In pharmaceutical industry the difference between assay potency and purity is major problem to understand for the newbies and sometime experienced also.

In pharmaceutical all (Assay, Potency and Purity) three are different terminologies and has different calculations.

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**Difference between assay potency and purity:**

Here we will understand what is the major difference between assay potency and purity. Before that we will see the meaning of all terminologies.

**What is Assay Or Definition of Assay:**

Assay is an analytical technique to determine the quantity of the drug product.

Or

It is a procedure to analysed and determine the percentage of the drug present in the product.

In the pharmaceutical industry, an assay is the main test for any drug product. As there is multiple tests for the drug product but the assay is a test that gives proper information of drug content percentage.

**How to calculate assay?**

Assay can be calculated on three different ways. Such as **assay on as is basis,** **assay on dried basis and assay on anhydrous basis.**

So here we will understand with the formulas and calculate the value of assay with the help of its examples.

**Assay on As is Basis: **

Here is a **calculation Formula for assay on as is basis**,

Sample area x standard concentration

=——————-–——–——―———————–x potency or Assay of Standard

Standard area x sample concentration

Now according to formula we can calculate the assay of the drug product.

With the help of example understand the calculation;

Suppose,

Area of the sample is = 5000

Area of standard is = 5200

Concentration of the sample is = 9.9

Concentration of standard is = 10

Potency or assay of standard is = 99.8%

Loss on Drying (LOD) of sample is = 2%

Water content of sample is = 2.4%

Then according to formula,

**Calculation formula for Assay on as is Basis**:

5000 x 10

=—————————- x 99.8 = 96.9308 %

5200 x 9.9

**Assay on dried basis:**

Now for the assay on dried basis, it is important to eliminate the LOD value of sample.

**Calculation Formula for assay on dried basis:**

Assay on as is basis

=———————————————x100

(100-LOD)

According to formula the calculation of on dried basis assay is;

LOD of sample is as mentioned above = 2 %

As is basis assay is calculated as above =96.9308%

96.9308

**Assay on Dried Basis** = ———————————— x 100 = 98.90897 %

(100-2)

**Assay on anhydrous basis:**

Now hare for assay on anhydrous basis, need to eliminate the water content of the sample.

**Calculation Formula for assay on anhydrous basis:**

Assay on as is basis

=————————————— × 100

(100 – water content)

So according to formula the calculated value of the assay is;

Water content of the sample is = 2.4%

As is basis assay is calculated above = 96.9308 %

**Calculation Formula for Assay on Dried Basis:**

96.9308

= —————————— x 100 = 99.3143 %

(100 – 2.4)

Here we understand the difference between assay on as is basis, assay on dried basis and assay on anhydrous basis.

*Also Read:*

*Difference Between Dissolution and Disintegration*

*Principle of Quality Control Instruments in Pharmaceutical*

Here described the only assay. In the difference between assay potency and purity we are looking ahead for potency and purity.

**What is potency?** Or **How to calculate Potency?**

Potency can be calculated by eliminating of % impurity, heavy metal, sulfated ash, residual solvent and LOD or Water content from the targeted drug.

It can be represent by following formula:

**Calculation formula for Potency:**

100-(%Imp.+RSL+Sulfated ash+Heavy metal+LOD or Water content)

Just understand with an example **how to calculate potency**.

Suppose,

% impurity present is = 0.3 %

Residual solvent is = 100 ppm or 0.01 %

Sulfated ash is = 0.2 %

Heavy Metal = 10 ppm or 0.001 %

LOD or Water content is = 2 % and

Target value of assay is = 100 %

So, by applying value in formula,

= 100-( 0.3+0.01+0.2+0.001+2)

= 100 – 2.511

= 97.489 %

This will be the potency of the drug after eliminating all impurities.

**What is purity? **or** How to calculate purity?**

Purity is the drug is determined by the area normalization method. It is directly calculated by chromatographic system and found on the chromatograms obtained from the HPLC system.

Purity can be represent by following formula:

**Calculation formula for Purity:**

Area of individual peak or desired peak to calculate

= ————————————————————————————– x 100

Sum of areas of all peaks (including principle peak)

Understand **how to calculate purity** with the help of following example:

suppose,

Area of individual peak is = 1000, and

Sum of area of all peaks is = 5000000

Then the calculated value will be as follow,

**% Purity Calculation:**

1000

= ———————– x 100 = 0.2%

500000

= 100-0.2

=99.8%

**Conclusion:**

Pharmaceutical industry is quality oriented industry that requires high accuracy while producing drug and the assay or potency or purity of sample is a key factor to confirm the quality of the drug.

Pharmaceutical industry producing life saving drugs due to that it is important to understand difference between assay potency and purity of the drug.

I hope this article will help to understand the **difference between assay potency and purity**.

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