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Review On Mass Balance


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Mass Balance:

“The assay value of drug and total responses of all degradants should equal the initial value i.e. it’s 100% is mass balance”.

Mass balance in the pharmaceutical analysis is very much important. It is representing the degradative losses of parent drug compare well with the measured increase in the degradation products. Equally, if one observes, for example, a 10% loss of parent drug but only measures a 2 % increase in degradation products, it is expected that additional degradation products formed are not accurately determined by the present method(s).

Why Mass balance?

Mass balance is an important consideration in assessing degradation pathways of pharmaceutical products. The related substances or organic impurities method is a stability-indicating method for degradants. it is important to have methods that detect all major degradation products. Thus, safety is the major reason for the study of mass balance.

Mass balance is also useful in method validation. In order to demonstrate that analytical methods are stability-indicating, unstressed, and stressed results were compared and evaluated. Any increase in degradation a product that correlates well with the loss of parent drug, aids in demonstrating that the methods can accurately assess degradation.

How much Degradation?

Usually, 5 % to 10 % of degradation is looked for as a forced degradation study during the different forced degradative conditions. There is not all compounds/formulations will degrade by 10% under even extreme conditions, however, a sufficient attempt must be taken to promote degradation. Any stability observed from the studies must be scientifically explained and justified in case of low degradation levels (i.e. < 10%). These types of criteria can be used within internal guidance documents where extreme conditions are defined.

Why Mass Balance Not Achieved?

During Stress studies it is “good to show the mass balance” but in reality, it is normally not achieved due to the following reasons,

  1. Forced degradation is to prove the selectivity of the method in terms of interference to the main peak and major known impurities.
  2. Since the conditions used are very harsh and not usually encountered in the real stability studies. Such stress conditions generate secondary and tertiary degradation.
  3. It will never know whether the resultant products are UV active or not, in certain cases gases liberate from the sample and end product might be an acid with no UV chromophore or just a precipitation.
  4. The degradant formed may not be eluted in the developed method (extend runtime or modify method conditions).
  5. The Relative Response Factor may be zero at method wavelength (select appropriate wavelength or develop separate method).

Figure: Fishbone diagram of the major causes of mass imbalance.

How Do We Achieve Mass Balance?

Primary aim is to separate related substance degradants from unrelated, to identify key degradation pathways by studying about chemistry to get MS and UV spectra on degradants. Develop a simple method to resolve the degradants from the drug and from each other.  Calculating the mass balance from this initial study tells us about how much we know.

We need to explore different detectors like GC/MS, LC/MS, LC-MS-NMR, RI Detector, ELSD Detector for possible degradants.  Later steps might include isolation of specific degradants to determine structure and response factors, giving us better understanding of chemistry and a quantitative correction to the degradant’s response. 

How Mass Balance is Calculated?

There is no guideline or industry-accepted practice for mass balance limit for forced degradation studies but we need to demonstrate the mass balance of the method, whatever is achieved if all other validation parameters are OK. Generally, 95 % to 105 % mass balance is well accepted.

The mass balance equation:
Total mass = (Amount of mass remained + Amount of known degradants + Amount of unknown degradants)
So, here I will convert the equation for better understanding. After each degradation study,
 Total % of drug = % drug remained + % known degradants + % unknown degradants

  1. Relative response factors play an important role to calculate exact mass balance.
  2. Molecular weight of Drug and molecular weight of degradant formed plays an important role to calculate exact mass balance.


  1. International Conference on Harmonization (ICH) and US Food and Drug Administration, Q1B: Photostability Testing of New Drug Substances and Products.
  2. ICH and FDA, Q1A(R2): Stability Testing of New Drug Substances and Products.
  3. ICH and FDA, Q2A: Validation of Analytical Procedures, Text for Validation of Analytical Procedure.
  4. Impurities in new drug substances, Q3A, Q3B and Q3C, ICH of Technical Requirements For Registration of Pharmaceuticals.
  5. ICH and FDA, Q2B: Validation of Analytical Procedures, Methodology.
  6. M. Nussbaum et al., “Determination of Relative UV Response Factors for HPLC by use of a Chemiluminescent Nitrogen-Specific Detector,” J. Pharm. Biomed.
  7. Baertschi, S.W., Alsante, K.M., Reed, R.A. (Eds.). (2011). Pharmaceutical Stress Testing: Predicting Drug Degradation (2nd ed.). London, UK: Informa Healthcare.

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