressed in “mg/ml”
Its role of a leader to create awareness within analysts about possible mistakes that can happen during calculation and one must ensure that team is not making any one of them.
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