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To determine loss on drying (LOD)

To determine loss on drying (LOD)

Definition: Loss on drying compares the weight of product sample before and after drying.
The result is the percentage of moisture in a product.
% Moisture = (Begging weight-Ending weight) X 100
                                   Begging weight 
Loss on drying determine by two method which is given below:

Method (1): Weigh accurately a dry empty glass Petri dish. Put the sample (about 0.5 to 5 gm as par requirement, Its mean if sample having less wet take 5 g weight. If sample having more wet than take 0.5 g weight) in dish and weigh. Note down the reading. Distribute the sample in Petri dish by gentle shaking. Place the loaded dish (without cover) in the drying chamber for two hours. Maintain the oven temperature 105 +/- 50C. 
After drying is completed, open the drying chamber, cover the dish and allow it to cool at room temp. Keep it in the desiccator for 15 minutes. Weigh it and calculate the loss on drying percentage as follow.
Calculation :-
Wt of Petri dish =W
Wt of Petri dish + sample = W1
Wt of sample= M
After drying wt of Petri dish + sample = W2

Loss on drying % = (W1 – W2)  x 100
Method (2): In the second method LOD determine through the moisture analyzer.
Moisture analyzer having two component: balance and heater.

  • Sample is placed in the analyzer.
  • Balance records initial weight.
  • Heater heats the sample.
  • Weight is recorded continuously during the test until the sample stops losing weight. 
  • The analyzer automatically shuts off at this point, using the final weight to calculate % moisture. 
Why does the heating the sample take out the moisture ?
The process of LOD moisture analysis is made possible by the phenomenon of evaporation. Evaporation is the process of changing moisture from a liquid to a gas or vapor without boiling. This process happens when kinetic energy in the material causes the water molecules to bounce off each other and fly out of the test sample. The process is driven by heating the sample in the automatic systems the heating continues until all the water molecules have gone and a sample weight stabilizes. Alcohol fat and other volatile can evaporate with the water to distort LOD results, likewise moisture captured in the structure of simple sample chemistry can also cause errors. 

Additional information for the purpose of interview

Question: What is difference between moisture content (M/C) and loss on drying (LOD) ?
Answer: Moisture content is determined by the Karl Fischer instrument. It is a titration method technique. It is determine only moisture content present in the sample.
Loss on drying test is designed to measure the amount of water and volatile matter in a sample when the sample is dried under specific condition. Sample should dry below its melting point.
Question: Why LOD content is always higher than moisture content.?
Answer: moisture result give only water content present in substance while LOD result gives water content, solvent in substance. Thus LOD content is always higher than moisture
Questions: What is moisture content, total moisture content & loss on drying?

Answer: Moisture content: It is amount of water present in a sample. It is determined by Karl Fischer titration.
LOD: Its loss in weight of a substance after drying under specific temperature and time. It is determined with moisture analyzer.
Total moisture content: Total moisture content includes bound water, free water, other volatile impurities and solvent. It is determined by moisture analyzer.

Some Important notes:

  1. Total moisture content is one of the critical control point during manufacturing of pharmaceutical product at granulation stage.
  2. It is critical process parameters (CPP) Refer annex 15 MHRA orange guideline for more detail on CPP
  3. It is used for continuous process verification of validated manufacturing process of Pharmaceutical product during commercial manufacturing of product.
  4. Moisture content is important before dispensing of active pharmaceutical ingredients. 
      5.Moisture content can be measured as:

                    M/W=Moisture content wet base
                    D/W= Dry content wet base
                    M/D= Moisture content dry base
                    W/D=Dry content dry base


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